Optimization of overall excavation costs in a coal strip mine

Coal strip mines are usually large scale operations that excavate millions of tons of rock per year; thus optimization of the overall excavation costs can bring quite significant savings. The Operations Research problem involved is explained, and a method of optimization is proposed. This method depends on the creation of a ‘diggability function’, which is obtained by correlating the shovel digging rate to the average displacement velocity of the rock broken by blasting with explosives. The correlation is made possible by the use of the blasting model BLASPA to simulate the rock velocity; it is converted into a mathematical relationship by means of a least square approach. The ‘diggability function’, together with simulated values of the percentage of rock cast far enough to require no handling by the shovel, are combined to calculate the average shovel costs per cubic yard of rock moved. These costs are added to the drill and explosives costs to arrive at a prediction of overall unit excavation costs. It is found that if the strength of the blasting be increased, the overall unit excavation costs go through a minimum value. The paper concludes that (i) savings which can be found by this optimization method are significant; and (ii) the method is practical in that it requires only a few field measurements, this being desirable because mines object to numerous field tests which might disturb operations.