Engineering Geology and Numerical Modelling of a Steep Rock Slope for the Ensi-Qianjiang Expressway, China

Abstract

This paper describes the site investigation and engineering geological characterization of steep engineered rock slopes at the site of a major 280 m single span arch bridge. The rock slopes comprised predominantly carbonaceous limestone with reduced rock mass quality due to the influence of faulting and shearing. The carbonaceous limestone occurs as lenticular bodies with smooth, tectonically disturbed, variably dipping (37°–57°) bedding planes. The rock slope was excavated to a total height of 90 m, including a bridge foundation excavation depth of 30 m and a final permanent engineered slope height of 60 m. The engineered rock slope was excavated at 60°–90° and the natural rock slope is between 50° and 70°. Construction of the bridge foundation required the excavation of three rock slopes, two perpendicular to bedding and one parallel to the strike of the bedding. In the two rock slopes excavated perpendicular to bedding, one exhibited potential bedding plane controlled failure and the other potential toppling instability. The intact rock properties were characterized using laboratory testing and selected discontinuities tested in direct shear. Using the derived properties and the results of the engineering geological mapping numerical modelling was undertaken using a discontinuum model, (UDEC). The deformation of the rock mass during the stages of excavation was simulated and the results used to develop plans for safe construction and reinforcement of the rock slopes.