13 June 2013 Our Ref: 351717 - Wigram Skieswigramskies.co.nz/images/documents/neighbourhood/paerangi...We note that ground levels were lowered across lots 573, ... Good Ground is defined

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.

Eliot Sinclair and Partners Ltd Page 2

351717_13151141338_LetterVemail_jta_5Y_Foundation_Report

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



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).



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


Canterbury earthquakes “Version 3, December 2012 (released 31 January 2013).



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”.



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.



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”.



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.



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.



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.


Canterbury earthquakes” Version 3, December 2012 (released 31 January 2013).

Documents

13 June 2013 Our Ref: 351717 - Wigram Skieswigramskies.co.nz/images/documents/neighbourhood/paerangi...We note that ground levels were lowered across lots 573, ... Good Ground is defined