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Built to Stand the Test of Time
Te Puna o Waiwhetu, the art gallery of Christchurch, New Zealand, is a striking landmark at the edge of the city’s historic cultural precinct. Designed by the Buchan Group of Melbourne, Australia, it opened its doors to the public in 2003. The gallery features a striking flowing glass and metal façade, or sculpture wall, on the outside; its forecourt incorporates a sculpture area, trees and recreational spaces. Inside there are nine exhibition areas, a reference library, an underground car park, a restaurant, retail outlets and extensive storage space for the gallery’s art collection. The gallery spaces are arranged across two floors connected by a dramatic marble staircase rising from the Gallery foyer.
But the gallery was engineered with more than form and function in mind–in response to Canterbury’s earthquake risk, the design brief for the gallery demanded very high levels of seismic tolerance. For this reason, the gallery’s foundation is a concrete raft slab that sits on the surface of the ground. This foundation evenly distributes earthquake forces through walls that are also designed and braced to withstand quakes.
Even the glass façade is built as a frame within a frame, so it can stand up on its own and is isolated from ground movement. The glass panels are connected by ball joints, which allow the facade to flex during vibration.
The Gallery’s Toughest Test
On September 4, 2010 Te Puna o Waiwhetu withstood the region’s first earthquake in many years. The 7.1 quake forced many other city buildings to close, but the gallery’s fortitude enabled it to temporarily house the Civil Defense emergency response headquarters and then re-open to the public within a few short weeks. Confidence in the safety of the gallery’s structure remained high.
However, on February 22, 2011 all of the gallery’s earthquake proofing measures were put to the ultimate test. The region’s most violent aftershock since the September quake hit the city at 12:51pm. Although smaller in magnitude at 6.3 on the Richter scale, the February quake was shallower and harder than the September event, with one of the world’s highest recordings of peak ground acceleration at 2.2g. It resulted in almost 200 fatalities and wreaked havoc on buildings, city infrastructure and land already weakened by the September quake.
Despite minor trauma–such as damaged ceiling panels, light tracks and cracking at wall joints–the Christchurch gallery stood strong. Every pane in the glass façade remained intact; just 22 art works sustained minor damage. Within 30 minutes the gallery was once again set up as the Civil Defense emergency headquarters.
But the ground beneath Te Puna o Waiwhetu was seriously compromised. The earth had liquefied and settled unevenly, eventually warping the building. The gallery became bowed in the middle and the iconic glass façade, which originally hung like a curtain, now touched the ground.
That Sinking Feeling
Due to its tectonic setting and active seismicity, the Canterbury region of New Zealand’s South Island has always been susceptible to earthquakes. And the area is wet–the land on which Christchurch city resides was once mainly swamp, estuaries and lagoons; two rivers, the Avon and Heathcote, gently meander through the city. So residents were shocked, if not surprised, when they woke to the intense shaking of a magnitude 7.1 earthquake on September 4, 2010. Since then, Canterbury residents have experienced over thirteen thousand aftershocks.
Arguably the most damaging result of all Canterbury’s quakes has been liquefaction, where wet sediment seeps to the surface with each sizeable quake. This loss of sediment from the ground has reduced the thickness of the crust between the surface and the ground water table, causing the level of the land to sink–in some areas by up to 1.5 m (4.9 ft)–and making it less able to support construction.
The Repair Plan: Float the Gallery Up
The Christchurch City Council committed to repairing Te Puna o Waiwhetu to 100 percent of the building code, including additional earthquake stabilization work. The repair plan entails lifting the building and correcting the level of its foundations, repairing the façade and interior, and retrofitting base isolation for further earthquake proofing.
Following a long design process, Mainmark Ground Engineering won the Council’s international tender to perform the lifting and level correction repairs–Mainmark have extensive experience in this work for residential and commercial buildings.
Russell Deller, technical project manager on the project, noted the importance and high visibility of the project. "This was a landmark project for a landmark building," he said. At 6,500 m² (70,000 ft²), Christchurch’s gallery is the largest building in the world to be leveled and lifted to this extent. It therefore required us to be visionary in our approach." So Mainmark sourced the technology and partners to develop a method to float the building upwards.
The project commenced in August 2013, with Mainmark setting up an extensive project headquarters in the underground car park of the gallery, from where all work would be performed.
Mainmark planned to use a process known as jet grouting, or jet crete, where cement columns are created beneath a building’s foundations and fast-acting grout is then used to lift the building. Christchurch’s gallery would be the first major building in New Zealand to use this technology.
From the underground car park, Mainmark’s team drilled 124 small (200 mm [8 in]) holes through the foundation to a depth of 6.5 m (21 ft). They then jet high-pressure grout into each hole. The grout mixed with the natural soils around it to create a column of about 4 m (13 ft) diameter. Each of the 124 resulting columns stopped about 2 m (6 ft) below the foundation. Notably, Mainmark was able to complete all its activities within the limited space of the carpark…a confined area with a height of just 2.5 m. (8 ft). Mainmark equipment is compact enough to enable this unique capability.
Once the jet-grout columns were complete, a technology known as JOG Integrated Computer Grouting was applied to re-level the building. "JOG (an acronym based on Japanese terminology) provides hydraulic lift," says Deller. "During each sequence, only a relatively small amount of material is injected at each pass, at each location, and so lifting is gentle and widespread. By pumping the grout into multiple spaces at the same time, we could slowly raise the building upwards a millimeter at a time."
Every step of Mainmark’s process was controlled above ground with a computerized system operating the grout injections.
Monitoring Progress and Measuring Success
The settling of the gallery was not even– in some places the building was 182 mm (0.6 ft) too low, in others just 40 mm (0.1ft), and in still others it had to be raised 90 mm (0.3 ft). In order to determine how much grout to inject and when, Mainmark needed to precisely know the gallery’s position at any given time. And accuracy was critical–if a section of the gallery was raised too quickly, the foundation would crack.
Before embarking on the project the Mainmark team, headed by Russell Deller, approached positioning solutions provider Geosystems New Zealand Ltd, with a vision of the monitoring solution they needed. Patrick Manson from Kevin O’Connor and Associ
ates was hired by Mainmark as a surveying consultant.
In designing a monitoring network for the job site, it was identified that a selfleveling network was not possible because the structure of the underground car park negatively impacted communications between the total stations, router/laptop and also line of sight to common targets between the total stations. Also, it was not possible to achieve consistent line of sight to the outside for control points to transfer the true levels into the basement.
This constraint was addressed by placing five robotic Trimble® S8 total stations as monitoring instruments inside the gallery’s car park. Each total station was connected via WiFi to a central access point and had its own power supply and router. A sixth total station was located outside. It was connected to the main access point by WiFi plus cable due to its distance from the central access point. Because Mainmark needed to monitor in as many places and as frequently as possible, 290 prisms were installed in the car park–on walls, columns, the floor and around the perimeter.
The team was primarily concerned with vertical measurements, so essentially the Trimble S8 instruments were to be used as robotic digital levels. "Given the innovation required in the network," says Manson, "our team relied heavily on the innate accuracy that the Trimble S8s provided."
Trimble 4D ControlTM software controlled the instruments in measuring rounds of angles to each point. Rounds were scheduled to finish within a few minutes of each other so that Trimble 4D Control could process all data at once. For example, one total station capturing 90 locations took approximately 40 minutes to complete its round while another instrument had to capture just 15 targets, so these required different start times.
Each round determined the exact position of the prism targets approximately every 45 minutes. To capture the same information using a "dumpy", or conventional level, would have taken Mainmark personnel at least four hours, plus at least 30 minutes processing time–a downtime that would have seriously impacted the project’s timeline.
The biggest challenge to the surveying team was that while they were measuring the movement of the building, the total stations were moving with the floor. This meant that each real-time round measured relative displacements only. To find the true level of the total stations, Manson and his surveying colleagues needed to check their control three times a day –and see how much lift the JOG Integrated Computer Grouting had attained–with a manual level. They used a Trimble DiNi® digital level, which enabled them to complete their checks in just 45 minutes.
Customized Output Streamlines Grout Injection
Once the monitoring network was established, Mainmark’s challenge was to communicate the project’s complex surveying data to the personnel injecting the grout. So a simple, fully automated traffic-light system was developed to identify any movement outside the project’s tolerances.
Manson, now the surveyor on site, monitored the Trimble 4D Control data on a 50-inch screen that was sufficient to display and compare every prism at once. "The ability to see a multitude of points very, very quickly was a huge advantage," Deller said. The raw data was displayed in the Trimble 4D Control Web interface and was overlaid over a plan of the gallery’s floor that showed Mainmark’s grout injection points. Mainmark’s personnel in the injection control room viewed the same data, also on the Web interface. As soon as a round was completed the Web interface was updated.
A green, yellow and red visual alarm system was set up in Trimble 4D Control Web. If one injection point deviated 4 mm (0.16 in) from its surrounding points, it would change from green to yellow on screen. Manson would then investigate why–often a yellow alert would indicate a simple survey issue such as line-of-sight to a target being temporarily blocked. If the point deviated by 8 mm (0.03 ft) from its surrounding points, then it would turn red. Mainmark would isolate the offending injection locations in order to investigate.
This easy-to-interpret traffic-light system significantly reduced the risk of damage to the building and facilitated communication between the surveyor on site and Mainmark personnel.
"Overall this monitoring system gave Mainmark and the client a high level of confidence that the building was being lifted in a safe manner," said Manson. "It increased the amount of data we could capture, and in a shorter amount of time. We were measuring so many points that if anything was happening to the building that shouldn’t be we were sure to pick it up."
A Groundbreaking System
"The job wasn’t without its challenges," Manson adds. "But now I can sit back and think, "actually, that was really cool"."
Mainmark was able to successfully complete the gallery’s level correction in less than three months, on tendered time and budget. Going forward, the company plans to employ their Trimble Monitoring solution on projects in other regions around the world.
"We developed this monitoring system because we wanted it to help us better manage the Christchurch gallery project," Deller said. "It proved itself to be absolutely invaluable, so now we believe we’ve set a new standard in our industry." Meanwhile, the still beautiful Te Puna O Waiwhetu will stand tall and strong at its re-opening in 2015, right on schedule.
Frances Mortimer is a freelance writer specializing in high-tech positioning solutions, including conventional, GNSS and spatial imaging survey systems. Frances is based in New Zealand.
Sidebar:
JOG Integrated Computer Grouting technology was developed in Japan in 1995. It is thoroughly proven, having raised huge culverts and re-leveled hundreds of single and multi-story buildings, including many affected by liquefaction in Japan’s 9.0 earthquake in 2011.
In JOG, one or more high-pressure grout pumps provide hydraulic lift; computers control a rapid succession of grout injections and determine the amount of liquid grout injected at each port. Amounts vary depending on the degree of settlement, width of footing, load, and voids. Setting times also vary from seconds to minutes.
At each pass, only a relatively small amount of grout is injected at each port, and so lifting is gentle and widespread. JOG experts speak of "floating" their project structures back to level. Multiple ports can be injected simultaneously–during each grouting sequence, individual ports are injected many times.
JOG is used under exacting environmental and performance controls, with conditions constantly monitored and reviewed, and information being fed back into the computers.
A 1.612Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE