Using Bender Elements to Measure Soil Stiffness at Small Strains in Laboratory Samples

Over the last two decades improved laboratory techniques have shown that soil stress-strain responses are highly non-linear over the range of strains and stresses that are relevant around foundations and excavations. Accurate predictions of settlement and of the stresses in the ground depend on knowing this non-linear stress-strain response. One of the parameters needed to describe this response is the stiffness at very small strains. A variety of techniques are available to measure this stiffness. In this paper the bender element method is described. One of the attractions of using bender elements is that they can be easily incorporated into existing equipment, and at least in principle, the test is very simple to conduct. A high frequency pulse is applied to one bender element and the time for this to be detected at the other end of the test specimen is recorded. It is shown how bender elements can be incorporated into existing tests with minimal hardware requirements. A method of automating the test interpretation is presented which should enable reliable stiffness measurements to be obtained from commercial laboratory environments. This method makes use of cross correlation between input and output signals. It is argued that the approach is easy to integrate into existing test control programs, and it is shown how the technique may be implemented at relatively low cost.