Analysis of Dissipation Response for the Piezoprobe and DVTP-P on ODP LEG 204

The Strain Path Method (SPM) is used to predict the initial undrained pore pressure response upon insertion of the Davis-Villinger Temperature/Pressure Probe (DVTP-P) and Fugro McClelland's tapered piezoprobe. Uncoupled-isotropic consolidation is assumed and the coefficient of consolidation is estimated from excess pore pressure dissipation. The modeled response to insertion in normally consolidated Boston Blue Clay (BBC) includes the following. The initial excess pore pressure at the port is 3.3 times the in situ vertical effective stress for the DVTP-P, whereas for a piezoprobe, it is 2.4 times the in situ vertical stress. Dissipation of the piezoprobe is much faster than that of a DVTP-P in the early stages due to the much narrower tip of the piezoprobe. The time to achieve 50% dissipation, t50, for the DVTP-P is 2.6 times that for the piezoprobe. Nevertheless, the piezoprobe dissipation is retarded by the arrival of a pore pressure front from above tapered section, which forms a platform on the dissipation curve. The DVTP-P and the piezoprobe were deployed in the Ocean Drilling Program (ODP) on ODP Leg 204 at approximately 53 mbsf in the same lithology, but in an adjacent borehole. The in situ pore pressure (9.55MPa) can be estimated accurately within 0.5 hours at this site by comparing the dissipated pore pressures measured at pressure ports of the piezoprobe and DVTP-P (Two-Point Intersection Method), which is approximately the hydrostatic pressure (9.53MPa). The peak excess pore pressures of the DVTP-P and piezoprobe are 3.0 and 2.2 times the in situ vertical effective stress, respectively. t50 of the DVTP-P is approximately 2.5 times that of the piezoprobe. The results show very good agreement between the predicted and measured dissipation curves for both the piezoprobe and the DVTP-P. The coefficient of consolidation is estimated to be 8x10-7 m2/s.