Pore Pressure Response Following Undrained uCPT Sounding in a Dilating Soil

The generation and dissipation of pore fluid pressures following standard piezocone sounding (uCPT) sounding in silty sands are observed to exhibit many of the characteristics of undrained penetration in dilatant materials; steady excess pore pressures may be subhydrostatic, or may become subhydrostatic during dissipation, and are slow to decay. Enigmatic pore pressure dissipation histories which transit from sub- to supra- and again to subhydrostatic before equilibrating at hydrostatic are consistent with a response where undrained pressures are maximally negative remote from the penetrometer tip. This surprising distribution of induced pore fluid pressures is accommodated in cavity expansion models for a dilating soil. A Mohr-Coulomb constitutive model is established for undrained loading of a soil with pore pressure response defined by Skempton pore pressure parameters. Defined in terms of effective stresses, this allows undrained stresses and pore pressures to be determined following cavity expansion in a $c–\varphi$ soil. Pore pressures are conditioned by the shear modulus, Skempton $A$ parameter, and the “undrained shear strength.” The undrained shear strength is additionally modulated by the magnitudes of $c$ , $\varphi$ , $A$ , and of the initial in situ effective stress, ${\sigma }_0^{\prime }$ . Cavity expansion stresses, and pore pressures may be backcalculated. Undrained pore pressures are shown to decay loglinearly with radius from the cavity wall; they may be either supra- or subhydrostatic at the cavity wall, and where suprahydrostatic may become subhydrostatic close to the transition to the elastic region. This initial pressure distribution contributes to the observed switching between supra- and subhydrostatic pore pressures recorded during dissipation. “Type curves” that reflect the dissipation response enable the consolidation coefficient, undrained strength, and shear modulus to be computed from observed pore pressure data, and confirmed against independent measurements. In addition to representing the dilatory response of cohesionless silts, the method applies equally to recovering the pressure generation and dissipation response of overconsolidated clays.