For clarifying the effects of relative density (Dr) and Atterberg’s limits on the cyclic shear-induced pore water pressure properties of soils, sandy soils with similar index properties and clayey soils with different Atteberg’s limits were collected from Vietnam and Japan and used for this study. Specimens at Dr = 50% of Nam O sand and Dr = 70% of Toyoura sand, and those of Hue clay and Japanese Kaolin clay were consolidated under the vertical stress of σvo = 49 kPa. They were then subjected to undrained cyclic shear for various cyclic shear directions and wide ranges of the number of cycles and shear strain amplitudes. Under the same cyclic shearing conditions, specimens of sand at higher Dr (Toyoura sand) and clay with higher Atterberg’s limits (Kaolin) show a lower pore water pressure ratio. The number of cycles and the cumulative shear strain at the starting point of pore water pressure generation were observed for different soils and testing conditions. In addition, using the cumulative shear strain, a new strain path parameter, the effects of shear strain amplitude and cyclic shear direction can be captured, resulting in a unique uacc/σ’vo - G*  relation on each soil. Based on this, fitting lines can be drawn and referred to promote a prediction of the cyclic shear-induced pore water pressure accumulation for the used soils under different cyclic shear conditions.

Biopolymer soil treatment has been introduced as the latest technological innovation in soil stabilization and improvement. Biopolymers with agar, guar, xanthan, casein, gellan and sodium alginate have been commonly studied to improve strength, reduce hydraulic conductivity and prevent erosion of highly permeable soil materials. Among those, xanthan gum is introduced to perform better in terms of lowering permeability. In this study, the effect of xanthan gum on slope stability under rainfall was evaluated via a 2D finite-difference program (FLAC) considering the wetting behavior of xanthan gum-treated soil. In order to obtain input parameters for numerical modelling, wetting soil-water characteristic tests, permeability tests, and direct shear tests were conducted on xanthan gum-treated soils. The numerical results support the application of biopolymers to soil erosion control during rainfall, mainly due to the significant decrease in the infiltration of xanthan gum-treated soil.