Stability, Failure, and Measurements of Boreholes and Other Circular Openings (1993) / Chapter Skim
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3 Stability Assessment
3 Stability Assessment
Pages 31-40
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From page 31... ...
Sometimes the damaged rock reman in place so the excavation more or less retains its original cross section For the remainder of this chapter, instability will be used to imply that the ratio of rock Cress to rock strength exceeds the elastic limit producing either a zone of degraded rock adjacent to the circular cross section or resulting in a substantial change in cross section of the originally circular borehole. Frequently, the cross sections of boreholes or tunnels are observed to elongate in the direction of the minimum principal stress as a result of preferred failure in orientations that produce a breakout.
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From page 32... ...
(1989) showed that the stress concentrations near the end approach a state of hydrostatic stress, as for a notch or mathematical crack, and that it is the reduction in shear stress and the increase in mean stress ahead of the end that stabilize the breakout cross section Closure Measurements The mechanical stability of the fractured rock adjacent to the opening can be inferred from measurements of the changes in the opening diameter and from wall scaruiing devices.
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From page 33... ...
The comparison provides insight into the effects of stress relief on the fracture aperture, mechanical spading of the fracture faces, and erosion of the infilling minerals and alteration minerals. An alternate method of borehole wall imaging has recently been introduced by expanding the number of small electrodes on the pad of a conventional resistivity logging tool to more than 20, and then constructing an electrical conductivity image of a vertical strip of borehole wall (Ekstrom et al., 1987; Laubach et al., 1988; Paillet, 1991a ~n<3 l991b)
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From page 34... ...
The method is useful when the core sample recovery and processing eliminate soft or brittle lithologies, so the continuous mechanical property profiles constructed from acoustic logs can be used to generate the profiles of the mechanical properties with the depth (PaiBet and Moran, 1988; Tarif et al., 19881. In such situations, the dynamic moduli inferred from the logs may not agree closely with those determined from the cores, but the relative distribution of the static moduli may be estimated from the dynamic moduli profile.
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From page 35... ...
Waveform amplitude is related to the fracture permeability through the transmission of a guided wave mode smiler to the Stoneley interface mode known to propagate along plane interfaces between elastic solids and fluids. Effective measurement of the tube-wave transmission and reflections requires tuning of the borehole logging systems to particular situations to ensure that the tube-wave modes are present and Weir amplitude can be measured without interference from other wave modes.
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From page 36... ...
In hard crystalline rocks, differential compressive stresses adjacent to the wall of the borehole produce extensile crack growth where the values of the tangential stress are greatest, resulting in local borehole wall failure (Zheng et al., 19891. Theoretical models demonstrate Mat in the case of a vertical borehole the orientation of the borehole wall breakouts measured in the borehole agree with the location of breakouts predicted by the shear failure theory (Zoback et al., 1985~.
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From page 37... ...
This application of geophysical logging appears to be an area where significant future progress in rock mechanics is likely to occur. However, considerable uncertainty remains in the interpretation of breakout observations especially when the borehole is not drilled parallel to a principal Dress direction (Roegiers anc]
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From page 38... ...
As in the case of borehole breakouts, successive slabs elongate the cross section of the excavation in the direction of the minimum stress in the rock. :[n hard rock under lower Cress conditions, Blabbing often occurs by subcritical crack grown without dynamic Stability again leading to an elongated breakout cross section Although the magnitudes of the tangential Cress adjacent to the end of the elongated cross section are greater than those around the origin cross section of the excavation, the elongated Mope is more stable than that of the original cross section.
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From page 39... ...
Often this is accomplished with a continuous steel or concrete lining of preformed sections installed immediately behind a shield t~eling machine.
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