In recent rock mechanics studies, more attention is being focused on the behaviour of rocks under varying straining conditions, as it is a vital factor in the design of rock structures as well as various mining operations like drilling, blasting and crushing. Many researchers have been investigating into the effects of strain rate on strength, Young's modulus, ultimate strain and also the failure behaviour of the rocks. However, conflicting results have been obtained by various investigators. An investigation, therefore, was carried out by the authors to study the effect of strain rate on mechanical behaviour of some Indian rocks. A third degree polynomial has been found to give the best fit to the plot between ultimate strength and strain rate, and Young's modulus and strain rate of rock samples. The failure strength was found to increase with increase in strain rate and an abrupt increase was observed at strain rates of 10 ¹/sec and 10 2/sec. The Young's modulus also increased with increase in strain rate. However, the ultimate strain was found to be independent of the strain rate.
The study of the mechanical behaviour of rocks deformed at various strain rates is very useful in the engineering problems of rock excavation. To optimize the two principal operations, drilling and blasting, a detailed knowledge of the mechanical properties and fracture characteristics of the rock material is needed as reported by Blanton (1981) and Chong et al (1980a, 1980b). The two variables that are of particular interest to the concerned engineer are the time involved and the energy consumed in fragmenting the rock. In drilling operations, if the failure stress of rock increases with increase in strain rate, then drilling techniques involving higher strain rates will consume more energy in removing a given amount of rock. In this case, these additional energy requirements at the higher strain rates may not be offset by the advantage gained, i.e. the saved time. If, on the other hand, the rock becomes more brittle at higher strain rates, then with the drilling techniques involving higher strain rates the energy required to remove a given amount of rock will be lower. This is so because, a more brittle rock would end to fragment at a lower strain, and the combination of relatively constant stress with decreasing failure strain at higher strain rates enables the energy necessary to break the rock to be lowered at higher strain rates. In this case, a high deformation rate would be advantageous from the point of view of both excavation rate and energy consumption. In blasting operation, fragmentation sizes and permeability extensions are strongly related to strain rates of loading. In regions of low strain rates, fragment sizes are large and cracks propagate to larger distances, whereas, in regions of high strain rates, fragment sizes are small and cracks are localized. An optimal fragmentation can be achieved at intermediate strain rates of 10° to 102 per second.
