A. Siddique

The use of hall effect semiconductors in geotechnical instrumentation

For the past five years or so Hall effect semiconductors have been increasingly used in the geotechnical engineering laboratories at the University of Surrey. They have been incorporated as sensing elements in local radial and axial strain measuring devices, for the small-strain instrumentation of triaxial specimens, and in small diameter boundary normal and shear stress cells. Triaxial internal load cells are currently being built incorporating Hall effect semiconductors. This paper describes the Hall effect principle and the methods of configuring magnet/sensor systems to achieve suitable measuring systems. Some geotechnical instruments built at the University of Surrey are detailed, and their characteristics discussed. The calibrations of the instruments described in the paper show a performance generally at least as good as might be expected from some commercially available instruments. Hall effect semiconductors are shown to be of use in a range of situations where displacement can form the basis of measurement. The displacements measured can vary from as little as 5 µm to as much as 10 mm, and the best repeatability so far obtained has been of the order of 1/100 of a micrometre.

THE EFFECTS OF VARYING CENTERLINE TUBE SAMPLING DISTURBANCE ON THE BEHAVIOR OF RECONSTITUTED CLAY

In order to simulate varying degrees of disturbance of soil at the centerline of a tube sampler, triaxial stress and strain path tests have been carried out on normally consolidated and overconsolidated reconstituted London clay specimens. The tests were carried out using an automated system capable of controlling both stresses and deformations imposed on specimens. The specimens were instrumented with local axial and radial strain and mid-plane pore pressure measuring devices. Test results indicate that the most pronounced effects due to imposed tube sampling strains in reconstituted normally consolidated London clay are significant reductions of mean effective stress and undrained small-strain stiffness, which are accompanied by reductions in the excess pore pressure generated during shearing. The higher the magnitudes of tube sampling strains, the greater the change in behavior. In over-consolidated reconstituted London clay, however, laboratory simulation of tube sampling strains together with deviatoric stress relief (i.e., “ideal” sampling) causes little change in effective stress, stiffness, and strength.