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Collaborating Authors
Results
Laboratory Determination Of Fracture Permeability
Ryan, Thomas M. (Department of Mining and Geological Engineering, University of Arizona) | Farmer, Ian W. (Department of Mining and Geological Engineering, University of Arizona) | Kimbrell, Allen F. (Engineering Geology and Rock Mechanics Division, USAE Waterways Experiment Station)
ABSTRACT 1 ABSTRACT Intact and fractured rock samples were studied in the laboratory in order to understand more fully the mechanism of closure of fractures subjected to high confining stresses and the resultant effect on sample permeability. Confining stresses applied to the samples ranged from 3.5 to 20.5 MPa, and the closure of fractures was observed by monitoring changes in the hydraulic conductivity of the specimens. Test results suggest that some resealing may occur due to crushing and realignment of mineral grains along a fracture surface. The closure of fractures is dependent upon the strength of the rock mass, the physical nature of the fracture, and the fluid pressure present in the fracture. Flow rates through fractures obeyed the cubic law although the induced permeability resulting from the fractures was very sensitive to changes in aperture, and was affected by the matrix permeability of the rock mass and the roughness of the fractures. 2 INTRODUCTION The general approach to analysis of fluid flow through fractures has been to equate fluid flow through fractures with viscous, incompressible flow between smooth parallel plates (see Snow, 1965). By considering the flow to be laminar, and the plates to be horizontal, the flow velocity and hydraulic gradient relationship becomes: (mathematical equation)(available in full paper) 3 LABORATORY PROCEDURES The Hassler cell apparatus was used for testing the hydraulic conductivity of the cylindrical specimens. Specimen dimensions were kept as uniform as possible to reduce the size effects that have been observed by other researchers including Gale and Raven (1980). Thirty-four samples were prepared from intact NX sized core specimens which ranged in diameter from 500 mm to 510 mm. The length of the specimens varied between 950 mm and 1150 mm. Four different lithologies were tested: Berea Sandstone, Hartshorne Sandstone, basalt from the Umatilla member in the Columbia Plateau Series, and gneiss from the Orofino Metamorphic Series near Dworshak Dam, Idaho. All samples were tested using straight flow along a single horizontal fracture that ran along the longitudinal axis. Fractures were induced in the specimens by a modified Brazilian test procedure, in which triangular platens were used to ensure even loading and splitting. All fracture surfaces were mapped using a LVDT and contour plots were generated to help characterize fracture roughness, and nonplanarity. Samples were _first saturated and initially tested for their hydraulic response under loading, in order to determine as accurately as possible their matrix permeability. The samples were then fractured and the surfaces of the induced fractures were characterized. The samples were resaturated and then reloaded into the Hassler cell permeameter to test their fracture permeability. Precautions were taken in order to ensure that the fractures were as horizontal as possible before final emplacement of platens and sealing of the pressure chamber. Water was employed as the confining medium, with nitrogen gas used to apply both the confining and infection pressures. Flow rates through the samples were measured as a function of both confining and head pressures, during both loading and unloading cycles.
- North America > United States > West Virginia (0.26)
- North America > United States > Pennsylvania (0.26)
- North America > United States > Ohio (0.26)
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ABSTRACT 1 ABSTRACT In 1980, wedge failure of the sea cliff adjoining Dana Point Harbor endangered a nearby restaurant and precipitated costly legal action against the County of Orange, California. Landslide debris partially buried a county road near the base of the slope forcing closure of the only traffic outlet from the western end of the harbor. The first phase of slope repair and restoration involved stabilization of the steep landslide headscarp, which threatened to undermine the southern corner of the restaurant. Principal control for the sole of the landslide was an easterly inclined fault which trends up and across the cliff face. Stabilizing forces are applied by a shotcrete restraining structure supported with long, flexible-tendon anchors bonded in competent rock beyond the fault surface. The sixteen primary anchors are between 80 and 100 feet long with design working loads of 175 kips each. Limited access, complex slope geometry, and difficult drilling conditions required many specialized and innovative construction techniques. However, ten anchors in the primary system failed to meet test loading requirements because a fundamental principle of grouting was apparently not considered in design of the anchor components. Grouting systems for the replacement anchors were redesigned to provide more positive control of the quality of bond at the grout/tendon interface. All replacement anchors met the specified acceptance criteria, allowing the second phase of landslide repair and road reconstruction to proceed as planned. 2 INTRODUCTION Dana Point is a prominent headland on coastal southern California about 60 miles south of Los Angeles. Richard Henry Dana, the area's namesake, described this historical headland and its protected cove in Two Years Before the Mast, his autobiographical book on early California shipping. In 1970, nearly two centuries later, a marina-harbor complex was constructed at Dana Cove. The once sleepy, seaside community has since become a bustling, beach resort and prime center for development in the megalopolis that may ultimately stretch from San Diego to Los Angeles. The top of the 150 to 200-foot-high sea cliffs that rim the harbor provide a striking view of the southern coastline and yachting basin below. It was this view that prompted construction of a restaurant near the top of the bluffs. Unfortunately, the desire to capture the view caused the structure to be positioned at the edge of the sea cliff, despite potentially hazardous geologic conditions. Aesthetic appeal apparently overrode geotechnical considerations. Dana Cove is a shoreline reentrant caused by differential coastal retreat in two distinct rock types: the San Onofre breccia and the Capistrano formation (see Figure 1). The relatively competent and resistant San Onofre breccia forms the headland (Dana Point), while the weaker, more erodible Capistrano formation is exposed along the inner margin of the harbor (cove). The contact between these rock units is an ancient (more than 125,000 years old), north-south trending fault that has deformed and broken an interval of Capistrano siltstone and sandstone along a zone approximately 100 feet wide. Conversely, the more competent San Onofre breccia has been relatively unaffected. The disturbed character of the rock and slopeward inclination of the fault (50 to 70 degrees) promoted more rapid erosion and commensurate bluff retreat along the fault trace.
- North America > United States > California > San Diego County > San Diego (0.24)
- North America > United States > California > Orange County > Orange (0.24)
- Geology > Rock Type > Breccia (0.66)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock (0.36)
- Law (1.00)
- Energy > Oil & Gas > Upstream (1.00)
- Consumer Products & Services > Restaurants (0.71)
- Reservoir Description and Dynamics > Reservoir Characterization (1.00)
- Well Drilling > Drilling Equipment (0.67)
- Management > Professionalism, Training, and Education > Communities of practice (0.40)
- Data Science & Engineering Analytics > Information Management and Systems > Knowledge management (0.40)
ABSTRACT ABSTRACT Controlled overcoring trials were carried out with a U.S. Bureau of Mines borehole deformation gage in different rock and rock-like materials possessing varying degrees of anisotropy and homogeneity. Full-scale overcoring equipment was used in laboratory tests on prismatic blocks of discontinuous as well as massive material. The validity of the experimental approach was confirmed and some interim conclusions have been drawn as to the suitability of the instrument under the different rock conditions. 1 INTRODUCTION Various types of instruments have been developed to measure the absolute state of stress in rock. Many of them utilize the overcoring stress relief principle, adopting a wide range of sensing techniques. Every technique appears to be subject to certain limitations of rock type and conditions; high instrument failure rate often results from the use of an inappropriate technique. This paper describes the first part of a series of performance trials on some of the commonly available overcoring instruments (Cai 1986). The U.S.B.M. borehole deformation gage is well documented; see for example Merrill (1967). 2 EXPERIMENTAL METHOD To simulate in-situ stresses on a laboratory scale, blocks of rock and rock-like materials were subjected to compressive biaxial pressure by two hydraulic rams acting in a plane normal to the axis of the installation hole (Figures 1 and 2). While pressure was applied, an installation hole was drilled in the material and a deformation gage installed and overcored with a standard 145 mm diameter thin-walled bit. Borehole deformation was continuously recorded during overcoring and stress-relief curves (graphs of deformation vs overcoring distance) were plotted to ensure that the measurements were not obviously influenced by factors other than the removal of the stress field (Blackwood 1978). The induced stress field was calculated and compared with the pressures applied to the block in order to evaluate the instrument for the specific conditions of the test. This simple experimental procedure enabled reproducible results to be obtained which are an indication of the in-situ conditions suitable for the instrument. Figure 1. Experimental arrangement, Figure 2. Details of laboratory apparatus (Desoe 1983).(available in full paper) 3 VALIDITY OF EXPERIMENT For valid computation of the observed stress from elastic theory, as well as to simulate an infinite rock mass, a plane strain condition must exist at the measurement plane of the gage. A boundary element analysis of a range of block sizes showed that plane strain is approximately met for blocks larger than about 250 x 250 mm (Desoe 1983). For practical reasons a block size of 380 x 300 x 300 mm was adopted. 3.1 Distribution of induced stress The BEM analysis assumed that the external pressure is applied uniformly over the faces of the block. In practice the pressure was applied by circular hydraulic rams; thus pressure plates were required to spread the load over the block faces. The design of these plates is described below. 3.1.1 Numerical modeling A 3-D FEM technique was used to model the experimental arrangement accurately in order to determine the induced stress in the block at the measurement point for different pressure plate thicknesses.
- North America > United States (0.85)
- Oceania > Australia > New South Wales (0.15)
- Research Report > Strength High (0.54)
- Research Report > Experimental Study (0.54)
ABSTRACT ABSTRACT The amount of brecciation of faults in mines is greater in felsic igneous rocks and shale than in quartzite, allowing for differences in localities and fault movement by normalizing breccia thickness by a direct log-log relation with fault displacements. Chlorite-cumming- tonite schist accommodates to regional stress by small-scale folding and brecciation without any large faults. 1 INTRODUCTION Field observations indicate perceptible differences in the amount of brecciation in fault zones among several rock types. The observations were made underground in several mines since publication of a proposed relation between amount of breccia formed (including gouge) and displacement of faults (Robertson, 1983), which permits comparison between rock types at different localities and among different faults, despite the uncertainties in field measurements, the petrography, and the physical and chemical environment. In addition rock deformation experiments can be utilized to explain variations in the field data. 2 BRECClATION AND ROCK TYPE In the first discussions of the log-log relation between displacement (d) and breccia and gouge thickness (t) (Robertson, 1982, 1983), the apparent scatter of observations did not seem to warrant drawing conclusions about differences in brecciation among rock types. However, based on the rough validity of the d/t relation, analysis of additional data and review of old data indicates that amount of brecciation does vary in small but perceptible amounts for certain rocks. Inherent differences among faults at different localities and of varying displacement and length can be accounted for if we assume that fault displacement is acceptable as a normalizing factor for thickness of breccia and gouge; discussion of this is given below, and the dashed trend line (d/t : 100) shown in Figures I and 2 can serve as a guide to greater or less brecciation. In Figure 1 the d and t data for faults in quartz monzonite in mines at Butte, MT (marked "B") plot nominally as a group on the high t side of the trend line, whereas the data for faults in quartzite in mines at Coeur d'Alene, ID (marked "C") plot in a nominal group at lower t. The two to five time's greater thickness of breccia in the quartz monzonite than in the quartzite correlates roughly with the two to five times higher compressive strength of the quartzite. Figure 1. Displacements (d) and thicknesses (t) for faults in quartz monzonite at Butte, MT and in quartzite at Coeur d'Alene, ID.(available in full paper) In Table I are listed new fault data from measurements made in mines by the author, plus data from the Ozark-Mahoning fluorspar mine (Robert Diffenbach, oral communication, 1984), from the Battle Mtn. gold mine (Rob G. Benson, oral communication, 1986), and from Equity gold mine (Steven and Ratte, 1965). The d, t points, marked with locality symbols, are plotted in Figure 2. Discussions of the problems with obtaining acceptable measurements of d and t, taking into account splitting and dying out of faults, are given in Robertson (1983, 1984). Rather subjective averaging of thickness has been used by the author (and other geologists) in all the measurements cited, those made underground and from maps.
- Geology > Structural Geology > Fault (1.00)
- Geology > Rock Type > Metamorphic Rock > Quartzite (1.00)
- Geology > Rock Type > Breccia (1.00)
- (2 more...)
- Materials > Metals & Mining (1.00)
- Energy > Oil & Gas > Upstream (1.00)
1 Abstract The Modified Ring Test is used to study the effect of confining pressure on fracture toughness. After an analysis of the decrease of the stress intensity factor as a function of the confining pressure, experimental results are presented. They show that the increase of fracture toughness with confining pressure depends on rock properties. The nature of the cement and preexisting microcracks appear to be the main parameters in this influence. 2 Introduction Few authors have studied the influence of confining pressure on the fracture toughness value (Schmidt and Hurdle, 1977; Abou-Sayed, 1977; Winter, 1983; M üller, 1986). The difficulties reside not only in performing the experiments, but also in interpreting the results. The general trend is to observe a non-negligible increase of the fracture toughness with an increase of confining pressure. Winter (1983) measured, for instance, an increase in fracture toughness value, at 20 MPa confining pressure, of 100% over the unconfined value for the Ruhr sandstone. Similar increases were reported by M üller (1985) for Iidate Granite. Schmidt and Hurdle only reported an increase of 50% in fracture toughness value at 20.7 MPa confining pressure for Indiana Limestone. Abou-sayed (1977) measured the same amount of increase, but at 6.9 MPa confining pressure. The increase of fracture toughness with confining pressure is generally attributed to a change in the behavior of the decohesion zone (or process zone) which exists ahead of the stress free crack tip. The microcrack model developed by Schmidt (1980) has been used to explain this influence. This model assumes that the process zone ahead of the crack tip results from tensile microcracks which are induced when the singular stress field exceeds the tensile strength of the rock. Using then, Irwin's formula, one can easily determine the size of the process zone: (mathematical equation)(available in full paper) where r and ¿ are the polar coordinates with respect to the crack tip (figure 1), KIC is the fracture toughness, st is the tensile strength of the rock, and sc is the confining pressure. This formula shows that an increase in confining pressure results in a decrease of the process zone, provided KIC remains constant. As KIC increases with confining pressure, one can assume that the size of the process zone r(and ¿) is a material property, indepeudent of the confining pressure. This assumption allowed Müller to derive an equation which relate KIC at ambient condition to KIC at given confining pressure: (mathematical equation)(available in full paper) This equation was found in excellent agreement with the data obtained by Winter (1985), on the Ruhr sandstone. However, such model is a simplified model and did not succeed to explain observed behaviors in fracture propagation. Indeed it largely underestimates the size of the process zone ;measured in rock by Swanson et al. (1984) and Labuz et al. (1985). Consequently, there is a need in determining rock fracture toughness as a function of confining pressure, and as a function of rock properties.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.50)
ABSTRACT 1 ABSTRACT This paper briefly describes an electromagnetic (EM) wave tomography process [The Radio Imaging Method (RIM)] which is used in coal seams to detect fractured roof rock zones and other geologic anomalies in advance of mining. It also compares a RIM reconstructed image of a fractured roof zone to in-mine geologic mapping obtained while mining. Since the coal seam is often bound above and below by more conductive rock, the layered coal formation forms a natural waveguide in the earth. EM waves propagate in the waveguide structure with an attenuation rate that depends on the coal seam height, electrical conductivity, and dielectric constants of the coal and surrounding rock layers. Coal seam anomalies such as fluvial sandstone channels, faults, dikes, and interbedding radically change the electrical properties of the waveguide. The RIM process measures the EM wave level at the end of each ray path in a coal seam. Computerized algorithms are used to calculate the attenuation rate and reconstruct tomography images of the geologic structure in the coal seam. 2 INTRODUCTION Sedimentary geologists have defined generalized depositional models that illustrate how valuable natural resources were formed throughout the geological history of the reserve (Ward 1984). Through the years these models have been refined to include the effect of various types of geologic disturbances on exploration, mineability and extraction cost. By knowing the predominate type of disturbance likely to occur in the reserve, exploration methods can be selected to increase the cost effectiveness of exploration and reliability of knowledge about geologic conditions prevailing in a reserve. To generalize the problem, layered models of the South African gold fields (Chamber of Mines 1986), oil and gas reservoirs, and coal seams (Scholle 1982) are similar in many respects. They lie between layers of rock with contrasting electrical properties (conductivity and dielectric constant). Often layers containing oil and gas, coal, trona or potash are surrounded by more conductive rock. The unique layering in the geologic model produces a waveguide structure in the formation. EM wave propagation in uniform and nonuniform waveguides is well known (King and Prasad 1986). To apply EM waveguide theory, the electrical properties of geologic disturbances in the waveguide must be known and added to the geologic model. The predominate geologic anomalies in a coal seam include dikes, faults, interbedding and fluvial sandstone channels. Deltaic coal depositional models are useful in illustrating the formation of the waveguide coal seam along with the nature of geologic disturbance. Deltaic peat coal swamps are often formed adjacent to ancient river fan tails that flowed into seas as illustrated in Figure 1. The distributary channel water is guided through the swamp region by sand and mud levees. The surface of the channel water is often several meters above the surface of the coal swamp. Peat coal thickness may be quite uniform over great distances tending to decrease near the margins of the deposit. Fry (1984) and Lloyd (1986) of Utah Power and Light Company found that subsidence of the river delta region has caused wide area mud flats to develop forming the first mudstone layer over the eastern mountain property near Huntington, Utah.
- North America > United States > Utah (0.45)
- North America > United States > North Dakota > Bowman County (0.24)
- North America > Canada > Alberta > Clearwater County (0.24)