Drilling and Excavation Technologies for the Future (1994) / Chapter Skim
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4. Drilling and Boring of Rock
4. Drilling and Boring of Rock
Pages 34-62
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From page 34... ...
Thus, in the proposed ideal smart drilling approach, the drill must sense the type of rock or stratum ahead of the drill bit; recognize its resistance; and automatically adjust the drilling process in terms of rate, contact pressure, and so forth. If necessary, a smart drill will divert around a particularly difficult heterogeneity or seek alternative directions to avoid premature termination of the drilling operation.
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From page 35... ...
Pore fluids play a significant role in engineering applications for energy resource recovery or dam construction. The effect of pore fluids on fracture behavior can be either mechanical Trough pore pressure diffusion or chemical through stress corrosion (NMAB, 1983~.
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From page 36... ...
Thus, considerable theoretical and mechanistic methodologies exist that are capable of relating elastic properties to heterogeneities in We rock Mat govern its fracture behavior. The apparent inelastic behavior of rocks known as ciast~cflow results from brittle fracture processes due to the formation and stable growth of brittle microcracks.
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From page 37... ...
of crack initiation to show that the uniaxial compressive strength of brittle solids must be many times (about eightfold) higher than Weir tensile strength, and Mat this compressive strength could be increased monotonically by the application of a confining pressure.
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From page 38... ...
Granite Granite · Diorite DTabase · Diorite · Peridotite Serpentinite ~ Serpent in ite o 3 Normalized Confining Pressure O3/Co 4 FIGURE 4.2 Relation between compressive strength and confining pressure for a series of rocks tested at room temperature (after Ohnaka, 1973~.
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From page 39... ...
The actual mechanism of brittle fracture in compression is a simple criterion of an equality between a crack driving force Kit and a material fracture toughness Kit at the tips of He microcracks, interacting en echelon. However, He development of a shear fault Hat produces eventual failure or chip formation in drilling obeys a phenomenological pressure~ependent deviatoric stress criterion (NMAB, 1983~.
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From page 40... ...
I Cr3 (b) FIGURE 4.3 Development of intrinsic fracture behavior by initiation and propagation of a shear fault in compression.
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From page 41... ...
has been investigated in the laboratory in a series of elegant experiments. These experiments have identified the key microstructural damage processes, which include fracturing of weak interface boundaries and grain boundaries; Hertzian diametral fracturing of grains; relative sliding across separated interfaces or boundaries, which act as inclined planes that "jack open" tensile wing cracks running parallel to the principal compression direction; and finally, en echelon action of interacting wing cracks that results in zones of shear faulting.
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From page 42... ...
This size effect, which clearly cart be important in the actual fracture response of rock under drill bits, is not well understood and needs furler study. Compressive strength measurements of about 140 MPa, which have been obtained from the laboratory borehole breakout simulations, are quite high.
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From page 43... ...
FIGURE 4.4 Relationship between shear and normal stresses and principal stresses at failure ranging from tensile, to uniaxial compression, to compression under a confining pressure (Hock and Brown, 1980~. Most of He empirical fracture criteria discussed earlier are formulated from conventional biaxial test data with the implicit assumption of the independence of the fracture phenomenon on He intermediate principal stress a2.
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From page 44... ...
~) Dependence of He fracture angle 8, between the shear fault plane and He direction of principal compression, on the principal stress difference from tests in biaxial compression (Mogi, 1972~.
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From page 45... ...
Evolution of Sources of Shear Faulting The sequence of processes of microcrack interactions leading to the evolution of shear faults has been studied in the laboratory both by techniques of systematic sectioning experiments and by a variety of acoustic techniques. For example, monitoring the acoustic emission (AK)
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From page 46... ...
predict Mat Me onset of localization under axisymmetric compression should occur when Me sample has been deformed well into the strain-softening stage another factor of importance in Me mechanistic rationalization of the drilling process. Surface-Active Agents in Rock Fracture In bribe fracture of solids, the work of fracture results largely from Me surface free energy of the solid (Griffith, 19201.
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From page 47... ...
Some evidence (Kranz, 1980) suggests, however, that such Rehbinder effects are considerably reduced when the part is under pressure or when the surface-active agent cannot penetrate to the crack tips, particularly when the key shear faulting events are of a subsurface nature.
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From page 48... ...
Quite clearly, a detailed understanding of these will be of key importance in the establishment of controlled cutting strategies for exploitation of the full potential of He smart drilling process. Quasi-static too} penetration tests (Evans and Murrell, 1958; Harunan, 1959; Reichmuth, 1963; Sikarskie, 1966; Singh and Johnson, 1967)
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From page 49... ...
Theoretical "Plasticity" Models for the Faulting of Rocks The process of chip formation as outlined above involves the repeated application of local shear faulting in the rock under the concentrated pressure of the tool. The mechanics and mechanisms of the evolution of microcracking processes leading to Me formation of a shear fault nucleus in a homogeneous compressive stress field under confining pressure apply locally in the rock that is to be chipped albeit in this case, the local stress field is highly inhomogeneous, and shear faults comprise curved surfaces.
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From page 50... ...
This will be a fertile area for immediate development for application to Me smart drilling process. Specific Models for Chip Formation Theoretical models using slip line plasticity approaches have been developed specifically for the purpose of dealing with chip formation in rocks by Me wedge indentation process.
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From page 51... ...
have studied this in some detail on models made of plaster of Paris to better understand the complex processes of vertical splitting, crushing, and chip formation. On the basis of their observations, Sikarskie and Altiero (1973)
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From page 52... ...
Here the problem is one of tensile fracturing of rock under a large welIbore pressure and the opposing confining pressures of the surrounding rock, made complicated by the flow processes of pore fluids. Problems of mine shaft failures and lateral displacement of large earthworks such as dams and embankments are direct applications of compressive fracturing of rock and soil flow, both obeying the general framework of shear faulting theories.
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From page 53... ...
2. Fracture processes: For drilling applications, He relevant rock behavior is purely elastic, with brittle fracture occurring under the drill bit.
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From page 54... ...
Every brittle fracture process is ultimately based on Me severance of chemical bonds. There are many examples of dramatic reduction of Me local work of brittle separation in active liquid environments.
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From page 55... ...
L., 1969, A note on the penetration of a rigid wedge into a nonisotropic bribe material: International Journal of Rock Mechanics and Mineral Science, v.
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From page 56... ...
B., and Sikarskie, D L., 1973, Penetration problems in rock mechanics: Annual ASME Applied Mechanics Division of Rock Mechanics Winter Symposium, Sikarskie, D
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From page 57... ...
Observations and analysis of deformations: International Journal of Rock Mechanics, Mineral Science, and Geomechanical Abstracts, v.
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From page 58... ...
L., 1980, The effects of confining pressure and stress difference on static fatigue of granite: Journal of Geophysical Research, v.
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From page 59... ...
G W., 1992, Extensile cracking in porous rock under differential compressive stress: Applied Mechanics Review, v.
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From page 60... ...
W., 1977, The effect of stress-induced anisotropy on a model of brittle rock failure as localization of deformation: Energy Resources and Excavation Technology: Proceedings of the ~ Sth United States Symposium on Rock Mechanics, Colorado School of Mines, Johnson Publishing Company, p.
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From page 61... ...
F., 1976, Development of stress-induced microcracks in Westerly Granite: International Journal of Rock Mechanics, Mineral Science and Geomechanical Abstracts, v.
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From page 62... ...
62 Drilling aru] Excavation Technologies for the Future Modeling of Quasi-Brittle Materials Behavior, Li, V
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