Uncertainty in overturning of precariously balanced rocks due to basal contact

Precariously balanced rocks (PBRs) are a type of naturally occurring freestanding structure that provides valuable information to constrain seismic hazard at return periods which are important for critical facilities such as nuclear power plants and nuclear repositories. Exposure ages have been established to be in excess of 10,000–30,000 years, which is why precarious rocks are one of the only available means to validate seismic hazard associated with long return periods. One critical component for constraining seismic hazard in this way is the overturning estimate of a given precarious rock as a function of earthquake intensity. However, current state‐of‐the‐art methods for modeling the seismic response of precarious rocks involve significant sources of uncertainty. One of the main sources of uncertainty stems from the interface of the rock, which is usually occluded during surveying and assumed during modeling. Through extensive shake table testing, this study analyzes the uncertainty in the overturning response of a granite precarious rock specimen incorporating various degrees of interface contact. The results indicate that a small variation in the contact geometry could result in a substantial increase in the stability of the specimen, which is significant given the difficulty of surveying the interface of PBRs in the field. Repeatability tests indicate that the overturning demand can vary up to nearly ±50%. The probabilistic overturning responses are compared across the interface changes to bound uncertainty; and, the effect of modeling parameters, namely the contact normal stiffness, is evaluated through a parametric study and comparison with experimental results.