Stability, Failure, and Measurements of Boreholes and Other Circular Openings (1993) / Chapter Skim
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4 Rock-Mass Assessment
4 Rock-Mass Assessment
Pages 41-50
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From page 41... ...
In almost every case, these rock-mass assessment method are bad on the measurement of physical properties other than those properties that are directly related to rock-mass stability. Furthermore, the effectiveness of a particular measurement in either penetrating a given formation to the desired kept or in providing information that can be related to the mechanical properties of interest depends on the properties of the rocks being investigated.
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From page 42... ...
Such boreholes can be directly drilled into the area where a large-scale excavation is planned, or may be used to sample the detailed geological structure of the formation in a representative area adjacent to the area of interest. The optimum resolution of rock-mass properties results when large-scale soundings are coupled with calibrations of a geophysical response by means of geophysical logs in boreholes, or based on the physical properties of recovered core samples.
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From page 43... ...
The remote sensing of rock-mass properties almost always involves the measurement of geophysical properties other than the mechanical properties of rocks that directly relate to the stability of cylindrical openings. For example, geophysical soundings often depend on the seismic velocities and electrical conductivity of rocks, while the elastic moduli, state of stress, and distribution of the fractures influence the stability of the openings.
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From page 44... ...
In those cases where reliable interpretations of the rock properties can be made use deep geophysical soundings or information from exploratory boreholes, the potential stability of cylindrical openings may depend on scale effects not indicated by Me measurements. In particular, deep seismic and electromagnetic soundings investigating the rock properties averaged over much larger scales than the diameter of most production boreholes, while the geophysical measurements made In most boreholes apply to scales of investigation smaller than those associated with tunnel stability.
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From page 45... ...
In many situations, the effective sounding of rock masses requires competent, unbactured rock because the high attenuation of poorly consolidated or intensely fractured materials prevents the propagation of seismic waves over the required distances. An additional area of progress in the ability to resolve the mechanical properties of relatively unconsolidated materials is the development of low-frequency techniques for shear-wave sounding and shear-wave tomography.
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From page 46... ...
The seismic tomographic method requires large computational storage and extensive processing even by modern standard, and interpretations are sensitive to the extent of illumination, as well as Me interpretation model used to relate seismic anomalies to the mechanical properties of rocks. Electneal Resistivity Sounding Electrical resistivity profiling was one of the earliest methods used to charact~enze the properties of subsurface formations.
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From page 47... ...
For example, the radar images may indicate the distortion of thin conductive beds associated with small movements along the Shear planes rather than the presence of fractures and shear planes. Directional radar measurements performed while injecting saline water in fracture systems adjacent to tunnels and boreholes is an especially promising method for characterizing the distribution of fractures and fracture permeability adjacent to tunnels and boreholes (Sandberg et al., 19911.
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From page 48... ...
Many of these geophysical measurements are scaled down versions of the Deface geophysical &oux~ing, techniques already discussed. These conventional geophysical logging methods provide measurements of the electrical conductivity, porosity, density, and compressiona1 velocity of a sample volume approximately one meter in diameter.
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From page 49... ...
Continued development of this technology will enhance the ability to identify formation properties during drilling and may permit recognition of unstable borehole conditions, or unpending changes in the mechanical properties of rocks before they are encountered, at reduced costs. Abnormal pore-pressure gradients in shale have been detected from surface seismic data.
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