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Measurement of in situ lateral stress during full-displacement penetration tests Sully, John Paul


The thesis considers the problem of the in situ measurement of horizontal stress with specific reference to the use of full displacement probes. It is generally accepted that correct measurement of the horizontal stress in situ should be performed under conditions of no disturbance since, depending on the soil characteristics, even small amounts of disturbance can significantly alter the in situ conditions. The self-boring pressuremeter is widely acknowledged to be the best available instrument for measuring horizontal stress but recent research has shown the data obtained to be very sensitive to the effects of probe installation. On the other hand full displacement probes cause repeatable degrees of disturbance and the induced stresses and pore pressures may provide a means of backfiguring the initial pre-penetration in situ stress condition. The thesis presents the results of a detailed programme of in situ testing using both self-boring and full-displacement probes during which measurements of both stress and pore pressure have been performed. In addition these measurements have been performed at various locations on the full-displacement probes to evaluate the stress distribution. Both platelike and cylindrical probes have been used in the study. Reference profiles of lateral stress have been established for each of the research sites based on both in situ and laboratory test results. The stresses measured by the full-displacement probes and the interpreted in situ conditions are compared to the reference profiles. The data suggest that in soft to stiff clay and sands reliable estimates of the reference lateral stress profile can be obtained from the large strain measurements using semi-empirical techniques which are based on the results of published case histories. Certain index parameters are also shown to provide consistent indicators of the variation in K₀ stress state as obtained from the reference tests. Using both calibration chamber and field data both the cone resistance and pore pressure gradient around the tip are shown to be dependent on the in situ horizontal stress. Theoretical approaches for evaluating the stresses around full-displacement probes are considered and cavity expansion formulations are applied to measurements in both sand and clay. The data clearly show the inadequacy of the theoretical approaches to consider the unloading that occurs around the penetrometer tip. The degree of unloading is shown to be a function of both soil and probe characteristics.

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