Stability evaluation of slopes with bedding planes parallel to the slope, considering saturation softening effect of undisturbed potential slip zones

The undisturbed soil in the slip zone is highly water sensitive. Elucidation of its strength properties and degradation mechanism is important for assessing the stability of slopes with bedding planes parallel to the slope. For this purpose, a series of direct shear test, ring shear test, scanning electronic microscope (SEM) test, and nuclear magnetic resonance (NMR) test were conducted on undisturbed slip zone soil samples sourced from a typical bedding slope along the Mabian River. Finally, an evaluation method of the bedding slope stability was investigated. The results show that the shear strength of undisturbed slip zone soils under saturated softening degrades sharply within the first hour. During this period, the moisture content of slip zone soil increased by 79.6%, and the cohesion and internal friction angle decreased by 45.0% and 36.2%, respectively. The occlusion of coarse grains in the slip zone soil hinders the formation of the shear plane, thus transforming the occlusal friction into sliding friction under saturated softening, leading to an obvious characteristic strain softening. The decreasing shear strength of the slip zone soil is caused by the shear failure of its internal structure. Under saturated softening, cementation between solid grains is destroyed when clay minerals are swelled by water absorption. Furthermore, the NMR shows the increase in the volume of small pores in the samples under submerged conditions, indicating a more losing structure, which provides a reasonable explanation for the significant strength degradation of undisturbed samples. A quantitative relationship is proposed for the bedding slope stability considering the effect of saturated softening.