Shakin' all over

Shakin' all over

A one-of-a-kind 'shaking table' is helping to make retaining walls more stable during earthquakes
March 23, 2011

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At the Royal Military College of Canada (RMC) in Kingston, Ont., Richard Bathurst is a mover and a shaker. Literally.

Over the past 20 years, Bathurst, a civil engineering professor who is cross-appointed to Queen’s University, has become an internationally recognized authority on earthquake-resistant retaining walls — the vertical or near-vertical “earth structures” used to contain rock and soil behind highway overpass abutments, reservoirs and buildings.

Critical to Bathurst’s research is a hydraulic “shaking table,” located in a cavernous basement laboratory at RMC. Commissioned in 2008, the solid steel three-by-three-metre table is hooked up to banks of computer sensors and video cameras, and on it, Bathurst erects model retaining walls about 1.5 metres high. The set-up allows him to mimic the violent back-and-forth motion of earthquakes of varying magnitudes and monitor and measure the behaviour of different wall designs.

The table is the only one in Canada dedicated specifically to the testing of earth structures, and the data it has generated have allowed Bathurst to improve the seismic performance and cost-effectiveness of retaining walls in British Columbia, parts of Quebec, Japan and other earthquake-prone locales.

One way to construct a sturdy retaining wall is to make it really thick. But that requires large quantities of concrete and steel, which are expensive.

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Canada's only shaking table devoted to studying how soil structure responds to earthquakes. A one-fifth-scale model of a geosynthetic einforced soil wall gets shaken up during a simulated earthquake (ABOVE).
Richard Bathurst, Royal Military College

An effective, less costly alternative involves the installation of a thick vertical sheet of expanded polystyrene — basically, Styrofoam — between the back of a conventional steel-reinforced concrete wall and the soil it retains. In a quake, the foam acts as a flexible buffer, like a car’s shock absorber, between the wall and the shifting mass of earth behind it. When this simple option is employed in the foundation of a tall building, Bathurst has found that it can boost the earthquake resistance of the retaining wall by 50 percent.

Another solution is used in walls made of large concrete blocks. Bathurst lays metre-wide strips of specially manufactured plastic “reinforcement” — think of thick plastic chicken wire — on top of every second or third course of blocks, with one end projecting straight out into the soil behind the wall. (The strips measure 70 percent of the height of the finished wall, so the strips for a 10-metre-high wall, for example, would be seven metres long.) When the strips are buried in the compacted backfill soil, they effectively make the earth an integral part of the wall itself. Put another way, the blocks and the earth act as a massive unit that is remarkably stable in an earthquake.

Back at his shaking table, Bathurst builds scaled-down retaining walls with differing construction materials and designs, soil types and wall heights. During each experiment, he gradually increases the frequency, duration and intensity of the shaking until the structure begins to crack and fail, all the while gathering data from a large array of sensors and a high-speed video camera. The data enable him to validate numerical models that are used to predict how full-size walls might respond to quakes in different parts of the world. 

“It’s part of the tool kit that geotechnical research engineers can use to develop structures that are safer and more cost-effective,” says Bathurst. “Safety and cost-effectiveness are an important objective in civil engineering, particularly for structures in earthquake areas.”