Earth Pressure Control in Mines by Stress Measurement

Abstract

Much effort has been made for the earth pressure control in metal mines as well as in limestone mines where the ores are mined by the room and pillar method. To obtain the fundamental data for the earth pressure control in these mines the measurement of the variation in stress in the rock has been made by the photoelastic stressmeter under the authors' guidance. Recently the absolute stress has come to be measured by the stress-relief technique to supply more substantial data.These measurements have been made mainly on the rock surface around underground openings, where the cracks initiated by excavation are frequently found. Although the measurement has served not a little for the earth pressure control, it is desired that the measurement of the rock stress will be made possible at the points at considerable distances apart from the wall surface by making use of boreholes. However no method to determine the stress in the rock from the measurements made in the boreholes has yet been established. The authors have therefore investigated this problem. The first part of this paper describes the results obtained.Based on the theory of elasticity, they have deduced formulae to be used in determining the amount of stress in the rock (1) from the changes in diameter, (2) from the strains on the bottoms of boreholes and (3) from the strains on the wall of a borehole. The application of these formulae in terms of the measured values gives the observation equations, and their solution yields the estimation required of the stress in the rock in question. The minimum number of boreholes that afford the necessary data for determining the amount of stress in the rock has been discussed by examining the rank of the matrix made up of the coefficients of the observation equations.Subsequently two practical examples of stress measurements are described. In the Nakatatsu mine, the absolute stresses on the walls of a number of drifts around the Stope 4-5 of the Nakayama pit were measured, from which it was discussed whether or not the rock around the shaft situated near the stope would fail by extracting the ore completely. The original stress state, which seems to depend more on the orogeny rather than the topography, was estimated. Judging from the stress concentration that might appear after a complete mining, it was found that complete extraction of ore would be allowed.In the Hikawa mine, the main haulage drift had been driven through the deep strata in the Tobo district before any other excavation was made. For the purpose of obtaining the data for the design of mining, the stresses on the wall of the drift were measured along the drift. Then the original stress states were determined mathematically, which were taken into consideration in the design of the mining. On the other hand, this measurement presented the data about the stress state in the rock in a mountain district as well as about the influence of faults on the stress state.