A cutting edge grouting technique has been used to lift and correct the foundations of an art gallery which was damaged in the 2011 Christchurch earthquake.
When Christchurch’s Te Puna o Waiwhetu Art Gallery reopens to the public this month, it will be the first time the striking building has functioned in the capacity for which it was built since the Christchurch earthquake of February 2011.
The gallery is the largest art institution on New Zealand’s South Island and has accumulated over 6,000 works of art since its foundation in 1932. It moved to its new home in the futuristic Te Puna o Waiwhetu building in 2004.
The curved structure was designed to reflect Christchurch’s two winding rivers. It is four storeys high and sits on a 600mm to 1.2m thick concrete raft foundation with a series of columns supporting the structure above.
The foundation design enables earthquake forces to be distributed evenly through walls which were designed and braced to withstand local seismic activity.
In addition, the building’s distinctive glass façade is built as a frame within a frame so that it is self-supporting and isolated from ground movement. The glass panels are also connected by ball joints allowing the façade to flex during vibration.
All of these measures were sufficient to keep the building standing during the initial magnitude 7.1 earthquake which struck Christchurch in September 2010 and the most violent aftershock in February 2011, measuring 6.3. Nevertheless the building still sustained substantial damage.
“Due to liquefaction, the building had settled about 182mm at its worst. It had more or less broken its back and bent through the centre,” says Mainmark Ground Engineering New Zealand technical project manager Russell Deller. His company won an international tender from Christchurch City Council to lift the building and correct the level of its foundations.
The company claims that the lifting operation it carried out on the building is the first in the world to use jet grouting techniques in combination with Jacking Off Grout (Jog) integrated computer grouting. Jog was a technology developed in Japan in the mid-90s.
In short, it is a process where multiple grout pumps precisely controlled by computers carry out a rapid succession of cementitious grout injections to provide hydraulic lift. The amounts injected are dictated by degrees of settlement, loads, voids and width of footing.
Setting times can also be varied to control the spread of the material and to make sure uniform support is maintained beneath the foundations. Because only a small amount is injected at each port, the lifting happens gently.
“We’ve discovered that the system works extremely well in soils that have been traditionally difficult for us with other methods,” says Deller.
“But we also like the ability to put almost unlimited injection points in and to be injecting them all simultaneously and to float the building up in a very controlled manner.”
For the gallery in Christchurch, Mainmark Ground Engineering New Zealand carried out most of the lifting work in the gallery’s underground car park.
From there it drilled multiple holes to jet grout cement columns beneath the building to create a reaction platform from which the Jog could lift off. But before it could do any of this, the construction team had to remove 124 ground anchors that were holding the building down.
“Because the gallery was constructed below the water table, in order to cast the concrete slab without it suffering from buoyancy issues during excavation of the basement or construction, there were 124 strand anchors which went down 16m or 20m,” says Deller.
“We had to locate those and we had to release them as we lifted the building.”
With the ground anchors released, the site team drilled 124, 200mm diameter holes through the foundation to a depth of 6.5m. They then fired high pressure jet grout into each hole to create a series of 4m columns stopping roughly 2m below the foundations, creating a reaction platform from which the building could be lifted using Jog.
For Deller, the benefit of the technique is the amount of control it provides.
“We had 350 Jog injectors installed across the entire footprint of the building,” says Deller. “It’s that ability to network or plumb the entire structure and inject into each and every one of those locations effectively at the same time which means that you can float a very large building or area in a very controlled manner incrementally, millimetres at a time.”
He adds that such an approach is preferable to other systems where the contracting team only injects in two to four locations at any one time, running the risk of introducing new stresses to the structure and of having an uneven lift.
Because the gallery settlement was uneven in places, the process for establishing how much grout to inject and where it would be had to be carefully monitored. To do this, Mainmark worked with positioning solutions provider Geosystems New Zealand to develop an automated monitoring system. Together they came up with a way of monitoring the structure in 350 locations using five robotic Trimble S8 Total Stations inside the car park.
In total 290 prisms were installed in the car park and around the walls and columns of the perimeter to monitor every injection point. Data was then relayed by wi-fi back to a central control room.
An added complication was created by the fact that the building was moving throughout the day in response to thermal temperatures and the total stations were moving with the floor.
To overcome this, they used a digital level to find the true level of the total stations three times a day.
The accuracy of the data and real-time feedback provided by the monitoring system allowed Mainmark to develop a fully automated traffic light warning system for the grout injection.
As soon as a round of grouting was completed, the Trimble T4D Control Web interface was updated to collate all of the data from the monitoring points.
“If one injection point was more than 4mm out from its surrounding points, the screen would turn to yellow from green. If it deviated by more than 8mm, a red warning would show. is allowed the site team to isolate the offending injection site for investigation and gave the client greater confidence that the building was being raised in a controlled fashion. “The Jog computer controls how much material each injector receives,” explains Deller. “The area of the building that needs to be lifted more than the adjacent areas receives more material and once that lowest point is up to the next contour, the adjacent contours all receive the same amount of material so that you lift it up gently.
“It’s literally at the touch of a button that we can change and alter the amount of material and the amount of lift.”
Because the Jog system uses a cementitious product, Deller says it can be used below the water table with limited environmental impact. But he is most excited by the potential to raise very large buildings by significant degrees.
“The accuracy and control of the system provides the ability to lift massive structures an unlimited amount,” he says.“There’s nothing to say you couldn’t lift a building a metre if you so required.”