Prediction and interpretation of pore pressure dissipation for a tapered piezoprobe

Abstract

This paper describes a combined theoretical and experimental study of a tapered piezoprobe device that has been designed to measure the in-situ pore pressures 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 model 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 section. The analyses suggest that more reliable predictions of in-situ pore pressures can be achieved in a shorter timeframe by introducing a second porous filter on the shaft above the tapered section and correlating the pore pressure dissipation at the two sensors. Entire dissipation responses of the tapered piezoprobe and piezocone devices have been measured concurrently at a well-documented site near Boston. The results show excellent agreement 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. In-situ pore pressures can be estimated accurately within 1 h at this site by correlating the dissipated pore pressures measured at the tip of the tapered piezoprobe and the response at the base of the piezocone.