Bender elements for the measurement of soil stiffness

The measurement of soil stiffness at small strains is assuming greater importance in the study of soil mechanics and its application to geotechnical design. Routine estimations of stiffness have traditionally been made in a stress path triaxial apparatus using local displacement transducers fixed directly on the sample. However, recent research has brought about the development of dynamic methods for the measurement of soil stiffness at very small strains using piezo-ceramic plates (bender elements). The value of bender element testing and its application to geotechnical design was highlighted by Prof. J.Atkinson [1] .

Piezo-ceramic elements distort or bend when subjected to a change in voltage. Two such elements are placed opposite one another and inserted a small distance into a soil sample (typically 3mm). The voltage in one element is varied creating shear waves through the sample, which are received by the opposite element. The input voltage, (created using a function generator) and the received signal are recorded continuously using an oscilloscope, allowing the travel time of the shear waves to be measured from which the dynamic elastic shear modulus (G max) can be determined.

Bender elements provide a reliable, cost effective alternative to undertaking locally instrumented stress path triaxial tests and can be readily performed on unconfined samples in the laboratory.

1. Atkinson, J.H. (2000) Non-linear soil stiffness in routine design. Geotechnique 50, No5, 487 - 508.

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