Multidirectional bender element measurements in the triaxial cell: equipment set-up and signal interpretation

Summary The paper presents a description of the arrangements of the vertical and horizontal bender elements and their implementation in stress-path triaxial cells, together with the comparison among three of the most commonly used interpretation methods of the bender elements signals to identify the travel time of the input wave to the receiver. The methods are the first arrival time, travel time between the characteristic points, cross-correlation method and π-point phase comparison method. For the material tested in this research and the test boundary conditions, the signals from bender elements demonstrate that the travel time should be taken as the time corresponding to that obtained by the first arrival method based on the visual identification of the wave arrival to the receiver. The horizontal and vertical bender elements implemented in stress-path triaxial cells have been used to investigate the evolution of shear moduli G(ij) of reconstituted specimens of Lucera clay (Southern Italy) under both isotropic and anisotropic stress states up to pressures higher than those usually achieved in similar studies. In this way the influence of long anisotropic stress paths on the clay stiffness will be highlighted. It is deduced that the different plastic straining resulting from the imposition of different virgin radial paths tends to modify the original pattern of G(ij).