Issue 4 - December 2013

Update 4 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. 

39. I have a foundation in TC3 that satisfies the criteria in Table 2.3 for a relevel. Can I use the methods in Appendix A1 to relevel the foundation because the appendix title states that it only applies to TC1 and TC2, implying that it doesn’t apply for TC3?

(Guidance document reference – Part C, section 14.2.2, section 4.3, and Appendix A1)

Because you are in TC3 the start point is Table 2.3 to determine whether a relevel is indicated. If this is possible then section 14 of the Guidance is relevant as this relates to TC3 repairs. Section 14.2.2 notes that for a Case 2 foundation relevel (and local repair) if less than 25-30% of the foundation beam does not require replacement then relevelling and repairs can proceed in accordance with Part A, section 4.3. Section 4.3.1 makes reference to the available relevelling methods in Appendix A1 (4th paragraph).

40. The process diagram for repairing foundation on TC3 sites in Part C, Figure 14.1 refers to section 5 in the bottom right blue box, is this correct?

(Guidance document reference - Part C, section 14, Figure 14.1)

The reference should be to section 15.

41. What guidance is available for design of new retaining walls for the Port Hills?

(Guidance document reference – Part A, section 6)

An MBIE working group is presently preparing detailed guidance for the design of new residential retaining walls for the Port Hills and these are expected to be released early in 2014. Limited guidance is available within the supporting documents of the Building Code (Acceptable Solutions or Verification Methods) for the design of earth retaining structures. The only specific provision for earthquake loading is provided in the wall foundation example given in Appendix C of B1/VM4. A load factor of 1.5 is specified in NZS 1170.0:2002 for earth pressure under gravity loading, which will provide a certain level of earthquake resistance for many cases.

The impending guidance provides more specific details on how to comply with Clause B1 of the Building Code when designing earth retaining structures and cut or fill batters for earthquake actions. These should be designed to resist earthquake effects in the following situations:

  • where failure or excessive deformation of the batter or retaining structure might contribute to loss of life within or safe egress from a dwelling (ULS) or loss of amenity of a dwelling (SLS), or
  • where the height of the batter or retaining structure is greater than 3m (including the height of batter above the retaining structure).

Recommendations will incorporate a procedure for determining design acceleration coefficients considering the importance of the structure and its tolerance for permanent earthquake induced deformations. The guidance will provide more detailed performance requirements for residential retaining walls with the most critical case being where a retaining wall is built integral with a dwelling or directly supports a dwelling. In such cases no movement can be tolerated at the SLS and only limited movement even at the ULS. Familiar quasi-static design procedures based on the Mononobe-Okabe equations will generally be appropriate with acceleration coefficients derived from the revised interim Z factor for Christchurch. For walls where more movement can be tolerated after earthquake shaking, acceleration reduction factors may be adopted from a Newmark sliding approach.

The document will also reference selected published research into the properties of loess soils of the Port Hills.

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: