Prediction and interpretation of pore pressure dissipation for a tapered piezoprobe

This paper describes a combined theoretical and experimental study of a tap ered piezoprobe device that has been designed to measure the in-situ pore p ressures in offshore geotechnical site investigations. Predictions of pore pressure dissipation are obtained using a non-linear coupled consolidation analysis, with effective stress-strain properties of the soil characterised by the MIT-E3 model, and initial conditions computed by a strain path mode l of undrained probe penetration. Although the initial dissipation times of the probe are much shorter than for a conventional piezocone, the response is retarded by the arrival of a pore pressure front from above the taper s ection. The analyses suggest that more reliable predictions of in-situ pore pressures can be achieved in a shorter timeframe by introducing a second p orous filter on the shaft above the tapered section and correlating the por e pressure dissipation at the two sensors. Entire dissipation responses of the tapered piezoprobe and piezocone devices have been measured concurrentl y at a well-documented site near Boston. The results show excellent agreeme nt between the predicted and measured dissipation curves at depths of 20-35 m. However, backfigured values of hydraulic conductivity are consistently less than laboratory measurements at the same elevation by a factor of 2. I n-situ pore pressures can be estimated accurately within 1 h at this site b y correlating the dissipated pore pressures measured at the tip of the tape red piezoprobe and the response at the base of the piezocone.