Critical Detachment of Cohesive Soil

Characteristics of soil erosion caused by rain water flow (I)

Abstract

The organic combination of friction force of the clod and sand particles forming the surface of a soil layer, and the cohesion temperately compacted with an appropriate amount of fine particle content, is closely related to the resistance of cohesive soil to the tractive force of rain water flow. The study was made on the resistance of cohesive soil to surface sheet flow, by expressing the contents of fine soil particles influencing such soil resistance as friction force, cohesion, etc. by clay ratio^* (C_r^*= clay (%)/[silt (%)+ sand (%)]) ; and soil layer compaction by dry density γ_d; and soil surface roughness by the average clod size, d_k. By letting u equal the average velocity on the surface roughness of soil layer, u_* the shear velocity, f_c the cohesion per unit of area, u/u^*=φ(h/d_k, u^*d_k/ν) equal the distribution of the velocity of surface sheet flow on the surface roughness of soil layer, and also taking into consideration the equilibrium between the tractive force of surface sheet flow and the soil resistance, the relationship of critical shear velocity was established by the formula u_*c²/gd_k=K_r(γ_d/ρ-1)+α₁f_c/ρgd_k·2cos θ/(C_Dφ_c²). The logarithms herein quoted are K_r, coefficient of friction ; α₁, area coefficient ; C_D, coefficient for flow resistance ; γ_t, wet density ; and g, gravity acceleration. For the purpose of obtaining the correlation in the respective dimensional quantity in Reynold's number of surface roughness, u_*cd_k/υ, u_*c²/(γ_d/ρ) gd_k, and C_r^*, an experiment of critical detachment was performed on 8 different native types of soil. The general susceptibility of soils to erosion was observed in the soils of different native types, and depended on the clay ratio and dry density. In most of the soils, with some exception like volcanic ash soil rich in organic matters, a considerable relation was discovered in the dimensional quantity of C_r^* for u_*c2/gd_k, u_*cd_k/υ, through which the characteristics of erosion of cohesive soil by surface sheet flow was clarified.