Issue 1 - May 2013

Update 1 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. 

 

1. Can I average liquefaction settlement results across a site and compare the average to the index numbers in the guidelines?

(Guidance document reference – various)

Where calculated liquefaction settlement results exceed a specified index number in Parts A - C of the guidelines, the more conservative calculated value should be used in most cases. However, if judged appropriate by the geotechnical professional, it is possible to average calculated settlement results over a single house as long as the calculated value exceeds the specified index value by only a small amount (i.e. in the order of 10% or less). The spatial variability of the data and the potential consequences of that variability should be considered when making that judgement. It may also be possible, through additional more detailed investigation, to isolate a problem area and demonstrate that it does not affect the issue being addressed.

2. If settlements or slopes measured for a house exceed those tabulated in column 3 of Table 2.3 in section 2.3, Part A, does that automatically mean the foundations need to be rebuilt?

(Guidance document reference – Part A, section 2.3)

No. If there is an appropriate relevel or repair methodology available that can be applied practically and economically, then the preference is to repair or relevel. Table 2.3 gives indicator criteria, not absolutes, and needs to be carefully applied with consideration to a number of other factors. If the deformations are well beyond the indicator criteria in Table 2.3 then relevelling or repair may be too difficult or expensive, but there will be many cases where this is not so. However, where these indicator criteria are being exceeded by a significant amount, or a repair solution is being used that is not covered by the guidelines, the repair should be considered as a specific engineering design and appropriate documentation provided by CPEng professionals.

3. Can we use thin layer correction factors in our liquefaction calculations?

(Guidance document reference – Part C, section 13.5 and Part D, section 16.4.1 and Appendix D1)

As outlined in section 13.5.1 and 16.4.1, you cannot use thin layer correction factors if you are calculating settlement figures that you intend to compare with the index numbers in the MBIE guidelines. These sorts of corrections need to be applied with a great deal of judgement. In order to get more consistency across all professionals when they calculate index numbers, it has been decided not to include the thin layer correction factors in the MBIE guidelines methodology.

4. For subdivisions where most of the land is found to be equivalent to TC2 but some areas are found to be equivalent to TC3 performance (based on geotechnical calculations), can the results be 'averaged' and the subdivision called 'TC2-like' on the whole?

(Guidance document reference – Part D, section 16.5)

No. The guidelines require that the subdivision is either classed conservatively as ‘TC3-like’, or micro-zoned on a conservative basis into multiple classifications of ‘TC2-like’ and ‘TC3-like’ behaviour. The latter classification may require additional investigation points to better identify the ‘TC2-like’ and ‘TC3-like’ areas.

5. Do I need to design piles for lateral displacement in TC1 and TC2?

(Guidance document reference – Part A, section 5.3.1, Option 5)

No. Not unless there is evidence of stretching having taken place across the site, or in the opinion of the geotechnical professional the site is obviously vulnerable to lateral stretch.

6. Do I need to use the sliding head pile detail in TC1 and TC2?

(Guidance document reference – Part A, section 5.3.1, Option 5)

Generally not. The sliding head detail is intended for TC3 sites, unless:

  • there is evidence of stretching having taken place across the site, or
  • in the opinion of the geotechnical professional, the site is obviously vulnerable to lateral stretch.

7. Is the sliding head detail only for certain cases of lateral stretch, and not required in other cases, or was it intended to be 'the standard detail’?

(Guidance document reference - Part C, section 15.2.5)

The sliding head detail is intended to be the standard detail for TC3 sites.

8. The sliding head detail is for cases where 'significant lateral stretch up to 200mm has occurred'. What about situations where lateral stretch is more than 200mm?

(Guidance document reference - Part C, section 15.2.5)

The MBIE solutions for deep piles do not cover situations where more than 200mm of lateral stretch, or 300mm of global lateral movement has occurred or is anticipated. Piles are not considered suitable in these situations without specific engineering design.

9. How do we design for differing depths of liquefaction versus crust thickness?

(Guidance document reference - Part C, section 15.2.5)

Under the current guidance, a designer must carry out a full kinematic interaction analysis to design for differing depths of liquefaction versus crust thickness. We anticipate that further updates will extend the current guidance to cover a slightly wider range of scenarios.

10. How does the 200mm relate to the 300mm 'lateral surface movement' that we imply in the design for the standard piles?

(Guidance document reference - Part C, section 15.2)

The 200mm relates to lateral stretch, the 300mm relates to global lateral movement.

11. We say 'where major or severe global lateral movement (>300mm) has occurred' - do we mean this, or do we mean 'or is anticipated'.

(Guidance document reference - Part C, section 15.2.2 (point 6)

We mean ‘has occurred or is anticipated’. Therefore, point 6 in section 15.2.2 should read, ‘Pile foundations are not considered suitable (without special engineering) for sites where major or severe global lateral movement (>300mm) has occurred or is anticipated (refer section 12.2)’.

12. What do the terms 'SS Sheet’ and ‘EPS’ mean?

(Guidance document reference – Part C, section 15.2.3, figure 15.3)

‘SS Sheet’ means ‘stainless steel sheet’, and ‘EPS’ means ‘expanded polystyrene’.

13. What do the letters H, M and L mean in Table 7.2, in Part A?

(Guidance document reference - Part A, section 7.8, Table 7.2)

These letters refer to cladding weights with ‘H’ meaning heavy, ‘M’ meaning medium, and ‘L’ meaning lightweight cladding. The glossary in the Reference material at the back of the guidance document gives specific definitions of the following terms: ‘heavy roof’, ‘heavy wall cladding’, ‘light roof’, ‘light wall cladding’, and ‘medium wall cladding’.

14. Ground Improvement – In Part C, section 15.3.1, the guidelines state that ‘It is intended that ground improvement carried out following these guidelines will allow the construction of either concrete or timber floors that are supported on foundations that meet the requirements of TC2.’ Does this mean that for types 1 and 2 (densified raft or stabilised crust), the depth of treatment needs to be extended beyond the specified 2 m depth to the point where the residual calculated settlements are less than 100mm (ULS) and 50mm (SLS) in the upper 10m of the soil profile?

(Guidance document reference - Part C, section 15.3)

No it does not. If the site meets the requirements of the guidance document for these ground improvement methods, then the depth of treatment needs only be to that specified in the document. (Refer to Table 15.4 for limitations).

15. Ground Improvement – can all types of ground treatment be applied on all sites?

(Guidance document reference - Part C, section 15.3.3)

No. A number of variables need to be taken into account to determine which ground improvement option to use, including soil types, site access, and potential effects of adjacent sites.

16. How do we relevel houses that are already attached to deep piles?

(Guidance document reference – Part A, section 4 and Part C, section 14)

Houses that are already supported on deep piles, where the piles have been cast into the foundation, are likely to be difficult and expensive to relevel. Consider why the piles have already failed and, once you have discovered the reason, take into account the future implications of this. If it is still considered feasible to relevel the house then, in most cases, it will be necessary to excavate beside the piles and detach them from the foundations. For internal piles this may require demolition of floor slabs although, in some cases, the floor slab piles may not be cast into the slab and therefore may not need detaching. Once detached, the foundations can be relevelled using a variety of techniques, as outlined in the guidelines. Re-establish a connection between the piles and the foundation that allows the transmission of vertical loads, and also debond the piles so that future relevelling will not require specific detachment (and to allow for possible global lateral movements if appropriate).

In some cases it might be feasible to relevel the foundations and piles using a grout injection method at the base of the piles. A specialist contractor should be consulted before recommending such a strategy.

17. The media reporting of the recent High Court case involving Tower Insurance seemed to imply that the use of injection relevelling methods was ‘not allowed’ – is this actually the case?

(Guidance document reference – Part A, section 4 and Part C, section 14)

No. In O'Loughlin v Tower Insurance Ltd (5 April 2013), the Court's ruling did not relate to the general viability of injection relevelling methods, which are now in common use in the Canterbury recovery. The ruling, which Justice Asher emphasised was specific to the case and evidence presented, related to the insurance company’s contractual obligations for payment of a particular insurance claim.

The decision whether to use an injection relevelling method for a repair should be determined based on the particular circumstances. In many cases, injection methods will be a viable, consentable solution for the relevelling of a building. Section 4 of Part A and section 14 of Part C of the guidance provide information to be considered when repairing foundations and considering relevelling options.

18. For Port Hills properties, does the presence of the Green Zone mean that we do not need to consider rockfall hazards or other external hazards that might affect a site?

(Guidance document reference – Part A, section 6)

No. The Green Zone does not diminish the normal obligation to consider and assess natural hazards that might affect the site, both from sources external and internal to the site boundaries. The Port Hills zoning considers the exposure of risk to life rather than property damage; a site may have particular local features that still render it vulnerable to natural or manmade hazards such as rockfall or land instability.

19. Regularity – Given that a 2:1 ratio base plan is allowed, and a 1:1 main projection, does this allow the combination of the two, along the long axis of the building to give an overall 3:1 aspect ratio rectangular building plan?

(Guidance document reference – Part C, section 11.2)

No. This goes against the obvious intent of the regularity guidelines. The 1:1 major projection needs to come off the long side of the base plan.

Repairing and rebuilding houses affected by the Canterbury earthquakes

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This information is published by the Your home’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: