A practical approach to a reliability-based stability evaluation of precariously balanced granite boulders

As new development moves into the vicinity of large granite boulders, new factors including vibrational loads caused by construction equipment or seismic shaking, loss of downslope support, and human activity begin to play a role in the stability of precariously balanced boulders. This study presents a practical reliability-based approach, using a Monte Carlo simulation, nine simple field measurements, and three “in house” estimates to determine the stability of a granite boulder, with respect to both sliding and rocking motion. The addition of a contact percentage parameter practically augments the widely accepted joint roughness coefficient (JRC) to account for the three-dimensional effects of real natural rock discontinuities. The Monte Carlo simulation allows for the uncertainty of the field measurements and rock strength properties, obtained using traditional geologic field tools, to be incorporated into the determination of the reliability, or probability of boulder movement. The method of analysis transforms the complexity of estimating potential boulder movements into a simple procedure, which can be conducted using a typical spreadsheet allowing for immediate implementation into geological/geotechnical engineering practice. A case study site is also presented, for which an active mitigation approach using rock bolts is employed.