Estimating the excavation disturbed zone in the permanent shiplock slopes of the Three Gorges Project, China

The stability and deformation of the permanent shiplock slopes are among the key issues in the design and construction of the Three Gorges Project. The permanent shiplock slopes, formed by deeply and steeply cutting into weathered and fresh granites, are 1607 m in length and 50-170 m in height. The disturbed zone in the permanent cut slopes induced by excavation is therefore one of the most important aspects in slope stability and deformation evaluation. A comprehensive investigation including non-linear finite element analysis, in-situ testing, instrumentation and monitoring, back analysis and rock mass quality rating has been carried out for the identification and evaluation of the excavation disturbed zone in the permanent cut slopes. The results of the investigation confirmed the existence of such an excavation disturbed zone in the permanent cut slopes. This zone is characterized by a considerable weakening in the mechanical properties of the rock mass. From the cut surface to the deeper region of the permanent cut slopes, the excavation disturbed zone can be further divided into a damaged zone, an affected zone and a slightly affected zone according to the extent of weakening in the rock mass. The damaged zone and the affected zone have thicknesses of 5-10 m and 10-20 m, respectively. The exact pattern of the excavation disturbed zone is variable in different parts of the permanent cut slopes and are generally similar to that of the plastic zones estimated by using the non-linear finite element analysis. The present investigation has provided both factual data and insights for the stability and deformation evaluation of the permanent shiplock slopes. The approach and methodology developed in the paper can be used to assess similar excavation disturbed zones in other large cut slopes.