The simulation of sampling disturbance and its effects on the deformation behaviour of clays
The importance of studying the behaviour of soil at small strain levels (« 0.1 %) has been increasingly recognised in recent years. In the laboratory, tests are usually conducted on tube samples retrieved from the ground, which inevitably suffer from disturbance. This thesis describes an investigation of the effects of sampling disturbance on the small strain behaviour of one-dimensionally consolidated kaolin. Following the suggestion of Baligh (1985), tube sampling disturbance was simulated by applying a strain cycle in the triaxial cell. Comparative tests on 'disturbed' and 'undisturbed' specimens were conducted in a specially designed 100 mm hydraulic triaxial apparatus. Tests were also carried out in a 38 mm triaxial cell to study the effects of loading rate and overconsolidation ratio on the small strain deformation behaviour of isotropically consolidated kaolin. In the 100 mm cell the local axial and radial strains were measured using proximity transducers mounted on adjustable fittings, enabling small strains to be measured accurately at any stage of a test. The axial strain was also measured between the end caps and externally. Comparisons of these three axial strain measurements showed that, under favourable conditions when bedding errors arc negligible, end cap or external strains may be considered satisfactory. Bedding errors are reduced by consolidating the specimen to a high stress level and providing a rigid connection to the top cap. External strains must be corrected for equipment compliance. Tests for the effects of sampling disturbance indicated that reconsolidating disturbed specimens to their initial stress conditions results in an adequate recovery of the small strain stiffness, but yields a higher stiffness at larger strains. The recovery of the small strain stiffness was better in compression tests than in extension tests and further work is required to understand this finding. Specimens were observed to approach failure during the simulation of sampling disturbance. For isotropically consolidated specimens, the relationships between normalised small strain stiffness at a given strain level and both overconsolidation ratio and rate of shearing were linear on a semi-logarithmic scale. This is in agreement with expectation on the basis of most previous research, but the effect of over consolidation ratio requires further study using one-dimensionally consolidated specimens. The experimental results were compared with the predictions from theoretical models based on critical state soil mechanics. These comparisons showed that the behaviour of kaolin under monotonic loading can be adequately predicted, even at small strains. Encouraging predictions were also made for behaviour during the loading cycle, as sampling disturbance was simulated. However, further theoretical developments are needed to take into account the rate of shearing. The critical state parameters derived from the present tests and used in the model predictions showed close agreement with those obtained by other researchers.