Crack-driving stresses and the propagation of natural joints

Conference · Wed Jan 01 04:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5579032

Engelder, T; Fischer, M P ^[1]

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1. Pennsylvania State Univ., University Park, PA (United States). Dept. of Geosciences

The premise of this abstract is natural joint propagation occurs under four combinations of loading conditions, each of which is called a crack-driving stress. The four crack-driving stresses are layer-parallel stretching, layer-parallel contraction, natural hydraulic fracturing, and axial splitting. Layer parallel stretching involves the development of a crack-normal tensile stress as the outer boundaries of the rock are pulled apart by tectonic processes such as folding. For layer-parallel contraction the crack normal boundaries are fixed and will not move regardless of the buildup of crack-normal stress. During thermal cooling, the fixed boundaries prevent contraction and, hence, a tensile crack-normal stress builds within the block until stress becomes large enough to cause crack propagation in the direction of S. The first two types of crack-driving stresses have traction free cracks. Exterior boundary conditions for natural hydraulic fracturing are the same as for layer-parallel contraction, however, the crack is filled with a pore fluid that exerts a traction on the crack wall. While the outer boundary for natural hydraulic fracturing follows a fixed-grips model, the P[sub p] on the crack wall acts as a dead-weight load so that this situation resembles a combination of the first two crack driving stresses. The final crack-driving stress, axial splitting, has neither a crack-normal traction nor a fixed crack-normal boundary. The driving stress for axial splitting arises because the crack-parallel stress, S, is concentrated as a tensile hoop stress at the crack tip with the same magnitude but opposite sign as S. If the initial crack is tilted relative to S, axial splitting is driven by crack opening upon slip of the initial crack. Axial splitting is easily suppressed by the concentration of a compressive hoop stress arising from a modest crack normal stress.

• OSTI ID: 5579032
• Report Number(s): CONF-921058--
• Conference Information: Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 24:7
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
BOUNDARY CONDITIONS
CRACK PROPAGATION
DEFORMATION
FRACTURING
GEOLOGIC FRACTURES
GEOLOGIC STRUCTURES
HYDRAULIC FRACTURING
MECHANICS
ROCK MECHANICS
STRESSES