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

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

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

Fundamental Investigation on Design of Tunnel Steel Arch Supports

For the purpose of contributing to a proper design of tunnel supports with steel arches, it is necessary to clarify their behavior under earth pressure. As a first approach, the authors investigated a method to calculate the deformation of, and the stress in, steel supports subjected to earth pressure. Special attention was paid to the treatment of the passive forces induced, the mechanical characteristics of the ends of these supports, as well as the joints of the members. As a result of theoretical investigations followed by experiments, the authors obtained a method that may be used in practice

Investigations on the Stress in Circular Shaft Linings

For the purpose of contributing data for the reasonable design of shaft linings, the authors have tried, at first, to measure the stress in the linings of several vertical shafts recently sunk in coal mines by means of a photoelastic stressmeter, and have obtained data for estimating the earth pressure that may act on shaft linings when they are sunk into ground of various conditions. Secondly the stresses in a circular shaft lining caused by various states of earth pressure are analysed by the theory of elasticity, and the stress concentration due to irregular earth pressure is clarified. Part I describes the results of the stress measurements, and Part II the results of the analysis

Measurement of Variation in Stress by a Photoelastic Stressmeter

Researches have been carried out to obtain a suitable apparatus and technique for field stress measurement in rock or concrete constructions by application of photoelasticity. The principle of this method is that a gage is beforehand fixed to a body whose stress is required to be measured. When the stress state of the body varies, there appears some stress in the gage. By observing the stress pattern in the gage by a polariscope, the variation in stress of the body can be determined. In the first place a polariscope and photoelastic gages suitable for this measurement were investigated, and three types of gages, i.e. a hollow cylinder, a solid cylinder and a rectangular prism of borosilicate glass and a portable polariscope, 500 gr in weight, were obtained. This polariscope has a detachable compensator so that it can be used for two purposes, namely to measure the relative retardation at an optional point on a gage by means of the compensator, and to observe stress patterns without the compensator. The stress in gages was analysed by the theory of elasticity to obtain fundamental data for finding a proper technique to determine variation in stress in a body. Referring to the results of analysis and taking into account the results of calibration, some techniques to determine variation in stress of the body that may be recommendable were obtained. Many experiences in practical application of this method of stress measurement have proved that the method is suitable for measurement extending over a long period of time

Preparation and Dielectric Properties of [Ba,Sr]TiO(3)-Al(2)O(3)-SiO(2) Glass-Ceramics

A series of ferroelectric glass-ceramics was elaborated by the controlled growth of Ba(1-x)Sr(x)TiO(3) crystal particles in the glass system 60[Ba(1-y)Sr(y)]TiO(3)-10Al(2)O(3)-30SiO(2)(0≦y≦0.2) in molar basis. Analysis of crystal phases by X-ray diffraction revealed that Sr content in Ba(1-x)Sr(x)TiO(3) increased with increasing content of SrO in glasses by its preferential transfer into the crystal phase, and the appropriate temperature for the crystal growth was 1100°C. Curie temperatures of glass -ceramics shifted to lower temperature with increasing SrO content in the crystal and comparatively high dielectric constant was obtained at room temperature for a glass-ceramics with y=0.2. Frequency dependences of dielectric constant and loss tangent were examined in the frequency range from 1 K to 1 M Hz