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Before a Board of Inquiry

MacKays to Peka Peka Expressway Proposal

Under the Resource Management Act 1991

In the matter of Notice of requirement for designation and resource consent

applications by the NZ Transport Agency for the MacKays to

Peka Peka Expressway Proposal

Applicant NZ Transport Agency

Requiring Authority

Statement of Evidence of

Dr Hugh Edward Cherrill

(Ground Settlement)

Dated 4th October 2012

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TABLE OF CONTENTS

QUALIFICATIONS AND EXPERIENCE .............................................................................................. 3

SCOPE OF EVIDENCE ..................................................................................................................... 3

EXECUTIVE SUMMARY ................................................................................................................. 4

GROUNDWATER DRAWDOWN ..................................................................................................... 5

SOURCES OF SETTLEMENT ............................................................................................................ 6

CONSOLIDATION OF THE PEAT ..................................................................................................... 7

PEAT SHRINKAGE AND OXIDATION ............................................................................................. 10

COMBINED SETTLEMENT ............................................................................................................ 12

EFFECTS OF SETTLEMENT ............................................................................................................ 13

MONITORING ............................................................................................................................ 16

CONTINGENCY MEASURES AND MITIGATION ............................................................................. 16

CONCLUSIONS ........................................................................................................................... 19

FIGURES ..................................................................................................................................... 21

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STATEMENT OF EVIDENCE OF DR. HUGH EDWARD CHERRILL ON BEHALF OF SAVE KAPITI

INCORPORATED

QUALIFICATIONS AND EXPERIENCE

1. My full name is Hugh Edward Cherrill.

2. I have an Honours degree in Civil Engineering and a PhD in Geotechnical Engineering from the City

University (London). I am a member of the Institution of Civil Engineers (UK) and I am a Chartered Civil

Engineer, Chartered Environmentalist and a Specialist in Land Condition.

3. I have 25 years experience in geotechnical, civil and environmental engineering in the UK, USA, Ireland,

New Zealand and elsewhere. My experience covers most ground related aspects of civil engineering

projects from site investigation through design and construction.

4. I am very familiar with the Kapiti district. I have lived in the district for nearly five years and have

worked professionally, based in Wellington, for the same period of time.

5. I confirm that I have read the “Code of Conduct for Expert Witnesses” contained in the Environment

Court Consolidated Practice Note 2011 and I agree to comply with it as if this Inquiry were before the

Environment Court. My qualifications as an expert are set out above. Other than where I state that I am

relying on the evidence of another person, I confirm that the issues addressed in this brief of evidence

are within my area of expertise. I have not omitted to consider material facts known to me that might

alter or detract from the opinions expressed.

SCOPE OF EVIDENCE

6. My evidence will deal with the following matters:

6.1. Groundwater drawdown predicted to extend beyond the footprint of the proposed expressway

construction. This includes drawdown due to temporary dewatering during construction and

long term drawdown resulting from changes in ground conditions caused by construction of the

road. In matters relating to groundwater modelling I rely on the evidence of Ms Helen Rutter.

6.2. Settlement of the ground surface outside the footprint of the proposed expressway construction

resulting from this groundwater drawdown.

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6.3. The potential effects of settlement.

6.4. Monitoring of groundwater and settlement.

6.5. Contingency measures and mitigation.

EXECUTIVE SUMMARY

7. I have reviewed the evidence of Mr Alexander and the documents to which he refers in relation to the

above issues. In making this review I have drawn on the evidence of others and information from

published literature where appropriate.

8. Ground settlements will be caused by drawdown of the groundwater level. The magnitude of

groundwater drawdown is a primary input into estimation of likely ground settelement. Based on the

evidence of Ms Helen Rutter I consider that there is considerable uncertainty in the estimated

groundwater drawdown. There is therefore the possibility that groundwater drawdown may have been

underestimated. Any underestimation of groundwater drawdown will result in underestimation of

ground settlements.

9. I consider that consolidation settlements may be significantly underestimated as a result of the peat

compression parameters and assumptions used in the settlement calculations described in Technical

Report 35.

10. I consider that peat shrinkage and, in particular, peat oxidation related settlement may be significant

and occur over a timeframe of many years following the construction of the Expressway.

11. As a result of potentially significantly greater settlements than predicted, I consider that the adverse

effects on buildings, services and transport infrastructure may be more significant than has been

recognised.

12. Due to natural variation in groundwater level, and probably ground surface level, I consider that reliable

warning of adverse effects from monitoring of groundwater levels and ground levels may not be

achieved as effects resulting from construction of the Expressway are likely to be masked by seasonal

variations. Additionally, as a result of seasonal fluctuations and the long timeframe over which some

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settlement processes occur, adverse effects may not manifest themselves during the period of post

construction monitoring proposed. Significant adverse effects may however occur over a longer

timeframe.

13. Even if warning of adverse effects is interpreted from monitoring, I consider that the proposed

contingency measures are not implementable as they consist primarily of changing the method of

construction which is clearly not possible after construction is completed.

14. It is therefore my opinion that there is considerable uncertainty with regard to the likely ground

settlement that may occur as a result of construction of the Expressway. I consider that the ground

settlements predicted are unconservative and that ground settlement and the associated adverse

effects may be more significant than is presented in Mr Alexander’s evidence and the supporting

documents to which he refers. I further consider that practical contingency/mitigation measures have

not been identified to mitigate adverse effects should they occur.

GROUNDWATER DRAWDOWN

15. Ms Ann Williams states in her evidence (at paragraph 83) that peat treatment (either surcharging or

excavation and replacement) will alter groundwater levels by typically less than 0.3m (but up, to 0.5m)

immediately adjacent to the Expressway, reducing to 0.1m at a distance of 50m to 70m, from the edge

of the embankment. These changes in groundwater level will be permanent. NZTA’s Technical Report

21, in Appendix E, includes a recommended groundwater drawdown profile for use in settlement

analyses. This design groundwater drawdown profile has been used to calculate settlements (refer

Technical Report 35, Section 4.4.1).

16. The groundwater modelling used to derive the ‘design’ groundwater drawdown profile has been

assessed by Ms Helen Rutter, and her conclusions are set out in her evidence.

17. Ms Rutter concludes, at section 32 of her evidence, that;

‘In summary, the calibration of the models is not good, and not all measures of calibration that should

be provided have been provided. The results that have been provided, suggest that little confidence

should be placed on the models in terms of interpreting water levels to within less than a metre’.

18. Based upon the evidence of Ms Rutter I conclude that there is considerable uncertainty in the

predictions of the magnitude of drawdown likely to be caused by the Expressway. In particular, the

design groundwater drawdown profile (Technical Report 21, Appendix E), predicts groundwater

drawdown of up to 0.3m. This predicted magnitude of groundwater level change is considerably smaller

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than the degree of uncertainty interpreted to exist in the modelling by Ms Rutter. Given that the design

groundwater drawdown profile is used as the basis for calculation of ground settlements this

uncertainty in the likely magnitude of groundwater drawdown raises the possibility that drawdown may

be greater than has been used in the calculation of settlements. This conclusion casts considerable

doubt on the validity of the calculated consolidation settlements.

19. Temporary groundwater drawdown will also occur during construction as a result of dewatering of

excavations. It is proposed that excavation to remove peat may extend to 3m below ground level. In

some locations the groundwater level is close to the ground surface and therefore excavation may

extend up to 3m below groundwater level.

20. Dewatering of the excavations will be required to allow backfilling with dune sand. Dewatering is stated

to be by pumping directly from sumps in the base of the excavations (refer to Section 10.0 Glossary, of

Technical Report 21). The base of excavations, after removal of peat will be in dune sand. Technical

Report 21 notes, at section 3.7, that ‘boiling’ of sand in the base of excavations may occur due to

upward inflow of groundwater.

21. If ‘boiling’ occurs, and I consider that it may in some situations, this will result in instability of

excavations, loosening of the sand deposits and difficulty keeping the excavation dry. This will need to

be avoided to progress the excavation and filling works.

22. In some circumstances it is possible that a different dewatering technique such as wellpointing may be

required. Such a technique would result in a greater degree and duration of drawdown than currently

envisaged. This issue needs to be addressed and detailed excavation methodologies confirmed so that

the assessment of potential adverse effects can be confirmed to be appropriate and reliable.

SOURCES OF SETTLEMENT

23. I will consider three sources of settlement in areas outside of the immediate footprint of the

Expressway construction that may result from drawdown of the groundwater level. The three sources

of settlement are:

Consolidation of Peat

Shrinkage of Peat

Oxidation of Peat

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These are discussed in the following sections.

CONSOLIDATION OF THE PEAT

24. Consolidation of soils, including peat, is the result of the expulsion of water from the soil matrix as load

is applied to the soil. Applied load is initially taken by water in the pores in the soil and the pore water

pressure increases. If the soil is free to drain, water is squeezed from the soil and the load is then

transferred to the soil structure, the stress in the soil is increased and the soil compresses. Similarly

when the water pressure in the soil is reduced, for example by drawing down the groundwater level,

water drains from the soil, load is transferred to the soil matrix, stress in the soil increases and the soil

compresses.

25. When a soil is loaded for the first time it compresses and water is forced from the soil pores. If this soil

is then unloaded it will swell, drawing water into its pores. However it will not swell back to its original

volume. On reloading its compression is also reduced compared to its first loading until the applied

loading reaches the load to which it was previously loaded, known as the pre-consolidation pressure.

This is shown in a simplified form in Figure 1.

26. In Figure 1 the line representing the change of soil volume with increasing stress in the soil for the first

time loading is known as the normal or virgin consolidation line, while the lines representing unloading

and reloading of the soil are known as recompression lines. When plotted on a log scale these lines are

straight and may be assigned compression parameters related to their gradients. Consequently the

compression parameter Cc for first time loading is greater than the recompression parameter Cr for

reloading reflecting the greater degree of volume change experienced on first loading.

27. I have reviewed the data from site investigations on the route of the Expressway (refer section 1.3 of

Technical Report 36) that have been used to derive the peat compression parameters used in the

estimation of settlements (refer Table 9 of Technical Report 35 for derived parameters).

28. I agree that the adopted compression index parameter Cc/1+eo of 0.35 is generally reasonable although

the data indicate that for some peat materials it may be significantly higher, by a factor of up to 60%.

29. I agree that the adopted recompression index parameter Cr/1+eo of 0.06 is also generally reasonable,

however the data indicate that for some peat materials it may be higher by a factor of up to about 25%.

30. I agree that the adopted pre-consolidation pressure of 15kN/m2 is generally reasonable however the

test data indicate that it is variable and may be as low as 10kN/m2 for some of the peats. It should be

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noted that the transition from the recompression line to the virgin consolidation line is generally not as

distinct as indicated in Figure 1 and determination of the pre-consolidation pressure is often difficult

particularly when the material tested has been disturbed during sampling.

31. Groundwater levels have been shown to vary seasonally (Refer section 2.5 of Technical Report 21) and

naturally in response to weather (i.e. rain) patterns. The degree of groundwater level fluctuation will

vary from year to year. In a dry summer groundwater will be lower than in a wetter summer.

32. It is reasonable to assume that in a very dry summer groundwater levels will drop to levels equal to

their lowest levels in the past. In this condition the vertical stress in the peat is likely to be equal to the

pre-consolidation pressure as a result of the increased stress in the soil at lower groundwater levels as

explained in section 24 above. The pre-consolidation pressures that may be interpreted from laboratory

testing of samples and the measured groundwater levels reported are consistent with this

interpretation.

33. It may be expected that there is also a seasonal movement of the ground surface where it overlies peat.

In the summer when groundwater levels drop there will be a corresponding settlement of the ground

surface which will recover in the winter as groundwater levels rise again. The magnitude of these

movements is likely to be small as the pre-consolidation pressure will not be exceeded and

recompression soil parameters will apply.

34. Any additional groundwater drawdown below the lowest previous groundwater level will result in the

pre-consolidation pressure in the peat being exceeded and compression will take place along the virgin

compression line in Figure 1, this being defined by the compression index parameter Cc/1+eo rather

than the recompression index parameter Cr/1+eo.

35. The assumptions made in the calculations presented in Technical Report 35 include an initial

groundwater level of 0.5m below ground level (refer section 4.4.3 of technical report 35). This

assumption means that stresses in the peat for the drawdowns modelled will not exceed the assumed

pre-consolidation pressure of 15kN/m2. Consequently the settlements calculated are all based upon the

recompression index parameter. As explained above this is appropriate for estimation of settlements

resulting from normal seasonal variation. However if groundwater drawdown, as a result of the

construction of the Expressway, causes drawdown of groundwater levels below the existing range of

seasonal fluctuation, use of the compression index parameter is appropriate in settlement calculations

and calculated settlements will be significantly larger.

36. The compression index parameter interpreted from the available data and presented in Technical

Report 35 (Table 9) is about six times greater than the recompression index parameter. This means that

for the same stress change in the peat the settlement will be six times greater. For particularly

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compressible peat, the compression index parameter may be as much as ten times higher than the

recompression index parameter used in the calculations with calculated settlement being

correspondingly greater for the same stress increment.

37. However, soil stresses will be larger for the scenario of a low dry weather groundwater level than those

assumed in the calculations presented in Technical Report 35 based upon a groundwater level of 0.5m

below ground level. Because of the log nature of the relationship between soil volume and stress,

settlements are unlikely to be as much as 6 to 10 times greater than those calculated using the

recompression index parameter at the lower stresses assumed to apply. However I estimate that

settlements may be as much as two to four times greater than those presented in Technical Report 35 if

the pre-consolidation pressure is exceeded, depending upon the compressibility of a particular peat

deposit.

38. In addition to the assessment described above, I have considered the magnitude of likely settlements

based upon values of the ‘coefficient of compressibility’, mv, of the peat reported in the factual

geotechnical investigation reports relating to the Expressway route. Technical Report 35, in section 4.2,

indicates that use of the ‘Mv approach’ provides a less good fit to historic data and field trials than the

compression index approach and it has therefore not been used in the calculation of settlements

presented in Technical Report 35.

39. mv, is a parameter that defines a linear stress/settlement relationship. As discussed above, the

relationship is actually a non-linear (log) relationship. Consequently a particular value of mv determined

over a specific stress range is applicable only to that stress range and mv changes for different stress

ranges. Consequently it is difficult to apply the mv approach to situations where there are large changes

of stress, such as construction of an embankment. However, for situations where there are only small

changes of stress, such as result from groundwater drawdown, this approach is valid if mv values

appropriate to the stress range being considered are used.

40. I have made an assessment of possible settlement using the mv approach and adopting appropriate mv

values from the available test data. This assessment yields the same conclusion as reached above, i.e.

consolidation settlements may be of the order of two to four times greater than predicted in Technical

Report 35.

41. In summary, should the predicted drawdown of the groundwater table coincide with naturally low

groundwater levels resulting in drawdown of the groundwater level below the normal natural range of

groundwater level fluctuation, consolidation settlements of the order of two to four times larger than

those predicted in Technical Report 35 and reported in Mr. Alexander’s evidence may occur.

Furthermore these settlements will be in excess of the greatest normal seasonal ground movements

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experienced as a result of natural groundwater fluctuations and will be settlements not previously

experienced by buildings, services and transport infrastructure.

42. As noted previously the settlements presented in Technical Report 35, and on which Mr. Alexander’s

evidence relies, are based upon the ‘design’ drawdown profile derived from groundwater modelling

which Ms Helen Rutter has concluded may not be accurate (refer section 17 above). If groundwater

drawdown is greater than estimated, consolidation settlements will be correspondingly greater.

43. Taking all of these factors into account I consider that the consolidation settlements presented in

Technical Report 35 and relied upon by Mr Alexander are unconservative and may be significantly

underestimated.

44. For example, if groundwater drawdown is greater than expected by 50% and the pre-consolidation

pressure is exceeded, locations where 10mm and 20mm settlement are predicted in Technical Report

35 may experience settlements in excess of 50mm and 100mm respectively.

45. Another factor of considerable importance that arises from the above review of likely ground

settlement is the timing of this settlement. Because significant settlement (in excess of normal

seasonal movements) may not occur until a particularly dry summer it is possible that settlements of

the magnitude that are possible may not occur for a period of years after construction. This is of

particular significance in terms of the proposed monitoring and implementation of contingency

measures. This matter is discussed later in my evidence.

PEAT SHRINKAGE AND OXIDATION

46. When the water table is lowered peat that was below the water table is drained and becomes

unsaturated and the organic fibres can dry out and shrink leading to a loss of volume and settlement of

the ground surface. Before drainage when the peat is saturated the conditions within the peat are

anaerobic (no oxygen) and degradation of the peat is very slow. When the peat becomes unsaturated it

also becomes aerated. Biochemical aerobic degradation processes oxidise the organic matter to carbon

dioxide and water. Aerobic degradation leads to loss of peat volume and settlement of the ground

surface.

47. Whereas consolidation settlement takes place quite quickly after groundwater drawdown, shrinkage

and particularly oxidation of peat takes place more slowly. Oxidation of peat results in ongoing

relatively slow settlement for many years.

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48. These processes have been documented12345

around the world, largely in relation to drainage of peat

land for agricultural use. Oxidation of peat above the groundwater level will ultimately lead to its

complete loss and the combined effects of consolidation, shrinkage, compaction due to agricultural

activities and oxidation have resulted in metres of settlement in agricultural peat lands in the UK,

Netherlands, Italy, Indonesia, USA and other parts of the world as the groundwater level is lowered

repeatedly to maintain drainage for agricultural use as the land subsides.

49. Settlement of peat agricultural land is also a problem in New Zealand. Settlement of peat farmland in

the Waikato region is occurring at rates of 18-33mm per year as a result of oxidation of peat6 resulting

from drainage and groundwater drawdown. Environment Waikato’s publication ‘For Peat’s Sake, Good

Management Practices for Waikato Peat Farmers’6, recommends that deep drainage and ground water

drawdown is avoided to minimise peat oxidation and settlement. The relationship between the depth

to the water table (depth of peat exposed to aeration above the water table) and the rate of settlement

due to oxidation of peat is well established.

50. One of the conclusions of Special Publication SJ 2007- SP5, Influence of Water Levels on Subsidence of

Organic Soils in the Upper St John’s River Basin4, is that any drawdown of the groundwater level in peat

will result in additional shrinkage and oxidation leading to settlement. Although peat may be kept wet

near the groundwater level by capillary action and by fluctuating groundwater levels rewetting the peat

periodically any reduction in the mean groundwater level will lead to some degree of shrinkage and

oxidation of peat.

51. In the agricultural situations referred to above, the largest contribution to settlement over many years

is oxidation, as ultimately all the peat can be lost to oxidation. One study in Sumatra5 concluded that

oxidation accounted for 92% of the total settlement in the 18 years after drainage. Volk (1972) is

reported4 as stating that microbial oxidation of peat soils can contribute from 58% to 73% of the total

settlement. Oxidation is estimated to have contributed to an average of 37% of the total settlement of

agricultural peat lands in the Waikato6 region over a period of 40 years.

1 Subsidence due to peat decomposition in the Netherlands, Kinematic observations from radar

interferometry. M Caro Cuenca and R Hansen, Delft Institute of Earth Observation and Space Systems, 2008. 2 Carbopeat Technical Report 3: Assessment of risk and vulnerabilities of tropical peatland carbon pools:

Mitigation and restoration strategies. Carbopeat, University of Leicester, UK. 3 The legacy of wetland drainage on the remaining peat in the Sacramento-San Joaquin Delta, Calfornia, USA. J

Drexler, C de Fontaine and S Deverel. Wetlands, Vol. 29 No.1, March 2009, pp372-386. 4 Special Publication SJ2007-SP5. Influence of water levels on subsidence of organic soils in the Upper St Johns

River Basin. St Johns Water Management District, Florida, 2006. 5 Recent findings on subsidence and carbon loss in tropical peatlands : Reducing uncertainties. A Hoojer, S

Page, J Jauhiainen, W lee and X Lu. Workshop on ‘Tropical wetland ecosystems of Indonesia : Science needs to address climate change adaption and mitigation’, Bali, 11-14 April 2010. 6 For peats sake, Good management practices for Waikato peat farmers. Environment Waikato, June 2006.

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52. The rate of oxidation in the context of the Expressway is difficult to estimate, however I conclude that,

in the long term, settlement resulting from oxidation of peat has the potential to be equal to or greater

than that resulting from consolidation.

53. Mr Alexander, in section 46 of his evidence, acknowledges that ‘permanent lowering of the

groundwater level will potentially result in drying induced volume change settlement.’ However, he

goes on to say that ‘complete drying is not expected to occur’ and he concludes that ‘the drying induced

volume change is expected to be relatively small in comparison with the settlements resulting from

consolidation due to groundwater lowering’. I agree that the degree of shrinkage is difficult to quantify

however, based upon the sources referred in this document I consider that it may be significant.

54. In Section 85.2 (a) of his evidence, Mr Alexander responds to a Section 92 request from the Board of

Inquiry relating to the susceptibility of organic matter in the peat to biological oxidation. Mr Alexander

quotes a reference that states that oxidation of peat has been found in the Netherlands to contribute

around 50% of the total subsidence arising from drainage. This is consistent with my preceding

assessment. However, Mr Alexander goes on to say that as the predicted groundwater drawdown

remains within the current seasonal range, oxidation and additional drying related settlement is not

expected. This view is not consistent with experience reported in the literature discussed above. Any

lowering of the mean groundwater level is likely to expose more peat to oxidation processes than is

currently the case, and is likely, over time, to result in oxidation and settlement.

COMBINED SETTLEMENT

55. Considering the preceding discussion of settlements resulting from consolidation, shrinkage and

oxidation it is clear that there is considerable uncertainty in the magnitude of actual settlements that

may occur. However it is also clear there is potential for settlements to be greater than estimated in

Technical Report 35.

56. Considering the same examples as in section 42 above, of locations where 10mm and 20mm of

settlement are predicted in Technical Report 35 and taking into account the combined effects of

consolidation, shrinkage and oxidation, the actual settlements may be of the order of 100mm and

200mm respectively i.e. ten times more than estimated in Technical Report 35.

57. As discussed in the preceding sections settlement of this magnitude may take many years to develop.

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EFFECTS OF SETTLEMENT

58. The effects of settlement have been assessed in Technical Report 35 which is referred to by Mr.

Alexander in his evidence. The approach that has been taken is described in section 4.8 of Technical

Report 35. The approach used is to estimate settlements resulting from embankment construction and

groundwater drawdown and to develop contour plans and sectional profiles of settlement. These

contour plans and section profiles have been used to assess horizontal strains that may be induced in

buildings and potential changes in grade and damage to services, roads and the railway.

59. The effects on buildings have been assessed in Technical Report 35 using the method developed by

Burland (1997) entitled “Assessment of risk of damage to buildings due to tunnelling and excavation”

and it is noted that this is a widely used approach. As implied by the title of Burland’s paper, this

assessment approach is most applicable to ground settlements resulting from deep seated causes such

as tunnelling and excavations that result in smooth variation of ground surface settlement e.g. a

smooth dished ground surface settlement above a tunnel.

60. In this instance the Burland methodology has been applied to the smooth profiles of predicted

settlement at various sections along the Expressway as presented in Appendix F of Technical Report 35.

61. The conclusions drawn on the degree of damage likely to result from the predicted settlements are

based upon the implicit assumption that the settlement of the peat deposits varies smoothly with

distance from the Expressway as shown on the cross-sections.

62. The actual settlement is unlikely to be characterised by such a smooth profile. Locally the settlement of

the peat, in response to drawdown of the groundwater will vary due to variations in the thickness and

compression properties of the peat. The smooth settlement profiles presented in Appendix F of

Technical Report 35 assume a uniform peat thickness and constant peat properties. These local

differential settlements, that may occur over small distances, are not considered by application of the

Burland method to the smooth profiles of predicted settlement.

63. This deficiency in the model used to assess settlement effects on buildings is recognised in Technical

Report 35 in section 5.0 where it is stated that;

‘The nature and thickness of these deposits is highly variable and is expected to result in variation of the

peat settlements. Typically, the differential settlements are estimated to be in the order of half of the

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calculated settlement magnitude. These potential differential settlements are an important

consideration in assessing the settlement effects.’

64. I agree that these local differential settlements are important in assessing effects as they have the

potential to cause much more severe distortion of buildings, services and transport infrastructure than

the smooth conceptual settlement profiles indicated by the sectional profiles in Appendix F of Technical

Report 35.

65. Estimation of local differential settlements as 50% of the total settlement is a commonly used ‘rule of

thumb’. However it should be noted that differential settlement can exceed 50% of the total

settlement. For example, where the area under consideration (e.g. a building or road) lies over the

boundary between an area with peat and one without peat, differential settlement may approach 100%

of the total settlement.

66. The conclusions of the assessment of effects on buildings are reported in section 6.2.1 of Technical

Report 35. Taking into account the small settlements predicted and the distance over which they are

predicted to decrease with distance from the Expressway it is not surprising that that the assessment

(by the Burland method) indicates negligible damage is likely. As discussed above this conclusion does

not take account of local differential settlements. This is addressed in section 6.2.1 of Technical Report

35 where it states that;

“The existing residential dwellings are located where estimated settlements are less than 25mm and

typically less than 12.5mm. As such, the estimated differential settlements resulting from the variable

nature of the peat deposits are relatively small. The total and differential settlements are consistent

with the assessed ‘negligible’ effects”

67. For the magnitudes of total settlement predicted in Technical Report 35, the conclusion that local

differential settlements are unlikely to result in damage is reasonable. However for larger total

settlements this conclusion may no longer be valid.

68. For buildings the effect of local differential settlement is often assessed on the basis of considering the

estimated differential settlement as a proportion of the length over which it occurs, commonly known

as the angular distortion. Eurocode 77, recommends a limit on angular distortion, to prevent damage, of

1 in 500. Although this approach is not as sophisticated as the Burland method, it is very widely used

and provides a simple method of assessment of the acceptability of local differential settlements.

7 EN 1997-1 : 2004 Eurocode 7 Geotechnical design – Part 1 : General Rules, Annex H. European Committee for

Standardisation (CEN) : Brussels.

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69. Applying this criteria to a 10m long building (typical of a residential dwelling) with greater settlement at

one end of the building than the other, the maximum acceptable difference in settlement (the

differential settlement) is 20mm. Over this sort of distance the 50% rule of thumb referred to in

Technical Report 35 is generally reasonable and therefore a maximum total settlement of about 40mm

is indicated as being the limiting total settlement before damage may occur to buildings of this size.

Eurocode 7 recommends a maximum acceptable total settlement of 50mm and a maximum differential

settlement of 20mm.

70. Recently published guidance from the New Zealand Ministry of Business, Innovation and Employment8

relating to rebuilding in Christchurch requires that anticipated settlement (from a liquefaction event)

should not exceed 15mm for a conventional shallow house foundation.

71. If the magnitude of settlement that I consider is possible (refer section 56 above) is assessed on these

criteria, it is clearly unacceptable and likely to result in building damage. This would be likely to

manifest itself as cracking of masonry walls and interior finishes. In fact it would only require an

increase in settlement over those predicted in Technical Report 35 by a factor of two to three times for

settlements to be potentially damaging. This is considerably less than the uncertainty in predicted

settlements that I have identified.

72. It should be noted that the preceding discussion on the effects of settlement on buildings relates to

buildings founded on conventional shallow foundations. It is likely that some dwellings are founded on

timber piles driven through the peat. Piled buildings are not considered in Technical Report 35. Piled

buildings should not settle if the piles are well founded below the peat stratum, however settlements of

the magnitude possible could still be damaging as a result of the settlement of the ground away from

the building structure. Such settlement can result in damage to services where they enter the building

and features such as steps.

73. Settlements of the magnitude that I consider are possible may also result in unacceptable changes in

the gradient of drains and distortion of road surfaces (similar to those on the Raumati straights section

of SH1 where it lies over peat deposits).

74. Based upon this assessment I consider that the possible adverse effects of settlements resulting from

groundwater drawdown caused by construction of the Expressway may be significantly greater than

concluded by the assessment in Technical Report 35.

8 Guidelines for the investigation and assessment of subdivisions on the flat in Canterbury. Minimum

requirements for geotechnical assessment for land development (‘flatland areas’ of the Canterbury region). Ministry of Business, Innovation and Employment, New Zealand. September 2012.

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MONITORING

75. Mr Alexander refers to the proposed monitoring of ground settlements and groundwater levels set out

in Technical Report 35 (section 7), the Settlement Effects Management Plan (SEMP) and the

Groundwater (level) Management Plan.

76. As previously noted groundwater levels fluctuate on a seasonal basis and this is likely to result in

seasonal ground movements over peat areas. These seasonal variations in levels are likely to make

interpretation of monitoring data difficult as they may mask construction effects. For example, if

groundwater levels are naturally high at the time of construction, although drawdown may occur it may

remain within the natural seasonal variation range and therefore not trigger a response. Similarly as

discussed previously if groundwater drawdown remains within the past natural range of groundwater

levels, settlements are likely to be small.

77. However, in a subsequent drier year, perhaps after monitoring has ceased, drawdown may drop the

groundwater level to below the natural range resulting in significant settlement as discussed previously.

78. I therefore conclude that the proposed duration of monitoring (two and a half years for ground

settlement and three years for groundwater) may be insufficient to identify potentially damaging

groundwater drawdown and settlement.

79. As discussed previously consolidation is not the only potential cause of settlement. Oxidation of peat, in

particular, may also result in significant settlement. This is likely to take place over many years at a slow

rate and is very unlikely to be identified by the proposed monitoring.

CONTINGENCY MEASURES AND MITIGATION

80. Mr Alexander refers, in section 70 of his evidence, to the contingency measures presented in Technical

Report 35 that may be implemented in the event of ‘greater than predicted damage occurring’.

Technical Report 35 (section 7.2.2) and the Settlement Effects Management Plan (section 3.1.2) present

a number of contingency measures, although no detail is provided to confirm how effective they may

be and under what conditions they would be implemented. However, it is clear that their

implementation is proposed in response to monitoring i.e. they ‘can be implemented should the

measured settlements or their effects require it.’ (Technical Report 35, Section 7.2 and SEMP, Section

3.1). This reactive approach is confirmed in section 3.3 of the SEMP.

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81. Specific comments on the proposed contingency measures are provided below.

82. Change the ground (peat) treatment methodology.

82.1. It is proposed that the peat treatment method may be varied. Options presented include:

Substitution of the ‘excavate and replace’ construction method with the ‘preload and

surcharge’ method or vice versa.

Use of a permeable starter layer for embankment construction for the preload and surcharge

approach.

Use of alternative approaches such as a load transfer platform to avoid loading the underlying

peat.

Reduction of the loaded footprint of the embankment by use of geogrid reinforcement to

steepen embankment slopes.

82.2. Any of these changes in construction methodology would be significant and I do not understand

how they will be implemented in response to monitoring. Notwithstanding my previous

comments relating to the effectiveness of the monitoring, the implementation of these changes

in response to monitored effects seems impracticable as the effects will not be evident until

construction has been completed. These measures are therefore not implementable as

contingency measures to mitigate observed effects. They may however be effective as mitigation

or avoidance measures if implemented as part of a planned strategy to minimise, as far as

possible, groundwater drawdown, determined and set out in advance of construction and

incorporated into the design and construction of the Expressway.

83. Lining temporary or permanent cuts below groundwater level, including stormwater storage ponds.

83.1. Lining of cuts below the water table is likely to be effective in preventing significant drawdown of

the groundwater level once the lining is installed but is unlikely to be a viable contingency

measure implemented in response to monitoring data. Lining of cuts, whether temporary or

permanent, will require considerable additional work, materials, planning and design that would

be difficult to institute quickly in response to monitoring data.

83.2. I note that permanent lining of some of the stormwater storage ponds is proposed as part of the

design rather than as a contingency measure where the degree of drawdown has been assessed

to be unacceptable. No detail of the lining technique proposed is provided. Any lining used will

require sufficient weight to overcome uplift pressures from the groundwater and this will

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require over-excavation, below the level of the base of the pond. It will also require dewatering

of the excavation to allow installation of the lining system.

83.3. Such over-excavation and greater depth and duration of dewatering would be counterproductive

for a temporary excavation and I therefore consider that this is not a viable mitigation measure

for groundwater drawdown resulting from temporary excavations.

83.4. With regard to permanent excavations (stormwater ponds), the temporary excavation and

dewatering required to ensure stability of the permanent lining system and to allow construction

of the lining, needs to be identified and the potential temporary drawdown of groundwater

levels away from the excavation and associated potential settlements need to be assessed. It is

not clear that this assessment has been made and therefore the viability of this measure is

unclear.

84. Limit the length and drained duration of temporary excavations below the groundwater level.

84.1. This is sensible and good practice and should be part of the standard construction methodology.

It should not be a contingency measure to be implemented based on monitoring data but should

be implemented for all such excavations as routine practice. Whilst this practice will minimise

temporary drawdown of groundwater level away from the excavation, it will not prevent it.

85. Local groundwater cut off.

85.1. The construction of groundwater cut offs can be effective in limiting the extent and magnitude

of groundwater drawdown and settlement. However the effectiveness of this measure in

relation to the particular hydrogeological conditions prevailing and the activities causing

groundwater drawdown needs to be confirmed. For example, whilst a cut off around a

temporary excavation may limit drawdown away from the excavation, the installation of a cut

off may have other undesirable long term effects such as the damming of groundwater flows.

This method may therefore have some application, in principle, for temporary excavations but

its application to control of long term drawdown effects and the implications of the long term

effects of cut offs installed for temporary control are unclear.

85.2. Construction of a slurry cut off wall is a relatively rapid (although expensive) technique that

might, conceivably, be implemented in response to monitoring data. However it requires

specialist plant and materials and would need to be designed and planned in advance with well

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defined monitoring triggers and mobilisation timeframes. In practice this may be difficult to

achieve.

86. Recharge trenches/wells.

86.1. In principle, recharge trenches and wells could be used to introduce water to the ground to raise

groundwater levels drawn down by construction activities. I consider that this may be a practical

short term measure to overcome temporary construction drawdown, subject to development of

a detailed design and implementation plan. As with the other contingency measures identified it

would be best implemented as part of the excavation methodology rather than attempting to

implement it in response to monitoring. However I do not believe it would be a viable long term

solution to control ongoing drawdown of the groundwater level resulting from the permanent

expressway construction as it would require an ongoing supply of water, operation, maintenance

and monitoring. Even for the short term, there would need to be very careful design and

management.

87. In summary I do not consider that the contingency measures, proposed to be implemented in response

to monitored effects, are practical. Nor have they been demonstrated to be effective. They might

however form the basis of a design to avoid unacceptable groundwater drawdown and the associated

risk of damaging ground settlement, implemented as part of the road construction rather than as

contingency measures.

CONCLUSIONS

88. Based upon the preceding evaluation of the proposal it is my opinion that:

88.1. There is considerable uncertainty in respect of the likely ground settlements that may occur as a

result of the Expressway construction. Areas of uncertainty include:

The magnitude of groundwater drawdown

The magnitude of consolidation settlements related to uncertainty of peat compression

parameters and groundwater levels

The magnitude of local differential settlements related to uncertainty of variation of

peat thickness and compression properties

The magnitude of shrinkage settlement

The magnitude of oxidation settlement

The time over which settlement will occur

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88.2. Because of the uncertainty around estimation of ground settlements and an unconservative

approach to this uncertainty in the calculation of settlements and assessment of effects, I

consider that the risk of damaging ground settlements has been underestimated and the effects

of settlement may be more than minor.

88.3. Significant settlements and adverse effects, should they occur, may not manifest themselves for

a period of years after construction is complete and may therefore not be detected by the

proposed monitoring.

88.4. There are no practical contingency measures proposed that may be implemented, following

construction and in response to monitoring, to mitigate or avoid unacceptable adverse effects.

89. As a consequence of the uncertainty of likely ground settlements and associated adverse effects,

coupled with the lack of practical mitigation measures, it is my opinion that a precautionary approach

should be adopted towards ground settlement. As ground settlements are the result of groundwater

drawdown, I consider that a precautionary approach to groundwater drawdown is required. I

therefore consider that, if the Expressway is to be built, it should be designed to avoid the occurrence

of any significant changes in groundwater levels.

Hugh Edward Cherrill

4th

October 2012

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FIGURES

Figure 1 : Consolidation