The objective of this paper is to summarize briefly the practical use of rock mechanics in design and construction of various types of structures. These range from foundation for industrial facilities, nuclear power plants, bridges and dams to tunnels and underground caverns. The use of rock mechanics has varied from performing a few laboratory unconfined compressive tests on core samples to complete field and laboratory tests for underground power caverns.
The primary difference between the rock mechanics studies we have for construction projects and those most commonly found in the literature is that our tests must provide immediate answers to specific questions facing a design or construction engineer. For example, when we make a plate-bearing test we arrive at a load-deformation relationship. We realize that this is not a true modulus of elasticity. Yet, when considering the resistance the rock will provide against forces occurring in a pressure tunnel it is a far more useful value than the true modulus of elasticity of the rock.
Another example of this difference in interest is in the use of fiat jacks. Many people criticize the use of fiat jacks because they measure the stresses in the disturbed zone very near the surface and do not represent the undisturbed stress condition in the rock. However, in actual construction it is frequently those near-surface stresses with which we must cope. We are interested in both the stress conditions present prior to excavation and those as modified by construction of an exploratory adit or drift. Fundamentally, we are-interested in the practical application of rock mechanics in the solution of problems of design and construction of civil engineering works.
As an example of how our firm applies rock mechanics to civil engineering, Bechtel Corporation maintains a laboratory containing the usual equipment used t o determine specific gravity, porosity, absorption compressive and tensile strengths, modulus of elasticity, Poisson's ratio, triaxial and direct shear strengths.
An evaluation of the physical properties of rock samples enables us to determine the suitability of rock for riprap. Unconfined compressive tests, hardness absorption, specific gravity, freezing and thawing determinations and the wet-shot tests are used to make such evaluations.
The old Duval test ASTM-289 utilizing equipment such as that shown in Fig. I is used to evaluate the resistance to abrasion during a very severe test. The cylinder (diameter-20 cm) is charged with the rock sample, water and steel balls. The mass is then subjected to 10,000
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revolutions. The chart shown in Fig. 2 is used to evaluate the suitability of the rock samples for riprap. We believe these data are sufficient to determine if the riprap will remain suitable during the life of the project.
The behavior of rock at cryogenic temperatures was unknown to us until a few years ago when we were asked if it was feasible to store
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cryogenic fluids, such as liquid methane, in underground caverns.