Soil under CTV was 'soft' - engineer

JOELLE DALLY
Last updated 11:05 25/07/2012
Tim Sinclair/Ctv
Fairfax NZ

SOFT GROUND: Tim Sinclair (inset) says in one bore hole he drilled at the CTV site there was only about 100mm of gravel, then silt. There should have been 4 metres of gravel.

Dr Brendon Bradley
STACY SQUIRES/Fairfax NZ
EVIDENCE: Dr Brendon Bradley at the royal commission hearing into the CTV building collapse.

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The soil under the Canterbury Television building was "soft" in the northeast 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 building collapse during the February 2011 earthquake. The collapse claimed 115 lives.

Sinclair prepared and contributed a report on the soil at the CTV site for structural engineers Clark Hyland and Ashley Smith for their report on the collapse for the Department of Building and Housing.

Sinclair said the original interpretation of the soil under the CTV building in 1986 was that there was a "consistent'' gravel layer of about four metres across the site, with silt 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 that in one area - the northeast quadrant - the gravel was largely "absent''.

In one bore hole drilled in that area, there was only about 100 millimetres of gravel, then silt.

This meant the ground in that corner was softer.

About 13 bore holes were machine and hand-drilled on the CTV site for the building's construction.

In areas where the gravel went down 4m, Sinclair found some bore holes did not reach the silt below.

Earlier, the commission heard from Brendon Bradley, director of Bradley Seismic and a lecturer in civil and natural resources engineering at Canterbury University.

Bradley said that despite suggestions that the CTV site was by nature particularly vulnerable to the vertical forces experienced during the February 2011 quake, it was in fact consistent with results seen in similar quakes elsewhere.

Based on the evidence to date, the vertical nature of the quake was not unique, Bradley said.

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