Undrained Cyclic Pore Pressure Response of Sand-Silt Mixtures: Effect of Nonplastic Fines and Other Parameters

Literature regarding the pore pressure generation characteristics and in turn the cyclic resistance behaviour of silty sand deposits is confusing. In an attempt to clarify the effect of nonplastic fines on undrained cyclic pore pressure response of sand-silt mixtures, an experimental programme utilising around 289 stress-controlled cyclic triaxial tests on specimens of size 50 mm diameter and 100 mm height was carried out at a frequency of 0.1 Hz. Specimens were prepared to various measures of density through constant gross void ratio approach, constant relative density approach, constant sand skeleton void ratio approach, and constant interfine void ratio approach to study the effect of nonplastic fines on pore pressure response of sand-silt mixtures. The effect of relative density, confining pressure as well as the frequency and magnitude of cyclic loading was also studied. It was observed that the pore pressure response is greatly influenced by the limiting silt content and the relative density of a specimen corresponding to any approach. The influence of other parameters such as relative density, confining pressure and magnitude of cyclic loading was as usual but an increase in frequency of cyclic loading was seen to generate excess pore pressure at a higher rate indicating an impact load type of behaviour at higher frequency. Utilising the entire test results over a wide range of parameters a new pore pressure band for sand-silt mixtures in line with Lee and Albaisa (1974) has been proposed. Similarly another pore pressure band corresponding to 10th cycle of loading as suggested by Dobry (1985) and up to a shear strain of around 25% has been proposed. These two bands can readily be used by researchers and field engineers to readily assess the pore pressure response of sandsilt mixtures.