Measurement of the deformations of the tunnels, during construction, showed horizontal inward movements of the rock walls greatly in excess of those predicted on the basis of elastic behaviour of the rock influenced only by its weight^{(12, 13)}. Also apparently the excavation of the first tunnel had a marked effect upon rock movement in the region of the second tunnel, even though they were so far apart that this effect should have been negligible. In the same area, inward movements of the rock walls of a canal 50 to 60 ft. deep were about one inch. All of these observations suggest the existence of considerable horizontal compressive stresses in the area.

Origin of the High Horizontal Stresses

In most of the examples quoted, from several widely separated localities, the high horizontal stresses cannot be explained merely as an effect of the weight of material, at present overlying the sites. Surface topography is important in some cases, but in others it is apparent that the cause of the high stresses had to be sought in the geological history and in the present state of activity of tectonic forces.

Except perhaps for the comprehensive measurements made at some sites by Hast, the stress measurements themselves are insufficient, and there is not enough known about the geological history, rock structure, and terrain, for a discussion of possible causes of the high horizontal stresses to be more than very general and speculative.

Some rocks, granites for example, at an early stage of their history were probably at a sufficiently great depth to be in a hydrostatic state of stress. Subsequent regional uplift and reduction of the thickness of overlying rock by erosion, would have permitted concurrent relief of vertical compressive stress by upwards expansion of the rock. But in the horizontal direction no such relief by expansion would be possible (unless some geological process in addition to vertical uplift occurred to permit it). The horizontal compressive stresses would tend to decrease in accordance with Poisson's ratio, but only at one-quarter to one-third the rate of decrease of vertical stress. Thus the horizontal stresses would become relatively higher than the vertical, that is, N would become greater than 1. Rock creep, if it occurred, would tend to equalize the stresses.

However, as already mentioned, many rocks exhibit internal evidence, in the form of particular kinds of faults, folds, and joints of having been subjected to high compressive forces and it is considered that the effects of these probably overshadow the gravitational effects in most cases. Rocks compressed by these tectonic forces at some time in the past would remain compressed in the horizontal directions unless other crustal disturbances permitted them to expand, or the compression was relieved by creep.