An approximate correction for the influence of strength anisotropy on conventional shear vane measurements used to predict field bearing capacity

Abstract

The author derives a correction to be applied to shear vane measurements to allow the more accurate estimation of the field bearing capacity of anisotropic soils. He first outlines the need for a correction factor and discusses bjerrum's empirical factor that has been used to obtain a more realistic forecast of field stability. He points out that an ideal factor should compensate for the effects anisotropy and progressive failure as well as the effects of testing rate, size of test zone and any other effects resulting from differences between the test and field conditions. He argues that because bjerrum's correction is based on the result of plasticity index measurements on remoulded samples it cannot correct for inherent anisotropy, only for the other factors and that a further correction is necessary. A mathematical relationship is derived between this additional correction and the degree of anisotropy of undrained shear strength and includes a curve depicting this relationship. He concludes by setting out a relationship between field and vane strengths that is similar to bjerrum's correction, but incorporates the additional correction based on the degree of anisotropy. The latter may be estimated from undrained compression tests made at different orientations on block samples or, more directly, by tests with diamond-shaped vanes. It is pointed out that, although useful, the correction is only approximate as it assumes equal strengths in the three shearing modes. /TRRL/