4

ROCK-MASS ASSESSMENT

Pre-knowledge of the character of geological formations that will be penetrated by drilling or excavation is an important factor in assessing the potential stability of cylindrical openings. A wide range of methods for vertical sounding or remote imaging is currently available for depths ranging from a few tens of centimeters away from existing shafts and boreholes to several kilometers beneath the land or ocean surface. In almost every case, these rock-mass assessment methods are based 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 depth or in providing information that can be related to the mechanical properties of interest depends on the properties of the rocks being investigated. It may be difficult to determine whether a specific method of investigation will provide the required results if these properties are not well constrained before the investigation begins. In spite of all of these qualifications and limitations, seismic and electromagnetic soundings can provide important and useful indications of the 3D distribution of mechanical properties within a large volume of rock. The effectiveness of these soundings can be improved by the calibration of geophysical measurements (depth scales and geophysical response) using information from carefully selected boreholes. Geophysical soundings can be especially useful in identifying major structural features (such as contacts, folds, and faults) that may have a significant effect on the stability of openings, i.e., tunnels.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 41
Stability, Failure, and Measurements of Boreholes and Other Circular Openings 4 ROCK-MASS ASSESSMENT Pre-knowledge of the character of geological formations that will be penetrated by drilling or excavation is an important factor in assessing the potential stability of cylindrical openings. A wide range of methods for vertical sounding or remote imaging is currently available for depths ranging from a few tens of centimeters away from existing shafts and boreholes to several kilometers beneath the land or ocean surface. In almost every case, these rock-mass assessment methods are based 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 depth or in providing information that can be related to the mechanical properties of interest depends on the properties of the rocks being investigated. It may be difficult to determine whether a specific method of investigation will provide the required results if these properties are not well constrained before the investigation begins. In spite of all of these qualifications and limitations, seismic and electromagnetic soundings can provide important and useful indications of the 3D distribution of mechanical properties within a large volume of rock. The effectiveness of these soundings can be improved by the calibration of geophysical measurements (depth scales and geophysical response) using information from carefully selected boreholes. Geophysical soundings can be especially useful in identifying major structural features (such as contacts, folds, and faults) that may have a significant effect on the stability of openings, i.e., tunnels.

OCR for page 41
Stability, Failure, and Measurements of Boreholes and Other Circular Openings Scale of Investigation and Resolution In most cases, the assessment of the rock-mass properties involves a trade-off between the depth of the investigation and the scale of the resolution. Large depths of penetration are associated with long wavelengths or low frequencies, whereas finer scale resolution requires short wavelengths or higher frequencies. In situations where both a high resolution and great depth of penetration are required, exploratory drilling or “pilot” boreholes can provide an acceptable alternative. Measurements made with high-resolution geophysical probes can be made on the interior of the formation using the access provided in this way. 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. Mechanical Properties and the Stability of the Openings The physical properties and conditions that determine the stability of the openings include the strength (elastic moduli), state of stress, integrity (density of fractures, porosity, and permeability), and sensitivity to alteration in contact with the borehole fluids. None of the geophysical sounding techniques currently available is capable of providing a direct measurement of these properties. However, some measurements (compressional and shear velocity) are closely related to elastic moduli, and other measurements can often be correlated with mechanical properties. The rock samples recovered from pilot boreholes can be subjected to laboratory tests that directly measure rock strength, permeability, etc. Overcoring techniques can be used to estimate in-situ stresses. However, there are still questions about how well the rock mass is characterized by a few samples, and about the bias introduced by core loss, desiccation, etc. Geophysical logs in boreholes and borehole-to-borehole soundings serve as an indispensable means for putting data from discrete samples into the larger geological context. One area where geophysical measurements may be especially difficult to apply to rock-mass assessment is in distinguishing different mineral types. For example, kaolinite and smectite may appear to

OCR for page 41
Stability, Failure, and Measurements of Boreholes and Other Circular Openings have similar properties in seismic or electric soundings, but would have very different properties when exposed to borehole fluids or the atmosphere in the walls of a shaft. Some of the latest geotechnical applications in borehole geophysics, such as cryogenic gamma spectral logging and downhole neutron activation using neutron generator sources, may provide an important capability in this area. Primary Factors Affecting the Remote Assessment of Rock-Mass Properties In assessing the rock-mass stability by means of remote geophysical sensing or geophysical measurement in boreholes, the preceding comments indicate that three important considerations influence the interpretation of any given data set: (1) uniqueness, (2) relation of the geophysical properties to the mechanical stability, and (3) scale of investigation. There is no formal uniqueness theorem in geophysics so that the peculiarities of the interpretations cannot be rigorously verified, and caution must be used in applying the results of a particular investigation. At the same time, the depth of the penetration may depend on the rock properties, so a lack of a particular signal could mean either that no zones of anomalous rock properties are present, or that the source signal did not fully penetrate the volume of interest. These comments highlight the importance of identifying and applying whatever additional geological constraints may be available, although many geophysical investigations are constrained enough that non-uniqueness of the inversion or interpretation does not present a serious problem. 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. In some cases, the interpretation of the borehole stability depends on a specific model relating the measured property (seismic reflection or electrical conductivity) to parameters of direct interest (apertures of fluid-filled cracks or volume fraction of conductive clay mineral deposits). In other cases, the interpretation of the geophysical data produces a qualitative assessment of the rock mass. The analysis does not yield specific information on stability, but indicates the location and extent of rocks which are characterized by

OCR for page 41
Stability, Failure, and Measurements of Boreholes and Other Circular Openings anomalous properties. This information may then be combined with a knowledge of the general background of the rock properties and the nature of the most probable anomalies, to make more specific conclusions about the stability of the excavations in a particular part of the rock mass. In those cases where reliable interpretations of the rock properties can be made using deep geophysical soundings or information from exploratory boreholes, the potential stability of cylindrical openings may depend on scale effects not indicated by the 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. High-Resolution Surface Sounding Various remote sensing methods, including seismic reflection, resistivity sounding, and high-frequency electromagnetic propagation, have been used to estimate the rock conditions in situ. These methods do not provide enough resolution to give more than a general indication of the formation properties when the depths of the investigation exceed a few 100 meters. Higher frequencies and shorter receiver offsets provide the resolution required to detect the features in the vicinity of shafts and passageways, giving more direct access to the rock volumes of interest. Under the best conditions, a number of boreholes or passages provide access to the target zone from several different sides, so a two- or three-dimensional image of the rock properties can be generated by tomography. Although the tomographic imaging of rock-mass properties produces an image of such physical properties as electrical conductivity or seismic velocity, the locations of the anomalies within the rock mass can be interpreted by such features as fault or shear zones, or lenses and layers of mechanically weak rocks (such as shales and alteration clays) that could impact the stability of the openings. Seismic Methods Surface seismic soundings (reflection and refraction) have been used for decades to interpret subsurface conditions. The soundings provide a direct indication of the vertical distribution of seismic velocities.

OCR for page 41
Stability, Failure, and Measurements of Boreholes and Other Circular Openings These velocities (compressional and shear) can be directly related to elastic moduli of rock if independent measurements of rock density are known. However, seismic velocities indicate dynamic moduli, which are almost always significantly larger than the static moduli that control rock deformation and failure adjacent to the openings. In most situations, conventional surface seismic profiling does not yield a spatial resolution anywhere near that desired for the interpretation of the potential shaft stability. For example, conventional seismic profiling frequencies are usually less than 100 Hz, corresponding to wavelengths of more than 100 meters. High-resolution seismic reflection methods have been developed for relatively shallow depths of investigation in relatively competent rocks to provide much greater resolution (Green and Mair, 1983; Wong et al., 1983). Improvement in resolution has been achieved by modifying the seismic methods to allow signal detection in boreholes adjacent to the rock volume of interest, a technique known as vertical seismic profiling (VSP). This method can be used to generate images of rock properties based on compressional and shear velocity and attenuation (Turpening, 1986). Recent progress, using VSP, has been made in identifying properties, especially the extent of fracturing adjacent to and below boreholes (Majer et al., 1988; Salo and Schuster, 1989). The most recent applications of VSP methods to such interpretations require the extension of existing seismic techniques to include diffraction, scattering, and mode conversion, in addition to coherent reflection (Beydoun and Mendes, 1989). In many situations, the effective sounding of rock masses requires competent, unfractured 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. These methods are based on the application of shear sources that induce nonsymmetric (multipole) motion adjacent to a borehole (Kitsunezaki, 1980; Winbow, 1988). The seismic waves generated by these sources contain very little excited P-wave energy, so the analysis of shear-wave propagation through shallow, poorly consolidated materials can be completed without interference from compressional waves. At the same time, the relatively low excitation frequencies and long wavelengths allow penetration over distances up to several thousands of meters. These methods appear especially suitable for the direct sampling of the shear velocity and Poisson's ratio of unconsolidated sediments and the inference of the presence of clay.

OCR for page 41
Stability, Failure, and Measurements of Boreholes and Other Circular Openings Under optimum conditions, shafts or boreholes provide a means for seismic “illumination” of the target volume from a number of different directions. In that case, tomographic methods may be used to construct a 2D or 3D image of the volume of investigation. Higher frequencies can be used such that the seismic inversion resolves zones of weakness or anomalous rock properties associated with faults or major fractures (Bregman et al., 1989; Gelbke et al., 1989). The seismic tomographic method requires large computational storage and extensive processing even by modern standards, and interpretations are sensitive to the extent of illumination, as well as the interpretation model used to relate seismic anomalies to the mechanical properties of rocks. Electrical Resistivity Sounding Electrical resistivity profiling was one of the earliest methods used to characterize the properties of subsurface formations. Standard electrical resistivity profiling methods are still used to provide general information on the distribution of rock properties with depth; the success of the method depends on the distribution of the resistivities beneath the surface array, and the uniqueness of the interpretations may be difficult to establish. More advanced profiling systems are based on the induction techniques with the depth of the investigation based on the source-to-receiver coil spacing being used. Such methods are useful over depths ranging up to 100 meters or more, except where either highly resistive or shallow conductive formations prevent penetration of the source signal. These methods are most effective at delineating the general stratigraphy which may be related to the distribution of the rock properties rather than the location of such anomalous features as faults and fractures. Electrical tomography is useful and effective in defining the properties of a rock mass located between a number of boreholes or passages. In this method, the electromagnetic propagation between the source and receiver is related to the electrical conductivity of the intervening volume. These anomalies in the conductivity may be related to the presence of clay alteration minerals in faults and shear zones, or to the presence of relatively conductive groundwater in fractures or solution openings (Ramirez et al., 1982). In the interpretation of such images of the distribution of electric conductivity, the distribution of rock conductivity needs to be related to the mechanical properties relevant to rock stability. These may be based on an assumed relationship between the conductivity and extent of alteration or an empirical relationship between the electrical properties and

OCR for page 41
Stability, Failure, and Measurements of Boreholes and Other Circular Openings elastic moduli. Results may be significantly improved if both electrical and seismic tomography are performed on the same volume of rock, eliminating ambiguities associated with only one physical measurement (Saito et al., 1990). Radar Sounding and Radar Tomography High-resolution electromagnetic measurements of rock properties can be made using much higher frequencies than those used in surface resistivity or conductivity soundings (100 to 1000 MHz compared to 20 kHz). These measurements depend on physical properties different from those governing low-frequency conductivity and provide a much greater resolution associated with very short wavelengths. However, the depth of the penetration remains a significant problem, with the effective depths of investigation ranging to more than 100 meters under optimum conditions (small conductive bodies within resistive formations). Surface radar profiling is of use in defining the shallow structure of unconsolidated formations or soils where materials are resistive enough to allow radar penetration to more than a few meters. Radar sounding and tomography provide a useful, high-resolution method for imaging anomalous rock properties in the immediate vicinity of shafts and boreholes (Annan et al., 1988). As in the case of low-frequency electrical conductivity measurements, the radar images are related to contrasts in the electrical properties of rocks, rather than to the elastic moduli, state of stress, and fractures that influence the stability of the openings. 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., 1991). Rock-Mass Exploration by Means of Pilot Boreholes In some situations, pilot boreholes provide the only means by which to assess the properties of rock masses in the required detail. Samples recovered from these boreholes can be tested in the laboratory, but a large number of samples are required to be tested to generate a

OCR for page 41
Stability, Failure, and Measurements of Boreholes and Other Circular Openings statistically reliable data base (Paillet and Morin, 1988; Tarif et al., 1988). However, a variety of geophysical measurements can be made in the borehole over scales ranging up to tens of meters away from the borehole, which more closely correspond to the scales of circular openings such as shafts and production boreholes. Many of these geophysical measurements are scaled down versions of the surface geophysical sounding techniques already discussed. These conventional geophysical logging methods provide measurements of the electrical conductivity, porosity, density, and compressional velocity of a sample volume approximately one meter in diameter. Most are discussed in detail in standard geophysical logging texts such as Hearst and Nelson (1985). In addition to the conventional suite of geophysical logs developed for the exploration industry, several geophysical logs have been developed for the assessment of the mechanical properties of rocks for various geotechnical applications. Foremost of these is the acoustic full-waveform logging system where the complete pressure signal (the waveform) is recorded at successive depth stations in the borehole. This method provides a means to measure the dynamic moduli, Poisson 's ratio, etc., of geological formations adjacent to the borehole (Cheng and Toksoz, 1981; Paillet, 1985; Paillet and Cheng, 1991). The full-waveform logging technique can be performed in deviated boreholes as long as the logging tool is centered in a fluid-filled berehole, but is somewhat sensitive to rough borehole walls, and the applications are limited in cased boreholes. One important application is the recognition that the guided tube-wave mode can be used to infer permeability in situ (Paillet and White, 1982; Cheng et al., 1987). Early research had indicated that the method would not work in cased boreholes or if the borehole walls were sealed with filter cake (Rosenbaum, 1974), but studies show that nonsymmetric modes are not affected by borehole wall sealing (Schmitt et al., 1988; Winbow, 1988). Another important class of geophysical logging tool that is very useful in assessing the mechanical properties of rocks adjacent to a pilot borehole is borehole wall imaging. The most consistently useful of these tools is the acoustic borehole televiewer (Zemanek et al., 1970). This device produces a photographic image of the pattern of the borehole-wall acoustic reflectivity at ultrasonic frequencies (1 to 1.5 mHz). The televiewer has been used for many years to measure the extent of fracturing and as a qualitative indicator of fracture permeability (Paillet et al., 1985). More recently, the televiewer has been applied to in-situ stress measurement because the incidence of nonisotropic horizontal principal stresses can produce a borehole wall spalling, with these breakouts aligned along the borehole azimuth

OCR for page 41
Stability, Failure, and Measurements of Boreholes and Other Circular Openings corresponding to the direction of minimum horizontal stress (Zoback et al., 1985). The distribution of breakouts observed in the boreholes has proved especially useful in understanding the magnitude and orientation of stresses in the vicinity of bed contacts and fracture zones where other stress measurement techniques have been difficult to apply (Paillet and Kim, 1987). Various experimental studies demonstrate that rock-mass inhomogeneities and other factors can produce significant local variations of in-situ stress on a scale that may affect borehole and shaft stability (Martin, 1990). Borehole breakout logging by means of the borehole televiewer or four-arm caliper can be an especially effective means for identifying and mapping such inhomogeneities (Paillet and Kim, 1987). Measurement-While-Drilling (MWD) The potential for borehole collapse or excessive fluid influx during deep drilling has led to an intensified interest in the identification of the mechanical properties and the state of stress in situ. Detection of abnormal geopressures initially required withdrawal of the drillstring so conventional acoustic well logs could be run, and possible reverses could be identified in the expected increase in velocity with depth. In recent years, rapid advances in geophysical measurements while drilling have provided the ability to sample the formation properties as the drill bit advances. Continued development of this technology will enhance the ability to identify formation properties during drilling and may permit recognition of unstable borehole conditions, or impending 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. In favorable tectonic environments, the magnitude of this pore pressure may be assessed with great accuracy before drilling (Weakley, 1989).

OCR for page 41
Stability, Failure, and Measurements of Boreholes and Other Circular Openings This page in the original is blank.