Investigation of relevant soil parameters useful in predicting tractive ability of off-road vehicles

Abstract

An instrumented portable soil device that measures soil shear, sinkage and frictional soil properties in-situ was developed (Garciano et al., 2006). The specific objectives of this project were: 1) Perform extensive laboratory tests using the instrumented soil device and Cohron sheargraph; 2) Analyze the results from the soil device to develop composite soil parameters that may be useful in predicting tractive ability of off-road vehicles; and, 3) Relate the soil parameters derived from the soil device to fundamental engineering properties of soil derived from the Cohron sheargraph. The tests were conducted by pushing the device into the soil to a desired depth and then rotating it. Torque developed on the cone tip was due to soil adhesion and soil-metal friction and on the shear vane unit was related to cohesion and soil internal friction angle. Maximum torque at soil failure on the cone tip was related to soil adhesion and soil--metal interface friction and that on the vane was related to cohesion and soil internal friction angle. Cohron sheargraph was also tested under the same experimental conditions. Soil adhesion and soil-metal friction were derived from the friction head while soil cohesion and angle of internal friction were derived from the shear head of the Cohron sheargraph. The experimental results obtained from the device showed that typical torque characteristics increased exponentially until the soil sheared and then increased linearly as the device was rotated further. The experimental data were fitted to a combination of exponential and linear functions using an optimization procedure. Composite soil parameters gross friction and shear indices were developed by dividing the maximum torques developed on the cone tip and vane unit by their effective radius and then further dividing the result by their cone index values. A regression analysis conducted between gross friction and shear index indicated higher coefficient of determination when soil moisture condition and soil moisture-friction index interaction effects are taken into consideration. The analysis of the experimental data indicated that the maximum torque values on the vane unit as well as the cone unit showed linear relationship against the cone index values. The intercept and slope of this torque versus cone index values for the cone and the vane unit were found to be valuable soil parameters. In particular, soil adhesion and soil-metal friction were found to be functions of the intercept and slope values of cone torque versus cone index plot (r2= 0.94 and 0.95 respectively). Furthermore the gross friction index also related quite well to the intercept and slope (r2=0.67) of the cone tip tests. Following Hettiaratchi (1974, 1997) we were able to relate soil cohesion to adhesion, soil internal angle of friction and soil-metal friction, and soil metal friction to soil-internal angle of friction for polished metal.