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:

Deep piles down 25 metres, which was rejected due to the potential for future earthquake movement to cause soil lurch (or ground displacements) which could affect the structural integrity of the building
Digging out the surface soil and replacing with hard fill imported from local quarries. The extent of dig out was not precisely known and would have involved about 900 truck movements on local roads
Remaking the ground to a depth of about three metres by adding cement to the site soils. This is a viable option being used elsewhere in the Christchurch CBD but this particular site is three metres above the water table and major dewatering was likely if soil was removed down to the three-metre level. It was rejected because of cost
Rammed gravel columns, which penetrate the surface soils down to the upper gravel layer. This option was rejected because of cost

"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

Stage 1 will appear as three buildings on Oxford Tce and one on Hereford St. It will be hospitality on the ground and all of the first floor areas in the Oxford Tce buildings. It will include office space on the upper levels in these buildings
Stage 2 will be the major large office building on the corner of Cashel and Oxford Tce that will contain approximately 6,000m² of office space and retail plus hospitality on the ground floor. It is due for completion in early 2016
Stage 3 will complete the Cashel St frontages and is likely to contain retail on the ground floor and a 100 bedroom hotel above
Stage 4 will be the final stage and it is expected to contain a 320 bay car park sleeved with apartments to sell on both its north face facing Hereford St and the west face facing the central piazza and the Avon River

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