岩石の塑性についての課題
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概要
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Although surfacial rocks appear brittle, the earth's crust the large aggregate of rocks is supposed to be very viscous. In the realm of rock mechanics, however, the aspect of plasticity cannot be overlooked. Rock deformations that accompany any kind of failures of fractures are declaredly plastic. Fine slips and incipient failures of rocks may take places where locally concentrated stresses satisfy the yielding condition, while the apparent general stresses are consistently below the elastic limits. The crypto-plastic flow as a result of such minor slips in the geological materials throw many problems not yet settled. In a polycrystalline rock, boundary surfaces of grains are obstructive for moving inside dislocations and apt to concentrate local stresses matchable to the yield values. Similar concentration processes may occur in the matrices of sedimentary rocks regardless whether the cementing substance is a solid mineral or adhesive water. Brittle tensile fractures created in homogeneous fine-grained rocks exhibit uneven surfaces with special patterns of (1) concentric undulations - conchoidal fractures, (2) radial striatures at right angles with the conchoidal waves - plumose fracture and (3) columnar fringes. Tension cracks under the high confining pressures show none of such figures. The so-called angle of internal friction is merely expressing increase in strength with increasing lithostatic pressure or the grain to grain average rock pressure has been neglected hitherto. Reduced effective stresses are given in that the deviatoric stress minus the pore fluid pressures. The Berea Sandstone under the total given hydrostatic pressure - lithostatic pressure plus pore water pressure= 2kbar - is perfectly plastic with a material constant k_0 = 2.2 kbar, but the rock yields at k =k_0 -pore water pressure. The value of k_0 increases with increasing total pressures. If all the principal values of reduced effective stress are tensile, the porosity increases contrary to the triaxial compression. Strain hardening is revealed at higher confining pressures than that corresponding the fixed value of pressure, while thixotropy is evident below. Most of the subconsolidate sedimentary rocks that are indurated by means of adhesive water are perfectly plastic even at very low confining pressure.
- 1965-06-15