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Room temperature creep in saturated granite
280A 935086 Possible secondary fracture patterns due to a change in the direction of loading Wu, H; Pollard, D D Proc Conference on Fractured and Jointed Rock Masses, Lake Tahoe, 3-5 June 1992 P505-512. Publ California: Lawrence Berkeley Laboratory, 1992 Many sedimentary rocks contain natural flaws. The effect of such a flaw on tangential stress distribution along the surface of a short fracture has been simulated using a circular hole. Results of numerical analysis and physical model tests using stress coat on P M M A substrates are presented. When loading direction is changed (rotated), flaws triggering the first set of fractures also dominate the second set, until rotation of a stress axis reaches a critical value, when fractures initiate from tips of existing cracks. 935087 SEM study of fracture development in rock materials Yonghong Zhao; Jiefan Huang; Ren Wang Proc Conference on Fractured and Jointed Rock Masses, Lake Tahoe, 3-5 June 1992 P513-518. Publ California: Lawrence Berkeley Laboratory, 1992 Uniaxial compression tests have been carried out on plates of marble, sandstone, and limestone containing slots at 30, 45, and 60 deg to the loading axis. Effects of lithology and grain size on the mode of fracturing are discussed. At low magnification (50x), multi-order propagation was seen in marble, single order in sandstone and no propagation in limestone. At higher magnification, multi-order propagation became evident in sandstone, single order in limestone. The role of grain diameter as a measure in evaluating the strength of energy barriers and the effects of capability of crack proliferation on apparent ductility and crack propagation speed are examined. 935088 Pore-based mechanism of dilatancy and fracture of rocks under compression Dyskin, A V; Germanovich, L N; Ustinov, K B Proc 33rd US Symposium on Rock Mechanics, Santa Fe, 3-5 June 1992 P797-806. Publ Rotterdam: ,4 ,4 Balkema, 1992
935090 Energy dissipation processes affecting crack growth in quasi brittle materials Stefter, E A; May, G B; Epstein, J S Proc 33rd US Symposium on Rock Mechanics, Santa Fe, 3-5 June 1992 P837-846. Publ Rotterdam: ,4 A Balkema, 1992 Although initiation energy for crack growth in brittle materials may be low, energy for continued growth increases, making the materials tougher (rising R curve). Dissipation mechanisms not local to the crack tip, crack deflection, bridging, and face friction, which may explain this phenomenon are discussed. These effects are observed in laboratory tests on amorphous and crystalline calcium silicate rocks, in which Moire interferometry was used to study surface crack tip displacement fields.
Time dependent behaviour See also." 935344, 935450 935091 Effects of ageing and overconsolidation on the elastic stiffness of a remoulded clay Athanasopoulos, G A lnt J Geotech Geol Engng Vll, NI, March 1993, P51-65 Resonant column measurements were made of the low amplitude shear modulus of remoulded kaolinite clay as a function of time and at different values of confining stress and OCR. Once primary consolidation ceased, values of modulus increased linearly with log(t) and, after 7 days of confinement, the value of normalized rate of secondary increase of modulus could be reliably estimated. These values decreased linearly with logarithms of OCR or time. Based on this, an equation correlating age and equivalent OCR is formulated. Confining stress and OCR have greater influence on modulus than the Hardin Equation predicts. 935092 Room temperature creep in saturated granite Lockner, D J Geophys Res V98, NBI, Jan 1993, P475-487
An exact asymptotic solution to the 2D problem for a couple of long cracks growing from a circular pore under uniaxial compression is derived. Comparison with published numerical results indicates the solution is valid even if crack length is less than pore radius. Two possible mechanisms of dilatancy and fracture in brittle rocks under uniaxial compression are compared on the basis of this work: growth of secondary cracks from pre-existing cracks or from pores. The latter is seen to be a significantly weaker source of secondary cracks.
Triaxial creep tests on granite were carried out at 20 deg C, constant confining pressure of 60MPa, with deviatoric stress cycled between 70 and 90% of short term failure strength. Axial and volumetric strain and resistivity parallel to sample axis were measured. Most experiments were in the steady state regime. Coefficients of the logarithmic creep law have been evaluated. Stress dependence took the same form as that for mode I crack growth in double cantilever beam tests. Behaviour is analysed in terms of stress corrosion and crack growth models.
935089 Microcrack interaction leading to shear fracture Lockner, D A; Moore, D E; Reches, Z Proc 33rd US Symposium on Rock Mechanics, Santa Fe, 3-5 June 1992 P807-816. Publ Rotterdam: .4 A Balkema, 1992
935093 New perspectives on soil creep Kuhn, M R; Mitchell, J K J Geoteeh Engng Div A S C E VI19, N3, March 1993, P507524
Recent laboratory data on nucleation and growth of shear fractures in brittle rocks are discussed. In homogeneous crystalline materials such as granite, fault nucleation occurs rapidly, with minimum change in the precursory microcrack pattern. Growth is also rapid, with damage largely restricted to the small zone close to the fracture front. A conceptual model has been developed for brittle fracture controlled by microcrack interactions on the basis of acoustic emission studies and observed microcrack densities.
A creep mechanism for soils is presented which is based on interparticle sliding which is both viscous and frictional. It is formulated using rate process theory and gives an expression for the sliding velocity of two contacting particles in terms of the ratio of tangential and normal contact force components. When the mechanism is incorporated in a discrete element model of a large assembly of circular particles, the following aspects of soil behaviour are reproduced: creep rate increasing with applied stress and decreasing with time; and creep rupture at elevated stress levels.
© 1993 Pergamon Press Ltd. Reproduction not permitted
935090 Energy dissipation processes affecting crack growth in quasi brittle materials Stefter, E A; May, G B; Epstein, J S Proc 33rd US Symposium on Rock Mechanics, Santa Fe, 3-5 June 1992 P837-846. Publ Rotterdam: ,4 A Balkema, 1992 Although initiation energy for crack growth in brittle materials may be low, energy for continued growth increases, making the materials tougher (rising R curve). Dissipation mechanisms not local to the crack tip, crack deflection, bridging, and face friction, which may explain this phenomenon are discussed. These effects are observed in laboratory tests on amorphous and crystalline calcium silicate rocks, in which Moire interferometry was used to study surface crack tip displacement fields.
Time dependent behaviour See also." 935344, 935450 935091 Effects of ageing and overconsolidation on the elastic stiffness of a remoulded clay Athanasopoulos, G A lnt J Geotech Geol Engng Vll, NI, March 1993, P51-65 Resonant column measurements were made of the low amplitude shear modulus of remoulded kaolinite clay as a function of time and at different values of confining stress and OCR. Once primary consolidation ceased, values of modulus increased linearly with log(t) and, after 7 days of confinement, the value of normalized rate of secondary increase of modulus could be reliably estimated. These values decreased linearly with logarithms of OCR or time. Based on this, an equation correlating age and equivalent OCR is formulated. Confining stress and OCR have greater influence on modulus than the Hardin Equation predicts. 935092 Room temperature creep in saturated granite Lockner, D J Geophys Res V98, NBI, Jan 1993, P475-487
An exact asymptotic solution to the 2D problem for a couple of long cracks growing from a circular pore under uniaxial compression is derived. Comparison with published numerical results indicates the solution is valid even if crack length is less than pore radius. Two possible mechanisms of dilatancy and fracture in brittle rocks under uniaxial compression are compared on the basis of this work: growth of secondary cracks from pre-existing cracks or from pores. The latter is seen to be a significantly weaker source of secondary cracks.
Triaxial creep tests on granite were carried out at 20 deg C, constant confining pressure of 60MPa, with deviatoric stress cycled between 70 and 90% of short term failure strength. Axial and volumetric strain and resistivity parallel to sample axis were measured. Most experiments were in the steady state regime. Coefficients of the logarithmic creep law have been evaluated. Stress dependence took the same form as that for mode I crack growth in double cantilever beam tests. Behaviour is analysed in terms of stress corrosion and crack growth models.
935089 Microcrack interaction leading to shear fracture Lockner, D A; Moore, D E; Reches, Z Proc 33rd US Symposium on Rock Mechanics, Santa Fe, 3-5 June 1992 P807-816. Publ Rotterdam: .4 A Balkema, 1992
935093 New perspectives on soil creep Kuhn, M R; Mitchell, J K J Geoteeh Engng Div A S C E VI19, N3, March 1993, P507524
Recent laboratory data on nucleation and growth of shear fractures in brittle rocks are discussed. In homogeneous crystalline materials such as granite, fault nucleation occurs rapidly, with minimum change in the precursory microcrack pattern. Growth is also rapid, with damage largely restricted to the small zone close to the fracture front. A conceptual model has been developed for brittle fracture controlled by microcrack interactions on the basis of acoustic emission studies and observed microcrack densities.
A creep mechanism for soils is presented which is based on interparticle sliding which is both viscous and frictional. It is formulated using rate process theory and gives an expression for the sliding velocity of two contacting particles in terms of the ratio of tangential and normal contact force components. When the mechanism is incorporated in a discrete element model of a large assembly of circular particles, the following aspects of soil behaviour are reproduced: creep rate increasing with applied stress and decreasing with time; and creep rupture at elevated stress levels.
© 1993 Pergamon Press Ltd. Reproduction not permitted