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The paper presents the experimental investigation and analysis of the non-linear elasto-plastic stress-strain behaviour of normally consolidated lacustrine clay. Drained triaxial stress path tests were performed on natural block samples of Swiss lacustrine clay. Data were analysed using plasticity theory and the shape and extent of kinematic yield and bounding surfaces were determined and found to be elliptical but not congruent. Cross-anisotropic elasticity was used to quantify elastic strains to permit plastic strain increment vectors and hence a plastic potential surface to be defined.

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Large suffusion and cover-collapse sinkholes, of up to 100 m in diameter, developed in the Lar valley in Iran. The sinkholes formed through overburden several hundred metres thick comprising coarse- and fine-grained deposits. The formation mechanism for these large sinkholes was studied based on the topography of the bedrock and on deep borehole log data. It could be determined that the large cover-collapse sinkholes are located above permeable bedrock berms and that they formed through a layer of fine-grained, cohesive soil that was both underlain and overlain by coarse-grained soil layers. The data analysis showed that suffusion from the coarse-grained layer at the bedrock and subsequent erosion of these particles through permeable zones in the bedrock to the karst channels caused settlement of these coarse-grained layers and the formation of open caverns above them, at the interface with the fine-grained layer. With continuing erosion, the caverns widened until the limit equilibrium of the fine-grained layer was reached and then collapsed to form a depression with a horizontal bottom surface, termed a cover-collapse sinkhole. An analytical model was developed for this mechanism, considering cohesive and non-cohesive soil layers. This analytical model confirmed that, for the given composition of the overburden, cover-collapse sinkholes of 80–100 m in diameter can be expected. The paper also analysed the formation mechanism of the suffusion sinkholes and it was shown that their formation took place through coarse-grained, non-cohesive formations, in this case alluvium and slope talus. Finally, it was analysed whether the formation of new cover-collapse and suffusion sinkholes after the rehabilitation of the reservoir leakage is likely.

This paper describes the development, design and use of a large diameter sampling tube. High quality test specimens are essential for the investigation of mechanical properties of a soil for high risk projects and when complex and expensive testing methods are to be used. Block sampling is recommended to give the highest sample quality for clayey soils, however, extracting blocks of normally consolidated lacustrine silty clay without excessive disturbance was challenging due to the inherent structure of the soft varved silty clay and difficulty in maintaining K o conditions, as well as no vertical strain, in the sample. A new sample tube, with an inner diameter of 196 mm, an area ratio of 4% and an outer cutting-edge angle of 11°was designed to offer a larger cross sectional area than conventional thin walled sampling tubes, to provide the necessary side support and to prevent water ingress at the sides of the sample. The length-diameter aspect ratio was 1.275 to optimise the amount of clay sampled for subsequent testing and in an attempt to minimize the pressure in front of the tube. Samples were taken in initially newly excavated trenches at a depth of c. 1 m with this new sampler and with conventionally sampled soil specimens, prior to the main testing programme with samples from 6 m depth. A comparative study was then performed including preliminary unconsolidated unconfined compression tests followed by anisotropically consolidated undrained triaxial compression tests. It was important to establish whether this approach had led to an improvement in sample quality prior to embarking on an extensive triaxial stress path testing programme on this varved soil (Messerklinger, Non-linearity and small strain behaviour in lacustrine clay, 2006; Messerklinger and Springman, Geotech Test J 30(6), 2007; Messerklinger and Springman, Geotech Geol J, 2008). The results showed that the undrained shear strength of the specimens from the new sampler was consistently around 20% higher than that of specimens extruded from conventional thin walled tube samplers. This confirmed that samples with a significantly higher quality could be extracted from normally consolidated, fine grained, varved lacustrine deposits with this large diameter 'block' sampling tube.