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olEGG <br /> I" Pore Pressure Dissipation Tests <br /> (PPDT) <br /> Pore Pressure Dissipation Tests (PPDT's) conducted at various intervals <br /> measured hydrostatic water pressures and determined the approximate depth of <br /> the ground water table. A PPDT is conducted when the cone is halted at specific <br /> intervals determined by the field representative. The variation of the penetration <br /> pore pressure (u) with time is measured behind the tip of the cone and recorded <br /> by a computer system. <br /> Pore pressure dissipation data can be interpreted to provide estimates of: <br /> • Equilibrium piezometric pressure <br /> • Phreatic Surface <br /> • In situ horizontal coefficient of consolidation (ch) <br /> • In situ horizontal coefficient of permability (kh) <br /> In order to correctly interpret the equilibrium piezometric pressure and/or the <br /> phreatic surface, the pore pressure must be monitored until such time as there is <br /> no variation in pore pressure with time (refer to Figure PPD). This time is <br /> commonly referred to as t100, the point at which 100% of the excess pore <br /> pressure has dissipated. <br /> Interpretation of either ch and kh from dissipation results can be most easily <br /> achieved using either of two analytical approaches: cavity-expansion theory or <br /> the strain-path approach. Comparisons of the available solutions and results <br /> from field studies suggest that the cavity-expansion method of Torstensson <br /> v <br /> (1977) and the strain-path approaches of Levadous (1980) and Teh (1987) all <br /> provide similar predications of consolidation parameters from CPTU dissipation <br /> data (Gillespie 1981; Kabir and Lutenegger 1990; Robertson et al. (1991). <br /> Robertson et al. (1991) have shown that these methods, although developed for <br /> normally consolidated soils, can be equally applied to overconsolidated soils. <br /> Furthermore, comparisons of field and laboratory data indicate that the trends in <br /> the measured (laboratory) and predicated (CPTU) data are consistent provided <br /> the micro fabric and nature of the soils being tested are taken into consideration. <br /> (Danziger 1990; Robertson et al. 1991). <br /> A complete reference on pore pressure dissipation tests is presented by <br /> Robertson et al. 1991. <br /> A summary of the pore pressure dissipation tests is summarized in Table 1. <br /> Pore pressure dissipation data is presented in graphical form in Appendix PPDT. <br /> v <br />