Innovative in-ground engineering and a New Zealand first, sums up the relevelling of some of Christchurch’s iconic buildings and vital infrastructure. Richard Silcock provides some insight into the technology and the projects where it has been successfully used.
FOLLOWING THE 2010/11 earthquakes that devastated many of the buildings, infrastructure and the ground in Christchurch’s CBD and surrounding suburbs, a number of in-ground engineering projects have drawn interest for their complexity and innovation.
One such project was the relevelling of the Christchurch Art Gallery. With a 6500 square metre footprint and an all-up weight of around 33,000 tonnes, the gallery, with its expansive and eye-catching metal and glass vertical ‘wave-like’ façade had sunk on its concrete raft foundation by as much as 182mm in some places.
Enter Mainmark Ground Engineering which, using a relatively unheard of technique, was able to relevel the building while at the same time strengthen the integrity of the ground beneath the iconic building which houses valuable New Zealand and international artworks.
This relevelling was achieved over a period of two months, which given the magnitude of the project, was significant in itself, using a method known as ‘jacking on grout’ (JOG), or integrated computer controlled relevelling using a ground injection system. This entails injecting a specially formulated cement-based grout into the ground beneath the concrete slab foundation.
Mainmark brought the technology to New Zealand four years ago from Japan, where it has been used successfully to relevel infrastructures such as large buildings, bridges, tarmacs, pipelines, culverts, houses etc, and it has been particularly useful in relevelling some of the structures affected by the earthquakes.
“In essence JOG is a computerised relevelling system that involves, in sequence and in rapid succession, inert cementitious grout or resin being injected at a number of strategically arranged locations into the ground beneath the concrete slab foundation,” says Russell Deller, Mainmark’s NZ general manager. “This action creates multiple localised hydraulic forces which effectively ‘floats’ a structure back to its original level.
“This allows us to relevel complex and heavy structures and buildings like the art gallery without having to drill and place new piles into the ground or use hydraulic jacks which can be invasive.
“It is very akin to modern keyhole surgery,” he says.
For the art gallery, a two-stage solution was used. Firstly, the ground beneath the building was strengthened by jet-grouting (also known as ‘soil-crete’, it is a technique used for creating columns or silos of cementitious grout in the ground to strengthen it and provide support for the structure above), and then JOG was used to relevel the building.
The soil-crete columns under the gallery’s internal floor area are 3000mm in diameter and those under the external perimeter foundations are 4000mm in diameter and were inserted to a depth of between 2000mm to 6000mm below slab level, with the grout injected through penetrations of only 200mm. For the JOG sequence, 350 strategically-placed, 40mm injection ports were created in the slab foundation under the building which doubles as the underground parking area, and the cementitious grout injected.
“In combination, the JOG and jet-grout technologies had not been used together in New Zealand prior to this,” says Russell, “so it was pretty groundbreaking stuff.”
“To achieve the lift we used nearly 1.5 million litres of injected grout material for the JOG process, which required over 16,000 25 kilogram bags of cement. On an average day we achieved lifts of 2mm.”
The process of injection was carefully programmed into a bank of custom designed Trimble industrial computers which both monitored the ‘lift’ incrementally via sensors located around the building and the flow of cementitious grout being injected. The bespoke monitoring system included five automated stations that could monitor the ‘lift’ wirelessly from over 400 locations simultaneously.
“Each computer was programmed to control up to 128 injection points at once, in a sequence,” says Russell.
This massive project did require some preliminary tasks, such as investigating and measuring where the ground beneath the foundations required strengthening and by how much. Also required, after the building had been lifted, was the filling of all the JOG and jet-grout penetration ports in the basement slab, the reinstatement of all utility services and reconnection of 72 ground anchors.
“As a result, not only is the art gallery now level, it also exhibits much improved stability and better behavioural characteristics under seismic stresses,” says Russell.
Other projects in Christchurch that Mainmark has been involved with following the earthquakes have included relevelling a supermarket building, a large shopping and office complex, a multi-storey office building in the CBD, a sewerage pipe-line, several apartment buildings and private houses, and projects to stop groundwater entering damaged underground service utilities.
One such project included a six-storey concrete apartment block in Carlton Mill that was suffering from differential settlement of up to 70mm. Jet-grouting was used to strengthen the ground and then JOG injected to raise the level of the building back to its design level.
At the shopping and office complex in Ferrymead Road, two buildings had tilted backwards by 153mm and sunk by 67mm. Mainmark installed 84 JOG injection ports to lift the structure then applied expanding resin underneath the internal floor slabs to gradually raise and relevel the structure in just 22 days.
“For the 1350mm in diameter sewerage pipe, which was under construction at the time of the earthquake, a 40-metre section had settled by 60mm due to liquefaction. We were able to raise the pipe back to design level spec in just two weeks by injecting specially engineered structural resins to fill voids and improve the bearing capacity of the saturated silty soil,” says Russell. “Laser equipment was used to monitor movement and control the relevelling.”
The Christchurch Art Gallery project won the International Project of the Year Award at the 2016 Ground Engineering Awards, which were announced in London in June with the judges commenting that the project showed a strong and innovative application of modern technologies.