October 31 2013 - As foundation work begins on the new entertainment and office complex The Terrace, in Christchurch, simple Kiwi engineering common sense from Aurecon has meant significant savings in time and money, and reassurance for the developer that the complex is going to be safe.
Geotechnical assessments of the site, on the banks of Christchurch’s Avon River, showed that layers of sand and silty sand approximately 20 metres below ground level were potentially liquefiable in a strong earthquake.
While close to the surface, a compressible soil layer - which is the remnant of an old stream bed - had potential to allow foundation settlement under the weight of the building.
How to make the ground stiffer was the challenge that faced Aurecon. Aurecon Technical Director, Stephen Hogg, says it was known from the outset that staying out of the water table was necessary in order to save time and money.
For the owner of the property, Antony Gough, the solution also had to ensure the safety of the building, which he describes as “paramount”.
Prior to Aurecon’s solution, four options had been considered and rejected:
“This is where Aurecon stepped in. They came up with an extremely effective solution similar to the old game of ‘stacks on the mill’” says Antony.
Aurecon’s Stephen Hogg says the site was overloaded with heaped demolition rubble to apply weight heavier than what the new buildings will be.
The fill was trucked on to the site from a few hundred metres away and stacked four metres high and left for about four weeks. About 5 000m3 of fill was moved (800 truckloads) with a total weight of about 9 000 tonnes. The total weight of the building will be about 6 000 tonnes.
Surveyors, using precise optical equipment, frequently measured the effect of the heaped demolition rubble on the site. These measurements continued until the settlement effect caused by the stockpiles ceased, at which time the rubble was removed from the site.
“The impact on the roading system and traffic flows was lessened and it was cost effective, good for the environment and good for the economy,” Mr Gough says.
The preloading treatment has meant that the buildings do not need complex or deep-seated foundations, as would have been the case. Aurecon also designed a reinforced concrete raft foundation that will allow the buildings to float on top of the pre-compacted soil, which will minimise the effects of differential settlement if deep liquefaction occurs in any future earthquake.
“We gained a month because the raft foundations did not have to be fully designed before pre-loading the site. The design was completed while we waited for the ground to settle“ says Stephen.
The concrete raft will mean the building will float on the upper levels of the ground, providing a robust foundation structure that will be strong and tolerant to settlement.
“It will also distribute the effects of potential differential settlement rather than concentrate the damaging effects, as would be the case with a piled solution or pad foundation solution.
“It will also mean we can easily re-level the building in the future should it be necessary.”
Mr Gough says the fact that work is already under way “is testimony to Aurecon’s smart thinking”.
About The Terrace
For further information please contact:
Technical Director, Buildings
M +64 27 2408212