Martin L. Lings

Horizontally Mounted Bender Elements for Measuring Anisotropic Shear Moduli in Triaxial Clay Specimens

Novel horizontally mounted bender element devices capable of high-quality transmission and reception of horizontally propagated shear waves polarized in orthogonal planes across the mid-height of a triaxial clay specimen are described. Mounting of these mini benders, in a T-shaped configuration, is in the pads of a radial strain belt on 100 mm diameter triaxial samples, or alternatively as bender probes (similar in concept to mid-height pore pressure probes) suitable for use on triaxial samples down to 38 mm diameter. This latter type may also be used in bench-top tests or on site. The effective fabrication procedures that have been developed are described. The instrumentation systems used to drive and receive signals are outlined, and estimates of the magnitude of the shear strains developed by the bender elements and the accuracy with which shear wave velocities can be determined are discussed. These new bender elements enable both anisotropic shear moduli to be measured over the same path length on a single triaxial specimen that can be taken to a wide variety of anisotropic stress states. Comparison with measurements using conventional platen-mounted bender elements suggests these may underestimate the shear modulus by up to 20%.

The Effects of Controlled Destructuring on the Small Strain Shear STiffness G0 of Bothkennar Clay

The important differences between natural and reconstituted soils are well recognised. In a natural structured clay, the microstructure enables it to exist at states outside the state boundary surface for the reconstituted soil, resulting in greater peak undrained strength and yield stress at a given void ratio. The structure of soft clays is gradually destroyed by strain; understanding the process of destructuring is important both in developing constitutive models and in understanding the differences between field and laboratory behaviour.