Understanding rock strength is essential when undertaking major excavation projects, as accurate as-sessments ensure 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 sta-tions, where key components are built within relatively deep excavations. Designing these engineered slopes is 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 commence-ment of works. However, the physical process of excavation weakens the remaining rock mass. Therefore, the model also requires an adjustment for the anticipated rock disturbance. In practice, this parameter is difficult to quantify and as a result it is often poorly constrained. This can have a significant impact on the final design and cost of excavation. We present results from passive and active seismic surveys, which image the extent and degree of disturbance within recently excavated slopes at the construction site of Hinkley Point C nuclear power station. Results from active seismic surveys indicate that the disturbance is primarily confined to 0.5 m from the excavated face. In conjunction, passive monitoring is used to detected seismic events corresponding to fracturing on the cm-scale and event locations are in agree-ment with 0.5 m of disturbance into the rock face. This suggests rock disturbance at this site is relatively low and occurred during and immediately after the excavation. A ratio of seismic velocities 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 with the magnitude of the disturbance diminishing more quickly than expected into the excavated slope. Seismic methods provide a low-cost and quick method to assess excavation related rock mass disturbance, which can lead to cost reductions in large excavation projects.