excavation, the compressive stress in one horizontal direction was about 2 ½ times that in the vertical direction. The latter was approximately equal to the weight of overlying rock. The stresses referred to are the initial natural stresses in the rock before excavation, computed from the measured values using photoelastic models.

Extremely interesting and important investigations have been made since 1952 by Hast⁽¹⁰⁾ in Sweden who has developed a technique enabling stresses to be measured deep in solid rock.

Measurements have been made at depths of 15 meters in from the face of an excavation, and even deeper measurements are possible. This technique now makes it possible to penetrate beyond the effects of blasting damage, and generally beyond the zone of stress concentrations around openings of moderate size, into undisturbed rock, thus avoiding some of the major difficulties of other methods.

The method briefly is as follows. A hole, approximately 1 inch in diameter, is drilled in the rock face. A combined measuring and loading cell is inserted in any required position, and a sufficient load in excess of the compressive stress in the rock is applied diametrically to the walls of the hole. A circular channel, usually about 4 inches in diameter, is cut co-axially with the cell hole, for a sufficient distance beyond the cell. This channel has the effect of isolating the cylinder of rock containing the cell from the surrounding stress field.

The cylinder of rock thus expands, and the load on the cell is simultaneously reduced. The difference between the original load on the cell and the last value read off on completion of the stress- release channel represents the absolute value of the stress in the rock around, the cell hole.

Measurements are usually made in three directions at adjacent points in each hole. By making measurements in three holes drilled in different directions through the same area, the magnitude and direction of the three principal stresses may- be determined.

From the results of measurements made by this method, in mines in several localities in Scandinavia, Hast has found that the rock down to great depths is subjected to horizontal compressive stresses several times greater than the weight of the overlying strata. At depths of 400 meters and in one case, 900 meters, horizontal stresses were 1.5-3.5 times the weight of overlying strata, and locally as high as 8 times.

Underground stress measurements on the Kolar gold field in India, reported by Isaacson⁽¹¹⁾ showed the existence of horizontal stresses in the rock, greater than could be accounted for by the weight of overlying rock.

The Sir Adam Beck Niagara Generating Station No. 2, Ontario, Canada, involved the construction of two parallel, essentially horizontal, 50-ft. diameter tunnels spaced 250 ft. apart, in horizontally stratified limestone, dolomite, shale, and sandstone, at some 300 ft. below the surface of the ground.