Seismic Evaluation of Excavated Rock Slopes at Hinkley Point C, UK

Understanding rock strength is essential when undertaking major excavation projects, with accurate assessments producing both safe and cost-effective engineered slopes. Balancing the cost-safety trade-off becomes more imperative during the construction of critical infrastructure such as nuclear power stations, when key components are built within relatively deep excavations. Integral to these stability assessments is a determination of the stress conditions at which these slopes fail, and degree of rock engineering required to prevent failure. The design of these engineered slopes is therefore reliant on rock strength models, which are generally parameterised using estimates of rock properties (e.g. Unconfined Compressive Strength, rock disturbance) measured prior to the commencement of works. The physical process of excavation however weakens the remaining rock mass, therefore a crucial component of rock strength model is an adjustment to account for the anticipated rock disturbance. This parameter is however difficult to quantify and as a result it is often poorly constrained, which significant impacts of the final design of the excavation.

In this paper we should results from pico-seismic monitoring and seismic surveys, which are used to image the extent and degree of disturbance within recently excavated slopes at the construction site of Hinkley Point C nuclear power station. Seismic survey results indicate that the disturbance is primarily confined to the initial 0.5m from the excavated face, while the passive monitoring has detected only a small number of events. This suggests that the rock disturbance at this site is relatively low and transient in nature, occurring immediately during and after the excavation. A ratio of seismic velocity recorded before and after excavations are used to determine the disturbance parameter required for the Hoek-Brown rock failure criterion, and we assess that rock disturbance is low within these excavated slopes. We further develop this approach by proposing a method of modelling the velocity variations for this geological setting, which allows for the assessment of disturbance prior to the commencement of excavations.