Prediction and Measurement of Pore Pressure Dissipation for a Tapered Piezoprobe

Abstract Reliable measurements of in-situ pore pressures in low permeability seabed soils are very difficult to achieve by conventional piezocone devices due to the length of time required for the dissipation of penetration-induced excess pore pressures. This problem has been addressed in recent deep water site investigations by using a tapered piezoprobe, which measures the pore pressures close to the tip of a 3.2 mm radius, 260 mm long extension piece that fits onto the end of a standard cone rod (17.8 mm radius). This paper summarizes a combined theoretical and experimental validation of the tapered piezoprobe design. Predictions of pore pressure dissipation are obtained using a non-linear coupled consolidation analysis, with effective stress-strain properties of the soil characterized by the MIT-E3 model, and initial conditions computed by a strain path model of undrained probe penetration. The predictions show that 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. A program of field experiments have validated these important findings by comparing the measured dissipation behavior of the tapered piezoprobe and piezocone devices at a well documented site near Boston, where in-situ pore pressures are well defined. The results confirm the characteristic features predicted by the analyses, while backfigured values of hydraulic conductivity are typically a factor of 2 smaller than reference laboratory values measured on vertically oriented samples. In situ pore pressures can be estimated within ±5% by comparing incomplete pore pressure dissipation records from the piezoprobe and piezocone devices over a period of 1–2 hours. Further calculations suggest that similar predictions can be achieved by incorporating a second porous filter on the shaft above the taper section of the piezoprobe. Predictions for typical Gulf of Mexico clays must now be validated through controlled field measurements. INTRODUCTION Reliable measurements of in-situ pore pressures in marine sediments are an essential component of offshore geotechnical site investigations. Recent site investigations for deepwater prospects in the Gulf of Mexico have revealed large upward hydraulic gradients within low permeability, clay sediments associated both with their rapid deposition (underconsolidation) and with high pressures in more permeable, underlying deposits ('shallow-water-flow': Pelletier et al., 1999). Current methods of measuring the insitu pore pressures are all based on monitoring the dissipation of penetration-induced excess pore pressures, below the base of the borehole. This process becomes impractical using conventional piezocone devices as the time required to achieve full dissipation can require in excess of 24hrs at a single elevation. In order to address this problem, Fugro-McClelland Marine Geosciences Inc. developed a tapered piezoprobe device which measures pore pressures close to the tip (radius, R1 = 0.32 cm) of a 26 cm long taper extension piece that fits onto the end of a standard cone rod (R2 = 1.78 cm).