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Engineering of geomaterials
Engineering of geomaterials
New research will change the way geotechnical engineers design civil infrastructure which interacts with unsaturated soils. Currently used design tools are only applicable when the interaction takes place with soils that are fully saturated or completely dry. However, often the soils are above the ground water table where they are variably saturated and may experience changes to their moisture content. Therefore, true margins of safety in retaining wall design, shallow foundation design or pavement design, for example, cannot be known using existing tools.
“When you are at the beach building a sand castle, the sand is strongest when a small amount of moisture is added to it. When the sand suddenly gets very wet, a lot of that strength is lost” says Associate Professor Adrian Russell, Senior Lecturer in the Centre for Infrastructure Engineering and Safety in the School of Civil and Environmental Engineering. “We’re looking at particularly complicated aspects of soil behaviour – what happens when the soil is unsaturated and when the amount of moisture changes a lot, for example through drought or flooding.”
When reflecting on widely used design tools based on Rankine earth pressure theory (1857) and Terzaghi bearing capacity theory (1943), Adrian says “the need is to update these so they are relevant to soils which vary in their moisture content”. “We have to seriously question established design procedures and possibly redo them all.”
For the past 15–20 years UNSW geotechnical engineers have been developing the mechanics of soil behaviour under different moisture conditions, but they are now modelling and developing practical applications that will feed into design codes.
“We’re building in our labs pieces of equipment to replicate how large structures operate and interact with soils while they are unsaturated and as they change from being very wet to very dry, and vice versa,” Adrian says. “We’re employing the latest mechanics of soil behaviour to solve real problems. Soil behaviour involving large moisture variations, and how unsaturated soils interact with infrastructure, is something the civil engineering industry doesn’t know a lot about yet.”
The work will be useful in everything from house construction to much larger projects, including embankment dams, airport runways and slope stability.
Adrian says that in the past, engineers basically just applied very large safety factors in design to deal with the uncertainties of soil behaviour. “In an extreme case you’d conduct a design, based on your knowledge and expertise, and increase its capacity by a factor of three for safety.”
This inaccurate method is unable to account for additional soil strength that may be present when the soil is unsaturated, resulting in unnecessarily conservative designs and expensive infrastructure. It also fails to account for soil strength losses that may occur due to sudden saturation of the soil, for example due to heavy rain or burst water pipes.
“At the very least we need to identify the risks and the likelihood of certain things happening as moisture conditions change, and if they do happen, how severe the impacts on our infrastructure will be,” Adrian says.
Further Information:
Associate Professor Adrian Russell
a.russell@unsw.edu.au
- See more at: https://www.engineering.unsw.edu.au/civil-engineering/engineering-of-geomaterials-0#sthash.KA8N6Bos.dpuf