Viscous property of loose sand in triaxial compression, extension and cyclic loading

The viscous property of sand under triaxial compression (TC), triaxial extension (TE) and cyclic triaxial loading conditions was evaluated by using reconstituted loose samples of three types of relatively fine uniform and relatively angular sand (Silica No. 8, Toyoura and Hostun). The viscous property was quantified mainly by changing stepwise the axial strain rate many times and partially by performing drained sustained loading, both during otherwise monotonic loading (ML) at a constant strain rate. Such a peculiar feature of viscous property as that the viscous stress increment decays with an increase in the irreversible strain during ML at a constant strain rate (i.e., the so-called TESRA viscosity), which has been observed with Toyoura and Hostun sands in previous studies, was observed also with Silica No. 8 sand. The magnitude of viscous property is represented by the rate-sensitivity coefficient beta, defined as the slope of the Delta R/R-log {(gamma(ir))(after)/(gamma(ir))(before)} relation, where Delta R is a sudden change in the principal stress ratio, R = sigma'(1)/sigma'(3), caused by a step change in the irreversible shear strain rate from (gamma(ir))(before) to (gamma(ir))(after) at a given R value during otherwise ML. The following was found. The same definition for beta is relevant to drained TC and TE tests. In drained cyclic triaxial loading, the beta value is obtained from the above-shown equation after re-defining the sign of Delta R and proportionally scaling the value of R. With the respective type of sand, the beta values under these different loading conditions are very similar. The effect of overconsolidation on the beta value is insignificant. The beta values of these three types of sand are similar to each other and also to those of other ordinary types of sand and gravel having largely different particle sizes. The decay rate of viscous stress increment with an increase in the irreversible shear strain, which is another factor of the viscous property, is rather similar among the three types of sand, while the decay rate is slightly lower in ML TC than in ML TE. The effect of overconsolidation on the decay rate is insignificant.