Guidelines for the geotechnical investigation and assessment of subdivisions in the Canterbury region
This document is issued as guidance under section 175 of the Building Act 2004. While the Department has taken care in preparing the document it should not be relied upon as establishing compliance with all relevant clauses of the Building Act or Building Code in all cases that may arise. The document may be updated from time to time and the latest version is available from the Department’s website.
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Minimum requirements for geotechnical assessment for land development (Canterbury region)
Introduction
In support of both Plan Change applications and Subdivision Consent applications, appropriate geotechnical investigations shall be carried out, and stand-alone geotechnical reports prepared by a suitably qualified and experienced geotechnical engineer (CPEng). The reports shall combine all relevant geotechnical information in both a factual and interpretive manner, provide justifiable statements about all pertinent geotechnical aspects and consider relevant RMA Section 106 issues.
In Canterbury, the requirements for geotechnical assessments for subdivisions are set out to a certain degree in the following documents (all available online):
- Christchurch City Council – Infrastructure Design Standards
- Selwyn District Council – Engineering Code of Practice
- Waimakariri District Council – Engineering Code of Practice.
Additional guidance is given in the following Standards (available from Standards New Zealand):
- NZS 4431 Code of Practice for Earth Fill for Residential Development
- NZS 4404: 2010 Land Development and Subdivision Infrastructure.
However, these documents do not give specific guidance on the assessment of liquefaction risk. In the first instance, for liquefaction assessments reference should be made to the following New Zealand Geotechnical Society publication (available online):
- NZGS guidelines (2010) Geotechnical Earthquake Engineering Practice Module, 1 – Guideline for the identification, assessment and mitigation of liquefaction hazards.
In conjunction with these documents, the minimum requirements for assessing liquefaction for land development in Canterbury are summarised below.
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Site investigation
Appropriate geotechnical investigations shall be carried out to enable the characterisation of ground forming materials to at least 15 m depth below ground level, unless the ground is known to be of acceptable quality from lesser depths (for example, in areas known to be underlain by competent gravels and deep groundwater profiles, or in hillside areas).
Following an appropriate desktop study to evaluate existing subsurface information in the vicinity of the site, deep investigations shall consist of one of, or an appropriate mix of:
- CPT (Cone Penetrometer Test) testing
- SWS (Swedish Weight Sounding) testing
- Dynamic Penetrometer Testing
- physical drilling and sampling with SPT (Standard Penetration Testing)
- testpit excavations (eg, in ground of acceptable quality from shallow depths)
- laboratory testing
as judged appropriate by a suitably qualified and experienced geotechnical engineer (CPEng).
Scala Penetrometer (‘DCP’ or ‘Dynamic Cone Penetrometer Testing’) testing is often useful as a shallow investigation tool in conjunction with the methods outlined above. However, Scala Penetrometer testing is not considered appropriate as the primary ground characterisation method.
In many areas of Canterbury, liquefiable deposits contain interbedded layers of relatively stiff gravel deposits and, therefore, CPT testing alone may not penetrate deep enough to achieve the depth of ground characterisation required.
Geophysical methods such as MASW (multichannel analysis of surface waves) can be useful in characterising ground conditions between borehole locations.
Knowledge of the geological depositional environment can also be a guide to identifying areas of likely liquefaction susceptibility.
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Site investigation density for overall ground characterisation
The following minimum investigation density guidelines are recommended for deep investigations:
| Investigation Stage |
Total Number of Test/Investigation locations (cumulative) |
| |
Site over 1 hectare |
Site less than 1 hectare |
| Plan change |
0.2 to 0.5 per hectare (minimum of 5) |
2 to 5 |
| Subdivision consent |
0.25 per lot (urban)
1 per house site (rural) |
0.5 per lot |
Note: The lower end of the recommended minimum range might be appropriate where investigations show ground conditions to be reasonably consistent (especially if MASW or the like is being used between investigation ocations), while the upper end of the range may be more appropriate if ground conditions prove to be highly variable.
For the purposes of this table, a minimum effective lot size of 600 m2 may be used.
If initial investigations demonstrate a lack of liquefaction potential, the engineer may judge fewer test locations or shallower depths of investigation to be appropriate. Conversely higher densities may be required where particular site conditions (subsurface complexities, site geometry etc) exist. In commercial or industrial land, specific development proposals may also lead the engineer to judge that fewer or more test locations are appropriate.
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Liquefaction assessment
In addition to standard geotechnical characterisation, the site data shall be analysed using recognised methods to determine liquefaction susceptibility and in particular likely ground deformations under design serviceability limit state (‘SLS’) and ultimate limit state (‘ULS’) ground motions.
Liquefaction analysis methodologies (minimum requirements):
Recognised standard liquefaction analysis methods shall be used, in conjunction with specified input ground motions and, where appropriate, observations of land damage from recent seismic events.
Ground input motions
Refer to the latest data published by GNS Science on Canterbury seismicity and design peak ground accelerations for geotechnical analysis – note that this supersedes the updated ‘Z’ and ‘R’ values published by the Department in May 2011, which should not now be used for geotechnical purposes. (in press)
Liquefaction hazard, liquefaction induced settlements and lateral spread.
Refer to the following documents or methodologies:
- The latest edition of NZGS guidelines, Geotechnical Earthquake Engineering Practice, Module 1 – Guideline for the identification, assessment and mitigation of liquefaction hazards (current edition July 2010).
- Idriss & Boulanger 2008, Soil Liquefaction During Earthquakes – EERI monograph MNO12
- Ishihara (1985), “Stability of Natural Deposits During Earthquakes”, Proc 11th International Conference in Soil Mechanics and Foundation Engineering, pp 321 – 376 – Figure 88, p 362. (Reproduced as Figure 107 on p 157 of Idriss & Boulanger (2008)) (optional).
- Observations of damage or lack thereof in areas deemed to have been ‘sufficiently tested at or near ULS’ by recent seismic events can be used to judge the applicability or not of settlements calculated at the design SLS level (optional).
- It is hoped that, with time, a modified methodology for liquefaction settlement/damage calculation will be derived from extensive site data and damage observations in the recent earthquake sequence. This will be incorporated into these requirements at an appropriate stage.
- Modification by reference to soil deposit ageing is not considered appropriate in the Canterbury region.
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Broad classification of land
The site’s liquefaction characteristics shall be assessed against the deformation limits in the Revised guidance on repairing and rebuilding houses affected by the Canterbury earthquake sequence, as summarised in the table below.
Liquefaction deformation limits and house foundation implications
| Technical Category |
Liquefaction deformation limits |
Likely implications for house foundations (subject to individual assessment) |
| Vertical settlement |
Lateral spread |
|
| SLS |
ULS |
SLS |
ULS |
|
| TC1 |
15 mm |
25 mm |
nil |
nil |
Standard 3604-like foundations with tied slabs* |
| TC2 |
50 mm |
100 mm |
50 mm |
100 mm |
The Department’s enhanced foundation solutions (Section 5.2) |
| TC3 |
>50 mm |
>100 mm |
>50 mm |
>100 mm |
Site specific measures – piles, ground improvement, etc |
* Note that certain foundation details included in NZS 3604 are precluded from use in Canterbury (refer to the Seismicity information sheet).
The geotechnical report shall identify likely requirements for construction of buildings to meet the design requirements as prescribed by the Department of Building and Housing, with respect to liquefaction and lateral spread. In addition to this, the geotechnical report should address all other normal geotechnical aspects (soil types, static bearing capacities, RMA section 106 hazards etc).
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Site investigation density at building consent stage
The density and depth of ground investigation at building consent stage will vary depending on the information already available from earlier stages of investigation.
For land that fits into TC1 and TC2, the Department’s guidelines require as a minimum a shallow investigation to be carried out at each house site (similar to a normal NZS 3604-type investigation), and as a minimum four test locations for each house site would be required. The geotechnical engineer may judge it appropriate to carry out deeper or more intense investigations than this, particularly in Technical Category 2 if the previous subdivision consent level of investigation indicated a high variability in the assessed liquefaction potential.
For building sites in TC3 (or for sites where no previous ground characterisation and liquefaction assessment has been carried out), deep investigations and liquefaction assessments similar to those carried out at subdivision stage should be initiated (a minimum of one to two per house site), as well as a shallow investigation as judged necessary by the geotechnical engineer.
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(Advisory only) Engineering Advisory Group recommendations regarding liquefaction performance for new subdivisions
The expectations of a now risk-adverse public (who will be increasingly aware of the significance and in particular the cost implications of the three foundation technical categories) is such that developers should consider the potential advantages of the following:
- Incorporating building consent-level investigations at subdivision consent stage.
- Undertaking subdivision-wide ground remediation to bring liquefaction deformation performance characteristics up to the equivalent of TC1 performance (ie, ready to receive NZS 3604-cited foundations). This is particularly important where multi-section remediation is the most appropriate approach (for example, along river margins).
- Where it is not considered practical or economic to provide TC1-compliant building platforms, to provide TC2-compliant building platforms.
- On land that will remain in the TC3 category (which will be well signalled on LIMs and the like), to provide (as a package with the land sale) a cost-effective means of compliance with the Department’s requirements for buildings on this type of land.
Please note: It is recommended that residential lots in new subdivisions be provided as either TC1 or TC2.
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