Analyses and interpretation of tapered piezoprobe and application to offshore pile design
Massachusetts Institute of Technology. Dept. of Civil and Environmental Engineering.
Andrew J. Whittle and John T. Germaine.
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This thesis presents theoretical predictions and experimental validation of a newly designed tapered piezoprobe penetrometer that is currently being used in offshore site investigation. The new device measures pore pressures at the tip of a 1/4" diameter extension piece that connects through a tapered section to conventional drill rods. By accelerating the dissipation of penetration-induced excess pore pressures, the design aims to provide reliable measurements of in situ pore pressures in low permeability marine clays in a much shorter timeframe than is possible with conventional piezocone devices. Predictions of probe performance are obtained using a non-linear coupled consolidation analysis, in which the effective stress-strain properties of the soil are characterized by the MIT-E3 model, and the initial penetration process is simulated using the Strain Path Method. The predictions show that the dissipation response can be divided into three stages: I) initial dissipation that is controlled by the radius of the extension piece; II) a transition stage, characterized by marked retardation of dissipation response, due to migration of pore water generated around the shaft of the drill rods; and III) long term response that converges to the behavior of a conventional piezocone and is not affected by the presence of the tip extension piece. Parametric studies show that the pore pressure dissipation is affected by the stress history of the clay and it's anisotropic flow properties, as well as details of the probe geometry. Although the design can be improved by increasing the length of the extension piece, the most useful modification of the design is to add a second pore pressure sensor located above the taper section on the driB shaft. Data from this designed dual sensor piezoprobe can then be interpreted using two new methods proposed in this thesis: I) Two-point intersection method that correlates the dissipated pore pressures at the two sensor locations in order to estimate the in situ pore pressures, u0, from an incomplete dissipation record. 2) Concurrent matching of dissipation data at the two monitoring points can be used to define the in-sim hydraulic conductivity. The theoretical predictions are evaluated through detailed comparisons with field dissipation measurements for piezoprobe and piezocone devices at MIT test site in Saugus, Massachusetts. In general, there is excellent agreement between the predicted and measured pore pressure dissipation for test performer at depths 65-1 15 ft. The backfigured hydraulic conductivities are consistently lower than laboratory measurements by approximately a factor of two. However, the in-situ pore pressure, u0 , can be estimated with 5% accuracy within I hr from the start of the dissipation test. Pore pressure dissipation is directly related to time dependent increases in the capacity of driven piles. Predictions of this set-up behavior are obtained by simulating the effective stress changes that occur in the soil close to the pile shaft during installation, coupled consolidation and undrained axial pile loading. Parametric studies establish the effects of stress history, pile geometry (radius and wall thickness) on predictions of the time dependent pile set-up. There is a significant difference in behavior predicted for sensitive, low plasticity clays and insensitive plastic clays. Site specific predictions are evaluated through comparisons with field data from instrumented model piles at four well-documented test sites. A generic design methodology is proposed for interpreting dissipation data from the tapered piezoprobe and predicting pile set-up at deepwater sites in the Gulf of Mexico. The proposed design charts are based on synthesized soil properties for an Average Gulf Clay. Preliminary calculations for pile load tests performed at the West Delta site confirm the importance of reliable measurements of in situ pore water pressures and soil pre-consolidation pressures.
Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 1999.Includes bibliographical references (v. 2, leaves 411-427).
DepartmentMassachusetts Institute of Technology. Department of Civil and Environmental Engineering
Massachusetts Institute of Technology
Civil and Environmental Engineering.