He also drew attention to an error in the 20th line on p. 167, which should read: 2.0 × 10⁶ p.s.i.

Dr. G. D. Aitchison:

Dr. Aitchison reported two cases from experience in Victoria which evidenced the existence of high horizontal stresses in residual soils.

In the first case, during a study of the swelling of soils, a comparison of the state of affairs in weathered basalt in the field and in the laboratory required high horizontal stresses in order to get reasonable agreement in moisture contents. In the second case, some Benoto pile casings were sunk into weathered Silurian shale for the King Street bridge in Melbourne. Several of the steel casings had been severely distorted to the order of ⅓ of their diameter by high horizontal pressure, showing that high stresses could remain even after a rock had been weathered.

Mr. D. G. Moye:

Mr. Moye, referring to Dr. Aitchison’s remarks, was reminded of two cases where there was evidence of high stresses in soils. In the first, observations of weathered Wianamatta shale exposed in cuttings for the Warragamba pipe near Sydney had shown that in places the residual soil was folded or even faulted, indicating very high pressures developing as a result of hydration and expansion of the shale. In the second, a fault in the slate bed-rock in the Snowy Mountains area had been observed to carry up through stratified surface soil right to the surface which indicated that the soil had been subjected to high horizontal pressures.

Mr J. K. Wilkins:

Mr. Wilkins described stress measurements made in connection with the underground power station for the Great Lake Power Development in Tasmania.

The excavation for the Power Station would be about 45 ft. wide, 90 ft. high and 300 ft. long. It was situated 500 ft. below ground surface and located near the base of the Western Tiers which rose some 3,500 ft. above it. It was expected that vertical stresses would amount to about 500 p.s.i. and horizontal stresses would be much lower.

Three sets of stress measurements were made using the flat jack method described by Mr. Alexander. The first set was made around the periphery of a horizontal gallery modelled on the section of the Power Station to a scale of 1 : 6; the second in a drive along the crown of the Power Station enlarged to a width of 25 ft. and with a height of 8 ft., and the third in a horizontal drive of circular sections. From the stress readings it was possible to set up a number of expressions for the horizontal and vertical rock stresses. Thirteen simultaneous equations were solved for these two unknowns, the solutions having a correlation factor of 0.85. The stresses were rather surprising, being 2,400 p.s.i. horizontally and 1,250 p.s.i. vertically.

Mr. Wilkins went on to show a number of slides illustrating the crushing effect of the high horizontal stresses in the trial galleries and showing the results of the stress measurements. He also described the way in which the cross section of the power station had been modified to reduce stresses and the cutting of slots to relieve peak stresses at the haunches of the roof section.

Dr. D. H. Trollope:

Dr. Trollope considered that both rock and soil masses were basically similar in that they behaved as discrete solids rather than homogeneous solids. Using this assumption he believed that it was possible to model rock masses using uniform cubes in the same way that he had used uniform spheres to model stresses in rock fills. He explained that models of slopes using cubes had shown stress patterns similar to those obtained with spheres or sand grains. He had been able to show that with deformation, full arching conditions would occur and horizontal stresses increased to large values. He considered that the Mohr criterion of shear failure could be used, neglecting cohesion for the present to establish the principal stress differences.

Dr. Trollope said that he was commencing an investigation of stresses around tunnels using a model built up of cubes. The extraction of cubes from a “semi infinite” mass represented the excavation of the tunnel.