Experimental Verification of CRS Consolidation Theory

The paper presents and analyzes results from nine constant rate of strain (CRS) consolidation tests on resedimented Boston blue clay (BBC) using a specially instrumented, computer-automated hydraulic consolidation device. Specimens were loaded using one of three vertical strain rates (0.1, 1, and 3%/h), and pore pressures were measured at three points through the specimen height and at the base. The CRS results were compared to the end of primary states in four incrementally loaded (IL) oedometer tests performed using standard procedures. At the two faster CRS rates, excess pore pressures (Δu) developed and two methods were used to compute vertical effective stress $\left({\sigma }_{\mathit{v}}^{\prime }\right):$ nonlinear CRS theory of Wissa et al. that depends in part on Δu measured only at the base; and direct integration of the measured Δu distribution through the specimen (the “isochrone method”). The results show that at the two faster rates, the CRS tests based on nonlinear CRS theory are consistent with those obtained using the isochrone method; results from CRS tests at all three rates compare well with the IL test results, despite base Δu values exceeding 70% of applied vertical stress in the 3%/h CRS tests. The CRS behavior of resedimented BBC is virtually independent of strain rate across the range tested; the value of preconsolidation pressure $\left({\sigma }_{\mathit{p}}^{\prime }\right)$ is rate independent as is the virgin compressibility. At ${\sigma }_{\mathit{v}}^{\prime }$ levels at and above ${\sigma }_{\mathit{p}}^{\prime }\text{,}$ CRS tests loaded at 1 and 3%/h yielded coefficients of consolidation and hydraulic conductivity values that agreed well with IL test values. The paper supports the validity of nonlinear CRS theory and suggests that less structured clays such as those tested may have less rate dependence during consolidation than soils with strong interparticle bonding and structure.