Investigating the influence of groundwater on the <i>in-situ</i> behaviour of Poisson's ratio using downhole array data

SUMMARY Poisson's ratio is a commonly used parameter in geotechnical engineering research. Although most studies on its dynamic behaviour have been performed using laboratory tests, which are not necessarily representative of the anticipative soil behaviour in practice, especially nonlinear behaviour. By contrast, the in-situ studies using seismic data are limited and inadequate. Hence, in this study, we first performed the in-situ estimations of Poisson's ratios at various strains for seven research cases by applying seismic interferometry to seismic data from three seismic downhole arrays. Then, we proposed a proxy named equivalent water content, Weq, which was calculated by using site information and used to evaluate the influence of groundwater. Finally, by calculating the Weq for each research case, the in-situ dynamic behaviours of Poisson's ratio and the ratio between compression and shear wave velocities were investigated for sites with low shear stiffness and high water content. It was found that there were significant relationships between the proposed proxy and the in-situ behaviours of Poisson's ratio and the ratio between compression and shear wave velocities. The small-strain Poisson's ratio and the ratio between small-strain compression and shear wave velocities increased with Weq at the change rates, which gradually decreased and increased, respectively. The nonlinear variation of Poisson's ratio at each shear strain decreased with increases in Weq and shear modulus ratio at the change rates, which gradually decreased and increased, respectively. By constructing an analysis model, three formulae were further proposed to describe the above relationships, which can be utilized to build the empirical equations for preliminarily estimating the small-strain Poisson's ratio, the ratio between small-strain compression and shear wave velocities and the nonlinear variation of Poisson's ratio without requiring compression wave velocity.