Geotechnical-focused Engineering Briefing - CTF
5:30pm to 8:80pm, Wednesday 15th June 2011
Christchurch Art Gallery Auditorium
This Canterbury Technical Clearinghouse meeting was opened by John Snook (John Snook Consulting, Canterbury Structural Group chair) who welcomed the 100+ attendees. The meeting was chaired by Bruce Deam (NZSEE and University of Canterbury), who provided this summary on behalf of the NZSEE.
Jarg Pettinga (University of Canterbury and NHRP Researchers) presented Progress in understanding the Canterbury Earthquakes, beginning with an update on the hazard analysis in light of the Christchurch earthquake and continuing aftershocks. He indicated that the Greendale fault is shutting down but the Port Hills Structure is still active. He explained the background to the aftershock probabilities that had caused controversy when abbreviated in the press and showed how the activity was within the expected range.
Prof. Pettinga then outlined the reflection survey results that clearly showed the buried volcanic topography and fault structures beneath the New Brighton Beach and Barbados St traverses. The data from other traverses is still being processed. GNS magnetic and gravity surveys over a much wider area (500 km2) hinted at the presence of structures, but needed further processing to improve the images. He concluded his presentation with images and maps of the off-shore structures identified by NIWA and older commercial surveys, but noted that none of the still active structures broke the surface.
Mark Yetton (Port Hills Geotechnical Group Leader) began by noting that Don Macfarlane is the new group leader. Dr Yeton's Port Hills Geotech Group Issues presentation began with graphic photographs of the boulder bounce damage and rockfall in Redcliffs and Sumner, progressed to maps of the cracks in land behind the cliff faces and concluded with images of what remains of the Sumner-Lyttleton road.
Chris Massey (GNS Science) began his presentation on their proposed method of assessing the life safety risks associated with potential house damage by summarising their work mapping the locations and movements of 7000 fallen rocks, ranging in size from 0.83 to 40 m3. The aftershock PGA's have provided indicative trigger levels for rock fall and they have produced suburb-scale risk maps that are now awaiting political decisions on the tolerable risk levels. He also briefly described the landslide management process and recordings.
Misko Cubrinovski outlined the rapid liquefaction surveys that had been conducted since the June 13 aftershocks, which had identified sand boils, rivers of water and house settlement in both Avonside and the CBD. He noted that liquefaction depends on both acceleration amplitude and duration, but that amplification within the deep alluvial soil made the land susceptible even with small base rock accelerations.
Graeme Beattie (BRANZ, EAG Housing Group Chair) outlined how the changing landscape was making it difficult to reconcile the technical and regulatory situations. While the aim for the guidelines published in December had been to repair and accommodate small foundation deformations, different guidance is now required. An intermediate limit state for habitability and repair was being considered but there are questions about the boundary between repair or rebuild. He completed his presentation by noting that the slab repair and hillside property annexes were nearing completion and that the chimney and URM annexes are being drafted.
John Hare (Holmes Consulting Group, SESOC Chair, EAG Commercial Group Chair) briefly outlined the qualitative assessment procedure within the current draft of the EAG Building Safety Evaluation. He indicated that the aim was to provide a qualitative triage process to reduce the more difficult (and therefore expensive) quantitative assessments. He also described some of the background behind Critical Structural Weaknesses identified in the guides. The technical guides had taken a back seat following the 13 June aftershocks.
Carl Devereux (CERA - Significant Buildings Unit, 021 369 376) briefly noted some of the risks with demolitions managed by owners and indicated that demolitions now required significant engineering input and approved methods to ensure they were carried out safely.
The meeting concluded with a number of questions to the presenters, and the expectation that another would be scheduled about mid-July.
Use of Information Disclaimer from the Canterbury Technical Clearinghouse