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Resonant column

The resonant column test was initially developed in the US to assess the changes in soil stiffness and damping that occur during earthquake loading. Although it is commonly specified in the US, for obvious reasons, it is still a relatively rare test in the UK, where, in contrast, it is often used to investigate the changes in stiffness that occur in soils at low shear strain levels.

A cylindrical specimen is fixed to the base pedestal of the resonant column machine, then saturated and consolidated. A vibration excitation device then twists the top of the specimen at varying frequencies until the resonant frequency of the soil is observed. By continuously measuring the applied torque and the resulting acceleration, it is possible to infer the stiffness of the specimen at resonance, and the corresponding shear strain. And by temporarily stopping the vibration and allowing the specimen to return to a steady state, it is possible to infer the viscous damping properties of the soil.

This is repeated for a range of applied torque giving a range of shear stresses and shear strains. As the test proceeds, and the shear strains increase, the specimen loses stiffness, with a similar increase in viscous damping.

Fugro's latest machine is capable of applying confining pressures of up to 3MPa, its two other machines being capable of up to 1MPa. The latest machine can also apply much higher torque and is intended to increase the scope of the test to include stiffer soils, soft rocks and even concrete. One disadvantage of the latest piece of equipment is that anisotropic consolidation of the test specimen is not currently possible. However, Fugro says that this shortfall will be rectified in the near future.

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