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Christchurch Art Gallery

Earthquakes that hit New Zealand’s South Island in 2010 and 2011 caused severe liquefaction of the ground under the foundations of the recently built Christchurch Art Gallery with consequent uneven settlement of the building. Mainmark Ground Engineering re-supported the building and then completely re-levelled the structure.

christchurch art gallery ground stabilisation

The Challenge

JOG 306x195 CAGsitelocationThe challenge was twofold. Firstly, to stabilise the sub-soil under the building to provide a sure foundation for the relevelling process. Secondly, to precisely re-level the entire 33,000 tonne, 6,500 square meter building to levels agreed with the client. During the strengthening and lifting, the utilities services to and from the building were to be maintained and the ground anchorage controlled.

All the works were carried out with the art gallery operational staff in occupation. The gallery had been closed to the public since the earthquakes.

What was done and achieved

Two new, leading edge technologies were used, Jet Grouting to strengthen the foundation sub-soil, JOG integrated computer grouting to relevel the structure. Neither of these technologies had been used in New Zealand prior to Mainmark introducing them in response to the 2010-11 earthquake remediation challenges.

There were several preparatory tasks. Sub-soil investigations were carried out to measure where the foundation sub-soil required strengthening and by how much. Also, working with the City Council engineers and engineering firm Aurecon, Trimble ‘Billion-point Plotting’ was employed to produce the 3D computer models of the building, defining positions and levels around and within the structure thereby establishing the benchmark data for the monitoring process. Areas of dilapidation in the building were recorded on the 3D models. The set-up of multiple wireless, automated, robotic survey stations enabled real-time monitoring of the levels of thousands of points across the entire building during the lifting.

JOG 306x195 ContourmapThe basement slab was below the water table, and the area was de-watering allowing the 72 ground anchors to be temporarily disconnected. They were reconnected when de-watering was stopped after the lifting operation.

124 jet grout columns were installed under the critical load points of the building. These columns can be described as ‘soil-crete’ some 3 to 4 metres diameter and 6.5 metres deep. They were formed through holes in the basement slab of just 200mm diameter. These jet grout columns stabilised the sub-soil forming a deep secure foundation to the building. Mess from the excess slurry and extracted soil was kept to a minimum by a continuous process of drawing it away through large hoses to waste tanks out side of the building.

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The JOG process was then brought into action to lift the building. 350 grout injection ports were installed into 40mm diameter holes in selected locations in the basement slab. 10 grout pumps delivered cementitious grout through multiple control valves set up in circuits enabling the control of grout delivery to each of the 350 injection points individually. Nearly 1.5 million litres of grout were pumped over a 44 day period. The average daily delivery was 34,000 litres with a peak one day to 71,000 litres. Lifting of up to 182 mm was required in places with an average lift per day of 2mm.

On completion of the lifting the basement was re-waterproofed, the ground anchors reactivated and the utility services reinstated.

The entire 6,500sqm structure was relevelled to within +/- 10mm and the stabilising of the foundation sub-soil will provide improved foundation behaviour under future seismic loads.

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