The simulation of sampling disturbance and its effects on the deformation behaviour of clays.

摘要

The importance of studying the behaviour of soil at small strain levels (« 0.1 %)udhas been increasingly recognised in recent years. In the laboratory, tests are usuallyudconducted on tube samples retrieved from the ground, which inevitably suffer fromuddisturbance.udThis thesis describes an investigation of the effects of sampling disturbance onudthe small strain behaviour of one-dimensionally consolidated kaolin. Following theudsuggestion of Baligh (1985), tube sampling disturbance was simulated by applying audstrain cycle in the triaxial cell. Comparative tests on 'disturbed' and 'undisturbed'udspecimens were conducted in a specially designed 100 mm hydraulic triaxial apparatus.udTests were also carried out in a 38 mm triaxial cell to study the effects ofudloading rate and overconsolidation ratio on the small strain deformation behaviourudof isotropically consolidated kaolin.udIn the 100 mm cell the local axial and radial strains were measured usingudproximity transducers mounted on adjustable fittings, enabling small strains to beudmeasured accurately at any stage of a test. The axial strain was also measuredudbetween the end caps and externally. Comparisons of these three axial strain measurementsudshowed that, under favourable conditions when bedding errors arc negligible,udend cap or external strains may be considered satisfactory. Bedding errorsudare reduced by consolidating the specimen to a high stress level and providing audrigid connection to the top cap. External strains must be corrected for equipmentudcompliance.udTests for the effects of sampling disturbance indicated that reconsolidatinguddisturbed specimens to their initial stress conditions results in an adequate recovery of the small strain stiffness, but yields a higher stiffness at larger strains. Theudrecovery of the small strain stiffness was better in compression tests than in extensionudtests and further work is required to understand this finding. Specimens wereudobserved to approach failure during the simulation of sampling disturbance.udFor isotropically consolidated specimens, the relationships between normalisedudsmall strain stiffness at a given strain level and both overconsolidation ratio and rateudof shearing were linear on a semi-logarithmic scale. This is in agreement with expectationudon the basis of most previous research, but the effect of over consolidationudratio requires further study using one-dimensionally consolidated specimens.udThe experimental results were compared with the predictions from theoreticaludmodels based on critical state soil mechanics. These comparisons showed that theudbehaviour of kaolin under monotonic loading can be adequately predicted, even atudsmall strains. Encouraging predictions were also made for behaviour during theudloading cycle, as sampling disturbance was simulated. However, further theoreticaluddevelopments are needed to take into account the rate of shearing. The critical stateudparameters derived from the present tests and used in the model predictions showedudclose agreement with those obtained by other researchers.