The external and internal measurement impact on shear modulus distribution within cyclic small strains in triaxal studies into cohesive soil

Silesian University of Technology, 44-100 Gliwice, ul. Akademicka 5/214, Poland

^a e-mail : malgorzata.jastrzebska@polsl.pl

Abstract

The paper deals with comparison of tangent shear moduli G_s of kaolin from Tułowice obtained from cyclic triaxial tests on the basis of external and internal reading in the small strains range (10^-5÷10^-3). The tests were carried out on a modernised test bed, enabling full saturation of specimens using the back pressure method as well as a precise internal measurement of strains by means of contactless microdisplacements sensors. The value of linearity factor L is one of adopted quality criteria for two measuring methods. Maintaining a constant deformation rate the influence of various cyclic process parameters (deviator stress amplitude – constant or variable; high or low; initial level of stress and strain, at which the unloading and reloading cycles were started; overconsolidation ratio OCR as well as cycles’ number and arrangement) on the "shear modulus – axial strain" characteristic was studied. The obtained values of G_int and G_ext (or L_int and L_ext) clearly show an underestimation (even 5 times) of G_s value within the range 10^-5÷10^-3 when using an external measurement. In addition, the differences between G_int and G_ext, which develop differently depending on specified cyclic process parameters, gradually decrease with increasing axial strains.