Field and Laboratory Tests in Rock Mechanics by L. G. Alexander
Summary.—The study of rock movements about excavations on the basis of the elastic theory comprises investigation of the rock movements, the deformation characteristics of rock (generally jointed), rock stresses, and the validity of the elastic hypothesis. The tests given in this paper relate to these different aspects, with emphasis on rock stress measurement.
1.—Introduction
The rock mechanics investigations were undertaken because of the number of tunnels and underground power stations to be constructed by the Snowy Mountains Hydro- electric Authority.
For tunnels, there is potential scope for taking advantage of rock support in the design of linings but knowledge of both the strain performance of tunnel linings and of rock deformation modulus in situ is limited. For underground power stations, chief interest is in methods of rock support, both temporary during excavation and for permanent roof and wall support. Very little is known of the strains developed in rock bolts used for stabilizing the surface layer of rock, which may be close- or open-jointed, the elasticity of this layer and of the underlying rock, or of the strains developed in concrete roof structures during excavation.
The rock movement which causes the above strains has been generally thought to be related to the stress in the rock and its changing concentration during excavation, together with the elasticity and creep characteristics of the rock. Methods of measuring these on rock in situ are at present only in an early stage of development. Stresses about excavations are investigated by electrical analogue tests and tests on photoelastic models. The latter may also be used for displacement studies. Some discussion is given of confirmation of this method by observed rock stresses.
*Engineering Physics Section, Snowy Mountains Hydro-Electric Authority. On Soil Mechanics and Foundation Engineering
2.—Rock Deformation Modulus.
In many uses rock behaviour is not elastic and the term “deformation modulus” is used here in place of Young’s modulus. This deformation modulus is calculated by applying the formulae of the elastic theory to the slope of the deformation-load test curve. For application in design, the deformation required is the total in a test from zero to a given load, inclusive of closing of fissures, creep, and increments from load cycling. In the plate bearing test it is the pseudo-elastic modulus of the generally fissured surface layer of rock which is measured.
In the flat jack jack test (in stressed rock) and in laboratory tests on rock cores, the behaviour is more nearly elastic, but the modulus is again calculated as a secant modulus from the overall deformation.
3.—The In-Situ Plate Bearing Test of Rock Deformation
The plate bearing test consists of applying a load to an area, circular or square, of an infinite plane rock face, and measuring the displacement of the loaded area. The modulus is computed using the Boussinesq formula (Refs. 1 to 9).
Although details of equipment and procedure vary, the plate bearing test is widely used, and fair agreement has been obtained between the modulus from the test and that deduced from tunnel lining strains (See the comparison given in section 7 of this paper and Ref. 6). Belin(9) has described tests in the Eucumbene-Tumut Tunnel, Snowy Mountains Area, using a 100-ton jack (Fig. 1).
There are some controversial aspects of the tests and their interpretation.
