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Though tunnel boring has found a permanent place in rapid excavation and has given promising results, percussive drilling and blasting, especially in hard rocks, is still the main excavating method. Advancements in this field are possible and probable. The use of energy in rock breaking is not wholly understood; the progress in this understanding leads to the better control and improvements in the whole excavating process.
The present paper describes the way in which to use the actual percussive drilling experiments for the determination of the specific energies in rock fracturing.
It is concluded in this work that the penetration rate depends primarily on the energy used to fracture the rock under the drill bit. Though the energy which is transmitted elastically to the rock is generally estimated to be negligible, in some rock types under this investigation the elastic energy is a considerable amount of the total energy transferred to the rock.
It is outlined in this paper that a new and better way to determine the specific energies of rocks should and can be established. This development has a great importance in improving the excavation methods.
INTRODUCTION
Percussive drilling and blasting is still the main excavating method in hard rocks, though rock cutting has given promising results also in medium and hard rocks. In both methods the determination of specific energies has an important role when considering drilling or boring results.
When rock drilling has been studied the energy per unit volume required in producing fractured rock has often been determined by so called crater experiments with a droptester.
The following type of equation for the penetration rate in percussive drilling has been presented [1] (Mathematical Equation)(Available in full paper)
The energy transferred to rock (K x W) is partly transmitted further as elastic energy causing no fracturing and partly used to produce fracture at the end of the drill hole under the drill bit. If the energy required to produce unit volume of new hole is obtained from the crater experiments it includes both elastic and fracture causing parts of the energy transferred to rock.
It has been shown, however, [2] that only a limited amount of energy can be transmitted through rock with a given pulse. The limit is determined by the maximum stress amplitude which can be transmitted unattenuated through rock. The excess energy is used in rock fracturing.
PENETRATION RATE AND THE ENERGY IN FRACTURE
It is reasonable to assume that the penetration rate depends rather on the energy used to fracture the rock at the end of a drill hole than on the total energy transferred to the rock. With this assumption, equation (1) should be represented in the form (Mathematical Equation)(Available in full paper)
When we do not know the real value of K, we limit our consideration to the ratios between penetration rates and the energies. In the same rock with the same machine and drill we should have (Mathematical Equation)(Available in full paper)
where indices 1 and 2 refer to two different operating pressures.
