The influence of saturation degree on soil elasticity modulus: application on prediction the reach of rockfalls

Abstract

Rock falls are one of the most destructive mass movements in hillslopes. Considering the low predictability of its triggering, attention has been paid in determining the paths of the blocks as a method to reduce risks. The coefficient of restitution (ε) is the primary physical parameter involved in the analysis of the reach of the blocks. It represents the loss of energy after each shock of a rock block with different geological materials in its trajectory and it is still little known in geological and geotechnical literature, especially when considering the interaction of rock blocks with soil at different humidities. This article aimed to evaluate the effect of soil moisture on the elasticity modulus, an elastic  parameter that affects the stiffness at the contact between rock and soil at the time of shock and, therefore, the actual coefficient of restitution. For comparison purposes, soils derived from acid rock and from basic rocks were sampled in the mountainous region of the State state of Rio de Janeiro. The procedure was to characterize the mineralogy of source rocks and its derived soils, as well as the physical indices of the soils. After determining the characteristic curve, the soils were molded at different moisture contents and subjected to unconsolidated and undrained triaxial compression tests. The results showed variation of elasticity modulus in more than an order of magnitude depending on the soil moisture, indicating that moisture has considerable influence on the stiffness of the contact  between soil and rock during shocks, making the length of trajectories dependent of the seasonal variation of moisture in hillslopes.