Strain-dependent mduli and pressuremeter tests

Abstract

High precision measurements of the deformation of stiff clay in the laboratory have shown that the response of these soils is non-linear from very small strains. This is a reflection of the occurrence of plastic irrecoverable strain in the soil and is an indication that the soil cannot realistically be described as linear elastic material. The object of this note is to show that when non-linear effects are to be studied care is necessary in order to ensure that the parameters that are being compared from pressuremeter and triaxial tests are truly comparable. The theory behind undrained triaxial compression tests and pressuremeter tests is outlined. The variation of secant shear stiffness with strain and its effect on the tangent shear modulus and pressuremeter shear modulus is examined. Problems associated with the pressuremeter test are noted and include the assumption of an undrained process and exclusion of the viscous element of response. It is also noted that non-linearity of the type considered here leads to different distribution of stress or strain around a test device or geotechnical structure from those which emerge from constant modulus elastic analysis. (TRRL)