Economic Sampling and Extraction of Undisturbed,High Quality Samples in Normally Consolidated Lacustrine Clays Using a Large Diameter Tube |
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Authors: | S Messerklinger S M Springman |
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Institution: | (1) Institute for Geotechnical Engineering, ETH Zurich, Wolfgang Pauli Str. 15, 8093 Zurich, Switzerland |
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Abstract: | 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 Ko 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. |
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Keywords: | Block sampling techniques Varved lacustrine clay High quality test sample |
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