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land development consultants | land & hydrographic surveyors | civil, structural, geotechnical & environmental engineers | resource management planners | landscape architects
Eliot Sinclair and Partners Ltd. 20 Troup Drive, PO Box 9339, Tower Junction, Christchurch 8149, Phone: 03 379-4014, Fax: 03 365-2449, [email protected]
Directors: Marton Sinclair BE BSc FNZIS MIPENZ CPEng IntPE RPSurv Bruce Sinclair BSc MNZIS RPSurv Maurice Perwick Dip Surv MNZIS RPSurv SSSI CP (HS1) Mark Allan BSurv NZCLS MNZIS RPSurv John des Brouard BE MIPENZ Principals: Richard Wilson BE MIPENZ Warren Haynes BSurv MNZIS RPSurv Samuel Cech BSpSc ANZIS Associates: Ken Burrowes NZCD ANZIS Alistair Cocks BSurv NZCLS MNZIS RPSurv Simon Ironside BSurv MNZIS RPSurv SSSI CP (HS1) John Henry BE (Hons) MIPENZ CPEng Keith Mills BSc MNZIS RPSurv John Aramowicz BE Mining (Hons) MIPENZ CPEng IntPE
351717_13151141338_LetterVemail_jta_5Y_Foundation_Report w w w . e l i o t s i n c l a i r . c o . n z
13 June 2013 Our Ref: 351717
Ngāi Tahu Property Ltd
PO Box 130060
Armagh
Christchurch 8141
Attention: Scott Johansson
Dear Sir
Re: Foundation Report - Lots 484-493, 515-518, 567-624, 660, 666-678 & 712
Wigram Skies - Paerangi (Neighbourhood 5Y), Wigram, Christchurch
1. Introduction
We are writing to report on the general ground conditions and the foundation
requirements for the above residential lots, which form part of Neighbourhood 5Y.
These lots are part of the subdivision development of the former Wigram Aerodrome site
which was previously used as a New Zealand Air Force base and training facility, and
more recently as an airfield for light aircraft.
The common site name for marketing purposes is Wigram Skies – Paerangi Subdivision.
2. Historic Services
Some parts of the site contained historic services which are now redundant. When
encountered these were removed, or were cut and sealed if necessary, before filling
over. There were also areas that were historically filled with clean uncontrolled fill, and in
places, this had been placed over buried topsoil.
The nature and degree of compaction of the historic fill is unknown, however, any
unsuitable historic fill materials that were observed by the inspecting engineer at the
time of subdivision construction were removed by the Contractor before placement of
controlled fill materials.
3. Earthworks and Ground Preparation
Excavation and filling earthworks were undertaken as part of subdivision construction to
remove old building services, access roads and airport taxiways or unsuitable historical
fill material. Excavations were made to remove topsoil and any obvious fill materials
down onto clean insitu subsoil materials, generally silty sand, sandy silt, sand or silt,
before placement of controlled compaction of fill materials to provide adequate falls for
drainage.
Insitu subsoil materials generally comprised sand, silt, silty sand and sandy silt.
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Fill materials were placed in layers not exceeding 200mm thickness, and compacted
using a large drum-type vibrating roller. The depth of controlled, compacted engineering
fill is up to 2.8 metres (Refer to attached Eliot Sinclair & Partners drawings 351717 G1
Sheets 1 of 1).
The fill material was generally placed in accordance with NZS 4431:1989 “Code of
Practice for Earthfill for Residential Development” and the Inspecting Engineers Report
dated 31 May 2013, and the Fill Certificate addresses the controlled fill materials.
Upon completion of the controlled filling operation, the finished ground was typically
surfaced with 350mm to 400mm of silty and sandy topsoil that was lightly rolled, straw
mulched, and sown with grass seed.
We note that ground levels were lowered across lots 573, 582 and 671-672 by stripping
the turf and topsoil, stripping the subgrade, re-shaping and then dressing with topsoil,
and were therefore not subjected to controlled filling.
4. Site Investigation
Our investigation of the subdivision was carried out in April 2013 at the completion of the
filling earthworks. Our investigation generally consisted of one machine auger hole on the
boundary between neighbouring sections to a target depth of between 2.5m to 3.2m
depth, although additional auger holes were carried out on some sites, and two to four
Hydraulic penetrometer tests to 2.7m below ground level, but with practical refusal at
shallower depth at some locations where the probe met practical refusal on gravels.
Some machine augers were only able to extend to between 1.0m to 1.6m depth due to
refusal on gravel fill or redundant pipe debris. Where this occurred additional holes were
undertaken nearby which extended to the target depth.
Hydraulic penetrometer testing was carried out by Canterbury Geotest using a Hydraulic
Scala-Mini CPT rig. The data is recorded by measuring the force applied by the hydraulic
rams to the probe and then an inferred penetration resistance in blows per 75mm depth
is plotted. The Hydraulic Scala-Mini CPT rig has been calibrated against a Scala
penetrometer by field testing.
These results indicate both the soil strata and the soil bearing capacity. Please refer to
the attached Site Investigation Records.
The machine auger holes generally revealed from 300mm to 500mm of silty topsoil fill,
overlying various layers of controlled fill comprising silt, sandy silt and silty sand to
between 0.4m to 2.7m depth. Underlying the fill material were various insitu layers of
sand, silt, silty sand and sandy silt to between 2.5m to 3.2m depth where the auger
holes were terminated.
The deep insitu silt contained relatively minor concentrations of organic material at Lots
516-518, 570-577, 582-593, 596-599, 617, 660, 666-667, 671-673 & 712 below 1.6m
to 2.9m depth. We also note Lots 590-591, 614, 617 & 621-622 contained dark brown or
dark grey silt or silty sand layers at between 0.9m to 2.6m below ground level and was
typically 0.2m to 0.6m thick but did not contain any obvious organic material. It is likely
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to represent darker silty sand under the former topsoil later that was stripped as part of
subdivision earthworks, or a thin layer of remnant silty topsoil.
5. Compliance with “Good Ground” Definition
Good Ground is defined in the Compliance Document for the NZ Building Code Clause B1
Structure and requires consideration of lateral and vertical ground movements and
bearing capacity.
5.1. Bearing Capacity
In terms of static bearing capacity only, the Scala penetrometer results, within or
immediately below the topsoil layer on the majority of the lots, generally exceeded the
minimum penetration resistance requirements to be termed acceptable as “Good Ground”
in accordance with the Compliance Document for New Zealand Building Code, Clause B1
Structure.
Where the minimum penetration resistance requirements were not met within or
immediately below the topsoil layer, “Good Ground” was generally encountered within
150mm below the underside of the topsoil fill layer.
5.2. Liquefaction and Lateral Spread
Tonkin & Taylor (T&T) have extensively investigated the site to assess the potential for
liquefaction, and the results of this are summarised in the report “Wigram Skies
Subdivision Geotechnical Assessment1”.
This report confirmed that no evidence of liquefaction was observed following the
Canterbury earthquake sequence even though the earthquake shaking exceeded the
Serviceability Limit State (SLS) (PGAM7.5 = 0.13g)2.
The site has been characterised by Tonkin & Taylor into two land classifications, being
areas of very low risk of liquefaction or areas with potentially liquefiable soil where
liquefaction induced ground damage is possible.
5.2.1. Lots 484-493, 515-518, 586-596, 605-612, 615-621, 660, 666-678 & 712
These lots in Neighbourhood 5Y are located outside the areas which have a risk of
lateral spread or are potentially liquefiable.
Subsequently, the report1 confirms that foundations complying with the provisions
of NZS 3604:2011 are suitable for these sites, provided shallow investigations for
each lot indicate a geotechnical bearing capacity of greater than 300kPa. These
foundation recommendations are consistent with land classified as Technical
1 Wigram Skies Subdivision Geotechnical Assessment, dated July 2012, prepared by Tonkin & Taylor Ltd.
2 Ministry for Business, Innovation & Employment “Guidance: Repairing and Rebuilding houses affected by the
Canterbury earthquakes” Table C2.1, Version 3, December 2012 (released 31 January 2013).
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Category 1 (TC1). TC1 land is defined as land where “Future land damage from
liquefaction is unlikely, and ground settlements from liquefaction effects are
expected to be within normally accepted tolerances”3.
5.2.2. Lots 567-585, 597-604, 613-614 & 622-624
T&T’s report1 concludes that lots 567-585, 597-604, 613-614 & 622-624 of
Neighbourhood 5Y are located in an area with a potentially liquefiable sand layer
at between 4-6m below ground level. However, land damage is only expected in a
large earthquake event with a return period greater than 350 years.
Subsequently, the report1 confirms that foundation Options 1-5 of the DBH
Guidelines (now MBIE) will be suitable. These foundation options are consistent
with land classified as Technical Category 2 (TC2). TC2 land is described as land
where “Liquefaction damage is possible in future large earthquakes.”3
As these sites are underlain by potentially liquefiable layers at 4-6m below ground
level, the ground cannot be defined as “Good Ground” and specific foundation
design will be required.
Refer to Eliot Sinclair and Partners drawing 351717 G4, dated 09/05/2013 for lot
zonings.
6. Foundation Requirements for Buildings Within the Scope of NZS 3604:2011
6.1. Lots 484-493, 515-518, 586-596, 605-612, 615-621, 660, 666-678 & 712
(Technical Category 1)
Tonkin & Taylor conclude that liquefaction induced land damage or lateral spread is not
likely in a moderate sized earthquake (1 in 25 year SLS event) for these lots.
For these sites, T&T recommends foundations can be constructed as per NZS 3604:2011
provided shallow testing confirms a geotechnical bearing capacity greater than 300kPa.
6.1.1. Lots 484-487, 489, 515-518, 586-595, 605-608, 610-612, 615-621, 668,
671-672 & 674-678
The testing has confirmed the subsoil materials below the topsoil layer at these
lots in Neighbourhood 5Y may be regarded as “Good Ground” in accordance with
the procedure set out with the Compliance Document for NZ Building Code Clause
B1 Structure provided that the foundations are excavated through the topsoil
layer to bear onto clean silts or sand, and to a minimum depth of 300mm below
existing ground level. The topsoil depths across these lots will be typically
between 300mm to 450mm depth, however on some lots this could be up to
3 Ministry for Business, Innovation & Employment “Guidance: Repairing and Rebuilding houses affected by the
Canterbury earthquakes “Version 3, December 2012 (released 31 January 2013).
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500mm depth. The subsoils below the topsoil layer can be considered “Good
Ground” for the purposes of NZS 3604:2011 “Timber Framed Buildings”.
For these lots we recommend that the turf and any organic-rich topsoil is removed
from beneath the area of the floor slab to at least 100mm below existing ground
level.
However on lots 620, 668 & 671, Scala penetrometer test results indicate the lots
are underlain by very soft topsoil. For lots 620, 668 & 671 we recommend all
topsoil is removed from beneath the area of the floor slab and be replaced with
compacted AP40 hardfill up to the underside of the floor slab. The AP40 hardfill
should be placed in layers not exceeding 200mm loose depth and compacted to
achieve a minimum target dry density of 2150kg/m3.
Alternatively, subject to specific engineering investigation and design, excavations
may be able to be reduced on lots 620, 668 & 671, but such design cannot be
done until the design and location for the proposed house is available.
6.1.2. Lots 490-493, 609, 666, 669-670, 673 & 712
The testing has confirmed these lots in Neighbourhood 5Y may be regarded as
“Good Ground” in accordance with the procedure set out with the Compliance
Document for NZ Building Code Clause B1 Structure provided that the foundations
are excavated through the topsoil layer and a further 100mm below the underside
of the topsoil to bear onto clean silts or sands at least 400mm below existing
ground level. The subsoil materials below the topsoil layer at these lots can be
considered as “Good Ground” for the purposes of NZS 3604:2011 “Timber Framed
Buildings”.
For these lots we recommend that the turf and any organic-rich topsoil is removed
from beneath the area of the floor slab to at least 100mm below existing ground
level.
However on Lots 666, 669-670 & 712, Scala penetrometer test results indicate
the lots are underlain by very soft topsoil. For Lots 666, 669-670 & 712 we
recommend all topsoil is removed from beneath the area of the floor slab and be
replaced with compacted AP40 hardfill up to the underside of the floor slab. The
AP40 hardfill should be placed in layers not exceeding 200mm loose depth and
compacted to achieve a minimum dry density of 2150kg/m3.
Alternatively, subject to engineering investigation and design, the foundation
depth may be able to be reduced to the underside of the topsoil, and for lots 666,
669-670 & 712, excavations underneath the floor slab may be able to be reduced,
but such design cannot be done until the design and location for the proposed
house is available.
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6.1.3. Lots 488 & 596
The testing has confirmed that these lots in Neighbourhood 5Y may be regarded
as “Good Ground” in accordance with the procedure set out with the Compliance
Document for NZ Building Code Clause B1 Structure provided that the foundations
are excavated through the topsoil layer and at least 100mm below the underside
of the topsoil bear onto clean silts or sands at least 450mm below existing
ground level. The lots may therefore be regarded as “Good Ground” for the
purposes of NZS 3604:2011 “Timber Framed Buildings”.
For these lots we recommend that the turf and any organic-rich topsoil is removed
from beneath the area of the floor slab to at least 100mm below existing ground
level. Were required, the excavated material can be replaced with compacted
AP40 hardfill placed in layers not exceeding 200mm loose depth and compacted to
achieve a minimum dry density of 2150kg/m3.
Alternatively, subject to specific engineering investigation and design, the
foundation depth may be able to be reduced to the underside of the topsoil, but
such design cannot be done until the design and location for the proposed house
is available.
6.1.4. Lots 660 & 667
The testing has confirmed that Lots 660 & 667 in Neighbourhood 5Y may be
regarded as “Good Ground” in accordance with the procedure set out with the
Compliance Document for NZ Building Code Clause B1 Structure provided that the
foundations are excavated through the topsoil layer to bear onto clean silts or
sands at least 150mm below the underside of the topsoil to a minimum depth of
500mm below existing ground level. The lots may therefore be regarded as
“Good Ground” for the purposes of NZS 3604:2011 “Timber Framed Buildings”.
For these lots the Scala penetrometer test results indicate the sites are surfaced
with very soft topsoil fill. We recommend all topsoil is removed from beneath the
area of the floor slab and be replaced with compacted AP40 hardfill up to the
underside of the floor slab. The AP40 hardfill should be placed in layers not
exceeding 200mm loose depth and compacted to achieve a minimum dry density
of 2150kg/m3.
Alternatively, subject to specific engineering investigation and design, the
foundation depth may be able to be reduced to the underside of the topsoil, and
excavations underneath the floor slab may be able to be reduced, but such design
cannot be done until the design and location for the proposed house is available.
6.2. Lots 567-585, 597-604, 613-614 & 622-624 (Technical Category 2)
These Lots are underlain by a potentially liquefiable soil layer at 4-6m below ground
level. Foundations will have to be designed to accommodate potential ground movement
associated with liquefaction-induced ground damage.
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Tonkin & Taylors report1 advises building foundations may achieve adequate performance
by using foundation Options 2 to 5 as described in the DBH Guidelines (now the MBIE
Guidelines3).
Essentially, the philosophy of the guidelines is to construct resilient type foundations so
that they do not split apart and are more easily able to be repaired and re-levelled if
there is ground deformation and foundation movement due to earthquake shaking.
With this philosophy in mind we recommend concrete foundations comprise a stiffened
waffle slab, being Option 4 of the Guidance Document3, that is designed for the bearing
conditions of the underlying ground.
For these lots in Neighbourhood 5Y, the testing confirms that the silty topsoil is soft and
loose, but the ultimate bearing strength of the underlying ground below the topsoil layer
is at least 300kPa. We recommend all topsoil is removed from beneath the area of the
waffle slab and be replaced with compacted AP40 hardfill up to the underside of the
waffle slab to suit the finished floor height. We anticipate excavations on these lots will
be between 300mm to 450mm below ground level and extend 600mm to 900mm beyond
the perimeter of the building (i.e. 1 vertical to 2 horizontal). The AP40 hardfill should be
placed in layers not exceeding 200mm loose depth and be compacted to achieve a
minimum dry density of 2150kg/m³.
With the waffle slab supported on the compacted AP40 hardfill, the foundations can be
designed for an ultimate bearing strength qu=300kPa, a strength reduction factor of
Фbc=0.5, and a design bearing strength qdbs=150kPa may be used. The foundation
system should be designed to accommodate the movements specified in Clause 5.4 of
the Guidance Document3.
Alternatively, subject to specific foundation design and/or additional testing, excavations
may be able to be reduced by the waffle slab being supported on the topsoil layer,
however such design cannot be done until the design and location for the proposed house
is available.
For this foundation system it would be necessary to obtain the foundation design
documentation from the supplier of the floor system to submit with the building consent
documentation. We recommend the waffle slab designer inspects the excavated
foundations in order to confirm the exposed conditions are consistent with the
parameters outlines in this report.
The designers of the waffle slab will need to design the foundations for any external
isolated columns and posts of the house that support the roof. We anticipate that in a
liquefaction event, differential movement could occur between an isolated post footing
and the waffle slab of the house due to the differences in the stiffness of the foundation
systems. To minimise the potential for differential settlement, any columns and posts
would need to be supported on an external waffle slab that is tied into the house waffle
slab.
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The services shall be designed to exit through the side wall of the waffle slab foundation
and shall not be laid within the ground below the foundation. Provision should be made
for easy repair of the services after a significant earthquake.
7. Inspection Requirements
7.1. Lots 484-493, 515-518, 586-596, 605-612, 615-621, 660, 666-678 & 712
As the ground in Neighbourhood 5Y has been subject to past earthworks activities during
its use as an airbase, each excavated foundation trench for lots 484-493, 515-518, 586-
596, 605-612, 615-621, 660, 666-678 & 712 is to be inspected by an engineer
experienced in foundation design in order to verify that all foundations extend through all
topsoil and bear onto clean, compact insitu subsoil or controlled fill. Where foundations
bridge filled and natural ground, special consideration may be required to minimize the
risk of differential settlement by additional excavation or providing upgraded reinforcing.
The foundation design engineer can confirm any specific requirements at the time of
foundation inspection.
7.2. Lots 567-585, 597-604, 613-614 & 622-624
Lots 567-585, 597-604, 613-614 & 622-624 are underlain by a potentially liquefiable
sand layer. As such, the ground cannot be considered “Good Ground” and each
foundation will be subject to specific engineering design by a Chartered Professional
Engineer (CPEng). Builders should refer to the waffle slab design documentation for any
specific construction inspection requirements.
However, as the ground in Neighbourhood 5Y has been subject to past earthworks
activities during its use as an airbase, we recommend each foundation excavation is
inspected by an engineer experienced in foundation design in order to verify the
foundation subgrade bearing conditions.
8. Foundation Requirements for Buildings Outside the Scope of NZS 3604:2011
Buildings outside the scope of NZS 3604:2011 will require specific foundation design.
Disclaimer
Comments made in this geotechnical and foundation report are based on our visual
inspection of the site on during subdivision construction, shallow Scala penetrometer
testing, machine auger test holes and shallow hand auger testing across Neighbourhood
5Y, Tonkin & Taylor’s July 2012 report, and reference to the most recent version of the
Ministry of Business, Innovation and Employments Guidelines4.
Whilst every care was taken during our investigation and interpretation of subsurface
conditions, there may well be subsoil strata and features that were not detected.
4 Ministry for Business, Innovation & Employment “Guidance: Repairing and Rebuilding houses affected by the
Canterbury earthquakes” Version 3, December 2012 (released 31 January 2013).