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CORRIB ONSHORE GAS PIPELINE ORAL HEARING REPORT ON GROUND MOVEMENT AND PEAT STABILITY

By

CONOR O’DONNELL, BA, BAI, MS, C.Eng, MIEI

for

An Bord Pleanála 64 Marlborough St.

Dublin 1 Tel: (01) 858 8100 Fax: (01) 872 2684

AGL Consulting Suite 2, The Avenue

Beacon Court, Sandyford Dublin 18

Tel: (01) 295 6532 Fax: (01) 295 6533

September 18th, 2009

Corrib Onshore Gas Pipeline Oral Hearing Report on Ground Movement & Peat Stability

September 18th, 2009 Page i


1.0 INTRODUCTION ........................................................................................................................... 1

2.0 GROUND CONDITIONS AND PROPOSED METHOD OF CONSTRUCTION ................... 4

2.1 INTRODUCTION .................................................................................................................................. 4

2.2 SECTION 1: - CH. 83+400 TO 83+910 (510 M): GLENGAD LANDFALL & DOONCARTEN MOUNTAIN .. 5

2.2.1 Ground and Groundwater Conditions ..................................................................................... 5

2.2.2 Proposed Method of Construction ........................................................................................... 5

2.3 SECTION 2: - CH. 83+910 TO 84+510 (600 M): SRUWADDACON BAY LOWER CROSSING ................... 7

2.3.1 Ground Conditions .................................................................................................................. 7


2.4 SECTION 3: - CH. 84+510 TO 85+990 (1480 M): ROSSPORT LANDFALL TO ROSSPORT COMMONAGE . 9

2.4.1 Ground and Groundwater Conditions ..................................................................................... 9


2.5 SECTION 4: - CH. 85+990 TO 88+350 (510 M): ROSSPORT COMMONAGE ......................................... 10

2.5.1 Ground Conditions ................................................................................................................ 10

2.5.2 Proposed Method of Construction ......................................................................................... 10

2.6 SECTION 5: - CH. 88+350 TO 89+600 (1250 M) - UPPER CROSSING SRUWADDACON BAY. .............. 16



2.7 SECTION 6: - CH. 89+600 TO 91+000 (1400 M): SOUTH OF SRUWADDACON BAY TO L-1202........... 18



2.8 SECTION 7: - CH. 91+000 TO 91+550 (450 M): L-1202 TO EXISTING STONE ROAD LINKING TO

BELLANABOY TERMINAL SITE. ........................................................................................................ 20



2.9 SECTION 8: - CH. 91+550 TO 92+550 (450 M): EXISTING STONE ROAD LINKING TO BELLANABOY

TERMINAL SITE. ............................................................................................................................... 21

2.9.1 Ground and Groundwater Conditions ................................................................................... 21


3.0 GROUND MOVEMENT RISK – NON PEAT AREAS ................................................................. 23

3.1 INTRODUCTION ................................................................................................................................ 23


September 18th, 2009 Page ii

3.2 SECTION 1: - CH. 83+400 TO 83+910 (510 M): GLENGAD LANDFALL & DOONCARTEN MOUNTAIN 23

3.2.1 Ground Movement Risks ........................................................................................................ 23

3.2.2 SEPIL Assessment .................................................................................................................. 24

3.2.2.1 Potential Impact of Landslides on Dooncarten Mountain on the LVI and gas pipeline at Glengad .. 24

3.2.2.2 Potential Impact of Rock Excavation Works on the Destabilised Material on the slopes of Dooncarten Mountain. ..................................................................................................................... 26

3.2.2.3 Coastal Erosion at the Glengad Landfall .......................................................................................... 27

3.2.2.4 Slope Stability and Pipe Settlement ................................................................................................. 27

3.2.3 Review and comment ............................................................................................................. 28

3.2.3.1 Potential Impact of Landslides on Dooncarten Mountain on the LVI and gas pipeline at Glengad . 28

3.2.3.2 Potential Impact of Rock Excavation Works on the slopes of Dooncarten Mountain: ..................... 30

3.2.3.3 Coastal Erosion at the Cliff Face at the Glengad Landfall ................................................................ 30

3.2.3.4 Slope Stability and Pipe Settlement ................................................................................................. 31

3.3 SECTION 2: - CH. 83+910 TO 84+510 (600 M): SRUWADDACON BAY LOWER CROSSING ................. 32



3.3.2.1 During construction – scour around an intervention pit. .................................................................. 32

3.3.2.2 After construction – natural scour in the bay. ................................................................................... 33

3.3.3 Review and Comment ............................................................................................................ 34



3.4 SECTION 3: - CH. 84+510 TO 85+990 (1480 M): ROSSPORT LANDFALL TO ROSSPORT COMMONAGE

36




3.5 SECTION 5: - CH. 88+350 TO 89+600 (1250 M) - UPPER CROSSING SRUWADDACON BAY. .............. 38









September 18th, 2009 Page iii

4.0 GROUND MOVEMENT RISK IN THE BLANKET BOG (PEAT STABILITY) ...................... 41

4.1 INTRODUCTION ................................................................................................................................ 41

4.2 GROUND MOVEMENT RISKS (PEAT STABILITY) ............................................................................... 41

4.2.1 Planar peat slide – Undrained Condition: ............................................................................ 42

4.2.2 Planar peat slide – Drained Condition (Uplift/Buoyancy): ................................................... 43

4.2.3 Bog Burst: .............................................................................................................................. 44

4.2.4 Local Shear Failure:.............................................................................................................. 45

4.2.5 Planar Sliding at the Base of the Stone Road: ....................................................................... 45

4.2.6 Settlement of the Stone Road: ................................................................................................ 46

4.3 SEPIL ASSESSMENT ........................................................................................................................ 47

4.3.1 Planar sliding – Undrained Condition (Short-Term/Total Stress) ........................................ 47

4.3.2 Planar sliding – Drained Condition (Long Term/Effective Stress) ........................................ 52

4.3.3 Bog Burst ............................................................................................................................... 54

4.3.4 Local Shear Failure ............................................................................................................... 55

4.3.5 Planar Sliding at the Base of the Stone Road ........................................................................ 56

4.3.6 Settlement of the Stone Road ................................................................................................. 59

4.4 REVIEW AND COMMENT .................................................................................................................. 61

4.4.1 Planar sliding – Undrained Condition (Short-Term/Total Stress) ........................................ 61

4.4.2 Planar sliding – Drained Condition (Long Term/Effective Stress) ........................................ 66

4.4.3 Bog Burst ............................................................................................................................... 68

4.4.4 Local Shear Failure ............................................................................................................... 68

4.4.5 Planar Sliding at the Base of the Stone Road ........................................................................ 69

4.4.6 Settlement of the Stone Road ................................................................................................. 71

5.0 QUANTIFIED RISK ASSESSMENT – GROUND MOVEMENT RISK .................................... 73

6.0 OTHER GEOTECHNICAL CONSIDERATIONS ........................................................................ 78

6.1 HAULAGE ON RAMPART ROADS ........................................................................................................ 78

6.1.1 Proposed haul route .............................................................................................................. 78

6.1.2 Pavement assessment – Falling Weight Deflectometer and Pavement Condition Surveys .... 79

6.1.3 Proposed improvement works ................................................................................................ 80


6.2 STONE ROAD – PERMEABILITY OF BASAL LAYER AND TRANSVERSE PLUGS...................................... 83

6.3 TUNNELING RATES – SRUWADDACON BAY CROSSINGS .................................................................. 84

6.4 BENTONITE – BREAK OUT. ................................................................................................................ 85


September 18th, 2009 Page iv

7.0 SUMMARY AND CONCLUSIONS ................................................................................................ 86

7.1 INTRODUCTION ................................................................................................................................ 86

7.2 BRIEF ............................................................................................................................................... 86

7.3 GROUND MOVEMENT RISK – NON PEAT AREAS .............................................................................. 88


7.2.2 Conclusions ........................................................................................................................... 89

7.2.2.1 Dooncarten Landslides: ..................................................................................................................... 89

7.2.2.2 Rock Excavation: .............................................................................................................................. 89

7.2.2.3 Coastal Erosion: ................................................................................................................................ 90

7.2.2.4 Slope Stability and Pipe Settlement: ................................................................................................. 90

7.2.2.5 Upper and Lower Sruwaddacon Bay Crossings: ............................................................................... 91

7.3 GROUND MOVEMENT RISK IN THE BLANKET BOG (PEAT STABILITY) ............................................. 92


7.3.2 Conclusions ........................................................................................................................... 94

7.3.2.1 Planar Sliding – Undrained Condition: ............................................................................................. 94

7.3.2.2 Planar Sliding – Drained Condition: ................................................................................................ 97

7.3.2.3 Bog Burst and Localized Shear Failure: ........................................................................................... 98

7.3.2.4 Planar Sliding at the Base of the Stone Road: .................................................................................. 99

7.3.2.5 Settlement of the Stone Road: ........................................................................................................ 100

7.4 QUANTIFIED RISK ASSESSMENT ..................................................................................................... 102

7.5 OTHER GEOTECHNICAL CONSIDERATIONS ..................................................................................... 104

7.5.1 Haulage on rampart roads .................................................................................................. 104

7.5.2 Stone Road – permeability of basal layer and transverse plugs .......................................... 105

7.5.3 Tunneling rates – Sruwaddacon Bay Crossings .................................................................. 106

7.5.4 Benonite break out during microtunneling .......................................................................... 106

7.6 PROPOSAL FOR ADDITIONAL INFORMATION................................................................................... 107

REFERENCES


September 18th, 2009 Page 1


By

CONOR O’DONNELL, BA, BAI, MS, C.Eng, MIEI

1.0 INTRODUCTION This report was prepared by Mr. Conor O’Donnell as specialist geotechnical adviser to the inspector for an Bord Pleanala, Mr. Martin Nolan, at the oral hearing application for the onshore section of the Corrib Gas Pipeline from the landfall site at Glengad, to the Terminal Site in Bellanaboy (Ch. 83+400 to 92+550) in Co. Mayo. The specific references for the Oral Hearing are: Pleanála Development: Corrib Onshore Gas Pipeline Type of Application: Strategic Infrastructure Development (16.GA.0004) Compulsory Acquisition Order (16.GA.0004) Applicant : Shell E&P Ireland Ltd. (SEPIL) My brief was to consider and advise on:

The adequacy of the pipeline design to withstand ground movement and peat instability which may arise from the construction of the pipeline as now proposed and which may arise during the operational life of the pipeline.

The adequacy of the proposed method of construction in peat

The impact of the proposed construction on the stability of the adjoining peat

The risk posed by the construction of the pipeline whereby surface water may gain access to the adjoining peat either during construction or post construction when the pipeline has been reinstated.

The risk posed to failure in the adjoining peat and how satisfactory the construction methods and mitigation measures proposed by the applicant are in eliminating this risk.

Has the risk of ground movement been adequately assessed in the Quantified Risk Assessment.

I have also been requested to comment on some other geotechnical considerations for the project, including:



Haulage on the rampart roads

The effectiveness of the basal layer and transverse plugs in preventing vertical and longitudinal drainage along the stone road

Tunneling rates for the Upper and Lower Crossings of Sruwaddacon Bay, and

The risk of bentonite break out for the microtunnel crossings. The format of the report is as follows:

The report will give a summary of the ground conditions and the proposed method of construction for each section of the pipeline in sequence from the landfall site at Glengad, to the Terminal at Bellanaboy.

The report will then look at the risk of ground movement in the non-peat areas, specifically focusing on the risk and potential impact of landslides at Dooncarten Mountain in Glengad during and after construction, and the risk of scour undermining the pipeline at the Sruwaddacon Bay Crossings.

The risk of peat instability will then be considered in the peat areas in Rossport Commonage and South of Sruwaddacon Bay to the Terminal site. This section will specifically address the potential impact that the proposed stone road method of construction will have on peat stability, both during and after construction, as well as the potential impact that peat instability or settlement of the stone road could have on the pipeline after it has been constructed.

Specific comment will be presented on the way in which Ground Movement was assessed in the Quantified Risk Assessment.

Commentary will be provided on other geotechnical considerations for the project, including the impact of haul routes on the stability of narrow rampart roads on peat in Rossport, the permeability of the proposed hydraulic barriers and peat plugs along the stone road, tunneling rates for the Sruwaddacon Bay crossings, and the risk of bentonite break out on the crossings.

Conclusions will then be drawn on the risk of ground movement and peat stability along the pipeline, and on the other geotechnical considerations addressed in the report.

This planning application covers the section of onshore pipeline between Ch. 83+400 and 92+530 at the landfall site. For the purpose of this report, the alignment has been subdivided into the following eight sections:

Section 1: - Ch. 83+400 to 83+910 (510 m): Glengad Landfall & Dooncarten

Section 2: - Ch. 83+910 to 84+510 (600 m): Sruwaddacon Bay - Lower Crossing

Section 3: - Ch. 84+510 to 85+990 (1480 m): Rossport Landfall to Rossport Commonage



Section 4: - Ch. 85+990 to 88+350 (2360 m): Rossport Commonage

Section 5: - Ch. 88+350 to 89+600 (1250 m): Sruwaddacon Bay - Upper Crossing

Section 6: - Ch. 89+600 to 91+000 (1400 m): South of Sruwaddacon Bay to L-1201

Section 7: - Ch. 91+000 to 91+550 (450 m): L-1201 to Existing Stone Road

Section 8: - Ch. 91+550 to 92+550 (1000 m): Existing Stone Road Linking to Belanaboy Terminal



2.0 GROUND CONDITIONS AND PROPOSED METHOD OF CONSTRUCTION

2.1 Introduction This section will present a concise summary of the ground conditions and the proposed method of construction for each section of the pipeline between the Glengad Landfall and the Terminal site at Bellanaboy. The ground conditions have been assessed based on the ground investigation reports presented in the applicants submissions, as well as from the additional geotechnical information submitted during the Oral Hearing. A full list of references is included at the end of this report. Where specific reference is required it will be identified in the text. The proposed method of construction is also a concise summary of the relevant information from the various documents presented in the Applicants submission, as well as from points that were clarified during questions and from additional information submitted by SEPIL during the Oral Hearing.



2.2 Section 1: - Ch. 83+400 to 83+910 (510 m): Glengad Landfall & Dooncarten Mountain

2.2.1 Ground and Groundwater Conditions The ground conditions in the area generally consist of loose to dense gravelly sand and medium dense to very dense sandy gravel over weak to strong moderately weathered psammite bedrock at a depth of 3.85 m to 5.0 mBGL. The deeper rock up to 5.0 mBGL was encountered near the LVI site and the quality of the rock down to a depth of 6.2 m was poor with 65% core recovery and 0% RQD (Rock Quality Designation). Groundwater observations in the piezometers and boreholes in the area show that the depth to water ranges from 2.5 to 4.5 mBGL. 2.2.2 Proposed Method of Construction The pipeline, umbilical and services will be constructed in an open trench battered back to stable side slopes at 1V:1.5H within a 40 m wide working area, as illustrated in Figure No. 2.1. The pipeline will have a minimum cover of 1.2 m, increasing to 1.6 m at road and ditch crossings, so the depth of the trench will be about 2.0 m along most of the pipeline. At the Landfall Valve Installation (LVI) the compound will be set down into a “dished” area approximately 3.0 m below original ground level to minimize the visual impact of the site. The pipeline will be below ground level inside the compound so that the total depth of excavation at the LVI will be up to 5.5 m deep. Rock is at a depth of approximately 5.0 m.

Figure No. 2.1 Typical Temporary Working Area (RPS[1])



It is proposed to excavate the trench and LVI dished area using mechanical excavators, possibly locally breaking rock with a light hydraulic breaker inside the trench. However, significant rock excavations are not anticipated and no blasting will be used to remove the rock from the trench.



2.3 Section 2: - Ch. 83+910 to 84+510 (600 m): Sruwaddacon Bay Lower Crossing

2.3.1 Ground Conditions The ground conditions across along the lower Sruwaddacon Bay crossing consist of loose to very dense granular deposits of silty gravelly sand and sandy gravel with occasional cobbles, small boulders and seams of silt and clay. The granular soils were underlain by Psammite, Quartz-Schist or Quartzite bedrock at variable depths across the bay. In Glengad, where the launch pit for the tunnel drive will probably be located, the depth to Psammite bedrock is about 3.85 m and the rock is described as strong and moderately weathered with 95-100% core recovery and RQD of 33-64%. Trial pits TW-1 to TW-4, which were excavated along the alignment of the pipeline on the foreshore of the bay at Glengad were excavated to depths of 2.5 to 3.0 m in running sand without encountering rock. There is no information on the depth to rock in this area. However, rock was encountered at a depth of 1.1 m in TP-W5 to the south, which is an indication that rock could be shallow in the area. Across the bay rock drops off to depths of 8.2 m to 24.8 m below sea bed level with the deepest rock at the centre of the channel and sharp changes in the rock level on either side of the bay. There is a layer of weathered non-intact rock 1.1 to 2.7 m thick at rockhead and this is generally underlain by very strong to extremely strong, slightly to moderately weathered Psammite, but Quartz Muscovite Schist was encountered at the deepest point in the channel. The level of rock rises sharply on the east side of the bay and there is a low cliff face of Quartzite exposed above high tide level. The ground conditions at the Rossport landfall are comprised of about 1.2 m of medium dense gravelly sand over Psammite bedrock. The rock is moderately strong to very strong and slightly weathered with an RQD of 42% to 91%. Groundwater level at the location of the launch pit and reception pit appears to be perched on rock at a depth of 2.5 to 3.2 m on the west side, and 1.1 m on the east side, although a piezometer in the rock on the east side showed tidal fluctuation in the water level between a depth of 1.1 m and 3.6 m. On the foreshore the groundwater level in the granular soils would be tidal and similar to the sea level in the Bay. Groundwater seepage was encountered at depths of 1.0 to 1.5 in the trial pits on the west side of the bay. Most of the trial pits on the east side were excavated below 0.5 to 1.0 m of water. 2.3.2 Proposed Method of Construction The proposed method of construction of the pipelines in this area is by the Direct Pipe microtunneling methods using a tunnel boring machine (TBM). The tunnel boring machine will be used to install an approximately 2.0 m diameter sleeve pipe between a



launch pit in Glengad and a reception pit in Rossport – a distance of approximately 600 m. The exact dimension of the sleeve pipe will be determined by the contractor selected to carry out the works. Bentonite will be injected into the annulus around the sleeve pipe as lubrication. A bentonite drilling mud will also be used for cooling the drill shield, removal of cuttings and stabilization of the tunnel face. When the sleeve pipe has been completed the gas pipeline, water outfall and umbilical pipes will be bundled together into a cluster and pulled through the sleeving pipe. The annular space around the pipes will be filled with cement grout upon completion. The depth of the pipeline below ground level will depend on the bend radius of pipes and the topography along the pipeline alignment chosen during detailed design. However, the minimum depth of cover in the bay is 3.0 to 4.0 m at Rossport and Glengad sides, respectively. The depth of cover increases below the channel due to the curvature of the pipe, but the pipe will probably be at or close to the normal depth of cover of 1.2 m at or near to the launch and reception pits on either side. The ground conditions along the tunnel horizon are likely to range from moderately strong to very strong Psammite or Quartzite rock, to coarse granular deposits of sandy gravel and gravelly sand with occasional cobbles and small boulders. Therefore, the proposed method of tunneling will have to be able to bore through granular soils and rock. The rock will most likely be encountered on land either side of the bay, possibly in the launch shaft and reception pit. The launch shaft will most likely be constructed on the west side of the Bay in Glengad, and the reception pit will most likely be on the east side in Rossport. SEPIL state that the contractor selected to do the work may elect to reverse the direction of the microtunneling operation. However, this is not considered to have a significant impact on the potential impact of the works with respect to ground movements or environmental considerations. The launch and reception pits will be constructed in open excavations, possibly supported by sheetpiles or some other form of temporary support in the overburden. Some rock excavation will probably be required on the Rossport side, where rock was encountered at a depth of 1.2 m. However, rock is at a depth of about 3.85 m on the Glengad side, so rock excavation will only be required in the pit if the pipe will be deeper than this. Some drilling will be carried out at each location to construct anchors into rock for the thrusting and pulling operations. An intervention pit would be required in the middle of the bay if an obstruction was encountered that impeded the tunneling and could not be removed by other methods such as manual excavation by men working inside the tunnel. However, SEPIL would consider that this is a very unlikely scenario. The intervention pit would consist of a sheetpile cofferdam around the TBM at the obstruction with associated dewatering, excavation, backfill and reinstatement by mechanical means from the surface.



2.4 Section 3: - Ch. 84+510 to 85+990 (1480 m): Rossport Landfall to Rossport Commonage

2.4.1 Ground and Groundwater Conditions The ground conditions along this section of the route generally consist of granular deposits of medium dense and dense gravelly sand and sandy gravel over moderately strong to strong Psammite bedrock. A 0.5 m thick layer of soft clay was encountered below the topsoil in most of the boreholes in the area, and very stiff boulder clay was encountered below the gravel at a depth of 4.8 m in one of the boreholes (Ch. 85+000). The depth to rock varies from 1.4 m to 10.1 m along this section of the pipeline, with the shallowest rock at the reception pit in Rossport. The rock is moderately strong to very strong slightly weathered Psammite with an RQD typically >50%. The groundwater strikes in the boreholes, and the water level observations in the piezometers indicated that the water level is generally at a depth of 1.5 to 5.0 m with a tidal fluctuation of about 1.5 to 2.0 m, particularly in the granular overburden. 2.4.2 Proposed Method of Construction The pipeline, umbilical and services will be constructed in an open trench battered back to stable side slopes at 1V:1.5H within a 40 m wide working area, as illustrated in Figure No. 2.1. The pipeline will have a minimum cover of 1.2 m, increasing to 1.6 m at road and ditch crossings, so the depth of the trench will be about 2.0 m along most of the pipeline, increasing to about 2.5 m at the road and ditch crossings. Some rock excavation may be required where rock is shallow in the vicinity of the reception pit in Rossport. This would probably be achieved by mechanical excavation and hydraulic breaking, similar to the Glengad ladfall and LVI site.



2.5 Section 4: - Ch. 85+990 to 88+350 (2360 m): Rossport Commonage 2.5.1 Ground Conditions This section of the pipeline crosses the lowland blanket bog of the Rossport Commonage. The section between Ch. 87+550 and 88+350 is part of the Glenamoy Bog Complex cSAC, whereas the section between Ch. 85+990 and 87+550 is a non-designated habitat. Both areas contain sections of eroding, cutover and intact lowland blanket bogland. There is limited ground investigation along this section of the pipeline. It is limited to hand probes that were carried out at regular intervals along the corridor of the pipeline to determine the depth of peat; and vane shear tests with an SL810 hand vane to determine profiles of undrained shear strength with depth at discrete points along the route. AGEC also produced geomorphological plans for the route based on a visual inspection of exposures and ground conditions along the route. The depth of peat through the Rossport Commonage ranges from <1.0 m in the cutover peat at the edges of the commonage and adjacent to the access road across the centre, to between 3.0 and 5.4 m in the following areas of relatively deep peat:

Ch. 86+190 and 86+550 Ch. 87+010 and 87+380 Ch. 87+940 and 88+240

The undrained shear strength of the peat recorded with the hand vane ranged from 1kPa to 25 kPa, but was generally between 4 kPa and 12 kPa, with an average of 8 kPa. There are no boreholes to confirm the stratigraphy of the soil or rock below the peat. AGEC have mapped cohesive mineral soil at the base of the peat at limited exposures in shallow peat cuttings along the route. However, within Rossport Commonage this appears to be limited to one area south of the pipeline at about Ch. 86+750. The mineral soil in this area is described as firm grey brown sandy gravelly silt with some tree roots. 2.5.2 Proposed Method of Construction Mr. Turlough Johnston of AGEC confirmed that the stone road method of construction will be used throughout the peat areas in the Rossport Commonage and South of Sruwaddacon Bay to the Terminal site at Bellanaboy. It will apply equally to the areas of eroding, cutover and intact blanket bog in the Glenamoy Bog Complex cSAC and in the non-designated blanket bog. However, the level of reinstatement of the surface vegetation will depend on the condition of the existing acrotelm or surface vegetated layer of the peat.



Typical details of the stone road construction are illustrated on Drawing Nos.864_02_004 to 007, which were submitted by AGEC as part of an Addendum to the EIS during the Oral Hearing (RPS[9]) and are included here as Figures Nos. 2.2 to 2.5. Stone road method of construction: The following is a summary of my understanding of the main aspects of the stone road construction method that will be used in the sections of non-designated and designated intact blanket bog:

The vegetated acrotelm layer of the peat will be machine cut in 0.5 -1.0 m thick turves across the width of the excavation for the stone road and stored in a single layer on timber bog mats adjacent to the excavation. Wherever possible the turves will be stored upslope from the excavation and outside the zone of influence of local shear failure at the sides.

The minimum width of the excavation will be sufficient to construct a platform

9.0 m wide at the top with 1V:1H side slopes down onto the mineral soil below the peat. For example, in peat up to 5.0 m deep the minimum width of the excavation will be 19.0 m (5.0 m + 9.0 m + 5.0 m). The excavation may need to be wider at the top to batter back the sides of the peat to stable temporary slopes.

Where possible the peat will be excavated in an open trench with the sides of the

peat cut back to stable temporary side slopes.

A minimum thickness of 0.5 m of peat will be left in place at the base of the trench as a hydraulic barrier between the road and the underlying mineral soil and the base of the stone road will then be formed by pushing coarse rockfill into the peat until firm resistance is achieved with the bucket of the excavator.

In deep very soft peat, where the sides of the excavation would not be stable over the full depth of excavation, the peat will be excavated down to the maximum depth at which the peat slopes will be stable, and then rockfill will be pushed into the peat below this level until firm resistance is achieved with the bucket of the excavator. Experience previously on the site would indicate that this method will be used in peat greater than about 3.0 m in depth. In these areas the base of the stone road below a depth of 3.0 m will consist of a mix of remoulded peat in a matrix of coarse rockfill.

All of the excess excavated peat will be transported directly to the peat repository at Srahmore. A small volume of peat will be stored locally at the compound sites for reinstatement. There will be no sidecasting of excavated material.

Similarly, the rockfill will only be stockpiled on the completed road or in the compound areas. None of the fill material will be stored on the peat.



Once the base of the road has been formed the rest of the stone road will be constructed in layers of well-graded rockfill up to within 0.6 to 0.7 m of the original peat surface. There will only be nominal compaction of the material by tracking over the fill with an excavator. No vibratory rollers will be used.

The drawings indicate that the sides of the excavation outside the rockfill will be filled with peat with a v-notch drain on the downslope side. The drawings do not clearly illustrate that the excavation will extend out to the toe of the rockfill slopes, which will be required to construct the 1V:1H slopes for the road.

During or following completion of the stone road transverse ‘plugs’ or hydraulic

barriers of low-permeability material will be constructed across the width of the stone road at a maximum spacing of 50.0 m to reduce longitudinal drainage along the road within the rockfill. The plugs will be typically a minimum of 1.0 m wide at the base. AGEC stated that the plugs will probably consist of remoulded peat mixed with rockfill.

After the road and transverse plugs have been constructed the gas pipeline, umbilical and water outfall pipes will be constructed in a trench within the stone road with a minimum depth of cover of 1.2 m. The depth of excavation will be on the order of 2.0 m below ground level, or about 1.5 m below the top of the stone road.

The transverse plugs will be reinstated around the pipes during and after installation of the pipeline and associates service pipes and umbilical.

Upon completion of the pipeline, the surface of the stone road will be covered in a regulating layer 100-200 mm thick of remoulded peat. The peat turves will then be reinstated across the surface of the regulating layer in a staggered pattern and any gaps between the turves will be hand-filled with turf off-cuts.

Where the transverse ‘plugs’ have been constructed across the stone road the peat turves will laid 100 mm higher than the surrounding area to control surface runoff. Peat turves will also be hand cut along a 0.5 m wide strip for a distance of 4 m upslope and 2 m downslope of the excavated area. These turves will be reinstated on a 100 mm thick regulating layer to raise the strips 100 mm above the surrounding ground to create “wings” on either side of the road.

In the areas outside the designated and non-designated intact bogland, where the acrotelm surface layer of the peat is cutaway or damaged, the method of construction for the stone road will be the same as in the intact bogland. However, the peat turves will not be stored for reinstatement unless the acrotelm layer is largely intact. Instead SEPIL will endeavour to restore the surface to a similar condition using the excavated peat, which will be stored locally in the site compounds or on the bog mats adjacent to the excavation. However, it is not clear from the information provided where exactly it is proposed to store peat on bog mats outside of the zones of intact bog.



Figure No. 2.2 Stone road construction – typical details

Figure No. 2.3 Typical reinstatement details along stone road



Figure No. 2.4 Plan view of reinstatement details for stone road in intact blanket bog

Figure No. 2.5 Transverse “plugs” along stone road. Site Compound: There is a site compound – SC5 – on the west side of road crossing RDX-2 on the L-52453-0. The compound will be constructed in an area of cutover blanket bog where the depth of peat is typically <1.0 m.



The method of construction of the compound is not detailed in the EIS. However, in evidence from RPS presented at the oral hearing it is stated that peat will be excavated from under the site and the site will be temporarily surfaced with a crushed rockfill. The peat will be stored for reinstatement. On cross questioning regarding the ecohydrogeological controls on groundwater movement and drainage in the compounds, RPS stated that the 0.5 m thick lower layer of peat would be left in place at the base of the rockfill, similar to the stone roads, and that peat plugs around the perimeter of the compound could be considered to prevent drainage of the surrounding peat.



2.6 Section 5: - Ch. 88+350 to 89+600 (1250 m) - Upper Crossing Sruwaddacon Bay.

2.6.1 Ground Conditions On the north side of the Bay the thin cover of peat has been largely cutaway and the ground conditions consist of < 1.0 m of peat over firm brown slightly sandy, slightly gravelly silt, and silty to very silty gravelly sand or sandy gravel with occasional to many cobbles. One borehole in the area was terminated at refusal on possible Sandstone rock at a depth of 5.4 m. The shoreline is formed by a 5-6 m wide terrace of silty gravelly sand. Within the bay there are estuarine deposits of very loose to loose and medium dense silty fine and fine to medium sand with seams of silt, clay and peat at depths of 8.0 – 10.0 m below sea bed level where the overburden is deepest in the channel. The sand grades to a coarser gravelly sand at the sides of the channel and in some of the boreholes the estuarine deposits were underlain by very stiff boulder clay at a depth of about 10.0 m below sea bed level. Bedrock classified as Psammite, Psammitic Schist or Semi-Pelitic Schist was encountered at a depth of 3.0 m to 16.3 m below sea bed level in the rotary coreholes, increasing in depth towards the centre of the channel. On the north shore the rock rises to a depth of 3.0 m, whereas on the south shore it is at a depth of 7.5 to 9.2 m. The quality of the rock is very variable, ranging from highly weathered non-intact rock at rockhead in some of the coreholes, to very strong slightly or moderately weathered thinly foliated rock with RQD of 0-50%. On land at Aghoos on the south side of the bay the ground conditions consist of up to 1-2 m of peat over granular deposits of loose to medium dense gravelly sand and very sandy gravel. Cohesive deposits of stiff light grey slightly sandy clay with lenses of fine to medium gravel were encountered below a depth of about 6.5 m. Very weak highly to completely weathered Psammite bedrock was cored below a depth of 9.3 m. SPT N-Values in the rock ranged from 12 to 25. Groundwater was struck at ground level in the borehole on the northern and southern shores. A piezometer was installed in the borehole on the southern shore with a response zone extending into the cohesive overburden and rock but there are no water level readings reported. 2.6.2 Proposed Method of Construction The proposed method of construction of the pipeline in this zone is by the Direct Pipe microtunneling methods using a tunnel boring machine (TBM), as described in Section 2.3.2 for the Lower Crossing of Sruwaddacon Bay. The length of the tunnel drive in this area will be approximately 1.03 km from Ch. 88+520 to 89+550. It is currently proposed that the tunnel will be constructed in a single drive from south to north across the bay.



The depth of the pipeline below ground level will depend on the bend radius of pipes and the topography along the pipeline alignment chosen during detailed design. The vertical profile drawings included with the EIS show that the actual depth to the pipeline crown ranges from 3.0 to 4.0 m at the edge of the water on either side of the Bay, to 5.0 to 6.3 m under the Bay. The pipe will probably be at or close to the normal depth of cover of 1.2 m at or near to the launch and reception pits on either side. The ground conditions along the tunnel horizon are likely to generally consist of estuarine deposits of very loose silty to very silty fine and fine to medium sand, grading to loose to medium dense gravelly sand and very sandy gravel at the edges of the channel. Depending on the tunnel depth on the south side, Semi-Pelitic Schist may be encountered in the tunnel bore. The quality of the rock is very variable and ranges from highly weathered non-intact rock recovered as angular fine to coarse gravel and cobbles sized clasts from 3.0 to 6.0 m below sea bed level, to strong moderately weathered rock with 100% core recovery and RQD of 0-68% below a depth of 6.0 m. Therefore, the proposed method of tunneling will have to be able to bore through granular soils and rock. The launch shaft will most likely be constructed on the north side of the Bay in Rossport, and the reception pit will most likely be on the south side in Aghoos. SEPIL state that the contractor selected to do the work may elect to reverse the direction of the microtunneling operation. However, this is not considered to have a significant impact on the potential impact of the works with respect to ground movements or environmental considerations. A large work area is identified around the site compound at the reception pit in Aghoos for peat turving, stockpiling and pipeline stringing. The pipes will probably be strung out on the access road to the compound. The launch and reception pits will be constructed in open excavations, possibly supported by sheetpiles or some other form of temporary support in the overburden. Some rock excavation may be required on the Rossport side, where rock was encountered at a depth of 5.4 m, depending on the depth of the tunnel. Some drilling will be carried out at each pit to construct anchors into rock for the thrusting and pulling operations. An intervention pit would be required in the middle of the bay if an obstruction was encountered that impeded the tunneling and could not be removed by other methods such as manual excavation by men working inside the tunnel. However, SEPIL would consider that this is a very unlikely scenario. The intervention pit would consist of a sheetpile cofferdam around the TBM at the obstruction with associated dewatering, excavation, backfill and reinstatement by mechanical means from the surface.



2.7 Section 6: - Ch. 89+600 to 91+000 (1400 m): South of Sruwaddacon Bay to L-1202.

2.7.1 Ground Conditions This section of the pipeline crosses the lowland blanket bog on the south side of Sruwaddacon Bay. It is a non-designated habitat of recovering and eroded blanket bog with some intact blanket bog between Ch. 90+200 and 90+400. The section between Ch. 90+400 and 91+000 has been forested with some clearings along the route of the pipeline. There is limited ground investigation along this section of the pipeline. It is limited to hand probes that were carried out at regular intervals along the corridor of the pipeline to determine the depth of peat; and vane shear tests with an SL810 hand vane to determine profiles of undrained shear strength with depth at discrete points along the route. AGEC also produced geomorphological plans for the route based on a visual inspection of exposures and ground conditions along the route, and there are a number of boreholes, trial pits and probes from the ground investigation for the previous alignment of the pipeline at Ch. 90+700, just west of the bend in the pipeline. The depth of peat through this section of the pipeline generally ranges from 1.0-2.0 m in the cutover and eroding peat from Sruwaddacon Bay up to Ch. 90+400. Between Ch. 90+400 and the L-1202 at Ch. 91+000 the depth of peat increases from 2.0 to 4.0 m. The depth of peat is > 3.0 m between Ch. 90+750 and 91+000 and it is described as considerably to highly amorphous. The undrained shear strength of the peat recorded with the hand vane ranged from 1kPa to 19 kPa, but was generally between 4 kPa and 15 kPa, with an average of 11 kPa. At Ch. 90+700 the minimum undrained shear strengths recorded with the Geonor H-10 vane ranged from 7.0 to 8.0 kPa. The boreholes along this section of the pipeline indicate that the peat is underlain by loose and medium dense sand and gravel near the shore of Sruwaddacon Bay and very soft to soft light grey slightly sandy, slightly gravelly clay with occasional cobbles (SPT N-Value = 0 & 6) in the forested area at Ch. 90+700, near the bend in the pipeline. TP-06 in the same area was terminated in running sands below 2.3 m of peat, which indicates that the ground conditions below the peat in the area are very variable. A trial pit to the north of the pipeline (TP-07) at Ch. 90+700 was terminated at a depth of 2.9 m in very soft peat up to 3.55 m deep due to squeezing of the sides and heave at the base of the trial pit. The peat probes in this area were pushed to refusal at depths of 2.5-2.9 m, and one of the probes adjacent to the L-1202 refused on wood in the peat below a depth of 2.3 m, possibly indicating the presence of relic tree stumps at the base of the peat. The pipeline crosses the estuary of the Leenamore River at Ch. 90+060 to 90+110. The geomorphology maps by AGEC also identify possible watercourse crossings at Ch. 91+350, 90+550 and near the crossing of the L-1202 at Ch. 91+880.



Based on the groundwater monitoring carried out by QMEC at Aghoos, groundwater level during the winter period is at or close to the ground surface and there is a downward hydraulic gradient over the depth of peat with a lower phreatic level in the underlying mineral soil. 2.7.2 Proposed Method of Construction The proposed method of construction for this section of the pipeline is the stone road construction method described in Section 2.5.2. However, the peat turves will only be preserved where the acrotelm layer is intact in the sections of intact blanket bog along the route between Ch. 90+200 and 90+400. Elsewhere, SEPIL will endeavour to restore the surface to a similar condition using the excavated peat, possibly by turving. However, it is not clear from the information provided where exactly it is proposed to store peat on bog mats outside of the zones of intact bog. All of the surplus excavated peat will be taken directly to the Srahmore Repository site. The peat is a non-designated habitat generally classified as recovering and eroded blanket bog. The zone between Ch. 90+000 and 90+200 is in shallow peat about 1.0 m deep on the relatively steep slopes at the crossing of the Leenamore River Estuary. There is a zone of intact blanket bog between Ch. 90+200 and 90+400, but the section between Ch. 90+400 and 91+000 has been forested with some clearings along the route of the pipeline. There are special construction techniques proposed for the estuary crossing of the Leenamore River at Ch. 90+050 to 90+100 and for the stream crossing at Ch. 90+880. These will involve temporarily diverting the river through a flume, turving the salt marsh vegetated surface layer in the estuary, open trenching with dewatering for the pipeline, and installation of the pipeline with increased depth of cover (1.6 m) with a protective precast concrete slab 0.5 m above the crown of the pipe. Site compounds will be constructed in the peat to provide a stringing and stockpiling area around the reception pit on the south side of Sruwaddacon Bay (SC-7), and adjacent to the L-1202 near road crossing RDX-4 (SC-8). The method of construction of the compound is not detailed in the EIS. However, in evidence from RPS presented at the oral hearing it is stated that peat will be excavated from under the sites and that the sites will be temporarily surfaced with a crushed rockfill. The peat will be stored for reinstatement. On cross questioning regarding the ecohydrogeological controls on groundwater movement and drainage in the compounds, RPS stated that the 0.5 m thick lower layer of peat would be left in place at the base of the rockfill, similar to the stone roads, and that peat plugs around the perimeter of the compound could be considered to prevent drainage of the surrounding peat.



2.8 Section 7: - Ch. 91+000 to 91+550 (450 m): L-1202 to Existing Stone Road Linking to Bellanaboy Terminal Site.

2.8.1 Ground Conditions This section of the pipeline crosses sections of the lowland blanket bog that have been heavily forested. Apart from the short section between Ch. 91+000 and 91+200 which has been re-routed around an area of intact blanket bog, the rest of this section follows the original route of the pipeline where the trees have been felled. The ground conditions between Ch. 91+000 and 91+550 consist of 3.0 to 4.55 m of very soft considerably to highly amorphous peat with some to many fragments of bog wood below a depth of 2.1 to 2.3 m. Trial pits TP-04 and TP-05 in this area were terminated at depths of 2.1 to 2.3 m due to slumping and collapse of the peat. The undrained shear strength of the peat was determined using a Geonor H-10 Vane and ranged from 4.5 kPa to 26.5 kPa, but was generally <18 kPa. The average was 11.5 kPa. There is limited information on the ground conditions below the peat. TP-03 penetrated 1.6 m into slightly silty or clayey, gravelly to very gravelly sand below 3.5 m of peat on the north side of the watercourse at Ch. 91+500. QMEC Environmental Ltd. Carried out groundwater monitoring through the depth of peat at Bellagelly, however the groundwater levels had not reached equilibrium at the time of reporting. Based on monitoring at Aghoos and between Ch. 91+700 and 91+800 groundwater level in the area during the winter period is at or close to the ground surface and there is a downward hydraulic gradient over the depth of peat with a lower phreatic level in the underlying mineral soil. Groundwater seepages were noted in the peat at depths of 1.0 to 3.1 m in the trial pits. Running sand conditions were encountered at the base of trial pit TP-03. 2.8.2 Proposed Method of Construction The proposed method of construction for this section of the pipeline is the stone road construction method described in Section 2.5.2. The peat in the area has been heavily forested. Therefore, the acrotelm layer will not be preserved for reinstatement by turving. SEPIL will endeavour to restore the surface to a similar condition using the excavated peat. However, it is not clear from the information provided where exactly it is proposed to store peat on bog mats outside of the zones of intact bog. All of the surplus excavated peat will be taken directly to the Srahmore Repository site. There are special construction techniques proposed for the watercourse crossing at Ch. 91+500. These will involve temporarily diverting the river through a flume, open trenching with dewatering for the pipeline, and installation of the pipeline with increased depth of cover (1.6 m) with a protective precast concrete slab 0.5 m above the crown of the pipe.



2.9 Section 8: - Ch. 91+550 to 92+550 (450 m): Existing Stone Road Linking to Bellanaboy Terminal Site.

2.9.1 Ground and Groundwater Conditions This section of the pipeline will be constructed within an existing stone road that was completed along the original alignment of the pipeline between Ch. 91+550 and the terminal at Ch. 92+350. The road passes through an area of lowland blanket bog that has been heavily forested. There will also be a short section approximately 200 m long between Ch. 92+350 and 92+550 where the stone road will have to be constructed to connect to the terminal at Bellanaboy. The original ground conditions along the road between Ch. 91+550 and 92+550 consisted of 2.1 to 4.55 m of peat which was classified as very soft considerably to highly amorphous peat below the top 0.5 to 1.0 m acrotelm layer. Wood fragments were encountered in a number of the probes below a depth of about 2.0 to 2.5 m. Trial pits TP-06 and TP-07 in this area were terminated at depths of 2.6 to 2.7 m due to slumping and collapse of the peat below a depth of 2.3 to 2.4 m. The undrained shear strength of the peat was determined using a Geonor H-10 Vane. The strength ranged from 2.0 kPa to 26.5 kPa, but was generally <15 kPa. The average strength was 8.8 kPa. There is limited information on the ground conditions below the peat. However, TP-9 and TP-10 penetrated 0.6 to 0.9 m into the underlying mineral soil. An iron pan layer was identified at the base of the peat in both trial pits and the underlying soil is classified as medium to soft or loose pale greenish grey micaceous sandy silt with occasional gravel and cobbles. The fines content (%<0.063 mm) in the sample from TP-9 was 39%, which is consistent with the sample classification, but the grading of the sample from TP-10 is more representative of a granular deposit of silty very gravelly sand with a fines content of 11%. QMEC Environmental Ltd. Carried out groundwater monitoring in the peat on either side of the stone road between Ch. 91+700 and 91+800. The records show that groundwater level in the area during the winter period is within 0.5 to 1.0 m of the ground surface, with localized drawdown adjacent to the stone road. The piezometers also indicate that there is a downward hydraulic gradient over the depth of peat with a lower phreatic level in the underlying mineral soil. Groundwater seepages were noted in the peat at depths of 1.2 to 2.7 m in the trial pits. The saturated silt below the peat in TP-9 and TP-10 was unstable causing the overlying peat to slump in the pits.



2.9.2 Proposed Method of Construction The stone road has been completed in this area between Ch. 91+550 and 92+350, but the gas pipeline and associated umbilical and service pipes have not been constructed. The following is a summary of some of the other pertinent details related to the completed road:

A minimum thickness of 0.5 m of peat was left in place as a hydraulic barrier at the base of the road and the rockfill was pushed to refusal on the underlying mineral soil with the bucket of the excavator.

In the deeper areas of peat along the route (>2.5-3.0m) the construction of the road would have involved the controlled displacement of peat below the depth of stable excavation. In these areas the base of the road would be made up of a matrix of coarse rockfill mixed with remoulded peat possibly up to 2.5 m deep.

The transverse hydraulic barriers or “plugs” were not constructed across the road.

However, it is proposed to construct them prior to installation of the gas pipeline.

Remoulded peat that was excavated from the trench was sidecast on the east side of the road to depths up to 1.0 m. This peat is not fully contained by the road as the road runs perpendicular to the slope down to the watercourse at Ch. 91+550.

There is a row of sheetpiles across the base of the slope on the west side of the

road adjacent to the watercourse at Ch. 91+550. These sheetpiles would improve the stability of the peat on the slope. However, there are no sheetpiles across the slope on the east side of the road where the excavated peat was stored.

No peat turves from the surface acrotelm layer were preserved for reinstatement

and the road has been left exposed since it was constructed.

Significantly the stone road has not been constructed as far as the stream at Ch. 91+550 so the drainage effect of the rockfill would be minimized. This is demonstrated by the water level monitoring carried out by QMEC within the stone road, which shows that the water level is near the surface of the rockfill, similar to the water level in the adjacent peat, albeit with some drawdown adjacent to the road.

There is a short section approximately 200 m long between Ch. 92+350 and 92+550 where the stone road has not been constructed. The proposed method of construction for the pipeline in this area is the stone road method described in Section 2.5.2. As the peat has been forested the peat turves in the acrotelm layer will not be preserved for reinstatement but SEPIL will endeavour to restore the surface to a similar condition using the excavated peat. However, it is not clear from the information provided where exactly it is proposed to store peat on bog mats outside of the zones of intact bog. All of the surplus excavated peat will be taken directly to the Srahmore Repository site.



3.0 GROUND MOVEMENT RISK – NON PEAT AREAS 3.1 Introduction This section deals with the risk of ground movement on the pipeline in the non peat areas, as well as the potential impact of the pipeline construction on ground movement in these areas. The non peat areas were generally assessed for SEPIL by RPS. However, in an additional submission to An Bord Pleanála during the Oral Hearing, AGEC also carried out a review of the potential impact of landslides at Dooncarten Mountain on the LVI and gas pipeline, and Mr. Ian Wilson of RPS Energy (UK) and Benthic Solutions Ltd. carried out the analysis of potential scour in Sruwaddacon Bay in association with HR Wallingford. 3.2 Section 1: - Ch. 83+400 to 83+910 (510 m): Glengad Landfall & Dooncarten

Mountain 3.2.1 Ground Movement Risks The landfall at Glengad, the LVI site, and the section of pipeline between Ch. 83+400 and 83+910 are on the coastal at the base of Dooncarten Mountain where a series of landslides occurred on the steep upper slopes of the mountain during a period of intense rainfall on the night of September 19th 2003. Some remedial works and protective measures have been implemented in the area since the slides occurred, as described by Tobin [13]. However, there is still a risk that further slides could occur under similar rainfall events and possibly under less intense rainfall. Therefore, the following primary risks with respect to ground movement would need to be considered in the design and construction of the gas pipeline and LVI in this area:

Inundation of the LVI due to a debris flow from further slides on Dooncarten Mountain.

Erosion and scour in the watercourses along the pipeline during a period of intense rainfall similar to the event that triggered the landslides.

The impact of rock excavation works for the gas pipeline and LVI on potentially unstable material on the slopes of Dooncarten Mountain.

During questions to SEPIL at the Oral Hearing, it was confirmed that the potential impact of an above ground debris flow reaching the LVI would probably be limited to damage to the above ground actuators for the pressure control valves on the buried gas pipeline. If the actuators are damaged they would automatically close, which could lead to a shutdown situation at the terminal. If there were no replacement actuators stored on site then the lead time on replacing the actuators could be on the order of a few months, which could lead to a build up in pressure on the upstream side of the valve, possibly up to 345 bar, due to leakage in the valves at the wellhead. However, SEPIL stated that they



would be using a type of valve and actuator that they are already using on other gas pipelines and would maintain a stock of replacement valves, which would reduce the time required to replacing damaged valves or actuators at Glengad. The following risks would also need to be considered due to the proximity of the LVI site to the coast, and due to the relatively gently sloping topography along the pipeline:

Coastal erosion of the embankment at the landfall site. Slope stability along the alignment of the pipeline Pipe settlement

3.2.2 SEPIL Assessment 3.2.2.1 Potential Impact of Landslides on Dooncarten Mountain on the LVI and gas

pipeline at Glengad RPS [2] assessed the potential impact of a debris flow failure from Dooncarten Mountain on the pipeline. Their assessment is primarily based on a review of the report on the landslides prepared by Tobin Consulting Engineers (2003) for Mayo County Council. Based on this report, RPS concludes that (for the Glengad area):

The landslides occurred following a 1 in a 100 year rainfall event and were initiated on the steep upper slopes of Dooncarten Mountain.

The debris, once mobilized, behaved as a fluid due to the amount of entrained water.

The topography of the area affected the flow path of the debris material and drainage channels in the area became conduits for the debris.

A pre-existing berm between commonage lands and privately owned properties on the higher ground above the L-1202 road absorbed a significant portion of the energy of the flow.

No debris material reached the relatively flat lying coastal strip where the proposed pipeline is to be laid.

Remedial works carried out by Mayo County Council and the OPW since the failures, including reinstating the berm between the commonage lands and privately owned properties, and improved/repaired drainage systems at Glengad, will further reduce the prospect of a landslide reaching Glengad Headland where the pipeline is proposed.

RPS carried out an assessment of the potential impact of three different heights of failure – 1m, 2 m and 3 m - on the buried pipeline in the event of an “unlikely” debris flow failure reaching the pipeline. They state that the analysis demonstrates that the movement determined at the locations of the buried pipe will not jeopardize the integrity of the pipeline. However, the results of the analyses are not included in the report.



In questioning during the Oral Hearing, RPS stated that they had not carried out an independent inspection of the slopes either at the time of the landslides or since then as part of their assessment of the potential impact of the landslides on the gas pipeline. AGEC [10] also submitted a brief review of the Dooncarten Mountain Landslides in additional information submitted during the Oral Hearing. Their review was also primarily based on the report by Tobin (2003). However AGEC did carry out part of the investigations into the landslides in 2003 a few days after the event. In addition to the general comments on the cause of the landslides, AGEC made the following conclusions regarding the distribution of landslides at Glengad and the risk of a landslide on the LVI and pipeline route in the area:

Figure No. 3.1 – Risk Zoning of Dooncarten Mountain (AGEC – Ref)



On the steep northwest facing slopes of Dooncarten Mountain above Glengad there were 10 significant landslides and several small gully washouts.

The landslide debris was mostly comprised of peat, soil and boulders that passed directly downslope, initially overland, and in places became channelized.

Due to the shape of the ridges and the susceptibility of the slopes to failure, the landslides were essentially located on the northwest and northeast sides of the mountain and this was the direction of the associated landslide debris trail, as shown on Figure no. 3.1.

The landslides had less of an effect on the fields to the north of the mountain where the LVI is located and the LVI site was not affected.

Figure No. 3.1, which was produced by AGEC based on the Tobin report, shows that the north side of the mountain in Glengad is classified as a low risk area and the area of the LVI was at such a distance from the mountain that it was not considered within the risk zoning.

Mitigation measures included re-constructing the existing berm and improving the drainage on the mountain. The berm proved effective at containing debris during the failure event.

Debris impact/accumulation is associated with mostly the upper and mid-slopes of the mountainside and would not be considered a direct risk to the LVI taking into account the LVI’s distance from the failed slopes, slope morphology, slope inclination and previous failure tracks.

Erosion of watercourses occurred on the lower slopes. It is noted that the LVI is not sited on a watercourse. However, it is also recognized that there is a risk of erosion where the pipeline crosses watercourses in the area.

To protect the pipeline it will be placed 1.6 m below the watercourse beds and a concrete slab will be placed 0.5 m above the pipe. If there is still a risk of further erosion then the pipe can be buried deeper or the extent of the concrete protection can be increased.

It is also noted that there are several streams passing close to the tunneling work areas and that protective measures such as temporary bunding may be required to protect against flooding during construction.

3.2.2.2 Potential Impact of Rock Excavation Works on the Destabilised Material on the

slopes of Dooncarten Mountain. Some rock excavation may be required in Glengad for the gas pipeline at the LVI site and possibly at the launch pit for the microtunnel section at the Lower Crossing of Sruwaddacon Bay, depending on the depth of the pipe. RPS [2] have carried out an assessment of the excavatability of the rock and concluded that it should be possible to excavate the rock by mechanical excavator or with a medium weight hydraulic breaker . They state that the offshore pipeline works carried out in the area in 2008 required excavations 5.0 to 7.2 m deep in 2.5 to 3.0 m of coarse granular overburden over rock and the excavations were not problematic for conventional earthworks plant. There was no analysis in the EIS of the potential impact of vibrations



generated by rock excavation on the stability of the destabilized material on Dooncarten Mountain. At the Oral Hearing RPS confirmed that rock blasting will not be carried out for the construction of the onshore gas pipeline. When questioned about the potential impact that rock excavation works could have on the destabilised material on Dooncarten Mountain RPS stated that, based on their experience with rock breakers in similar conditions, vibrations generated by mechanical excavation or hydraulic breakers in the weathered rock at Glengad would be expected to be very low, of short duration, and would be unlikely to have any adverse effect on the stability material on the slopes of Dooncarten Mountain several hundred meters away. 3.2.2.3 Coastal Erosion at the Glengad Landfall In evidence presented at the Oral Hearing, Mr. Turlough Johsnton of AGEC gave details of an assessment carried out of the rate of erosion of the cliff face at the foreshore of the Glengad Landfall. He concluded that:

The cliff face is typically 3-4 m high and formed mostly of glacial soil with some colluvium and bedrock.

Visual inspection of the cliff face showed that the cliff was slowly retreating inland as a result of repeated minor failure and erosion from the face, particularly along the west face, where the gas pipeline comes ashore.

Based on field observations and a historical review over a period of 160 years, the rate of regression of the natural cliff is estimated at 0.01 m to 0.03 m/year per meter run of cliff face.

They state that locally greater rates of infrequent regression up to 2 m per year would be expected due to localized minor slumping.

Slope stability analyses indicate that the local stability of the cliff is marginal, as evidenced by the localized slumping, but that the factor of safety for the LVI site is >3, which is acceptable.

Set back distances of 5 m and 7 m are proposed for temporary and permanent works for the Corrib Gas Pipeline.

The LVI is set back approximately 50 m from the cliff face. 3.2.2.4 Slope Stability and Pipe Settlement RPS [2] carried out a slope stability analysis of the gas pipeline in a trench in the granular overburden along the Glengad coastal strip using the infinite slope method of analysis. The factor of safety against planar sliding at a depth of 5.5 m was calculated as 2.5 for a worst case slope of 10 degrees, which indicates that there is a very low risk of slope instability along the route of the pipeline in this area. With regard to pipe settlement, RPS state that the settlement of the pipe would be insignificant on the medium dense to very dense granular overburden in the area.



3.2.3 Review and comment 3.2.3.1 Potential Impact of Landslides on Dooncarten Mountain on the LVI and gas

pipeline at Glengad Based on a review of the Tobin report, I would agree with the combined conclusions of RPS and AGEC with regard to the potential impact of future landslides at Dooncarten Mountain on the LVI and the buried gas pipeline at Glengad, i.e.:

The LVI and the section of the pipeline along the Glengad headland is in a low risk zone with regard to the potential impact of landslides on Dooncarten Mountain.

It is unlikely that a debris flow from a landslide on the mountain would reach the LVI or pipeline due to the topography of the area, and due to the protection offered by the berm and drainage system at the base of the steep slopes on Dooncarten Mountain.

The main risk with regard to the LVI and pipeline is limited to the potential for erosion and scour in the watercourses along the route of the pipeline and there are established design measures that can be implemented to protect the pipeline from this risk.

This is based on the following main conclusions from the Tobin report with regard to the landslides at Glengad:

The landslides on Dooncarten Mountain were triggered by an exceptional and extreme rainfall event.

Although the failures that occurred in 2003 could have lowered the threshold of a rainfall event required to move previously destabilised peat deposits, mitigation measures have been implemented to reduce the risk of a potential landslide at Glengad. This involved reinstating the berm and drainage system at the boundary between the commonage land and private land holdings at the base of the steep slopes on Dooncarten Mountain, and upgrading culverts along watercourses on the L-1202 lower road through Glengad, as illustrated on Figure No. 3.2.

The earthen berm and drainage system at Glengad proved to be effective at intercepting the debris flow during the September 2003 landslides. Material which reached the LP-1202 was limited to fluidized flows confined to, or in the vicinity of, existing drainage channels.

The area at Glengad downslope from the earthen berm has been classified as low risk by Tobin after completion of the remedial works to the berm and drainage system in the area. Therefore, it is unlikely that a debris flow from a landslide on the mountain would reach the LVI or pipeline on the relatively gently sloping coastal strip at Glengad.



Aerial photos from after the slide would indicate that some of the fluidized flow from the slide did reach the streams and ditches along the field boundaries on the north side of the LP-1202, particularly near the location of the launch pit, as shown in Figure No. 3.2. Therefore, there is a risk that further intense periods of rainfall could lead to erosion and scour in these watercourses, and possibly some flooding in the area.

Figure No. 3.2 – Location of remedial works at Glengad and area where fluidized flow entered watercourses in the area. Note that we have not carried out an independent inspection of the slopes and this review is based on the data presented in the EIS, at the Oral Hearing and in the report on the landslides by Tobin. AGEC did carry out part of the investigations into the landslides a few days after the event but the slopes were not inspected recently as part of the design process for the EIS. Therefore, we would recommend that AGEC or RPS carry out an independent inspection of the slopes as part of the detailed design process to ensure that the conclusions in the Tobin report are still valid.

Launch Pit

Berm and Drainage

Cuvlerts

Area in vicinity of pipeline where aerial photos indicate that fluidised flow entered the watercourses



3.2.3.2 Potential Impact of Rock Excavation Works on the slopes of Dooncarten Mountain:

I would agree with the opinion of RPS that vibrations generated from the mechanical excavation and hydraulic breaking in the rock at the LVI site and possibly in the launch pit for the microtunnel section at the Lower Crossing of Sruwaddacon Bay are unlikely to have any significant effect on the destabilized material on the upper slopes of Dooncarten Mountain. This is based on my own experience with monitoring vibrations generated by rock excavations and the following conclusions:

No blasting will be carried out. The rock excavation at the LVI site will be in weathered rock and it should be

possible to remove much of this by mechanical excavation with only localized rock breaking with a hydraulic breaker.

If rock excavation is required in the launch pit for the Lower Sruwaddacon Bay crossing the quality of rock is better than at the LVI site. However, it should be still possible to excavate the rock by hydraulic rock breaking.

Vibrations generated by rock breaking would attenuate with distance from the site and would generally be of short duration.

The destabilized material is on the upper slopes of Dooncarten Mountain which is at a distance of over 1 km from the LVI site and launch pit. At this distance the vibrations generated by rock breaking would be nominal and below any significant threshold that might cause movement on the slopes.

This is also consistent with the assessment of Tobin Consulting Engineers, who ruled out vibrations from traffic and rock breaking as a contributory factor for the landslides on Dooncarten Mountain in 2003. To address the concerns of local residents I would recommend that vibration monitoring be carried out at two locations at a distance of about 25 m and 50 m from the site to establish a response curve for the attenuation of vibrations generated from the rock excavation and to demonstrate that the vibrations are unlikely to have an effect on the adjacent properties or the destabilized material on the upper slopes of Dooncarten Mountain. 3.2.3.3Coastal Erosion at the Cliff Face at the Glengad Landfall AGEC have carried out an analysis of the potential rate of regression of the cliff face based on a visual assessment and a historical review over a period of 160 years. The evidence presented does not state how the historical review was carried out, but we assume that this was based on a comparison of the extent of the shoreline over this time. I would consider that the historical review would be more accurate than an assessment based on visual inspection of the current condition of the cliff, provided that the review was based on reliable mapping of the surveyed coastline over the period of the study. If



the period exceeded 160 years it would include the potential impact of a storm with a 100 year return period, which should be adequate for the design of the LVI. Having said that there are a number of other factors that should be considered in the analysis:

The coastline protection has been altered by the removal of large boulders on the beach in front of the western cliff face at the pipeline landfall. These would have absorbed some of the wave energy during a storm.

The overburden on the cliff face has been disturbed due to the open cut for the gas pipeline, which makes it more susceptible to erosion.

Climate change could result in more severe storms over the design life of the pipeline.

This would seem to indicate that some form of coastal protection should be implemented along the cliff face to control the rate of erosion at the pipeline landfall. However, we also note that the LVI is set back 40 m in from the cliff face, which significantly exceeds the minimum recommended setback of 7 m for permanent works. This would mean that an observational approach could be used to monitor the rate of erosion over the design life of the pipeline and protective measures could be implemented if necessary to arrest the rate of erosion if it was likely to have an impact on the pipeline or the LVI. This may be the preferred option given that the area is a Special Area of Conservation. However, a more conservative design option would involve constructing some form of natural coastal protection to prevent erosion of the cliff face. This work would need to be carried out outside the boundary of this application before the Board and is really a matter for the foreshore licence. However, it does highlight the need for overlap between the two elements of the project. 3.2.3.4 Slope Stability and Pipe Settlement I would agree with RPS that slope stability and pipe settlement should not be a concern in the medium dense to very dense granular overburden over shallow rock on the relatively gentle slopes on the coastal strip of Glengad Headland.



3.3 Section 2: - Ch. 83+910 to 84+510 (600 m): Sruwaddacon Bay Lower Crossing

3.3.1 Ground Movement Risks This section of the pipeline will be constructed by direct pipe microtunneling techniques using a tunnel boring machine. Drawings submitted with the EIS show that the vertical alignment of the tunnel is curved with a 4m cover shown in the channel near Glengad water edge and a 3m cover near the Rossport water edge. The tunnel is deeper under the main length of channel because of its curved path. The ground conditions along the tunnel horizon across the bay are likely to generally consist of coarse granular estuarine deposits of sandy gravel and gravelly sand with occasional cobbles and small boulders. Moderately strong to very strong Psammite or Quartzite rock will be encountered at the edges of the bay on the Glengad and Rossport side. The main risk related to ground movement along this section of the pipeline would be due to the potential for scour in the bay to expose and undermine the gas pipeline. During construction this risk could occur if an intervention pit is required to remove an obstruction to the TBM. The pit would obstruct the natural flow in the bay, which could result in localized erosion and scour around the pit. After the pipeline has been constructed there could be a risk of natural scour under variable flow conditions in the bay, particularly in the deep channel. 3.3.2 SEPIL Assessment 3.3.2.1 During construction – scour around an intervention pit. In Section 14.7 of the EIS [1], and in evidence presented at the Oral Hearing, Mr. Ian Wilson of RPS/Benthic Solutions Ltd. stated that scour up to 7.5 m deep could occur around an 11.0 x 12.0 m intervention pit located in the strongest part of the current flow in the fast flowing deeper waters of the channel at the lower crossing, which is considered the worst case scenario. The analysis was carried out using a finite element hydrodynamic flow model that was developed for Sruwaddacon Bay and calibrated using observations recorded at four moorings in the Bay. Mr. Wilson states that the scoured granular materials will be deposited immediately outside the scour footprint within the channel and that, in most cases, the scour is expected to be temporary and will naturally infill very quickly on removal of the pit with the tidal processes. Nevertheless he also says that further mitigation may be required to fully reinstate sediment for deeper areas of scour. The potential impact of the scour on the gas pipeline is not addressed. However, some appropriate mitigation measures, including the use of scour protection are identified in Section 14.7.8 of the EIS.



3.3.2.2 After construction – natural scour in the bay. At the request of An Bord Pleanála, Mr. Ian Wilson presented the results of an analysis to determine the likelihood of the two proposed Sruwaddacon Bay crossings becoming exposed due to bed level changes arising from natural marine and estuarine sediment movement over time. The analysis was supported by Mr. Richard Whitehouse and Mr. Jort Wilkens of HR Wallingford. The report considers sources of sediment movement due to oceanographic factors including:

Tide Storm surges Waves Wind-driven currents River discharge, and Sea level rise

in addition to sedimentological factors such as:

Seabed sediment particle size and type Presence of harder/coarser ground Presence of bed forms Existing suspended sediment load.

The main conclusions of this assessment are that:

Tidal action is considered to be the dominant oceanographic control factor for bed level changes within Sruwaddacon Bay.

The generally coarse, granular material in the bay is in dynamic equilibrium with the dominant tidal action.

The potential for morphological changes in Sruwaddacon Bay over time was assessed by considering the historical changes to the river channel and the potential for future change to the channel. The analysis indicated that some variability in smaller channels was found to have taken place historically and that recent bathymetric data has identified three areas where there is potential for future changes in the course of the channel, particularly in the wide central section of the bay between the two crossings. However, there is no evidence of large scale changes to the bed level within the bay and it is considered that the location of the proposed Lower crossing would not be effected by possible future changes in the channel. The potential for natural scour at the lower pipeline crossing was assessed by considering the potential impact of increased tidal energy in the deep main tidal channel along the Rossport Bank. The analysis indicated that the maximum reduction in the sea bed level at this location would be no more than 2 m. It is concluded that the risk of exposing the pipeline buried to a minimum depth of 4.0 m below the sea bed is negligible.



3.3.3 Review and Comment 3.3.3.1 During construction – scour around an intervention pit. The analysis of scour is a very specialized area of expertise. The analysis that was carried out on the potential scour for the intervention pit would appear to be site specific, advanced and reasonably comprehensive as it is based on a numerical model using a finite element hydrodynamic flow model for Sruwaddacon Bay that was developed using hydrographic data for the bay and then calibrated based on measurements recorded at four mooring points in the bay. Scour depths were estimated based on the flow data calculated from the finite element analyses. The analyses indicated that up to 7.5 m deep could occur around an 11.0 x 12.0 m intervention pit located in the strongest part of the current flow in the fast flowing deeper waters of the channel at the lower crossing, which is considered the worst case scenario. However, the potential impact of this scour on the pipeline is not assessed. If the pipeline is constructed at depths less than 7.5 m to the crown of the sleeve pipe, then there is a risk that the pipeline could be exposed adjacent to the pit. RPS state that this scour is temporary in nature and would recover very quickly under natural tidal processes. However, eddies created by flow around the exposed pipeline could prevent this from occurring, which would restrict the natural restoration of the sea bed, potentially leaving the pipeline exposed after the intervention pit is removed, or possibly left covered but with a void below the pipe. In general I would consider that based on the ground conditions encountered in the boreholes at the tunnel horizon, the risk of requiring an intervention pit would be very low, in which case the potential for scour around a pit would be an unlikely scenario. In the unlikely event that an intervention pit is required, then the risk that the scour would have on the completed gas pipeline should be negligible if the problem is identified during construction so that the appropriate mitigation measures could be implemented, which may include grouting or controlled filling below the pipe if it is exposed. Some appropriate mitigation measures, including the use of scour protection are identified in Section 14.7.8 of the EIS. It should also be recognized that the potential for scour outside the deep water channel would be significantly less due to the lower tidal currents. Also, due to the curvature of the vertical alignment of the pipe, the actual depth of cover below the channel could be greater than 4.0 m and may exceed the estimated depth of scour. As a minimum it would advisable to identify the potential for scour in the Geotechnical Risk Register for the unlikely event that an intervention pit is required.



3.3.2.2 After construction – natural scour in the bay. RPS concludes that in general the coarse, granular material in the bay is in dynamic equilibrium with the dominant tidal action and that alignment of the pipeline at the Lower Crossing will not be effected by potential future changes to the channel alignment. However, when the potential for scour under an increased tidal energy is considered in the deep channel along the Rossport Bank the maximum scour depth is limited to 2.0 m, which should not expose the pipeline which will be buried with a minimum depth of cover of 4.0 m at the edges of the Bay and a little deeper under the channel due to the bend in the pipe. It is not stated what method of analysis is carried out, but I assume that the analysis was carried out using the hydrodynamic flow model that has been developed for the bay. If this is the case, then analysis would seem to be reasonable as it is very comprehensive and site specific. The analysis also considers the potential for scour in the event of increased tidal energy, presumably to account for a storm surge in Broadhaven Bay or an increase in sea level, although the assumptions are not stated in the report. Based on the analysie presented it would appear that the risk of exposing the pipe due to natural scour in the bay would be negligible.



3.4 Section 3: - Ch. 84+510 to 85+990 (1480 m): Rossport Landfall to Rossport Commonage

3.4.1 Ground Movement Risks In this section the pipeline will be constructed using conventional open-cut shallow trench techniques. The ground conditions consist of glacial till overburden with shallow rock in places. The potential ground movement risks that could apply to this area would include:

Slope stability on the coastal plain along the pipeline route. Shear failure on the slopes along the coast Erosion along the coastline Pipe settlement

There is no peat so peat stability is not an issue. 3.4.2 SEPIL Assessment RPS [2] carried out an analysis of the stability of the shallow slopes of the coastal plain along the alignment of the pipeline in this section. The analyses were carried out using the infinite slope method of analysis for slopes of 7 degrees in glacial till. The calculated factor of safety was 4.0. RPS also assessed for post-construction pipe settlement and concluded that, due to the medium to dense nature of the overburden material at the proposed invert level of the pipe, settlements of the pipe should be negligible. This was supported by a pipe bearing test in the area where settlements up to 6 mm were recorded at a depth of 2.1 m. RPS did not carry out an assessment of the potential for erosion along the coastline. However, in response to questions during the Oral Hearing RPS stated that they considered that the risk of coastal erosion along this section was negligible as there is rock at the base of the low cliff along the fast moving waters in the deep channel near the Rossport Landing, and along the rest of this section there are tidal mudflats along the coast, which suggest that it is a depositional environment, or an area sheltered from coastal erosion, which should pose no threat to the pipeline. These features are recorded in the aerial photographs and in the geomorphological plans produced by AGEC, which show the extent of rock along the coast.



3.4.3 Review and Comment In general this would be considered a negligible risk zone with regard to the potential impact of ground movements on the pipeline. Slope stability should not be an issue on the gently sloping coastal plain along the alignment of the pipeline. The pipe would also be set back far enough so that it is outside the limit of potential localized slope failures along the coast. Pipe settlements should also be negligible in the glacial till overburden and this is supported by site-specific pipe bearing tests. I would also agree with RPS that the risk of coastal erosion along this section of the pipeline is negligible due to the rock at the base of the low cliff above the water adjacent to the deep fast-flow water in the channel at the Rossport landing. Over the rest of this section there are tidal mudflats which suggest that it is a depositional environment, or an area sheltered from coastal erosion, which should pose no threat to the pipeline, particularly as the Bay is sheltered from the wave action offshore in the Atlantic.



3.5 Section 5: - Ch. 88+350 to 89+600 (1250 m) - Upper Crossing Sruwaddacon Bay.

3.5.1 Ground Movement Risks This section of the pipeline will be constructed by direct pipe microtunneling techniques using a tunnel boring machine. Drawings submitted with the EIS show that the vertical alignment of the tunnel is curved. At the Aghoose side the depth to the tunnel crown is shows as 4.0 m near the water edge and 5.0 m to 6.3 m in the crossing due to the curvature of the pipe. The depth then reduces to 3.0 m near the edge of the water on the Rossport side. The ground conditions along the tunnel horizon across the bay are likely to generally consist of estuarine deposits of very loose silty to very silty fine and fine to medium sand, grading to loose to medium dense gravelly sand and very sandy gravel at the edges of the channel. Semi-Pelitic Schist may be encountered in the tunnel bore on the south side of the bay. As described in Section 3.3.1, the main risk related to ground movement along this section of the pipeline would be due to the potential for scour in the bay to expose and undermine the gas pipeline. During construction this risk could occur if an intervention pit is required to remove an obstruction to the TBM. The pit would obstruct the natural flow in the bay, which could result in localized erosion and scour around the pit. After the pipeline has been constructed there could be a risk of natural scour under variable flow conditions in the bay, particularly in the deep channel. 3.5.2 SEPIL Assessment 3.5.2.1During construction – scour around an intervention pit. In section 14.7 of the EIS and in evidence presented at the Oral Hearing, Mr. Ian Wilson of RPS/Benthic Solutions Ltd. stated that scour up to 5.0 m deep could occur around an intervention pit with a 50.0 x 14.0 m pontoon moored alongside the pit. For the analysis the pit is located in the strongest part of the current in the shallower slower moving waters of the Upper Crossing, which is considered the worst case scenario. The analysis was carried out using a finite element hydrodynamic flow model that was developed for Sruwaddacon Bay and calibrated using observations recorded at four moorings in the Bay. Mr. Wilson states that the scoured granular materials will be deposited immediately outside the scour footprint within the channel and that, in most cases, the scour is



expected to be temporary and will naturally infill very quickly on removal of the pit with the tidal processes. Nevertheless he also says that further mitigation may be required to fully reinstate sediment for deeper areas of scour. The potential impact of the scour on the gas pipeline is not addressed. However, potential mitigation measures, including the use of scour protection are presented in Section 14.7.8 3.5.2.2After construction – natural scour in the bay. As described in Section 3.3.2.2, at the request of An Bord Pleanála, Mr. Ian Wilson presented the results of an analysis to determine the likelihood of the two proposed Sruwaddacon Bay crossings becoming exposed due to bed level changes arising from natural marine and estuarine sediment movement over time. The analysis was supported by Mr. Richard Whitehouse and Mr. Jort Wilkens of HR Wallingford. The main conclusions of this assessment are that:

Tidal action is considered to be the dominant oceanographic control factor for bed level changes within Sruwaddacon Bay.

The generally coarse, granular material in the bay is in dynamic equilibrium with the dominant tidal action.

There is no evidence of large scale changes to the bed level within the bay and it is considered that the location of the proposed Upper crossing would not be effected by possible future changes in the channel.

The assessment of the potential for natural scour at the upper pipeline crossing appears to have been based on the results of bathymetric data and historical changes to the channel over the course of the last 150 years rather than with the hydrodynamic finite element model. Based on this data it was concluded that the possible changes to the sea bed could consist of natural changes in the alignment of the two existing shallow channels to revert back to a single channel, similar to the alignment in 1852-1853. They state that, even allowing for a reduced channel width and a subsequent enhanced flow it is not likely that the bed levels will alter by more than 1 to 2 meters. It is therefore concluded that the risk of exposing the pipeline buried to a minimum depth of 4.0 m below the sea bed is negligible. 3.5.3 Review and Comment 3.5.3.1During construction – scour around an intervention pit. As described in Section 3.3.1, the analysis that was carried out on the potential scour for the intervention pit would appear to be site specific, advanced and reasonably comprehensive as it is based on a numerical modeling using a finite element



hydrodynamic flow model for Sruwaddacon Bay that was developed using hydrographic data for the bay and then calibrated based on measurements recorded at four mooring points in the bay. Scour depths were estimated based on the flow data calculated from the finite element analyses. The analyses indicated that up to 5.0 m deep could occur around an intervention pit located in the strongest part of the current flow in the shallower slower moving waters of the Upper Crossing, which is considered the worst case scenario for this area. This would indicate that the depth of scour would be less if the intervention pit was located along other sections of the crossing outside the channel. RPS state that the minimum depth of cover for the pipeline is 4.0 m. However, the vertical profile drawings included with the EIS show that the actual depth to the pipeline crown ranges from 5.0 to 6.3 m across the bay due to the curvature of the pipeline and only reduces to about 3.0 to 4.0 m at the edge of the water on either side of the crossing. This would mean that the risk of exposing the pipeline in the event that an intervention pit is required would be negligible. In general I would also consider that based on the ground conditions encountered in the boreholes at the tunnel horizon, the risk of requiring an intervention pit would be very low, in which case the potential for scour around a pit would be an unlikely scneario. 3.5.3.2After construction – natural scour in the bay. RPS concludes that possible changes to the sea bed could consist of natural changes in the alignment of the two existing shallow channels to revert back to a single channel, similar to the alignment in 1852-1853. They state that, even allowing for a reduced channel width and a subsequent enhanced flow it is not likely that the bed levels will alter by more than 1 to 2 meters. It does not appear in this case that the analysis was based on a numerical analysis of the tidal flow in the bay, but rather on an interpretive assessment of the bathymetric data and historical changes to the channel over a period of about 150 years. Therefore, the results are subjective and there is some uncertainty in the analysis. Nevertheless, the potential for scour in the slower moving waters of the Upper Crossing would be lower than at the Lower Crossing. Therefore, given the proposed depth of the pipeline across the bay at the Upper Crossing, the risk of exposing or undermining the pipe due to natural scour would appear to be negligible.



4.0 GROUND MOVEMENT RISK IN THE BLANKET BOG (PEAT STABILITY)

4.1 Introduction This section of the report addresses the risk of peat stability in the lowland blanket bogs along the route of the pipeline. In particular, it covers the sections of the pipeline that crosses the Rossport Commonage and the blanket bog South of Sruwaddacon Bay to the Gas Terminal site at Bellanaboy, i.e.:

Section 4: - Ch. 85+990 to 88+350 (510 m): Rossport Commonage Section 6: - Ch. 89+600 to 91+000 (1400 m): South of Sruwaddacon Bay to L-

1202. Section 7: - Ch. 91+000 to 91+550 (450 m): L-1202 to Existing Stone Road

Linking to Bellanaboy Terminal Site. Section 8: - Ch. 91+550 to 92+550 (450 m): Existing Stone Road Linking to

Bellanaboy Terminal Site. The risk of ground movement in these areas was addressed by AGEC for SEPIL. 4.2 Ground Movement Risks (Peat Stability) The main ground movement risk along these sections of the pipeline are associated with the risk of peat stability. There are 4 main modes of failure that need to be considered:

Planar peat slide – Undrained Condition Planar peat slide – Drained Condition (Hydrostatic uplift/Buoyancy) Bog burst Localised shear failure

Other geotechnical issues with regard to ground movement that need to be considered for the design and construction of the gas pipeline in the blanket bogs include:

Planar sliding at the base of the stone road Settlement of the stone road.

The following is a brief overview of the principal mechanisms for each of the above forms of peat failures or ground movement in peat with particular comment on how they would relate to the design and construction of the gas pipeline.



4.2.1 Planar peat slide – Undrained Condition: A planar peat slide is a translational downslope movement of a large block of unconfined peat. Due to the relatively high frictional resistance of peat compared to its undrained shear strength, this type of failure typically occurs in the undrained condition due to shear failure along a plane of weak amorphous peat near the base of the peat, or at the interface with the underlying mineral soil where there is an existing discontinuity, such as a smooth rock surface or a pre-existing failure plane. However, recent experience in Ireland has also identified that undrained planar failure can occur due to progressive shear in a layer of weak sensitive clay below the peat. The term ‘sensitive’ refers to a material that exhibits low residual or post-yield shear strength relative to its peak strength. In these materials when the peak shear strength is exceeded the failure plane can progressively develop downslope due to the low residual strength of the soil. The main risk of planar slides due to undrained shear failure typically occurs in locally weak peat 2-4 m deep with an undrained shear strength less than about 4-5 kPa on intermediate slopes (4-7º) upslope from a watercourse in a defined valley. The risk of a slide can also be exacerbated by other contributory factors such as:

Groundwater conditions and site drainage – particularly man-made drains cut across slopes.

Subsurface groundwater flow through natural pipes in the peat. Machine cutting for peat harvesting, particularly sausage cutting Topographical features such as a convex break in slope. Weather – e.g. heavy or intense rainfall after an extended period of dry weather. Previous slides in the area

A planar slide by undrained shear failure can occur naturally under the self-weight of the peat, or more significantly, by loading the peat to the point where the downslope component of the load combined with the self weight of the peat exceeds the shear strength of the soil along the failure plane. In this case the highest risk is within a period of about 24 hours after initial loading as the soils would gain strength with time as they consolidate under the applied load. For the Corrib Onshore Gas Pipeline there will be no sidcasting or stockpiling of surplus excavated material on the peat. Therefore, the main risk of an undrained planar slide would be where the applied load of the peat turves on the bog mats adjacent to the excavation for the stone road exceeds the shear strength of the peat or underlying soil. SEPIL intend to place the turves upslope from the stone road wherever possible to protect against a planar slide. However, the stone road will not be in place when the turves are placed on the peat and there are areas where the road runs perpendicular to the slope. Therefore, there is a risk of a planar slide if there is not an adequate margin of safety against failure under the applied load.



4.2.2 Planar peat slide – Drained Condition (Uplift/Buoyancy): Planar peat slides can also occur in the drained condition where the frictional resistance along a failure surface within or just below the peat is exceeded by the downslope component of the combined load of the self weight of the peat and any external applied load. The drained shear strength properties (e.g. friction angle) of peat are relatively high compared to its undrained shear strength. Therefore, where planar slides occur in the drained condition the peat is normally underlain by a relatively permeable granular soil or weathered highly fractured rock layer, or where there is a network of natural pipes within the peat at the interface with the underlying soil. Also, the failures tend to occur during or immediately following a period of intense rainfall where restricted flow along preferential drainage paths in the pipes or more permeable soils below the peat lead to a build up of water pressures that can reduce the frictional resistance along the interface between the two soils until shear failure occurs. In an extreme case the water pressures can exceed the dead weight of the overlying peat leading to an uplift or buoyant failure with negligible frictional resistance along the sliding plane. The risk of failure is exacerbated after an extended period of dry weather where the natural water level in the peat is lower and the peat dries out. This reduces the self-weight of the peat and can allow rainwater to penetrate more rapidly into the underlying soils through shrinkage cracks at the surface. This is the mechanism that Tobins [12] have attributed to the failures that occurred on the upper slopes of Dooncarten Mountain in Spetember 2003. This type of failure can also occur on shallower slopes at lower elevations, but the areas of higher risk generally occur in relatively shallow peat (<2 m) at a convex break in slope where there is a preferential line of drainage at the base of the peat in granular soils or through natural pipes in the peat upslope from a defined watercourse or valley. For the Corrib Onshore Gas Pipeline, the undrained shear strength of the peat and underlying soil will probably govern the stability of the peat under hydrostatic groundwater conditions. Therefore, the risk of a planar slide under drained conditions would probably be limited to those areas where there is potential for a build up in water pressure in granular soil or fractured rock below the peat, or in natural pipes in the peat at the interface with the underlying soil. In particular, the stone road could create a path for groundwater to penetrate more rapidly into the pipes or more permeable soils below the peat. This could lead to a planar slide where there is a build up in excess pore water pressures during a period of intense rainfall that reduces the frictional resistance along a sliding plane at the base of the peat downslope from the stone road. The risk would be highest during construction when the stone road is exposed. The design of the stone road includes leaving a 0.5 m thick layer of peat mixed with rockfill at the base of the road as a low-permeability hydraulic barrier. If effective this should reduce the rate of infiltration of groundwater into the underlying soils. Transverse barriers will also be constructed across the stone road at approximately 50 m centres to limit longitudinal drainage, which could otherwise lower the water level and



dry out the peat. However, in periods of heavy rainfall during construction the transverse barriers will assist the build up of water in the stone road, which could increase the risk of a drained planar slide if the basal hydraulic barrier is not effective at controlling groundwater infiltration into the underlying soil in an area where there is potential for a drained planar slide due to the underlying ground and hydrogeological conditions. 4.2.3 Bog Burst: Bog bursts can occur in deep deposits of very weak peat with a high water table. They are often characterised by a flow slide of slurried or fluidised weak amorphous peat from below the fibrous acrotelm crust through a tear or rupture on the downslope side of the deposit. Some of the acrotelm layer is left in-situ in large tufts or “islands” at the base of the depression that was occupied by the peat, although much of the layer gets carried along with the slide. The mechanisms by which bog bursts can occur under natural conditions are not clearly understood but one theory is that the surface layer of fibrous peat can form shrinkage cracks during extended periods of dry weather which can allow greater infiltration of rainwater during subsequent periods of heavy rainfall. The resulting increase in water content of the underlying weak amorphous peat can cause swelling which can lead to a rupture in the surface peat, releasing the underlying fluidised amorphous peat and trapped water into a flow slide on the downslope side. Other theories suggest that the flow slide can be triggered by a collapse or blocking of natural pipes within the peat, or that the rupture can occur due to long term creep of the amorphous peat. Bog bursts can also be initiated by adjacent planar slides. The risk of a bog burst can be increased by anthropogenic activities such as mechanical cutting for peat harvesting, particularly cross cutting with a sausage cutter, and excavation of drains across the slope of the deep peat area. A flow slide can also be triggered by peat excavations at the margin of the deep peat area. For the Corrib Gas Pipeline the risk of a bog burst would be confined to the deep peat areas between:

Ch. 86+190 and 86+550 Ch. 87+010 and 87+380 Ch. 87+940 and 88+240 Ch. 90+750 and 92+050

There could be a risk of a bog burst when carrying out excavation for the stone road along the downslope margins of these areas, particularly where:

the depth of peat increases to greater than 4 or 5 m, the peat is very weak and highly amorphous with a very high natural moisture

content,



the peat has been machine cut for peat harvesting or drainage, particularly where the cuts are across the slope.

there is a high water table with poor drainage, surface water, bog pools, or a watercourse directly downslope from the excavation.

4.2.4 Local Shear Failure: There is a risk of shear failure around the sides of the excavations for the stone road in weak peat if the sides of the excavations are not adequately supported below the stable depth of open cuts. This risk would be confined to the same areas of deep peat identified for bog bursts. Initially the risk is of squeezing or collapse at the sides of the excavation. However, if this is not adequately controlled it could lead to a progressive failure that could extend for some distance from the excavation and possibly lead to a planar slide or bog burst at the margins of the deep peat areas. 4.2.5 Planar Sliding at the Base of the Stone Road: In areas of relatively shallow peat (<2.5 -3.0 m) the bottom 0.5 m of the peat will be left in place under the stone road and rockfill will be pushed to refusal in the peat with an excavator to form the base of the road embankment. In this case the bottom 0.5 m of the road will consist of peat within a matrix of rockfill. In deeper peat the sides of the excavation would generally not be stable at depths greater than 2.5 to 3.0 m so the base of the stone road will be constructed by the controlled displacement of peat with the rockfill below this level. The rockfill will again be pushed to refusal with the bucket of the excavator until a stable platform consisting of peat within a matrix of rockfill is formed below a depth of about 2.5 m. Where the peat is between 4.0 and 5.4 m deep, the thickness of this basal layer will range from about 1.0 to 2.5 m and possibly up to 3.0 m. Above this basal layer the road will be constructed in controlled lifts of rockfill up to within about 0.6 m below the original ground surface with nominal compaction by tracking across the surface of each layer with the tracked excavator. With this construction method the soil at the base of the stone road will not be verified by inspection and there would also be some uncertainty over the integrity of the basal layer of the road. There is a risk that very soft peat could be trapped under the rockfill to form a weak layer at the base of the road. With the limited ground investigation carried out to date in the blanket bogs, there could also be a risk that a weak sensitive clay layer could be left in place under the road. Under these conditions there is a risk of planar sliding at the base of the stone road during construction depending on the undrained shear strength of the soil at the base of the road, the depth of fill and the slope angle at the base of the



peat. The undrained shear strength of the fine-grained soils or peat would increase with time due to consolidation under the weight of the fill material. Therefore, the highest risk would occur within about 24 hours of placing the fill. 4.2.6 Settlement of the Stone Road: The stone road will not comprise of well-compacted layers of well-graded rockfill constructed directly on an inspected and approved subgrade of rock, firm or stiff cohesive glacial till, or medium dense to dense sand and gravel. Instead, in areas of peat up to 5.4 m deep the lower 0.5 to 2.5 or 3.0 m of the rockfill embankment will consist of an uncompacted matrix of rockfill pushed to refusal in peat using the bucket of the mechanical excavator. Above this level the road will be constructed in layers of rockfill given nominal compaction by tracking across it with the excavator. Under these conditions there is potential for significant settlements to occur within the rockfill during and after construction due to shifting of the coarse fill and consolidation of the peat in the basal layer of the road, and due to downward migration of fines and collapse of void space in the poorly compacted rockfill above this level. The settlements will be non-uniform and will vary along the length of the road due to the varying depth of peat. If the settlements are not largely built out before the pipeline is constructed within the fill, then there is a risk that non-uniform differential settlements could occur along the length of the pipeline which could induce significant bending stresses on the pipe.



4.3 SEPIL Assessment This section gives a summary of the analyses that were carried out by SEPIL to assess the risk of peat stability along the alignment of the Gas Pipeline. The geotechnical analyses were carried out by AGEC. J.P. Kenny carried out an assessment of the tolerance of the pipeline to differential settlements in the stone road. 4.3.1 Planar sliding – Undrained Condition (Short-Term/Total Stress) AGEC [3] carried out an analysis of the stability of the natural peat slopes in the blanket bog along the alignment of the pipeline. The analysis was based on:

A walkover survey along the route of the pipeline with detailed mapping of the geomorphology along the route.

A desk study review of aerial photographs and previous peat failures in the area. An interpretation of the soil, rock and groundwater conditions along the route on

the basis of the walkover survey, geomorphology mapping and ground investigations that have been carried out.

An interpretation of characteristic undrained and drained strength parameters for the peat and underlying mineral soil from the ground investigation data.

Stability calculations at representative locations along the alignment using the infinite slope method of analysis.

The walkover survey was carried out to record salient geomorphological features along the pipeline route, including:

General ground conditions Morphology, such as slope inclination, direction and break in slopes Surface hydrology e.g. streams, drainage ditches etc. Indications of active, incipient or relict instability within the peat. Drainage conditions and wet areas General land use, such as agriculture and peat cuttings General peat conditions including description, thickness and strength.

The survey was generally based on a visual inspection combined with probing of peat depth, but the survey was supplemented with interpretation of aerial photographs. The results of the survey were recorded on geomorphological plans included in the EIS (Ref – AGEC Drawings 864_01_002 to 004). The desk study included a review of the peat slides that occurred on the upper slopes of Dooncarten Mountain in September 2003, and a cursory inspection of the peat failure that occurred during road widening works on the L-1202 road in Aghoos, Erris in May 2008. The peat stability analysis was carried out by calculating the factor of safety against sliding along an interface at the base of the peat and in the underlying mineral soil using the infinite slope method. The analysis was carried out in the undrained and drained



condition. The undrained analysis applies to short-term conditions that occur during construction. The following assumptions were made in the analysis:

The undrained shear strength parameters for the peat were determined from the results of the in-situ vane shear tests.

The undrained shear strength parameters for the mineral soil were determined from strength descriptions on the borehole and trial pit logs, correlations to SPT N-Values, and experience. A minimum undrained shear strength of 15 kPa was assumed in the analyses to represent a generalized worst case.

The failure surface is assumed to occur at the base of the peat, as determined from the probes.

For failure in the mineral soil the failure surface is assumed to be 0.1m below the base of the peat.

The angle of the sliding plane is assumed to be parallel to the ground surface. Two load cases were analysed - no applied loading, which represents the undisturbed condition of the peat slopes, and a live load of 10 kPa on the surface of the peat to represent the surcharge loading of up to 1.0 m of peat on bog mats adjacent to the stone road. No partial factor was applied to the live load. A minimum Factor of Safety of 1.3 is required from BS 6031:1981 – British Standard Code of Practice for Earthworks. However, AGEC state that a minimum FoS of 1.5 would be preferable for the undrained condition. The following is a summary of the main conclusions that AGEC make on the basis of the walkover survey and the peat stability analyses in the undrained condition. (Note that only the results of the analyses that were carried out in Rossport Commonage are included in the AGEC report):

The findings of the walkover survey identified no apparent signs of peat instability along the route of the onshore pipeline that would pose a risk to the pipeline.

The following critical areas with respect to peat stability were, however, identified along the route:

o The extensive areas of machine cut peat in Rossport Commonage, where local stability of the excavation for the stone road could be an issue (e.g. Ch. 86+250 – 86+600 Ch. 87+300 – 87+450

o Areas of weak/wet peat – e.g. the shallow bog pools within an area of relatively intact peat at Ch. 87+200 to 87+300, where the peat is notably weak and deep at the crest of a slope, which represents a greater risk to peat stability.



o An area of deep peat around Ch. 88+100 where the peat is up to 5 m deep and is likely to have a significant thickness of weak amorphous peat at depth.

The characteristics of the peat slides on the upper slopes of Dooncarten Mountain were not considered representative of the peat conditions along the alignment of the onshore gas pipeline.

The localized failure adjacent to the L-1202 during road widening works was probably due to bearing failure in the in-situ peat due to excessive loading from excavated peat and quarry stone placed on the peat during road widening. In comparison, limits will be placed on storage of peat on undisturbed peat for the construction of the stone road for the onshore gas pipeline.

The results of the stability analyses showed that the natural peat slopes along the

proposed pipeline route have an acceptable FoS of 1.5 or greater for undrained shear failure at the base of the peat in accordance with the relevant design standards where the peat is not loaded.

The analyses also indicated that the FoS for undrained shear failure in the mineral

soil below the peat was >1.5 even when the peat was loaded with a 10 kPa surcharge, assuming a minimum undrained shear strength of 15 kPa in the mineral soil.

In the case where the peat is loaded with a 10 kPa surcharge the FoS was less than

1.5 for undrained failure within the basal zone of peat at three locations: o Ch. 87+219 – Rossport Commonage: FoS = 1.3 o Ch. 89+867 – South of Sruwaddacon Bay: FoS = 1.3 o Ch. 91+688 – on the approach to the Terminal at Bellanaboy: FoS = 1.0

The low FoS were calculated in areas where locally low undrained shear strengths of 1kPa and 2 kPa were recorded in the vane shear tests.

At Ch. 87+219 it is noted that the low strength was recorded close to a series of shallow bog pools with an increasing slope inclination to the east, which represents an increased risk of peat instability.

At Ch. 89+867 the low strength was recorded in an area of relatively intact peat. The low shear strength of 1kPa was at a shallow depth of 1.0 m and AGEC note that the value is very low and may not be representative of the peat conditions.

At Ch. 91+688 the low strength was recorded at a slight increase in slope inclination. However, AGEC note that the stone road has already been constructed in this area and conclude that the peat strength of 2 kPa may not be representative of the peat conditions.



Recognizing that the ground investigations that have been carried out to date were limited due to access constraints in some areas AGEC’s report also contains recommendations for additional investigations to consist of in-situ vane shear tests and undisturbed sampling in the peat and mineral soil to establish strength and general soil characteristics. The investigations are recommended for Rossport Commonage and in particular within the critical areas identified in the machine cut areas, in the vicinity of the bog pools (Ch. 87+200 to 87+300) and in the area of deep peat around Ch. 88+100. At the request of the inspector, AGEC also submitted additional information to An Bord Pleanála during the Oral Hearing consisting of a qualitative assessment of the relative potential for peat failure along the route of the onshore gas pipeline based on a combination of up to 32 No. environmental factors that can affect peat stability, including factors relating to:

Ground conditions (e.g. peat depth, undrained shear strength, classification, strength of underlying mineral soil)

Topography (e.g. peat slope, slope of underlying mineral soil, morphology) Water conditions (hydrology, hydrogeology, site drainage) Stability analyses of the stone road assuming that it is constructed on a weak

sensitive clay layer with a peak undrained shear strength of 5 kPa (FoS infinite slope – undrained condition)

Peat slide history (e.g. Dooncarten Mountain, L-1202, evidence of incipient failure)

Land use (forestry growth, peat workings) For the purpose of the assessment the alignment of the pipeline in the peat areas was divided into 19 No. different sections. The relative peat failure potential for each section was categorised as low, medium or high on the basis of a risk rating system for the environmental factors considered in the analyses. Particular emphasis was given to the infinite slope factor of safety for planar sliding at the base of the stone road under a 10 kPa surcharge load. Where an FoS <1.3 was calculated along the route of the pipeline, the relative peat failure potential for the corresponding section of the pipeline was increased to the next highest category. The relative peat failure potential for each section of the onshore gas pipeline is shown on Figure No. 4.1 for Rossport Commonage, and on Figure No. 4.2 for the area South of Sruwaddacon Bay to the gas terminal at Bellanaboy. The potential direction of peat movement in the event of a failure is indicated on the figures as well as proposed discharge points for temporary dewatering during construction.



Figure No. 4.1 – Relative Failure Potential Categories – Rossport Commonage

Figure No. 4.2 – Relative Failure Potential Categories – South of Sruwaddacon Bay to the Gas

Terminal at Bellanaboy



The following is a summary of the main conclusions that can be made on the results of the assessment:

The relative failure potential category for the majority of the alignment in the Rossport Commonage is interpreted as Low. This is mainly due to relatively favourable morphology, hydrology and hydrogeology with an acceptable factor of safety against sliding for the stone road on weak clay below the peat in the undrained condition.

The relative failure potential category for Section 2 in Rossport Commonage (Ch. 86+250 to 86+600) has been interpreted as High on the basis of a low FoS for the stability of the stone road corresponding to locally higher slope angles at the west end of this section. The higher risk rating is also influenced by peat cuttings in the area and some higher ratings for unfavourable conditions related to surface hydrology and morphology.

In Section 8 (Ch. 88+100 to 88+270) the risk rating has been increased to Medium on the basis of the infinite slope analysis for the stone road on weak clay. However, it is noted by AGEC that the low FoS may be due to a locally anomalous peat probe depth.

On the South side of Sruwaddacon Bay the relative failure potential for the majority of the alignment is Medium based on less favourable topographical and drainage conditions. However, the failure potential for Sections 10 and 11 (Ch. 89+520 to 89+950) has been characterized as Low due to locally more favourable conditions. It is also assumed that the stone road is not constructed on soft clay in this area due to exposures of the underlying soil at the foreshore and in peat cuttings.

The highest relative failure potential was calculated for Section 18 downslope from the Terminal (Ch. 91+560 to 91+920) where it was increased to High on the basis of the Low FoS calculated for the stone road on weak clay (FoS = 1.2). The stone road has already been constructed in this area.

4.3.2 Planar sliding – Drained Condition (Long Term/Effective Stress) The analysis for planar sliding in the drained condition is included in the peat stability assessment report by AGEC [3], which was included in the EIS and discussed in Section 4.3.1. The peat stability analysis was carried out by calculating the factor of safety against sliding along an interface at the base of the peat and in the underlying mineral soil using the infinite slope method with drained (effective stress) strength parameters. The drained or effective stress condition applies to long-term conditions after construction when construction-induced pore water pressures have dissipated to hydrostatic conditions.



Drained strength parameters were determined for the peat on the basis of a desk study of published data. For the mineral soil the drained strength parameters were determined from experience and the strength descriptions on the logs. Two load cases were analysed - no applied loading, which represents the undisturbed condition of the peat slopes, and a live load of 10 kPa on the surface of the peat to represent the surcharge loading of up to 1.0 m of peat on bog mats adjacent to the stone road. No partial factor was applied to the live load. The analyses were carried out under hydrostatic conditions assuming three different groundwater levels:

No groundwater – groundwater level at the base of the peat. Groundwater level at mid-height of the peat. Groundwater level at ground surface

A minimum Factor of Safety of 1.3 is adopted from BS 6031:1981 – British Standard Code of Practice for Earthworks. The following is a summary of the main conclusions on the results of the peat stability analyses in the drained condition from AGEC’s report. (Note that only the results of the analyses that were carried out in Rossport Commonage are included in their report):

The factor of safety against planar sliding at the base of the peat and in the underlying mineral soil was >1.5 in the drained condition for the natural peat slopes (0 kPa surcharge) and with a surcharge of 10 kPa on the surface of the peat for all of the different water levels that were assumed.

The results would indicate that the peat slopes have an adequate factor of safety against sliding in the long-term condition, which is consistent with the findings of the walkover survey which identified no signs of rainfall-induced failures of the peat in the area.

The analyses by AGEC does not consider artesian conditions in the soil below the peat, or a build up of excess water pressures in granular soils below the peat due to restricted drainage along preferential drainage in the granular soils. However, the proposed stone road construction method does include a minimum 0.5 m thick basal layer of rockfill pushed into the peat to create a matrix of rockfill mixed with remoulded peat. The purpose of this is to reduce the potential for drainage from the stone road into the underlying soils. Transverse plugs on 50 m intervals along the stone road will limit longitudinal drainage. Further ground investigation will be carried out in the areas of blanket bog to confirm the material characteristics and strength properties of the mineral soil below the peat, which will determine where there are granular soils below the peat.



In additional information submitted to an Bord Pleanála during the Oral Hearing, SEPIL also confirmed that the surface water management system for the temporary works during construction will be designed for the 10 year 24 hour storm event in accordance with CIRIA guidelines (e.g. CIRIA Report No.C648,2006 – Control of Water Pollution from Linear Construction Projects). Data from the Belmullet Met Eireann Weather Station was used for the design, which allows for up to 51 mm rain in the 10 year 24 hour storm event. 4.3.3 Bog Burst The risk of a bog burst is not specifically identified in the peat stability assessment by AGEC. However, the report does identify the following critical areas with regard to peat stability:

The extensive areas of machine cut peat in Rossport Commonage, where local stability of the excavation for the stone road could be an issue, e.g.:

o Ch. 86+250 – 86+600 o Ch. 87+300 – 87+450

Areas of weak/wet peat – e.g. the shallow bog pools within an area of relatively

intact peat at Ch. 87+200 to 87+300, where the peat is notably weak and deep at the crest of a slope, which represents a greater risk to peat stability.

An area of deep peat around Ch. 88+100 where the peat is up to 5 m deep and is

likely to have a significant thickness of weak amorphous peat at depth. These areas would correspond to areas where there is a risk of a bog burst, locally exacerbated in the machine cut peat or in areas upslope or adjacent to notable hydrological features such as bog pools or watercourses. The deep peat area to the South of Sruwaddacon Bay where the stone road has not been construction has not been identified in the report, ie:

Ch. 90+750 and 92+050 AGEC recommend further ground investigation during detailed design for the critical areas that they have identified in their report. The proposed method of construction for the stone road should also limit the depth of excavation in the peat to the depth at which the sides remain stable when unsupported, with the controlled displacement of peat below this level using rockfill pushed into the peat. In evidence presented at the Oral Hearing AGEC also propose comprehensive geotechnical supervision and monitoring for the construction stage of the project by experienced personnel on a full-time basis to prevent:



Uncontrolled deposition of excavated materials Collapse or development of tension cracks around potentially unstable

excavations. Placement of fill or excavations in the vicinity of steeper peat slopes.

Areas of machine cut peat will be inspected in advance of construction and there will be on-going assessment of the ground conditions to confirm the findings with respect to peat stability in the EIS. Geotechnical instrumentation will be installed and monitored in areas of possible poor ground. The risk of unstable excavations is also addressed in the Geotechnical Risk Register (e.g. Items 5, 7 and 14). 4.3.4 Local Shear Failure The risk of local shear failure would occur in the same critical areas of deep, weak, highly amorphous peat identified in the Peat Stability Assessment report by AGEC where there could be a risk of a bog burst. Therefore, it has been addressed by AGEC with the following measures, as described in Section 4.3.3:

Additional SI in the blanket bogs, particularly in the critical areas. Controlled displacement of peat with rockfill below the depth of stable open

excavation in peat for the construction of the stone road. Full-time supervision and monitoring during construction by experienced

personnel. On-going assessment of ground conditions to confirm the findings in the EIS with

installation and monitoring of instrumentation in critical areas. Items 5, 7 and 14 of the Geotechnical Risk Register which relate to work in soft

ground and unstable excavations. In response to questions during the Oral Hearing AGEC confirmed that the stone road will be constructed through the areas of deep weak amorphous peat using the controlled displacement of peat below the stable depth of open excavation in the peat and that steel sheetpiles would probably not be used as temporary support for the peat. The deep peat area to the South of Sruwaddacon Bay where the stone road has not been construction has not been identified in the report by AGEC, ie:

Ch. 90+750 and 92+050



4.3.5 Planar Sliding at the Base of the Stone Road AGEC[4] presented a report on the stability of the stone road construction in the peat areas in the EIS. The report presents a summary of the results of stability analyses for the stone road in the short-term during construction, in the long-term drained condition after construction, and for sliding on the mineral soil at the base of the road under the applied lateral load of a peat failure upslope from the road. The analyses for the short-term undrained condition during construction included limiting equilibrium slip circle analyses to calculate the factor of safety for a slip circle to develop within the stone road for the following load cases and groundwater conditions:

1. No load on the stone road. Partially saturated rockfill (ru = 0.24).

2. A 20 kPa surcharge on the surface of the stone road to represent heavy construction traffic. Partially saturated rockfill (ru = 0.24).

3. A uniform load of 230 kPa over the 0.7 m width of the tracks for the pipelaying plant. Partially saturated rockfill (ru = 0.24).

4. An eccentric load on the tracks with 33% of the total load on the track near the

centre of the stone road, and 67% of the load on the track closer to the outer edge of the road. This analysis was carried out to simulate the potential eccentric load distribution during pipe lifting. The eccentric loading condition was analysed for three different depths of water in the stone road: 3.4 mBGL, 1.5 mBGL and 0.0 mBGL.

All of the analyses were carried out for the stone road at Ch. 88+095 where the peat depth is up to 5.0 m and the slope inclination is 1 degree to the southwest. The height of the stone road in this area is 4.4 m. It is 9.0 m wide at the surface with 1V:1H side slopes. The rockfill was assigned a friction angle of 45 degrees for the analyses and the basal layer of rockfill mixed with remoulded peat is not considered. It is assumed that the road is constructed on a 1.0 m thick basal layer of cohesive glacial till with an undrained shear strength of 40 kPa, increasing to 75 kPa below this. The undrained shear strength profile for the peat is taken from the in-situ vane shear tests. The following conclusions were drawn from the analyses:

The factor of safety for the road for Load Cases 1 to 3 ranged from 1.29 to 3.67, which should be acceptable for the temporary condition.

The highest loading intensity occurs during the pipe lifting, when there will be an eccentric load on the tracks of the pipelaying plant. The factor of safety for a circular slip circle-type failure is sensitive to the groundwater level in the stone road and ranges from 1.22 with about 1.0 m of water at the base of the rocfill (3.4 mBGL), to 1.04 when the water is at the road surface.



AGEC recommend monitoring the water level in the stone road prior to and during but conclude that, provided that the tracks are kept more than 1.0 m from the edge of the road, or close to the pipe trench, then the loading envelope will be within competent stone fill which should have sufficient capacity to support the crane.

AGEC [4] also carried out an undrained analysis to calculate the factor of safety of the stone road at Ch. 88+095 against planar sliding in the mineral soil below the road under the force of the upslope peat pushing against the road. It is not stated how this force was calculated, but it is assumed that the analysis was carried out for a sliding plane in the cohesive glacial till at the base of the road, which had an undrained shear strength of 40 kPa. The calculated factor of safety against sliding was 3.67, which indicates that the stone road is stable against sliding for the assumed ground conditions. In additional information submitted to An Bord Pleanála at the request of the inspector, AGEC [10] considered the stability of the road to planar sliding in the mineral soil at the base of the stone road assuming that it was constructed on a 1.0 m thick layer of weak cohesive soil with an undrained shear strength of 5 kPa. Two sets of analyses were carried out:

1. The factor of safety against failure in was calculated by limiting equilibrium slip circle analysis in plane strain undrained conditions for the stone road at Ch. 91+560 to 91+920 (Section 18 in Figure No. 4.2), where there is the greatest area of upslope peat.

2. Infinite slope analyses were used to calculate the factor of safety against sliding for the stone road for each section of the pipeline in Figures Nos 4.1 and 4.2 using the slope angle at the base of the peat and at the ground surface.

For the slip circle analysis at Ch. 91+560 to 91+920 the depth of peat was taken as 3.0 m and the height of the stone road was about 2.5 m. The 0.5 m thick basal layer of rockfill mixed with remoulded peat was modeled in the analysis but it was given a high undrained shear strength of 50 kPa compared to the underlying weak layer of mineral soil. A 10 kPa surcharge was applied to the surface of the road, and on the peat upslope from the road to model the sidecast peat. The force from a peat slide upslope from the road was modeled as the maximum passive force that could be applied by 3.0 m of peat (75 kN/m). The analyses indicated that, for the assumed condition with a weak clay layer at the base of the stone road, the factor of safety against failure for the road was 0.8 for the full passive pressure of the peat on the upslope side of the road, which indicates that failure could occur under these conditions. The factor of safety increases to 1.89 for the corresponding condition using assumed properties for the clayey gravelly sand that was encountered below the peat in this area.



For the qualitative assessment of the relative potential for peat failure along the route of the onshore pipeline through the blanket bogs, the infinite slope factor of safety calculated using the slope angle at the ground surface for the stone road on a layer of weak clay with an undrained shear strength of 5 kPa generally ranged from 1.4 to 2.6 with some locally higher values. The corresponding range for the slope angle at the base of the peat was generally between 1.2 and 3.4 with some locally higher values and one very low value of 0.7 for Section 8, which AGEC attribute to anomalous peat depth probing in the area. These factors of safety reflect the stability of the stone road in the short-term condition without any construction surcharge and without any lateral loading from the peat upslope from the road. Those areas where low factors of safety were calculated were identified as medium or high potential areas on the risk mapping in Figures 4.1 and 4.2. AGEC also comment on the plausibility of the model with the soft sensitive clay layer at the base of the peat, stating that:

Where the pipeline is constructed close to the watershed in Rossport Commonage it limits the potential for peat failure upslope.

Some of the original peat stability analyses were carried out using low peat strengths of 1-3 kPa, which would be more conservative than the assumed strength of 5 kPa for a weak clay layer at the base of the peat.

The likelihood of peat failure upslope from the road is considered remote. There is no evidence of a weak sensitive clay layer at the base of the peat on the

basis of the ground conditions that were recorded on the logs for the boreholes, trial pits and probes, as well as in exposures in peat cuttings in the blanket bogs and on the cliff faces at the foreshore.

For an undrained shear strength of 5 kPa to apply for a clay layer at the base of the stone road the following assumptions would have to apply:

o The failure would have to occur during construction of the stone road, when the pipeline would not be in place.

o There is no penetration of stone into the clay layer. o Consolidation of the clay under the weight of the stone road would not

result in an increase in the shear strength of the road.

AGEC consider that penetration of the stone into the clay layer should increase the shear resistance at the base of the road and a weak clay layer at the base of the stone fill would increase in shear strength due to consolidation under the weight of the fill. AGEC also note that the presence of a weak sensitive silt or clay has been identified in the Geotechnical Risk Register with appropriate design and construction controls to mitigate against the risk. In the worst case, AGEC state that the weak sensitive layer could be excavated out from under the road or the pipe could be buried deeper, although these measures are considered unlikely and are not included on the risk register.



4.3.6 Settlement of the Stone Road Differential settlement of the gas pipeline in the stone road was not addressed in the EIS or in the Addendum to the EIS. In additional information submitted in the Oral Hearing at the request of the inspector, AGEC[10] presented the results of an analysis carried out to calculate the potential level of settlement that could occur in the stone road due to consolidation of peat in the basal layer of the road, which will consist of a matrix of rockfill mixed with remoulded peat ranging in thickness from 0.5 m to about 2.5 m. AGEC have calculated the potential settlement of the basal layer for three cases to represent the uncertainty in the analysis and the possible maximum range of the settlements that could occur:

Case 1: Peat with no stone: 0.3 x peat thickness Case 2: Peat with some stone: 0.2 x peat thickness Case 3: Peat with much stone: 0.1 x peat thickness

The calculated settlements are presented in Figure No. 4.3a) and 4.3 b). The maximum settlements in the areas of deep peat along the route of the pipelines ranges from 450 mm to 750 mm for Case 1, and from 100-250 mm for Case 3. AGEC consider that the settlements calculated for Case 1 are unrealistic as they assume that no stone penetrates the peat and that no settlements occur before the pipe is constructed. The settlements calculated for Case 3 are considered to represent the likely settlement of the stone road and pipe. JP Kenny carried out a finite element analysis to calculate the stresses that would be induced in the gas pipeline using the maximum (Case 1) differential settlements estimated by AGEC for the pipeline at Ch. 88+134 (750 mm). They calculated the stresses in the pipeline for an operating gas pressure of 144 bar, as well as during hydrostatic testing of the pipe at up to 504 bar, The analyses by J.P Kenny shows that, even in the worst case using very conservative differential settlement parameters the maximum Von Mises equivalent stress in the pipeline is less than the allowable limits for the testing and operating phase of the pipeline. In the worst case scenario for maximum and differential settlements the equivalent stress in the pipeline reaches 96% of the allowable limit in the testing phase, and 73% of the limit in the operational phase. If an anomalous probe reading at Ch. 88+134 is neglected in the analysis then the differential settlements would reduce significantly, which would also reduce the induced stresses in the pipeline to 88% and 38% of the maximum allowable stresses for the construction stage and operational phase of the pipeline.



Figure No. 4.3 – Estimated settlements for the stone road.



4.4 Review and Comment This section provides a review of the analyses that were carried out by SEPIL to assess the potential for ground movements and peat stability for the sections of the onshore gas pipeline that will be constructed using the stone road method in the lowland blanket bog. The following potential failure mechanisms and sources of ground movements have been considered in this review:

Planar sliding – Undrained Condition (Short-Term/Total Stress) Planar sliding – Drained Condition (Long Term/Effective Stress) Bog Burst Local Shear Failure Planar Sliding at the Base of the Stone Road Settlement of the Stone Road

4.4.1 Planar sliding – Undrained Condition (Short-Term/Total Stress) For the onshore gas pipeline the main risk with regard to peat stability during construction is related to the undrained shear failure at the base of the peat or in very soft clay below the peat under the surcharge of the peat turves that will be stored up to 1.0 m high on bog mats adjacent to the excavation for the stone road. Although the peat turves will be stored upslope from the road, the critical time for undrained shear failure is at the time that the load is applied up to about 24 hours after and it is unlikely that the finished road will be in place within that time. There are also areas where the road is perpendicular to the slope and will not act as a barrage to prevent shear failure under the applied load even after the road has been constructed. Therefore, it is essential that there is an adequate margin of safety against shear failure in the peat or underlying mineral soil in the undrained condition under the weight of the surcharge load. Notwithstanding the limited ground investigation information in the blanket bog in the Rossport Commonage and South of Sruwaddacon Bay, AGEC have now carried out a comprehensive analysis of the stability of the peat slopes in the blanket bogs along the alignment of the onshore gas pipeline in the natural condition (0 kPa surcharge) and under the 10 kPa surcharge. The analysis included:








A qualitative assessment of the relative potential for peat failure along the route of the onshore gas pipeline based on a combination of up to 32 No. environmental factors that can affect peat stability.

This level of analysis is commensurate with the geotechnical risks associated with the proposed construction of the pipeline through the blanket bogs. The infinite slope method of analysis in the undrained condition gives an index of the stability of the peat slopes for the existing condition (0 kPa surcharge) and under the 10 kPa surcharge. The desk study, walkover survey and geomorphology mapping combined with the qualitative assessment of the relative potential for peat failure evaluates the potential impact of environmental factors related to hydrology, hydrogeology, topography, peat workings and slide history which have been found to contribute to a higher risk of peat instability. The comparative assessment between different sections of the pipeline also highlights specific areas where greater controls would be required during construction. This method of analysis is consistent with the guidelines recommend by the Scottish Forestry Commission for the risk management of peat slips on the construction of low volume/low cost roads over peat (Mac Culloch, 2006). The particular approach used by AGEC for assessing the interaction between different environmental factors is also consistent with more advanced geotechnical risk assessment procedures being developed for managing the risk of peat stability for the construction of windfarms on blanket bogs in Ireland to take account of experience from recent peat failures. AGEC have carried out the analysis in accordance with BS 6031:1981 – British Standard Code of Practice for Earthworks, which they say states that for a first time failure with a good standard of site investigation the design factor of safety should be greater than 1.3. BS 6031:1981 actually states that the design value for the factor of safety under these conditions should be between 1.3 and 1.4 with the added stipulation that this would only apply when the consequences of failure are average. An FoS of 1.4 would normally be more appropriate when using unfactored undrained strength parameters. In their report AGEC do state that a factor of safety of 1.5 would be preferable for the undrained condition. AGEC also state that the BS6031:1981 design approach is similar to EC7 – Eurocode 7 for Geotechnical Design. However, EC7 uses a partial factor approach which is different to BS6031 significantly in the way it treats external loads. For example, in Design Approach 3, which would be suited to slope stability, the undrained shear strength would be reduced by a factor of 1.4 and the live load surcharge would be multiplied by a factor of 1.3. This takes into account the potential variability in the strength parameters as well as the live load surcharge. With BS6031:1981 no partial factor is applied to the live load. Given the uncertainty in the undrained shear strength determined from the hand vanes



and the potential variability in the live load when the weight of the bog mats is included, it would be more conservative to use the Eurocode 7 approach for design in this case. Using the infinite slope analysis AGEC have calculated a factor of safety > 1.5 for the peat slopes in the undrained condition with no surcharge, indicating that there is generally a low potential for failure of the slopes in the natural condition. This is consistent with the shallow slope angles along the route of the pipeline, which are generally less than 3 degrees and only locally higher in areas of shallow peat at the boundaries of the bog and at the edges of the Leenamore River. When the 10 kPa surcharge on the peat is included in the analysis, the factor of safety over much of the pipeline is still >1.5 with the exception of a few areas where a factor of safety of 1.0 to 1.3 was calculated i.e.:

o Ch. 87+219 – Rossport Commonage: FoS = 1.3 o Ch. 89+867 – South of Sruwaddacon Bay: FoS = 1.3 o Ch. 91+688 – on the approach to the Terminal at Bellanaboy: FoS = 1.0

Again, this is consistent with the generally low slope angles along the route. The low factors of safety are attributed to locally very low values of undrained shear strength of 1-2 kPa, which AGEC state may not be representative of the peat conditions, although it is noted that the low factor of safety at Ch. 87+219 was recorded to a series of shallow bog pools with an increasing slope inclination to the east, which represents an increased risk of peat instability. Undrained shear strengths of 1-2 kPa are more representative of the remoulded strength of disturbed amorphous peat than intact peat, so it is possible that these values are uncharacteristically low, particularly if the tests were carried out within 1.0 m of the ground surface in the vegetated acrotelm layer of the peat, which usually has a higher strength (e.g. Ch. 89+867). Nevertheless, these areas have been identified as having a locally higher risk of peat instability and it would be conservative to use the low values unless additional information is provided. It is noted that the stone road has already been constructed at Ch. 91+688 and that up to 1.0 m of peat has been sidecast adjacent to the excavation even though the infinite slope factor of safety under the 10 kPa surcharge was 1.0, which would be unacceptable, even in the temporary condition. AGEC also identify the following critical areas based on particular characteristics of the peat, hydrology and land use which can contribute to a peat slide:

The extensive areas of machine cut peat in Rossport Commonage, where local stability of the excavation for the stone road could be an issue (e.g. Ch. 86+250 – 86+600 & Ch. 87+300 – 87+450)



Areas of weak/wet peat – e.g. the shallow bog pools within an area of relatively intact peat at Ch. 87+200 to 87+300, where the peat is notably weak and deep at the crest of a slope, which represents a greater risk to peat stability.


likely to have a significant thickness of weak amorphous peat at depth. However, they have not included any similarly critical areas in the blanket bog to the south of Sruwaddacon Bay, particularly the area of deep highly amorphous peat at the head of the watercourses between about Ch. 90+900 and 91+550, where the existing stone road ends. Using the qualitative assessment of the relative potential for peat failure along the route of the onshore gas pipeline, AGEC identify the following areas with a relatively high potential for peat failure:

Section 2: Rossport Commonage - Ch. 86+250 to 86+600 Section 18: South of Sruwaddacon Bay – Ch. 91+560 to 91+920

This is based on a particularly critical combination of factors such as soil properties, topography, groundwater conditions, peat cuttings and a low factor of safety for the stone road in the infinite slope condition assuming that it is founded on a weak clay layer. AGEC note that the stone road has already been constructed in Section 18. In the Rossport Commonage most of the remaining sections have been categorized with a low relative potential for peat failure, with the exception of the deep peat area between Ch. 88+100 and 88+270 (Section 8), which is categorized as having a medium potential. However, we note that, using the criteria specified by AGEC, Sections 6 & 7 should also be classified as having a medium potential based on the rating score >55.3. To the South of Sruwaddacon Bay most of the sections of the pipeline have been characterized as having a medium potential for peat failure, with the exception of the section between Ch. 89+500 and 89+950 where the peat depth is relatively low and the corresponding relative failure potential is low. The qualitative assessment that was carried out by AGEC is a valuable tool for identifying the relative failure potential for different sections of the pipeline. However, there is still some subjective interpretation of the risk bands and in this case we would consider that it underestimates the potential risk for peat failure for Section 15 (Ch. 90+870 – 91+210) & Section 16 (Ch.91+210 to 91+470) where there are deep deposits of wet highly amorphous peat directly upstream or adjacent to a defined watercourse. At Section 15 there is a particularly high risk combination of environmental factors where the deep peat is directly upstream from a watercourse. It also noted that the SPT N-Value in the cohesive mineral soil below the peat was 0 in borehole BH-009C-07 at Ch.90+750, which indicates the presence of a weak clay layer at the base of the peat at a point directly



upstream from another watercourse. The risk of a peat slide is somewhat offset by the shallow slope angles in this area. Nevertheless, the particular combination of risk factors in these areas would warrant a more conservative approach to the construction of the pipeline than other lower risk areas along the route with appropriate mitigation measures to reduce the risk of a peat failure such as not allowing the storage of peat turves on bog mats adjacent to the excavation for the stone road. There are a number of design and mitigation measures that are proposed by AGEC that could be used to successfully manage the geotechnical risk of peat stability during construction of the stone road, including:

Additional ground investigations, particularly in the critical areas that have been identified.

Full-time supervision and monitoring during construction by experienced personnel.

On-going assessment of ground conditions to confirm the findings in the EIS with installation and monitoring of instrumentation in critical areas.

There are also a number of mitigation measures identified in the geotechnical risk register for dealing with peat stability. However, there is a deficiency of information on how these risk mitigation measures would be implemented on site. For example, no specific mitigation measures have been specified for those areas where a relatively high risk of peat failure potential has already been identified, or where a low factor of safety has been calculated for the infinite slope condition under the 10 kPa surcharge. The problem with this is highlighted by the work that was carried out during the construction of the existing stone road through Section 18 (Ch. 91+560 to 91+920) where the relative peat failure potential was characterized as high and a factor of safety of 1.0 was calculated for the 10 kPa surcharge on the slope in the undrained infinite slope analysis. There are also other contributory factors that would identify this as a critical area with respect to peat stability, such as the deep highly amorphous peat at the break in slope upslope from a watercourse with the road running perpendicular to the slope, where it will not act as a barrage to prevent shear failure on the slope. Artesian water pressures were noted in some of the probes by AGEC (PL-09, PL-10) Adopting a reasonably conservative approach to risk management it would have been good practice to restrict sidecasting peat adjacent to the road in this area. However, during construction of the road up to 1.0 m of peat was stored on the peat adjacent to the road. I would still consider that the stone road can be safely constructed through the blanket bogs by an experienced contractor under the full-time supervision of an experienced geotechnical engineer. However, this will take a coordinated and more conservative effort to risk management during design to identify where specific mitigation measures are required prior to construction. A list of appropriate mitigation and control measures have been proposed by AGEC but it is not clear from the information provided where



these will be implemented based on the information that is available at this time. Specifically they have not specified areas where limits will be put on the storage of the peat turves adjacent to the excavation for the stone road. Having said that, it is also not clear from the information submitted if the peat turves will be stored on bog mats adjacent to the stone road outside the zones of intact bog, even in the forested areas south of Sruwaddacon Bay, or if the turves will only be stored adjacent to the road in the designated and non-designated intact bog with material stockpiled elsewhere on the site for the reinstatement of the sections of road through the remaining sections of the bog. There is a need to clarify the storage of surplus peat on the site for reinstatement and to integrate the design with the geotechnical risk assessment and mitigation measures. Therefore, if the Board decides to request more information from SEPIL to reach a decision on the application, then I would recommend that the following information be requested:

Precise section by section details of the proposals for temporary peat storage and reinstatement outside the areas of intact bog, which take into account the condition of the surface layer of the peat and specifically identify where peat turves or remoulded peat will be stored on bog mats adjacent to the road.

Details of the specific risk mitigation measures that would be proposed for each of Sections 1 to 18 in the qualitative assessment of relative peat failure potential, in particular noting where there would be limits on the storage of peat on bog mats adjacent to the stone road excavation and adopting a conservative approach to the assessment of peat stability.

4.4.2 Planar sliding – Drained Condition (Long Term/Effective Stress) The risk of planar sliding in the drained condition will generally be limited to areas where there is potential for a buildup of pore water pressures below the peat due to restricted drainage along preferential drainage paths in underlying granular soil or relatively permeable weathered/fractured rock during periods of heavy rainfall. This is illustrated by the relatively high factors of safety calculated by AGEC for the infinite slope analyses along the pipeline in the drained condition. The results demonstrate that the long-term stability of the peat should be adequate for 0 kPa and 10 kPa surcharge under hydrostatic groundwater conditions up to the ground surface, and that the stability of the slopes under the 10 kPa surcharge during construction will be governed by the undrained shear strength of the peat and underlying cohesive soil, as described in Section 4.3.1. AGEC have not specifically considered artesian conditions below the peat in their stability analyses. However, they have specified the hydraulic barrier at the base of the



stone road to limit drainage from the road into the underlying mineral soil, and additional ground investigation will also determine the extent of those areas where there are granular soils below the peat. The hydraulic barrier will consist of a matrix of rockfill mixed with remoulded peat that will range in thickness from a minimum of 0.5 m up to about 2.5 to 3.0 m in the deep peat areas. The permeability of this matrix is uncertain. It will reduce the potential for drainage into the underlying mineral soil. However, there could be preferential flow paths around the rockfill that would make it more permeable than the undisturbed peat. Based on the limited exposures and gouge auger probes recorded by AGEC in Rossport Commonage the underlying soil would appear to be a cohesive soil, which would reduce the risk of drained planar sliding. Nevertheless, this would need to be confirmed by the proposed additional SI. Cohesive and granular soils have been identified below the peat in the blanket bog to the South of Sruwaddacon Bay. However, we note that in many cases there are no laboratory classification tests to support the visual classification. In general I would consider that the risk of planar sliding in the drained condition in the blanket bog would be relatively low and limited to the unconfined steeper slopes in relatively shallow peat at the boundary of the blanket bog or near the valley for the Leenamore River. There could also be a risk of drained planar sliding on the slope down to the watercourse between Ch. 91+500 and 91+800, where sub-artesian conditions were noted in some of the probes by AGEC and the trial pits in the area identified granular soils and possibly an iron pan at the base of the peat. However, the piezometers installed in this area by QMEC indicate that there is a downward gradient in the groundwater level with a lower hydrostatic head below the peat, so the groundwater observations in the probes may not be representative. Given the relatively high depth of peat towards the base of the slope (3-4 m), there would have to be a significant build up of water pressure to cause a failure in the peat, so the risk of a drained sliding failure is probably low. The risk of planar sliding downslope from the stone road due to drainage from the road is a temporary problem which could only occur during construction of the pipeline when the stone road is left exposed with no peat cover. Therefore, the risk is significantly reduced if the temporary drainage system is designed to adequately handle the volume of surface runoff anticipated over the construction period. In additional information provided to An Bord Pleanála during the Oral Hearing, SEPIL stated that the temporary works drainage system would be designed for the 10 year 24 hour duration storm event of 51 mm in a 24 hour period, which is based on the Belmullet Met Eireann Weather Station data. This design is based on the design criteria set out in the CIRIA Report No. C648 (2006) – Control of Water Pollution from Linear Construction Projects. Under normal circumstances this would seem reasonable for the proposed duration of the works on the onshore gas pipeline. However, given the potential for an associated risk of peat instability in some areas it would be more



conservative to design for a storm with a greater return period, such as the 50 or 100 year storm. 4.4.3 Bog Burst For the Corrib Gas Pipeline the risk of a bog burst would be confined to the areas where there is deep highly amorphous peat with very high natural moisture content between:


The risk would be particularly evident when excavating the road along the margins of those areas where

the depth of peat increases to 4-5 m the peat has been machine cut for peat harvesting or drainage (particularly across

the slope), there is a high water table with poor drainage, surface water, bog pools or a water

course directly downslope from the excavation. The proposed method of constructing the road through these areas is with the controlled displacement of peat below the depth of stable excavation in the peat, which should prevent any localised shear failure at the edges of the excavation progressing into a larger peat slide or bog burst. This is an accepted method of constructing stone roads through areas of deep peat that has been used extensively and succesfully across blanket bogs in Ireland. Therefore, it should be possible to construct the road through the areas of deep peat without causing a bog burst provided that the work is carried out by an experienced contractor under the full-time supervision of a geotechnical engineer with experience of similar works. Those areas where the surface of the peat is disturbed by machine cutting would require particularly care as the stability of the peat in the excavation is likely to be very poor. 4.4.4 Local Shear Failure The risk of local shear failure at the sides of the excavation would be highest in those areas where there is a risk of a bog burst, as described above. This risk would also be mitigated against by the proposed method of construction for the stone road by an experienced contractor under the full-time supervision of a geotechnical engineer with experience of similar works, as described in Section 4.3.3.



4.4.5 Planar Sliding at the Base of the Stone Road There could be a risk of planar sliding at the base of the stone road if the road was constructed on a weak layer of very soft peat or sensitive clay and if the topography of the ground along the base of the road was such that the downslope component of the self weight and surcharge exceeded the undrained shear strength of the weak peat/clay at this level. The highest risk would be immediately after constructing the road or possibly up to 24 hours after. After this time the material would consolidate and increase in shear strength under the weight of the fill I would consider that the risk of planar sliding at the base of the road during construction should be very low for the following reasons:

The road will be constructed on gently sloping ground near the watershed of the bog where the slope angles at the base of the peat are very low (Gen <3°, and even lower in the deep peat areas) so that, even with a weak clay layer with an undrained shear strength of 5 kPa at the base of the fill, AGEC have calculated an infinite slope factor of safety >1.3 for the stone road along most of the pipeline, with the exception of two areas where a FoS of 1.2 was calculated in relatively deep peat on locally steeper slopes i.e. Section 2 (Ch. 86+250 to 86+600) and Section 18 (Ch.91+560 to 91+920). The low FoS in Section 2 is attributed to the steeper slopes to the west of this section, where the peat depth gets shallower. The stone road has already been constructed in Section 18.

The ground investigations carried out to date, and the observations of the mineral soil from peat cuttings would seem to indicate that there is generally no weak sensitive clay layer at the base of the peat. AGEC also propose to carry out additional ground investigations during the detailed design stage of the project to assess the material characteristics and strength of the mineral soil below the peat.

If there is a thin layer of very soft peat or clay left in place at the base of the road some of the coarse rockfill is likely to penetrate the layer to support the road on the underlying soil, increasing the shear resistance along the base.

In the deep peat areas the rockfill will be pushed to refusal with the bucket of the excavator through weak amorphous peat up to 3.0 m deep, which should generally penetrate to the mineral soil below the peat.

The undrained shear strength of the material would also increase as it consolidates under the weight of the rockfill. Some of this gain in strength would occur relatively quickly during the construction of the road.

The geotechnical information that will be provided by the additional ground investigation will be necessary to carry out a detailed assessment of the risk of planar sliding at the base of the road. It would be normal to carry out this work during the detailed design



stage. The proposed methods of investigation and laboratory testing would be appropriate for assessing the necessary design parameters. However, a grid of probes or typical cross sections perpendicular to the slope would be required to determine the topography of the ground at the base of the peat along the road, which has not been proposed by AGEC. Although the ground investigations carried out to date would appear to indicate that there is generally no weak sensitive clay layer at the base of the peat, it should be noted that there is a layer of weak mineral soil with an STP N-Value of 0 at the base of the peat at Ch. 90+800 (GES: BH-009C-07), which has not been identified by AGEC. An N-Value of 0 would indicate that the soil has an undrained shear strength less than 10 kPa, and possibly as low as 5 kPa. Although this is in an area of very flat terrain, which would mitigate the risk of planar sliding, further investigation would be required to determine the characteristics, strength and extent of this material to assess the stability of the stone road in this area. AGEC have identified the risk of a sensitive clay layer at the base of the peat in the Geotechnical Risk Register in Appendix M4 of the EIS. The proposed mitigation measures include avoiding excessive loading and/or vibration. However, they have not included the removal of this material from below the stone road where necessary even though this would be an effective mitigation measure and has been identified as a worst case control measure in the additional information presented by AGEC during the Oral Hearing. In reality this would be difficult to implement in the deep peat areas. AGEC also indicate that a worst case control measure could also involve increasing the burial depth of the pipeline, but again this is not included on the Geotechnical Risk Register in the EIS. Having said this, the highest risk of planar failure at the base of the stone road would be immediately after constructing the road or possibly up to 24 hours after. After this time the material would consolidate and increase in shear strength under the weight of the fill. Therefore, provided that the road can be safely constructed, the risk of planar sliding at the base of the road after the gas pipeline has been constructed should be negligible. For health and safety reasons, it would be advisable to carry out an incremental load test on the roads in advance of pipe laying to verify that the road can support the proposed construction surcharge loads. In the additional information submitted to An Bord Pleanála during the Oral Hearing AGEC [10] give the results of an analysis of the stability of the stone road in the event that a peat slide occured upslope from the stone road between Ch. 91+560 and 91+920. The results indicate that, at this location there is a risk of planar sliding at the base of the stone road under the lateral load applied by the mobilized peat if there is a weak sensitive clay layer with an undrained shear strength of 5 kPa at the base of the road. However, they also demonstrate that the stability of the road should be adequate in the event of a peat slide based on the actual soil conditions identified in a trial pit in the area, where clayey gravelly sand was recorded below the peat.



I would agree with AGEC that the risk of a planar slide at the base of the stone road due to the lateral load applied from a peat slide upslope from the completed road and gas pipeline is negligible for the following primary reasons:

Provided that appropriate mitigation measures are implemented during the design and construction of the stone road the undrained shear strength or frictional resistance along the base of the road in the long-term after the pipeline has been constructed should be greater than prior to construction so that the road will act as a barrage to prevent a planar slide or bog burst in the peat upslope from the road.

Where the stone road is perpendicular to the slopes it will not act as a barrage. However, the impact of a peat slide parallel to the road in this case would be negligible.

4.4.6 Settlement of the Stone Road In additional information submitted to ABP during the Oral Hearing, AGEC [10] estimate that the maximum settlement of the gas pipeline due to consolidation settlement of the rockfill and peat in the stone road below the gas pipeline could range from 100-250 mm for the most likely situation, to 450 to 750 mm in the worst case scenario. The settlements are based on calculations of 0.1 to 0.3 times the thickness of the peat left in place under the road, which would reange from 0.5 to 2.5 m within the blanket bog. A worst case differential settlement is calculated based on the profile of settlement calculated based on the peat depth probes, including the possibly anomalous shallow probe result in the deep peat at Ch. 88+100. The analysis by J.P. Kenny[10] indicates that, even in the worst case scenario with the highest calculated differential settlements the pipeline should be able to withstand the stresses induced by the settlement in the rockfill. This assumes that all of the settlement occurs after the pipeline has been constructed. Based on this analysis I would consider that the risk of a pipeline rupture due to settlement of the stone road should be very low to negligible, as:

The lowest settlements calculated for Case 3 based on 0.1 times the peat thickness would probably represent a more reasonable estimate of the potential settlements that could occur whereas the J.P.Kenny report indicates that the pipeline could tolerate settlements up to three times these values.

The estimated differential settlements based on the depth of peat from the peat probes would also probably be reasonably conservative, particularly considering the variable peat depth and possibly anomalous probe in the deep peat area at Ch. 88+100.



The analyses assumes that all of the settlement occurs after the pipeline has been constructed when in fact some settlement would occur during construction of the stone road and before the pipeline is constructed.

However, it should be noted that there is still some uncertainty in the settlement estimates, and the nature of the rockfill mixed with peat could be vary variable which could increase the risk of non-uniform differential settlements along the length of the pipeline. Therefore, it would be good practice to monitor settlement of the rockfill prior to installation of the gas pipeline to validate the design assumptions and to demonstrate that the settlement has largely been completed before the pipeline and associated service pipes are installed. Similarly, it would be good practice to install stress gauges on the pipeline itself in critical areas in the deep peat to monitor the stresses induced by any differential settlements that occur over the design life of the pipeline. It should also be noted that the analyses by AGEC and J.P. Kenny do not take into account the potential impact that the settlements could have on the umbilical and service ducts, which would probably have significantly lower tolerance to differential settlement.



5.0 QUANTIFIED RISK ASSESSMENT – GROUND MOVEMENT RISK The Quantified Risk Assessment is presented in Appendix Q7 of the EIS [8]. The report was prepared by DNV Energy for SEPIL. The assumptions regarding the failure frequencies related to ground movement are outlined in Appendix I and Appenix II of the report. Appendix I: Summary of Main Points from Previous Studies: J.P. Kenny/Allseas Engineering Report – April 2005 (JPK) This was the initial report which predicted the risk to members of the public from the original route of the onshore section of the proposed gas pipeline. The pipeline failure frequencies for ground movement were determined using the UK Onshore Pipelines Operators Association (UKOPA) data. The values that were used in the analysis were:

Leak Frequency: 8.64 x 10-6 per km year Rupture Frequency: 9.6 x 10-7 per km year Total Release Frequency: 9.6 x 10-6 per km year

Leaks were represented by a 25 mm diameter hole; ruptures were modeled as a hole diameter equivalent to the pipeline internal diameter. Appendix II: Failure Frequencies Appendix II gives details of failure frequencies that are interpreted from published databases from two primary sources, i.e.

European Gas Pipeline Incident Data Group (EGIG) – 7th Report (1970-2004) – [15]

PD8010-3:2009 – British Standard Code of Practice for Pipelines – [16] Table No.5.1 gives a summary of the failure frequencies interpreted by DNV from these two sources. The frequencies from the EGIG report are for a 20” diameter pipeline which has a wall thickness of 27 mm. The data is interpreted from an incident database reported by 15 European Countries between 1970 and 2004. The data from PD8010-3:2009 gives failure frequencies for UK pipelines derived from UK data collated since 1962 and published by the UKOPA. For comparison, the failure frequencies assumed by DNV for the Corrib Onshore Gas Pipeline are also included in Table No. 5.1. these values are reported in Appendix VII of the DNV Report.



Table No.5.1 – Pipeline failure frequencies due to ground movement from the DNV Report Source Base

Frequency (per km year)

Probability of Rupture/

Hole/ Pinhole

Rupture Frequency

Hole Frequency

Pinhole Frequency

(per km year)

EGIG 2.3 x 10-5 0.68/0.16/0.16 1.56 x 10-5 3.68 x 10-6 3.68 x 10-6 PD8010-3 2.1 x 10-7 1/0/0 2.1 x 10-7 0 0 DNV Negligible - 0 0 0 Table No.5.1 clearly highlights that DNV have assumed a negligible base frequency for failure due to ground movement with a zero frequency for rupture, hole or pinhole failures based on the SEPIL design. They state that pipeline construction methods for the modified pipeline route have been developed so that the pipeline will not be affected by ground slides or flooding erosion, i.e:

Standard buried pipeline construction methods in farmland (minimal elevation, horizontal trace).

Trenchless crossings of the Sruwaddacon Bay with the gas pipeline, umbilical and service ducts in sleeving pipes min. 6-7 m below the bottom of the bay.

The pipeline will be constructed within a stone road through peatlands where the road will be constructed by removing the peat and placing gravel quarry run material on the underlying mineral soil.

The base frequency from the EGIG database is two orders of magnitude higher than the value quoted in PD8010-3. The EGIG have published a 7th Report dated December 2008, for the period 1970-2007. Based on this report, approximately 7.3% of all pipeline failures in the 15 European Countries that contributed to the incident database occurred due to ground movement. The following is a breakdown of the different sources of ground movement that are attributed to these failures:

Landslide 55% Flood 19% River 6% Mining 5% Other 2.5% Dyke Break 1.5% Unknown 11%

In questions at the Oral Hearing, Mr. Phil Crosswaite of DNV stated that he understood that approximately 90% of the failures recorded in this database were due to landslides in mountainous regions of one country that contributed to the database. This would appear to be an overestimate based on the data in the EGIG report.



PD8010-3 does not give a breakdown of the different sources of ground movement that gave rise to the failures that are recorded in the UKOPA database. However, at the Oral Hearing Mr. Crosswaite stated that the majority of the failures in the UKOPA database that were due to ground movement were caused by subsidence over mine workings. It should also be noted that the base failure frequency of 2.1 x 10-7 per km.yr quoted by DNV for ground movement actually only relates to rupture failures due to natural landslides (PD8010-3 – Section B.8.2), whereas Table B.1 (Section B.2) of PD8010-3 actually quotes a higher frequency of 9 x 10-6 per km.yr for pinhole, hole and rupture frequencies due to ground movement in general, which is almost two orders of magnitude higher and significantly closer to the comparable EGIG base failure rate of 2.3 x 10-5 per km.yr. It would seem to be misleading to omit the general base rate for all forms of failure and ground movement from PD8010-3 unless there is a specific reason why the lower base failure rate for natural landslides is considered more representative. However, in questions at the Oral Hearing Mr. Crosswaite could not explain why this figure was not quoted in the DNV report. Table B.15 of PD 8010-3:2009 gives a range of failure frequencies for a site specific assessment of the failure frequency due to natural landslides depending on the interpreted risk of slope instability. The ranges of failure frequencies are summarised in Table No. 5.2 Table No.5.2 – Pipeline failure frequencies due to natural landslides from PD 8010-3:2009 Description Pipeline Rupture Rate

(per km.year) Slope instability is negligible or unlikely to occur, but might be affected by slope movement on adjacent areas

0 to (9 x 10-8)

Slope instability might have occurred in the past or might occur in future, or is present and might occur in the future

(1 x 10-7) to (2.14 x 10-7)

Slope instability is likely and site specific assessment is required.

> (3 x 10-7)

Based on SEPIL design, DNV have interpreted a zero base failure rate for the site specific assessment on ground movement along the onshore gas pipeline. As we have summarised in preceding sections of the report, the main sources of ground movement that could have an impact on the gas pipeline after construction include:

Coastal Erosion at the Glengad Landfall site. Inundation or scouring from landslides on Dooncarten Mountain Scour in the upper and lower crossings of Sruwaddacon Bay Planar sliding of the stone road. Differential settlement of the pipeline in the stone road



Based on the information provided in the EIS it would appear that, with appropriate construction controls and design risk mitigation measures, the risk of ground movements from these sources affecting the completed gas pipeline is negligible. However, we would not consider that there is a zero risk, as there would still be some residual uncertainty related to the potential for scour along the watercourses at Dooncarten, and also with the construction of the stone road, particularly in the deep peat areas where there is a 2.5 to 3.0 m base of rockfill mixed with remoulded peat, which does not form the solid foundation implied by the description in the DNV report of a road constructed of rockfill on the underlying mineral soil. It is difficult to interpret a representative base failure frequency rate that would be appropriate to the Corrib Onshore Gas Pipeline. If, as Mr. Crosswaite stated, approximately 90% of the EGIG data is related to failures that occurred primarily in a mountainous region of one country in Europe, then this would probably not be directly applicable to the ground conditions along the low lying land on the Corrib Onshore Gas Pipeline route, even along the coastal plain at Glengad. Subsidence due to mining or karst features would not apply to the route of the onshore gas pipeline, and Ireland is not a seismically active zone so the risk of ground movements due to tectonic movement would be negligible. Similarly, if the UKOPA data in PD8010-3 is primarily related to failures that occurred due to mining subsidence, then this would also not be directly applicable to the ground conditions along the route of the Corrib Onshore Gas Pipeline. In the absence of another database that would be directly applicable to the route of the Corrib pipeline, but recognising that the risk of ground movement affecting the pipeline is probably very low to negligible, then it would seem unconservative to apply a zero base failure frequency to the risk of ground movement in the QRA. As a minimum, for the integrity of the analysis, it would be appropriate to calibrate the QRA with a very low base failure frequency for ground movement. In his evidence at the Oral Hearing, Mr. Crosswaite of DNV stated that failure frequencies on the order of 1 x 10-8 per km year or lower could be considered negligible. On this basis it would seem reasonably conservative to calibrate the QRA using the upper bound base frequency of 9 x 10-8 per km year quoted in Table 5.2 for areas where slope instability is negligible or unlikely to occur, but might be affected by slope movement on adjacent areas. In additional information submitted to An Bord Pleanála at the Oral Hearing, DNV submitted risk transects calibrated to show the effect of introducing a base failure frequency of 9 x 10-8 per km.year at 100 bar and 144 bar for the onshore gas pipeline. The results clearly showed that even a negligible base failure rate can significantly increase the risk of fatalities from a pipeline rupture, particularly at a distance of about 100-200 m from the pipeline, where the risk increases by several orders of magnitude.



Whereas this would be an important exercise to assess the sensitivity of the QRA, the fact that the incident databases may not be representative of the ground movement risks along the route of the Corrib Onshore Gas Pipeline could mean that the QRA may not be the most appropriate way of assessing the impact of these risks. In situations like these it may be more appropriate to carry out a qualitative risk assessment, which could also be more compatible with the geotechnical risk assessment, which uses a qualitative method of analysis.



6.0 OTHER GEOTECHNICAL CONSIDERATIONS 6.1 Haulage on rampart roads 6.1.1 Proposed haul route Figure No. 6.1 shows the proposed haul route for the construction of the onshore gas pipeline, which was presented in evidence by RPS at the Oral Hearing. The route follows local and regional roads including the:

R313 & R314 L 1202, L1203, L1203-40 & L1204 L5245-0, L52453-0 & L52453-25

Most of these roads are constructed as floating roads on peat. Some improvement works have been carried out on sections of the L1202, R314 and L1204. However, a number of the roads have not been upgraded and are still only suitable for light local traffic. Of particular note are the local roads in the vicinity of Rossport (L5245-0, L52453-0 & L52453-25), which are generally narrow (3.0 - 4.5 m wide) rampart roads on peat.

Figure No. 6.1 – Proposed haul route for the construction of the Onshore Gas Pipeline



6.1.2 Pavement assessment – Falling Weight Deflectometer and Pavement Condition Surveys

During the Oral Hearing, at the request of the inspector SEPIL submitted a report to An Bord Pleanála by TOBIN Consulting Engineers [12] which gave details of the Falling Weight Deflectometer (FWD) analyses and Pavement Condition Surveys that were carried out on the local and regional roads along the haul route as well as the proposed pavement overlay design for the roads that have not been strengthened. The FWD analyses were carried out in 2002 by PMS Pavement Management Surveys Ltd., prior to the strengthening works that have been carried out on some of the roads. The analyses were carried out on the following roads that are along the haul route for the onshore gas pipeline:

R313 & R314 (Road Width = 5.9 to 6.1 m) L 1202, L1203 & L1204 (Road Width = 4.7 to 5.4 m) L5245-0 (Road Width = 4.0 m) L52453-0 & L52453-25 (Road Width = 3.1 to 3.5 m)

As noted above, at the time that the surveys were carried out, the width of the local roads ranged from 3.0 - 4.0 m in the Rossport Area (L5245-0, L52453-0 & L52453-25) to 4.7 to 5.4 m outside this area (L-1202, L-1203 & L-1204). Based on limited trial hole information, the road pavement along most of these roads ranges from 300-500 m of imported granular material of varying quality over peat. There are no details of the bituminous pavement surface layers. Some improvements have been carried out along the L-1202 and L-1204 to strengthen the pavement and increase the width of sections of the roads along the haul routes to the Srahmore Peat Repository site to 5.5 m. However, no improvement works have been carried out in the local roads in the Rossport area and land constraints along the roads precludes any widening works and limits the improvement options to strengthening of the road pavement. The road width along the regional roads (R313 and R314) ranges from 5.9 to 6.1 m. Based on the limited trial hole information the road thickness along the R314 is in the range of 300-500 mm, as on the local roads, but this increases to 500-800 mm along the R313. The FWD analyses indicate that, prior to any strengthening works on the L-1203 and L-1204 the subgrade strength along the L5245, L52453-0, L-1203 and L-1204 was very poor, particularly along the L-1203, indicating that the roads are constructed on peat or soft subgrade. Similar results were recorded on the R313 and R314 regional roads. A slightly higher subgrade strength was recorded on the L52453-25 along the coast in Rossport, where the subgrade is classified as “Fair”.



All of the local roads exhibited extremely poor maximum deflection under the falling weight, and very poor or poor upper pavement condition. The maximum deflection along some sections of the R313 was classified as very poor, but elsewhere along the R313 and R314 it was extremely poor. The upper pavement condition on the regional roads was typically poor and locally very poor. PMS also carried out pavement condition surveys along the haul roads for the construction of the Terminal and the Onshore Gas Pipeline. Surveys were carried out in February 2005 and March 2009. The surveys consisted of a video survey and an assessment of the Pavement Condition Index (PCI) and International Roughness Index (IRI). The PCI is a measure of the pavement condition which reflects the degree of distress that is recorded in terms of structural and surface defects on the pavement. The IRI is a measure of the variation in surface elevation that induces vibration in moving vehicles. Of particular interest in these reports are the March 2009 surveys of the local roads in the vicinity of Rossport where no improvement or strengthening works have been carried out, i.e. the L5245-0, L52453-0 & L52453-25. Based on the PCI, the pavement along these roads has been classified as follows:

L5245-0 Fair (Average PCI = 62, St. Dev = 26) L52453-25 Poor (Average PCI = 47, St. Dev = 23) L52453-0 Very Poor (Average PCI = 33, St. Dev = 16)

The majority of the defects that are noted are structural (39-70%), including potholes, road disintegration, alligator cracking, edge breakup, rutting or depressions, which indicate load-related structural distress to the pavement. The rest (16-27%) are surface defects such as bleeding, raveling or patching. These results generally indicate a high level of structural damage to the pavement on lightly trafficked local roads probably due to damage to the weak subgrade on peat. The average IRI of these roads ranged from 6.2 for the L5245-0, to 7.8 to 8.1 for the L52453-0 and L52453-25, respectively indicating that there would be some perceptible movement with some swaying and wheel bounce on the roads with the higher IRI of about 8.0. 6.1.3 Proposed improvement works The proposed improvement works will generally consist of an overlay design to strengthen the pavement. Where the land is available the roads have been widened to 5.5 m to allow two-way HGV traffic. However, due to land constraints along the local roads in the Rossport area these roads will only be improved by strengthening the pavement



with the overlay. To compensate for this a convoy system will be implemented to control 1 way movement of up to 3 trucks at a time along the narrow roads. The proposed overlay design consists of:

100 mm Wet Mix Macadam Regulating Layer Geogrid reinforcement (‘Mesh Track’) 100 mm deep base course of Dense Bituminous Macadam (DBM) Double layer of surface dressing

Mesh Track is a galvanized steel hexagonal weave wire mesh interwoven with twisted flat wires for lateral reinforcement. This design is based on the overlay design specified by Mayo County Council in the May 2009 “Report on Infrastructure”, which Tobins state is consistent with the previous improvements carried out on the L1204, R313, R314 and L1202. Tobins also state that this is excess of the requirement for the HGV generation envisaged for the haulage operations in the Rossport Area. Their assessment is based on the overlay design for the 201 to 1000 one way AADT (Annual Average Daily Traffic) design band (comprising 10% HGV) in Table 5 of the Department of Environment and Local Government “Guidelines on the Depth of Overlay to be used on Rural Non-National Roads”. The overlay design specified for this traffic band in the DOE report comprises 200 mm of wet mix macadam assuming a CBR of 1.5 to 3% and an existing granular base of 100-200 mm. Table 6 of the DOE report specifies that the 200 mm of wet mix macadam can be substituted with 100 mm of DBM. This compares with the proposed design which includes 100 mm of DBM in addition to 100 mm of wet mix macadam and a steel mesh reinforcement. Tobins state that based on the Department of Environment and Local Government “Guidelines on the Rehabilitation of Roads over Peat” and on experience, where a road is to carry significant volumes of heavy vehicles the appropriate recommended maintenance method for both regional and local roads is to reinforce the pavement with a geogrid, overlay with cold or hot mix bituminous material, and then apply a double surface dressing to achieve skidding resistance and to seal the pavement from water ingress. Tobins state that the use of geogrid reinforcement will spread the wheel loads and, where there are existing cracks in the pavement due to settlement of the underlying peat, the reinforcement can aid in retarding the propagation of reflection cracks through the overlay layer and can restrict the width of cracks that may develop. They also say that the geogrid has an effect in reducing tensile strain in the overlay layer and in reducing differential movement.



6.1.4 Review and Comment Without reviewing the specific sections of the DOE “Guidelines on the Rehabilitation of Roads over Peat” or “Guidelines on the Depth of Overlay to be used on Rural Non-National Roads” which were referenced by Tobins but not included in their report, I would make the following observations and comments on the proposed strengthening works for the local roads along the haul route for the Corrib Onshore Gas Pipeline based on the information provided by SEPIL for the Oral Hearing, on my own experience with the earthworks design for roads, and on observations that I have made on the impact of construction traffic on low-volume regional and local roads, including rampart roads on peat:

It is not clear from Tobins report if the design in the DOE “Guidelines on the

Depth of Overlay to be used on Rural Non-National Roads” refers to roads that are constructed on peat. Peat would have a CBR significantly less than the 1.5-3.0% criteria used by Tobins. Therefore, the proposed overlay design from Mayo County Council is not necessarily in excess of the requirement for the HGV traffic envisaged for the construction of the Onshore Gas Pipeline.

The overlay design is also not based on a site specific assessment of the FWD data using the design methodology outlined in LR1132 and the analytical method involving pavement and subgrade layer modulii and limiting strains at the bottom of the bituminous layers and the top of the subgrade, as described in the FWD report by PMS Pavement Management Services, which was included in the Appendix of Tobins report.

The proposed method of strengthening the pavement with a wet mix macadam regulating layer, steel geogrid reinforcement and double surface dressing would appear to be an acceptable method of strengthening pavements of local and regional roads over peat that has been used successfully elsewhere in Ireland. However, it appears to be a general design based on experience and it is not clear from the information provided if the method would be effective in preventing subgrade failure in narrow rampart roads with steep sides close to the edge of the road, particularly along the L52453-0 through the Rossport Commonage.

The overlay and steel mesh geogrid reinforcement will stiffen the deflection of the

pavement under the wheel loads and will significantly increase the resistance of the pavement to cracking under the dynamic loads from HGVs, including the point loads directly under the wheels and the tensile forces that are created in the pavement from vibrations and from bow waves that can form in front of the trucks where the roads are constructed on a weak subgrade of peat.

However, the strengthening design does not provide any additional reinforcement to the subgrade of the road and narrow rampart roads with steep side slopes would be particularly vulnerable to shear failure at the sides of the road where the road cannot be widened. Although the steel reinforcement would spread the wheel



loads over a wider area and would provide some resistance to shear failure at the edges of the road, it may not be sufficient to prevent shear failure or deep rutting at the sides of the road.

In summary, the proposed overlay design will provide a significant improvement to the design and performance of the local and regional roads on peat along the haul route for the construction of the Onshore Gas Pipeline with improved ride quality and increased resistance to surface wear, cracking and formation of potholes under the concentrated wheel loads of the HGV. However, the overlay and upper level of steel reinforcement will not provide any basal reinforcement to increase in the subgrade strength of the road. Therefore, it may not be effective in preventing rutting due to yielding in the peat below the road or shear failure at the edges of the road, particularly where the haul route is on narrow rampart roads with steep slopes close to the edge of the road, as in the Rossport Area. Maintenance work can be carried out to repair rutting with additional overlays. However, the weakened subgrade in these areas would be susceptible to progressive failure on subsequent loading and a more effective repair would probably involve excavation of the weakened soil with associated road reconstruction and widening.

6.2 Stone Road – permeability of basal layer and transverse plugs As part of the proposed design for the stone road construction AGEC have proposed to incorporate low permeability hydraulic barriers to limit the potential drainage effect of the road. A minimum thickness of 0.5 m of peat will be left in place at the base of the stone road to impede vertical drainage, although in deeper peat areas the depth of peat left in place at the base of the road could be up to about 2.5 to 3.0 m. Rockfill for the stone road will be pushed to refusal into this peat so that the basal layer will actually consist of a matrix of coarse rockfill mixed with very soft remoulded amorphous peat. Transverse peat plugs will be constructed across the road at approximately 50 m centres to impede longitudinal drainage along the road. Similar to the basal layer, the plugs will actually be comprised of a matrix of rockfill mixed with very soft remoulded amorphous peat. The barriers that are formed with this composite material will have a lower permeability than the coarse rockfill alone and will reduce the potential for vertical and longitudinal drainage from the road. However, they are not controlled engineered materials that would normally meet a specification for a low permeability hydraulic barrier such as a well-compacted clay material (e.g. puddle clay or boulder clay with low stone content). They are non-uniform uncompacted soils that would have a higher permeability than the undisturbed peat and could exhibit preferential seepage around the coarse rockfill within the peat, or along the gas pipeline.



Vertical drainage will not be an issue if the underlying material is cohesive, as expected by SEPIL in the Rossport Commonage. However, this has not been proved as yet due to the limited SI information available and there are still some areas on the south side of Sruwaddacon Bay where granular soils have been identified at the base of the peat. Longitudinal drainage would still be an issue along the length of the route and currently there is no data to demonstrate that the barriers will be effective at limiting drainage along the stone road to prevent drawdown in the adjacent peat. Information presented by SEPIL in the addendum to the EIS seems to suggest that the section of stone road that has already been constructed near the terminal has a high water level in the rockfill and had limited drawdown in the adjacent peat, even without the transverse plugs, but with the lower hydraulic barrier, which was constructed along the length of the road. However, it should be noted that the road was stopped short of the stream at the base of the slope in this area, which would form a natural outfall to the water within the rockfill. The construction details of the stone road at stream crossings will be a key element of the design to limit the drainage effects of the road. As a minimum it would seem appropriate to have an effective transverse plug across the road on either side of the stream to prevent drainage from the road upslope from the stream. In summary, although the transverse plugs and basal barrier may be adequate in impeding drainage from the stone road to prevent drawdown in the peat adjacent to the road, the proposed design does not appear to be an engineered solution that has been validated by an analytical model or by precedent. It would be difficult to assess the permeability of the composite materials due to their variable nature. There also do not seem to be any proposals to validate the effectiveness of the barriers through an observational approach involving comprehensive groundwater monitoring. If necessary it would be possible to carry out remedial works or to install more effective barriers such as compacted clay, or bentonite plugs with sheetpiles. Details of the hydrological controls at watercourse crossings will also be a key issue, and drainage controls around the site compounds will also need to be considered in more detail. 6.3 Tunneling Rates – Sruwaddacon Bay Crossings It is proposed to complete the microtunneling for the Upper and Lower Crossings of Sruwaddacon Bay in 12 weeks and 8 weeks, respectively. This equates to a tunneling rate of 11-12 m/day for both drives based on continuous work 24hrs/day and an approximate length of 1030 m and 600 m, respectively. Based on recent experience on a tunneling contract for a 1.8 m ID tunnel in stiff cohesive glacial till, gravel and alluvial silt & clay, the average rate of drilling for a 100-200 m long tunnel drive could be on the order of 18-22 m/day when the progress of drilling is smooth without interruption. This would seem to indicate, that even if the efficiency of operations reduces with the longer tunnel drives, or some minor difficulties or delays are encountered, it should still be possible to achieve a tunneling rate of 11-12 m/day.



It is unlikely that the tunnel drives would be completed in the allocated time period if an intervention pit is required as this would involve operations to construct the supply jetties, mobilizing offshore sheetpiling rigs, driving sheetpiles, excavation and groundwater control, which would delay the tunneling by a few weeks at each location. However, based on the ground investigation information for the crossings it would seem unlikely that the intervention pits would be required. 6.4 Bentonite – break out. Bentonite break out is a risk in the Sruwaddacon Bay Crossings. They could typically occur in granular soils where there was an interruption in the tunneling which could lead to a void forming at the tunnel crown which could migrate up to the ground surface. The bentonite that is at risk of break out is the more viscous clay that is injected into the annulus around the pipes to lubricate the tunnel drive, which is different than the bentonite drilling fluid that is circulated at the head of the tunnel to transport material from the face. The volume of clay that is injected around the annulus of the pipe is low and the pressures are also relatively low, therefore, the risk of breakout would be limited to a puddle of the clay reaching the ground surface. With a rapid response to loss in pressure, as proposed by SEPIL, the potential size of a puddle could be limited to something on the order of about 1.0-2.0 m in diameter and <0.1 m deep. The bentonite would be inert. However, the material could be transported in suspension in the water in the bay, which could pose a threat to fish life. In the EIS SEPIL identify this risk (Section 14.6.1) and proposed a monitoring programme in the unlikely event of a bentonite release.



7.0 SUMMARY AND CONCLUSIONS 7.1 Introduction This section of the report provides a summary of the main conclusions and comments from Sections 3.0 to 6.0 of the report. Sections 3.0 and 4.0 address the issues of ground movement in the non-peat areas and peat stability in the blanket bog, respectively. The way in which ground movement is incorporated into the Quantified Risk Assessment is dealt with in Section 5.0, and Section 6.0 addresses the potential impact of other geotechnical considerations such as haulage on the narrow rampart roads on peat, the permeability of the hydraulic barriers along the stone road, and tunneling rates and the risk of bentonite break out for the tunnel drives across Sruwaddacon Bay. For a more comprehensive comment on each of these topics the reader is referred to the individual sections of the report. 7.2 Brief This report was prepared by Mr. Conor O’Donnell as specialist geotechnical adviser to the inspector for an Bord Pleanála at the oral hearing application for the onshore section of the Corrib Gas Pipeline from the landfall site at Glengad, to the Terminal Site in Bellanaboy (Ch. 83+400 to 92+550) in Co. Mayo. The specific references for the Oral Hearing are: Development: Corrib Onshore Gas Pipeline Type of Application: Strategic Infrastructure Development (16.GA.0004) Compulsory Acquisition Order (16.GA.0004) Applicant : Shell E&P Ireland Ltd. (SEPIL) My brief was to consider and advise on:

The adequacy of the pipeline design to withstand ground movement and peat instability which may arise from the construction of the pipeline as now proposed and which may arise during the operational life of the pipeline.

The adequacy of the proposed method of construction in peat

The impact of the proposed construction on the stability of the adjoining peat

The risk posed by the construction of the pipeline whereby surface water may gain access to the adjoining peat either during construction or post construction when the pipeline has been reinstated.



The risk posed to failure in the adjoining peat and how satisfactory the construction methods and mitigation measures proposed by the applicant are in eliminating this risk.

Has the risk of ground movement been adequately assessed in the Quantified Risk Assessment.

I have also been requested to comment on some other geotechnical considerations for the project, including:

Haulage on the rampart roads

The effectiveness of the basal layer and transverse plugs in preventing vertical and longitudinal drainage along the stone road

Tunneling rates for the Upper and Lower Crossings of Sruwaddacon Bay, and

The risk of bentonite break out for the microtunnel crossings. This planning application covers the section of onshore pipeline between Ch. 83+400 and 92+530 at the landfall site. For the purpose of this report, the alignment has been subdivided into the following eight sections:





Section 5: - Ch. 88+350 to 89+600 (1250 m): Sruwaddacon Bay - Upper Crossing






7.3 Ground Movement Risk – Non Peat Areas This section of the report addresses the risk of ground movement in the non-peat areas along the route of the pipeline, including:




Section 5: - Ch. 88+350 to 89+600 (1250 m): Sruwaddacon Bay - Upper Crossing 7.2.1 Ground Movement Risks Section 1 and Section 3 will be constructed in a shallow open trench along the narrow coastal strip along Sruwaddacon Bay using standard pipeline construction techniques. The LVI and Section 1 of the pipeline are located in Glengad, at the base of Dooncarten Mountain, where a series of landslides occurred on the steep upper slopes during a period of intense rainfall on the night of September 19th 2003. Therefore, the following ground movement risks have been considered for these sections of the pipeline:

Inundation of the LVI due to a debris flow from further landslides on Dooncarten Mountain (Section 1).

Erosion and scour in the watercourses along the pipeline in Section 1 during a period of intense rainfall similar to the event that triggered the previous landslides on Dooncarten Mountain.

The impact of rock excavation works for the gas pipeline and LVI on potentially unstable material on the slopes of Dooncarten Mountain (Section 1).

Coastal erorion of the cliff face at the landfall site in Glengad (Section 1), and along the coast in Rossport (Section 3).

Slope stability and pipe settlement along the alignment of the pipeline (Section 1 & 3).

The Sruwaddacon Bay crossings in Section 2 and Section 5 will be constructed by direct pipe microtunneling. The length of the tunnel drives for the Upper and Lower crossings will be 1030 m and 600 m, respectively, and it is proposed to complete each tunnel in a single drive. The following ground movement risks have been assessed for these areas:

Scour in the bay during construction in the event that an intervention pit is required to remove an obstruction at the head of the tunnel boring machine.

Scour in the bay after construction has been completed due to natural erosional processes.



7.2.2 Conclusions With the additional information that was presented during the Oral Hearing I am satisfied that sufficient information was provided to assess the potential impact of ground movement on the pipeline in the non-peat areas. 7.2.2.1 Dooncarten Landslides: I would agree with the combined conclusions of RPS and AGEC with regard to the potential impact of future landslides at Dooncarten Mountain on the LVI and the buried gas pipeline at Glengad, i.e.:

The LVI and the section of the pipeline along the Glengad headland is in a low risk zone with regard to the potential impact of landslides on Dooncarten Mountain.

It is unlikely that a debris flow from a landslide on the mountain would reach the LVI or pipeline due to the topography of the area, and due to the protection offered by the berm and drainage system at the base of the steep slopes on Dooncarten Mountain.

The main risk with regard to the LVI and pipeline is limited to the potential for erosion and scour in the watercourses along the route of the pipeline and there are established design measures that can be implemented to protect the pipeline from this risk.

The assessment carried out by RPS and AGEC is largely based on the report on the landslides produced by Tobin Consulting Engineers in 2003 rather than a recent independent inspection of the slopes. AGEC inspected the slopes in 2003 as part of the investigations into the cause of the landslides. However, I would recommend that AGEC or RPS carry out an independent inspection of the slopes during detailed design to ensure that the conclusions in the Tobin report are still valid. 7.2.2.2 Rock Excavation: I also agree with the opinion of RPS that vibrations generated from rock excavation at the LVI and possibly in the launch pit for the Lower Sruwaddacon Bay crossing are unlikely to have any significant effect on the destabilized material on the upper slopes of Dooncarten Mountain. Much of the excavation will be in weathered rock and the rock will be removed by digging and hydraulic breaking. No rock blasting will be carried out. Vibrations generated by the rock breaking would attenuate with distance from the source so that the magnitude of the vibrations would be negligible at the destabilized material on the mountain slopes, which is more than 1km away from the pipeline and LVI. To address the concerns of local residents I would recommend that vibration monitoring be carried out at two locations at a distance of about 25 m and 50 m from the site to establish a response curve for the attenuation of vibrations generated from the rock excavation and to demonstrate that the vibrations are unlikely to have an effect on the



adjacent properties or the destabilized material on the upper slopes of Dooncarten Mountain. 7.2.2.3 Coastal Erosion: The rate of erosion on the cliff face at the Glengad Landfall was assessed by inspection and by a historical review of the cliff face over a period of 160 years. It was concluded by AGEC that the rate of erosion is on the order of about 0.01 to 0.03 m/yr and set back distances of 5 m and 7 m were recommended for the temporary and permanent works, respectively. The LVI is set back approximately 40 m from the cliff face. I would consider that the historical review would be more reliable than a recent inspection of the cliff face, provided that the review is based on reliable mapping of the surveyed coastline over the period of study. However, the basis for the study was not stated. Other factors that should be taken into account would include:

The coastline protection has been altered by the removal of large boulders on the beach in front of the western cliff face at the pipeline landfall. These would have absorbed some of the wave energy during a storm.

The soil on the cliff face has been disturbed due to the open cut for the gas pipeline, which makes it more susceptible to erosion.

Climate change could result in more severe storms over the design life of the pipeline.

Therefore, there is some uncertainty in the analysis carried out by AGEC. The set back distance for the LVI significantly exceeds the specified minimum setback, which would mean that an observational approach could be implemented to monitor erosion at the cliff face, provided that appropriate inspection procedures and mitigation measures were established. However, some consideration should be given to constructing some form of natural coastal protection at the cliff face or on the beach. I accept the opinion of RPS that the risk of coastal erosion along the north shore of Sruwaddacon Bay is negligible along Section 3 of the pipeline as rock is exposed at the base of the cliff face along the fast flowing channel near the Rossport Landing, which would be resistant to erosion, and to the east of this there are tidal mudflats along the shore which indicate that this is a depositional environment. 7.2.2.4 Slope Stability and Pipe Settlement: The risk of pipe settlement and slope stability in the non-peat areas are also discussed in the report but are considered to be negligible due to the gently sloping topography and competent ground conditions along the route of the pipeline which consist of medium dense to dense granular soils and stiff cohesive glacial till with shallow rock in places.



7.2.2.5 Upper and Lower Sruwaddacon Bay Crossings: The risk of scour exposing the pipeline was considered for the event that an intervention pit is required to remove an obstruction to the tunnel boring machine during construction of the pipeline, and also for natural scour in the bay after construction has been completed. Scour is a very specialized area of expertise. The analyses were generally carried out by advanced numerical modeling of the tidal flow in the Bay using a finite element hydrodynamic flow model that was calibrated by measurements recorded at four mooring points in the bay. This is a very sophisticated and site specific method of analysis so I would consider that it is a reasonable approach for assessing flow data in the bay. Scour potential was assessed on the basis of this data. Based on the ground conditions along the tunnel horizon, I would agree with RPS that it is unlikely that an intervention pit would be required to clear an obstruction to the TBM. Nevertheless, the scour analyses carried out by RPS/HR Wallingford indicate that, in the worst scenario of an intervention pit location in the fastest flowing part of the channel, the potential scour depth around the intervention pit ranges from 5.0 m at the Upper Crossing, to 7.5 m at the lower crossing. This is within the depth of the pipeline for the Lower Crossing. Therefore, there is a risk that the pipeline could be exposed at this location outside the intervention pit. Eddies created around the pipeline could prevent the natural restoration of the sea bed, potentially leaving the pipe exposed after the intervention pit is removed, or possibly left covered but with a void below the pipe. Some appropriate mitigation measures are identified in the EIS, including the use of scour protection. I would also consider that the impact that this could have on the completed gas pipeline would be negligible provided that the problem is identified during construction so that appropriate mitigation measures can be implemented, which may include grouting below the pipe. I have recommended that the risk be included in the Geotechnical Risk Register for detailed design and construction. Based on the analyses carried out by for the scour potential in the natural condition after the pipeline has been completed RPS conclude that in general the coarse, granular material in the bay is in dynamic equilibrium with the dominant tidal action and that alignment of the pipeline at the Lower Crossing will not be effected by potential future changes to the channel alignment. However, when the potential for scour under an increased tidal energy is considered in the deep channel along the Rossport Bank the maximum scour depth is limited to 2.0 m, which should not expose the pipeline which will be buried with a minimum depth of cover of 4.0 m. At the Upper Crossing the maximum depth of natural scour is estimated at 1-2 m in the event that there is a change in the location of the channel in the Bay. On the basis of these analyses the interpreted risk of exposing the pipeline is negligible.



7.3 Ground Movement Risk in the Blanket Bog (Peat Stability) This section of the report addresses the issue of ground movement along the route of the pipeline through the blanket bogs, including:





The main issue in these sections is related to peat stability. However, potential impacts of the stone road construction method are also discussed. 7.3.1 Ground Movement Risks The pipeline will be constructed through the blanket bog using the stone road method of construction. The method is described in some detail in Section 2.5.2 of the report. In general it will involve excavation of the peat to construct a road comprised of granular rockfill supported on the underlying mineral soil. However, the mineral soil will not be exposed in the excavation as a minimum depth of 0.5 m of peat will be left in place at the base of the road to prevent vertical drainage into the underlying soil. The base of the road will then be formed by pushing coarse rockfill into the peat with the bucket of the excavator until there is firm resistance, which will create a matrix of rockfill mixed with remoulded peat. In deep, very soft peat the peat will be excavated down to the maximum depth of stable excavation and the rockfill will be pushed into the peat below this level to depths of up to 2.5 to 3.0 m. Transverse hydraulic barriers or “plugs” of remoulded peat mixed with rockfill will be constructed at maximum 50 m centres across the road to prevent longitudinal drainage. All of the surplus excavated peat will be removed directly to the Srahmore repository site. However, peat turves 0.5 to 1.0 m thick will be stored on bog mats adjacent to and upslope from the road in the intact non-designated and designated bog for resinstatement over the stone road after construction of the pipeline. In the areas outside the designated and non-designated intact bogland, where the acrotelm surface layer of the peat is cutaway or damaged, the method of construction for the stone road will be the same as in the intact bogland. However, the peat turves will not be stored for reinstatement unless the acrotelm layer is largely intact. Instead SEPIL will endeavour to restore the surface to a similar condition using the excavated peat, which will be stored locally in the site compounds or on the bog mats adjacent to the excavation. However, it is not clear from the information provided where exactly it is proposed to store peat on bog mats outside of the zones of intact bog.



The main risk with regard to peat stability is for a planar peat slide in the undrained condition under a surcharge of up to 1 m of peat stored on bog mats adjacent to the road. Failure could occur where there is an inadequate factor of safety against shear failure at the base of the peat under the applied load. Failure could also occur in the underlying mineral soil if there is a weak sensitive clay layer below the peat. The undrained condition would apply to the short-term condition during construction. The highest risk of failure would occur immediately on loading, or up to about 24 hours after. A planar peat slide could also occur in the drained condition where the frictional shear strength at the base of the peat or in the underlying mineral soil is exceeded under the self weight of the peat with an applied surcharge of up to 1 m of peat stored on bog mats. For this type of failure to occur the peat would generally be underlain by a relatively permeable granular soil or fractured rock layer, or where there is a network of pipes in the peat at the interface with the underlying soil. The drained condition would apply to the long-term condition after pore water pressures generated by construction loading have dissipated. However, in general the risk of drained failure under hydrostatic ground water conditions is very low and drained failures typically occur after periods of intense rainfall, where restricted flow along preferential drainage paths in the pipes or more permeable soils below the peat lead to a build up of water pressures that can reduce the frictional resistance along the interface between the two soils until shear failure occurs. In an extreme case the water pressures can exceed the dead weight of the overlying peat leading to an uplift or buoyant failure with negligible frictional resistance along the sliding plane. The risk of failure is exacerbated after an extended period of dry weather where the natural water level in the peat is lower and the peat dries out. This reduces the self-weight of the peat and can allow rainwater to penetrate more rapidly into the underlying soils through shrinkage cracks at the surface. In deep, highly amorphous weak peat with a high water table there is a risk of a bog burst or localized shear failure at the face of the excavation for the stone road. Bog bursts are typically characterized by a flow slide of slurried or fluidized weak amorphous peat from below the fibrous crust of the acrotelm layer through a tear or rupture on the downslope side of the deposit. The risk of a bog burst or shear failure can be increased by mechanical cutting for peat harvesting, particularly cross cutting with a sausage cutter, and by excavation of drains across the slope of a deep peat area. Bog bursts can also be triggered by adjacent planar peat slides or excavations at the margins of deep peat areas. In areas of deep weak peat, localized shear failure at the face of an excavation can extend for a considerable distance from the face of the excavation, leading to tension cracks and slumping of the peat over a wide area, which could lead to a larger scale peat failure if left unsupported. The other ground movement risks that have been considered for the stone road include planar sliding at the base of the road, and differential settlement of the rockfill after the pipe has been constructed.



There could be a risk of planar sliding at the base of the stone road if the road was constructed on a weak layer of very soft peat or sensitive clay and if the topography of the ground along the base of the road was such that shear forces on the slope at the base of the road exceeded the undrained shear strength of the weak peat/clay at this level. The highest risk would be immediately after constructing the road or possibly up to 24 hours after. After this time the material would consolidate and increase in shear strength under the weight of the fill There is a risk that differential settlement of the rockfill in the stone road could induce bending stresses in the gas pipline that could potentially exceed the yield strength of the steel. 7.3.2 Conclusions 7.3.2.1 Planar Sliding – Undrained Condition: Notwithstanding the limited ground investigation information in the blanket bog in the Rossport Commonage and South of Sruwaddacon Bay, AGEC have now carried out a comprehensive analysis of the stability of the peat slopes in the blanket bogs along the alignment of the onshore gas pipeline. The analysis included:






A qualitative assessment of the relative potential for peat failure along the route of the onshore gas pipeline based on a combination of up to 32 No. environmental factors that can affect peat stability.

This level of analysis is commensurate with the geotechnical risks associated with the proposed construction of the pipeline through the blanket bogs, and the method of analysis is also consistent with the guidelines recommend by the Scottish Forestry Commission for the risk management of peat slips on the construction of low volume/low cost roads over peat (Mac Culloch, 2006), which is a key publication for the current standard of practice for this type of work. The particular approach used by AGEC for assessing the interaction between different environmental factors is also consistent with more advanced geotechnical risk assessment procedures being developed for managing



the risk of peat stability for the construction of windfarms on blanket bogs in Ireland to take account of experience from recent peat failures. AGEC have carried out the analysis in accordance with BS 6031:1981 – British Standard Code of Practice for Earthworks. However, given the uncertainty in the undrained shear strength determined from the hand vanes, and the potential variability in the live load when the weight of the bog mats is included, it would be more conservative in this case to use the Eurocode 7 approach for design as a partial factor is applied to the live load as well as the soil strength parameters. Using the infinite slope analysis AGEC have calculated a factor of safety > 1.5 for the peat slopes in the undrained condition with no surcharge, indicating that there is generally a low potential for failure of the slopes in the natural condition. This is consistent with the shallow slope angles along the route of the pipeline, which are generally less than 3 degrees and only locally higher in areas of shallow peat at the boundaries of the bog and at the edges of the Leenamore River. When the 10 kPa surcharge on the peat is included in the analysis, the factor of safety over much of the pipeline is still >1.5 with the exception of a few critical areas where a low factor of safety of 1.0 to 1.3 was calculated i.e.:

o Ch. 87+219 – Rossport Commonage: FoS = 1.3 o Ch. 89+867 – South of Sruwaddacon Bay: FoS = 1.3 o Ch. 91+688 – on the approach to the Terminal at Bellanaboy: FoS = 1.0

AGEC also identify the following critical areas based on particular characteristics of the peat, hydrology and land use which can contribute to a peat slide:

The extensive areas of machine cut peat in Rossport Commonage, where local stability of the excavation for the stone road could be an issue (e.g. Ch. 86+250 – 86+600 & Ch. 87+300 – 87+450)

Areas of weak/wet peat – e.g. the shallow bog pools within an area of relatively

intact peat at Ch. 87+200 to 87+300, where the peat is notably weak and deep at the crest of a slope, which represents a greater risk to peat stability.


likely to have a significant thickness of weak amorphous peat at depth. However, they have not included any similarly critical areas in the blanket bog to the south of Sruwaddacon Bay, particularly the area of deep highly amorphous peat at the head of the watercourses between about Ch. 90+900 and 91+550, where the existing stone road ends.



Using the qualitative assessment of the relative potential for peat failure along the route of the onshore gas pipeline, AGEC identify the following areas with a relatively high potential for peat failure:

Section 2: Rossport Commonage - Ch. 86+250 to 86+600 Section 18: South of Sruwaddacon Bay – Ch. 91+560 to 91+920

This is based on a particularly critical combination of factors such as soil properties, topography, groundwater conditions, peat cuttings and a low factor of safety for the stone road in the infinite slope condition assuming that it is founded on a weak clay layer. The qualitative assessment that was carried out by AGEC is a valuable tool for identifying the relative failure potential for different sections of the pipeline. However, there is still some subjective interpretation of the risk bands and in this case we would consider that it underestimates the potential risk for peat failure for Section 15 (Ch. 90+870 – 91+210) & Section 16 (Ch.91+210 to 91+470) where there are deep deposits of wet highly amorphous peat directly upstream or adjacent to a defined watercourse with a weak underlying clay layer. The risk of a peat slide is somewhat offset by the shallow slope angles in this area. Nevertheless, the particular combination of risk factors in these areas would warrant a more conservative approach to the construction of the pipeline than other lower risk areas along the route with appropriate mitigation measures to reduce the risk of a peat failure such as not allowing the storage of peat turves on bog mats adjacent to the excavation for the stone road. There are a number of design and mitigation measures that are proposed by AGEC that could be used to successfully manage the geotechnical risk of peat stability during construction of the stone road, including:

Additional ground investigations, particularly in the critical areas that have been identified.

Full-time supervision and monitoring during construction by experienced personnel.

On-going assessment of ground conditions to confirm the findings in the EIS with installation and monitoring of instrumentation in critical areas.

There are also a number of mitigation measures identified in the geotechnical risk register for dealing with peat stability. However, there is a deficiency of information on how these risk mitigation measures would be implemented on site. For example, no specific mitigation measures have been specified for those areas where a relatively high risk of peat failure potential has already been identified, or where a low factor of safety has been calculated for the infinite slope condition under the 10 kPa surcharge.



The problem with this is highlighted by the work that was carried out during the construction of the existing stone road through Section 18 (Ch. 91+560 to 91+920), where the relative peat failure potential was characterized as high and a very low factor of safety of 1.0 was calculated for the infinite slope analysis for planar sliding in the undrained condition under the surcharge of up to 1 m of peat (10 Pa). There are also other contributory factors that would identify this as a critical area with respect to peat stability. Adopting a reasonably conservative approach to risk management it would have been good practice to restrict sidecasting peat adjacent to the road in this area. However, during construction of the stone road up to 1.0 m of peat was stored on the peat adjacent to the road. I would still consider that the stone road can be safely constructed through the blanket bogs by an experienced contractor under the full-time supervision of an experienced geotechnical engineer. However, this will take a coordinated and more conservative effort to risk management during design to identify where specific mitigation measures are required prior to construction so that they can be effectively implemented in appropriate areas during construction. A list of appropriate mitigation and control measures have been proposed by AGEC but it is not clear from the information provided where these will be implemented based on the information provided in the EIS and at the Oral Hearing. Specifically they have not specified areas where limits will be put on the storage of the peat turves adjacent to the excavation for the stone road. It is also not clear from the information provided where exactly it is proposed to store peat on bog mats outside of the zones of intact bog. 7.3.2.2 Planar Sliding – Drained Condition: Based on the results of stability analyses carried out by AGEC for planar sliding along an interface at the base of the peat or in the underlying mineral soil using the infinite slope method in the drained condition, AGEC conclude that:

The factor of safety against planar sliding at the base of the peat and in the underlying mineral soil was >1.5 in the drained condition for the natural peat slopes (0 kPa surcharge) and with a surcharge of 10 kPa on the surface of the peat for all of the different water levels that were assumed.

The results would indicate that the peat slopes have an adequate factor of safety against sliding in the long-term condition, which is consistent with the findings of the walkover survey which identified no signs of rainfall-induced failures of the peat in the area.

However, the analyses by AGEC does not consider artesian conditions or a build-up of excess water pressures below the peat in relatively permeable soils or fractured rock due to restricted drainage along preferential drainage paths in the event of an extreme rainfall condition.



In general, the risk of a peat slide under these conditions would be limited to areas of relatively shallow peat at a break in slope near the margins of the bogs or along the edges of the Leenamore River Estuary. It is also a temporary condition that would apply during construction when the surface of the stone road is left exposed to rainfall. The risk of drained failure would be partially mitigated by the proposed 0.5 m thick hydraulic barrier along the base of the road consisting of rockfill mixed with the remoulded peat, which should limit vertical drainage into granular soils below the peat. However, it could be more effectively mitigated with a temporary drainage system that would prevent the build-up of groundwater in the stone road. In additional information submitted to an Bord Pleanála during the Oral Hearing, SEPIL confirmed that the surface water management system for the temporary works during construction will be designed for the 10 year 24 hour storm event in accordance with CIRIA guidelines (e.g. CIRIA Report No.C648,2006 – Control of Water Pollution from Linear Construction Projects). Under normal circumstances this would seem reasonable for the proposed duration of the works on the onshore gas pipeline. However, given the potential for an associated risk of peat instability in some areas it would be more conservative to design for a storm with a greater return period, such as the 50 or 100 year storm so that the risk of planar sliding in the drained condition would be negligible. 7.3.2.3 Bog Burst and Localized Shear Failure: For the Corrib Gas Pipeline the risk of a bog burst or localised shear failure at the sides of the excavation for the stone road would be confined to the areas where there is deep highly amorphous peat with very high natural moisture content and a high natural water table between:


Particularly where there are other contributory factors such as machine cut peat, poor drainage, surface water, bog pools or a watercourse directly downslope from the excavation. The proposed method of construction for the stone road in these areas involves the controlled displacement of peat with rockfill below the depth of stable excavation in the peat, which should prevent any localised shear failure at the edges of the excavation progressing into a larger peat slide or bog burst. This is an accepted method of constructing stone roads through areas of deep peat that has been used extensively and succesfully across blanket bogs in Ireland. Therefore, it should be possible to construct the road through the areas of deep peat without causing a



bog burst provided that the work is carried out by an experienced contractor under the full-time supervision of a geotechnical engineer with experience of similar works. Those areas where the surface of the peat is disturbed by machine cutting would require particularly care as the stability of the peat in the excavation is likely to be very poor. 7.3.2.4 Planar Sliding at the Base of the Stone Road: I would consider that the risk of planar sliding at the base of the road during construction should be very low for the following reasons:

The road will be constructed on gently sloping ground near the watershed of the bog where the slope angles at the base of the peat are very low (Gen <3°, and even lower in the deep peat areas).

The ground investigations carried out to date, and the observations of the mineral soil from peat cuttings would seem to indicate that there is generally no weak sensitive clay layer at the base of the peat.

AGEC propose to carry out additional ground investigations during the detailed design stage of the project to assess the material characteristics and strength of the mineral soil below the peat.

If there is a thin layer of very soft peat or clay left in place at the base of the road some of the coarse rockfill is likely to penetrate the layer to support the road on the underlying soil, increasing the shear resistance along the base.

In the deep peat areas the rockfill will be pushed to refusal with the bucket of the excavator through weak amorphous peat up to 3.0 m deep, which should generally penetrate to the mineral soil below the peat.

The undrained shear strength of the material would also increase as it consolidates under the weight of the rockfill. Some of this gain in strength would occur relatively quickly during the construction of the road.

The highest risk of planar failure at the base of the stone road would be immediately after constructing the road or possibly up to 24 hours after. Therefore, provided that the road can be safely constructed, the risk of planar sliding at the base of the road after the gas pipeline has been constructed should be negligible. For health and safety reasons, it would be advisable to carry out an incremental load test on the roads in advance of pipe laying to verify that the road can support the proposed construction surcharge loads. Although the ground investigations carried out to date would appear to indicate that there is generally no weak sensitive clay layer at the base of the peat, it should be noted that there is a layer of weak mineral soil at the base of the peat at Ch. 90+800 (GES: BH-



009C-07), which has not been identified by AGEC. Although this is in an area of very flat terrain, which would mitigate the risk of planar sliding, further investigation would be required to determine the characteristics, strength and extent of this material to assess the stability of the stone road in this area. AGEC also submitted the results of a stability analysis that showed that there was a risk of planar sliding at the base of the stone road between Ch. 91+560 and 91+920 in the event that the road was constructed on a layer of weak sensitive clay and a peat slide occured upslope from the stone road after construction of the gas pipeline. However, I would agree with AGEC that this analysis is not representative of the actual conditions, and that the risk of a planar slide at the base of the stone road due to the lateral load applied from a peat slide upslope from the completed road and gas pipeline is negligible because:

Provided that appropriate mitigation measures are implemented during design and construction of the stone road, the undrained shear strength or frictional resistance along the base of the road in the long-term after the pipeline has been constructed should be greater than prior to construction so that the road will act as a barrage to prevent a planar slide or bog burst in the peat upslope from the road.

Where the stone road is perpendicular to the, the impact of a peat slide parallel to the road in this case would be negligible.

7.3.2.5 Settlement of the Stone Road: AGEC estimate that the maximum settlement of the gas pipeline due to consolidation settlement of the rockfill and peat in the stone road below the gas pipeline could range from 100-250 mm for the most likely situation, to 450 to 750 mm in the worst case scenario. A worst case differential settlement is calculated based on the profile of settlement from the peat depth probes. J.P. Kenny then present a series of calculations to determine the stresses that are induced in the pipeline under the estimated settlement profiles. Based on these analyses I would consider that the risk of a pipeline rupture due to settlement of the stone road should be very low to negligible, as:

The J.P.Kenny report indicates that the pipeline could tolerate settlements up to three times the values estimated for the most likely situation.

The estimated differential settlements based on the depth of peat from the peat probes would also probably be reasonably conservative.



The analyses assumes that all of the settlement occurs after the pipeline has been constructed when in fact some settlement would occur during construction of the stone road and before the pipeline is constructed.

However, it should be noted that there is still some uncertainty in the settlement estimates, and the nature of the rockfill mixed with peat could be vary variable which could increase the risk of non-uniform differential settlements along the length of the pipeline. Therefore, it would be good practice to monitor settlement of the rockfill prior to installation of the gas pipeline to validate the design assumptions and to demonstrate that the settlement has largely been completed before the pipeline and associated service pipes are installed. Similarly, it would be good practice to install stress gauges on the pipeline itself in critical areas in the deep peat to monitor the stresses induced by any differential settlements that occur over the design life of the pipeline. It should also be noted that the analyses by AGEC and J.P. Kenny do not take into account the potential impact that the settlements could have on the umbilical and service ducts, which would probably have significantly lower tolerance to differential settlement.



7.4 Quantified Risk Assessment DNV have interpreted a zero base failure rate for the site specific assessment on ground movement along the Corrib Onshore Gas Pipeline, which is based on the following key elements of SEPIL’s design:

Standard buried pipeline construction methods in farmland. Trenchless crossings of the Sruwaddacon Bay with the gas pipeline, umbilical and

service ducts in sleeving pipes min. 6-7 m below the bottom of the bay. The pipeline will be constructed within a stone road through peatlands where the

road will be constructed by removing the peat and placing gravel quarry run material on the underlying mineral soil.

Based on the information provided in the EIS it would appear that, with appropriate construction monitoring and design risk mitigation measures, the risk of ground movements affecting the gas pipeline after construction should be negligible. However, I would not consider that there is a zero risk, as there would still be some residual uncertainty related to the potential for scour along the watercourses at Dooncarten, and also with the construction of the stone road, particularly in the deep peat areas where there is a 2.5 to 3.0 m base of rockfill mixed with remoulded peat, which does not form the solid foundation on the underlying mineral soil, as assumed by DNV. It is difficult to interpret a site-specific representative base failure rate for ground movement that would be applicable to the Corrib Onshore Gas Pipeline as the database of failures in the EGIG report and the database used to calculate the relevant failure frequencies in PD8010-3 may not be directly comparable to the ground movement risks along the route of the Corrib pipeline. Nevertheless, I would consider that it would still be unconservative to apply a zero base failure frequency to the risk of ground movement in the QRA. As a minimum, for the integrity of the analysis, it would be appropriate to calibrate the QRA with a very low or negligible base failure frequency for ground movement to determine the effect that this would have on the results. For site specific analysis of the failure frequencies due to natural landslides PD8010-3 quotes a pipeline rupture rate up to 9x10-8 per km.year where slope instability is negligible or unlikely to occur but might be affected by slope movement on adjacent areas. In the absence of more representative site-specific records of failures due to ground movement for the Corrib Onshore Pipeline, I would recommend that this value be used to calibrate the QRA. In information submitted to An Bord Pleanála at the Oral Hearing, DNV showed that even this very low failure rate can have a significant impact on the interpreted risk of fatalities from a pipeline rupture, particularly within about 100-200 m from the onshore gas pipeline, where the risk increases by several orders of magnitude. However, the results have not been carried through the rest of the risk assessment.



Whereas this would be an important exercise to assess the sensitivity of the QRA, the fact that the incident databases may not be representative of the ground movement risks along the route of the Corrib Onshore Gas Pipeline could mean that the QRA may not be the most appropriate way of assessing the impact of these risks. In situations like these it may be more appropriate to carry out a qualitative risk assessment, which could also be more compatible with the Geotechnical Risk Assessment in Appendix M4, which also uses a qualitative method of analysis.



7.5 Other Geotechnical Considerations 7.5.1 Haulage on rampart roads The proposed haul route for the construction of the onshore gas pipeline follows a number of local and regional roads between Rossport, Glengad and the Srahmore Peat Repository site, including the:

R313 & R314 L 1202, L1203, L1203-40 & L1204 L5245-0, L52453-0 & L52453-25

Most of these roads are constructed as floating roads on peat. Some improvement works have been carried out on sections of the L1202, R314 and L1204. However, a number of the roads have not been upgraded and are still only suitable for light local traffic. Of particular note are the local roads in the vicinity of Rossport (L5245-0, L52453-0 & L52453-25), which are generally narrow (3.0 - 4.5 m wide) rampart roads on peat.

Falling Weight Deflectometer surveys indicated that the subgrade strength along these roads was very poor, although a slightly higher subgrade strength classified as “fair” was recorded on the L52453-25, along the coast in Rossport, indicating that this road may be constructed on the underlying mineral soil, which is consistent with the shallow depth of peat in this area. Pavement condition surveys on the local roads in Rossport indicated that the pavement would be classified as “Fair” to “Very Poor”. The majority of the defects that were recorded were structural, including potholes, road disintegration, alligator cracking, edge breakup, rutting or depressions, which indicate a high level load-related structural distress to the pavement, probably due to damage to the weak subgrade on peat. The survey of the surface roughness on these roads indicated that there would be some perceptible movement with some swaying and wheel bounce. SEPIL have proposed to carry out strengthening and improvement works along the local roads along the haul route with an overlay design that consists of:

100 mm Wet Mix Macadam Regulating Layer Geogrid reinforcement (‘Mesh Track’) 100 mm deep base course of Dense Bituminous Macadam (DBM) Double layer of surface dressing

However, due to land constraints, no road widening works will be carried out on the narrow rampart roads in Rossport. The proposed method of strengthening the pavement with a wet mix macadam regulating layer, steel geogrid reinforcement and double surface dressing would appear to be an



acceptable method of strengthening pavements of local and regional roads over peat that has been used successfully elsewhere in Ireland. However, it appears to be a general empirical design based on experience rather than a site-specific analytical design based on the results of the Falling Weight Deflectometer results along the haul route. It is also not clear from the information provided if the design would be would be applicable to narrow rampart roads on peat. The proposed overlay design will provide a significant improvement to the design and performance of the local and regional roads on peat along the haul route with improved ride quality and increased resistance to surface wear, cracking and formation of potholes under the concentrated wheel loads of the HGV. However, the overlay and upper level of steel reinforcement will not provide any basal reinforcement to increase in the subgrade strength of the road. Therefore, it may not be effective in preventing rutting due to yielding in the peat below the road or shear failure at the edges of the road, particularly where the haul route is on narrow rampart roads with steep slopes close to the edge of the road, such as on the L52453-0 across the Rossport Commonage. Maintenance work can be carried out to repair rutting with additional overlays. However, the weakened subgrade in these areas would be susceptible to progressive failure on subsequent loading and a more effective repair would probably involve excavation and replacement of the weakened soil with associated road widening. 7.5.2 Stone Road – permeability of basal layer and transverse plugs As part of the proposed design for the stone road construction AGEC have proposed to incorporate low permeability hydraulic barriers to limit the potential drainage effect of the road. A minimum thickness of 0.5 m of peat will be left in place at the base of the stone road to impede vertical drainage, although in deeper peat areas the depth of peat left in place at the base of the road could be up to about 2.5 to 3.0 m. The basal layer will consist of a matrix of coarse rockfill mixed with very soft remoulded amorphous peat. Transverse peat plugs will be constructed across the road at approximately 50 m centres to impede longitudinal drainage along the road. Similar to the basal layer, the plugs will be comprised of a matrix of rockfill mixed with very soft remoulded amorphous peat. The barriers that are formed with this composite material will have a lower permeability than the coarse rockfill alone and will reduce the potential for vertical and longitudinal drainage from the road. However, they are not controlled engineered materials that would normally meet a specification for a low permeability hydraulic barrier such as a well-compacted clay material (e.g. puddle clay or boulder clay with low stone content). They are non-uniform uncompacted soils that would have a higher permeability than the undisturbed peat and could exhibit preferential seepage around the coarse rockfill within the peat, or along the gas pipeline.



Although the transverse plugs and basal barrier may be adequate in impeding drainage from the stone road to prevent drawdown in the peat adjacent to the road, the proposed design does not appear to be an engineered solution that has been validated by an analytical model or by precedent. It would be difficult to assess the permeability of the composite materials due to their variable nature. However, there do not seem to be any proposals to validate the effectiveness of the barriers through an observational approach involving comprehensive groundwater monitoring. If necessary it would be possible to carry out remedial works or to install more effective barriers such as compacted clay, or bentonite plugs with sheetpiles. Details of the hydrological controls at watercourse crossings will also be a key issue, and drainage controls around the site compounds will also need to be considered in more detail.

7.5.3 Tunneling rates – Sruwaddacon Bay Crossings It is proposed to complete the microtunneling for the Upper and Lower Crossings of Sruwaddacon Bay in 12 weeks and 8 weeks, respectively. This equates to a tunneling rate of 11-12 m/day for both drives based on continuous work 24hrs/day and an approximate length of 1030 m and 600 m, respectively. Based on recent experience on a tunneling contract for a 1.8 m ID tunnel in stiff cohesive glacial till, gravel and alluvial silt & clay, the average rate of drilling for a 100-200 m long tunnel drive can be on the order of 18-22 m/day when the progress of drilling is smooth without interruption. This would seem to indicate, that even if the efficiency of operations reduces with the longer tunnel drives, or some minor difficulties or delays are encountered, it should still be possible to achieve a tunneling rate of 11-12 m/day. It is unlikely that the tunnel drives would be completed in the allocated time period if an intervention pit is required to remove an obstruction to the TBM. However, I would consider is an unlikely scenario based on the ground conditions encountered along the tunnel horizon. 7.5.4 Benonite break out during microtunneling Bentonite break could occur during microtunneling on the Sruwaddacon Bay Crossings. An interruption in the tunneling could cause a void to form at the tunnel crown which could lead to break out of the lubricating bentonite from around the tunnel annulus up to the sea bed. The volume of clay that is injected around the annulus of the pipe is low and the pressures are also relatively low. Therefore, with a rapid response to loss in pressure, as proposed by SEPIL, the risk of breakout would be limited to a puddle of the clay about 1.0-2.0 m in diameter and <0.1 m deep reaching the ground surface or sea bed. The bentonite would be inert. However, the material could be transported in suspension in the water in the bay, which could pose a threat to fish life. In the EIS SEPIL identify this risk (Section 14.6.1) and proposed a monitoring programme in the unlikely event of a bentonite release.



7.6 Proposal for Additional Information In general, SEPIL have now supplied a comprehensive body of information and analyses to assess the risk of ground movement along the route of the onshore gas pipeline. However, if the Board were to request additional information from SEPIL to reach a decision on the application, then I would recommend that the following information be requested to clarify some of the risks that have been identified in this report:

Precise section by section details of the proposals for temporary peat storage and reinstatement outside the areas of intact bog, which take into account the condition of the surface layer of the peat and specifically identify where peat turves or remoulded peat will be stored on bog mats adjacent to the road.

Details of the specific risk mitigation measures that would be proposed for each of Sections 1 to 18 in the qualitative assessment of relative peat failure potential, in particular noting where there would be limits on the storage of peat on bog mats adjacent to the stone road excavation and adopting a conservative approach to the assessment of peat stability.

An assessment of the potential impact of the estimated stone road settlements on the umbilical pipeline and service ducts that will also be constructed within the stone road, including an assessment of the risks associated with failure due to rupture of the pipes.

SEPIL have carried out a sample parametric study to quantify the potential impact of incorporating a very low base failure frequency due to ground movement in the QRA. Risk transects were calculated for a failure frequency of 9x10-8 per km.year at 100 bar, 144 bar and 345 bar and compared to the comparable transects for a zero failure rate, as assumed by SEPIL. The results show that the impact of the very low failure frequency could be significant. However the parametric study has not been carried through the whole QRA. I have recommended that this assessment should be carried out, but I assume that this will be dealt with in more detail in the report by Mr. Nigel Wright. Signed: Conor O'Donnell BA, BAI, MS, C.Eng, MIEI

Date: 18/9/09



References:

1. RPS (February 2009) – “Corrib Onshore Pipeline Environmental Impact Statement”, Volume 1

2. RPS (February 2009) – “Geotechnical Assessment of the Non-Peat Areas Along

the Proposed Route of the Corrib Onshore Pipeline”, Environmental Impact Statement, Appendix M1

3. AGEC (January 2009) – “Report on Corrib Onshore Pipeline - Peat Stability

Assessment”, Environmental Impact Statement, Appendix M2

4. AGEC (January 2009) – “Report on Corrib Onshore Pipeline – Geotechnical Assessment of Stone Road Construction in Peat Areas”, Environmental Impact Statement, Appendix M3

5. RPS/AGEC (January 2009) – “Geotechnical Risk Register”, Environmental

Impact Statement, Appendix M4

6. RPS (January 2009) – “Hydrological Impact Assessment”, Environmental Impact Statement, Appendix M5

7. RPS (February 2009) – “Eco-Hydrological and Eco-Hydrogeological Impact

Assessment of Proposed Corrib Onshore Pipeline”, Environmental Impact Statement, Appendix M6

8. DNV (January 2009) – “Corrib Pipeline Quantified Risk Analysis”,

Environmental Impact Statement, Appendix Q7

9. RPS (May 2009) – “Corrib Onshore Pipeline Environmental Impact Statement - Addendum”, Environmental Impact Statement

10. AGEC (June 2009) – “Corrib Onshore Pipeline - Additional Information”

11. JP Kenny (June 2009) – “Onshore Pipeline Stone Road Settlement Analysis”

Environmental Impact Statement

12. Tobin Consulting Engineers (June 2009) – “Haul Road Pavement Design and Road Cross Section, Supplementary Information for June Oral Hearing”

13. Tobin Consulting Engineers (October 2003) – “Report on the Landslides at

Dooncarton, Glenagad, Barnachuille and Pollathomais, County Mayo”

14. Mac Culloch, F. – Forestry Commission (January 2006) – “Guidelines for the Risk Management of Peat Slips on the Construction of Low Volume/Low Cost Roads over Peat”



15. European Gas Pipeline Incident Data Group (EGIG) (December 2008) – “7th

EGIG Report on Gas Pipeline Incidents 1970-2007”

16. BSI British Standards - (December 2008) – PD8010-3:2009 – Code of Practice for Pipelines, Part 3: Steel pipelines on land – Guide to the application of pipeline risk assessment to proposed developments in the vicinity of major accident hazard pipelines containing flammables – Supplement to PD 8010-1:2004.

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