The Stress and the Earth Pressure Phenomena in the Rocks around Shaft Bottom Spaces

In general, shaft bottom spaces in mines are large and of complex shape, and are the most important places for haulage and winding. Though it is essential to clear up the earth pressure phenomena around these spaces in order to plan rational linings, very few investigations have been carried out into this problem. The authors have, therefore, studied the stress and the earth pressure phenomena around shaft bottom spaces by means of barodynamics and three dimensional photoelastic experiments. In the latter experiments, special consideration has been given to the treatment of statically indeterminate stresses. It is found, from this study, that if the rock is not sufficiently strong, failure takes place on the roof, especially at the part of the roof between a shaft and a horizontal level, due to tensile stress, and in a few cases on the side walls due to compressive stress ; that the degree of stress concentration is not so high as expected ; and that these failures do not progress further once the corners of rock have been rounded off. Furthermore, there is a discussion of how to line the shaft bottom spaces

Determination of the Stress in Rock Unaffected by Boreholes or Drifts from Measured Strains or Deformations

The present paper treats the problems regarding determination of the stress in the rock unaffected by boreholes or drifts from measurements made in them. As the items of measurement, we can mention variations in borehole diameter, strains on the bottom surface of boreholes, strains of the wall surface of boreholes, variations in oblique dimension of boreholes and combinations of these as well as strains on the wall surface of drifts or shafts. The formulae to be used in practice to determine the stress in the rock from several measurements have been presented. On deducing them, the elastic constants of the rock, the rigidity of the measuring instruments used and the irregular distribution of strain within the range of each strain gauge were taken into account. The least number of boreholes or drifts necessary for each stress determination and the accuracy in the results obtained have also been discussed

Stress Around a Shaft or Level Excavated in Ground with a Three-Dimensional Stress State

The stress around underground openings is much affected by the state of stress in the ground at which the opening is made. The present paper describes the results of an investigation of the stress distributions around a vertical shaft, an inclined shaft and a level, taking into account the fact that the ground is in a three-dimensional stress state. First the stress around a circular inclined shaft is analyzed strictly and it is proved that some components of the stress are indeterminate. Secondly the general method of experimental analysis of stress by means of two-dimensional and three-dimensional photoelastic experiments is discussed, paying special attention to the evaluation of indeterminate stresses. By the method thus obtained, the stress is found around a shaft or level with a square or rectangular cross section having rounded corners, from which the influence of the state of stress in the undisturbed ground upon the stress around a shaft or level is discussed

Earth Pressure near a Longwall Working Place

The earth pressure around a longwall working place has been studied, taking into account several factors such as the balance of force, the supporting capacity of the overlying layers, and the retardation of the surface subsidence. As the results, the authors have derived a theory on the transition of stress conditions in the ground above a mined area ; that the earth pressure distribution in a goaf is at first very irregular but it becomes more and more uniform as time passes, as illustrated in Fig. 7, and that less irregularity is seen on the plane farther from the coal seam as shown in Fig. 6. This theory has proved serviceable to explain various facts that had otherwise never been understood

The Fracture of Rock around Underground Openings

The authors have carried out researchs on the mechanism of the failure of rocks which is found at underground openings by means of experiments with models and stress analysis by photoelasticity as well as by field observations and arrived at the following conclusion. The fracture of rock on the inner surface of underground openings takes place when the theoretical maximum stress, tensile or compressive, under the assumption that the ground is perfectly elastic, multiplied by a corresponding factor reaches the tensile or compressive strength of the rock. There is a great difference between the factors for tension and compression fracture, namely that the former is about 0.45, while the latter is 0.95. This theory explains many rock pressure phenomena in solid ground which have not been heretofore understood, such as the compression fracture frequently found on the side wall of a drift. It is pointed out that the state of rock pressure can be inferred to some extent by observing the state of fracture of rock around underground openings