An engineer from UCD has been studying how earthquakes topple buildings. Dick Ahlstrom reports
The ground beneath your feet can turn nasty when it's shaken by an earthquake. Soil can weaken, causing foundations to crumble and bridges to tumble. It can even "liquefy", allowing heavy objects to sink without trace.
A researcher from the department of civil engineering at University College Dublin has become an expert in the way soil behaves when an earthquake strikes. She spent five years in California, studying how the soil above an active fault line deforms when there is an earthquake. She also knows what an earthquake feels like, having experienced five of them during her stay in the San Francisco area.
"Soil is a very complex substance," explains Dr Catherine O'Sullivan, a geotechnical engineer who lectures in soil mechanics at UCD. She is using computer modelling to try to predict how soil will perform, to reduce the risk of catastrophic building collapse. "To avoid failure, you have to be able to predict it," she says.
Deciding how soil will perform is difficult, because rather than being uniform it is a collection of discrete particles, each of which can behave in a unique way. Dr O'Sullivan's response is to model soil behaviour using discrete element modelling, a form of computer simulation in which you try to model each element in a structure - in this instance, the constituents of soil. She has already had some success with the technique, using "idealised materials" in the laboratory.
Essentially, she bundled steel rods together and squashed them, watching how they reacted in cross-section where they appeared as discs. Then she used a computer model to see if it would predict a similar disc movement. "And we were able to replicate what we saw in the lab using that simple model," she says.
Her latest research involves a three-dimensional set-up using steel balls put under the kinds of loads caused during an earthquake. Again, she is looking for a match between experimental responses and the computer model's results.
"What I am trying to do is start from the physics and model it from the inside out. The reason why we want to do this is that if you can model from the inside out, you know you understand the physics of soil."
Being able to tune the model to take account of various soil types would help reduce earthquake damage and be of benefit in many other fields, she believes. Dr O'Sullivan studies soil at a micro level in order to understand how it would respond at a macro level.
Many gas and water pipelines criss-cross above fault lines in California, for example. It would be a help to know how the soil would move around them. "You want to be able to design a pipe that can cope with that level of deformation," says Dr O'Sullivan.
Understanding exactly how soil will react is a powerful tool for reducing damage, she believes. Loose soils tend to "shrink" as they settle during disturbance; firm ones tend to break, due to sheer forces. Wet soils can liquefy during earthquakes, as water pressure causes particles to float freely, allowing heavy objects to sink.
Using this knowledge might point a way towards improving soil characteristics under buildings and around pipes. It might also bring savings by reducing a tendency to over-engineer structures in earthquake zones.
Surprisingly, Dr O'Sullivan's findings could be used in many other contexts, including helping to explain the behaviour of a range of particulate materials, such as grain or coarse powders.
It is also applicable to the food industry, she believes, including the handling, packing and transportation of apples, which respond like discrete soil elements under load. It may even be useful for offshore installations, as waves can put pressure on seabed footings.
O'Sullivan this month won an Irish Research Council (IRCSET)grant worth €120,000 to fund two PhD students to pursue soil modelling under her direction.