This paper presents the results of a research programme in which the transfer functions of bender element/soil systems have been measured for various materials, set-ups and stress levels. The resulting database has allowed the characterisation of the inherent multi-vibrational nature of the system, and the interpretation challenges that this imposes. The excitation of multiple modes of vibration may introduce signal distortion and consequently differences between group velocity and phase velocity. This means that the common assumption of conventional interpretation techniques that group velocity is equal to shear-wave velocity is in error. Based on the features observed, system parameters have been introduced and a parametric analysis carried out in order to assess the performance of the system and to better understand the effects of multimodal vibration as an attempt to improve signal interpretation.

A laboratory investigation was made of the effects of isotropic unloading on the mechanical behaviour of cemented sandstones, modelling in a simple way the influence of sampling on hydrocarbon reservoir sandstones that have been cemented at depth. Artificial samples of sandstone were created by allowing a cement to hydrate while a sand sample was under a high confining stress in a triaxial apparatus. Comparisons were then made between the behaviour of samples that had been unloaded to zero confining stress with that of samples tested at stress levels around that during cementing. It was found that the isotropic unloading damaged the cement, which had a significant effect on the small-strain and yielding behaviour but not on the strength and large-strain behaviour. Comparisons were also made with tests on natural samples of reservoir and analogue sandstones, and it was found that the artificial sandstone had reproduced the key elements of their behaviour. The intrinsic properties of all of the sandstones tested were found to be similar, although there were significant differences in the peak strengths and gross yielding envelopes. Cement content was found not to be the sole factor that influenced the peak and gross yield envelopes, and significant anisotropy was also revealed in one sandstone.

The objective of the paper is to study the shear stiffness of Bío Bío sand. To this end, a system to measure the travel time of shear waves in Bío Bío sand samples using bender elements was designed and setup in an adapted oedometer device. Measurements were carried out for sand samples with different relative density, pressure/deformation, diameter/height, dry/saturated and varying the frequency, amplitude, and type of the triggered electric signals. The shear wave velocity and elastic shear modulus increased with relative density and effective vertical stress as previously found by other authors. In addition, Hardin type empirical formulas for estimating the elastic shear modulus are used to compare with the experimental results. Estimations proved to be good only in loading for effective vertical stresses around 100 kPa, since underestimation and overestimation of the shear modulus occurred for stresses below and above that value, respectively. Soil shear stiffness during unloading/reloading cycles was underestimated with the expressions used for loading.

No abstract