Shear Wave Measurements Using Bender Elements in Argillaceous Rocks

The bender element technique for the measurement of the small strain shear stiffness of soils is here extended to measurements in very stiff argillaceous rocks that will be subject to artificially induced weathering. Moduli between 3 and 17 GPa are thus determined in natural samples of two different intact materials: Lilla claystone and Opalinus clay. It is explained how practical difficulties related to bender insertion and high frequency testing were overcome. Theoretical difficulties, particularly those related with possible near field noise, are examined in detail. Travel time is established by the first break technique by jointly examining the output from several input signals of different characteristics. The error associated with the technique is bounded by making similar measurements in several dummy samples of materials (aluminum and Lucite) whose well-known elastic properties lie in a similar range to that of the tested geomaterials. The effect of suction on the small strain stiffness of the homogenous Lilla clay samples is shown to be similar to that previously observed—at a lower suction range—in unsaturated compacted soils.