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6.2 Estimation of soil strength
A number of penetration devices have been used at The University of Western Australia to assess the strength of soft soils in centrifuge models. These include several 10 mm diameter cone penetrometers with tip resistance capacities ranging from 2 to 150 MPa, t-bar and ball penetrometers. The general design of the cone and t-bar penetrometers is described by Stewart and Randolph (1991). A cross-section through a 2 MPa capacity cone penetrometer of more recent design is shown in section 4.1.1. This penetrometer was designed specifically for soft clay and has an area correction (due to pore pressure on the shoulder behind the cone tip) of only 13 % due to the very thin sleeve extending down to behind the cone tip. A load cell located at the top of the penetrometer shaft measures the total installation force including the shaft friction.
The t-bar (Photo 1) and ball (Photo 2) penetrometers were developed as alternatives to cone and vane tests. Equilibrium of insitu stress can be achieved across the bar or ball, and thus a correction for the insitu vertical stress level, which is usually done for the cone test, is not required, unlike the cone. The results of the t-bar test are interpreted by dividing the bearing pressure on the bar by a factor of 10.5 to determine the undrained shear strength. This factor was derived from plasticity solutions as described by Stewart and Randolph (1991). The original t-bar had 7 mm in diameter and 35 mm in length, with a 5 mm diameter load cell and shaft attached to it. More recently, t-bars of 5 mm diameter and 20 mm length have been used routinely.
The ball penetrometer is newly developed, and interpretation of the test data is similar to that for the t-bar. Balls of various sizes have been used, although a 10 mm diameter ball has generally been adopted.
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Reference
Stewart, D.P. and Randolph, M.F. (1991) : A new site investigation tool for the centrifuge, Proc. of CENTRIFUGE 91, pp. 531-538.
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