Update 3 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.
37. Do the following statements in Part C, section 14.2.2 of the guidance mean that houses in major or severe lateral stretch areas cannot be repaired or relevelled?
"Repairs and relevelling can be considered if a site is assessed as having moderate (refer Table 12.1) lateral stretch potential” and also
"If a site is assessed as having major or severe lateral stretch potential (ie, >200mm at ULS), then neither repairs nor relevelling should be undertaken without careful engineering analysis and consideration".
(Guidance document reference - Part C, section 14.2.2)
No. An engineer should take account of a number of factors in making this assessment and a five-step process is outlined in section 12.2.2 of the guidance. There are inherent uncertainties in predicting lateral spread.
However, if a site has been relatively well tested by the earthquake series, and if no lateral stretch damage or only minor lateral stretch damage has occurred to the house, then a repair or relevel may well be appropriate. In other cases, it might be appropriate to make some simple upgrades to the foundation system where this is feasible, to reduce the risk of damage from such future events. (An example would be to add external reinforcing to a concrete edge beam to enhance its ability to withstand stretch).
38. Where the guidelines indicate the need for a surface structure Type 2A, could a Type 2B detail be used instead, without the underlying gravel raft?
(Guidance document reference - Part C, section 15.4.4)
Yes it can, in a modified format. The 300mm reinforced concrete slab will need to be underlain with a minimum of 150mm compacted hard fill (well-graded sandy gravels) that is in turn underlain with geotextile. The 150mm compacted gravel layer will need to extend 1m outside the foundation line, with the geotextile wrapped over the 1m extension and returned a minimum of 1m under the concrete slab.