“Existence of High Horizontal Compressive Stresses in Rock Masses”, by D. G. Moye—p. 19.

“Field and Laboratory Tests in Rock Mechanics”, by L. G. Alexander—p. 161.

Reporter—Mr. D. Campbell-Allen:

The Reporter emphasized the need in design for knowledge about the properties of, and stresses in, large masses of rock, and commended the papers as making contributions in a field in which there was a world wide lack of information. In summarizing the papers he raised the following points:

• (1) The assumption that stresses are tending to the hydrostatic with increase of depth and time is dependent on an assumption about the creep behaviour of rock under shearing stresses which may not be justified.
• (2) Is the hydrostatic stress associated with pore pressures?
• (3) The existence of measuring equipment presupposes the presence of a hole and therefore of disturbance of the existing stresses. Even the Hast method must be subject to this objection, and he enquired as to the technique used to overcome it.
• (4) The assumption that the vertical stress and the stresses along the line of the tunnel are principal stresses seems to have been made by Mr. Alexander and to be unjustified.
• (5) In view of the wide variation of stresses and deformations modulus in the rock, how is the designer to choose values for use in his design?
• (6) Can rock bolts of mild steel be regarded as permanent in view of their capacity for relaxation, and that of the rock for creep?

Mr. L. G. Alexander:

In reply, Mr. Alexander stated that water pressures in drilled holes had been found to be quite low relative to the rock stresses, of the order of 200 lb. per sq. in.

He suggested that the stresses of interest in design were, in fact, those near the surface of a hole during excavation and that the undisturbed stresses were of interest in the prediction of the surface stresses. The measurements were normal stress only, but they were recognized as such and some progress has been made towards the measurement of the associated shear stresses. The causes of the scatter of the values obtained were unknown in detail, but the averages were reasonable, and the chief interest of measurement of stresses away from the surface, he thought, lay in the possibility of explaining the scatter.

There seemed no geological explanation of the distribution of stress around openings, except that the overall average was related to the overburden. He regretted that the paper did not discuss the effect of the work on design, and pleaded lack of space. He pointed out that the results for the extension of rock bolts on p.168 of the paper showed no evidence of relaxation over considerable periods.

After describing the procedure used by Hast (see Moye’s paper p. 21) he explained the basis of Hast’s contention that his gauger which responds to changes in deformation, measured “absolute“ values of stress as opposed to changes in stress.

Mr. Alexander said that the equation in Section 8 of his paper should read: