Compression of Unsaturated Clay under High Stresses

The isotropic compression response of compacted, low-plasticity clay specimens having various initial degrees of saturation $S_{r,0}$ to high stresses under drained and undrained conditions was investigated using the approach of Mun and McCartney (2015). The compression curves in terms of void ratio $e$ versus mean effective stress $p^{\prime }$ or mean total stress $p$ are shown in Fig. 1(a).

After an initial elastic response, the drained specimens reach an apparent mean effective preconsolidation stress $p_c^{\prime }$ that increases with decreasing $S_{r,0}$. As $p^{\prime }$ increases further, the compression curves for the unsaturated specimens converge with the curve for the saturated specimen as air is expulsed. The value of $p^{\prime }$ required to reach the point of pressurized saturation increases with decreasing $S_{r,0}$. At higher $p^{\prime }$, the initial soil structure induced by compaction has an effect on the shape of the compression curves, which are also distorted by the logarithmic scale for $p^{\prime }$.

The compression curves for undrained specimens with a lower $S_{r,0}$ have a softer response due to the compression of the air-filled voids. With increasing mean total stress, the pore air dissolves into the pore water until reaching the point of pressurized saturation, which depends on $S_{r,0}$. After this point, the specimens are water-saturated and the shapes of the compression curves are dominated by the pore-water compressibility. The point of pressurized saturation can be assessed by revisiting the model of Hilf (1948). Specifically, by considering a pressure-dependent solubility of air in water ($h=u_a/k_h$, where $u_a$ is the pore-air pressure and $k_h$ is Henry’s constant), the value of $\mathrm{\Delta }{u}_a$ for a change in mean total stress $\mathrm{\Delta }p$ can be calculated, as follows:where $n_0$ = initial porosity; $u_{a0}=101.3\mathrm{kPa}$; and $m_{v,u}$ = coefficient of volume compressibility of the soil in undrained conditions. The change in mean total stress required to reach pressurized saturation ($\mathrm{\Delta }{p}_{ps}$) can then be estimated as follows:

The predicted values of $\mathrm{\Delta }{p}_{ps}$ for specimens having different values of $S_{r,0}$ is shown in Fig. 1(b) along with the experimental points of pressurized saturation from the undrained compression curves in Fig. 1(a). A good match is observed, with differences due to the choice of $m_{v,u}$ for different $S_{r,0}$ values.