Canterbury Structural Group Meeting

CSG 5:30pm to 6:30pm & CTC 6:30pm to 7:30pm, Wednesday 18th May 2011
Christchurch Art Gallery Auditorium

The Canterbury Structural Group (CSG) meeting was opened by the group's chair, John Snook. John reminded structural engineers that they should email all Christchurch building evaluations to mailto:cdrescue@ccc.govt.nz

Mike Stannard (Department of Building and Housing) outlined the changes to Building Code Clause B1 that were required to accommodate new knowledge of the seismic risk in Christchurch. Complete details of the changes that were effective from 19 May in the areas administered by the Christchurch City, Selwyn District and Waimakariri District Councils, are given in the Department's Information Sheet. Changes for this Canterbury Earthquake Region include: Significant changes to NZS3604:1999 include: There are similar types of changes for the 1998 and 1999 editions of NZS 4229.

Mike also indicated that, although not legally mandated, the recommendations in Guidance on house repairs and reconstruction following the Canterbury earthquake – A summary of geotechnical and structural engineering recommendations to guide house repairs and reconstruction are current best practice.

Graeme Beattie (BRANZ, EAG Housing Group Chair) outlined how the group are aiming to publish a revised edition of the guidelines by the end of June. He indicated that it will include annexes on slab repair, chimneys and retaining walls on hills (covered in Nick Taylen's later presentation).

Graeme indicated that field surveys had identified severe vertical movements of several heavy houses founded on deep-piled house foundations that require better engineering. Conversely, light houses on piles (without concrete slabs) were readily re-levelled.

Graeme would like feedback on the performance of DPM's under floors. mailto:graeme.beattie@branz.co.nz

John Hare (Holmes Consulting Group, SESOC Chair, EAG Commercial Group Chair) outlined the steps of the procedure within the current draft of the EAG Building Safety Evaluation. He indicated that the technical guides were being written and would be published in draft form in early June.

John posed some significant questions that he needs feedback on:
Nick Harwood (Coffey Geotechnics and NZGS Canterbury Branch Coordinator) introduced and chaired the Geotechnical part of the meeting.

Helen Rutter (Aqualinc Research Ltd) outlined three main causes of ‘springs’. In some cases capped artesian bores had burst, to the surprise of their current ‘owners’, in other cases sand volcanoes are still spouting water, and in other cases, new springs had appeared without any liquefaction. She showed photos of several examples and a map of the new springs, which showed a line of springs close to the edge of the loess around the Port Hills and another group in the Hoon Hay area. Some springs have decreased flows and some have dried up since the earthquakes, but most have shown increased flows. There is some concern about possible foundation movements near springs that are continuing to eject soil particles.

Helen is compiling a list of spring locations and welcomes feedback about new springs and changes in existing springs. ECan are not collating spring information and residents have difficulty finding who to contact about them.

Nick Traylen (Geotech Consulting) presented draft evaluation guidelines being considered by the EAG for hillside properties and retaining walls. The hillside property guidelines provide a general guide on what an Engineering Geologist or Geotechnical Engineer needs to be looking at when evaluating a property for rebuilding or repair, and also guidelines on what damage to be looking for in evaluating retaining walls. In some areas of the hills, issues such as potential large scale landslides and large rockfall sources are being investigated by the Port Hills Geotechncial Group and other parties.

Nick would like engineering feedback on the proposed text, particularly (but not only) structural engineering feedback on proposed damage limits for aspects such as crack widths and wall and foundation rotations

Mark Yetton (Geotech Consulting) reported that the April aftershock and high intensity rain caused some rockfalls. He presented an overview of the probabilistic risk analysis method being developed by Chris Massey (GNS) to placard houses subject to rockfall hazards.

Mark outlined how they are monitoring five large landslides in the hill areas, with particular focus on the two with steady movement (one with 7 mm over 10 days). The movement is less than the 5 mm/day that will trigger an evacuation of the affected land. The other landslides are not moving (yet!).

Mark also outlined the response of the Port Hills Geotechnical Group that provided part of the Civil Defence response assessing landslide and rockfall risk to engineering lifelines and life safety of residents. He described how the group divided the Port Hills into nine geographic sectors and assigned each a leader to be primarily responsible for assessments and work in that sector.

The sectors leaders, a map showing the sector boundaries and other details are available on the Port Hills Geotechnical Group's page. The group can be contacted via mailto:porthillsgeotech@ccc.govt.nz

The next meeting was expected in a fortnight. [Later postponed to 15 June. Ed.]

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