Determination of Effective Stress Parameters for Clayey Silt Using a Borehole Shear Test

The determination of shear strength parameters for use in geotechnical analysis is integral to safe and cost effective design. In general practise, shear strength parameters are frequently determined using published correlations and relationships with insitu field tests that are highly variable and are not intended to be used to develop shear strength relationships. Laboratory testing for measuring shear strength parameters is often minimal due to the inherent expense. Commercial drivers, sample quality assurance and turnaround timeframes greatly influence the quantity and quality of laboratory testing which is used to directly measure the shear strengths of soils. The Borehole Shear Test (BST) is an insitu testing apparatus that provides near instantaneous shear strength parameters, directly measured by shearing the side walls of a borehole at various normal stresses. Peak shear stresses are used to develop a Mohr Coulomb failure criterion and provide a direct measurement of shear strength. This paper summarises the results obtained from a series of Borehole Shear Tests undertaken on clayey silt in South East Queensland. Shear strengths were measured at various depths using the BST and were used to develop shear strengths parameters for the tested material. Thin walled Shelby tubes were used to recover insitu material samples which were used to determine the laboratory shear strength of the material. Consolidated Undrained Triaxial tests, Index testing and Standard Penetration Testing (SPT) were compared with the field BST results to assess relationships between the different testing methods. Results of the BST agree well with those tested in the laboratory. Results suggest that the BST could be an effective and alternative, cost efficient investigation tool which can be used to assess insitu strength parameters in conjunction with traditional laboratory testing.