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UBC Theses and Dissertations

The analysis and interpretation of the cone pressuremeter in cohesive soils Hers, Ian


The cone pressuremeter is a promising new in situ testing device which combines the well known capabilities of a piezocone with a full displacement pressuremeter (FDPM). The focus of this thesis is to present results from FDPM tests performed as part of a cone pressuremeter sounding at three cohesive soil sites in the Vancouver area. The insertion of a cone pressuremeter results in a substantial amount of disturbance and the generation of excess pore pressures. As a result of the changing stress conditions, the length of the relaxation time or time delay between insertion and testing has a significant effect on the lift-off pressure and shape of the FDPM curve. Results indicate that increased relaxation periods lead to lower lift-off pressures. The strain rate used during a test is also significant with lower rates resulting in higher limit pressures and undrained shear strengths. Comparisons were made between the FDPM, self-boring pressuremeter (SBPM) and dilatometer lift-off and expansion pressures. FDPM test results are also influenced by the design and performance of the pressuremeter. Important equipment related considerations discussed are compliance, strain arm design and pressuremeter L/D ratio. The results of FDPM tests were used to estimate the undrained shear strength, shear modulus, stress history and in situ horizontal stress of cohesive soils and when possible compared to SBPM, field vane and dilatometer results. The use of cavity expansion theory for the analysis of the FDPM test is made difficult by the unknown stress conditions created by disturbance. Nevertheless, reasonable estimates of the undrained shear strength were made using cavity expansion methods with the FDPM undrained shear strength generally greater than the field vane and similar to those obtained from the SBPM test. Cavity contraction theory was also used to estimate the undrained shear strength with the results generally being less than the field vane undrained shear strength. Good comparisons were obtained between the FDPM and SBPM unload-reload shear moduli. Both the unload-reload shear moduli and the rigidity index were shown to attenuate with increasing shear strain. Two new methods using the rigidity index and normalized pressuremeter limit pressure were proposed to estimate stress history. Both techniques appear to be promising. Attempts to use the FDPM to estimate the in situ horizontal stress were unsuccessful when compared to the results of other available tests.

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