Existence of high horizontal stresses in rock masses
In a rock mass divided into vertical columns by open joints, for instance columnar basalt, the N value might be close to zero. Where rock has carried a temporary heavy load such as a thick ice sheet, or a layer of material subsequently removed by erosion, N might be higher than ⅓.
Non-elastic behaviour, such as creep in the rocks, also tends towards an equalization of the stresses and an N value of 1.
However, in addition to these gravitational effects, many rock masses exhibit evidence of having been subjected to very strong horizontal forces. Many kinds of meta- morphic rocks, for example, show by the orientation of the minerals of which they are composed, that they were formed under conditions of high shearing stress, with the greatest principal stress horizontal or nearly so. Many rock masses have been subjected to the action of strong horizontal compressive forces during the past, which have exceeded the strength of the rock, resulting in plastic deformation by folding, or failure by fracturing, with the formation of thrust faults, strike-slip faults, and shear-joints. The occurrence of earthquakes originating within the earth's crust is evidence of the continuing activity at the present time of these so-called tectonic forces.
In these rocks it appears that N might have any value compatible with the strength of the rock.
Stress Determinations
During recent years there has developed a widespread and growing interest in the determination of the actual state of stress around openings and also the initial state of stress of the undisturbed rock mass. Investigations have been made mainly in civil engineering organizations concerned with the design of pressure tunnels, pressure shafts, and the large complex openings for underground power stations, and in mining organizations concerned with problems of large-scale underground extraction of coal and ores and with problems of deep mining.
Techniques of measurement of stresses and methods of analysis of results are undergoing development, and there are still many uncertainties associated with both the methods and results. Nevertheless the results so far available are considered to be of considerable interest and value.
Pioneer work in this held was carried out by the U.S. Bureau of Reclamation in a tunnel under Boulder Canyon in 1932(5) and later in Prospect Tunnel, Colorado, and Gorge Tunnel, Washington(6). The measurements under Boulder Canyon showed horizontal compressions several times greater than the vertical compression. In Prospect Mountain Tunnel, located 900 ft. beneath the peak of Prospect Mountain, the measurements indicated rock stresses,
