Publication

The shear strength of Opalinus Clay shale in the remoulded state

Alessio Ferrari
2020
Journal paper
Abstract

The Opalinus Clay shale formation is considered as a potential host geomaterial for the Swiss deep geological repository for radioactive waste. It presents different facies and it is characterised by a multi-scale heterogeneous composition, by a typical fissile structure with well-defined bedding planes and by anisotropic hydro-mechanical behaviour. This peculiar complexity makes it difficult to assign a unique set of geomechanical parameters to the material. This paper presents an experimental study aimed at characterising the lowest values of the shear strength parameters. In this sense, the shear behaviour was investigated on remoulded samples where the fabric and the diagenetic bonds of the intact material were eliminated. The results of a triaxial test campaign belonging to different facies of Opalinus Clay shale (‘‘shaly’’ and ‘‘sandy’’facies) are presented. Furthermore, with the aim to study the mechanical properties of fault zones in Opalinus Clay, the effect of large cumulated shear displacements on shear strength was also investigated. Ring shear tests were performed to determine the residual shear strength. The main geotechnical property which discriminates the different facies of Opalinus Clay is the grain-size distribution; in this sense, well-defined correlations between this intrinsic characteristic and shear strength angles of the remoulded material are presented.

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Related concepts (35)
Shear strength (soil)
Shear strength is a term used in soil mechanics to describe the magnitude of the shear stress that a soil can sustain. The shear resistance of soil is a result of friction and interlocking of particles, and possibly cementation or bonding of particle contacts. Due to interlocking, particulate material may expand or contract in volume as it is subject to shear strains. If soil expands its volume, the density of particles will decrease and the strength will decrease; in this case, the peak strength would be followed by a reduction of shear stress.
Direct shear test
A direct shear test is a laboratory or field test used by geotechnical engineers to measure the shear strength properties of soil or rock material, or of discontinuities in soil or rock masses. The U.S. and U.K. standards defining how the test should be performed are ASTM D 3080, AASHTO T236 and BS 1377-7:1990, respectively. For rock the test is generally restricted to rock with (very) low shear strength. The test is, however, standard practice to establish the shear strength properties of discontinuities in rock.
Triaxial shear test
A triaxial shear test is a common method to measure the mechanical properties of many deformable solids, especially soil (e.g., sand, clay) and rock, and other granular materials or powders. There are several variations on the test. In a triaxial shear test, stress is applied to a sample of the material being tested in a way which results in stresses along one axis being different from the stresses in perpendicular directions.
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