Stability, Failure, and Measurements of Boreholes and Other Circular Openings (1993) / Chapter Skim
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2 Observational Evidence
2 Observational Evidence
Pages 13-30
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From page 13... ...
The unportance of an appropriate support system design and the characteristics of the failure processes have been documented. Fracture of rock adjacent to the surface of the openings has been recorded in deep excavations where the existing stress field overcame the rock strength freeman, 1960; Hoek, 1964; Ortlepp et al., 1972; Karwoski, 1973; LaJtai and Lagtai, 1974; Hoek and Brown, 1980)
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From page 14... ...
are suggestive of axial splitting seen in laboratory experimentation ~ those environments where the in-situ stress regime and the lithologic features favor shear fracturing, it is important to note that the angle of incidence between a representative fracture and the wall of the opening seedy to be much less than what would be predicted by the plasticity theory (logarithmic spirals)
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From page 15... ...
Fig 1. _1 amid ~ l~ge-sc~e Beam Mien Wed, ~p 70~, dew ~ 1200 ~ ~10W Ices -~k co~lomer~e, CO = 250 Id.
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From page 16... ...
Extension stabbing is top right. Scrubbed whitened surface (upper left)
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From page 17... ...
D Ortlepp, Anglo American Corporations
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From page 18... ...
18 ST~ILI~ OF BOREHOLES AD OTHER OPENING Figure 4. Breakout failures in the vertical hole extending between the 7,020 and 7,200 levels of the Star Mine (av = 7,280 psi, aHMAx = 10,430 psi, OHMIN = 6,880 psi, aHMAx acting parallel to the fracture cleavage in steeply dipping, interbedded, fine-grained quartzite and argillite, 12,300 psi < CO < 42,000 psi, 1,200 psi < To ~ 3,800 psi)
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From page 19... ...
Apical shear failure patterns in a five-foot-diameter raised bore. [Photograph courtesy of E
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From page 20... ...
Curved fractures in the vicinity of a circular hole, observed in laboratory testing. [Photograph courtesy of A
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From page 21... ...
Foremost among these are the magnitude and ratio between the horizontal principal stresses, extent of geological discontinuity and rock-mass anisotropy, time lag between excavation and support installation, and influence of intersections into the primary opening, such as shaft stations and loading pockets. Analysis of the field data, using displacement discontinuity approaches developed by Cundall (1983)
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From page 22... ...
, using a 3D finite element approach, verified the influence of rock-mass anisotropy, as well as the effect of face advance and intersections. These observations substantiate the necessity for sophisticated modeling and for invoking considerations beyond established linear elastic formulations to rationalize excavation stability.
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From page 23... ...
Direct observation of the instability is supplemented with numerous examples in the Ding literature of stuck drillpipe due to localized inward movement. As in the context of the civil and mining structures, a paramount influence on the stability in oilfield drilling is the relative orientation of the wellbore with respect to the directionality of the principal stresses.
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From page 24... ...
Laboratory experimentation has been performed to define the Ornamental behavior, validate theoretical and numerical models, and discriminate between Me relative influence of stress levels and material characteristics associated with the response of a circular opening. Experiments have been performed on plates containing a central circular hole loaded either uniaxially (Abel and Lee, 1973; Rummel, 1975; Worotnicki and Denim, 1976; Kennedy and Lindberg, 1978; Mohring-Erdmann, 1982)
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From page 25... ...
Further e~penments on thick-walled hollow cylinders of Berea sandstone and Tntliana limestone incorporated additional features plane strain loading (zero axial deformation) , the ability to impose different stress paths on the rock using independently controlled pressures on the inner and outer diameters, and preservation of the fracture geometry under load using a metal saturation technique (Ewy et al., l98Sa)
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From page 26... ...
Oriented Br~ian compression tests determined that He failure location was controlled by rock strength arusotropy, with failure occulting where the tangential stress around the hole was parallel to the bedding plane, corresponding to the weakest direction in compressive loading. In all samples, the hole closure measured along the failed diameter was significantly greater than that measured along the perpendicular (unfailed)
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From page 27... ...
, these cracks have been reported to coalesce to form macroscopic splitting fractures at a fairly uniform spacing oriented parallel to the free surface, and also form en echelon patterns which approach and meet the free surface, combining to define the outline of a spelled piece. These spelled pieces progressively form and detach from the surrounding rock.
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From page 28... ...
Stabbing failure of Indiana limestone under servocontrolled uniaxial loading conditions.
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From page 29... ...
Shear failure (faulting) of Indiana limestone under servocontrolled 1=ia~nal conditions at a confining pressure 20 MPa.
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