The most important information required for the engineering design of structures in rock is its inherent rock properties. Without providing adequate parameters on the rock property, high quality of the engineering design cannot be expected. Many laboratory rock testing methods have been developed, but they are mostly for testing intact homogeneous rock without appreciable discontinuities. In reality, the intact rock properties do not represent the behavior of rock masses because rock masses include irregularities such as joints, cracks, interlayers, variations in mineral composition, etc. In general the type and intensity of the rock defects may be much more important than the type of rock which will be encountered (Terzaghi, 1977; Bieniawski, 1984). In-situ rock testing can provide better information on rock parameters, but it takes a tremendous amount of effort, time and cost. In recent years, numerical modeling techniques are gaining popularity as a design tool for structures in rock bodies due to the enormous progress in computer technology (Wang, 1985; Kripakov and Melvin, 1983; Park and Ash, 1985), but it is quite dangerous to use numerical techniques without having proper input data. Bieniawski (1984) states as follows:
Some design methods such as
numerical techniques have
outpaced our ability to provide
the input data necessary for
the application of these methods.
In order to provide an adequate amount of input data, an extensive testing program may be required, thus major efforts in time and expenses are commonly spent. In this paper, a new rock testing method, which utilizes the principles of laser holographic interferometry, is introduced. This method requires only a few minutes to measure the modulus of elasticity and quality of a rock core sample, which does not have to be cut or ground. The Poisson's ratio can also be measured in a few minutes from a sliced rock specimen.
Principles of Holographic Interferometry
In 1948, Gabor invented holography, which is the technique of reviving three dimensional images using a monochromatic light source. A full demonstration was not made then because a clean monochromatic light source was not available. Later, in the early 1960's, the laser was adopted as a monochromatic and coherent light source (Leith and Upatnieks, 1963). Since then, applications have been made in many different fields ranging from crime prevention to three dimensional television. In this technique, an image is usually recorded by means of constructing a hologram which is a record of the interference pattern formed on a high resolution photographic system as shown in Figure 1. A beam splitter divides the laser beam into two beams: (1) an object beam which is expanded while passing through beam expander and illuminates the object, and (2) a reference beam which is expanded and illuminates the holographic plate. Subsequent to an exposure, the holographic plate is developed and illuminated by the expanded reference beam. The original scene of the object is revived in a three-dimensional image. The entire instrument should be placed on a vibration free environment.
Ip, Chun K. (Department of Mining Engineering, University of Newcastle upon Tyne) | Johnson, Simon T. (Department of Mining Engineering, University of Newcastle upon Tyne) | Fowell, Robert J. (Department of Mining Engineering, University of Newcastle upon Tyne)
Laboratory conducted cutting tests at traverse speeds up to 1 m/s have been undertaken with high pressure water jets assisting the cutting action of drag tools. Four sandstones and one limestone have been investigated, which represents a wide range of compressive strength. Variables investigated include water jet pressure, nozzle diameter, depth of cut, traverse speed, jet position, and tool bluntness. The test programme has allowed a model to be developed which describes the action of water jet assistance for three conditions dependent on the ability of the jet to penetrate the rock: where no significant penetration occurs, where penetration is greater than 50% of the tool depth, and an optimum condition around 30% of tool depth.
The application of high pressure water jets to supplement the cutting action of drag tool tunneling machines has now become a reality. Both purpose-built machines and kits that allow water jets to be added to existing machines are becoming available. The interest in water jet cutting has stemmed from the need to increase the economic application of drag tool machines in harder and more abrasive strata where the limits of tungsten carbide tipped cutting tools have been reached. At the Department of Mining Engineering in the University of Newcastle upon Tyne, research on water jet assisted rock cutting has been undertaken since 1978. Initially, tests were undertaken at low traverse speeds which showed great promise in terms of force reductions Tecen, 1982). Later research conducted at speeds up to 1 m/s showed that traverse speed was a most important factor influencing the benefit to be derived from hybrid cutting. The paper will describe the recently completed large test programme on four sandstones and a limestone covering a wide range of strength and abrasivity. A hybrid cutting model is proposed which explains the findings from the experimental programme and supports the existence of optimum conditions of water jet/tool interaction. This depends on jet penetration and positioning, tool bluntness and traverse speed. A number of other studies on hybrid cutting have been conducted in the USA (Ropchanet al.,1980), France (Fairhurst et al., 1985), and Germany Baumann et al., 1982), following the work under- taken by Hood (1976) in South Africa and the promising field results with a boom-type selective tunnelling machine obtained by Plumptonet a1,(1982) in the United Kingdom.
RESEARCH RIG AND EXPERIMENTAL PROGRAMME
The rig is an instrumented 50-tonne rock planer, modified to cut at depths in excess of 30mm for moderately strong sandstones (Figures 1 and 2). Jet pressures up to 70 MPa were employed, though a capability to cut rock, using pressures up to 207 MPa is available (Fowell et al., 1985). The main programme was to investigate the influence of traverse speed, mechanical depth of cut, and rock properties on component force reductions during the initial few metres of cutting with a new tool and to explain the mechanisms of rock breakdown under the action of water jet assisted cutting. The main programme was supplemented by a number of smaller programmes to investigate the influence of specific variables; for example, jet position and nozzle diameter.
A thorough knowledge of coal permeability and its variation with factors like stress conditions, and time, is useful in simulation studies of mine ventilation and coalbed methane recovery, as well as characterizing coalbed methane reservoirs. An experimental investigation was carried out to study the gas flow characteristics of coal to enhance the under- standing of the mechanism of methane flow through it. The work consisted of measurement of permeability of coal samples under triaxial stress and an examination of the physical structure of coal using electron microscopy. Effort was made to fit the experimental data to several phenomenological models that have been used to describe fluid flow through porous media. It was realized that gas flow through coal cannot be described by models that assume this flow to be through a bundle of capillary tubes. However, results suggest that this gas flow could be predominantly through fractures and cracks in coal, and this could be represented by flow between parallel plates.
The release of methane from coal has become increasingly important in underground mine ventilation and safety over the last 30 years as mines have achieved greater depths and higher productivities. This trend, and especially the potential for commercially recovering methane in advance of mining or from unminable coal, has prompted the development of highly sophisticated research methods to analyze the phenomena of methane desorption and migration through coal and its surrounding strata. For any simulation of desorption, flow, or coal seam stimulation, one must not only determine the coal permeability and its variation with factors like stress, gas pressure, and time, but also have a nowledge of the mechanism of this flow through coal and how it is affected by external factors. This paper briefly discusses the influence of applied stress on coal permeability and the results obtained from an experimental investigation to study the relationship between the two. The various mathematical relationships used in the past to describe flow of fluids through porous media are examined using the experimental results. Microscopy work carried out to obtain information concerning the physical structure of coal, like size and distribution of the pores, etc., has been described briefly.
This section describes the results of the experimental investigation carried out to study the dependence of coal permeability on applied stress. Methane flowrate was measured through cylindrical specimens of coal while the stress conditions were varied in steps, and the permeability was calculated. Figure 1 shows the variation in permeability with changes in axial stress for three stressing/destressing cycles performed at constant gas pressure and confining radial stress. The effect of axial stress on coal permeability is evident in each of the three cycles. The upper segment, the stressing phase, shows that the amount of change in the permeability of coal gradually decreased with increase in stress, producing a curve. However, the destressing curve falls below the stress curve and, upon returning to the initial applied stress, shows a net loss of permeability. Another important observation is the effect of leaving the specimen in a lightly stressed condition (2.07 MPa/300 psi).
The Bureau of Mines, in cooperation with the Illinois Mine Subsidence Insurance Fund, is monitoring the response of two foundations to ground movements induced by subsidence from a high-extraction retreat room-and-pillar operation in southern Illinois. The objective of this monitoring program is to study the interaction between the ground surface and a structure during a subsidence event. Data from such a study should enhance the understanding of the mechanisms that produce structural damage and aid in the design of structures that resist such damage. This paper describes the monitoring instrumentation and techniques as they relate to the mining and site conditions. The capability of a tiltmeter to detect and follow the tilt of the foundations caused by the mining subsidence sequence is demonstrated. Using data from this tiltmeter, the development of curvature in the foundations as they conformed to the displaced ground surface is shown. This curvature is represented by differences in tilt at various points on the foundations, indicating a change in tilt over a horizontal distance. Also, even though subsidence damage has not been fully analyzed, preliminary observations of the cracks that formed in the foundations are reported. These cracks resulted in the separation of one footing into distinct pieces; therefore, their effect on the tilt readings is cited.
The investigation of subsidence and its effects is an active area of research within the Bureau of Mines. The objective of this particular project is to study the interaction between the ground surface and a structure during a subsidence event and, as a corollary, to determine the effectiveness of commonly recommended damage mitigating techniques. Data from such a study should enhance the understanding of the mechanisms that produce structural damage and aid in the design of structures that resist such damage. It should be noted that this project was not an attempt to completely delineate the subsidence event. rather, in order to study the ground surface-structure interaction, two foundations were built and instrumented above an active mine, and a survey net was designed to accurately measure the subsidence around the structures. One of these structures was built to simulate foundations of homes in southern Illinois. The second foundation was built to weather the subsidence event by incorporating commonly recommended damage-mitigating techniques (NCB, l975; Brauner, 1973). The footing was reinforced with steel rebar to increase its resistance to tension and poured on top of a compacted sand layer covered with polyethylene sheeting to reduce the friction between the soil and footing. Also, a trench around the structure was filled with vermiculite to reduce lateral pressure on the foundation. The structures were built during the late summer of 1984. Monitoring was initiated in December 1984, and is continuing. At this time, it is possible to show that a tiltmeter can detect and follow the deflections of the foundations caused by mining subsidence. Also, the development of curvature in the foundations as they conformed to the displaced ground surface can be demonstrated. Finally, observations of the cracks that formed in the foundations can be reported and related to the tilt readings.
Pacific Northwest Laboratory (PNL) currently supports the U.S. Department of Energy's Office of Civilian Radioactive Waste Management in developing and evaluating analytical methods for assessing the suitability of sites for geologic disposal of high-level radioactive waste. The research includes consideration of hydrological, geomechanical, geochemical, and waste package components and the evaluation of the degree of coupling that can occur between two or more of these components. The PNL effort and those of other research groups investing potential waste sites in the U.S. and abroad are producing a suite of computer codes to analyze the long-term performance of the proposed repository sites. This paper summarizes the ongoing research in rock mechanics at PNL involving flow through jointed rock. The objective of this research is to develop a methodology for modeling the coupled mechanical-hydrological process of flow through joints and then attempt to validate a "simple" model using small-scale laboratory test data as a basis for judging whether the approach has merit. This paper discusses the laboratory tests being conducted to develop a joint behavioral constitutive model for the numerical method under development and the modeling approach being considered.
The mechanical-hydrological characteristics of rock joints are of special interest to researchers investigating underground nuclear waste disposal in jointed rock. Basalt, tuff, and granite all contain interconnected joints that form the dominant flow paths for the ground water. Because ground water flow is the primary mode of radionuclide contaminant transport to the biosphere, the understanding of fluid flow through fractures is critical for evaluating the suitability of a site for waste disposal. Laboratory research efforts have investigated the effect of several variables on the hydraulic conductivity of rock joints. These variables include normal stress across the joint, dislocation (shear) of the joint surfaces, scale effects, surface expression or roughness, and rock type. Normal stress-conductivity experiments have been performed using a number of different rock types (Gale, 1982; Raven and Gale, 1985; Kranz et al., 1979). Other studies have considered the scale effect on fluid flow properties of joints (Raven and Gale, 1985) and the effect of fracture roughness (Tsang and Witherspoon, 1983). Maini and Hocking (1977) and Makurat (1985) investigated the effect of shear displacement on joint conductivity characteristics. In laboratory tests using rock samples containing rough natural joints, Makurat found that i to 2 mm of shear displacement was sufficient to cause nearly two orders of magnitude increase in joint conductivity. Relying on these and additional studies, Barton, Bandis and Bakhtar (1985) have proposed a constitutive model of joint behavior to simulate shear- displacement-dilation-conductivity coupling and normal stress-closure-conductivity coupling. The model incorporates estimates of asperity characteristics and the conducting aperture of the joint based on empirical formulas involving simple tilt and rebound tests. This research is leading to a better under- standing of fluid flow through jointed rock. The governing equations for fluid flow in numerical models will need to contain many of the above factors to accurately simulate fluid flow through a jointed rock mass. PNL is conducting laboratory tests to evaluate the effects of several of these factors, including normal stress, dislocation, cycling, and joint roughness on the conductivity of joints.
The heavy oil reservoirs of Alberta and Saskatchewan include sequences of unconsolidated silts and fine sands which respond in an unusual manner when subjected to the high temperatures and pore-fluid-pressures associated with steam injection and other thermal enhanced recovery techniques. Apparent transient connections are suddenly established between producers and injectors and there is an associated enhancement in oil (and sand) production. These phenomena have been attributed to local fluidization and piping processes which are known to occur in situations where high pore-fluid-pressures prevent the mobilization of intergranular friction. These breakdown processes may be associated with the penetration of tongues of the heated reservoir fluids with suspended sand into the creep sensitive intact reservoir since the viscosity ratios are sufficiently high for viscous fingering instability of the fluid-fluid interface. A program of laboratory tests has been carried out to measure the strength characteristics of saturated oil sands at a variety of temperatures and fluid pressures. These tests suggest, that in common with the coarser Athabasca sands, the Waseca sands have very little cohesive strength and show the characteristic dilational failure response of dense sands. Some special tests, devised to determine the relative ease with which samples can be fluidized, confirm the expected susceptibility of the fine- sand silts. Both piping and fluidization can be readily induced at in-situ reservoir temperatures and pressures. Analysis of stresses around wellbores suggests that fluid-pressure gradients may be sufficient to initiate piping and that backward erosion along streamlines may link producers and injectors. Such pipes may act as well 'adits' gathering oil from the heated reservoir and contributing to enhanced production rates. Mechanical breakdown of the reservoir by drilling, injection and production processes provides a consistent possible explanation for the sand-cut response of production wells.
The Lloydminster heavy oil district of western Canada lies on the Alberta-Saskatchewan provincial border about half way between Saskatoon and Edmonton on the Yellowhead highway. The region comprises a large number of small unconsolidated sand reservoirs containing viscous heavy oil at shallow depths of 400-500m. The oil sands under discussion lie to the south and east of the more extensive Alberta tar sand deposits of Athabasca, Wabasca and Cold Lake as shown in Fig. 1.
Fig.1 : Location of heavy oil reservoirs in Alberta and Saskatchewan.(available in full paper)
Although the Lloydminster heavy oil is very viscous, it can be produced conventionally from wells. Primary recovery is poor (2 - 8 m3/d) and increased production rates are obtained using thermal enhanced recovery techniques including fireflood, steamflood and cyclic steam stimulation.
EFFECTS OF INSTABILITY PHENOMENA ON PRODUCTION
The use of thermal EOR techniques places unusual thermal and fluid pressure stresses on the reservoirs which may lead to mechanical instability and failure, particularly close to the wellbore. This in turn can generate a number of practical production problems :
To evaluate the influence of uniformly tensioned roof bolts on roof stability, mechanically anchored roof bolts were installed using the conventional bolter and a specially developed thrust and torque controlled retrofitable box mounted on a bolting machine. Thirteen thousand bolts were installed over 19 crosscuts in a 4 heading development in an operating coal mine. Geological, bolt and mine parameters were recorded at bolt installation, and bolt tension histories, roof sag and other changes evident in the rooms, crosscuts and intersections were recorded over a 2 month period. In all, 24 variables and 11 responses were quantified at 67 bolt clusters.These data were analyzed statistically to determine the significant variables influencing roof stability. As the mine development was safe and stable during the monitoring period, roof sag after two months was used as an index to quantify "stability." The variables of significance were shown to be coefficient of variation of bolt tension, bolt anchorage capacity, roof lithology, roof shape, standard deviation of bolt tension, and dip of roof lithologies. It was concluded that of the variables with which the mine operator has control, bolt tension uniformity was the most significant parameter at this mine. Other variables, such as intersection type and time between mining and bolting, were not significant, whereas, some other potentially significant variables such as horizontal stress or roof joint frequency did not vary sufficiently for their influence on stability to be evaluated.
Roof bolts form a major component of underground coal mine roof control programs with the most commonly installed roof bolt being the mechically anchored tensioned roof bolt. The mechanically anchored roof bolt is anchored essentially at a point by an expansion shell that is activated and the bolt tensioned by rotating and torqueing the bolt head. This bolt system is generally the quickest to install with the lowest material cost but is not suitable for all geologies. Combinations of mechanical anchors and chemical grouts or mechanical full contact bolts (such as the Swellex or Split Sets) are used in roof lithologies where the simple point anchor mechanical bolt is unsuitable. This paper summarizes the results of a three year effort to evaluate the impact of improved techniques for the installation of mechanical anchored and torque-tensioned bolts on roof stability for roof lithologies typical of western United States coal mines. This research was conducted for the U.S. Bureau of Mines (USBM) in a program entitled, "Field Tests of Uniformly Tensioned Roof Bolts," with Mr. Robert Thompson as the Technical Project Officer. The USBM has supported several research and development projects to evaluate roof bolts reinforcement mechanisms and thus to increase bolt effectiveness for ground support. Programs related to the present study were initiated in 1975 with a project to develop accurate torque and thrust control for roof bolters. Subsequent programs evaluated in the laboratory and through field testing, the effect of hardened washers on the installed bolt tension (Rosso, 1977), and the effect of torque and thrust control with and without hardened washers on installed bolt tension and bolt tension variability. These programs resulted in a retrofitable control box to monitor and control torque output of the existing bolting machines (Brest van Kempen and Sweet, 1978). Further studies in 1979 and 1980, revealed a significant improvement in bolt tension uniformity (as measured by the ratio of bolt tension to standard deviation of bolt tension at installation) by improving the retrofitable control box for torque- thrust cont