Abstract
In order to understand the underground earth pressure phenomena in competent rock, it will be essential to clarify the stress around the underground openings. This stress depends upon various conditions such as the original state of stress, the shape and size of openings, the mechanical properties of rock and so on. Therefore it may be hard to evaluate the stress under all possible conditions.
A number of investigations have hitherto been carried out on the stress around the underground openings, but they have so long been undertaken on the premise that the ground is perfectly elastic and that the original state of stress was rather simple.
Measurements of stress in the rock around the underground openings, show however, that the original stress was threedimensional and the principal stresses did not always take vertical and horizontal directions. Therefore the authors have attempted to analyze the stress under such an original state of stress.
The theoretical study has shown that at least five kinds of photoelastic experiments are necessary to evaluate the stress around each opening, no matter what the original stress state may have been. Three of them may be two-dimensional photoelastic experiments, but the remaining two must be three-dimensional. When the direction of one of the principal stresses in the original ground coincides with the direction of the axis of a drift, we need only three kinds of two-dimensional photoelastic experiments. From each experiment we can determine a stress coefficient for any point in the model. The stress components at any point in the model can be expressed in terms of these stress coefficients whatever the original state of stress may have been.
Necessary photoelastic experiments have been carried out to find stress coefficients at several points on the wall surface of drifts having several shapes of cross section. For drifts with three kinds of rectangular cross section, five stress coefficients have been determined at several points on the wall surface of each drift, and for drifts with other five kinds of cross section, three stress coefficients have been determined. They are all tabulated.
As for the stress around a circular opening such as a circular shaft, drift or a bore hole, such strict analysis as had never been contemplated has been attempted, and the authors have succeeded in obtaining a general expression for each stress component.
Meanwhile to succeed in maintaining the earth pressure control during the operation of mining massive deposits, it is required to estimate the stress concentration in the pillars. Since the shape and arrangement of the rooms and pillars are various, it is extremely difficult to establish any strict formulas for use to find the stress. The authors have, however, proposed approximate formulas on the basis of the results of photoelastic experiments carried out by themselves.
All the results of investigation described above hold true, provided that the ground is perfectly elastic, that the stress induced around the openings is comparatively small and is therefore within the elastic limit of the rock. If, however, the stress exceeds this limit, the results of investigation must be duly corrected.
Model experiments as well as theoretical investigations have been carried out to find on what condition fracture will appear in the rock around the underground openings. It has been found that tensile fracture will take place wherever half the tensile stress computed on the assumption that the rock is perfectly elastic reaches the tensile strength of the rock, whereas compressive fracture will take place wherever 0.95 times the compressive stress computed under the same assumption reaches the compressive strength of the rock.
