A new analytical method for stability analysis of rock blocks with cavity in sub-horizontal strata by considering eccentric effect

Abstract. The basal cavity of rock block formed due to differential weathering is an important predisposing factor for rockfall, in hard-soft interbedded rocks. The rock block falling due to eccentric effect with the failure modes of toppling or sliding is defined as biased rockfall in this study. Considering the non-uniform stress distribution due to eccentric effect, a new analytical method for three-dimensional stability analysis of biased rockfall is proposed. In addition, a set of factors of safety (Fos) against partial damage (compressive and tensile damage of soft underlying layer) and overall failure (toppling and sliding of hard rock block) are used to determine the rockfall susceptibility level. The analytical method was applied and validated with the biased rockfalls in the northeast edge of Sichuan basin in Southwest China, where a large amounts of rockfalls develops, composed of overlying thick sandstone and underlying mudstone. The evolution process of biased rockfalls is divided into four stages, initial state, cavity formation, partial unstable and failure. The proposed method is validated by calculating Fos of the typical unstable rock blocks in the study area. It is indicated that the continuous retreat of cavity causes the stress redistribution between hard and soft rock layers. Consequently, the development of eccentric effect leads to the damage of underlying soft rock layer and the further failure of hard rock block. The critical cavity retreat ratio is determined as 0.33 to classify the low and moderate rockfall susceptibility. The proposed analytical method is effective for the early identification of biased rockfall, which is significant for rockfall prevention and risk mitigation.