Probabilistic and Deterministic Analysis of an Excavation Supported by Tiebacks and Nailing in Residual Soil of Gneiss

Abstract

With the growing economic appreciation of urban areas in large cities, building techniques that are technically and economically feasible have been widely employed to maximize utilization of spaces. Such techniques resort to large excavations to expand the land occupation rate. In this context, this paper analyzes a 24-m deep excavation located in São Bernardo do Campo, a city in the metropolitan region of São Paulo Metropolitan Region, Brazil. The local subsoil is composed by residual gneiss soil, and the retainment was executed in soil nailing. The length of the nails is variable in depth; the longer nails are introduced in the first 8 m of the excavation, and then reduced in greater depths. During the development of the retainment design, numerical analyses via limit equilibrium using deterministic and probabilistic methods considering the variations in properties of materials, demonstrated that the nails close to the end of the excavation are weakly influenced by the changes in nail lengths and diameters. In this sense, new analyses were performed to simulate the replacement of 1/3 of the nail rows for one or two rows of tiebacks at the end of the excavation to maintain stability of the retainment and reduce costs. The results for safety factor were obtained in deterministic and statistic (mean) terms. In the latter case, the reliability index and the probability of failure of the analyses performed are also presented. The results demonstrate the importance of conducting investigative analyses to determine the critical rupture surface by means of statistical analyses. In the case shown, this type of analysis proved to be more critical than the deterministic method of analysis of stability of retainment.