Small strain-stiffness of cement-mixed gravel during various loading histories

Abstract

The stiffness at small strains of compacted cemented-mixed well-graded gravelly soil was studied previously by other researchers [1]. In this present study, the effect of global unloading/reloading history on the Eeq values of the same material was studied. The small strain stiffness of geomaterial is usually evaluated by either dynamic tests (i.e., resonant column tests and wave propagation tests) or static tests (i.e., monotonic or cyclic loading tests measuring stresses and strains) or both. In the present study, by taking into account the fact that the specimen of compacted well-graded gravelly soil are not sufficiently homogeneous so that the wave velocity be representative of average small strain stiffness of a given specimen, the static tests were employed. To this end, a single or multiple unload/reload cycle(s) with a single amplitude axial strain of an order of 0.001 % is (are) usually applied at different isotropic stress states or anisotropic stress states during otherwise monotonic loading, ML, of triaxial compression (TC) test, for example. The elastic young’s modulus, Ee, is ideally defined as the slope of a fully reversible cyclic stress-strain relation. However, it is actually not the case due to viscous property and non-linear inviscid stress – strain relation. For this reason, when a small unload/reload cycle is applied during otherwise ML, the overall stress-strain relation is shifted toward larger strains due to the viscous behaviour of the test material. The peak-to-peak secant modulus from a unload/reload cycle is defined as the equivalent Young’s modulus, Eeq, as shown in Figure 1. However, this value may be somehow different from the truly elastic modulus for the reason described above.