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EGG Pore Pressure Dissipation Tests (PPDT) Pore Pressure Dissipation Tests (PPDT's) conducted at various intervals measured hydrostatic water pressures and determined the approximate depth of the ground water table A PPDT is conducted when the cone is halted at specific intervals determined by the field representative The variation of the penetration pore pressure (u) with time is measured behind the tip of the cone and recorded by a computer system Pore pressure dissipation data can be interpreted to provide estimates of • Equilibnum piezometric pressure • Phreatic Surface • In situ horizontal coefficient of consolidation (ch) • In situ horizontal coefficient of permability (kh) In order to correctly interpret the equilibrium piezometnc pressure and/or the phreatic surface, the pore pressure must be monitored until such time as there is no variation in pore pressure with time (refer to Figure PPD) This time is commonly referred to as t1Lv, the point at which 100% of the excess pore pressure has dissipated Interpretation of either ch and kh from dissipation results can be most easily achieved using either of two analytical approaches cavity-expansion theory or the strain-path approach Comparisons of the available solutions and results from field studies suggest that the cavity-expansion method of Torstensson (1977) and the strain-path approaches of I_evadous (1980) and Teh (1987) all provide similar predications of consolidation parameters from CPTU dissipation data (Gillespie 1981, Kabir and Lutenegger 1990, Robertson et al (1991). Robertson et al (1991) have shown that these methods, although developed for normally consolidated soils, can be equally applied to overconsolidated soils : Furthermore, comparisons of field and laboratory data indicate that the trends in the measured (laboratory) and predicated (CPTU) data are consistent provided the micro fabric and nature of the soils being tested are taken into consideration. (Danziger 1990, Robertson et al 1991) A complete reference on pore pressure dissipation tests is presented by Robertson et al 1991 A summary of the pore pressure dissipation tests is summarized in Table 1 Pore pressure dissipation data is presented in graphical form in Appendix PPDT