Strain rate effects in Ballina clay: implications for settlement and stability of embankments

This thesis evaluates the influence of strain rate on the mechanical properties of Ballina clay, an estuarine soft clay found on the eastern coastline of northern New South Wales, Australia. Emphasis has been given in this thesis to the consequences of rate effects on the predicted embankment behaviour of a trial embankment, constructed on PVD-improved Ballina clay at the National Field Testing Facility (NFTF) Ballina, New South Wales. The experimental work carried out in this study combined high-quality block sampling techniques with advanced laboratory and in situ testing methods. Experimental results were used to assess the influence of strain rate effects in geotechnical design. Predictions of: (i) settlement, (ii) excess pore pressure, (iii) lateral deformation and (iv) short-term embankment stability were carried out for an embankment with and without PVDs. Therefore, simulations made for the embankment without PVDS are considered ‘blind’ predictions. This study shows a strong dependency of the predicted embankment behaviour on strain rate effects, mainly as a consequence of changes in yield stress (OCR) and undrained shear strength. It is demonstrated that a close fit of the behaviour of the trial embankment improved with PVDs is possible without making refinements to the laboratory results. This was achieved by using soil parameters obtained using high quality samples and correction of the yield stress for rate effects (i.e. using the yield stress for a reference strain rate of 10-7 s-1).