Existence of high horizontal stresses in rock masses
Existence of High Horizontal Compressive Stresses in Rock Masses
Summary.—Recent work on the measurement of rock stresses underground, reported from different parts of the world show that initial natural compressive stresses in horizontal directions are usually either equal to, or often greater than, the compressive stress in the vertical direction. The latter is usually equivalent to the weight of overlying rock.
These high horizontal stresses are probably due to the operation of tectonic forces of the kind which in the past, caused folding and faulting of the rock, and whose activity at present is indicated by earthquakes and tremors. It appears reasonable to suppose that in many regions these tectonic forces are still active to a degree insufficient to cause failure of the rock, but sufficient to produce high horizontal stresses.
The magnitude of the horizontal stresses and the ratio of the horizontal principal stresses to the vertical principal stress are of fundamental importance in the design of underground openings and sequence of excavation operations, and in the design of pressure tunnels and shafts.
The relatively few determinations of rock stresses which have so far been made, forcibly demonstrate the need to make actual measurements.
Introduction
In the investigation of the effect of making openings in stressed material, the ratio of the principal stresses in the material initially, before the openings are made, is of primary importance. This applies particularly to the design and construction of excavations in rock, where making an opening causes a re-adjustment of the initial natural state of stress of the rock mass, resulting in stress concentrations in a zone surrounding the opening. The position and magnitude of these stress concentrations, in relation to properties of the rock mass, are the basic factors which determine the behaviour of the opening.
For example, the effect of the ratio, N, of the horizontal principal stress, σh, to the vertical stress, σv on the magnitude of the stress concentrations in the surface at the crown of the roof and at the mid-point of the side wall of a circular tunnel, is shown in Fig. 1 (from Terzaghi and Richart(1), Fig. 7), for ideal perfectly elastic, homogeneous material. It will be noted, for instance, that where N becomes less than ⅓, tension is present in the crown of the tunnel and there is compression in the side-wall, whereas where N becomes greater than 3, there is very strong compression in the crown and tension in the side-wall.
The ratio of the principal stresses is equally important in investigations made by purely mathematical methods and by photoelastic methods.
Until fairly recently, no methods had been developed for determining, by actual measurements, the state of stress of rock in place, and assumptions had to be made about the magnitude of the stresses and their ratios.
Assumptions Concerning the State of Stress
It was obvious that a major factor in determining the state of stress of rock at depth, and one which was always present, was the weight of overlying rock. It was assumed that, at least where the ground surface was horizontal, one of the principal stresses was vertical and was just the weight of rock of unit area above the point considered.
*Snowy Mountains Hydro-Electric Authority On Soil Mechanics and Foundation Engineering
