Effective stress and shear strength parameters of unsaturated soils as affected by compaction and subsequent shearing

Wheeling induced compression and shearing forces are main stresses accounting for soil deformation and changes of hydraulic, gaseous and thermal properties. There are reports about the combined effects of compaction and subsequent shearing on soil hydraulic properties, but their consequences on soil strength properties (i.e., effective stress and shear strength) need to be further analysed. This study investigated the dynamics of soil mechanical properties as affected by pore water pressure ($u_{\mathrm{w}}$) during compaction and shearing. Soil samples from an A-horizon of Gleysols derived from glacial sediment and a Stagnic Luvisol from loess were analysed. The repacked and structured samples were compressed under static and cyclic loading and then sheared at two speeds (0.3 and 2.0 mm min⁻¹) with three loading levels (50, 100, and 200 kPa). During each stress application, the $u_{\mathrm{w}}$, chi factor (χ) and effective stress (${\sigma }^{\prime }$) were measured and calculated. The shear strength ($\tau$), angle of internal friction ($\phi$) and cohesion ($c$), were determined and fitted by the Mohr–Coulomb failure criterion. The results showed that compaction and shearing increased $u_{\mathrm{w}}$ and χ in all homogenized soils while on structured soils this phenomenon only occurred when the applied loading stress exceeded the soil precompression stress. The increased $u_{\mathrm{w}}$ resulted in soil hydraulic and mechanical stresses, which ultimately reduced the ${\sigma }^{\prime }$, especially at −6 kPa initial matric potential. Soils with finer texture, higher loading stresses and faster shear speed normally exhibited more reduced ${\sigma }^{\prime }$ values. The structured soil had higher $\tau$ values with higher $\phi$ and $c$ compared to the homogenized soils. The changes of $u_{\mathrm{w}}$ at high loading stress (i.e., 200 kPa) may overlap the normal pattern of the Mohr–Coulomb failure line that results from the theoretical Mohr envelope. Thus, to minimize the destruction of soil structure and stability induced by wheeling, it is important to consider field water content, traffic loading and wheeling speed.