Studies on the Control of Slope Failure and Soil Loss by Rainfall due to Land Development in the Red Soil Area in Okinawa

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In the red soil (commonly called Kunigami Maaji) area in Okinawa, slope failures occur controled by the strike of weak planes such as bedding and joint plane. The shear strength along weak planes is lowered to the value close to residual strength. The peak strength is mobilized in shear zones where weak plane does not exist. The peak strength was measured for undisturbed samples from slope failures using a triaxial compression apparatus. The fully-softened strength and residual strength were measured for slurry of disturbed samples using a ring shear apparatus. Peak strength parameters were c_f=3.0kPa16.1kPa, phi_f=28.5? A1-43.5? A1. Fully-softened strength parameters were c_<sf>=0kPa-7.9kPa, phi_<sf>=35.0? A1-40.0? A1. Residual strength parameters were c_r=0kPa, phi_r=16.5? A117.9? A1 for soils containing vermiculite and high clay content, and c_r=0kPa, phi_r=19.8? A1-33.3? A1 for quartz and small gravels-dominated soils, both higher than thosef or landslide soils. This is because cla … More y particles in the red soil are poorly oriented, and, furthermore, the quartz and small gravels in the soils affected to make the residual strength high.The field shear strength (a average shear strength along the slip surface) and shear strength parameters of the soil from the slip surface were evaluated for the Makiya slope failure occurring in the red soil area based on the stability analysis method in which the residual factor was incorporated.A back-analysis of this slope failure showed that feild shear strengths are greater than residual strengths but smaller than peak strengths. The design strength parameters for control works were calculated by the proposed method and also by the graphic solution based on the back-analysis and shear strength parameters of the soil.The soil loss testing was performed by using a rainfall simulator in order to clear up that organic materials have the effect of erosion control on red soils. The quantity of runoff soil loss was small in the order of charcoal mixed soil, organic material mixed soil, wood shavings mixed soil and original soil. 10%charcoal mixed soil showed the more effective erosion control than original soil by 70%. Less