Overburden pressure

About: Overburden pressure is a research topic. Over the lifetime, 7627 publications have been published within this topic receiving 150998 citations.

Dilatancy in the Fracture of Crystalline Rocks

Abstract: Volume changes of a granite, a marble, and an aplite were measured during deformation in triaxial compression at confining pressure of as much as 8 kb. Stress-volumetric strain behavior is qualitatively the same for these rocks and a wide variety of other rocks and concrete studied elsewhere. Volume changes are purely elastic at low stress. As the maximum stress becomes one-third to two-thirds the fracture stress at a given pressure, the rocks become dilatant; that is, volume increases relative to elastic changes. The magnitude of the dilatancy, with a few exceptions, ranges from 0.2 to 2.0 times the elastic volume changes that would have occurred were the rock simply elastic. The magnitude of the dilatancy is not markedly affected by pressure, for the range of conditions studied here. For granite, the stress at which dilatancy was first detected was strongly time dependent; the higher the loading rate the higher the stress. Dilatancy, which represents an increase in porosity, was traced in the granite to open cracks which form parallel with the direction of maximum compression.

The frequency-magnitude relation of microfracturing in rock and its relation to earthquakes

Abstract: During the deformation of rock in laboratory experiments, small cracking events, i.e., microfractures, occur which radiate elastic waves in a manner similar to earthquakes. These radiations were detected during uniaxial and triaxial compression tests and their frequency-magnitude relation studied. They were found to obey the Gutenberg and Richter relation log N = a + b M Where N is the number of events which occurred of magnitude M , and a and b constants. The dependence of the parameter b on rock type, stress, and confining pressure was studied. It was found to depend primarily on stress, in a characteristic way. The frequency-magnitude relation for events which accompanied frictional sliding and deformation of a ductile rock was found to have a much higher b value than that observed in brittle rock. The Gutenberg and Richter formulation of the frequency-magnitude relation was derived from a statistical model of rock and crustal deformation. This analysis demonstrates the basis of similarity between rock deformation experiments in the laboratory and deformation of the crust.

Mechanics of Hydraulic Fracturing

Abstract: A theoretical examination of the fracturing of rocks by means of pressure applied in boreholes leads to the conclusion that, regardless of whether the fracturing fluid is of the penetrating or nonpenetrating type, the fractures produced should be approximately perpendicular to the axis of least stress. The general state of stress underground is that in which the three principal stresses are unequal. For tectonically relaxed areas characterized by normal faulting, the least stress should be horizontal; the fractures produced should be vertical, and the injection pressure should be less than that of the overburden. In areas of active tectonic compression, the least stress should be vertical and equal to the pressure of the overburden; the fractures should be horizontal, and injection pressures should be equal to, or greater than, the pressure of the overburden. Horizontal fractures cannot be produced by hydraulic pressures less than the total pressure of the overburden. These conclusions are compatible with field experience in fracturing and with the results of laboratory experimentation.

Elastic Wave Velocities in Granular Soils

Abstract: Laboratory tests, using the resonant column method, were conducted to evaluate the longitudinal and shear wave velocities in specimens of Ottawa sand, crushed quartz sand, and crushed quartz silt. The variables considered were the confining pressure, and the moisture content, void ratio, and grain characteristics of the materials. The wave velocities for the sands varied with approximately the 1/4 power of the confining pressure. At a given confining pressure, the velocity varied almost linearly with void ratio. A diagram is included that shows these relationships over a range of void ratio from 0.3 to 1.3 and confining pressures between 500 psf and 6,000 psf. The effects of relative density, grain size, and gradation entered only through their effects on void ratio. The wave velocities in the quartz silts were found to be greatly dependent on time.

An exact effective stress law for elastic deformation of rock with fluids

Abstract: The exact expressions for the effective stress 〈σij〉 and, in particular, pressure 〈P〉 that cause elastic strain of material with pore fluids are, assuming only that Hook's law is valid, 〈σij〉 = σij - αPδij and 〈P〉 = Pc - αPp, where α = 1 - (K/Ks), Pc and Pp are confining and pore pressures, and K and Ks are the bulk moduli of the rock and grain, respectively The equation for 〈P〉 was first suggested by Geertsma (1957) and by Skempton (1960) on empirical grounds The expression does not depend directly on porosity, but when pores vanish the effective pressure 〈P〉 equals the confining pressure Pc, because then K=Ks Thus the strain of a porous solid with pore pressure can be completely determined from the elastic modulus of the solid without pore pressure, if the effective stress law in the equation for 〈σij〉 is used The exact expression for the effective stress describes quite accurately the measured strains in sandstone and granite samples at confining and pore pressures to 25 kb The results are not applicable to inelastic processes, such as fracture, or elastic processes other than strain