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New testing system for seismically sensitive soils

ELE INTERNATIONAL has developed a new cyclic triaxial soils testing system that it claims could lead to more reliable foundation design for structures built on seismically sensitive soils.

The system uses closed loop control and is designed to measure the liquefaction criteria for non-plastic silts.These materials are increasingly used in landfill, reclaimed land and coastal protection schemes and are particularly sensitive to stress and cyclic loading generated by earthquakes.

The laboratory equipment manufacturer says traditional tests using open loop control have been shown to produce inaccurate and over-optimistic results. It says this could result in 'inadequate foundation design and structures ill-equipped to withstand seismic shocks' Closed loop control uses servo systems to continuously monitor and modify applied stress, which gives a more realistic measurement of the cyclic strength.

A research team from Sheffield University, led by Dr Adrian Hyde, compared the new system with open loop apparatus on identical samples as part of the development.

It found that in an open loop test, as the peak of the cyclic stress path approached a critical stress ratio (CSR), the amplitude of the cyclic deviator stress began to decay.

At the same time, corresponding cyclic strain amplitude began to increase over several cycles until failure.

In the closed loop test, cyclic stress amplitude was maintained and failure occurred within two cycles of stress path touching the CSR line.This difference in behaviour is critical in defining cyclic strength of a silt for earthquake conditions, ELE says.

Data can be used to produce cyclic strength curves of liquefaction potential and to determine the most appropriate remedial measures.

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