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1 <br /> 1 <br /> EGG Pore Pressure Dissipation Tests <br /> 1 ®� (PPDT) <br /> Pore Pressure Dissipation Tests (PPDTs) 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 (k,,) <br /> In order to correctly interpret the equilibnum 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 tioo, 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 /> (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 fabnc 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 /> I ' 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 />