Statement of Rebuttal Evidence of Kenneth John Read on ... · Statement of Rebuttal Evidence of...
Transcript of Statement of Rebuttal Evidence of Kenneth John Read on ... · Statement of Rebuttal Evidence of...
Before a Board of Inquiry Ruakura Development Plan Change
IN THE MATTER of the Resource Management Act 1991
AND
IN THE MATTER of a Board of Inquiry appointed under section 149J of the Resource Management Act 1991 to consider a Plan Change requested by Tainui Group Holdings Limited and Chedworth Properties Limited
Statement of Rebuttal Evidence of Kenneth John Read on behalf of Tainui Group Holdings Ltd and Chedworth Properties Ltd
16th April 2014
1
Introduction
1. My full name is Kenneth John Read. I am a Senior Engineering
Geologist with Opus International Consultants.
2. I have the qualifications and experience set out at paragraphs 2
and 3 of my statement of evidence in chief (EIC), dated 26th
February 2014.
3. My rebuttal evidence is given in relation to the Plan Change
requested by Tainui Group Holdings Limited and Chedworth
Properties Limited (the Plan Change).
4. I repeat the confirmation given in my EIC that I have read the
‘Code of Conduct for Expert Witnesses’ as contained in the
Environment Court Practice Note 2011 and that my evidence has
been prepared in compliance with that Code.
5. I attended expert witness conferencing held on 11 April 2014 in
relation to Geotechnical matters. I was a party to the outcomes
of that conference recorded in the joint witness statement dated
Friday 11 April 2014.
Scope of Evidence
6. In this statement of rebuttal evidence I will respond to the
evidence of:
(a) Mark Thomson Mitchell On behalf of Greame Goodwin and
himself (submitter No 106473), in particular paragraphs 12 and
13, paragraphs 15 to 19, and paragraphs 37 and 38.
(b) Mohammed Hassan on behalf of Waikato Regional Council
(submitter No 106894), in particular paragraphs 3.1(d), and 8.1 to
8.4 inclusive.
(c) William Roy Cowie on behalf of The Ruakura Residents Group
and himself (submitter number 106774), in particular paragraph
5.16.
2
7. The fact that this rebuttal statement does not respond to every
matter raised in the evidence of submitter witnesses within my
area of expertise should not be taken as acceptable of the
matters raised. Rather, I rely on my EIC, Opus’ technical reports
reference G3121, G3127 and G3134 and this rebuttal statement
to set out my opinion on what I consider to be the key
geotechnical issues matters for this hearing.
Mr Mark T Mitchell on Behalf of Mr Greame Goodwin and Himself.
(Submitter No 106473)
Number of soil test locations and presentation of findings.
8. In paragraph 15 of his evidence Mr Mitchell comments that “the
density of the soil test locations is sparse in relation to the size of
the development”. The geotechnical investigation carried out to
date has been a preliminary investigation to characterise the
ground conditions beneath the Stage 1 Area. The aim of this was
to determine if there are any major geotechnical constraints that
could prevent or have serious detrimental effects on the proposed
development. I consider the level of investigation and number of
soil testing points investgiated to be sufficient to have achieved
those aims and provide an appropriate level of information.
Paragraph 4 of the Introduction to the Opus Interpretative Report
(ref G3127) clearly states “Further investigation of each zone at
the site will be necessary to form in depth geotechnical
assessments and essentially to confirm and detail the design
measures required for the various developments. These
investigations may be required to be carried out over a number of
phases.”
9. With respect to the inclusion of geological maps and plans raised
by Mr Mitchell I confirm that these were considered during
preparation of the report but considered to be of little benefit in
this case. We instead relied on the preparation of geological
cross sections as we consider that those are the best means of
graphically demonstrating strata variation within each zone of the
Stage 1 Area investigated. I consider that the absence of such
geological plans makes no difference to the results of the
3
investigation and the conclusions and recommendations drawn
from them.
10. In paragraph 16 of his evidence Mr Mitchell states his opinion that
considerable emphasis has been placed on the residential areas
in contrast to a lesser emphasis on the inland port zone. Whilst
there may appear to be a greater density of test locations in the
residential zone the majority of these are scala auger bores,
which are shallow and used widely as the basis for residential
housing foundation design. Elsewhere in the Stage 1 Area
investigated greater emphasis has been placed on determining
the deeper geology and soil properties using boreholes and
CPTS which provide the more sophisticated and reliable levels of
information necessary for the level and types of development
proposed in those Areas. With respect to the interpretation and
reporting of the information obtained from the investigation I
consider that we have investigated all Areas appropriately.
Liquefaction of soils during a seismic event.
11. In paragraphs 12, 13, 17, 18 and 19 of his evidence Mr Mitchell
discusses the liquefaction assessment carried out to date.
12. In paragraph 12 Mr Mitchell compares the Stage 1 Area to low
lying areas of Christchurch which have been seriously adversely
affected by liquefaction following the recent series of seismic
events there. The geology, geomorphology and geography of
those parts of Christchurch to which Mr Mitchell refers are
significantly different from that at Ruakura, and I do not consider
it an approriate comparison.
13. In Paragraph 13 Mr Mitchell states that in his opinion there has
been insufficient investigation of the probability of liquefaction. In
the reports reviewed by Mr Mitchell the probability of liquefaction
has been assessed using the procedures described in
NZS1170:2004 “National Standard for Structural Design Actions,
Part 5: Earthquake Actions – New Zealand” to determine the
appropriate Peak Ground Acceleration (PGA), and current
industry standard computer software to calculate if, and to what
4
degree, any liquefaction could occur. This software in turn uses
current industry standard methods and procedures to calculate
the potential for liquefaction.
14. Our assessment methodology is consistent with the New Zealand
Geotechnical Society Guidelines1. We have also used 3 methods
of assessment:
(a) using Standard Penetrations Test results in boreholes, (SPTs);
(b) data from Cone Penetrometer Tests (CPTs); and
(c) shear wave velocity measurements using seismic CPTS.
15. The results obtained from each of these methods of assessment
were cross checked as a means of verifying the results. A
number of separate assessments were carried out. We have
also used seasonally adjusted groundwater levels in the
calculations. I therefore believe that in the reports presented to
date we have adequately assessed the probability of liquefaction
occurring. Notwithstanding this we recommended in our reports
that further assessment be carried out at detailed investigation
and design stage to further refine the liquefaction hazard and to
develop mitigation strategies. I consider this approach
appropriate.
16. In our reports there is comment on the estimated total and
differential ground surface settlements that may occur as a
consequence of liquefaction. However it has come to light in
further review for this evidence that there has been an error in
transposing data into the Interpretative Report and in particular
the units of measurement for the potential liquefaction induced
settlement. Settlements reported in millimetres (mm) should read
centimetres (cm). However I consider this transposing error does
not affect the conclusions I draw from the calculations for the
reasons given in subsequent paragraphs below.
1 Geotechnical Earthquake Engineering Practice, Module 1 – Guideline for the
identification assessment and mitigation of liquefaction hazards. NZGS July 2010
5
17. We also comment on the presence of a near surface crust in all
areas where there is a low risk of liquefaction in the near surface
soils. The presence of a non-liquefied crust of sufficient
thickness to prevent surface expressions of liquefaction is
important in limiting the surface effects of liquefaction.
18. The reports also recommend that TGH seek access to a site
specific seismic assessment risk assessment prepared by GNS
on behalf of the New Zealand Transport Agency for the Waikato
Expressway. This work included the Ruakura area. I have viewed
that assessment2 and the PGAs calculated in that site specific
assessment are lower than those estimated in the Opus reports
for the Ruakura Plan Change Area using the procedures in
NZS1170:5, implying that the assessment presented to date is
conservative. In terms of the local seismicity the GNS site
specific assessment for the Waikato Expressway describes the
whole area in which the expressway lies (which encompasses the
Ruakura Area) to have a ‘low general level of seismic hazard’.
19. In paragraph 17 of his evidence Mr Mitchell comments on
groundwater levels being low at the time of our soil testing. This
is recognised in the Interpretative Report (Report ref G3127,
page 16 and 3rd bullet point on Page 30) and was allowed for in
the liquefaction assessment carried out by using higher
groundwater levels than were observed at the time of the
investigation.
20. Also in paragraph 17 of his evidence Mr Mitchell states “the fact
that over a substantial period of the year, groundwater levels are
likely to be relatively close to ground level, except for the rounded
hills” and “in my opinion there is a relatively high risk of
liquefaction of some site soils under moderate seismic activity”.
With respect to groundwater levels used in our liquefaction
assessment we have used elevated groundwater levels based on
measured observations over a winter period provided by Harrison
Grierson. I consider that by this action we have accounted for the
2 GNS Science Report “Seismic hazard maps and spectra for Waikato Expressway”
GNS Science Consultancy Report 2010/206
6
seasonal high water levels described by Mr Mitchell in the
assessment of potential liquefaction.
21. The calculations presented in our reports have demonstrated that
some liquefaction of soils at depth will occur in the worst case
(ultimate limit state, ULS, 1 in 500 year earthquake) assessed
under the current New Zealand Standard. By so doing so we
have quantified the risk and potential effects of liquefaction rather
than relying on opinion and subjective assessment.
22. Subsequent to Mr Mitchells filing eveidence I ca nconfirm that the
liquefaction risk for the “serviceability limit state” (SLS, 1 in 25
year earthquake) has been assessed. Using the NZS1170.5
criteria we estimate a PGA of 0.045g. CPTs 103, 105 and
SCPT1 (presented in Opus Report G3127) which showed the
greatest degree of liquefaction in the ULS analyses were selected
for analyses. Near surface groundwater levels were assumed as
before, and it has been calculated that liquefaction will not occur
at the locations of these CPTs in the SLS case. I therefore
consider that this confirms and further demonstrates that there is
a low risk of liquefaction under ‘moderate’ seismic activity. A copy
of the relevant calculation for CPT 103 is attached in Appendix A.
23. In paragraph 18 Mr Mitchell questions whether or not the
liquefaction assessment undertaken is a ‘full seismic assessment’
on the basis of not including seasonal variation in groundwater
levels, groundwater level in relation to the ground surface, and
the local site geology. Each individual assessment and
calculation presented in the reports is based on an elevated
groundwater level to reflect winter conditions, took into account
the ground level at each borehole and CPT location, and was
based on the geology at the specific location of each borehole
and CPT. The software outputs presented a conservative ‘first
cut’ of the potential settlement which has been assessed
separately in more detail. The other effects such as potential loss
of bearing capacity have to be assessed separately from these
outputs. This is normal practice.
7
24. I believe it worth noting that the liquefaction assessment of the
borehole and CPT data in the Inland Port Area yielded the lowest
level of liquefaction observed in those areas investigated and
assessed to date and that in the Inland Port Area ground level is
to be raised by filling by approximately 1 to 2m. This additional
weight of fill will act to reduce the potential for liquefaction of
underlying soils from that calculated and presented in the reports
to date.
25. From the results of our analyses I confirm our conclusion that
widespread liquefaction is not expected to occur in a
serviceability level seismic event (i.e. a 1 in 25 year event).
26. Whilst the revision of the units of measurement relating to the
degree of potential soil liquefaction that could arise in an ultimate
limit state seismic event (1 in 500 years) would potentially be
significant, I consider that there are a range of engineering
measures available to prevent soil liquefaction and to mitigate its
effects to appropriate levels.
27. In paragraph 19 of his evidence Mr Mitchell questions the
practicality of using detention ponds to manage stormwater as a
consequence of potential liquefaction induced ground settlement.
Liquefaction is not anticipated in the serviceability level
earthquake. In a major event I anticipate that the ponds will be
quickly and easily repaired if necessary. This philosophy has
been widely adopted in the region, and is demonstrated by the
wide use and continued construction of such ponds in Waikato
and Waipa Districts in similar ground conditions with relatively
level ground and potentially liquefiable soils.
Soil and groundwater conditions – Graeme Goodwin Site.
28. In paragraph 35 of his evidence Mr Mitchell describes soils
conditions determined by soil testing on Mr Goodwin’s property. I
have not seen the results of the testing Mr Mitchell refers to
however it is broadly consistent with the results of soil testing
done in that vicinity as part of the Stage 1Opus investigations.
Since preparation of my EIC we have also carried out seven
8
auger boreholes around the enclave formed by Ryburn and
Percival Roads which included Mr Goodwin’s property. A copy of
our letter report on the findings of those auger boreholes is
presented in Appendix B, together with a plan showing the
location of Mr Goodwin’s property. The findings of these auger
holes are in general agreement with Mr Mitchell’s statement. No
buried peat horizons were observed in the new auger holes.
29. In paragraph 37 of his evidence Mr Mitchel states that “it will first
be necessary to excavate and remove the upper 1.8m of soft soil”
and to replace them with imported granular fill. This method of
ground improvement is one of a number of means of dealing with
potential settlement issues associated with the relatively poor
near surface soils in this area. Surcharging these with additional
fill and allowing settlements to occur prior to construction on the
fill is an accepted and widely used alternative. This method of
overcoming settlement risks associated with near surface poorer
soils is currently being employed on the Waikato Expressway
Tamahere to Cambridge Section which is under construction over
very similar soils. I have been heavily involved with the peer
review of that work. I can confirm TGH have already been in
discussions regarding a site trial to determine the efficiency of
this method for development in the Inland Port Area.
30. There are also other alternative ground improvement techniques
that could be employed such as installing vibro-stone columns to
support the new fill. I expect that final decision will be made on a
balance of cost, efficiency, risk, ground improvement
effectiveness, and sustainability considerations. I have not
attempted to verify the figures quoted by Mr Mitchell in paragraph
38 of his evidence however it is clear that the excavation and
replacement method described is likely to be high cost, have low
geotechnical risk and to be questionable with respect to
sustainability. I therefore expect alternatives to be thoroughly
explored before a decision is made.
9
Mohammed Hassan on behalf of Waikato Regional Council
(submitter No 106894)
31. In paragraphs 3(d) 8.3 and 8.4 of his evidence Mr Hassan
comments on settlement of peat and refers to a plan (Annexure 5
of his evidence) showing the indicative locations of peat in the
vicinity of the Plan Change Area. I am in general agreement with
his comments regarding on going peat settlement but cannot
comment on the figures he has quoted. I note however that the
area of peat shown on the plan that encroaches onto the Plan
Stage Area we have investigated has only thin surface organic
soils.
William Roy Cowie on behalf of The Ruakura Residents Group and himself
(submitter No. 106774)
32. In paragraph 5.16 of his evidence Mr Cowie comments that “the
Residents Land is consolidated peat and other soft soils in depth
from 0.5m to 1.5m”. This is broadly consistant with the findings
of the Stage 1 investigations and investigations carried out
around the Percival and Ryburn Road properties since
preparation of my EIC. A copy of the findings of that work
including a plan showing where the additional investigations were
located is presented in Appendix B, together with plan showing
the location of Mr Cowies property.
33. I note however that the organic soils proved in these holes were
generally less than 200mm thick and limited to surface deposits.
If this is typical of the soils within the boundaries set by Ryburn
and Percival Roads then I expect the housing to be founded
beneath these surface organic soils. Based on our investigations
elsewhere in this area there is a possibility of thin buried organic
soils below the depths investigated by these additional augers.
On the assumption that these thin burried organic soils are
localised and discontinous, as I believe to be the case, I
anticipate that potential surface settlement caused by further
consolidation of these soils, brough on by changes in
groundwater level or additional surface loading with be both
10
localised and marginal. These can be fully assessed at the
detailed design stage.
34. I cannot comment on the ground vibration issue raised by Mr
Cowie as that is outside my area of expertise. I understand that
this issue has been addressed in the rebuttal evidence of Mr
Carpenter.
Conclusion
1. Having read all the statements of evidence provided by the submitters in
relation to potential liquefaction, the level of investigation carried out in
the preparation of the Opus technical reports, and the presence of peat
and soft soils in the Ryburn Road and Percival Road area, I have no
reason to alter my overall conclusions expressed in my EIC that in my
expert opinion and based on the evidence available to me, there are no
major geotechnical constraints that could preclude development of the
Plan Change Area for the proposed land uses.
Kenneth John Read
16 April 2014
Appendix A
Calculation of liquefaction potential at CPT103,
Serviceability Limit State, PGA 0.045G
Appendix B
Submitters Properties and Auger location plan.
Additional Auger investigation at Rydale and Percival Roads.