Progresive destruction of the Bothkennar city: implications for sampling and reconsolidation procedures

Triaxial stress and strain path tests have been carried out on high-quality Laval and Sherbrooke samples of lightly overconsolidated Bothkennar clay. The specimens were instrumented with local axial and radial strain and mid-plane pore pressure measurement, with precautions being taken to retain the existing pore water chemistry. The test programme imposed shear and volumetric strains of various magnitudes to assess the reduction in strength and stiffness caused by sampling and laboratory reconsolidation procedures. It is concluded that for this clay even very high quality tube samples will suffer a significant loss in mean effective stress, and some loss of structure during sampling, as a result of the imposed undrained shear strain cycle. Re-establishment of the initial effective stress level, by an appropriate stress path, will recover a proportion of the undisturbed undrained compressive strength that depends on the amount of destructuring: stiffness cannot be fully recovered. Reconsolidation procedures risk taking the specimen through the current yield surface, in which case large volumetric strains will occur and be accompanied by significant destructuring and, hence, irrecoverable loss of strength and stiffness. For natural clays, such as Bothkennar clay, the variability of structure within the deposit means that normalization solely with respect to effective stress cannot be used to allow for the disturbance caused by tube sampling, or to recover in situ soil properties

Contribution of fines to the compressive strength of mixed soils

10.1680/geot.54.9.561.56936Geotechnique549561-569GTNQ

Disturbance of the Bothkennar clay prior to laboratory testing

Interpretation of laboratory tests in the characterization study of the Bothkennar clay required an assessment to be made of the effects of disturbance prior to testing. Disturbance is shown to have occurred as a result of tube sampling, sample transport and specimen preparation. The effects of disturbance in the clay are to reduce the mean effective stress in the sample and shrink the initial bounding surface. The consequences of these changes are illustrated for the case of unconsoli-dated undrained triaxial tests, undrained triaxial compression and extension tests after reconsolidation to in situ stress, and oedometer tests. Reduction in peak strength, stiffness, yield stress and post-yield compressibility are shown to be a consequence of disturbance. The evaluation of sample quality on the basis of strains during reconsolidation is examined