Recent applications of explosives and blasting agents to rubble rock have led to requirements for more elaborate design and analysis methods. In most blasting uses, it is necessary not only to fracture the rock, but also to move the broken rubble in a predictable manner. Many in situ extraction techniques require rubblization to take place in a confined region where rock motion is a predominate factor in creating a permeable broken bed. In this paper, two analytical methods are presented which describe the large rubble motion during blasting. These methods provide the blast designer with a tool for evaluation and further refinement of blasting patterns and timing sequences. In both these methods, the rock medium is represented by a series of discrete, discontinuous regions. The use of discontinuous techniques rather than the classical continuum methods, results in better approximations to the rubble motion. These regions are set in motion by pressure loads from the explosive. The motion of these regions is then calculated numerically using interaction laws between regions in contact. The basis for these models or methods is presented along with the background for selecting explosive pressure loads and rock mass material behavior. Typical examples, including both cratering and bench blasting geometries, are discussed which illustrate the use of these models to predict rubble motion. Such engineering representations appear to provide a practical method for use in predicting rubble motion and a tool for design evaluation of blasting in confined geometries.
Rubblization of rock begins with fracturing due to high amplitude stress waves followed by translation and rotation of the broken fragments due to explosive gas pressures. These events are interrelated and together form the basis for behavior of rubble blasting. In many blasting situations the final location and the porosity of the rubble or "muck" pile are critical. Blasting in confined regions, such as underground formation of rubble filled rooms for in situ processing, is very strongly influenced by the rubble motion. During blasting operation, the rock motion is dictated by the response of rock to pressures generated by detonation of the blasting agents. The blast pressure is related in an interactive manner to the degree of confinement. The rock surrounding a blasting agent must provide enough confinement such that the energy from the explosive is imparted into the adjacent rock. If enough confinement is not provided, a great deal of energy is converted into undesirable high rubble "fly" velocities or into shock wave energy in the air. If the explosive is buried too deeply, most energy is used in crushing a very small amount of rock and in generating high rock vibration levels. In this case, the resulting muck pile remains in place and the desired rock rubblization will not occur. Thus rubble motion is interrelated with the explosive energy dissipation. In most blasting designs (Langefors and Kihlstrom, 1978), these confinement effects are accounted for by using empirical and semi- empirical formulas which provide a means to select appropriate spacing, depth, size and timing of explosives. These methods have evolved through years of experience, mainly with quarry and tunnel blasting.
Directional dependence of strength and deformation properties of the Mount Waldo granite pluton in southeastern Maine is controlled by joints and microfractures whose orientations are linked to flow- induced alignment of microcline crystals. Longitudinal and cross joints which formed during cooling of the mass, as well as much younger sheeting fractures, are commonly associated with subparallel, discontinuous microfractures. The orthogonality of microfractures (open, closed; filled, unfilled; bubble planes) establishes the ease of splitting of the granite. The easiest splitting direction (rift) is subhorizontal, the second easiest direction (grain) is parallel to the cross joints, while the direction of least easy splitting (hardway) is parallel to the longitudinal joints. Static and dynamic measurements made in the field and laboratory established that structural features are responsible for the marked stiffness anisotropy of the granite. In the central part of the pluton the static Young's modulus in the north-south direction averages 31.1 GPa, and 20.9 GPa in the east-west direction; the stiffer direction being parallel to the long C-axes of microcline phenocrysts and microfractures as identified in outcrop, core samples, and thin sections. Vertical downhole sonic measurements of dynamic moduli ranging from 2.5 to 16.2 GPa appeared to be anomalously low when compared to the static horizontal values. However, the field sonic values were confirmed by laboratory sonic pulse testing of oriented cores. The field results reflected closely spaced layers of open horizontal microfractures. Those deformation and strength properties that are closely tied to bubble planes, filled microfractures, and microcline orientation could be extrapolated to greater depths with good hope of success.
The Mount Waldo granite pluton, in midcoastal Maine, is a batho-lith of Devonian age (390 m.y.) that intrudes tightly folded, north- easterly striking Precambrian and Lower Paleozoic metamorphic rocks. The pluton has been extensively quarried and has been suggested as a site for underground oil storage. Our research has concentrated on recording the microfabric and macrofabric elements in the granite and determining their influence on rock strength and deformation, in situ stresses, and permeability. This report describes a reasonably consistent relationship between jointing and microcline phenocryst alignment and the influence of these fabric elements on the mechanical behavior of the granite. The results of preliminary pressure injection tests are given. Clearly, if a consistent relationship between structure and mechanical properties could be established it could be helpful in forecasting excavation conditions.
The granite is light to medium gray, medium to coarse grained, and porphyritic; it contains potassium feldspar (perthitic microcline), plagioclase, quartz, and biotite in order of decreasing abundance. The microcline occurs as anhedral grains and as subhedral phenocrysts As much as 4 cm in length enclosing laths of plagioclase and biotite, which generally parallel the crystallographic planes of the microcline. The microcline crystals have a faint to distinct flow alignment that was established during emplacement of the pluton, resulting in a steeply dipping north-south-striking foliation that is most evident in the central part of the pluton (Fig. 1). Sweeney (1976) constructed a three-dimensional model of the pluton based on gravity data and structural features. He found the body to be steep sided with its horizontal dimensions shrinking slightly with depth.
A rapid in situ stress measurement technique was developed for exposed underground surfaces. The method applies radial stress in small diameter (38mm) boreholes, initiating fracture propagation which is monitored by measuring ultrasonic pulse velocity. Theoretical relations for stress concentrations near boreholes provide independent expressions for unknown principal field stresses at incipient fracture; fracture orientation defines principal stress orientation. Accuracy and precision were evaluated using blocks of brittle material subjected to known stresses. Results were comparable to those obtained by other in situ stress measurement techniques.
The U.S. National Committee for Rock Mechanics reported a need to make existing test methods more cost effective. In situ stress data is rarely used because of high cost and questionable accuracy. Some in situ stress measurement methods require costly non-recoverable components; others demand tedious slot cutting or overcoring. In some materials, usable results come from only 20% of the tests performed (Van Heerdon and Grant, 1967). This paper presents preliminary work toward a rapid, accurate technique for in situ stress evaluation at a free surface. The method may also serve in deep hole applications.
In situ stress by induced fracture involves assessment of pressures required for incipient crack formation on the periphery of a borehole. Crack initiation during borehole pressurization is controlled by the material's tensile strength and in situ field stresses at the borehole boundary. However, cracks reopened upon repressurization are no longer dependent on tensile strength, so stress concentration theory taken with known borehole pressures as cracks are caused to reopen provides information about in situ field stress. A two stage process providing information sufficient to calculate both principal stresses in a plane perpendicular to a borehole axis is proposed. Cracks are first induced from a single borehole, where theory predicts fracture should propagate in a direction normal to the minor principal stress (Hubbert and Willis, 1957). Next, simultaneous pressurization of a pair of bore- holes placed close to each other and aligned parallel to the minor principal stress is performed. This should result in a fracture normal to the major principal stress. Theories for stress concentration near single and double holes provide two relations in terms of the two unknown principal stresses.
Jeffrey (Timoshenko and Goodier, 1967) showed that the tangential component of a uniform radial pressure, Pi, acting on the inside boundary of a cylindrical hole is: (mathematical equation) (available in full paper)
The negative sense of Eq. 1 indicates tension. The tangential stress component as provided by Kirsh (Timoshenko and Goodier, 1967) for field stress concentrations at the wall of a single hole is: (mathematical equation) (available in full paper)
where: s1 = major principal stress, s2 = minor principal stress and ¿ = angle measured from the direction of major principal stress. For unequal compressive principal stresses, Eq. 2 shows minimum compressive stress occurring at ¿ = 0 and p as: (mathematical equation) (available in full paper)
In this paper, a numerical model for determining the stress and displacement fields around a crack of arbitrary geometry in a linear elastic medium is described. Accuracy and computational speed were the main considerations in selecting the numerical method and propagation criteria used to model the growing crack. Some results regarding the stresses and displacements around a two-dimensional crack of arbitrary geometry are discussed.
Hydraulic fracturing is widely used in such areas as the secondary recovery of oil and gas, enhancement of geothermal energy recovery and evaluation of in situ stress fields. The objectives in modeling a hydraulic fracturing operation are to predict the extent and geometry of the fracture as a function of time and field parameters, such as pumping and pore pressure, fluid viscosity and field stresses. This paper focuses on an important step in modeling the fracture propagation process, i.e. determining the stress field in the vicinity of the crack.
A number of numerical methods are presently used in geomechanics for performing stress analysis in rock. The Finite Element (FE) method is a well-known, comprehensive and powerful numerical tool for dealing with the complex differential equations and boundary conditions typical of many practical rock engineering problems. This technique is particularly useful when the rock mass to be modeled either exhibits a distinct non-linear behavior or is very inhomogeneous. However, because it is a domain method, it suffers from the need to discretize the interior of the rock mass. Considerable effort is required to set up and input the mesh coordinates and, as the problem size increases, the number of equations rapidly becomes prohibitive, especially in three dimensions. Moreover, for fracture problems the mesh must be changed at each propagation step and boundaries at infinity are difficult to account for accurately. The point force Boundary Element (BE) method has many advantages over FE because only the boundaries have to be discretized, but has difficulty in model ling problems in which the "boundaries" are in close proximity as is the case when modeling a fracture. The Displacement Discontinuity (DD) technique (Crouch, 1976) makes use of a Green's function based on a point displacement discontinuity instead of a point force. The fact that the fundamental unknowns of the DD method are the closure and ride of the discontinuity makes it ideally suited for crack-type problems. The Green's function for the DD method has been shown (Wiles and Curran, 1982) to be equivalent to the quadripole singularity introduced by Brady and Bray (1978). For these reasons, the authors have chosen the DD method as the most appropriate technique for modeling fracture propagation problems.
The criteria used to determine if a fracture will propagate, and in which direction, can be classified into three main categories (Ingraffea, 1977):
Because of the hazards associated with flammable methane gas and coal dust, the shooting of mudcaps (adobes) or other unconfined explosive charges in underground bituminous coal mines is prohibited; all explosives must be fired in stemmed boreholes. However, there are situations where it would be advantageous from a safety standpoint to fire open shots. This would be in the areas of dislodging loose roof slabs, overhangs, rock-fall leveling, slab or boulder breaking, crib removal, and crevice shooting. The Bureau of Mines, U.S. Department of the Interior, has been developing an explosive charge that is non-incendive; that is, detonation of the charge will not ignite a flammable atmosphere. This explosive charge could be applied to the blasting conditions mentioned above. As of this writing, the explosive charge has not yet been certified as "permissible" for use in flammable atmospheres, but the necessary protocol is being finalized with the Mine Safety and Health Administration (MSHA), U.S. Department of Labor. The explosive charge described in this paper is merely a prototype sheathed rock-breaker charge (sheathed permissible explosive rockbreaker charge) demonstrating the feasibility of such a device. Industry may modify the design and composition of the charge in producing a commercial product so long as it is still safe, as defined by a testing schedule being developed by the Bureau of Mines and MSHA.
EXPERIMENTAL PROCEDURES AND RESULTS
Research conducted by the Bureau of Mines has shown that a properly designed, prepackaged explosive charge could be safely fired openly in a flammable atmosphere if the explosive were covered with a layer of sodium chloride (NaC1) that would be dispersed as a fine cloud upon firing. For an explosive charge of 650 grams, equivalent to two 3.2- by 40-cm cartridges of permissible water gel explosive, a 1.3-cm layer of NaC1 proved adequate (Mainiero and Hay, 1982). Included in this research was an investigation into the proper shape for an explosive charge that would effectively break stone. Charges of various shapes with lined and unlined cavities were tested with the result that these were inefficient rock breakers. The most effective charge proved to be one in the form of a short cylinder 17.8 cm in diameter and 2.2 cm high (not including the NaC1 layer). This shape spread the explosive over a large surface area of the rock, yet provided a thickness adequate to provide for efficient detonation. Based on the research described above, the charge illustrated in figures 1 and 2 was developed. The explosive charge, packaged in a polystyrene container, is covered with a 1.3-cm layer of damp NaC1 (88 pct Nacl-12 pct water), which in turn is encased in a housing consisting of latex rubber reinforced with cheesecloth. This configuration provides a charge package that is soft enough to conform to the irregular surface of a rock yet strong enough to withstand rough handling.
Figure 1. Diagram of sheathed explosive rock-breaker charge. (available in full paper)
Figure 2. Exploded view of sheathed explosive rock-breaker charge (available in full paper)
The incendive characteristics of the new sheathed rock-breaker charge in flammable gassy atmospheres were evaluated by firing charges against concrete slabs in a cylindrical steel gallery 1.8 m in diameter, 3 m long.
Zanbak, Caner (Geology and Geological Engineering Department, South Dakota School of Mines and Technology) | Arthur, Randolph C. (Geology and Geological Engineering Department, South Dakota School of Mines and Technology)
Deformational behavior of a rock mass can be modeled with a constitutive law based on theories of elasticity, and plastic and viscous deformation. For most rock types, such constitutive models may be used with confidence because there is no geochemical interaction between the rock-forming minerals and water. However, if volume changes in the rock due to geochemical phase transitions are predicted, then these volume changes must be converted into mechanical parameters and must be included into the constitutive laws. In this paper, geochemical phase transitions between anhydrite (CaSO4) and gypsum (CaSO4.2H20) will be brought to the attention of researchers in rock mechanics. Hydration of anhydrite to gypsum may yield volume increases up to 62.6 percent and dehydration of gypsum to anhydrite may cause volume decreases up to 38.5 percent. If one were to convert the volume increases in hydrating anhydrite into strains and calculates the required stresses to restrict the expansions, the magnitude of stresses will be found at GPa levels. Such stresses on swelling anhydrite layers cannot be provided by geologic media. Therefore, the host rocks will deform under these high stresses. On the other hand, under increasing stresses, geochemical transition of anhydrite to gypsum may stop after some hydration of anhydrite, and anhydrite and gypsum systems may become stable. Changes in temperature and solution compositions in the anhydrite/ gypsum system also control the stability of the geochemical system, and therefore, the extent of vol, me changes. A similar conceptual model can be drawn for volume decreases due to dehydration of gypsum to anhydrite. In this case, stress reduction on the gypsum layer may cause extensive fracturing and changes in the state of stress in the host rock mass.
Evaporites have been reported from all continents, and approximately 25 percent of the continental areas are underlain by evaporitic rocks. Gypsum, anhydrite and halite are the most prominent minerals in evaporitic deposits. The original source of these evaporitic minerals is seawater. Precipitation of evaporitic minerals is usually generated either by direct evaporation of brine or by an indirect manner involving dissolution, transportation and reprecipitation of primary evaporitic deposits in waters circulating in the upper crust. Anhydrite and gypsum deposits generally occur in the vicinity of bedded and domal salt deposits. Phase transitions in calcium sulfate minerals are controlled by pressure, temperature and the composition of coexisting aqueous solutions. Phase diagrams showing the stability ranges of gypsum and anhydrite as a function of these parameters can be used to predict whether a hydration or a dehydration reaction could occur when environmental conditions in a rock system are changed. The geochemical stability of various anhydrite/gypsum systems and conceptual models for mechanical behavior of calcium sulfate bearing rock masses for their potential applications to tunnels, foundations and nuclear waste repositories in evaporitic rocks are discussed in the following paragraphs.
GEOCHEMICAL ASPECTS OF HYDRATION/DEHYDRATION OF ANHYDRITE AND GYPSUM
Solutions to the troublesome engineering problems associated with phase transformations of gypsum and anhydrite require basic information on the factors controlling the transition and on the geologic environment in which the transitions are likely to occur.
A specimen of Westerly granite was cyclically loaded to near failure at 50 MPa confining pressure. Holographic interferometry provided detailed measurements of localized surface deformations during loading and unloading. The data are consistent with deformation occurring primarily elastically at low differential stress; in conjunction with one incipient fault zone between approximately 360 and 520 MPa differential stress; and in conjunction with a second incipient fault zone above 580 MPa and/or during creep. During unloading only one fault zone, that which is active at the inter-mediate-stress levels during loading, is seen to recede.
Localization of deformation prior to macroscopic failure in geologic materials is well documented in laboratory studies. Localization, as detected by the migration of acoustic emission sources to the incipient fault plane, occurs during uniaxial constant strain rate tests (Scholz 1968) and during tertiary creep in igneous rocks (Mogi, 1968; Lockner and Byerice, 1980). Sondergeld and Estey (1981) found acoustic emission sources to cluster during uniaxial cycling of Westerly granite. Building along an incipient fault plane has been documented by strain anisotropy measurements (Hadley, 1975), slit diffraction optical techniques (Liu and Liranos, 1976) and holographic interferometry (Spetzler et al., 1977; Sobolev et al., 1978; Sondergeld et a1.,1980; Granryd et al., 1983; Kurita et al., 1983). Using holographic inter- ferometry, strain localization can be detected at lower stresses than by other techniques, providing a more sensitive measurement.. Granryd et al. (1983) were able to easily identify strain localization with holography when velocity measurements along different paths through the rock showed none. Simultaneous development of both the incipient fault plane and a conjugate plane has been seen under triaxial load using holography in Westerly granite and in pyrophyllite (Spetzler et al., 1977; Sobolev et al., 1978). During creep of Westerly granite under uniaxial load, the zone of active deformation may alternate between two conjugate planes, as documented by Spetzler and Martin (1974). Patterns of localized deformation observed in-situ in the earth are less reproducible. Tight clustering of seismic activity in the vicinity of a mainshock or alternatively, seismic quiescence have both been documented prior to earthquakes. (For a review see Kanamori, 1981.) Scaling between laboratory and field results requires a better understanding of many parameters including sample dimension, time scale, pressure, temperature, compositional variations, fault plane geometry and cyclic loading. In this study we use holographic interferometry to examine the development and migration of localized deformation zones during cyclic loading of Westerly granite. To the authors' knowledge this work is the first to document precisely the activation of multiple localized deformation zones during cycling under triaxial conditions.
A rectangular prism of Westerly granite, 57.8 mm in length and 17.0 mm in width and depth, has been subjected to three cycles of axial compression at 50 MPa confining pressure. Tests were conducted in a specially designed pressure vessel containing a glass window to permit optical holography. The window permits viewing through a 50.8 mm diameter circular opening obscuring the upper and lower few millimeters of the specimen. Force was measured external to the vessel.
Some unique hydrogeologic issues are inherent in the problems of siting, designing and licensing a mined geologic high-level nuclear waste repository. The problems involve hydrogeologically unfamiliar rocks for which little proven technology has been developed, long containment periods, elevated temperatures, and externally-imposed management constraints. In evaluating the existing national program, it should be recognized that hydrogeology is only one aspect of the total nuclear waste disposal problem, that the use of multiple barriers requires that the hydrogeologic system be considered in its entirety as a total system, and that engineering modifications can improve the isolation capabilities of certain barriers. Important research questions involve the mechanics of transport through dense fractured and unfractured rocks, methods for measuring vertical hydraulic conductivity, and a variety of geochemical questions. Current expectations for "high quality data" from either in situ or laboratory testing are unrealistic, and a modified approach to repository hydrogeology is needed. The dilemma is that the hydrogeological characteristics that make a potential host rock more satisfactory for respository siting are the same characteristics that are difficult to characterize and make the particular site more difficult to license. Finally, evaluation of the containment potential of any candidate site will become meaningful only with detailed underground exploration, mapping and testing in the host rock formations at the repository levels.
Subsurface hydrogeologic transport is central to the problem of disposing high-level nuclear wastes in a mined geologic repository. This is because subsurface hydrogeologic transport is the most likely mechanism by which radionuclides might reach the accessible biosphere from such a nuclear waste repository. Some unique technological and management issues are inherent in the hydrogeologic problems of siting, designing and licensing a mined geologic high-level nuclear waste repository. The origins of such problems are discussed below together with some general guidelines that are sometimes lost sight of in the evaluations of the existing national program. Current expectations for "high quality data" from in-situ or laboratory testing are unrealistic because of technological limitations, and modified approaches to repository hydrogeology are needed. Finally, some hydrogeologic caveats and research needs relevant to the waste isolation problem are discussed.
UNIQUE HYDROGEOLOGIC CHARACTERISTICS OF THE NUCLEAR WASTE DISPOSAL PROJECT
The high-level geologic nuclear waste disposal project is unlike any other engineering project for a variety of technological and management reasons. We consider first the technological problems that make this an unusual hydrogeologic project. Groundwater hydrology has been developed for permeable rocks capable of providing water supplies. The concern in nuclear waste isolation is with the capabilities of relatively impermeable rocks to contain wastes. The characteristics that make a rock formation desirable for water supply are the same characteristics that make it unsuitable for containing wastes. In fact, if some of the available technology, such as multi-well pumping tests, can be applied to a potential host rock, that rock is too permeable for use in repository siting. Relative to historical research, hydrogeologists are dealing with hydrologically unfamilar groups of rocks without a large body of proven technology. It is not surprising, therefore, that standard groundwater concepts and methods are sometimes applied to these unfamiliar hydrogeologic systems without sufficient questi
Boyce, Glenn H. (Earth Sciences Division, Lawrence Berkeley Laboratory, University of California) | Doe, Thomas W. (Earth Sciences Division, Lawrence Berkeley Laboratory, University of California) | Majer, Ernest (Earth Sciences Division, Lawrence Berkeley Laboratory, University of California)
This paper discusses the results of a laboratory testing program to determine the validity of hydraulic fracturing stress measurements in salt. Tests were performed on 15 cm diameter samples loaded under hydrostatic stress conditions to determine the influence of time, confining pressure, flow rate, borehole diameter, and packers on breakdown pressure. Tests were also performed in a polyaxial loading frame to determine whether or not hydraulic fracturing could be used to measure non-hydrostatic stresses. The test results indicate that the breakdown pressure is not affected by time delays of up to 64 hours between the application of load and fracturing the rock. Breakdown pressures were found to fall short of predicted elastic values at higher confining pressures for the hydrostatic tests. The ratio of the horizontal stresses in non-hydrostatic tests had no effect on breakdown pressures. Flow rate and borehole diameter variations were found to have a marked effect on breakdown pressures. Shut-in pressure values, which are generally used as an indication of the minimum stress value, exceeded the applied minimum stresses by 10 to 60% depending on the method of determination used. The lack of a relationship between breakdown pressure and horizontal stress ratio may preclude hydraulic fracturing from being used in salt as it is conventionally applied in brittle rocks.
Most conventional stress measurement techniques, such as overcoring and hydraulic fracturing, are based on elastic formulations which may not be applicable in salt, a rock well known for its non-elastic behavior. To determine the effects of non-elastic behavior on hydraulic fracturing stress measurements, we have performed a series of laboratory hydraulic fracturing tests in salt under controlled stress conditions. The determination of the maximum horizontal stress by hydraulic fracturing is based on the breakdown pressure (the pressure required to fracture the rock) being equal to the minimum tangential stress concentration in the borehole wall plus a tensile strength. For non-porous, elastic materials, the minimum tangential stress is (mathematical equation)(available in full paper) where s¿0 is the minimum tangential stress, sHmin is the minimum horizontal stress, and sHmax is the maximum horizontal stress. If the material deforms in a linear viscoelastic manner, the stress concentrations around the hole should be the same as for the elastic case (Goodman, 1980). However, the tangential stress will be reduced from the elastic value if the deformation is non-linearly viscoelastic (Goodall and Chubb, 1970 ; Hata, 1975) or plastic (Robertson, 1955). If the deformation of salt is non-linearly viscoelastic, the tangential stresses in the borehole wall decrease with time, and the breakdown pressure should also decrease. In conventional interpretation of hydraulic fracturing, sHmin is determined from the shut-in pressure and sHmax is inferred from the magnitude of the breakdown pressure. The lower the breakdown pressure for a given shut-in pressure, the higher is the inferred value of the maximum stress. If the breakdown pressure is low because creep has reduced the stress concentration, the application of elastic theory to the stress measurement data reduction could result in significant errors, such as the inference of large non-hydrostatic stresses where the stresses are, in fact, hydrostatic.
Serious floor heave of up to 2.4 m in a 2.4-m high mine entry was eliminated by applying the stress control method of mining, as a last resort, at the No. 5 coal mine of Jim Walter Resources, Inc., in the Black Warrior coal basin near Birmingham, Alabama. Underground observation of the first 3-room entry created using the stress control method is discussed here. The behavior of the test entry, which eliminated the heave problem, is analyzed in relation to similar studies conducted in a salt mine under similar ground conditions by utilizing finite element analysis.
SIGNIFICANCE OF TRIAL APPLICATION OF STRESS CONTROL METHOD
Failures of roof and floor present serious rock mechanics problems in many underground coal mines. These problems have been dealt with mainly by using artificial supports, such as roof bolts, wooden cribs, and concrete linings. These conventional means of ground control become increasingly difficult to use as the ground instability intensifies. Such instability encountered at the No. 5 mine of Jim Walter Resources, Inc., operating in the Black Warrior coal basin in Alabama, was so intense that none of the conventional means of ground control seemed feasible. The stress control method of mining was applied as a last resort. This method in fact proved to be successful in eliminating the intense ground failure problems without requiring any artificial means of ground control. The results of this trial demonstrate that the stress control method is not only a powerful and scientific method but also that it is economical and practical for solving ground failure problems in underground mines. The scientific significance of this development lies in the field verification of the rock mechanics principles of complex ground upon which the stress control method is based, demonstrating that the method is applicable to most underground mining, regardless of the type of ore body to be mined.
HISTORICAL BACKGROUND AND THEORETICAL ISSUES
The stress control method has a long history. The origin of the method may be traced back to coal mining in northern England in the early 1950s, when Charles Holland first introduced the concept of the stress arch and yield pillar (1). But his method did not receive wide acceptance at that time, despite his penetrating vision. In the meantime, the yield pillar was successfully adapted in underground trona mining in Greenville, Wyoming, through the creative work of Bill Fischer of FMC (2). Fischer utilized the yield pillar to sustain temporary stability of failing roof strata in his total extraction retreat mining. The use of yield pillars to provide long-term stability of an advancing new entry system was not developed until the late 1960s in potash, where the stress control method originated. Many potash mines at that time were suffering from extensive floor heave and roof failure problems. The conventional use of yield pillars and stress arches was found to be inadequate to solve these intensive ground failure problems. It was at this time that the stress control method was first introduced, successfully stabilizing the ground and eliminating the saturation roof bolting requirements which had been believed to be essential.