GROUND CONDITIONS
New Zealand is known as the shaky isles, for good reason. Up to 2010 Christchurch didn’t think it was susceptible to earthquakes. The whole of the country is on a plate boundary so we are all prone to sizable shakes. The learnings from the Canterbury Earthquake Sequence are being implemented right across the country.
A Healthy Home must have good foundations and the only way to achieve that is to understand the ground conditions beneath.
Assessing Ground Conditions to Design Resilient Foundations
The correct assessment of ground conditions is vital when designing resilient healthy homes and will have an impact on the overall build cost.
There are at least three sets of documentation to review as follows:
1. NZS3604:2011 Timber Framed Buildings; Definition of Good Ground.
2. Repairing and rebuilding houses affected by the Canterbury earthquakes, commonly referred to as the MBIE Guidance. (latest issue December 2012) Technical Categories TC1, TC2, TC3.
3. MBIE Acceptable Solutions and Verification Methods for New Zealand Building Code Clause B1 Structure. (28th November 2019) Update for expansive soils.
NZS3604:2011
This acceptable solution is the ‘Builders Bible’. It also forms the basis for most residential designs in New Zealand. Section 3 deals with Site Requirements and introduces the terminology of “good ground.”
Definition of Good Ground
Good ground means any soil or rock capable of permanently withstanding an ultimate bearing pressure of 300 kPa (i.e. an allowable bearing pressure of 100 kPa using a factor of safety of 3.0), but excludes:
a. Potentially compressible ground such as topsoil, soft soils such as clay which can be moulded easily in the fingers, and uncompacted loose gravel which contains obvious voids.
b. Expansive soils being those that have a liquid limit of more than 50% when tested in accordance with NZS 4402 Test 2.2, and a linear shrinkage of more than 15% when tested, from the liquid limit, in accordance with NZS 4402 Test 2.6.
c. Any ground which could foreseeably experience movement of 25mm or greater for any reason including one or a combination of: land instability, ground creep, subsidence, liquefaction, lateral spread, seasonal swelling and shrinking, frost heave, changing ground water level, erosion, dissolution of soil in water, and effects of tree roots.
Testing for Good Ground
Soils (excepting those described in a), b) and c) above) tested with a dynamic cone penetrometer in accordance with NZS 4402 Test 6.5.2, shall be acceptable as good ground for building foundations if penetration resistance is no less than:
d. a) 5 blows per 100mm at depths down to twice the footing width.
e. b) 3 blows per 100mm at depths greater than twice the footing width.
Depths shall be measured from the underside of the proposed footing.
For the quantum and location of testing required, please refer to NZS3604:2011. https://www.standards.govt.nz/sponsored-standards/building-standards/nzs3604/
Note that there are also important requirements in relation to the top of banks and certified fill to comply with to section 71 of the Building Act to meet the criteria of good ground. For more information please read NZS3604:2011.
The reality is that using the Stockwell scala penetrometer correlation 300kPa can be achieved with 3 blows, but NZS3604 allows for a factor of safety, acknowledging the wide range of experience that people likely to use this document may have.
A geotechnical engineer will be able to provide other testing solutions such as shear vane testing in cohesive soil that could provide more accurate results compared with penetrometer testing.
MBIE Guidance
The MBIE Guidance document introduced three foundation technical categories (TC1, TC2 and TC3) for assessment of residential properties on flat land, which reflected both the liquefaction experienced to date and future performance expectations. Any land not assessed was given the N/A category.
Definition of Technical Categories
The foundation technical categories are defined as follows:
● TC1: Future land damage from liquefaction is unlikely, and ground settlements from liquefaction effects are expected to be within normally accepted tolerances. Once the technical category is confirmed, shallow geotechnical investigations may be required. If the ‘good ground’ test is met, NZS 3604 foundations (as modified by B1/AS1) can be used.
● TC2: Liquefaction damage is possible in future large earthquakes. Once the technical category is confirmed, shallow geotechnical investigations may be required subject to establishing minimum bearing capacities, suspended timber floor or enhanced slab foundation options per Section 5 can be used.
● TC3: Liquefaction damage is possible in future large earthquakes. Deep geotechnical investigation (or assessment of existing information) may be required and depending on the geotechnical assessment, might require specific engineering input for foundations.
It is important to note that these categories do not automatically determine the foundation design options or actual future performance for that property. This can only be determined by the appropriate geotechnical assessment.
It is worth noting that within area wide mapped zones, physical geotechnical investigation can produce a different result to that indicated by the assigned technical category; for example a TC2 mapped site could require TC3 type foundations or a TC3 site could require TC2 foundations.
Criteria for Defining Technical Categories
The index criteria for foundation technical categories is as follows:
If a comparison is made with the NZS3604:2011 good ground category above, it can be seen to equate with TC1 subject to testing. TC2 and TC3 are outside the scope of NZS3604 in terms of foundation design and require a Chartered Professional Engineer (CPEng) to design the foundations.
Testing for Technical Categories
Testing in TC1 & TC2 land categorisations
Shallow testing is required as outlined in Section 3 of NZS3604:2011 with the following additions:
● While the prescribed depth of investigation of 2m is typically acceptable, it is recommended that, where practical, 50mm diameter boreholes for the examination of soil materials extend further, to between 3 and 4m below ground level to check for organic peat deposits.
● ‘Soft or very soft peat’ in the defined exclusions from ‘good ground’ is to be replaced with ‘peat’ in the list of unacceptable materials.
● Scala blows per 100 mm shall be minimum 2 blows (ie, 50mm per blow) for ground deemed to have 200 kPa geotechnical ultimate bearing capacity. For other foundation types (eg, in TC1), 300 kPa will need to be confirmed in accordance with NZS 3604.
● If 200kPa is not achievable then, 150 kPa geotechnical ultimate (raw) should be considered as the lower limit of acceptable soil strength, in the absence of specialist geotechnical advice.
Shallow subsurface investigations can be carried out by a soils technician or other suitably trained and supervised person. In TC2 this needs to be under the guidance of a CPEng qualified engineer.
From a designing for resilience point of view it is preferred practice to undertake deep testing on TC2 land to confirm that the correct technical category has been assigned and to quantify expected liquefaction induced settlements for informed foundation design.
Testing in TC3 land categorisations
The scope of a deep geotechnical investigation must be determined by a CPEng geotechnical engineer, in consultation with guidance provided in this document.
Testing should be carried out in accordance with Part C Section 13.2 but generally at least two deep investigation points (CPTs, boreholes with SPTs, etc) to 10 to 15 m depth would be expected, where practical, supplemented by shallower investigation points using shear vanes, hand augers and /or Scala Penetrometer testing. Groundwater measurements following the investigations should also be taken. Lateral and vertical movement assessments in accordance with Section 12 and liquefaction assessments following the guidelines contained in Section 13.5 can then be carried out. The appropriate foundation or ground remediation solutions can then be suggested by the geotechnical engineer in discussion with the structural engineer and client.
Expansive Soil Requirements
Updated guidance was issued in the form of an amendment to Acceptable Solution B1/AS1 at the end of November 2019. This document amends and adds clauses within NZS3604:2011.
Definition of Expansive Soils
Simply put expansive soils are generally clays or silts that shrink or swell due to changing seasonal climatic conditions resulting in differential foundation movement. There are now four categories based on the level of soil expansiveness or reactivity:
a. Slightly ‘S, having an Iss range of 0-1.9% and a 500 year design characteristic surface movement return (ys) of 22mm, or
b. Moderately ‘M’, having an Iss range of 2.0–3.7% and a 500 year design characteristic surface movement return (ys) of 44mm, or
c. Highly ‘H’, having an Iss range of 3.8–6.5% and a 500 year design characteristic surface movement return (ys)of 78mm, or
d. Extremely ‘E’, having an Iss range of 6.6–7.5% and a 500 year design characteristic surface movement return (ys)of 90mm.
Testing for Expansive Soils
Lab testing is carried out on site won samples in accordance with AS1289:1998 Methods of Testing Soils for Engineering Purposes and NZS4402:1987 Methods of Testing Soils for Engineering Purposes.
The outcome of the test results will determine which category of expansiveness is appropriate for the site and will inform the foundation design.
Peat Soils
Peat soils require specialist geotechnical advice so please consult your geotechnical professional.
Ideally, you wouldn’t try to design a Healthy Home on peat.
Recommended Healthy Home guidelines for Ground Conditions are set out in the table below:
