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:

·         Equilibrium piezometric pressure
·         Phreatic Surface
·         In situ horizontal coefficient of consolidation (c_h)
·         In situ horizontal coefficient of permability (k_h) 

In order to correctly interpret the equilibrium piezometric 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, Figure PPDT.  This time is commonly referred to as t₁₀₀, the point at which 100% of the excess pore pressure has dissipated.

Figure PPDT 

 A complete reference on pore pressure dissipation tests is presented by Robertson et al. 1991.