D.2 Constraints Mapping ReportBull Wall and north-eastwards to the Bull Island Causeway at St....

Appendices Ringsend WwTW EIS

D.2 Constraints Mapping Report

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Ringsend Wastewater Treatment Works Extension

Constraint Mapping of Dublin Bay

April 2010

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Constraints Mapping of Dublin Bay Apr 10

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Document Code: 75461-DG45-Dft02

Document Control Sheet

Client Dublin City Council

Project Ringsend Wastewater Treatment Works Extension

Report Constraint Mapping of Dublin Bay

Date April 2010

Project No: 75461 Document Reference: (74979) 75461/ 40/ DG45

Version Author Checked Reviewed Date

Draft 01 L. Gaston/ A. O’Connell

Anthony Kerr Bob Gaudes 25 Feb 10

Draft 02 L. Gaston Anthony Kerr Bob Gaudes 14 Apr 10

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Table of Contents

1 Introduction ................................................................................................. 1 1.1 Progress to Date ................................................................................................................... 1 1.2 Dublin Bay ............................................................................................................................ 2 1.3 Report Outline ..................................................................................................................... 3

2 Operational – Map A .................................................................................. 4 2.1 Dublin Port ........................................................................................................................... 4 2.2 Dun Laoghaire Harbour Company ................................................................................... 6

3 Environmental – Map B ............................................................................. 7 3.1 Protected Areas .................................................................................................................... 7

3.1.1 Natura 2000 Sites ........................................................................................................ 7 3.1.2 Natural Heritage Areas .............................................................................................. 8 3.1.3 Notifiable Actions ....................................................................................................... 9

3.2 Fauna and Fisheries not Covered by Designations......................................................... 9 3.2.1 Mammals ..................................................................................................................... 9

3.3 Water Body Classifications .............................................................................................. 11 3.3.1 Transitional Waters .................................................................................................. 11 3.3.2 Coastal Waters........................................................................................................... 11 3.3.3 Nutrient Sensitive Waters ........................................................................................ 12

4 Structures and Obstructions – Map C ................................................... 14 4.1 Pipelines and Cables ......................................................................................................... 14

4.1.1 Sewage Pipelines ....................................................................................................... 14 4.1.2 Gas Pipelines ............................................................................................................. 15 4.1.3 Telecommunications Cables .................................................................................... 15 4.1.4 Electricity Cables ....................................................................................................... 15

4.2 Shipwrecks ......................................................................................................................... 16 4.3 Unexploded Ordnance...................................................................................................... 17

5 Amenity – Map D ..................................................................................... 18 5.1 Sailing/ Leisure Boating ................................................................................................... 18 5.2 Bathing Waters .................................................................................................................. 18 5.3 Recreation (including water sports) ................................................................................ 21

6 Inshore Fisheries – Map E ....................................................................... 22

7 Geological – Map F ................................................................................... 23 7.1 Geology ............................................................................................................................... 23 7.2 Bathymetry ......................................................................................................................... 23

8 Conclusions and Recommendations ..................................................... 25 8.1 Conclusions ........................................................................................................................ 25 8.2 Recommendations ............................................................................................................. 25

9 References .................................................................................................. 26

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List of Tables

Table 1: Operational Constraints - Dublin Port Company .......................................................... 5Table 2: Operational Constraints - Dun Laoghaire Harbour Company .................................... 6Table 3: Environmental Constraints in Dublin Bay ................................................................... 13Table 4: Seabed Constraints – Sewage Pipeline .......................................................................... 14Table 5: Seabed Constraints – Gas Pipeline ................................................................................ 15Table 6: Seabed Constraints – Telecommunication Cables ....................................................... 15Table 7: Seabed Constraints – Electricity Cables ........................................................................ 16Table 8: Seabed Constraints – Shipwrecks .................................................................................. 16Table 9: Amenity Constraints in Dublin Bay .............................................................................. 20Table 10: Inshore Fisheries Constraints – Marine Institute Datasets ....................................... 22

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1 Introduction

Dublin City Council intend to upgrade the Ringsend Wastewater Treament Works (WwTW) to meet the future demand projected for the agglomeration and to ensure compliance with relevant legislation. The plant is presently experiencing periodic loading in excess of current capacity. The plant will be upgraded from its present capacity of 1.64 million PE to a design capacity of 2.2 million PE.

The selected effluent disposal option is to construct an extension of the existing final effluent outfall by 7 to 10 kilometres long to discharge the secondary treated effluent. Preliminary hydraulic modelling of two extended effluent outfall options demonstrated that the associated secondary treated discharges would not have any impact on Coastal or Transitional water bodies. Appropriate Ecological Assessments of these outfall discharge points concluded that there would be changes in water quality in the immediate vicinities of the discharges, but that no significant impacts were predicted for any Natura 2000 site. In fact, by extending the existing final effluent discharge outside of the Liffey River Estuary, water quality within the estuary and the inner bay, where Natura 2000 sites and bathing waters are located, will improve.

This report deals with the constraints for the construction of any proposed new effluent discharge extension within Dublin Bay. At this stage it is envisaged that any new effluent discharge outfall extension would be constructed in a tunnel. Construction of an effluent discharge outfall would require two periods of field operations in Dublin Bay, for both the:

Phase I – Project Planning and Investigation Preliminary assessment of potential tunnel routes by geophysical surveys and drilling investigative boreholes; and

Phase II – Project Construction Tunnel and termini construction phase.

All possible constraints must be identified and examined prior to any field operations in the Bay. The objective of this report is to locate and examine the constraints to both phases of field operations in Dublin Bay. The results of the comprehensive constraints studies will also feed into both the tunnel route and the termini location selection process and associated environmental studies.

1.1 Progress to Date CDM with subcontractor DHI (Danish Hydraulic Institute) completed a preliminary outfall discharge location modelling in October 2009, entitled ‘Modelling the Impact of Ringsend Discharges in the Liffey and Tolka Estuaries and Possible Long Sea Outfall Discharges in Dublin Bay’ (CDM, 2009a). The objective of this modelling study was to assess the impact of five alternative outfalls into Dublin Bay discharging at five different locations. Two locations were

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found to be more technically viable for the outfall discharge location whilst the other three locations were found to be less viable.

The discharge location points of the outfalls were somewhat indicative in order to assess whether an outfall would be potentially viable; a lot more detailed investigations will be required prior to the outfall construction of any outfall extension. An extensive field campaign will be required to provide adequate hydrodynamic and water quality data for a full and reliable calibration before any selected outfall location options are examined in detail.

The environmental considerations for the technically viable extended outfall discharge locations identified from the preliminary modelling were assessed in a report called ‘Preliminary Assessment of Long Sea Outfall Locations’ (CDM, 2009b). The examination of the preliminary modelling results has shown that apart from a mixing zone in the vicinity of the outfall points the receiving waters will meet the Environmental Quality Objectives for coastal water nutrients - Dissolved Inorganic Nitrogen (DIN). It was determined that there would be no deterioration in the bathing water quality in the Dublin area from either locations. The quality is expected to improve as a result of the ceasing of discharge to the Liffey Estuary.

Appropriate Ecological Assessments in accordance with Article 6 of the Habitats Directive were also undertaken for both outfall discharge location options. No significant effects on Natura 2000 sites were predicted. No significant impact is predicted on this habitat which is located over 5 km from the nearest of the two outfall discharge locations. Neither of the two technically viable options were found to conflict with any of the 16 priority objectives set out in the Dublin Bay Water Quality Management Plan.

1.2 Dublin Bay Ringsend WwTW is located by the mouth of the River Liffey as it enters Dublin Bay. The Liffey enters Dublin Bay between Clontarf and Ringsend in the channel formed by the North Bull Wall and the Great South Wall. The North Bull Wall is a natural bank reinforced by a stone embankment that is only inundated at half tide. It therefore holds back the water flowing out of the harbour at and after half ebb. The navigation channel runs close to the South Wall and extends from the Port area through the mouth of the harbour. This navigation channel is maintained at a depth of 7 to 8 metres below chart datum by dredging and natural scouring.

Dublin Bay is a small, shallow sandy embayment. It is enclosed by two headlands Howth to the north and Dalkey to the south. It is approximately 10 kilometres across the mouth of the bay and narrows to the mouth of the River Liffey.

The intertidal zone of the bay occupies the inner third of the bay. The bay slopes gently reaching depths of 20m at the mouth of the Bay. The water depth decreases towards the harbour with depths of less than 5m occurring in the inner half of the bay. The Burford Bank sits centrally across the mouth of Dublin Bay. The Burford Bank is a linear sand ridge about 5km in length, which rises to within 5m of the surface.

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The North Bull Island is a prominent physical feature in the bay which developed due to sedimentation accumulation after the construction of the North Bull wall in 1821. To the north of the channel are extensive areas which dry out at low water. These mudflats extend from the mouth of the River Tolka almost to the end of the Bull Wall and north-eastwards to the Bull Island Causeway at St. Annes.

Dublin Bay is currently home to a range of activities including fisheries, bathing recreational sports such as watersports and boating. Maritime traffic is busy in Dublin Bay as it is home to Ireland’s largest port.

1.3 Report Outline This report was prepared following a desk study to identify of the constraints that are likely to affect the field operations and potential construction activities in Dublin Bay. The potential constraints in Dublin Bay have broadly been split into six categories, as listed below:

A Operational (Section 2)

B Environmental (Section 3)

C Structures and Obstructions (Section 4)

D Amenity (Section 5)

E Fisheries (Section 6)

F Geological (Section 7)

Each category is discussed in a separate section of the report and a bespoke map corresponding to each of the six categories can be found in Appendix A. A key map summarising all the potential constraints is contained and discussed in Section 8.

Constraints to field investigations and to the design/ planning of the tunnel route and termini locations will differ from one another and this is highlighted where necessary throughout the report.

Note: It must be emphasised that the constraints study is an on-going process and will involve various consultations with the relevant parties.

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2 Operational – Map A

To prevent disruption to Dublin Port Company and Dún Laoghaire Harbour operations, whilst undertaking investigation or construction in the Bay, consultations have been held with both and the constraints are summarised here. Once survey locations have been decided they will have to be approved by Dublin Port Company and Dún Laoghaire Harbour.

The Department of Transport will be given advanced notice of any activities. Marine Notices will then be circulated by the Department to individuals and organisations ranging from state agencies and the fishing industry to international shipping and water-based recreational interests. The Marine Notice will detail the location, date of commencement and time period of activities.

The Harbours Act 1966 as amended by the Harbours (Amendment) Act 2009, sets the limits of Dublin Port Company as extending from Rory O’Moore Bridge (by the Guinness Brewery) on the Liffey Estuary to an imaginary line connecting Howth to Sorrento Point, Dalkey through Burford Bank. This line is shown on Map A. It should be noted that the boundary excludes the limits of the harbour of Dún Laoghaire Harbour Company and also the harbours of Coliemore and Sutton.

Dublin Port Authority have been named as the Pilotage Authority for the area shown on Map A. Dublin Port ensures the safe passage of a vessel through the pilotage district. Under the Harbours (Amendment) Act 2009, the boundary of the Dublin Pilotage District consists of “the waters of the River Liffey below the Matt Talbot Memorial Bridge and so much of the sea westward of the sixth meridian as lies between the parallels of latitude passing through the Baily Lighthouse on the North and through Sorrento Point on the South including all bays, creeks, harbours and all tidal docks within such area”.

2.1 Dublin Port Dublin Port Company (DPC) operates extensive port operations in the Upper Liffey Estuary and maintains deep navigational channels into the port. CDM met with DPC on two occasions; 21 January 2010 and 3 February 2010 to discuss operational constraints (Internal Ref: 22825/67511/MM66 and MM69). There are six areas shown on Map A that have designated uses for DPC. These have been sourced from the Admiralty Chart 1415 and confirmed with DPC.

DPC stated that no investigative drilling can take place in the area immediately adjacent to the ‘roundabout’ buoy in the centre of Dublin Bay. A circular exclusion zone (A.1 Exclusion Zone) of radius 800m around the buoy is shown on Map A <Exclusion zone to be confirmed with DPC>. In addition investigative drilling should be avoided in the Traffic Separation Scheme Zones marked A.5. on Map A excepting A.1. and A.5., DPC state that the investigative drilling could take place in the majority of locations in the Bay over which they have authority, subject to co-ordination with appropriate stakeholders.

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Document Code:: 75461-DG45-Dft02

Table 1: Operational Constraints - Dublin Port Company Map Ref. Area Description Constraint

A.1 Navigational Channel

Navigational channels which enter Dublin Bay to the north and south of Burford Bank meeting at a central roundabout buoy which then lead to Dublin Port via a dredged channel of 7.8m maintained depth. Travel around the central buoy is anti-circular.

DPC state that a rig/boat can move through the navigational channel. Drilling could be permitted in most parts of the navigational channel, subject to co-ordination with DPC except around the central ‘roundabout’ buoy. The exclusion area around the central buoy has been labelled “A.1 Exclusion Zone” on Map A.

A.2 Anchorage Area

A circular area in a south-easterly position in Dublin Bay for anchorage.

DPC state that drilling could be permitted in this area, subject to co-ordination with DPC.

A.3 Spoil Ground DPC have delineated a 2.15km2 spoil disposal area west of Burford Bank. DPC have a permit from the EPA to dump dredged material in this area..

DPC state that drilling is permitted in the spoil disposal area although EPA should be contacted prior to any works.

A.4 Inshore Traffic Zone

Vessels of less than 20 metres in length, sailing vessels and vessels engaged in fishing may use the inshore traffic zone. Larger vessels may use the inshore traffic zone if en-route to port or in immediate danger. DPC note that since there are no ports in the Inshore Traffic Zones in Dublin Bay, no vessel larger than 20m can access them. (Collision Regs., 1993)

DPC state drilling is permitted in Inshore Traffic Zone subject to co-ordination with DPC.

A.5 Traffic

Separation Scheme

The traffic separation scheme is zones which separate directions of navigation. (Collision Regs, 1993)

DPC state that investigative drilling should be avoided in these areas.

A.6

Burford Bank – Area to Avoid for Mariners

This area is marked on the Admiralty Chart as an area to avoid for Mariners.

As an area to avoid for Mariners, it is therefore an area to be avoided or entered at caution for investigative vessels.

Contact for DPC is Capt. David Dignam, Harbour Master (Tel: 01 8876000)

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It should be noted that A.6. Burford Bank is an area to be avoided by mariners due to the sandbar below, creating water depths as low as 4.6m.

Although investigative drilling the operational constraints areas marked on Map A (excluding A.1. ‘Exclusion Zone’ and A.5.) may be permitted as the works are temporary it is perhaps not feasible or desirable to construct a permanent structure such as a diffuser in these zones.

2.2 Dun Laoghaire Harbour Company Dun Laoghaire Harbour Company limit includes the area within the east and west piers and extends to 600m outside the mouth of the harbour. CDM met with Dun Laoghaire Harbour Master on 4 February 2010 (Internal Ref: 22825/67511/MM70).

The HSS Stena Line operates from March – December in 2010. The HSS will sail once a day in July/August and the small Stena Lynx will sail twice a day. The Stena Lynx does not sail in bad weather.

Table 2: Operational Constraints - Dun Laoghaire Harbour Company Map Ref. Area* Data Source Description Constraint

A.7.

Dun Laoghaire Harbour to Holyhead

Route

Coordinates from Dun Laoghaire Harbour and the route is inferred

Two routes leading from Dun Laoghaire Harbour to Holyhead – the choice of route being weather dependent. (Normal route shown on map)

Drilling is permitted along these routes subject to co-ordination with DLHC and DPC. The Stena Line ships can manoeuvre easily.

Contact for DLHC is Simon Coate, Harbour Master (Tel: 01 2808681)

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3 Environmental – Map B

The environmental considerations for the two technically viable outfall locations were assessed in a report called ‘Preliminary Assessment of Long Sea Outfall Locations’ (CDM, 2009b). This preliminary study included the assessment of the impact of the two indicative outfall locations on protected areas and Water Framework Directive water bodies in Dublin Bay.

3.1 Protected Areas Within Dublin Bay there are several areas designated for the protection of species and habitats of national or European importance, these are described below.

3.1.1 Natura 2000 Sites There are four existing Natura 2000 sites in Dublin Bay; two Special Protected Areas (SPAs) and two candidate Special Areas of Conservation (SACs).

The SPAs are areas of conservation value for the protection of rare and endangered bird species designated internationally under Council Directive 79/409/EEC on the Conservation of Wild Birds.

The SACs are habitats of EU importance designated for conservation under Council Directive 92/43/EEC on the Conservation of Natural Habitats and of Wild Fauna and Flora.

It is also understood that the NPWS intend to propose the Kish Bank as an SAC under the Habitats Directive (and possibly as an SPA under the Birds Directive). This area is being considered for designation due to the presence of the habitat sandbanks ‘which are slightly covered by sea water all the time’ (code 1110), which is listed in Annex I of the EU Habitats Directive.

‘Preliminary Assessment of Long Sea Outfall Locations’ (CDM, 2009b) included Appropriate Ecological Assessments in accordance Article 6 of the Habitats Directive for both outfall options. No significant effect on Natura 2000 sites were predicted. No significant impact is predicted on this habitat which is located over 5 km from the nearest sea outfall option.

South Dublin Bay and River Tolka Estuary SPA The South Dublin Bay and River Tolka Estuary SPA comprises a substantial part of Dublin Bay. It includes the intertidal area between the River Liffey and Dun Laoghaire, the estuary of the River Tolka to the north of the River Liffey, Booterstown Marsh and an area of grassland at Poolbeg, north of Irishtown Nature Park. A portion of the shallow marine waters of the Bay is also included. The site is of special conservation interest for a number of bird species (Light-Bellied Brent Goose, Oystercatcher, Ringed Plover, Golden Plover, Grey Plover, Knot, Sanderling, Dunlin, Bar-tailed Godwit, Redshank, Black-Headed Gull, Roseate Tern, Common Tern and Arctic Tern) and is important for wintering

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waterfowl and wintering gulls. An internationally important population of Light-bellied Brent Goose feed on the Eelgrass bed at Merrion and is also known to feed on the grassland at Poolbeg. The SPA is of international importance for Light-bellied Brent Goose and of national importance for nine other waterfowl species. It is also of international importance as an autumn tern roost.

The EU Birds Directive pays particular attention to wetlands, and these form part of the SPA, the site and its associated waterbirds are of special conservation interests for wetlands and waterbirds.

North Bull Island SPA North Bull Island is a sand spit that developed after the construction of the North Bull Wall. This island is covered in dune grassland. Other important ecosystems associated with the island are salt marsh and mud flats. The reserves are of international scientific importance for Brent Geese and also on botanical, ornithological, zoological and geomorphological grounds.

North Bull Island SPA is of international importance for waterfowl on the basis that it regularly supports in excess of 20,000 waterfowl. It also qualifies for international importance as the numbers of two species exceed the international threshold – Brent Goose and Bar-tailed Godwit. A further 15 species have populations of national importance – Shelduck, Teal, Pintail, Shoveler, Oystercatcher, Ringed Plover, Golden Plover, Grey Plover, Knot, Sanderling, Dunlin, Black-tailed Godwit, Curlew, Redshank and Turnstone. The North Bull Island SPA is a regular site for passage waders, especially Ruff, Curlew Sandpiper and Spotted Redshank.

North Dublin Bay cSAC Annex I Habitats include fixed dunes, marram/shifting dunes, embryonic shifting dunes, dune slack, annual vegetation of drift lines, salicornia mud and sand flats, Atlantic salt meadows, Mediterranean salt meadows, mud and sand flats. Annex II species include Petalwort. The site overlaps with North Bull Island SPA.

South Dublin Bay cSAC The site has extensive areas of sand and mudflats, a habitat listed on Annex I of the EU Habitats Directive. The largest stand of Eelgrass on the east coast occurs at Merrion Gates. New habitats are developing just south of Merrion Gates including embryonic dunes and a sand spit. This area is becoming increasingly important as a high tide roost site for waterfowl. The site overlaps with South Dublin Bay and River Tolka Estuary SPA

3.1.2 Natural Heritage Areas Natural Heritage Areas (NHAs) are protected under the Wildlife Act (2000). The basic designation for wildlife is the NHA. This is an area considered important for the habitats present or which holds species of plants and animals whose habitat needs protection. There are two pNHAs within the Bay the South Dublin Bay NHA and the North Dublin Bay pNHA. They are both ‘proposed’ NHAs published on a non-statutory basis. The NHAs in Dublin Bay largely overlap with SACs.

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3.1.3 Notifiable Actions To carry out certain activities, not covered by licence or consent from another statutory body, within the designated area, they must consult with, and get consent from, the Minister for the Environment, Heritage and Local Government (DEHLG). These activities are listed as “Notifiable Actions” for each habitat.

Notifiable actions for mudflats and sandflats, sandy coastal beaches, shingle beaches, boulder beaches and bedrock shores include:

“driving vehicles over the area, except over rights of way or over access to licensed aquaculture facilities; and

digging, ploughing or otherwise disturbing the substrate.”

The site investigations carried out by a Local Authority have to notify the Minister for the DEHLG of the proposed works, under Section 228 f the Planning and Development Act 2000. For the construction of the proposed outfall extension a Foreshore Consent will have to be obtained from the DEHLG.

3.2 Fauna and Fisheries not Covered by Designations Natura Environmental Consultants (2009) were commissioned to carry out a desktop study to determine the implications of the technically viable extended outfall options for the achievement of objectives set out in the Dublin Bay Water Quality Management Plan (DBWQMP, 1991) for fauna and fisheries not covered by designations.

They determined that the removal of the discharge from the Liffey Estuary will ensure that excessive dissolved oxygen deficits do not occur in the waters of the Liffey Estuary as a result of the WwTW, and that the dissolved oxygen standards are met; thereby protecting migratory fish.

Effluent discharging from the extended outfall will be treated to the same standards as the existing effluent discharging to the Liffey Estuary. Due to the location of the two technically viable extended outfall discharge location options, the quality of effluent discharging and the location and distribution of the modelled plumes there are no significant impacts predicted to fish populations within Dublin Bay and adjacent waters as a result of discharge from either outfall.

Birds and mammals, occurring in Dublin Bay, which are dependent on fish as a food source will not be affected by the two outfall options. There will be no significant impacts to the ecology of Dublin Bay as a result of the two technically viable extended outfall discharge locations.

3.2.1 Mammals Seals Both grey (Halichoerus grypus) and harbour (Phoca vituline - common) seals are found around the majority of the Irish coast, although most of the important breeding sites are located on the west and south-west coasts. Both seals are

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protected under Annex II and Annex V of the EU Habitats Directive and are listed under Appendix III of the Bern Convention. The Habitats Directive requires Ireland to establish SACs for conservation of both species of seal. Any activity likely to impact upon the seal population requires consent from the Minister. This legal protection affords seals considerable security from hunting and disturbance at haul-out sites. Dublin Bay is not designated as an SAC for the protection of seals.

Both grey seals and harbour seals feed in small numbers in Dublin Bay although there are no significant breeding groups here. There is a regular haul-out (resting place) for both species at the north-east end of North Bull Island. Occasional harbour seal pups have been reported from here but the site is not suitable as a breeding location for either species due to the high level of human disturbance. Grey seals regularly haul-out in small numbers on the rocks between Dun Laoghaire Harbour and Dalkey Island. There are no suitable pupping beaches or caves on these islands.

The nearest large breeding assemblage of seals to Dublin Bay is on Lambay Island, and Ireland’s Eye off north Co. Dublin. The all-age population there was estimated to be in the region of 203 to 261 animals. There is also a significant haul-out of harbour seals on the west side of Lambay Island with approximately 30 animals recorded in 2003. These animals disperse widely outside the breeding and moulting season and could be feeding anywhere in Dublin Bay. Their primary prey is fish but they also feed on squid and crustaceans such as crabs. They are not highly sensitive to water quality changes although they would be negatively affected by any significant reduction in fish populations in the inshore waters in which they feed.

Cetaceans (whales, dolphins and porpoise) All cetacean species are protected under Annex IV of the EU Habitats Directive and harbour porpoise and bottlenose dolphin are also listed under Annex II of the same directive. The most common near-shore species found within Dublin Bay are the harbour porpoise (Phocoena phocoena), bottlenose dolphin (Tursiops truncates), and minke whale (Balaenoptera acutorostrata).

Harbour porpoise are mainly confined to shelf waters, although sightings have occurred in deep water. They are the most common cetacean in Irish Waters; despite this it can often be difficult to observe due to its small size. The diet of harbour porpoise comprises a wide range of small fish, such as small gadoids, whiting, poor cod, sprat, sandeel, herring, saithe, pollack, dab, flounder, and sole. Harbour porpoise have been surveyed by the Irish Whale and Dolphin Group (IWDG) in Dublin Bay in 2008. Acoustic monitoring and estimates of diversity and abundance suggest that the numbers of porpoise off Howth Head are the highest recorded in Ireland (S. Berrow, pers. comm.).

Marine noise may have the potential to affect cetaceans. Seismic surveying is the geophysical exploration by acoustic methods. Sound energy (pressure pulses) is released from a source being towed by a vessel, and this signal is reflected off the seabed. There are various procedures that can be followed in a surveying

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programme to minimise the impact such as a soft start, using initial very low levels of sound to encourage marine mammals to leave the survey area and having Marine Mammal Observers (MMO). The “Code of Practice for the Protection of Marine Mammals during Acoustic Seafloor Surveys in Irish Waters” from the Department of Environment Heritage and Local Government (DEHLG) should be followed for seismic activities in Dublin Bay.

3.3 Water Body Classifications The general objective of the WFD is to achieve ‘good status’ for all surface waters by 2015. ‘Good status’ means both ‘good ecological status’ and ‘good chemical status’. The Environmental Objectives (Surface Waters) Regulations 2009 came into effect in July 2009 in order to implement aspects of the Water Framework Directive. EQS means the concentration of a particular pollutant or group of pollutants in water, sediment or in aquatic life which should not be exceeded in order to protect human health and the environment.

EQS for each water body type are set down in the regulations. The technically viable extended outfall options being considered have discharges that are located some distance out into the Irish Sea. It should be noted that the locations of the discharge points are outside the areas delineated for consideration under the Water Framework Directive.

Under the EU Bathing Waters Directive (76/160/EC) four stretches of beach have been designated as bathing water protected areas within Dublin Bay. These are discussed further in Section 5.

The examination of the preliminary modelling results has shown that apart from a mixing zone in the vicinity of the outfall points the receiving waters will meet the EQSs for coastal water nutrients - Dissolved Inorganic Nitrogen (DIN). It was determined that there would be no deterioration in the bathing water quality in the Dublin area. The quality is expected to improve as a result of the ceasing of discharge to the Liffey Estuary. (CDM, 2009b)

3.3.1 Transitional Waters Under the WFD coastal waters are defined as bodies of surface water in the vicinity of river mouths which are partly saline in character as a result of their vicinity to coastal waters, but which are substantially influenced by freshwater flows. The principal quality standard of concern in relation to wastewater discharges to transitional waters is Molybdate Reactive Phosphorus (MRP).

3.3.2 Coastal Waters Under the WFD coastal waters are defined as surface water on the landward side of a line, every point of which is at a distance of one nautical mile on the seaward side from the nearest point of the baseline from which the breadth of territorial waters is measured, extending where appropriate up to the outer limit of transitional waters.

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The principal quality standard of concern in relation to wastewater discharges to coastal waters are nutrients in the form of Dissolved Inorganic Nitrogen (DIN). DIN (rather than MRP) is considered to be the limiting nutrient in coastal waters and a breach of the EQSs may lead to eutrophic conditions (algal blooms, etc) and consequently the only nutrient standards in place for coastal waters are for DIN.

The EPA propose to designate the whole of Dublin Bay as a coastal water body, the boundary to which is not yet known.

3.3.3 Nutrient Sensitive Waters Nutrient Sensitive Areas are waters designated as sensitive under the Urban Waste Water Treatment Directive (91/271/EEC). The Liffey Estuary has been designated as sensitive from Islandbridge weir to Poolbeg Lighthouse, including the River Tolka basin and South Bull Lagoon. Nutrient removal to achieve 10 mg/L total nitrogen and 1 mg/L total phosphorus, in addition to normal secondary treatment standards, is required for continued discharge at the existing outfall.

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Table 3: Environmental Constraints in Dublin Bay Map Ref. Protected Area Data Source Site Investigation Constraints Outfall Design/ Construction Constraints

B.1 Special Area of Conservation NPWS Website Notify DEHLG

No adverse impacts – will be dealt with under EIS/ appropriate assessments and consultation with DEHLG/ NPWS

B.2 Special Protected Area NPWS Website Notify DEHLG

No adverse impacts – will be dealt with under EIS/ appropriate assessments and consultation with DEHLG/ NPWS

B.3 Natural Heritage Area NPWS Website None - the NHAs in the Bay are ‘proposed’ and non-statutory

B.4 WFD – Transitional Waters EPA WFD data None

Demonstrate compliance with Environmental Objectives (Surface Waters) Regulations 2009 S.I. No. 272 of 2009

B.5 WFD – Coastal Waters EPA WFD data None

Demonstrate compliance with Environmental Objectives (Surface Waters) Regulations 2009 S.I. No. 272 of 2009

B.6 Nutrient Sensitive Waters EPA WFD data None Demonstrate compliance with Urban Waste Water

Treatment Regulations 2001 (S.I. No. 254 of 2001)

n/a Fisheries Habitat n/a Need to contact Eastern Region Fisheries Board to verify

No adverse impacts – will be dealt with under EIS/ appropriate assessments and consultation with DEHLG and Fisheries Boards

n/a Marine Mammals n/a Follow - “Code of Practice for the Protection of Marine Mammals during Acoustic Seafloor Surveys in Irish Waters” from the DEHLG

No adverse impacts – will be dealt with under EIS and consultation with DEHLG/ NPWS

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4 Structures and Obstructions – Map C

4.1 Pipelines and Cables Pipelines and cables are marked on the Admiralty Chart of Dublin Bay (1415). Mariners should not anchor or carry out dredging or drilling close to pipeline or cables refer to Annual Notice to Mariners No.24/06 for further guidance. The locations of the pipelines and cables on Map C are taken from the Admiralty chart unless otherwise stated.

Further work:

The Admiralty chart pipeline and cable route locations need to be verified with the relevant parties prior to any works being carried out; and

Find out more details about the pipelines and cables from relevant parties.

4.1.1 Sewage Pipelines Mariners are advised to exercise caution when navigating in the vicinity of the pipelines as charted depths may be reduced by as much as 0.5 metres (Admiralty chart 1415).

There are two cross bay sewerage pipelines on Map C. Pipeline C.1 carries sewage from the Sutton pumping station in the North of Dublin to Ringsend WwTW. Pipeline C.2 carries sewage from the West Pier pumping station in Dun Laoghaire Rathdown to the Ringsend WwTW. Details of pipeline material and diameters was sourced from CAPCIS (2008). The depth of the Sutton to Ringsend cross bay pipeline is the approximate range for the length of the pipeline taken from the as built drawing (DCC ref: DBP-C4-03).

Table 4: Seabed Constraints – Sewage Pipeline Map Ref.

Constraint Name

Owner/ contact Details Material Diameter

(m) Depth Restriction

C.1

Sutton – Ringsend cross bay

sewage line

Dublin City Council Steel 1.422

4 – 8 m below bed level

?

C.2

Dun Laoghaire – Ringsend cross bay

sewage line

Dun Laoghaire Rathdown Steel 0.965 ? ?

C.3 Dodder Twin Lines

Dun Laoghaire Rathdown

Steel/ Pre-stressed

concrete

0.9144/ 1.2192 ? ?

C.4. Long sea

Outfall - Dun Laoghaire

Dun Laoghaire Rathdown ? 0.9 ? ?

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4.1.2 Gas Pipelines There is a high pressure gas main that runs into the ESB site at Poolbeg. The information of this gas main was sourced from CAPCIS (2008). The route was also sourced from this report and the pipeline is not in fact shown on the Admiralty chart.

Table 5: Seabed Constraints – Gas Pipeline

Map Ref. Constraint Name

Owner/ contact Details Material Diameter

(m) Depth Restriction

C.5. Bord Gais Pipeline Bord Gais Steel 1.422 ? ?

4.1.3 Telecommunications Cables There are five telecommunications cables in Dublin Bay labelled Cable C.6 - Cable C.11 on Map C, except C.8 which is a proposed cable. CDM attended a meeting with the GSI on 2nd February 2010. The GSI mentioned a planning application for a new fibre optic cable to be laid in the Bay and exiting the Bay between Howth and the northern tip of the Burford Bank.

Cables C6 and C7 are active cables and the status of cables C9 to C11 is currently unknown. The routes of these cables were taken from the Admiralty chart and will have to be verified with the relevant parties. Emergency contact information for the submarine cables can be obtained from Kingfisher Information Service at http://www.kisca.org.uk/Charts/Web_IrishSea.pdf.

Table 6: Seabed Constraints – Telecommunication Cables

Map Ref. Constraint

Name

Owner/ contact Details

Material Depth Restriction

C.6. Hibernia D Hibernia Atlantic,

01 867 3600 ? ? ?

C.7. Esat 2 Esat BT, 01 432 6555 ? ? Meeting required to

discuss restrictions

C.8. Proposed Fibre optic

cable ? ? ? ?

C.9. ? ? ? ? ? C.10. ? ? ? ? ? C.11. ? ? ? ? ?

4.1.4 Electricity Cables None identified at present.

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Table 7: Seabed Constraints – Electricity Cables

Map Ref. Constraint Name

Owner/ contact Details

Material Depth Restriction

C.X ? ? ? ?

4.2 Shipwrecks Shipwreck information as shown on Map C was obtained from three sources. The datasets differ slightly so all three have been shown on the map, however it is believed that the Underwater Archaeology Unit (UAU) have the most comprehensive and accurate dataset. The three datasets include:

1. Under water Archaeology Unit (UAU) of the DEHLG - have produced a draft Inventory of Recorded Shipwrecks for the East Coast of Ireland

2. INFOMAR – have been able to identify possible shipwrecks and from their multibeam marine mapping. This has been a co-operative effort between GSI and the Underwater Archaeology Unit of the National Monuments Section (NMS), at the Department of Environment Heritage and Local Government.

3. Admiralty Chart - published by the UKHO, contains the location of some wrecks. In certain cases an approximate position and date of sinking may be available.

The UAU manage and update the Shipwreck Inventory of Ireland. These databases will need to be investigated as part of the EIA process upon the selected engineering solution to scope the potential requirements for underwater survey work. This work should be undertaken by a suitably qualified archaeologist to ensure that the potential for archaeological impact is fully investigated prior to construction.

Wrecks over 100 years old and archaeological objects found underwater are protected under the National Monuments (Amendment) Acts 1987 and 1994. Significant wrecks less than 100 years old can be designated by Underwater Heritage Order on account of their historical, archaeological or artistic importance.

Table 8: Seabed Constraints – Shipwrecks Map Ref.

Constraint Name Data Source Owner/ contact

Details Restriction

C.10. Shipwrecks

1. Underwater Archaeology Unit

2. INFOMAR 3. Admiralty Maps

UAU - Karl Brady 01 4189757 [email protected]

?

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Further work:

Brady (2008). Shipwreck Inventory of Ireland.

Determine restrictions related to working with them e.g. Buffer zones or requirements for side scan sonar

4.3 Unexploded Ordnance No military or naval operations have resulted in the presence of unexploded ordnance in Dublin Bay. There are no official records of historical unexploded ordnance kept but there have not been any reports of such in Dublin Bay (Dept. of Defence - Press Office, pers. comms.).

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5 Amenity – Map D

Dublin Bay is host to many water based amenities, including sailing and boating, water sports and bathing beaches. There are constraints associated with each such as stakeholders that are required to be consulted and notified or certain water quality standards that need to be met.

5.1 Sailing/ Leisure Boating There are eight clubs in the Bay which are engaged in the organisation of sailing and/or leisure boating activities for their members and are affiliated with the Irish Sailing Association, as listed below:

Sailing in Dublin;

Clontarf Yacht & Boat Club;

Dun Laoghaire Motor Yacht Club;

National Yacht Club;

Poolbeg Yacht Club;

Royal Irish Yacht Club;

Royal St George Yacht Club; and

Sutton Dinghy Club.

The locations for these clubs were obtained from the Coastal and Marine Resources Centre and they are displayed on Map D (CMRC, 2009).

Dublin Bay Sailing Club which is not shown on the map as the club possesses no premises. With permission from Dublin Port Company, the club lays yacht racing marks within the Port of Dublin from April to October 2010. Each is a pillar-mark, 8' high (Notices to Mariners, No.10 of 2010). The marks cover a large area, extending from Salthill and Seapoint on the western side of Bay to near the Burford Bank on the east. The racing season usually starts at the end of April and continues up to the end of September. There are mid-week races on Tuesdays and Thursdays and as well as weekend races. The Irish Sailing Association have a calendar of the race dates.

5.2 Bathing Waters Under the EU Bathing Waters Directive (76/160/EC) four stretches of beach have been designated as bathing water protected areas within Dublin Bay;

Dollymount Strand;

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Sandymount Strand;

Merrion Strand; and

Seapoint.

The bathing water season runs from 1 June to 15 September. During this time the bathing waters are monitored. The Bathing Water Directive sets water quality standards that must be met in these areas. These standards include total and faecal coliforms, colour, mineral oils, phenols, transparency and floatables.

The Bathing Water Quality Regulations 2008 (S.I. 79 of 2008) will repeal and replace the Quality of Bathing Waters Regulations, 1992 (S.I. No. 155 of 1992) with effect from 31st December 2014 (Both regulations remain relevant).

The Blue Flag is an exclusive eco-label awarded for beaches and marinas by the Foundation for Environmental Education (FEE). The Blue Flag Programme is operated in Ireland by An Taisce – The National Trust for Ireland with support from the DEHLG and on behalf of the FEE. Blue Flag Beaches are selected through strict criteria dealing with water quality, environmental education and information, environmental management, and safety and other services. The water quality standards for Blue Flag Beaches are more stringent than the Bathing Water Regulations. In 2009 a total of 74 beaches of 133 were awarded the Blue Flag in the Republic of Ireland, only one of which was within Dublin Bay and that was Dollymount Strand (Blue Flag, 2010).

The Dublin Bay Water Quality Management Plan written in 1991 set out priority objectives for certain areas of Dublin Bay to which the objects should apply. The priority object for bathing waters in the marked areas is to meet the requirements of the Bathing Water Regulations with particular emphasis to microbial parameters (DBWQMP, 1991).

The ’Preliminary Assessment of Long Sea Outfall Locations’ (CDM, 2009b) assessed the potential impact of discharge from a number of extended outfall locations using modelling for an extreme case scenario. The results of the simulations showed that the discharge plume from the assessed outfall locations will remain offshore and that bathing water beaches in Dublin Bay will not be impacted by discharge from the extended outfall locations assessed. There will be no discernible increase in the bacteriological quality at the beaches as a result of the discharge via the extended outfalls. However, the beaches will still be vulnerable to bacteriological contamination from other sources such as the Tolka and Liffey. Nonetheless, the fact that the discharges of treated effluent to the estuary will be discontinued should result in an overall improvement in bathing water quality at the beaches around Dublin Bay.

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Table 9: Amenity Constraints in Dublin Bay Map Ref. Constraint Name Data source Site Investigation Constraints Tunnel Design/ Construction Constraints

D.1 Racing Buoys

DPC website. Notices to Mariners, No.10 of 2010

Sailing Amenity should be considered when selecting drilling and construction sites. Contact Dublin Bay Sailing Club. Donal O’ Sullivan 087 6524761

Sailing amenity will part of the stakeholder process of the EIS

D.2 Bathing Waters EPA WFD data None Bathing Water Regulations 1992 and 2008

D.3 Recreational Areas – Water sports

Dublin Bay Water Quality Management Plan, 1992 None

In the absence of water quality standards the Bathing Water Regulations 1992 and 2008 can be referred to

D.4 Recreational Areas Dublin Bay Water Quality Management Plan, 1992 None

In the absence of water quality standards the Bathing Water Regulations 1992 and 2008 can be referred to

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5.3 Recreation (including water sports) In the Dublin Bay Water Quality Management Plan (1991) many of the objectives were directed at protecting recreation and water sports areas within the Bay and the areas cover virtually the entire coastline in the Bay area.

The area from Dalkey to Seapoint is a zone of water sports recreation and the objective set was to protect the microbiological quality of the waters for water sports such as wind surfing. The objective for the other areas of recreation was to protect the recreational uses of the areas.

The ‘Preliminary Assessment of Long Sea Outfall Locations’ (CDM, 2009b) assessed the consequences of the technically viable extended outfall discharge location options in terms of the contribution that they will make to the overall faecal coliform counts in the these amenity areas. There will be no discernible impact whatsoever on the bacteriological quality of the amenity areas defined in the Priority Objectives. If anything there will be noticeable improvement due to the fact the discharge of treated effluent to the Liffey Estuary will cease.

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6 Inshore Fisheries – Map E

Presently fishing activities in Dublin Bay are limited. The boundaries for areas where different methods of fishing are commonly used were obtained from the Marine Institute from their Inshore Fisheries Atlas and are displayed on Map E.

Potting for lobster and velvet crab is the main activity in the south of the Bay and it is seasonal, however there are only a few pots located here and there. Line fishing is permitted throughout the Bay. Dublin Bay is a no trawl zone and so there is no activity such as trawling or dredging anymore (D. Rehan, Bord Iascaigh Mhara (BIM), pers. comm.)

The Fishery Monitoring Centre from Irish Naval Services monitors the fishing activity in Dublin Bay. All fishing vessels over 15 metres have a transponder system fitted since 1st Jan 2005. Information is collected regarding their position, effort and catch. The area to the east of Burford Bank was found to be busy but with passing traffic only and not with active fishing for the month of July 2009 (M. McGrath, pers. comm.).

It is not envisaged that the site investigations will have an impact on fisheries as the works are temporary and the fishermen will be advised in advance of the works with by a Department of Transport Marine Notice.

Further Work:

The MI/ BIM/ Irish Naval Service will be contacted to determine the status of these fisheries and any restrictions in the fishing areas.

Table 10: Inshore Fisheries Constraints – Marine Institute Datasets Map Ref. Constraint Name Description Constraint

E.1. Aquaculture There are three areas of aquaculture in Dublin Bay for the growth of mussels. ?

E.2. Dredge Part of the area of the Burford bank is where dredge fishing occurred for scallops. n/a

E.3. Pots

There are three large areas of Dublin Bay where pot fishing occurs for large crustaceans such as lobsters and velvet crab and whelks. The largest whelk fishery in Ireland is the one located just south of Howth.

?

E.4. Trawl Trawl fishing using mobile nets is carried out around the Burford Bank area and south of Howth Head for fish and/ or molluscs.

n/a

E.5. Line Hook and Line fishing is carried out in the entire Bay. None

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7 Geological – Map F

7.1 Geology CDM carried out a preliminary desk study of the geology of Dublin Bay, entitled ‘An Overview of the Geology of Dublin Bay’ (CDM, 2010). There are three types of bedrock that can be expected in the Bay – limestone, metasediments, and granite and these all have very different physical properties. As such, additional geological and geotechnical characterisation of the Bay is necessary.

The predominant bedrock in the inner Bay is likely to be Calp Limestone. This is the more easily solubilised, less resilient limestone that has eroded gradually, leaving a well-defined bay. The Leinster Granite formation to the south of the Bay, from Dun Laoghaire to Dalkey, that may lie in the path of any proposed tunnel routes. The Rathcoole Fault has been inferred in the nearshore from onshore geology, and it is predicted that it runs diagonally across the mouth of the Bay from the Rathcoole Fault in Dun Laoghaire to the Dalkey Fault. This fault is likely to be encountered with either of the outfall alignments.

While there have been a number of subsurface investigations conducted within the Bay, no boreholes have met bedrock with the deepest being 25 m below the surface.

It cannot be determined the amount of changes in the bedrock type that are present in Dublin Bay and changes in bedrock type has an impact on the tunnel boring process. Further information on the geology of Dublin Bay needs to be acquired prior to the detailed design phase. This will potentially inculde:

Interpretation of INFOMAR geophyiscal survey data; and

A field programme involving further geophyics and investigative drilling.

7.2 Bathymetry Dublin Bay is a shallow sandy embayment on the east coast of Ireland. Admiralty Chart 1415 of Dublin Bay shows depth to chart datum in metres for Dublin Bay. Contours from the Admiralty Chart are shown at 2m, 5m, 10m and 20m on Map C. The Bay slopes downwards to the east gently reaching depths of 20m at the mouth of the Bay.

The Burford Bank sits centrally across the mouth of Dublin Bay. The Burford Bank is a linear sand ridge about 5km in length, which rises to within 5m of the marine water surface. Bathymetric comparisons suggest that the offshore banks are quasi-stable over time probably maintaining their position due to the interaction between wave and current regimes (Wheeler et al., 2000).

The Marine Institute worked in partnership with the Geological Survey of Ireland (GSI) on the Irish National Seabed Survey (INSS). The survey aimed to map

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Ireland's 220 million acres of territorial seafloor. Phase 1 of the Irish National Seabed Survey (INSS) is now complete and includes bathymetric mapping of approximately 65 percent of Dublin Bay.

The final 35 percent of Dublin Bay will be mapped in Phase 2 of the survey under the INFOMAR project (INtegated mapping FOr the sustainable development of the MArine Resource). While Phase 1 concentrated on outer deep-sea territorial waters, Phase 2 has moved inshore to coastal waters. Dublin Bay Phase 2 will be available from the GSI in non-quality controlled format in the summer of 2010. The accurate bathymetry data will feed into the hydrodynamic and water quality modelling.

Further Work:

Acquire bathymetric data from INFOMAR once it has been processed

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8 Conclusions and Recommendations

8.1 Conclusions Six maps displaying the constraints for site investigations and tunnel construction have been developed. The categories include operational, environmental, structures and obstructions, amenity, fisheries and geology. Perhaps the most important constraints will be cooperating with the requirements of the port companies and accurately locating utilities/ shipwrecks on the seabed. The Key Map summarises all of the identified potential constraints within Dublin Bay.

The objective of the preliminary modelling (CDM, 2009a) was to assess the impact of five extended outfall discharge location options in Dublin Bay. Two locations were found to be more technically viable for an outfall location whilst the other three locations were found to be less viable, these locations are shown on the Key Map.

Keeping the results of the preliminary modelling in mind and all the potential constraints shown on the map there are several areas which stand out as potentially suitable for the outfall terminus. The general area of these potential termini locations are indicated on the Key Map with an asterisk. Three potential locations for the termini for the outfall discharge location are located within the Bay itself and one is located outside the Burford Banks. The potential location outside the Burford Bank could be located anywhere outside the bank as there are minimal constraints in that area. The suitability of these potential outfall termini locations will have to be tested with detailed hydraulic and water quality modelling, and the results may produce more of an envelope of potentially suitable areas. These results will also need to feed into a cost benefit analysis of the viable options.

As can be seen from Key Map, the Bay is densely populated with constraints, therefore in each category further investigation and consultations are required to determine the restrictions for works.

8.2 Recommendations The main recommendations include the following:

Close out of gaps in the constraints information;

Complete hydraulic and water quality modelling for the selection of the outfall termini location;

Feed the constraints information into EIS and site investigation documents; and

Carry out site investigations to aid the selection and truth check information for the tunnel alignment and termini location.

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9 References

Admiralty Chart 1415. Dublin Bay. United Kingdom Hydrographic Office.

Blue Flag, 2010. http://www.blueflag.org/Menu/Blue+Flag+beaches%2fmarinas/2009/Northern+Hemisphere/Ireland/Dublin/

CAPCIS (2008). Dublin Bay Stray current review and monitoring plan. Dun Laoghaire Rathdown County Council.

CDM 2009a. Modelling the Impact of Ringsend Discharges in the Liffey and Tolka Estuaries and Possible Long Sea Outfall Discharges in Dublin Bay. Internal Ref: 22825/67511/40/DG 16.

CDM, 2009b. Preliminary Assessment of Long Sea Outfall Locations. Internal Ref: 22825/67511/40/DG 19.

CDM 2010. Overview of Geology in Dublin Bay. Internal Ref: 22825/67511/40/DG 20.

Coastal and Marine Resources Centre (CMRC), 2009. Irish Sailing Association – Sailing Club Members. http://mida.ucc.ie/pages/dataLayers.htm

Collision Regulations (Ships and Water Craft on the Water) (Amendment) Order 1993 (S.I. No. 287/1993).

Dublin Pilotage Order, 1925 (Amendment) Order, 1963 (S.I. No. 24/1963).

INFOMAR www.marine.ie/home/services/surveys/seabed/

Jeffrey, D. W. 1991. Dublin Bay Water Quality Management Plan, 1991

Marine Institute. Inshore Fisheries Atlas. http://www.maps.marine.ie/inshore/default.aspx

Meeting with Dublin Port Harbour Master. Meeting Minutes Internal Ref: 22825/67511/30/MM 69.

Meeting with Dun Laoghaire Harbour Master. Meeting Minutes Internal Ref: 22825/67511/30/MM 70.

Meeting with GSI (INFOMAR) regarding Geophysical data in Dublin Bay. Meeting Minutes Internal Ref: 22825/67511/30/MM 73.

Natura, 2009. Ecological Impact Assessment on Dublin Bay Water Quality Management Plan Priority Objectives. Appendix C in: CDM, 2009b. Preliminary Assessment of Long Sea Outfall Locations. Internal Ref: 22825/67511/40/DG 19.

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Notices to Mariners, No.10 of 2010. http://www.dublinport.ie/information-centre/notice-to-mariners/

Wheeler, A.J., Walshe, J. and Sutton, G.D. (2000). Geological Appraisal of the Kish, Burford, Bray and Fraser Banks, Outer Dublin Bay Area. Marine Institute.

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Draft V2

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Source: © Ordnance Survey Ireland. All rights reserved. Licence No AR 0095908

© CopyrightThis drawing and any design hereon is copyright and should not be reproduced without the owner permission.© British Crown and SeaZone Solutions Limited. All rights reserved. Products Licence No. 012010.005This product has been derived in part from material obtained fromthe UK Hydrographic Office with the permission of the Controller ofHer Majesty's Stationery Office and UK Hydrographic Office(www.ukho.gov.uk). “NOT TO BE USED FOR NAVIGATION”.

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Legend

k Potential Termini Locations

Preliminary Modelling - Technically Viable Site

Preliminary Modelling - Technically Unviable Site

Boundary of Dublin Pilotage District

Boundary of Dublin Port Company

Boundary of Dun Laoghaire Harbour Company

Map A - Operational Constraints

Map B - Environmental Constraints

Map C - Seabed Constraints

GF Map C - Seabed Constraints - Shipwrecks

Map D - Amenity Areas

&R Map D - Racing Marks

Map E - Fishery Constraints (excl line fishing)

Map F - Structural Geological Linework

Map F - Inferred Fault

Land

Sea

Bathymetry

Depth contour (m)

Scale 1:55,000

Rathcoole Fault

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Page 28


Appendix A

Constraint Maps by Category

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LegendIreland Outline

Boundary of Dublin Pilotage District



!( A.1. Central 'Roundabout' Buoy

A.1. Dublin Port Company Navigational Channel

A.1. Drilling Exclusion Zone - 800m around Buoy

A.2. Dublin Port Company Anchorage Site

A.3. Dublin Port Company Spoil Ground

A.4. Inshore Traffic Zone

A.5. Traffic Separation Scheme

A.6. Burford Bank - Area for Mariners to Avoid

A.7. Dun Laoghaire Stena Route

Dun Laoghaire to Holyhead

Hollyhead to Dun Laoghaire

A.7. Inferred Dun Laoghaire Stena Route

Depth contour (m)

Depth area, Drying

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Depth area, 10 - 20m


Burford Bank

Scale 1:55,000

Limit of Dublin Port Company

Limit of Dublin Pilotage District

Limit of Dun Laoghaire

Harbour Company

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6°7'46"E, 53°18'56"N30

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LegendBoundary of Dublin Pilotage District



B.1. Special Areas of Conservation

B.2. Special Protected Areas

B.3. Natural Heritage Area

B.4. Water Framework Directive - Transitional Waters

B.5. Water Framework Directive - Coastal Waters

B.6. Nutrient Sensitive Waters

Bathing Water Beaches

Bathymetry

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Burford Bank

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Harbour Company

Water Framework DirectiveWater Body Classifications

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C.1. - C.4. Sewerage Pipelines

C.5. Gas Pipeline

C.6. - C.7. Active Telecommunications Cables

C.8. Proposed Fibre Optic Cable DRAFT

C.9. - C.11. Unknown Status of Telecommunications Cables

GF C.10. Shipwrecks - Underwater Archaeology Department

GF C.10. Shipwrecks - INFOMAR

GF C.10. Shipwrecks - UKHO Admiralty Chart

Bathymetry

Depth contour (m)

Depth area, Drying

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Burford Bank

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Harbour Company

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[h Irish Sailing Association - Sailing Clubs

&R D.1. Dublin Bay Sailing Club Racing Marks

\ D.2. 2009 Blue Flag Beach

\ D.2. 2009 Non Blue Flag Beach

D.2. Bathing Water Beaches

Dublin Bay Water Quality Management Plan (1991)

D.2. Bathing

D.3. Water Sports

D.4. Recreation

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E.1. Aquaculture

E.2. Dredge

E.3. Pots

E.4. Trawl

E.5. Hook and Line

Bathymetry

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Harbour Company

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LegendInferred Fault (source Dobson and Whittington, 1979)

Structural Linework

Bedrock Outcrops

Bedrock Formations (100K)Calp

Tober Colleen Formation

Ballysteen Formation

Waulsortian Limestones

Butter Mountain Formation

Drumleck Formation

Elsinore Formation

Gaskins Leap Formation

Hippy Hole Formation

Pipers Gut Formation

Type 1 granodiorite

Type 2e equigranular

Type 2p microcline porphyritic

Type 3 muscovite porphyritic

Scale 1:55,000

Rathcoole Fault

Geology is currently uncertain in foreshore. Further research currently underway

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Appendices Ringsend WwTW EIS

D.3 Effluent Outfall Extension Tunnel ‐Concept Design Report

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Ringsend WwTW Plant Upgrade | Effluent Outfall Extension Tunnel Concept Design Report

Doc Ref: 75461/DG44/ Draft 02 Page i

Document Control Sheet

Client Dublin City Council

Project Ringsend WwTW Plant Upgrade.

Report Effluent Outfall Extension Tunnel ‐ Concept design report

Date October 2011

Project No: 75461

Document Reference DG 44 – Draft 02

Version Author Reviewed Checked Date

Draft 01 U. Burbaum Michael Loeffler A Kerr August 2011

Draft 02 U. Burbaum Juergen Schmitt A Kerr October 2011

Distribution Copy No.

Master (CDM) 01

DCC 02

DCC 03

DCC 04

© 2011 CAMP DRESSER & MCKEE ALL RIGHTS RESERVED

Reuse Of Documents: These documents and designs provided by professional service, incorporated herein, are the

property of CDM and are not to be used, in whole or part, for any other project without the written authorization of CDM.

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Doc Ref: 75461/DG44/ Draft 02 Page ii

Table of Contents SECTION 1 GENERAL ........................................................................................... 1

1.1 Project Overview ............................................................................................ 1 1.2 Preferred Tunnel Route .................................................................................. 1 1.3 Tunnel Diameter ............................................................................................. 2

SECTION 2 REFERENCES ....................................................................................... 3

2.1 Project Documents ......................................................................................... 3 2.2 Communications ............................................................................................. 3 2.3 Project Datum ................................................................................................ 3 2.4 Cost Estimates ................................................................................................ 3

SECTION 3 GEOLOGICAL, GEOTECHNICAL AND HYDROGEOLOGICAL CONDITIONS .............. 4

3.1 General ........................................................................................................... 4 3.2 Scope of Site Investigation ............................................................................. 4 3.3 Geology ........................................................................................................... 5

3.3.1 General ................................................................................................. 5 3.3.2 Overburden .......................................................................................... 6 3.3.3 Bedrock ................................................................................................ 8

3.4 Geotechnical Properties ............................................................................... 13 3.4.1 Sediments ........................................................................................... 13 3.4.2 Glacial Till /Boulder Clay .................................................................... 13 3.4.3 Bedrock .............................................................................................. 14

3.5 Hydrogeological Conditions ......................................................................... 14 3.5.1 Onshore .............................................................................................. 14 3.5.2 Offshore ............................................................................................. 16 3.5.3 Ground Water Quality ........................................................................ 16

3.6 Contamination of Soils and Water ............................................................... 17

SECTION 4 OVERALL CONSTRUCTION LAYOUT AND DESIGN ....................................... 18

4.1 Linking of Outfall Elements .......................................................................... 18 4.2 Design Life .................................................................................................... 18 4.3 Exposure Class for Concrete ......................................................................... 18 4.4 Geotechnical Baseline Report ...................................................................... 18 4.5 Baseline Tender Reference Design (BTRD) ................................................... 19

SECTION 5 ONSHORE TUNNEL INLET SHAFT ........................................................... 21

5.1 General ......................................................................................................... 21 5.2 Ground Conditions ....................................................................................... 21 5.3 Vibrations ..................................................................................................... 21 5.4 Inner Shaft Diameter .................................................................................... 21 5.5 Minimum Depth of Shaft .............................................................................. 22 5.6 Shaft construction ........................................................................................ 23

5.6.1 Shaft Construction Techniques In General ........................................ 23 5.6.2 Shaft Concept Design ......................................................................... 29

5.7 Serviceability Test for Shaft Lining Prior to Excavation ................................ 33 5.8 Excavation .................................................................................................... 33

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Doc Ref: 75461/DG44/ Draft 02 Page iii

5.9 Survey ........................................................................................................... 33 5.10 Optimum Shaft Construction Technique ...................................................... 34 5.11 Overall Costs ................................................................................................. 38

SECTION 6 TUNNELLED SECTION .......................................................................... 39

6.1 General ......................................................................................................... 39 6.2 Tunnel Mechanics Requirements ................................................................. 39 6.3 Tunnel Environment, Costs Of Tunnelling And Advance Rates .................... 41 6.4 Vertical Tunnel Alignment ............................................................................ 42

6.4.1 General Guidelines ............................................................................. 42 6.4.2 To Keep The External Water Pressure On The Tunnel As Low As Possible,

The Tunnel Alignment Should Be As Elevated As Possible. Tunnel Gradient ............................................................................................. 42

6.5 Geotechnical Conditions .............................................................................. 44 6.6 Tunnelling Method ....................................................................................... 44 6.7 Slurry ............................................................................................................ 49 6.8 Minimum Diameter Of The Tunnel .............................................................. 50 6.9 Tunnel Lining Segments ................................................................................ 50 6.10 TBM Facilities For Probing And Ground Improvement ................................ 51 6.11 TBM Maintenance ........................................................................................ 52 6.12 Connection To The Diffuser Shaft ................................................................ 52 6.13 Compound Requirements ............................................................................ 53 6.14 Excavated Soil ............................................................................................... 53 6.15 Survey ........................................................................................................... 53

SECTION 7 OFFSHORE MARINE TUNNEL OUTLET DIFFUSER SHAFT .............................. 54

7.1 General ......................................................................................................... 54 7.2 Ground Conditions ....................................................................................... 54 7.3 Inner Shaft Diameter .................................................................................... 54 7.4 Depth of Shaft .............................................................................................. 55 7.5 Dewatering / Buoyancy ................................................................................ 55 7.6 Construction ................................................................................................. 56 7.7 Shaft Mechanics ........................................................................................... 56 7.8 Connection Between Shaft and Tunnel Section ........................................... 57 7.9 Onshore Construction Compound/Berthing Areas ...................................... 57 7.10 Soft Ground Condition Problems for Plant................................................... 57 7.11 Construction Sequence Diffuser Shaft ......................................................... 58 7.12 Survey ........................................................................................................... 58

SECTION 8 SUMMARY ....................................................................................... 59

8.1 General ......................................................................................................... 59 8.2 Geological Risk .............................................................................................. 59 8.3 Diffuser Shaft Location ................................................................................. 60 8.4 Additional Site Investigation ........................................................................ 61 8.5 Risk Assessment ........................................................................................... 61

SECTION 9 CONCLUSIONS .................................................................................. 62

APPENDIX A DRAWINGS ...................................................................................... 63

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Doc Ref: 75461/DG44/ Draft 02 Page iv

APPENDIX A – DRAWINGS

Drawing 1: Drawing Number ‐ 75461/Tunnel/Concept Design Report/01

Drawing Title ‐ Plan of tunnel alignment


Drawing Title ‐ Geological cross section


Drawing Title ‐ Vertical tunnel alignment

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Doc Ref: 75461/DG44/ Draft 02 Page 1

Section 1 General

1.1 Project Overview

Currently the existing final effluent outfall from the Ringsend Waste Water Treatment Works

(Ringsend WwTW) discharges into the Liffey Estuary at the North East corner of the ESB Ringsend

Poolbeg Power Station.

The current Ringsend WwTW capacity is 1.8 million PE, which generates an average daily flow of 5.7

m3/s. Current peak flow rate to the outfall is 11.1 m3/s. The current Ringsend WwTW treatment

capacity has to be increased to accommodate an average future capacity of 2.4 million PE (2.1 million

PE, firm), which generates an average daily flow of 7.0 m3/s to the year 2025 horizon.

As part of the review of upgrading options for the Ringsend WwTW the option of diverting the

existing final treated effluent discharge culvert into a purpose built long sea outfall tunnel (Ringsend

LSOT) is being considered. Under such a scenario the existing final effluent discharge outfall location

at the North East corner of the Ringsend ESB Poolbeg site will become substantially redundant and

the final treated effluent will be diverted eastwards to the outer reaches of Dublin Bay via the

Ringsend LSOT for final discharge into deep marine waters approximately 9 km offshore and close to

Burford Bank.

The constructed LSOT will involve three key components;

A large onshore tunnel inlet shaft in the Ringsend Poolbeg peninsula area in the vicinity of the

ESB Poolbeg power station.

A large diameter tunnelled section.

A large diameter offshore marine tunnel outlet diffuser riser shaft and diffuser head (diffuser

shaft and diffuser head).

As part of the scoping and development exercises to determine the viability and issues associated

with the construction of the LSOT three previous technical workshops were held during March and

May 2011.

A large site investigation was undertaken starting in September 2010. Fieldwork was completed in

July 2011, laboratory testing and reporting is still ongoing. The final SI report is expected in late

October 2011.

1.2 Preferred Tunnel Route

Currently, the optimum location for the diffuser shaft and diffuser head outfall location is at location

B3 (See Fig 1). The concept design described within this document is based upon the tunnel

terminating at location B3. This may be subject to change in subsequent project phases should the

current optimum location B3 be relocated elsewhere.

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Page 2 Doc Ref: 75461/DG44/ Draft 02

Fig. 1 Marine site investigation borehole locations and potential diffuser outfall discharge locations

1.3 Tunnel Diameter

All statements and designs of this report are based upon an assumed finished internal tunnel

diameter of 5 m. This final finished internal tunnel diameter will be confirmed after a subsequent

design phase. Therefore, all statements on depths, lengths etc. in this document must be reviewed

once the finished internal diameter is finalised at the subsequent design stage. For the purposes of

this report the finished outer diameter of the tunnel section is assumed to be 6.5 m.

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Section 2 References

This report refers to some project documents, communications and publications. Unless otherwise

stated, all data referring to soil or rock properties are based on the preliminary site investigation

reports from the marine and onshore drilled boreholes received to date from the SI contractor.

2.1 Project Documents

The working documents used in the preparation of this report are as follows;

Constructability Workshop for Onshore Shaft & Tunnel Elements, March 7‐8, 2011, Output

report and recommendations, Draft 02, May 2011, DG 32

Constructability Workshop for Offshore marine Diffuser Shaft, May 12, 2011, Output report

and recommendations, Draft 02, July 2011, DG 39

Tunnel Spoil Disposal report, Draft 02, June 2011, DG 34

Various working drafts of reports, borehole logs and laboratory results obtained during the

progress of the marine site investigation works.

2.2 Communications

Limits of vibration for electrical/mechanical plant at the ESB Poolbeg Ringsend site were provided by

email from Denis McCabe/ESB to Anthony Kerr/CDM, 2011‐07‐21 and 2011‐07‐25 (75461/20/CI 577

& CI 578)

2.3 Project Datum

All work for this project refers to Irish grid coordinates and to Malin Head Ordnance Datum. Lowest

Astronomical Tide (LAT) is ‐2.61 m below Malin Head Ordnance Datum.

2.4 Cost Estimates

All costs provided within this report are estimates based upon current costs for comparable works as

per information provided by a number of tunnelling contractors. The cost estimate information

provided by these tunnelling contractors may therefore not fully reflect competitive market

conditions. Final submitted tender costs/prices will vary from the costs in this report due to either

changes in the market or due to strategic interests of particular contractors etc that will prevail at the

time of tendering.

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Section 3 Geological, Geotechnical and Hydrogeological Conditions

3.1 General

The following description of geological, geotechnical and hydrogeological conditions is based upon a

preliminary interpretation of the current findings of the marine site investigation. Since most of the

reporting of results is still ongoing no overall detailed interpretation of the results has yet been

undertaken. Therefore, as part of any subsequent project stage it is possible that some descriptions

or values of geotechnical properties quoted within this report may change.

Design parameters used within this report are based upon the preliminary results of the ongoing

laboratory testing and represent the potential top/bottom range of potential characteristic values.

These are subject to further review and will be detailed fully as part of the geotechnical baseline

report to be prepared at a later stage.

3.2 Scope of Site Investigation

A site investigation for this project was undertaken. The extent of the site investigation comprises:

19 marine boreholes in Dublin Bay (Offshore)

2 onshore boreholes on the proposed onshore inlet shaft location at Poolbeg/Ringsend where

piezometers have been installed in both boreholes

Geophysics investigation (seismic reflection) offshore

Geophysics investigation (seismic reflection and refraction, electric resistivity) intertidal zone

and onshore

Seabed surveys, bathymetry, magnetometer, side scan sonar, at marine borehole locations

Laboratory tests

The marine site investigation fieldwork is completed. Laboratory testing is still ongoing (final

reporting due for completion in late October 2011) and comprises determination of the following:

Density by immersion in water or water displacement

Particle size distribution

Moisture content

Liquid limit, plastic limit and plasticity index

Organic Content

Mass loss on ignition

Sulphate content

Shear Strength

Schmidt rebound hardness

Uniaxial compressive strength

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Point load strength of rock specimen

Abrasivity

Quartz content

Clay minerals of cohesive soils (sort and percentage of each sort of mineral)

Rock minerals on rock samples using thin section microscopy

Swell tests on cohesive soils

Calcium carbonate content of water samples

Slake durability tests on rock samples

Indirect tensile strength by Brazilian test

Contamination in overburden materials above bedrock

Water quality in the overburden and the deep bedrock

Sulphate content

Field testing is completed (final reporting due for completion in late October 2011) and comprised

the following:

Standard Penetration tests

Menard Pressuremeter tests

Dilatometer tests

Packer tests

Rising head tests (onshore boreholes BH O 01 and BH O 02 only)

A technical assessment of the bedrock strata encountered is subject to ongoing review and may

change following receipt of all final marine site investigation reports.

The location of all drilled boreholes is shown on Fig. 1.

3.3 Geology

3.3.1 General

The marine geology (19 marine boreholes drilled) in the project area consists roughly of marine

sediments which are followed by glacial till. In some boreholes (for example Boreholes M20, M21),

glacial till was not encountered.

The marine sediments/glacial till are underlain by bedrock. The bedrock surface is very uneven and

was found between ca. ‐15 m and – 55 m below seabed level which is ca. ‐18 and ‐65 m to OD Malin

Head.

The underlying bedrock of the project area is mostly comprised of the Dublin Calp basin consisting of

Calp limestone which belongs to the Visean series of the Lower Carboniferous. Older limestone

formations of the Tournesian series of the Lower Carboniferous were encountered in boreholes M 21

and M 11.

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In boreholes M 22 and M 24 (not part of the current tunnel route – running between onshore and

offshore location B3, Fig 1) rocks of older Paleozoic series, potentially Cambrian and/or Silurian

formation, were encountered. This change in strata is likely to be fault controlled.

The older formation forms the eastern rim of the Dublin Calp basin.

The most eastern borehole, M 23 (not part of the current tunnel route – running between onshore

and offshore location B3, Fig 1) has a proven presence of bedrock belonging to much younger series

of the Jurassic era. This major change of strata is fault controlled.

In onshore borings, heavily fractured dolomitised limestone was encountered which might be

interpreted as a possible continuation of the Howth fault. It is to be expected that heavily fractured

bedrock around the proposed onshore shaft location will be encountered.

Granitoidic rocks, initially predicted prior to the commissioning of the marine site investigation, to be

present in the project area of Dublin Bay have been not encountered.

A more focused geological bedrock determination will be undertaken at a later stage to support the

preparation of the geotechnical baseline report. Currently the Geological Survey Ireland (GSI) are also

considering the possibility of funding a Research Masters project to look at the marine geological

findings in greater detail. If this Masters project does progress, and in a timely fashion, it may provide

further information for inclusion in the geotechnical baseline report.

The proposed tunnel route between onshore boreholes BH O 01 & O 02 and marine borehole BH

M11 (at the optimum diffuser location B3) is shown on Drawing 1 in Appendix A. A geological cross

section for the proposed tunnel route is shown on Drawing 2 in Appendix A

3.3.2 Overburden

Marine Sediments

The depth of marine sediments varies across the Bay to approximately between 8 ‐ 20m below

seabed level. However in BH M09 marine sediments were encountered to a greater depth of 32.4m

below seabed level.

The marine sediments comprised a mixture of muds, grey coloured silty ‐ gravelly sands, silty – sandy

clays and fine‐ coarse gravels with very occasional cobbles of a grey argillaceous limestone and

occasional quartzite. Cobbles when present ranged from 40mm to 130mm in size. Gravels were

described as being sub‐angular to sub‐rounded in shape and were composed of grey limestone. Sand

is described as fine or medium in most of the samples. Layers of sandy gravel were also described as

containing occasional pockets (<60mm) of a dark grey silty clay. Marine sediments sampled in BH

M10 consisted of silt or sand and were described as having a slightly organic odour. Shell fragments

(5‐10mm) were present in the sand layers on occasion and identified as bivalve shells, or in some

cases as razor shells.

Glacial Till

The glacial till comprises layers of sands, clay, gravels, cobbles and boulders. Glacial till is usually

described as a grey slightly clayey sandy gravel with occasional cobbles and boulders of limestone,

see Figs. 2a‐2e. Gravel is usually angular to rounded in shape, and can be fine to coarse and usually of

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a light grey coloured limestone. Cobbles found in the till are described as being sub‐angular in shape

and are of medium strong or strong dark grey argillaceous limestone. Boulders in general were

described as a dark grey argillaceous limestone with calcite veining (<20mm) present and in some

cases contained rare pyrite crystals (<5mm). The occasional cobble of green quartzite is also

described. The glacial boulder clay is usually described as firm grey colored gravelly sandy clay

(occasionally soft) with occasional to many cobbles and boulders of limestone. Boulders described in

BH M11 are large (300mm x full core circumference) and are of strong dark grey argillaceous

limestone.

The glacial till is a highly heterogenous type of soil. It consists of all kinds of grain sizes, comprising

clay size grains to boulder size grains. Layers or lenses with more or less pure clay, pure sand, pure

gravel occur as well as layers containing a mixture of the above (Figs. 2a – 2e). The maximum size of

the boulders varies up to 1 ‐ 2 m.

Fig 2a: Glacial Till – Clay grain size to gravel grain size (BH M09, 42.50 – 44.00 m)

Fig 2b: Glacial Till – Clay grain size to Cobble grain size (BH M09, 44.00 – 45.50 m)

Fig 2c: Glacial Till – Clay (BH M08, 17.80 – 19.30 m)

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Fig 2d: Glacial Till – Coarse gravel to cobble grain size (BH M05, 29,95 – 31.45 m)

Fig 2e: Glacial Till – Boulder (BH M05, 33.45 – 34.95 m)

3.3.3 Bedrock

Calp Limestone

The Calp limestone consists in particular of layers of limestone as well as of layers of claystone. A

section of typical cores is shown in Figs. 3a – 3c.

The limestone is a medium strong dark grey coloured argillaceous limestone. Crinoid stems and shell

fragments are frequently found in the bedrock, usually measuring < 4mm diameter. It displays calcite

veining in many locations and this veining is sometimes described as closely spaced subvertical

veining. These veins can range in size between <0.5mm to < 5mm.

The top of the Calp limestone was generally quite fractured and often recovered as fragmented non‐

intact moderately weathered limestone. The deeper limestone is mostly intact and competent.

However, fractured zones occur in the deeper sections as well.

Fractures in the limestone can range from locally very closely spaced to medium spaced, and can be

described as rough to smooth, and are generally inclined. Some fractures in the limestone are

reported as infilled locally with either clay or sand layers.

Pyrite crystals were also present in small amounts in the limestone, and are described as rare to

frequent in some cores. The pyrite crystals were usually < 2mm in size.

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Fig 3a: Calp limestone, unweathered, few fissures (BH M 07, 37.00 – 38.40 m)

Fig 3b: Calp limestone, unweathered, few fissures (BH M 10, 59.25 – 60.80 m)

Fig 3c: Calp limestone, weathered, fractured, fissured, fissure infilling consisting of sand, silt and clay (BH M 14, 60.30 – 70.80 m)

Limestone formations of the Tournesian series of the Lower Carboniferous

Limestone formations of the Tournesian series of the Lower Carboniferous were found at boreholes

BH M 21 and BH M 11*, see Fig. 4a – 4e.

* It should be noted that BH M 11 corresponds to the location of the proposed offshore marine tunnel diffuser riser shaft.

The rock is in general slightly more weathered, fractured and fissured than the Calp limestone.

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Fig 4a: Limestone formations of the Tournesian series of the Lower Carboniferous, unweathered, fractured, fissured (BH M 11, 33.40 – 36.40 m)

Fig 4b: Limestone formations of the Tournesian series of the Lower Carboniferous, unweathered, fractured, fissured (BH M 11, 46.80 – 48.80 m)

Fig 4c: Limestone formations of the Tournesian series of the Lower Carboniferous, unweathered, fractured, fissured (BH M 21, 26.00 – 29.00 m)

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Fig 4d: Limestone formations of the Tournesian series of the Lower Carboniferous, weathered, fractured, fissured (BH M 21, 44.75 – 48.00 m)

Fig 4e: Limestone formations of the Tournesian series of the Lower Carboniferous, heavily weathered (BH M 21, 56.75 – 58.25 m)

Rock at onshore site locations

Bedrock at the proposed onshore shaft location site was in general very severely fractured (see Fig.

5a – 5e). A fault zone has likely been encountered at this location.

Fig 5a: , BH O 01, very heavily fractured bedrock (BH O 01, 45.5 – 47.00 m depth)

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Fig 5b: , BH O 01, very heavily fractured bedrock (BH O 01, 48.50 – 50.00 m depth)

Fig 5c: , BH O 02, very heavily fractured bedrock (BH O 02, 55.50 – 56.50 m depth)

Fig 5d: , BH O 02, very heavily fractured bedrock (BH O 02, 57.50 – 59.00 m depth)

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Fig 5e: , BH O 02, very heavily fractured bedrock, (BH O 02, 69.50 – 70.80 m depth)

3.4 Geotechnical Properties

3.4.1 Sediments

The SPT results in the marine sediments range in general between N = 20 – 30 blows which indicates

soils of considerable density/consistency. In some of the boreholes a standard penetration test (SPT)

“N” value of 0 blows/450 mm penetration was recorded which indicates presence of extremely very

soft material. The expected presence of such soft materials to significant depth had been established

previously as part of the previous geological desk study exercise, which was undertaken in advance

of tendering the marine site investigation contract. Additionally, experience gathered from very deep

leg penetration of the legs of the jack‐up drilling barges used in the marine site investigation also

shows that the marine sediments are very heterogenous and soft to depths of up to 20 m or more

below seabed.

3.4.2 Glacial Till /Boulder Clay

The density ranges between 2.0 and 2.6 g/cm3. Plasticity of the clay fraction is in general low to

medium. Consistency ranges from soft to firm. N values from SPT tests generally range from about 22

to more than 50 blows. The till is therefore mostly dense to very dense.

The fines consists mostly (more than 70 %) of clay minerals (kaolinite, illite, interstratified

illite/smectite and chlorite). Remaining parts are quartz and calcite. Sand and gravel grains consist

mainly of quartz and limestone. Swelling tests indicate in general no considerable swell behavior. In

one sample of BH M 06, swelling pressure of 10 kN/m² was recorded, in one other sample (BH M 23,)

swelling pressure of 7 kN/m² was recorded. The final value for swelling pressure selected for the

static analyses design work will be determined following a detailed analysis of the site investigation

results. This information will subsequently be included as part of the geotechnical baseline report.

The elasticity moduli range between 5 MPa and 36 Mpa.

Effective friction angle ranges between 20 and 40 degrees, effective cohesion ranges in general

between 5 to 30 kN/m². In sections containing less clay and more sand and gravel fraction the

friction angle ranges between 30 and 40 degrees, whilst the cohesion is less than 5kN/m².

Abrasion tests show results with abrasivity indices up to 1776 g/t which indicates that the till may be

classified as “highly abrasive”. The clay from the clay sections may exhibit high clogging potential due

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to their plasticity and consistency. This will be determined following a detailed analysis of the site

investigation results. This information will subsequently be included as part of the geotechnical

baseline report.

3.4.3 Bedrock

The Calp limestone consists mostly of limestone rock but as well as of some claystone rock sections.

The limestone rock sections are in general very stable and competent. In cases of presence of clayey

fillings or fractured zones, the stability is reduced. The claystone rock sections are in general more

weathered and less stable than the limestone rock sections. The limestone rock parts do not change

in general in water storage tests. Slake durability tests on limestone parts indicate that limestone is

of high to very high durability. Slake durability tests on the claystone parts indicates a medium

durability

The rock is in general of a very stiff type. Elasticity moduli of more than 50,000 MPa in intact rock

were encountered. In more weathered/disturbed sections an elasticity modulus of ca. 5,000 to

10,000 MPa was found.

The unconfined compression strengths range from ca. 20 to 170 MPa.

A rock abrasity index (RAI) of less than 30 in general has been derived from the reported data so far

(with single highest values of ca. 40). The abrasivity is therefore classified as low. Some minor

sections may be classified as medium abrasive.

The bedrock around the potential diffuser location differs from the properties described above.

Unconfined compression strength and stiffness is lower. A final description of the properties must be

worked out as part of the geotechnical baseline report.

The bedrock around the onshore location differs from the properties described above. The bedrock is

highly fractured. No sufficient samples could be cored to perform unconfined compression tests.

Based on this and on visual inspection of the cores it is believed that the stiffness of the bedrock is

lower than described above (less than 5,000 MPa) A final description of the properties must be

worked out as part of the geotechnical baseline report.

3.5 Hydrogeological Conditions

3.5.1 Onshore

Piezometers have been installed in the two onshore boreholes BH O 01 and BH O 02 with response

zones at depths of ‐42.65 – ‐47.65 m OD (BH O 01) and ‐23.30 – ‐29.80 m OD (BH O 02) respectively

one in bedrock (BH O 01) and one in overburden (BH O 02).

The water levels are being continuously monitored in the two onshore boreholes. The water levels in

both BH O 01 and BH O 02 continue to correspond to the main sea level and are oscillating

continuously in unison with the tides (Fig. 6.)

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Fig 6: Sea level and ground water level – Onshore BH O 01 and BH O 02

Rising head tests in the bedrock sections of the onshore boreholes have been undertaken in order to

determine the permeability. It was not possible to undertake packer tests due to adverse ground

conditions – i.e. the highly fractured nature of the rock.

Coefficients of permeability ranges from K = 2.8*10‐4 m/s to 1.4*10‐5 m/s. For preliminary

calculations provided in this document a coefficient of permeability of K = 1*10‐4 m/s as a likely

medium value is used to represent bedrock at the onshore shaft site.

The fissures in bedrock at the onshore shaft location are hydraulically connected since a response in

ground water level in BH O 01 was recorded during the rising head pump tests in BH O 02. This is

demonstrated in Fig. 78. At 10:05 hrs the pumping for the rising head test in BH O 02, 86.5 m depth,

starts. The ground water level (in borehole O 01, red curve), normally linked to the tides (blue curve)

and supposed to rise at the time of the test, decreases. When pumps turned off at 10:30 hrs the

ground water level in borehole O 01 rises immediately and significantly to reflect the tide level at

that time. The same behaviour is apparent when the rising head test at 92.5 m depth in borehole O

02 was undertaken at 15:57 hrs.

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Fig 7: Response of ground water level in BH O1 whilst undertakingrising head test in BH O2

3.5.2 Offshore

Assuming that the marine sediments and the glacial till strata in the marine environment have the

same properties as those encountered for the onshore boreholes, it is likely that the ground water in

the bedrock is connected to the main sea level, too in the marine area.

Packer tests were undertaken successfully in the marine boreholes to assess bedrock mass

permeability. Permeability of bedrock is in general low (coefficient of permeability of less than 1*10‐6

m/s). For some packer tests, as for example in BH M16 (two tests) or in BH M17 (one test), no water

absorption was recorded, the tested sections are therefore impermeable. However, in some highly

fissured / fractured sections of bedrock higher permeability was encountered (coefficient of

permeability of more than 1*10‐6 m/s). For this reason as part of the site investigation interpretation

exercise a range of permeabilities should be developed for further inclusion in the geotechnical

baseline report.

3.5.3 Ground Water Quality

Water analyses taken from the onshore boreholes BH O 01 and BH O 02 show that the ground water

is sea water dominated since the chloride content is high (BH O 01, 4.5 m depth: 8,200 mg/l, BH O

02, 10.20 m depth: 17,000 mg/l, 104.00 m depth: 13,000 mg/l).Normal chloride content of sea water

ranges from ca. 18,000 to 21,000 mg/l with variations for humid/arid sea area conditions as well as

for estuary conditions.

Sulphate content is elevated (BH O 01, 4.5 m depth: 1,300 mg/l, BH O 02, 10.20 m depth: 2,300 mg/l,

104.00 m depth: 1,700 mg/). This correlates to the pyrite encountered in the limestone and sea

water environment.

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Aggressive Carbon Dioxide content is in general low (BH O 01, 70.00 m depth: 2 mg/l, BH O 02, 74.50

m depth: 7 mg/l), only one result displays a slightly elevated level of 30 mg/l (BH O 02, 104 m depth).

3.6 Contamination of Soils and Water

Chemical analyses on soils/rock samples and water samples has been undertaken for the purposes of

establishing potential levels of contamination. This information is still being collated and will be

provided in detail in a separate report at a later stage.

However, preliminary assessment of the results so far indicate that the level of contamination

encountered will not affect the likely selection of the final shaft construction technique, or tunnel

alignment selected. This issue will be reviewed at a later stage following a further detailed

assessment of the results from the soil/water contamination test samples when the separate report

is available.

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Section 4 Overall Construction Layout and Design

4.1 Linking of Outfall Elements

The construction of the tunnelled section is the most critical part of the outfall structure in terms of

risks, costs and time frame. The critical elements of tunnelling are the geotechnical and

hydrogeological conditions in which the tunnel is driven. It is therefore crucial to choose a vertical

tunnel alignment according to the geotechnical and hydrogeological conditions offering the best

conditions in terms of lowest geotechnical risk, lowest cost and highest advance rates. The most

promising geotechnical conditions for the tunnel are in the underlying bedrock (see section 6). The

preliminary assessment of the results of the geotechnical conditions to date indicate that the tunnel

should be advanced at a level of ca. ‐60 m to ‐70 m to OD approx.

However, the constructability of the shafts (onshore and offshore) cannot be separated from the

tunnelled section and vice versa. The ground conditions encountered at the onshore shaft location

were found to be unexpectedly very poor in terms of stability and permeability of the bedrock (see

section 5). That means that the deeper the onshore shaft is, the higher both the risks and the costs of

construction are. The onshore shaft may therefore become a programme critical path element of the

construction as it has to be completed in advance of the tunnel drive. To keep both the programme

risks and the construction costs of the onshore shaft as low as possible, the onshore shaft should

therefore be as shallow as possible.

4.2 Design Life

The entire long sea outfall system, onshore shaft, tunnel, offshore shaft should have a design life of

120 years.

4.3 Exposure Class for Concrete

Although laboratory testing is still ongoing a preliminary analyses of a groundwater sample at boring

BH O 01 indicates an elevated level of Sulphate content, Chloride and partly of Carbon dioxide.

Therefore as a minimum the concrete exposure class to meet these requirements has to be specified

as part of the Baseline Tender Reference Design.

4.4 Geotechnical Baseline Report

It is essential for tendering and awarding of the tunnel/shaft construction contract that the

geotechnical data gathered from the marine site investigation is assessed and interpreted. The main

reasons for this are to:

allocate the results to geological and/or geotechnical units of homogenous properties

analyse results statistically,

interpret and compare results from different testing methods and

provide a comprehensive characterization and description of the geotechnical and

hydrogeological properties and behaviour.

As part of this detailed assessment and interpretation characteristic design parameters will be

provided to determine characteristic design loads and resistances.

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This output will form a key input to the geotechnical baseline report. The comprehensive

geotechnical baseline report will be required as a key support document for inclusion with the

tunnel/shaft contract tender documentation.

Geological/Geotechnical/Hydrogeological technical specifications to be used as a basis for tendering,

awarding and design will be detailed as part of this geotechnical baseline report.

4.5 Baseline Tender Reference Design (BTRD)

An Employer prepared baseline tender reference design (BTRD) will be required for tendering

purposes.

The key elements of the project – onshore shaft, tunnel and diffuser shaft ‐ should be dealt with in

separate sections.

Part of this BTRD is for each key element to assess and interpret the geotechnical data gathered in

the marine site investigation in terms of construction requirements, which is essentially for tendering

and awarding of the construction contract.

A key requirement is to define sections of homogenous geotechnical and hydrogeological properties

along the tunnel route so as to determine:

a) Tunnel driving requirements such as:

Tunnel face stability (preliminary tunnel static analyses must be undertaken)

Tunnel face support

Water pressure

Water ingress (Permeability)

Abrasivity / Cutting tools

Clogging potential

Additional measurements (for example grouting)

Settlements

b) Design of tunnel lining

Rock mass dead load

Stiffness

Strength

Deformation of lining

Water quality

Additional supporting measurements as well as some “emergency tunnel driving measurements” in

case of unexpected ground conditions must be defined also.

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A number of tunnel face conditions for no: of homogenous tunnel driving requirements has to be

defined and a prediction of their location in the tunnel route and their percentage of the total tunnel

length.

All these assessments lead to the development of the technical specifications as a basis for tendering

awarding and design which will be provided within that report. The output has to be reported in a

Tunnelling Baseline Report (TBR).

Similar work should be undertaken for both onshore and diffuser shaft.

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Section 5 Onshore Tunnel Inlet Shaft

5.1 General

The purposes of the onshore shaft are:

1. Temporarily use as launch pit for tunnelling operations and access to tunnel during tunnel

construction stage, i.e the onshore shaft construction must guarantee a safe working

environment.

2. Permanent use as conveyance pipe for final WwTW plant effluent

The temporary use as a launch pit for tunnelling operations ‐ TBM/Other ‐ will determine the inner

shaft diameter, and the shaft construction technique to be adopted including the retaining wall

system / shaft dewatering system.

5.2 Ground Conditions

Existing ground level at the onshore shaft location is ca. + 4.7 m OD. The first 2 ‐ 5 metres below

existing ground level consists of made ground. Below the made ground there is approximately ‐18 to

‐19.7 m of marine and/or fluvial sediments. This is underlain by approximately 14 m to 16 m of glacial

till. The rockhead level for the underlying bedrock is at ca. ‐34 m OD.

The rock is significantly heavily fractured up to the borehole site investigation depth of ca. 106 m

(102 m OD) and highly permeable. Coefficients of permeability are in an order from K = 1*10‐2 m/s to

1 *10‐5 m/s. (See Section 3.5). For the preliminary calculations provided in this report a coefficient of

permeability of K = 1*10‐4 m/s has been assumed for the bedrock at the onshore shaft location. This

must be reviewed once the marine site investigation reporting is finalized.

The ground conditions encountered at the onshore shaft location are unexpectedly very poor in

terms of stability and the bedrock exhibits very high levels of permeability.

5.3 Vibrations

The vibration limit at the ESB steam turbine adjacent to the red brick admin and control building ca.

200 m in distance from the proposed onshore shaft location is 11 mm/sec (Limitation of vibration at

ESB site, Emails Denis McCabe/ESB to Anthony Kerr/CDM, 2011‐07‐21 and 2011‐07‐25). Vibrations in

excess of this limit will trip the turbine using via the inbuilt seismic probes. For design purposes it has

been assumed that this limit will apply for vertical and horizontal directions as well as for all

frequencies.

5.4 Inner Shaft Diameter

The inner shaft diameter has to match the requirements for its use as a tunnel launch pit TBM/Other.

Based on current project experience and the results of the constructability workshop for onshore

shaft and tunnel elements, March 7‐8, 2011, the inner shaft diameter will be in the range of 15 and

20 m, depending upon the final tunnel construction design and its assembly requirements.

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For the purposes of this report, both limits (15 m and 20 m) are used to provide the likely extreme

scenarios.

5.5 Minimum Depth of Shaft

The minimum depth of the shaft is controlled both by the requirements of the vertical tunnel

alignment as well as by the level of risks on the programme and the construction costs as well as the

need to keep the H&S impacts low. It is therefore prudent to keep the onshore shaft as shallow as

possible.

Due to poor ground conditions the rock mass overburden above the top of the tunnel should be at

least three times the externally excavated tunnel diameter (6.5 m, see above) as a minimum so as to

match tunnel mechanics requirements. This is roughly at:

‐ 34 m to OD (rock head level) – 3*6.5 m = ‐ 53.5 m to OD.

The tunnel bottom would then be at:

‐ 53.5 m to OD – 6.5 = ‐60 m to OD

The shaft bottom should also be kept roughly 2 m below the bottom of the tunnel, this is at:

‐ 60 m to OD – 2 = ‐62 m to OD.

Considering that the existing ground level is roughly at + 4.5 m to OD, the depth of the shaft equals

to:

+ 4.5 m to OD – (‐ 62 m to OD) = 66.5 m approx

This is shown in Fig. 8.

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Fig 8. Depth of the onshore shaft (Blue line = ground water level)

5.6 Shaft construction

5.6.1 Shaft Construction Techniques In General

The construction of the onshore shaft and its excavation are strongly interlinked. For the onshore

shaft to be sunk in highly permeable ground conditions, the first things to decide are both how to

keep the shaft dewatered and provide ground support during construction. In general there are two

options:

1. Dewatering of the shaft by drawdown of the groundwater table during the construction phase.

Use of a retaining wall system as ground support.

2. Application of an impermeable vertical and horizontal shaft lining or lowering the permeability of

the ground by grouting (Ground freezing could be applied also). Use of a retaining wall system as

ground support. Vertical impermeable lining and retaining wall are normally combined.

The two systems have the following impacts.

1 Drawdown of the groundwater table

Whilst such systems are normally commonly applied for depths of drawdown only up to 15 – 20 m,

an application for this proposed deeper onshore shaft is theoretically possible. This requires lowering

the groundwater table to at least 1 m below the shaft bottom i.e . to ca. ‐ 63 m OD. This can only be

achieved using deep wells around the shaft in which submerged pumps are used to pump the water

out. These wells must be sunk significantly deeper than the target shaft depth. A retaining wall must

also be applied as a shaft lining to support the ground. Considering the ground conditions and the

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earth pressure at the target depth of the shaft (‐62 m OD), only a bored pile wall or a diaphragm wall

could be constructed as a retaining wall.

The theoretical discharge and the radius of the cone of depression of the groundwater table can be

calculated using the formulae for Dupuit‐Thiem (Discharge) and Sichardt (Radius of cone of

depression), see Fig. 9.

Fig. 9: Analysis of discharge for an unconfined aquifer.

A calculation of the discharge for an onshore shaft of 15 m in diameter (as this leads to the minimum

discharge) and of 66.5 m in depth is given is Fig. 10. This diameter was selected in order to determine

the likely minimum water flow rates which will have to be dealt with during construction. By

implication a contractor constructing a larger diameter shaft will have to deal with larger flow rates

than those calculated below.

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Fig. 10: Calculation of water discharge and radius of cone of depression for a shaft of 15 m in diameter and of 66.5 m in depth.

A minimum water discharge of roughly 2,500 m³/h must be assumed. This calculation represents a

minimum discharge as the minimum shaft diameter was chosen and this calculation does not take

account of any boundary condition such as the fact that the shoreline is only a couple of metres

away. Inclusion of these boundary conditions will increase the predicted discharge significantly. The

predicted discharge is also likely to increase because of increase of permeability caused by erosion

effects of fines in the rock fissures due to the high hydraulic gradients.

The dewatering system has to be kept in operation until an inner concrete structure is cast in place.

The predicted drawdown, the radius of the cone of depression and the discharge are very significant.

This will cause significant impacts:

Change of the groundwater regime for the entire construction period within a radius of ca. 1.9

km from the onshore shaft.

Lowering of groundwater table will increase the effective vertical dead load of the ground in

the area of drawdown due to loss of buoyancy of the ground. Settlements of the ground will

occur which will affect the buildings in the vicinity of the onshore shaft i.e. the nearby adjacent

ESB buildings and facilities in particular.

The onshore shaft is located in an old industrial area. It is very likely that spots of groundwater

contamination are present in the vicinity. (See section 3.6). Contamination levels are detailed in a

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separate report. However, irrespective of the level of contamination present, if dewatering is

adopted then any existing contamination will migrate towards the shaft. Based upon the level of

contamination encountered onsite water treatment would then be required prior to disposal off site

of the pumped groundwater.

Given however, that the preference is to use an impermeable system primarily because of other

more technical reasons as detailed below ‐ a dewatering system can be avoided. The issue of

dewatering and treating contaminated ground water should then be significantly mitigated and

become less of a concern.

2 Impermeable vertical and horizontal shaft lining / Reduction of ground permeability

The application of these systems combines both keeping the shaft dewatered and supporting the

ground. Two options for vertical lining elements will be applicable:

a Construction of a diaphragm wall

b Reduction of permeability of the ground by pre grouting (or ground freezing).

Both options require an impermeable shaft bottom (“horizontal lining element”) as well. This could

be achieved either by bedding the vertical shaft lining in suitable aquicludes or by technical measures

such as grouting, freezing or casting concrete underwater to form rafts for the shaft base. Both must

match the uplift stability criteria re. buoyancy.

The two systems are described below (see also Fig.11).

a Construction of a diaphragm wall

A diaphragm wall of reinforced concrete (RC) is constructed by excavating a trench in which

concrete is then cast. The trench is normally excavated in sections of 2 – 2.8 m in width.

Trench wall supporting during excavation is normally undertaken using bentonite slurry.

The trenches are excavated, RC cages installed, and then the concrete is cast.

To guarantee a sufficient overlapping at section transitions zones a minimum thickness of

the wall is required to overcome deviations from verticality of excavation. This determines

the thickness of the diaphragm wall rather than static requirements. A wall thickness in the

order of 1.00 to 1.20 m is normally applied for a shaft depth such as that being proposed.

b Reduction of permeability of the ground by grouting

Grouting is a permanent measure. Borings will be sunk down to target depth through which

grout is then pumped into the pores/fissures of the ground. The grout fills the

pores/fissures which in turn reduces the effective space for water flow, i.e. the

permeability. The strength of the grouted ground increases significantly as well due to an

increase of its cohesion if cement based grout is applied. This is a very common

construction technique and is often used if both functions (reduction of permeability and

increasing of strength) are required. The improved ground itself then works as an

impermeable retaining wall. The final shaft then only requires a thin inner lining of

shotcrete (sprayed concrete). However, grouting of the overburden, esp. the glacial till is

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only possible using jet‐grouting techniques, which is normally more expensive than other

techniques.

Buoyancy forces are normally controlled by the dead weight either of an aquiclude, or of the grouted

soil slab or the underwater cast concrete raft.

The principles of these construction techniques are shown in Fig 11.

Excavation starts once the vertical retaining walls (Grouted soil/rock, diaphragm wall) are completed.

The excavation can either be undertaken in the dry or under water. Underwater excavation can only

be undertaken without any visual inspection during excavation. The tear‐out/rippability force of any

under water excavator suitable for the envisaged depth is very limited and likely to be unsuitable for

this kind of bedrock. Therefore an underwater excavation technique is not recommended.

a) aquiclude as horizontal lining element

b) grouted soil/rock as horizontal lining element,

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c) underwater concrete raft as horizontal lining element

Fig 11: Shaft constructions principles grouting (left) and diaphragm wall (right) a) aquiclude as horizontal lining element, b) grouted soil/rock as horizontal lining element, c) underwater concrete raft as horizontal lining element

Although these systems are considered as impermeable from a technical point of view it is nearly

impossible to make the systems fully impermeable. A small amount of ground water inflow will

always be present but this can be controlled by technical measurements, for example by grouting

techniques. This unavoidable remaining water inflow can be specified as a target limit in the contract.

If the amount is lower than the limit during construction, it can be accepted, if it is above the limit,

the contractor has to take measurements to reduce the inflow. This procedure is quite common. The

standard agreed amount of remaining water inflow is normally considered as 2.0 – 3.0 l/s per 1,000

m² lining surface (surface in contact with water. This leads to a water inflow of

Q = [π * D * h + π * D² / 4] * 2.0 …. 3.0 l/(s*1,000 m²)

= [π * 15 * 63 + π * 15² / 4] * 2.0 …. 3.0 = 6.3 …. 9.4 l/s = 23…….34 m³/h

This level of water ingress is easily controllable. The impact on the groundwater regime must be

assessed and detailed in the design phase.

The groundwater in the shaft has to be discharged during excavation only. This is particularly

advantageous when contaminated water is present as the groundwater volumes to be

treated/disposed off are kept to a minimum. The amount of water equals roughly the inner shaft

volume times the pore content of the soil

Ground freezing has the same effect as grouting because the ground water in pores are frozen. The

ground will become fully impermeable and its strength increases. A cooling agent running through

(vertical) bored pipes is applied to freeze the ground water. The freezing equipment must be kept in

operation during the whole construction period of the shaft until a stable lining is completed. Once

the structures are completed, the ground freezing is switched off and the ice melts. It is a temporary

measure which is normally completely reversible. Ground freezing is a very expensive method due to

high costs for energy and cooling agents and mostly applied when no other techniques are possible,

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or if permanent measures like cement grouting are not allowed due to environmental reasons. For

these reasons ground freezing has not been considered further in this concept design report.

A construction system using drawdown of the groundwater table has significant disadvantages as

described above. It is therefore strongly recommended that the construction system adopted should

focus on the construction of an impermeable system.

5.6.2 Shaft Concept Design

A very basic preliminary design for the proposed onshore shaft based on the general guidelines on

shaft construction techniques outlined above is given within this section.

a. Horizontal lining element (shaft base)

A man made horizontal impermeable lining (shaft base) must be applied because the existence of an

aquiclude at the base of the proposed shaft has not been proven from the marine site investigation.

Considering the depth of the shaft and the implication for underwater casting of concrete at the

depths envisaged re the accuracy of raft geometry / thickness etc, a solution using grouting

techniques is likely to be the best option for constructing the shaft base.

The thickness of the horizontal lining has to meet buoyancy requirements. Normally the dead load of

the horizontal lining and the shaft wall friction is taken into account. A rough analysis of the required

thickness of the shaft base is given in Fig. 12. This is a very preliminary overview to demonstrate the

scale/nature of what may be required.

The thickness of the grouted bedrock section is assessed to be ca. 14 m in thickness to overcome

buoyancy issues, see Fig. 12. The final depth of the bottom is therefore ca. 76 m to OD. This will be

the subject of a more detailed analysis in a subsequent design phase.

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Fig 12: Calculation of bouyancy of shaft

b. Vertical lining elements (shaft walls)

The bottom of the vertical lining must reach the bottom of the grouted horizontal lining (shaft base)

to guarantee a sufficient connection. The total length of vertical lining from ground level is therefore

ca. 80.5 m depth (finishing at ca. ‐ 76 m OD).

There are three options to construct the vertical shaft lining:

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1. Diaphragm wall

2. Grouting

3. Combination of diaphragm wall in overburden and grouting of bedrock

The options are shown in Fig. 13.

Fig. 13: Options vertical shaft lining

1. Diaphragm wall

Application of a diaphragm wall is technically feasible in overburden as well as in bedrock, but is

mostly applied in overburden. A wall thickness in the order of 1.00 to 1.20 m is normally applied

for this order of shaft depth. The diaphragm wall should reach the bottom of horizontal lining

(grouted rock, see above) to minimise the risk of water inflow / ground stability failure.

The depth up to which a diaphragm wall trench can be excavated with standard equipment is ca.

45 m to 50 m. Extra equipment is required when this limit is exceeded. Excavating of rock will

require more effort as well as different equipment than that used for the overburden soils.

The costs for that kind of diaphragm wall ‐ up to 50 m depth – are in the order of 500 Euro/m²

for the overburden section and 600 Euro/m² for the bedrock section. Extra costs of the order of

roughly 200 Euro/m² must be added when the limit of 50 m depth is exceeded. The orders of

costs for the diaphragm wall are given in Table 1.

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Table 1: Costs for diaphragm wall construction (shaft 20 m in diameter) incl. excavation of

trenches, concreting, reinforcement steel.

Section / Element Quantity Costs per unit

[Euro/ m²]

Costs

[Euro]

Overburden section +4.5 m to ‐ 34 m OD

(0 ‐ 38.5 m depth)

20 m * π * 38.5 m =

2420 m² 500 1,210,000

Bedrock section – 34 m to 45.5 m OD

(38.5 m to 50 m depth)

20 m * π * 11,5 m = 723

m² 600 434,000

Bedrock section ‐45.5 m to ‐ 76 m OD

(50 m to 80.5 depth

(Estimated up to bottom of grouted section)

20 m * π * 30.5 m =

1,916 m² 800 1,533,000

Total costs 3,177,000

2. Grouting

Grouting is technically feasible in overburden as well as in bedrock. However, grouting of the

overburden is possible using jet‐grouting techniques only because the overburden soils are not

groutable by standard grouting techniques. Jet grouting is a very costly technique. The thickness

of the grouted ground should be in an order of 2.5 m.

The costs for jet grouting are in an order of 500 – 600 Euro/m³ (grouted ground). The costs for

standard grouting are in an order of 300 ‐ 400 Euro/m³ (grouted ground). The order of costs for

the grouted wall are as shown in Table 2.

Table 2: Costs for grouted wall (shaft 20 m in diameter) construction incl. Drilling, grout supply,

grouting operation


[Euro/unit]

Costs

[Euro]

Jet grouting in overburden section

+4.5 m to ‐ 34 m OD (0 ‐ 38.5 m depth)

((24/2)²‐(20/2)²) * π * 38.5 m = 5,322 m³

600 3,193,000

Grouting of bedrock section – 34 m to ‐ 62 m OD

(38.5 m to 66.5 m depth) assumed up to shaft bottom,

only. Grouting of vertical lining below shaft bottom =

grouting of horizontal lining

((24/2)²‐(20/2)²) * π * 28 m = 3,870 m³

400 1,548,000

Shotcrete lining normal grouted section 20 m * π * 66.5 m =

4,178 m² 80 334,000


3. Combination of diaphragm wall in overburden and grouting of bedrock

Application of a diaphragm wall in overburden and grouting of the bedrock combines the

advantages of both techniques / avoids their disadvantages. The overlapping between the

diaphragm wall and the grouted section should be in an order of 2 m. The length of the

diaphragm wall section is therefore ca. 40.5 m.

The order of costs for the combined wall solution are given in Table 3.

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Table 3: Costs for diaphragm wall/grouted wall (shaft 20 m in diameter) construction incl.

excavation of trenches, concreting, reinforcement steel, drillings, grout, grouting


[Euro/unit]

Costs

[Euro]

Diaphragm wall overburden section

+4.5 m to – 34 m OD (0 – 38.5 m depth)

20 m * π * 38.5 m =

2420 m² 500 1,210,000

Diaphragm wall bedrock section

– 34 m to ‐36 m OD (38.5 m to 40.5 m depth)

20 m * π * 2 m =

126 m² 600 76,000

Grouting of bedrock section – 34 m to ‐ 62 m OD

(38.5 m to 66.5 m depth) assumed up to shaft bottom,

only. Grouting of vertical lining below shaft bottom =

grouting of horizontal lining

(12²‐10²) * π * 28 m

= 3,870 m³ 400 1,548,000

Shotcrete lining grouted section 20 m * π * 28 m =

1,759 m² 80 141,000


This approximate cost assessment indicates that option 1 and option 3 are similar in broad

economical terms. The final decision on the system to be chosen must be left to a subsequent project

stage where a more detailed design of shaft geometry and depth has to be undertaken.

5.7 Serviceability Test for Shaft Lining Prior to Excavation

A serviceability test for the proposed shaft lining must be undertaken prior to excavation to prove

the suitability of the lining in terms of impermeability (see Fig. 14e).

The groundwater inside the yet completed shaft lining has to be pumped out of the (unexcavated)

shaft by means of a well which has to be sunk in the shaft down to the top of the horizontal lining.

Simultaneously the groundwater level outside the shaft will also be monitored by a number of

piezometers to be installed around the exterior of the shaft.

The serviceability of the lining will be confirmed when no significant change in groundwater level

outside the shaft is registered and the water level in the well inside the shaft does not rise

significantly after pumping.

5.8 Excavation

Excavation could be undertaken using a heavy excavator for soil and bedrock and / or a rotary cutter

in bedrock. Some sections in bedrock may reach a level of strength that explosives must be used. It is

technically feasible to match the specified ESB vibration limits (Section 2.2) with an accurate choice

of explosives, number of blast holes and blast sequence ignition.

5.9 Survey

The shaft construction has to be monitored using piezometers, seismic, inclinometers,

extensometers and geodetic measurements during the entire construction period.

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5.10 Optimum Shaft Construction Technique

On the basis of the foregoing it is CDM’s opinion that the onshore shaft would be best constructed

using a combination of diaphragm wall and grouted rock for the vertical lining walls/element as

described above and to grout the rock below the shaft bottom to form the shaft slab (Option 3 from

above). The construction sequence for this option is shown in Figs. 14 a – 14 h.

Fig.: 14a: Grouting of bedrock for vertical shaft lining

Fig.: 14b: Grouting of bedrock for horizontal shaft lining (“shaft slap”)

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Fig.: 14c: Construction of reinforced diaphragm wall

Fig.: 14d: Grouting of transition zone diaphragm wall – grouted bedrock lining

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Fig.: 14e: performing of serviceability test

Fig.: 14f: Excavation of overburden

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Fig.: 14g: Excavation of bedrock, application of shotcrete lining

Fig.: 14h: Excavation to final depth, concreting of a slab as work floor

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5.11 Overall Costs

Overall costs for the onshore shaft construction using Option 3 are given in Table 4.

Table 4: Costs for onshore shaft (20 m in diameter) construction incl. lining / excavation excluding

extra costs for disposal of contaminated soils


[Euro/unit]

Costs

[Euro]

Vertical lining as specified above (Option 3) 2,975,000

Horizontal lining (grouted rock) 10² * π * 14 m

= 4,400 m³ 400 1,870,000

Excavation (excluding extra costs for disposal of

contaminated soils)

10² x π * 66.5 m =

20,900 m³ x 2.5 t/m³ =

52,250 t

100 5,250,000

Monitoring facilities, maintenance and monitoring during

operation, dewatering 1,500,000


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Section 6 Tunnelled section

6.1 General

The key technical considerations for the tunnelling operations are the geotechnical conditions in

which the tunnel is driven. It is therefore advisable to choose a vertical tunnel alignment according to

the geotechnical and hydrogeological conditions offering the best conditions in terms of lowest

health and safety impacts, lowest geotechnical risk, lowest cost and highest advance rates.

The costs and risks associated with tunnelling normally rises the more the geological, geotechnical

and hydrogeological conditions vary. The general guidelines for selecting the optimum vertical

tunnel alignment are therefore to choose;

most homogenous geological, geotechnical and hydrogeological conditions along the tunnel

most stable tunnel face conditions

the lowest possible hydraulic head

the lowest risk in terms of health and safety requirements

Furthermore it is important to understand that the geotechnical/hydrogeological conditions

determine the tunnelling requirements. Normal procedure is to predict the geotechnical and

hydrogeological conditions as well as to predict the technical measurements/requirements that will

be required to overcome these ‐ which is part of the project Baseline Tunnel Reference Design

process. During tunnelling the behaviour of soils and rock masses must be monitored (for example

measurement of strain, stresses, settlements and displacement) in order to check if the monitored

behaviour is consistent to that predicted. The onsite tunnelling process/operations must be reviewed

immediately if predicted and monitored behaviours do not correspond. Technical measurements

must be taken if required.

6.2 Tunnel Mechanics Requirements

The concept of tunnel mechanics is to establish a ring zone in the soil / rock around tunnel of suitable

bearing capacity to overcome the rock mass load (Fig. 15). The soil/rock around the tunnel is the

structural element in tunnelling rather than the tunnel lining. Tunnel lining has the function to

prevent soil/rock to get loose and to overcome groundwater pressure.

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Fig 15: Principle of tunnel statics.

Therefore for stability purposes all tunnels require a minimum overburden of certain strength above

the top of the tunnel to allow the soils/rock to establish this ring zone. The size of overburden above

the top of tunnel to establish this ring zone depends on the stiffness and the strength of the

soil/rock. The less stiff and the less strength the soil/rock is the more overburden is required.

Normally an overburden depth of twice the tunnel diameter is sufficient. In very stable ground

conditions like unfissured, unweathered rock an overburden of one times the diameter could be

sufficient. In less stable ground conditions however, three times the diameter or more is required.

If the tunnel is below the ground water table a sufficient soils/rock dead load is required to prevent

buoyancy of the tunnel. The normal minimum overburden is at least twice the outer diameter of the

tunnel. In less dense soils like the encountered marine sediments the overburden depth would have

to be more than that.

The ground must have minimum bedding strength to carry the load of the tunnel lining.

Stress‐strain behavior of soil and rock is strongly time‐dependent. In general, the more plastic the

ground behaves the longer it takes to reach the final strain stage. At final stage, rock load is at a

minimum. If the tunnel contour isn’t supported at this stage, the ground loses its strength and the

rock mass dead load increases. A principle of tunnel mechanics is to allow a certain strain of the

soil/rock to match a minimum of rock load. The number of variations of stress states of the ground

must be reduced to a minimum. Therefore tunnel driving should be a continuous operation in terms

of hours. Night pauses or weekend pauses normally should therefore be avoided. Tunnel driving

should operate 24 hour / 7 days per week to meet these tunnel mechanic requirements. A pause for

a couple for days normally requires special measurements for temporary ground stabilization.

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In this project the construction stage and operation stage must be considered separately. During

construction the tunnel will be empty hence internal static forces in tunnel linings will be normal

compressive forces and bending moments. Tension forces suitable to create fissures in the tunnel

lining are normally overcome by normal compressive forces during this stage.

After commissioning the tunnel has to transport WwTW final effluent to the sea using gravity

induced hydraulic head. That means the inner water pressure will be higher than the outer water

pressure following completion and commissioning. Tension stresses may occur in the tunnel lining

which may result in fissures in tunnel lining which are potential voids for water moving out from the

tunnel into the environment. Static analysis must assess this and the lining has to be designed in

order to keep the tunnel lining watertight. The watertightness requirement given in the British

Tunnelling Society Specification for Tunnelling for drainage and sewer tunnels is 0.5 l/m2/day.

6.3 Tunnel Environment, Costs Of Tunnelling And Advance Rates

The first 10‐15 m of the overburden soil are mostly marine sediments proven as very soft and in

general unsuitable to have any considerable strength or stiffness. This section of upper material

should not be considered for tunnelling.

Below these marine sediments and based on the assumption of a minimum overburden of suitable

strength and stiffness of twice (three times in bad material as quoted above) times the outer

diameter, the top of a potential tunnel will be at

10 m marine sediments + (2 … 3) * diameter (= 6.5 m external diameter)

= 10 + 13 …. 19.5 = 23 …. 29.5 m below seabed level.

The bottom of the tunnel will be at

23 …. 29.5 m + 6.5 m = 29.5 … 36 m below seabed level.

The findings of the marine site investigation show mostly glacial till or even some bedrock at these

depths.

The glacial till has been shown to be extremely heterogeneous in terms of geotechnical and

hydrogeological properties – therefore it will be extremely difficult for tunnelling. The order of costs

for tunnelling in these geotechnical conditions in this depth range will vary from 8,000 – 12,000

Euros/m (direct production costs). There will be many stops for maintenance / cleaning of the

machine / cutting wheel, etc.; the advance rate will be less than 12 m /day average production rate.

Moving into deeper bedrock offers the best conditions for tunnelling because the marine site

investigation results show that the bedrock is mostly stable for the tunnel diameter being considered

and of low permeability i.e. it is likely that the major part of the tunnel can be advanced without

active face support. Therefore the costs of tunnelling in bedrock are of the order of 6,000 – 9,000

Euro per m (direct production costs). The advance rate will be at ca. 15 m/day (long average

production rate) with a highest production rate of ca. 30 m/day.

It is therefore reasonable to choose the bedrock as the optimum tunnelling environment. The marine

site investigation borings show the bedrock to be weathered / fractured in the top 5 – 10 m. It is

recommended that the top of the tunnel should be kept twice the diameter (= 13 m) below rockhead

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in good/fair rock quality conditions and three times the diameter (= 19.5 m) in poor conditions like

such as those encountered at the onshore location.

6.4 Vertical Tunnel Alignment

6.4.1 General Guidelines

As stated above, the tunnel should be kept in the bedrock at a level to match a rock overburden

above the tunnel of a minimum of 13 m.

To date the marine site investigation shows that the top of bedrock is very uneven (See Drawing 2 in

Appendix A). It should be noted from the drawing that the top of the bedrock is inferred and for this

reason geophysics work needs to be done along the tunnel alignment to try and prove the bedrock

head over the entire proposed tunnel length. It must be ensured that the tunnel is located in bedrock

at every part of its alignment, and equally to assure 2‐3 times overburden cover it should not elevate

above a level of ‐ 60 m OD at any point along its length with the exception of the section between

the onshore shaft and BH M 05.

6.4.2 To Keep The External Water Pressure On The Tunnel As Low As

Possible, The Tunnel Alignment Should Be As Elevated As Possible.

Tunnel Gradient

The rockhead level at the proposed diffuser location B3 was found at ca. – 51 m OD. The bedrock

overburden should be 13 m as minimum (see above 2 *6.5 = 13 m), so the top of the tunnel at the

diffuser location is at – 64 m OD.

The top of the tunnel at BH M 05 ‐ where the lowest rockhead level was found along the route ‐ is at

ca. ‐57 m OD. Given an overburden of 13 m as minimum (see above 2 *6.5 = 13 m), the top of the

tunnel at location BH M 05 is at – 70 m OD.

The difference in level between the BH M 05 and the marine diffuser shaft is therefore 6 m which

gives a gradient of

(64 m – 70 m)/8,000 m = 0,075 %.

For hydraulic purposes the gradient should not be less than 0,1 % which equals to a difference in

level of

8,000 m * 0,1 % = 8 m.

The top of the tunnel around BH M 05 should therefore be at

– 64 m OD (top of tunnel at B3) – 8 m= ‐ 72 m OD (=top of tunnel at BH M 05).

The rockhead level at the onshore shaft location was found at ca. – 34 m OD. The bedrock

overburden should be 19.5 m as a minimum (see above), so the top of the tunnel at the diffuser

location is at – 53.5 m OD. (see above). The top of the tunnel at borehole BH M 05 is ca. ‐‐72 m OD

(see above), the difference in level is therefore

– ‐72 m OD – (‐ 53.5 m OD) = 18.5 m

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which equates to a gradient of

18.5 m/1,000 m = 1.85 %.

The optimum vertical tunnel alignment is shown in Drawing 3 in Appendix A

It should be noted that the work in this report is preliminary concept design work which must be

revisited when both the marine site investigation/any additional site investigation and the baseline

tender reference design have been completed. For this reason the vertical alignment presented in

this report may be subject to future change.

The tunnelling layout has to be designed to accommodate a maximum water pressure according to

its operating depth below maximum sea level. An extra over of water pressure of a couple of metres

(for high tidal surges) should be included to meet a change to the vertical alignment.

It should be noted that the optimum vertical tunnel alignment is of a “V”‐shape type. This approach

is quite common since most of metro tunnels in urban areas (for example Channel Tunnel Rail Link /

London, TBM tunnelling) are of this shape. A 6.6 km long road tunnel of ca. 11 m (TBM) in diameter

crossing the Schelde estuary in the Netherlands (“Westerschelde Tunnel”) with comparable

conditions in terms of access possibilities (single access point from onshore with no intermediate

shaft access), water pressure, etc. was completed recently in 2003. A longitudinal section of this

project is shown in Fig. 16.

Fig 16: Longitudinal cross section of Westerschelde tunnel.

With regards to Health & Safety this type of V shape tunnel alignment under a marine sea

environment, with single access entry point from onshore, and no intermediate shafts, is acceptable

with the proviso that a robust H&S management plan is put into effect and comprehensively

managed during construction.

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6.5 Geotechnical Conditions

At this stage a preliminary characterization of the bedrock in terms of tunnelling needs can be given

although a detailed geological, geotechnical and hydrogeological compilation of data and

interpretation as the basis for a design has not been completed yet.

Whilst the proposed tunnel section in the vicinity of the onshore shaft location is not competent

either in terms of stability or permeability, the bedrock at borehole location BH M05 and further east

of BH M05 is in general mostly competent in terms of stability and permeability i.e. offers good

conditions for tunnelling potential along the proposed route. There are exceptions however, for

example sections of less strength and higher permeability (For example BH M 17, ca. 44 – 46 m

depth) were encountered and these must be considered as part of the tunnel drive planning.

However, a detailed prediction of the exact location of these sections along the entire proposed

tunnel route is not possible due to the spacing distance of the marine site investigation borings.

Whilst more boreholes would reduce the overall level of uncertainty regarding the change in

geotechnical conditions along the proposed tunnel route, uncertainty will always remain because of

the nature and limited scope of geotechnical site investigations.

Different geology has been investigated between BH O 01/O 02 and BH M 05 as well as between BH

M 08 and BH M 21 / BH M 11. It must be assumed that similar changes of geology accompanied by

transition zones of very poor ground conditions such as the onshore shaft location(although these

have not been proven during the marine site investigation) – cannot be discounted.

Water pressure will correspond to the mean sea level (0.0 m OD), i.e. ca. 7.2 bar at the top and ca. 8

bar at the bottom of the tunnel. Since combinations of daily tides/surcharges can be as high as + 4.0

m OD, water pressure may rise further up to 8.4 bar.

Whilst more boreholes would reduce the overall level of uncertainty regarding the change in

geotechnical conditions along the proposed tunnel route, uncertainty will always remain because of

the nature and limited scope that has to prevail when undertaking geotechnical site investigations.

A geological interpretation and prediction of the tunnelling conditions and requirements along the

proposed route will be the subject of the baseline tender reference design.

6.6 Tunnelling Method

There are in general two tunnelling methods:

1. Conventional tunnelling

2. Mechanized tunnelling

Either of these methods can be used for similar projects. A short description of the methods is given

below.

1 Conventional tunnelling

Conventional tunnel heading (also known as “Shotcrete Method” or as “New Austrian

Tunnelling Method”) uses shotcrete, steel beams, rock bolts and drill and blast techniques.

After each blast, the soil has to be transported out and the rock contour has to be supported

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by steel beams, shotcrete and rock bolts. A minimum stability of rock is required during the

setting up of these elements. This is related to the length of the blasts. In very stable

conditions the length of the blasts can reach up to 6 m, in less stable conditions only 80 cm is

achievable.

Shotcrete cannot be applied to the tunnel lining if water inflow is present. The water table

must be drawn down to the bottom of the tunnel, i.e. a drainage channel is induced in the

ground along the invert of the tunnel. The control of water inflow to the tunnel is achieved

either by working in low permeable ground or, in high permeable ground conditions, by

applying additional measurements such as drawdown of ground water table by wells, grouting,

freezing or pressurized air.

Some large size rock fractures were encountered during the marine site investigation which

will be likely to hold/transmit water which will in turn necessitate the additional

measurements as described above. However, it is not possible to determine these sections

prior to construction works. Horizontal borings are required within a distance of a couple

metres from the tunnel face to investigate ground conditions prior to the blasts.

The advance rate for this technique will not exceed 5 – 8 m/day.

For economical, technical and H&S reasons it is recommended that the conventional tunnelling

technique should not be used for this project but only mechanized tunnelling methods should

be considered for this project.

2 Mechanized tunnelling

Mechanized tunnelling, as opposed to conventional techniques, incorporates all of the

tunnelling techniques in which excavation is performed mechanically by means of teeth, picks

or disks. These tunnelling techniques comprise a wide range of different machines, from the

simplest, such as backhoe diggers to the most complicated such as shield TBMs. These

machines not only carry out the excavation of the ground, they mostly also provide support to

the ground during tunnelling. This support can be just peripheral (like in the case of shield

TBMs) or also be applied to the front (Earth pressure TBMs or Slurry Shields for instance). The

final tunnel lining using precast concrete elements which are assembled directly by the

machine can be applied as well. Tunnel driving control facilities, accommodation, toilets,

electric power facilities, emergency facilities, air supply, tunnel segments erector, etc. are all

part of the machine and located close to the extraction chamber, see Fig. 17.

To overcome potential water inflow to the tunnel and/or potential unstable ground conditions

(which cannot be excluded) only a shielded tunnel boring machine should be considered. A

shielded TBM with active face support is applied if the tunnel face is not stable and if rock

collapse may occur. The shield skin, which covers the entire machine, serves as a temporary

support. As final support, usually pre‐cast lining segments of reinforced concrete are used. The

lining segments are installed under the protection of the rear part of the shield, the so‐called

tail‐skin. A face support using a slurry (slurry TBM) or an earth mud (Earth pressure balanced

shield, EPBS), or a combined machine is normally applied.

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Fig 17. Model of a shielded TBM (Slurry TBM used to construct a metro tunnel in Antwerpen)

However, whilst both Slurry and EPB TBM’s normally apply in soft ground such as clay, silt,

sand and gravel it is equally possible to apply these tunnelling techniques to hard rock

tunnelling where face supporting and ground water control is required.

Slurry TBM

Slurry shields are TBMs fitted with a full face cutterhead which provides face support by

pressurizing fluid (“Slurry”) inside the cutterhead chamber (see Fig. 18). These machines are

most suited for tunnels through unstable material subjected to high groundwater pressure or

water inflow that must be stopped by supporting the face with slurry subjected to pressure.

The cutterhead acts as the means of excavation, whereas face support is provided by slurry

counterpressure, namely a suspension of bentonite or a clay and water mix (slurry). This

suspension is pumped into the excavation chamber where it reaches the face and penetrates

into the ground forming the filter cake, or the impermeable bulkhead (fine ground) or

impregnated zone (coarse ground) which guarantees the transfer of counterpressure to the

excavation face. This slurry mixed with the excavated soil/rock and is pumped (hydraulic

mucking) from the excavation chamber to a separation plant located on the surface which

enables the bentonite‐clay slurry to be recycled. In the closed slurry shield in which the

counterpressure is compensated inside the excavation chamber, there is the addition of a

metal buffer which creates a chamber partially filled with air and connected to a compressor.

The result is the possibility of adjusting the counterpressure at the face independent of the

hydraulic circuit (supply of bentonite slurry and mucking of slurry and natural ground). These

machines are specially suited to excavate ground with limited self‐supporting capacity.

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Fig 18: Slurry TBM

Where there is a risk of water inflows under high pressure, the possibility for immediate

permanent face support provided by the slurry TBM will give greater security than all other

tunnelling techniques. A slurry TBM option is more flexible as it provides:

more ability to deal with unknown geological changes that may occur at the tunnel face

more ability to deal with sudden water inflow/ingress

more certainty regarding overall rate of progress because of the flexibility to control

change

Earth Pressure Balanced Shield TBM (EPBS)

EPBS or earth‐pressure balance shields are TBMs used for the excavation of soils where face

support and counter‐effect of ground water pressure is obtained by means of the material

excavated by the cutting wheel, which serves as support medium itself (see Fig. 19). The

cutterhead serves as the means of excavation whereas face support is provided by the

excavated earth which is kept under pressure inside the excavation chamber by the thrust

jacks on the shield. These jacks transfer the pressure to the separation bulkhead between the

shield and the excavation chamber, and hence to the excavated earth. Excavated debris is

removed from the excavation chamber by a screw conveyor which allows the gradual

reduction of pressure. These machines are used to excavate grounds with limited or no self‐

supporting capacity.

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Fig 19: EPB ‐TBM

The ground conditions are mostly stable for the main part of the tunnel and will not need

active face supporting and/or control of ground water inflow. That means in most parts the

TBM can operate without providing active face supporting/control of ground water inflow in a

so called open‐mode. Only smaller lengths of the tunnelled section will require a face

supporting / control of ground water inflow in a so called closed‐mode. The machine should be

prepared for rapid change to close mode The reaction time will be subject to be more detailed

assessment in a subsequent design phase as part of the preparation of the baseline tender

reference design and as part of the technical specification development for tendering

purposes.

The outer tunnel diameter to be excavated is somewhat larger than the outer diameter of the

tunnel lining because the tunnel lining is assembled under the protection of the shield, i.e. the

shield diameter must be larger than the lining. In addition a gap between excavated tunnel

contour and shield is required for steering of the TBM. The annular gap between excavated

tunnel contour and tunnel lining is normally filled with a grout injected behind the tailskin of

the TBM.

Comparison Slurry/EPB‐TBM

Slurry and EPB‐TBMs have advantages and disadvantages. A general comparison between the

performance of both TBM types is not clear cut since they each have to be considered

independently of each other for the specific project conditions. A minor disadvantage for one

particular TBM type on one project may become a major and critical advantage for another

project.

However a rough comparison between both Slurry and EPB TBMs is given in Table 5, which

must be treated with extreme caution because of the above mentioned issues.

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Table 5: Comparisons between Slurry and EPB‐TBM

Slurry TBM EPB TBM

Price Normally cheaper

Procurement Equal

Advance rates Equal

certainty of overall advance rate because of

the flexibility to control change Normally higher

Operating Normally simpler

Consumption of additives Normally lower (no slurry

circuit)

Torque Normally lower

Power consumption Normally lower

Compound size Normally smaller

Reaction time to control face stability Normally much quicker

Ability to deal with unknown geological

changes Normally greater

Ability to deal with sudden water

inflow/ingress Normally greater

Hard rock conditions Normally more independently

High water pressure (up to 8 bars) Normally recommended Not normally recommended at

this pressure

6.7 Slurry

The slurry is a mixture of water and bentonite, a smectite clay mineral. Some additives improving

certain properties of the slurry can also be applied.

The slurry has two main properties;

Thixotropic properties A (bentonite) slurry is viscous under normal conditions but behaves like

a fluid over time when shaken, agitated, or otherwise stressed. Sea water cannot be used for

mixing of the slurry because the thixotropic properties are strongly related to the electrolyte

content of water. Clay minerals will coagulate and lose thixotropic properties when sea water

is used.

Filter cake / Sealing properties A filter cake is a thin layer of highly impermeable bentonite

mineral particles caked (or “plastered”) on the soil contour set up because of a hydraulic

gradient towards the soil. A filter cake is shown in Fig. 20.

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Fig 20: Filter cake

The main purpose of the slurry is to:

Seal the tunnel contours in high permeable ground conditions (“filter cake”)

Support the ground (thixotropy)

Transport of the cuttings

Cooling of the cutting tools

6.8 Minimum Diameter Of The Tunnel

The minimum diameter of the tunnel is determined by TBM supply and H&S requirements;

Tunnel segments have to be transported through the tunnel,

Transport vehicles for TBM supply require a minimum space,

Pipe, live wires, etc. requires space,

Spoil disposal requires space,

Emergency vehicles require minimum space.

Normally, the minimum inner diameter should therefore not be less than ca. 4.0 ‐ 4.5 m. However,

because of the environment and length of this proposed tunnel, a minimum inner diameter of less

than 4.5 m should not be permitted. This data is derived from practical experience.

If for some reason a smaller diameter tunnel than 4.5 m internal diameter is chosen, then an inner

lining of the required reduced diameter will have to be inserted after the 4.5 m diameter primary

tunnel lining has been constructed. The primary tunnel lining has then only a temporary function

which will reduce the cost of construction. The inner lining can be constructed using precast

elements. The annular space between tunnel lining and inner lining has to be filled using injected

mortar.

6.9 Tunnel Lining Segments

Precast concrete tunnel segments are used normally as tunnel lining elements for shielded

tunnelling, see Fig. 21 and Fig. 22. Normally 4 to 8 elements are used per ring. The thickness of

tunnel lining segments range normally between 30 cm and 60 cm. Preliminary indications are that a

40 cm thick lining will be required for this project. The segments are transported via the tunnel to the

TBM for assembly. Temporary bolts ‐ removed after closure of one ring ‐ are used for assembly. The

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assembly starts in the bottom. A special key segment of a conic form is used as final segment to

complete the ring. Sealing is performed using sealing elements.

Fig 21: Assembled precasted tunnel lining segments

Fig 22: Assembled tunnel lining segments (left) and segments ready for transport (right)

The precasted segments have pockets for their assembly. These pockets may have an impact on the

hydraulic (relative) roughness of the completed lining. Radial joints may have a similar effect – if

lipping occurs. If required, the pockets and joints can be filled with mortar. Normally the friction loss

impacts of pockets and joints on the hydraulic properties are minor and negligible. In case of doubt,

this issue can be investigated further as part of the detailed design requirements of the tunnel lining.

6.10 TBM Facilities For Probing And Ground Improvement

The TBM machine should be prepared for geophysical site investigation, probing and ground

improvement (i.e. grouting) ahead of the TBM if required. This is essentially necessary to reduce and

effectively manage the geotechnical risk.

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The machine should be equipped with 2 ‐ 3 permanent installed drill rigs, located inside the shield for

the horizontal drilling, the third one is located behind the ring erection area for the radial drilling. An

additional fourth drill rig to be installed on the erector should be considered.

Given the nature of this project a key risk is the issue of ground conditions. Therefore the level the

level of significance of ground condition change must be assessed as part of the baseline tender

reference design. This information will form a key component of the technical specifications for

tendering. To investigate this during construction geophysical investigation should be carried out

ahead of the TBM and subsequently by drilling from the machine face if results of geophysical

investigation indicate significant changes of geotechnical conditions.

Probing and drilling can be done in open mode and, for the majority of the positions, also in closed

mode conditions using blow out preventer units.

Probing by drilling will reduce the advance rate because the TBM cannot operate whilst drilling

ahead of the TBM.

Additional measures, like grouting for ground improvement, should be suggested by the contractor

based upon the findings and approved by the Client Representative.

6.11 TBM Maintenance

Whilst divers can be used to change the tools on the cutting head of a TBM by entry to a pressurised

face through an air lock, it is highly desirable to change the tools in a free air environment. This can

be done where water ingress is not too great. This will require the onshore tunnel inlet shaft to be

deep enough to ensure the tunnel is driven in competent bed rock and adequate ventilation will have

to be provided to the tunnel face.

Consideration should be given to utilising a tunnelling machine where tool/cutting head replacement

can be done from the back face of the cutter head – thus eliminating the need to send maintenance

staff into the tunnel face area at the front of the machine.

Whilst the tunnel will be driven against water pressures of potentially up to 7‐ 8.4 bar, compressed

air working should be avoided. Compressed air working should only be considered if there is no

technical alternative. The National standards for compressed air working vary by country – this issue

has to be treated and assessed with consistency in returned tenders. Currently in the UK for example

compressed air work is not done in tunnels where pressures are in excess of 8 bar. If compressed air

working is permitted then a full time hyperbaric doctor must be employed.

6.12 Connection To The Diffuser Shaft

One method is to pre‐drill the diffuser shaft/s to below proposed tunnel section invert and then drive

through it with the TBM.

The TBM will then be driven past the diffuser connections, stripped out to some degree and then

filled and abandoned (Stripping out and abandonment usually takes 3 months). Recovery of the

carcass and all TBM equipment is not usually cost effective. The value of the equipment that could

be recovered would be of the order of £0.25 million to £0.5 million against which would be set a

contract time cost of around £100,000 per week and, if relevant, liquidated damages charges.

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6.13 Compound Requirements

Main areas required for the compound area for:

Temporary excavated soil (spoil) storage

Tunnel segments storage

Mixing plant for mortar to fill annular gap

Mixing plant for slurry if a slurry TBM is used

Separation plant if a slurry TBM is used

The size of storage facilities for excavated soil (spoil) and tunnel segments is determined by

possibilities of their disposal (spoil) / delivery (segments). Tunnel driving has to stop:

if spoil disposal storage space is running low

if precast tunnel lining segment stock is running low

If mortar and slurry stock is running low

Construction site logistics and required compound space are strongly linked together. The contractor

should make provision as to how to meet delivery needs, available space at compound and risks of

tunnel driving stops.

6.14 Excavated Soil

Tunnel advance rate is ca. 15 m/day (long average production rate) and ca. 30 m/day (highest

production rate). This leads to an average excavated volume of:

(6.5 m/2)² * π * 15 m/day = ca. 498 m³/day (average rate)

(6.5 m/2)² * π * 30 m/day = 995 m³/day (highest rate)

6.15 Survey

The tunnelling has to be monitored using stress/strain‐ and geodetic measurements during the entire

construction period.

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Section 7 Offshore Marine Tunnel Outlet Diffuser

Shaft

7.1 General

The indications from the water quality marine modelling exercise are that a single large diameter

diffuser shaft with a diffuser head will suffice, to be located at preferred diffuser location B3, see

Drawing Nr 1.

The purposes of this diffuser shaft will be to:

1. Provide a temporary construction shaft area for making the final transition structure

connections between the diffuser shaft and the tunnelled section below, i.e the diffuser shaft

construction must guarantee a safe working environment for final connection purposes.

2. Use as the permanent WwTW effluent riser shaft diffuser outfall

The principles of offshore shaft construction are the same as those for the onshore shaft. However,

many restrictions will apply to the marine environment in which the diffuser shaft/s has to be sunk.

7.2 Ground Conditions

Seabed level is ca. – 26 m OD at the site of the proposed diffuser shaft B3. Marine sediments extend

to ca. 9 m below seabed level i.e. ca. ‐35 m OD. Marine sediments are underlain by glacial till down to

bedrock at ca. 25 m depth below sea bed, i.e. ‐51 m OD.

The marine site investigation has established that the marine sediments are exceptionally soft to

very soft. The glacial till was found to be soft as well. Leg penetrations of the jack‐up barge used for

the marine site investigation at BH M 11 were ca. 16 m depth below sea bed level. At this location

the underlying bedrock is likely to be limestone formations of the Tournesian series of the Lower

Carboniferous. The unconfined compression strength of the rock at BH M 11 and BH M 21 ranges

between ca. 30 MPa and ca. 80 MPa.

The bedrock is mostly stable. Coefficients of permeability derived from packer testing in BH M 11

range between 1.4 to 4.9 * 10‐6 m/s. For the preliminary calculations provided in this report a

coefficient of permeability of K = 2*10‐6 m/s has been estimated for the bedrock at the offshore shaft

location.

7.3 Inner Shaft Diameter

The inner shaft diameter has to meet the hydraulic requirements for the diffuser outlet structure.

Whilst hydraulic analysis is still ongoing early indications are that the riser shaft internal diameter will

be of the order of 4.0 m or less.

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7.4 Depth of Shaft

The depth of the riser shaft is controlled by the requirements of the vertical tunnel alignment as well

as by the level of risks on the programme and the construction costs. It is therefore prudent to keep

the shaft as shallow as possible.

The rock mass overburden above the top of the tunnel at the offshore diffuser location is much more

stable than at the onshore shaft and must therefore match roughly only twice the tunnel diameter

because of tunnel static requirements (see section 6 above). At the proposed diffuser location B3 this

is roughly at:

‐ 51 m to OD (rock head level) – 2 * 6.5 m = ‐ 64 m to OD.

The tunnel bottom is then at:

– 64 m – 6.5 m = ‐ 70.5 m to OD

Considering the seabed level at location B3 is roughly at ‐26 m to OD, the depth of the shaft equals to

‐26 m to OD – (‐ 70.5 m to OD) = 44.5 m approx

This is shown in Fig. 23.

Fig 23. Depth of the offshore diffuser shaft

7.5 Dewatering / Buoyancy

The bedrock at the potential diffuser location is less permeable (as opposed to the onshore shaft

permeability conditions) and could be considered as an aquitard. The bedrock itself is considered to

be stable enough to carry the loads, therefore only a thin support structural shaft lining is likely to

required, most likely comprising of and outer large diameter steel drilled casing with an inner steel

casing inserted with the annulus between both casings concreted/grouted up.

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Based upon the working assumption of an external drilled diameter of 5 m for the shaft and using the

same calculation methodology set out in section 5 above to determine likely inflow rates the

information from the site investigation currently suggests that the water inflow discharge rate maybe

of the order of 66 m³/h. This level of water ingress is easily controllable. Therefore to keep the shaft

dry dewatering by pumping will be required.

In the event that the inflow discharge rates are much higher than 66m³/h grouting of the bedrock

section maybe required.

7.6 Construction

The diffuser shaft sinking and lining construction methods will be governed by the depth of the

seabed overburden and bedrock materials. The operation will also be restricted to a large diameter

drilling operation using a machine drill (with multiple drill bits/heads mounted within a single

machine drill face) within a pre installed large thick walled steel liner of extended length. This

approach is required because of the marine working environment whereby extended continuous

shaft lining will be required.

The pre‐installed large thick walled steel liner will extend continuously up to the deck level of the

drilling jackup barge. Additional steel liners will be welded from topside on the jackup deck as the

diffuser shaft advances downwards.

The principal advantages of such systems are:

Man‐entry is not required.

Groundwater lowering is not required.

The cutter head works submerged below the cutter ring.

Spoil is removed as a pumped slurry.

The shaft lining can be provided in long lengths – floated to site – to minimise jointing/welding

and provide leaks.

There could be limitations to the machine drilling system with regard to the maximum diameter and

the strength of rock that can be excavated continuously without excessive cutter tool wear and

maintenance costs. The boulders in the glacial till are likely to present problems for the machine drill

and cutting heads for sinking the diffuser shaft. This has to be considered in the subsequent design

and tender process.

7.7 Shaft Mechanics

The marine sediments around the diffuser location have been proven to be very soft. The horizontal

bedding of the shaft is therefore very low. A certain bedding of the shaft is maybe required to

overcome horizontal loads on the diffuser head by water currents and their variation in direction. An

improvement of the strength and density of the marine sediments must therefore be considered in

order to set‐up an economic design approach by reduction of the internal forces. This is subject to a

detailed static analysis and subsequent design phase.

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7.8 Connection Between Shaft and Tunnel Section

The method of forming the diffuser connection into the tunnelled section will be dependent upon

contractor choice of operations/plant. One method is to pre‐drill the diffuser shaft/s to below

proposed tunnelled section invert and then drive through this with the TBM. Tunnel driving through

steel shaft lining is not possible.

An alternative (and probably most preferred) is to pre‐drill the diffuser shaft/s to several meters

above the underlying tunnelled section and then mine through from the completed tunnel below

into the underside of the completed diffuser shaft. The final choice of diffuser connection/s method

should be left solely with the contractor.

7.9 Onshore Construction Compound/Berthing Areas

Whilst the majority of the diffuser shaft construction operation will take place at sea from jackup

platform barges there will be a need to provide a significant onshore berthing area. The onshore

berthing area will be in addition to the onshore compound area to be provided for the overall

Ringsend Long sea outfall project.

However, with regard to the onshore berthing area a single berth area will be required to include the

following:

Berth area for storage of elements.

Berth area for jackup platform barges and other marine support vessels.

Berth area for laydown space, materials handling, and making up RC detailing cages etc.

Fully serviced (with crane) berth area at least 100 m x 80 m approx.

7.10 Soft Ground Condition Problems for Plant

It is known that there are deep marine muds/sediments at the location of the proposed diffuser

shaft. Equally it is known that these are highly heterogeneous and vary significantly over short

distances. Depths of up to 23 m leg penetration have been encountered in one location in Dublin Bay

during the marine SI – which were in excess of the effective working leg length of the jackup barge

being utilised.

For the construction of the diffuser shaft it is therefore certain that very deep leg penetration of the

jackup barges being utilised will occur. This will cause problems in terms of providing suitable very

large rigs with long enough legs. This problem must be clearly understood and addressed within the

tender specifications which should include information on the expected range of seabed sediment

conditions.

The presence of deep marine muds will also present an operational challenge because the legs are

likely to become stuck in the deep sediments – particularly when the jackup is in a fixed location for

at least three months during construction of the diffuser shaft. One way of attempting to overcome

this problem would involve dredging out softer materials in the vicinity of the diffuser shaft to create

a deep trench on which to site the jackup barge/s. In this way less of the jackup barge legs would be

in the marine muds/sediments. This option does however, present potential problems because of

the possibility for the dredged trench to fill back up with silts/sediments over time due to sea bed

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movement – i.e. it becomes a silt trap. Equally the diffuser shaft is located close to the designated

spoil dump for Dublin Bay – which may also introduce silt/sediment material into the dredged trench.

Consideration could be given to lowering the seabed level locally to reduce the potential for the

jackup barge legs to stick in the marine muds/sediments.

7.11 Construction Sequence Diffuser Shaft

The overall sequence of operations for the diffuser shaft construction is likely to be:

Inspect seabed locally.

Potential improvement of soft sediments.

Grout of bedrock.

Prepare seabed locally (Localised dredging of marine sediments may be required).

Position jackup barge platform/s.

Jackup barge platform to legs to be pre‐loaded to establish ground resistance levels.

Position and drive outer steel liner into overburden sediments – shaft casing for machine drill.

Use machine drill with slurry return to jackup barge platform.

Add (weld) additional lengths to outer steel liner casing.

Progress and complete drilling – remove machine drill.

Insert inner steel liner.

Tremie in concrete between inner and outer liner (and RC cages if necessary).

Inner liner reaming exercise to drill 2 – 3 m plug shaft below diffuser shaft liner for eventual

breakthrough into tunnel below.

Fit out and cap diffuser shaft with new diffuser head and isolation mechanism for re entry

activities associated with the connection operation to the underlying tunnel.

7.12 Survey

The shaft construction has to be monitored using geodetic measurements during the entire

construction period.

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Section 8 Summary

8.1 General

This report provides a concept of design for the long sea outfall tunnel and its linked onshore inlet

and offshore outlet diffuser shafts comprising the main depths, vertical alignments, dimensions and

seizing of its elements. This approach is based upon rough static analyses and/or practical

experiences gathered from similar projects.

More detailed analysis and concept design work will be essential to inform the baseline tender

reference design and tender specifications.

In order to identify the final tender specification requirements and the design needs the following

reports have to be carried out:

geotechnical baseline report (GBR),

baseline tender reference design (BTRD)

8.2 Geological Risk

Site investigation has proven the general technical feasibility of the project. However there are still

some key issues related to ground conditions which are unresolved. Geological risks and related costs

are strongly linked to the density of site investigation borings along the tunnel route. The “normal”

distance of borings in land‐based tunnelling projects normally does not exceed ca. 200 m in distance.

Sections of homogenous conditions require less dense SI borings, sections of heterogeneous

conditions more dense SI borings.

Unfortunately, the average distance between borings in this project is ca. 900 m which is

comparatively high. However, this final spacing was dictated by a combination of cost constraints but

more importantly operating conditions within Dublin Bay – including Environmental/ Operational/

Marine shipping.

On balance, but with a few exceptions, the findings from the marine SI indicate that for the most part

the 900 m spacing was adequate given the homogeneous nature of the underlying bedrock.

However, there are two locations in particular along the tunnel route whereby an increased

geological risk still remains:

Between Onshore shaft location and BH M 05.

Whilst very poor bedrock conditions have been identified in the onshore borings at both BH O

01 and BH O 02, the bedrock conditions at BH M 05, which is located in the marine waters

some ca. 1,000 m due East, are fair to good.

Geophysical survey investigation results show heterogeneous conditions up to the shoreline

but could not provide detailed prediction of their extent and their properties along the

potential tunnel route. A more dense site investigation over this 1km length could reduce the

geological risk significantly and thereby provide greater certainty in contract tendered costs.

However, there are significant environmental constraints/restrictions in this area which is both

a designated Special Area of Conservation and a Special Protected Area. These

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constraints/restrictions are the reason for the current lack of marine boreholes West of BH O

05. At this stage there is no evidence that these constraints/restrictions will be lifted to

accommodate further drilling.

Between boreholes BH M 08 and BH M 21

The distance between these two boreholes is ca. 2.5 km in length. No borings were undertaken

over this length, which bisects across the main Dublin Bay shipping lane, because of

restrictions imposed by Dublin Port. To date Dublin Port have resisted requests to drill along

this section.

In tunnelling terms this is an extremely long uninvestigated section which results in a

significant geological risk. A more dense SI along this length would reduce the geological risk

significantly. It is unlikely however that the Dublin Port will permit any infill borehole drilling

along this length.

A change in bedrock strata has also been identified between BH M 08 and BH M 21. Therefore,

it is highly likely that transition zone may be present along this length. The location and

conditions/properties of this suspected transition zone are unknown.

Other changes in strata may also occur within this 2.5 km undrilled length.

Whilst to date the preliminary geophysical survey investigation results have been inconclusive

in the marine environment in Dublin Bay because of the strata encountered it would be

prudent to investigate if alternative geophysical survey techniques are available to assess this

2.5 km length.

Investigations whilst tunnelling as described above are possible but reduce the advance rates

and leave a greater uncertainty to the construction process – and will inevitably result in

higher contract costs..

8.3 Diffuser Shaft Location

Ground conditions at the potential diffuser location B3 have been investigated by a single one boring

(BH M 11). If the location does not change significantly, this will be sufficient as the main parameters

of the soil and rock properties can be inferred from other borehole results.

The issue of jackup barge legs sticking in the marine muds/sediments must be clearly presented in

the tender documents with all risk transferred to the contractors. Tenderers must be required to

respond with a comprehensive method statement in their returned tender detailing how they intend

to plan for, deal with and resolve this issue in the event that jackup barges become stuck. The

method statements must include a detailed recovery plan for how the legs will be removed if they

become stuck – including techniques to be used such as air/water injection through the legs etc.

Consideration should be given in the tender documents to permitting the contractor to advance an

independent SI around the proposed diffuser site as part of the early contract start‐up operations

before the jackup barge/marine equipment is specified and mobilised.

The seabed local to the proposed diffuser shaft should be resurveyed – side sonar, magnetometer

before a contractor mobilises marine construction equipment.

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The proposed location for the diffuser, B3, is subjected to geomorphological sea bed movements

during various marine/tide/storm conditions. Although the extent or frequency of these events is

unknown the existence of seabed dunes (and the movement of such dunes) has long been known

about in the Bay close to Burford Bank. The presence of these seabed dunes has also been identified

during the bathymetry survey which has been undertaken as part of the marine site investigation.

The presence of this type of seabed movement may lead to large buildup of materials around the

jack‐up legs during the construction period (which may last up to six months. This issue must be

clearly presented in the tender documents with all associated risks transferred to the contractor.

8.4 Additional Site Investigation

If possible we recommend that some local additional infill site investigation works should be

undertaken including:

Marine geophysics (This is technically feasible and will be permitted)

Intertidal geophysics (This is technically feasible and will be permitted)

Onshore geophysics (This is technically feasible and will be permitted)

Cored borings between onshore shaft location and BH M 05 (This is technically feasible and

unlikely to be permitted)

Cored borings between boreholes BH M 08 and BH M 21 (This is technically feasible but

unlikely to be permitted)

Install a well at the onshore site location (This is technically feasible and likely to be permitted)

Though to date Dublin Port have refused requests to drill between borehole BH M 08 and BH M 21

we recommend that one final attempt should be made to engage with Dublin Port (to advise them of

the site investigation findings to date) to drill over this length. If permitted, between 4 and 8 borings

should ideally be obtained over this length.

Additional borehole and laboratory testing should be undertaken to determine the geotechnical and

hydrogeological properties for any additional boreholes.

8.5 Risk Assessment

It is recommended that a risk assessment workshop is held to undertake a comprehensive review which deals with all aspects of Health & Safety issues regarding tunnel alignment, construction sequencing, construction methods, etc.

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Section 9 Conclusions A comprehensive site investigation was completed between September 2010 and September 2011.

The coverage of the site investigation was constraint because of significant environmental and operational constraints in Dublin Bay.

Following a preliminary review of the site investigation results a concept design has been developed for the onshore shaft construction, the tunnel alignment and the offshore shaft construction.

At this stage of the concept design work shows that the tunnel system can be constructed. The key issue however is the scale of geological risk remaining.

We believe that the geological risk can be reduced significantly if some further site investigation work is undertaken identified under section 8.

Due to environmental and operational constraints scope for undertaking further site investigation work may be limited. This matter needs to be investigated with the relevant statutory authorities and the Dublin Port authorities and the ESB.

A geotechnical baseline report (GBR) must be prepared based upon the findings from the site investigation. This report would be updated in accordance with the findings from any additional site investigation work which may be undertaken.

A baseline tender reference design (BTRD) must be prepared based upon the findings from the site investigation. The BTRD will include the tunnelling baseline report. This report would be updated in accordance with the findings from any additional site investigation work which may be undertaken.

The concept design presented in this report should be refined following a detailed interpretation of the site investigation works done to date and any future site investigation works.

The refined concept design will be incorporated within the finalized baseline tender reference design included in the tender.

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Appendix E Planning and Policy Context

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Appendix F Human Beings

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