Corner of CTV building sat on 'soft' ground
The soil under the Canterbury Television building was "soft" in the north-east corner, a royal commission of inquiry has heard.
Tim Sinclair, a chartered professional engineer for Tonkin & Taylor and an expert in foundation engineering, is giving evidence during a public hearing into the CTV collapse during the February 2011 earthquake. The collapse claimed 115 lives.
Sinclair prepared and contributed to a report on the soil at the CTV site. His findings were given to structural engineers Clark Hyland and Ashley Smith for their co-authored report to the Department of Building and Housing (DBH).
Sinclair said the interpretation of the soil under the CTV building in 1986 was that there was a "consistent'' gravel layer of about 4m across the site, with silt (sand) below.
Soil "stiffness'' was important for the resilience of the ground as well as its connection to the building above, he said.
However, he found in one area - what he called the north-east quadrant - the gravel was largely "absent''.
In one bore hole he drilled there was only about 100mm of gravel, then silt.
That meant the ground in that corner was softer, Sinclair said.
About 13 bore holes were drilled by machine and by hand on the CTV site for the building's construction.
In areas where the gravel did go down to 4m, Sinclair found some bore holes did not reach the silt below.
Earlier the commission heard from Dr Brendon Bradley, director of Bradley Seismic and a lecturer in civil and natural resources engineering at Canterbury University.
Bradley said in spite of suggestions the CTV site was, by nature, particularly vulnerable to the vertical forces experienced during the February, it was in fact consistent with results seen in similar events elsewhere.
Based on the evidence to date, the vertical nature of the February 2011 quake was not "unique", Bradley said.
- © Fairfax NZ News
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