Pore Pressure and $K_{\sigma }$ Evaluation at High Overburden Pressure under Field Drainage Conditions. II: Additional Interpretation

This article is the second of two companion papers studying the effect of a high overburden pressure on the liquefaction behavior of saturated Ottawa sand. A series of four centrifuge experiments were conducted simulating a 5-m prototype layer of this sand in the field, having two relative densities and subjected to overburden effective pressures, ${\sigma }_{v0}^{\prime }$, of ~100 and 600 kPa (1 and 6 atm). The layer was on a rigid impervious base and could drain freely at the top. This was supplemented by undrained stress-controlled and strain-controlled cyclic triaxial tests on the same sand consolidated at 1 and 6 atm. The laboratory undrained overburden pressure factor at 6 atm obtained from the triaxial tests on loose sand, $K_{\sigma }=0.85$, is consistent with the state of practice (SoP), which assumes that $K_{\sigma }<1.0$ and $K_{\sigma }$ decreases with ${\sigma }_{v0}^{\prime }$. However, in the centrifuge experiments and ${\sigma }_{v0}^{\prime }=6\mathrm{atm}$, the field $K_{\sigma }=1.28$ for loose sand and $K_{\sigma }>1.15$ for dense sand. The discrepancy is due to more significant partial drainage during shaking in the 6-atm centrifuge models. Although the excess pore pressures at the bottom of the sand layer seem to have been close to undrained in the four experiments, they were much smaller at shallower elevations in the 6-atm tests compared with the 1-atm tests. Further analysis is conducted by evaluation in the four centrifuge experiments of the coefficient of consolidation, $c_v$, during the dissipation phase. This is done using the recorded pore pressure and settlement data. It is concluded that $c_v$ was two to four times greater during dissipation in the 6-atm centrifuge tests. The reason is the increase—also by a factor of two to three—of the drained constrained volumetric stiffness of the sand, $M^{\prime }=1/m_v$, when going from 1 to 6 atm. This finding plus other data from the literature suggest that for a range of sands, layer thicknesses, field conditions, earthquake shaking, and values of ${\sigma }_{v0}^{\prime }$, both $M^{\prime }$ and $c_v$ may increase proportionally to $\surd {\sigma }_{v0}^{\prime }$, with the field $K_{\sigma }>1.0$, and with $K_{\sigma }$ increasing instead of decreasing with ${\sigma }_{v0}^{\prime }$.