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Berea sandstone, Marianna limestone, Hasmark dolomite, Repetto siltstone, and Muddy shale have been subjected to triaxial compression tests in which the external confining pressures and internal pore pressures (to 2 kilobars) are applied and measured independently. The interstitial water pressure is maintained constant throughout the test, and porosity changes are determined as functions of permanent shortening.
The ultimate strength and ductility of porous rocks are found to depend on effective confining pressure--the difference between external and internal pressures when the pore fluid is chemically inert, the permeability is sufficient to insure pervasion and uniform pressure distribution, and the configuration of pore space is such that the interstitial hydrostatic (neutral) pressure is transmitted fully throughout the solid framework.
At high effective pressures (about 1 kilobar) porosity decreases with progressive permanent strain. At intermediate pressures (about 500 bars) the porosity remains essentially constant for compressions as great as 20 per cent, and at low pressures (about 200 bars and less) the rocks are dilatant. Microscopic examination of the sandstone reveals that grain breakage becomes progressively less important as pore pressure is increased until the deformation becomes entirely intergranular. Since the aggregate is initially closely packed, the shortening leads inevitably to increased void volume.
An explanation of the pore pressure effects on the basis of Coulomb friction is consistent with the empirical data. High pore pressure reduces internal friction (but does not modify the coefficient). The rock is weak and relatively brittle; faulting is favored. These facts support the Hubbert-Rubey theory of large-scale overthrust faulting and enhance our understanding of diapir structures in shales, of sandstone dikes, of localized faulting in zones of "abnormal" formation pressures, and of high interstitial pressures in young intensely folded rocks of low permeability.
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