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Sunday, February 20, 2011

EQC - Interpretative Geotech Report for Tai Tapu

As with the previously released interpretative reports, the Tai Tapu report consists of 8 pages of text followed by references, maps and diagrams. The report is readable, and worth the effort. The report is here.

For now, the only part of the text immediately relevant to us homeowners is in Section 7.3 Reconstruction Considerations - Land (page 6 of the report). Also of interest is the Section 9 Summary of the report (page 8, reproduced in full below). In reading the material please keep in mind that I am not a geotechnical specialist.

7.3 Reconstruction Considerations - Land

Future risk of land damage

In Section 7.3 - Land we find out whether the land is at greater risk to earthquake damage than before 4 September. The ideal answer is that it is not.  For Tai Tapu the report says:
In terms of Section 106 of the Resource Management Act 1991, and Section 71 of the Building Act 2004, we consider that the risk of land damage due to a future seismic event is sufficiently low that building consents should continue to be issued for residential developments within the subject area in accordance with existing procedures.  
Good news and, my assumption, no need for an adverse comment on a LIM report.

Foundation design for repairs and rebuilding

Section 7.3 - Land also discusses the need for any special foundation designs for repairs or rebuilding. All land in the area that had previously been determined as acceptable for residential building, has now returned to that condition:
Therefore properties requiring repairs and rebuilding should be subject to the normal foundation design, assessment and consenting requirements as determined by the local council building regulations and Building Act 2004. It needs to be recognised that the land has previously been determined by the Council as acceptable for residential building development and the ground is no less resistant to damage now than it was before the Darfield Earthquake.
Properties with land damage

Section 7.3 - Land also covers property with land damage. In the extract below I have underlined the bit that will have relevance to those of us with land damage. 
Some earthquake related land damage has resulted in ground surface disturbance (such as settlement and cracking of the surface crust), which may require minor surface levelling and recompaction before building repairs and rebuilding commences. Some further limited geotechnical input will be required to confirm that the surface crust on an individual property provides adequate bearing capacity for the static case and provide recommendations for foundation design to improve future seismic performance, where appropriate.
9   Summary
"The results and interpretation of the currently available site investigation data indicates the following:
  1. The subsurface materials which underlie the Tai Tapu subject area generally comprise sands and silts and are assessed to have generally returned to their pre-earthquake condition.
  2. As the materials beneath Tai Tapu have not substantially altered as a result of the Darfield Earthquake, the susceptibility of the ground to liquefaction under future seismic shaking is considered to be similar to, but no worse than, the pre-earthquake condition.
  3. In some areas, earthquake-related land damage has resulted in ground surface
    disturbance (such as settlement and cracking of the surface crust), which may require minor surface levelling and recompaction before house repairs and rebuilding commences.
  4. All repair and rebuilding works should be undertaken in accordance with the guidance provisions set out by the Department of Building and Housing where appropriate and in addition to any normal foundation design, assessment and consenting requirements as determined by the local council building regulations and the Building Act 2004."

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