Soil-water characteristic of unsaturated granite weathered soil in Rong county, Southeast Guangxi province

Background  Collapsing hills and landslides frequently break out in the granite weathered soil in Rong county, Southeast Guangxi, causing serious damage. The soil-water characteristics and relative permeability coefficients of weathered soil are not only an important basis for unsaturated seepage calculations, but also a necessary basis for the erosion process and mechanism of collapsing hills and landslides formation.

Methods  In this paper, the curve of particle gradation accumulation and the pore size distribution was obtained by using sieving test and mercury intrusion test respectively. In addition, the pressure film instrument was used to carry out the dehumidification test to study the soil-water characteristics of the granite weathered soil with different initial dry densities. Then the applicability of Van Genuchten, Gardner and Brook & Corey model was analyzed. Finally, the relative permeability coefficients of unsaturated weathered soil were predicted by using Van Genuchten and Gardner model, and the differences were explored by comparing the permeability coefficients of granite residual soil and fully weathered soil.

Results  The soil-water characteristic curves of residual soil and fully weathered soil both presented a "double-step" feature, and the dominant pores were inter-aggregate pores and inter-particle pores. However, the effect of initial dry density on the dehydration process of residual soil was more significant than that of fully weathered soil. Though the pore distribution coefficient of fully weathered soil was larger than that of residual soil, the pore size dividing point was smaller than that of residual soil with the same dry density. The applicability of the Van Genuchten and Gardner models to the two soils was the best (R²>0.93), and the applicability of the Brook & Corey model was second. The relative permeability coefficient and permeability coefficient of residual soil both decreased with the increase of dry density, and were smaller than those of fully weathered soil.

Conclusions  The granite weathered soil slope in Rong county presents uneven permeability with upward and strong downward, which is related to particle size composition. It affects the deformation and failure mode of the slope and the erosion characteristics of the collapsing hill.