The anisotropy of Leighton Buzzard sand under general stress conditions
A hollow cylinder apparatus was used to investigate the inherent anisotropy and membrane penetration of Leighton Buzzard sand test-specimens. The degree of inherent anisotropy was found to be independent of the mean effective stress but dependent on the initial relative density of the test-specimen. In the vertical-tohorizontal directions, the degree of inherent anisotropy decreases with increasing relative density. A wet-pluviation, sample preparation technique produces cross-anisotropic test-specimens, with the degree of cross anisotropy independent of the initial relative density. The effects of membrane penetration (expressed in terms of the normalized unit membrane penetration) are found to be independent of the initial relative density and the mean effective stress.
Local measurements of the polar deformation response in a hollow cylinder apparatus
This paper describes the set-up, calibration and performance of the local instrumentation that measures the deformation response of the test-specimen in a new hollow cylinder apparatus (HCA). Customised, single- and double-axis inclinometers measure the axial and twist deformations of the outer specimen wall surface. Proximity transducers in the cell chamber and the specimen bore cavity measure the radial wall‐surface displacements and precision gearing allows the relocation of the transducers from outside the pressure-cell. The inclinometers were modified (reduced gauge of 45 mm) to suit the smaller-sized test specimen and the more compact pressure cell of the HCA. The instruments were calibrated using an optical table and laser distance-measurement system to achieve the necessary accuracy for strain measurements over the pseudo-elastic range. The calibration in water data for the proximity transducers was non-linear and strongly influenced by the specimen wall curvature. Equations are presented to compute the true torsional shear strain response from the measured inclinometer tilt angle, specimen dimensions and deformation response. External measurements include significant errors due to apparatus compliance, specimen end-restraint and bedding effects, inaccuracies in measuring volume changes and the fact that the deformations are calculated assuming that the entire specimen deforms as a right cylinder.
An alternative method for determining the membrane penetration correction curve
Test results show that the previously used linear relationship between the unit membrane penetration and logarithmic effective confining pressure has limitations at low confining pressure. An alternative method is described, based on a hyperbolic type of curve fitting, which can provide more suitable membrane penetration correction curves for various initial confining pressures. An example is given of triaxial strain path test results corrected using both the logarithmic and hyperbolic methods.