Orosur Mining operates the San Gregorio Gold Mine in northern Uruguay. The 300m deep open pit, flooded for eleven years, was dewatered in 2015 to carry out underground mining. The exposed pit walls showed a large wedge failure, leaving a 25-ton block hanging high above the projected mine portal. Taking the block down, though feasible, would have overly delayed mining works. SRK was called in to assess the risk of leaving the block in place.
SRK’s rockfall analysis tool RFall3D, developed by William Gibson, was employed. The software can carry out deterministic and probabilistic calculations and assess the trajectory, velocity and energy of falling boulders. Impact points can be singled out and their distance to any reference point can be calculated. The real volume of blocks and their rotational inertia can be incorporated, highly enhancing the precision of the computed trajectories.
A probabilistic analysis was performed using two thousand realisations, each one defined by the stochastic position and velocity of the block immediately after being released, and by changes in the internal parameters of the model.
Thousands of simulations produced data points in the range of several millions, handled by means of a Python code developed in-house. Results were grouped in ranges and the probability of impact was calculated for each range.
It was concluded that the chances of a fall could not be overlooked. The most probable landing zone was located some 30m away from the portal with a +50% chance of being hit. At the portal, the probability of impact was estimated as 0.35%, a low and tolerable value.
This little story has a funny ending: weeks after the analysis was delivered, when construction of the portal had not yet begun, the block spontaneously came down one Saturday at lunch time and happened to end its journey right where RFall3D had predicted. Good job, William!
