Sedimentary soft rocks such as mudstones are possibly deteriorated due to dry/immersion cycles, known as slaking, which cause slope instability. However, the slaking-induced deformation characteristics of crushed mudstones under cyclic and slope stress conditions have not been understood well. In order to investigate the effects of immersion on cyclic loading and creep deformation characteristics of crushed mudstones (JGS slaking index = 1, 2), a series of direct shear tests were performed under the different stress ratio, confining pressure and initial water content. A series of cyclic direct shear tests were conducted under the dried and immersed conditions to measure a residual strength (τres) of the specimen. In a direct creep shear test, the specimen was subjected to immersion under creep shear stress conditions (σv = 25, 100 or 200 kPa, τ = Rσv, R = 0.7~1.0). After the creep shear displacement became stable, a monotonic shear loading was applied to measure a peak strength. In the cyclic direct shear test, the significant negative dilatancy was observed in the immersed conditions compared with the dried conditions. τres declined by immersion despite the increase of the density. The degree of particle breakage after the test was more remarkable in the case of the immersed conditions than that of the dried conditions. In the direct creep shear test, the creep shear deformation by immersion became larger as an increase in the stress ratio. When the stress ratio became almost equal to τres/σv obtained from the cyclic direct shear test, a creep failure was observed by immersion. It may indicate that τres is a threshold for the creep failure caused by immersion.
Slaking of mudstones is an important potential factor to cause slope instability and leads to slope failure by precipitation or earthquake (e.g., Mark, 2010). In general, the slaking characteristics of geomaterials are evaluated by the observation of immersed samples (JGS 2124 and 2125). On the other hand, several studies have been conducted to investigate the change in physical properties by slaking (e.g., Sakai and Nakano, 2018). However, the effects of slaking on the strength and deformation characteristics of crushed mudstones under the cyclic and slope stress conditions have not been understood well.