Issue 6 - July 2014

Update 6 provides clarification and further information on technical issues relating to the residential guidance (Repairing and rebuilding houses affected by the Canterbury earthquakes). These issues result from new information or feedback received on the guidance since its publication in December 2012. 


47. In section 13 of the guidelines it states that the scope of a deep geotechnical investigation in TC3 must be determined by the geotechnical professional responsible for giving advice on the property in question, and that person must be either a CPEng geotechnical engineer or a PEngGeol with competence, suitable relevant training and experience in foundation investigations and liquefaction assessment. Do the same requirements apply for professionals who are carrying out investigations for foundation repairs on TC3 sites, where only a shallow investigation is being carried out?

(Guidance document reference – Part C, section 13)

It is preferable to have a specialist geotechnical professional involved in foundation work on TC3 sites. However, where remediation only involves simple repairs or relevelling, for which the MBIE guidelines do not require a deep investigation or liquefaction assessment, a shallow investigation can be carried out under the oversight of a CPEng engineer. The CPEng engineer must have relevant experience in ground investigation and the interpretation of the results of such investigations, and also enough relevant experience to be able to recognise on a site if further investigation, or a different approach is warranted.

The CPEng engineer must be familiar with the requirements of section 3.4.1 of the guidelines (which covers shallow investigations), and in particular the need to take investigations as deep as is practicable.

48. Recent public articles in ASCE Civil Engineering and NZ Geomechanics News, as well as data coming out of the EQC ground improvement trials, provide new information regarding the scope and limitations of LMG. How does this affect the guidelines?

(Guidance document reference – Part C, section 15.3)

The EQC trials showed that where LMG is injected as a ground improvement method at shallow depths with little control (which can result in heave and dilation of the ground), then little or no improvement results, and in fact there may be a slight increase in the liquefaction vulnerability of the ground. However where it is injected for lifting purposes (refer Appendix A1.9 of the guidelines), then for typical house foundation loads, lifts up to about 75 - 100mm are possible without adversely affecting the ground.

MBIE recommends that, in the absence of additional specialist input and analysis, lifting of houses with LMG should be limited to about 100mm. Greater lifts are possible where there is additional confinement from a heavy building, or the lifting is carried out at depths of greater than 4m. It is important to consider the integrity of the surface crust under such circumstances. This should be carried out only by a specialist contractor with appropriate experience, skills and technical expertise.

The EQC field trials found that LMG as a ground improvement technique was often not effective at shallow depths. As a result of these trials LMG has now been removed as a ground improvement option from section 15.3 of the guidelines (ie the methodology ‘Low mobility grout columns (Type 5)’ specified on page 15.30 of section 15.3 of the guidelines is no longer part of the MBIE guidelines). There has only been very limited use, if any, of LMG as a ground improvement method for houses to date.

Wharmby N (2014): “Relevelling Residential Properties Using Low Mobility Grout (LMG)” NZ Geomechanics News (Issue 87), 26 – 29.
Wilcox K (2014): “Team Tests New Zealand Soil Improvements” ASCE Civil Engineering online article id 23622330636.

49. What area of subfloor ventilation opening is required where a type 2A or 2B foundation has been constructed with a double polythene slip-layer?

(Guidance document reference – Part C, section 15.4)

If the under-slab is buried and overlaid with backfill (eg as shown in Figures 15.19 and 15.20 of the Guidance) then the NZS3604 Clause 6.14.1 ventilation requirements of 3500mm2 per m2 of floor area apply.

Where the under-slab is 50mm or more above ground level (eg as shown in Figure 23.33 of the Guidance), then the amount of ventilation can be reduced. This reduction is only applicable where the slab top does not get inundated with water or where a pond cannot be formed. To ensure compliance the top of the slab is required to be a minimum of 50mm above ground level.

The Types 2A-300 and 2B foundations can make use of the double DPM layer to resist vapour transfer similar to having a vapour barrier on bare ground beneath the house. Therefore the ventilation requirement is 700mm2/m2 of floor area (see clause 6.14.3 of NZS 3604).

However, the 2A-300 and 2B foundation designs have a 300mm deep edge to the perimeter where moisture can find its way into the slab and then the subfloor space. This moisture path is taken into account on the basis that this edge area would need ventilation at 3500mm2/m2, and this ventilation is added to that required for the slab plan area.

A range of Type 2A-300 and 2B floor plans has been investigated to determine the vapour contribution from beneath the slab and the sides of the slab. It was determined that the provision of 1100mm2/m2 was sufficient to ventilate the space on these floors.

Repairing and rebuilding houses affected by the Canterbury earthquakes

This information is published by the Ministry of Business, Innovation and Employment’s Chief Executive. It is a general guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. Expert advice may be required in specific circumstances. Where this information relates to assisting people: