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1 <br /> I <br /> EGG Pore Pressure Dissipation Tests <br /> (PPDT) <br />' <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 />' 0 In situ horizontal coefficient of consolidation (ch) <br /> ® In situ horizontal coefficient of permability (kh) <br /> I In order to correctly interpret the equilibrium piezometric pressure and/or the <br /> phreat€c 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 t1co, 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 at (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 /> I ' <br />