Summary Rebound of rock masses, defined as the expansive recovery of surficial crustal material, either instantaneously, time-dependently, or both, initiated by the removal or relaxation of superincumbent loads, is found in most geological terrains. The applied loads resulting from past or present geological processes are removed or relaxed by (1) natural processes such as valley erosion (long term) or (2) artificial processes such as excavations (short term). Rebound features are most pronounced if the time period of load removal is short compared to the relaxation time of the material, assuming that relaxation time is a material property. Some factors that affect rebound and its intensity are: Initial fabric, lithology, crack density, anisotropy, density and moisture content. Magnitudes of deviatoric stresses (stress differences, stress ratios) operating at the time of load removal, their orientation with respect to the free surface and their rate of unloading. Initial strength and time-dependent strength. Environmental conditions, such as moisture, temperature and other weathering agents. The near-surface stress distribution and time-dependent strain release measured within different geological terrains suggest that the redistribution and/or release of stored-strain energy within rock masses may be a major factor contributing to rebound. Strains, stored in the rocks as a result of past and present geological loads, can be released by excavation processes. As new surfaces are formed, the rock mass changes volume, creating physical property changes that include increased void space and higher permeabilities, thus allowing more rapid access for chemical agents that accelerate the weathering process. Rebound is a rate-dependent process that is manifested differently in various geological terrains. Long-term stability is dependent on changing fracture density and the rate at which strength decreases within near-surface rock material. The strength reduction may be, among other things, a function of the internal decay of cohesion due to the development or propagation of fractures. The time-dependent processes of rebound and the ambient stress fields are important in the development of criteria for the long-term design of engineering installations that are placed within or on disturbed rock masses.
A geotechnical investigation of the Pierre Shale near Hayes, South Dakota, was conducted by the U.S. Geological Survey as a basis for evaluating problems in deep excavations into that formation. The physical and mechanical properties of the shale were determined through use of core holes drilled to a maximum depth of 184 m. In situ borehole determinations included a gravimeter survey, pressuremeter testing, thermal profile measurements, and borehole velocity measurements. Onsite and offsite laboratory measurements included rebound measurements, sonic velocity measurements of shear and primary waves, X-ray mineralogy and major element determinations, size analyses, fracture analyses, fabric analyses, and determination of thermal properties.Below 15–22 m, the shale is an unweathered, saturated, overconsolidated, underpressured clay shale with a clay-mineral content ranging between 50 and 100%, dominantly composed of mixed-layer illitic smectites. The physical and mechanical properties vary widely. The variation is related to the clay mineral content (especially in bentonite zones), a large transverse mechanical anisotropy, and zones of fractures and microfractures, which may result from rebound caused by erosion. These may contribute to slope instability over large areas. The thermal and mechanical properties change markedly if the shale is permitted to dry out. The state of stress and overconsolidation appear to be functions of the depositional and erosional history of the deposit. Both are markedly affected by the large fracture zones. The properties of the clay shale indicate problems that may be encountered in excavation and use of deep underground facilities. Key words: anisotropy, characterization, clay shale, consolidation state, physical properties, rebound, relaxation, stress state, thermal properties.
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