Effects of partial saturation on the liquefaction resistance of sand and silty sand from Christchurch

The liquefaction resistance of partially saturated soil was experimentally investigated for one clean sand and one silty sand collected from a site in Christchurch, in an area severely affected by liquefaction in the 2010–2011 Canterbury earthquakes. A series of cyclic undrained tests were performed on fully and partially saturated sand and silty sand specimens, in conjunction with evaluation of saturation conditions in situ based on comprehensive field measurements of P-wave velocity (V_p) in Christchurch deposits. The Skempton’s B-value and P-wave velocity were comparatively used as measures for partial saturation in the laboratory. B-value - V_p relationships from the test results indicate that V_p steadily increases with the B-value until a threshold B-value is reached beyond which V_p remains unchanged at values indicating full saturation, i.e. V_p ;≥ ;1600 ;m/s. In general, the liquefaction resistance of tested sand and silty sand increases with a decrease in the B-value or V_p, i.e. with a reduction in the degree of saturation. Furthermore, test results suggest existence of threshold B-values and V_p for tested soils beyond which no significant increase in the liquefaction resistance was observed. This threshold B-values and V_p were found to be dependent on soil type and applied confining stress. The effects of partial saturation on liquefaction strength are different for the sand and silty sand when using V_p as a measure for the degree of saturation. While a gradual rate of increase in liquefaction strength with decreasing V_p is observed for the tested sand, the liquefaction strength of silty sand shows similar gradual increase with a decrease in V_p up to about 800 ;m/s, which is then followed by an abrupt increase in the liquefaction strength for V_p ;< ;800 ;m/s. Generally good agreement between liquefaction strength of tested soils and published data was observed, with a clear distinctive feature in the behaviour of the silty sand as compared to clean sands.