AGAPITO
ASSOCIATES, INC.Mining & Civil Engineers & Geologists
Home | Contact Us | Profile | Client List | Personnel | News | Employment | Publications | Links
|
AGAPITO
ASSOCIATES, INC. Mining & Civil Engineers & Geologists |
||
|
Home | Contact Us | Profile | Client List | Personnel | News | Employment | Publications | Links |
||
|
Project Summaries - Mineral |
This paper describes part of a geotechnical study based on stress determinations and convergence measurements conducted to assess the long-term stability of an experimental room-and-pillar oil shale mine. A rock mechanics program was initiated in 1971 at the Colony Mine as part of a large study conducted for the design of a commercial operation. The Colony Development Operation is a joint venture currently consisting of four active members: Ashland Oil Inc., Atlantic Richfield Company (operator of the project), Shell Oil Company, and The Oil Shale Corporation. Although oil shale commercialization plans are presently suspended, rock mechanics instrumentation has been continued as part of Colony's policy in maintaining readiness to proceed with a commercial shale oil plant when national energy policies became better defined. The property is located in the southern edge of the Piceance Creek Basin in northwestern Colorado, approximately 200 miles west of Denver. Experimental mining operations were conducted from 1965 to 1972, with one of the major objectives of the pilot mine being the assessment of opening and pillar sizes for the determination of extraction ratio and life of the property. Mining was conducted in a 60-ft-thick portion of the Mahogany Zone in the Parachute Creek Member of the Green River Formation, at depths of 600 and 860 ft by a one-bench system. Pillar dimensions were 58 ft by 58 ft and rooms were 55-ft wide. A well-defined system of joints and bedding planes is present in the flat-lying oil shale beds. The pillars at Colony have two joint sets approximately 90 degrees to each other. The mean strike of the major set is E-W, with 42 percent of the dips being vertical ±10 degrees and 33 percent dipping south at 24±6 degrees from the vertical. The other dips range between these two orientations. Major pillar slabbing has taken place along the south-dipping joints. Joint spacing may vary from one foot to more than 15 feet. The minor joint set dips mostly vertically and has a wider spacing than the major set. The roof shows only one joint system parallel to the major pillar joint set. Most joints are small and very tight, but a small percentage are open and can be traced across some pillars. Joint filling material when present consists mostly of calcite. Laboratory tests indicate that the uniaxial compressive strength is dependent on oil content. Above 30 gallons per ton, strength seems to remain constant at 13,000 psi, but below this grade the strength increases with a decrease in oil content. The immediate roof rock is 3,000 psi stronger than the mine horizon because of a marked difference in grade. Field instrumentation was described in detail in another paper and basically consisted of in-situ stress determinations by the overcoring technique, and rock mass displacement measurements by means of borehole and tape extensometers. Design data was greatly enhanced by unplanned pillar and roof failures. |
|
|
||
|
Project Summaries - Capability |
||
|
|
||
|
Characterization of the Performance of Large-Capacity
Face Ventilation Systems for Oil
Shale Mining The performance of two large-capacity face ventilation systems was measured using sulfur hexaflouride tracer gas tests in a large dead-end heading at the Colony shale oil mine. The test room was 17-m wide by 9-m high and 98-m long. Testing compared the effectiveness of a free-standing, jet fan and a reversible fan with rigid duct. The jet fan performance was similar to the ducted system while using less power. Tracer gas was used to simulate the production of mine air pollutants including blasting fumes, hot diesel emissions, free methane from surrounding strata, and methane desorbing from rubblized shale. The tests showed both systems could provide effective ventilation during oil shale mining.
Computer Simulation of Face Ventilation to
Dilute High Methane Concentrations Developed by Blasting Oil Shale Cooperative research efforts by the Bureau of Mines and Agapito & Associates, Inc., Grand Junction, CO, used a one-dimensional, finite-element computer model to simulate turbulent mass transfer in face ventilation of oil shale mines. The objective was to study the dilution of methane released by rubbled oil shale during blasting. A methane release rate function was developed from the back-analysis of field measurements, and the finite-element model was calibrated using tracer gas characterization data from tests of full-scale face ventilation systems.
Development of a Dry Exhaust Conditioner
for Large Diesel Equipment The requirement for large diesel equipment to be certified for operation in potential gassy mine conditions in western oil shale mines motivated a design engineering project to develop a dry exhaust conditioner system to replace traditional wet scrubber technology. A dry exhaust conditioner for a 600-hp diesel engine was designed and fabricated for retrofitting into a 600-hp underground haul truck. The engine conditioner package was taken through a simulated certification process at the Mine Safety and Health Administration (MSHA) Approval and Certification Center to refine test procedures and to uncover any flaws in the dry exhaust conditioner technology. The engine/exhaust system was then installed in an underground haul truck to demonstrate mine-worthiness of the system. No inherent features were discovered that would prevent the system from meeting test requirements. However, the tests did point out several deficiencies in the prototype design.
Effect of Pillar Reinforcement on
Long-Term Stability of an Oil Shale Mine
Stability in Exxon's Colony Pilot Mine in the Piceance Creek Basin, Colorado, has been monitored by periodic pillar stress determinations and roof-to-floor convergence measurements since 1971. Pillar failure has caused marginal stability in some areas of the mine. Measurements taken during the process of failure provided valuable information on the in situ pillar strength and long-term behavior of the mine structure. Four pillars were reinforced by 32-mm (1.25-in.) diameter tensioned grouted bolts to increase long-term stability in the northern area of the mine. Stability of this area is necessary because future plans envisage its use as an exit/access and ventilation exhaust. Computer analyses were performed to help evaluate the effect of pillar reinforcement on long-term stability. Results indicate that bolting is particularly useful in stabilizing failed pillars, but its effects are significant only in the immediate area of the pillar. The projected significant decrease in deformation of two extensively failed pillars near a main entry indicates that bolting reinforcement will assist the long-term stability of the opening.
Evaluation of Mining Methods for Thick
Tabular Ore Deposits Tabular ore deposits form a major portion of the reserves of the coal, potash, trona, and limestone that are mined by underground methods, while some copper and lead-zinc deposits are also tabular in shape. Oil shale mining represents a new addition to this group. Mining of tabular, extensive, ore bodies becomes difficult when the thickness of the ore body increases. For thin seams, longwall methods or room-and-pillar methods results in good extraction ratios, but as the seam thickness increases, longwall methods are not practical (with present mining machinery) and for room-and-pillar methods extraction ratios decline because pillar strengths decrease with increased height. Alternative mine layouts must be found to mine thick seams with high extraction. This paper details two such possible mining methods and discusses the major factors in the selection of roof spans and pillar sizes that control the overall extraction ratios. These two methods are referred to as the 'Lane and Pillar' and the 'Rib, Slab and Fill method'. The lane and pillar method involves mining long parallel lanes between barrier pillars then pillaring by mining cross cuts on the retreat to leave 'small' yielding pillars. Because miners do not need access to the yielding pillar area, these pillars can be designed with a low safety factor. Design for this method includes not only room span and rib pillar dimension selection for the first pass mining, but also estimation of yielding pillar 'post failure' characteristics and ultimate load carrying capacity. Analysis has been performed using the displacement discontinuity model to illustrate the influence of the post failure characteristic of the yielding pillars on the safety of the mining method. The rib, slab and fill method is similar to the above method but here high extraction is gained on the retreat by slabbing the rib pillars. Additional extraction is attained because backfill is introduced to the lanes after the slabbing operation. This fill acts to improve the post failure load-bearing characteristics of the rib pillar, thereby limiting the ultimate room closure. Primary mining is always adjacent to undisturbed ore under very stable conditions. Slabbing is in the adjacent room between the primary mining and a backfilled room. Analysis of this mining method depends heavily on the assumed characteristics of the pillar behavior. The fill acts only to improve the behavior of the pillar after the peak load has been reached. Analysis using the displacement discontinuity method is presented to demonstrate the significance of this pillar-fill interaction on the overall allowable extraction ratio.
Geotechnical Characterization
for Underground Mining at the Colony Shale Oil Property
Rock structural and mechanical properties data developed in geotechnical investigations at the Colony shale oil property are presented and compared to geotechnical data developed in rock mechanics studies at the Colony Pilot Mine. The comparison indicated that rock structure, rock quality, and mechanical properties were similar throughout Colony, and that design methodology developed from back analysis of conditions in the Mahogany (R-7) zone at the pilot mine could be applied with a high degree of confidence. Geotechnical data was developed from core drilling, fracture mapping, and remote imagery analysis. Rock quality data from core holes was compared cross hole, and correlated with data from the pilot mine. Fracture mapping identified dominant sets that were vertically pervasive, although discontinuous, through the stratigraphic section. Trends of lineaments identified in air photographs correlated with dominant fracture sets, indicating that fracture trends identified in localized exposures could be extrapolated over the entire property. These trends also correlated with regional tectonic grain in Landsat imagery. Rock mechanical properties exhibited grade dependence similar to that reported in other studies, and grade/mechanical properties correlations were similar from hole to hole in the Mahogany (R-7) zone.
Horizontal
Stresses as Indicators of Roof Stability
(click to view complete paper in PDF
format) High horizontal stresses were recognized to impact roof stability more than 60 years ago. Since then, numerous measurements associated high horizontal stresses with difficult ground conditions. This paper presents case histories illustrating the practical usage of roof stress determinations for helping assess stability, not only in the case of high horizontal stresses but also of low stresses. Examples are given of high stresses associated with faults, mine design changes, quantification of stress shadow effect, and anistrophy. The paper concludes with a comparative evaluation on the effects of various stress fields on ground support requirements.
Impact of Excavation Technique on
Strength of Oil Shale Pillars The load-carrying capacity of oil shale pillars excavated by conventional blasting can be increased significantly by presplit blasting and mechanical mining. Comparisons of in situ vertical stresses and fractures obtained from overcoring horizontal holes in the Colony Mine, Piceance Creek Basin, Colorado, indicate that conventional blasting causes a strength loss in a zone of damage approximately 3-m (10-ft) thick. Presplit blasting reduces damage significantly, and increases the load-carrying capacity in the 3-m (10-ft) thick zone by 5.93 MPa (860 psi). Mechanical mining causes little or no rock damage, and an increase of 9.83 MPa (1425 psi) in strength in the same 3-m (10-ft) thick zone. Pillar design using presplit blasting and mechanical mining techniques can increase the extraction ratio by at least 3% and 5%, respectively, as compared to conventional blasting. It is speculated that comparable increases in extraction should also occur due to increases in span dimensions.
Induced Horizontal Stress Method of
Pillar Design in Oil Shale Pillar design in oil shale by the induced horizontal stress method is based on in situ stress determinations of pillars before and during failure, on computer analysis incorporating site-specific rock properties, and on the pre-mining stress field. An empirical strength equation which relates vertical and horizontal stresses at failure was developed from stress determinations through the center of 60-ft cube pillars. Induced horizontal stresses within the pillars are then evaluated for different width-to-height ratios by simple finite element analysis. Design curves are developed relating pillar stresses and strength with pillar widths and extraction ratios. The induced horizontal stress method, which is based on the in situ strength of large pillars, has been used for planning and resource recovery evaluations throughout the Piceance Creek Basin.
Laboratory and In Situ Mechanical Behavior
Studies of Fractured Oil Shale Pillars
Preliminary results are reported of a cooperative research agreement between the U.S. Bureau of Mines and the Colony Development Operation of the Atlantic Richfield Corporation. The overall objective of this program is to correlate laboratory and in situ behavior of fractured oil shale pillars and to use this information to develop design criteria for underground room-and-pillar mining of oil shale. In situ stress measurements in the Colony mine have been made in pillars of oil shale containing joints of which the planes of weakness are oriented at low angles (<45°) to the pillar axis. Four findings of importance have resulted from these measurements. (1) Some pillars are in a post-failure condition, that is, an increase in deformation in the pillars results in a decrease in the load the pillars can sustain; (2) Pillar failure begins when the maximum and minimum stresses within the pillar become approximately 5,000 psi and 800 psi, respectively; (3) In situ measurements suggest that the stresses carried by the "solid" portion of a pillar become constant when the fractured pillar is in a post-failure condition; (4) Severely fractured pillars in a post-failure condition can be stabilized, that is, further fracturing can be prevented by the addition of small radical confinement such as provided by rock bolting normal to the major joint pattern in the pillar. The latter three observations are predicted from laboratory studies on model pillars of oil shale containing joints oriented with respect to the applied stress field as its in situ counterpart. The application of these results and problems associated with applying them to design room-and-pillar mining in oil shale is discussed.
Leakage Testing of Large
Ventilation Control Structures for Room and Pillar Oil Shale Mining
Full-scale stopping and simulated overcast structures were constructed in large opening (30-ft high by 55-ft wide) room-and-pillar excavations in an oil shale mine. The program was undertaken to develop construction techniques and to generate data on structure costs and leakage rates. Leakage rates were measured using both sulfur hexafluoride tracer gas and "brattice window" methods at varying differential pressures. The measurements were conducted before and after a full-scale face blast to characterize the effects of blasting on leakage, and blast air pressures were measured across the stoppings. The operating performance of different combinations of permanent and temporary stoppings was evaluated by developing an operating model for a two-panel oil shale mine. The operating performance for the different stopping combinations was then compared to the system cost to evaluate the overall performance.
Oil Shale Grade Variability in Close-Spaced
Core Holes in the Mahogany Zone of the Colony Shale Oil Project Area
Fourteen close-spaced core holes were drilled through the Mahogany mine zone on the Colony property to evaluate lateral oil shale grade variability. Lateral grade variation was evaluated using horizontal variograms, developed for the average grade of the 60-ft Mahogany mine zone, the 25-ft upper heading, and 35-ft lower bench. Variograms for the close-spaced holes did not indicate any reduction from the overall sample variance at the minimum spacing of 90 ft. However, the variance of the close-spaced holes as a group is roughly one-half the overall variance of all 59 holes located throughout the property for the 60-ft zone and the lower bench. The variance of the close-spaced holes is similar to the overall variance of all holes for the upper heading. The variograms indicated that predicting retort-feed-grade variability with vertical boreholes drilled from the surface would be impractical. The evaluation of the grade of daily or monthly production would require the development of some type of in-mine sampling program. The shape of the variograms for all the drilling data was not consistent with the variogram models commonly employed in geostatistics. This was probably due to the relatively small number of holes that are available for the analysis, especially at close and intermediate spacings. Thickening and thinning of stratigraphic subzones in the Mahogany mine zone was investigated in an attempt to identify the mechanism for lateral grade variability. Correlations between average grade of the subzones and thickness were poor, indicating that this was not a primary reason for lateral grade variation.
Performance Characteristics of
Large-Capacity Face Ventilation Systems for Oil Shale Mining
The performance of two large-capacity ventilation systems was compared through tests conducted in a large dead-end heading at a pilot oil shale mine in Colorado. Sulfur hexafluoride tracer gas was used to measure the performance of the two systems: a free-standing jet fan and a reversible fan with rigid duct. The tracer gas was used to simulate the production of mine air pollutants, including blasting pollutants, hot diesel emissions, free methane from surrounding strata, and methane desorbing from muck piles. The test room was 55-ft wide, 30-ft high, and 320-ft long. The performance of the two fans was similar, but the jet fan used less power. The tests showed that either system could provide effective ventilation during oil shale mining.
Pillar Design in Underground Oil Shale Mines
The Colony pilot mine provided valuable pillar strengths that should form the basis of any subsequent mine designs. As no commercial oil shale mine has yet been developed, neither a comprehensive empirical pillar strength formula can be developed, nor a proposed empirical formula tested. This paper summarizes the strength data available for oil shale pillars and suggests a strength formula similar to that in common usage for coal pillar design, but with constants compatible with the pillar strength available data in oil shale. Confidence in the use of this formula is limited to situations where the ground conditions are similar to that at the Colony pilot mine, so in application of this formula to a new oil shale area, a comprehensive geotechnical data collection program must be undertaken so that a valid comparison can be made. The paper points out the possible conservative nature of the strength formula for barrier pillar design where the pillar width to height ratio is high. Finally, additional stress measurements and back analysis in existing and planned pilot mines is encouraged to improve pillar strength design formulae and to clarify the interaction of individual pillars in the mine.
Reinforcement of Large Pillars by Bolting
An analysis of bolting reinforcement of several large [approximately 18-m (60-ft) cube] pillars was performed. The many overcoring stress profiles in pillars at the mine were used to produce generalized stress distributions at various stages in pre- and post-failure. A complete stress-deformation curve was estimated from this data. The effect of pillar bolting on the load-deformation behavior was extrapolated from the observed unbolted behavior. Bolting can increase pillar strength by up to 10 percent. The effect of bolting on general mine stability was examined with a short computer analysis. The computer model was calibrated against measurements performed at the mine, and produced good agreement between measured and modeled stresses. The analysis indicated that bolting increases the bolted pillar safety factors in the event of additional pillar failure by a small amount (approximately 5 percent). Progressive pillar failure is unlikely, because arching transfers most load from yielding pillars to large unmined abutments rather than to the smaller panel pillars.
Risky Business
(click to view complete paper in PDF
format) The current "Crisis in the Gulf" has brought the need for a long-term U.S. energy policy back onto the national agenda. Publications of the oil industry now abound with calls for the development of an effective policy that avoids the mistakes of the past. Other industrial nations have developed and implemented policies to encourage conservation and development of alternative energy sources while discouraging the consumption of imported oil. Some of these nations have become dominant economic powers, while being almost totally dependent on imported oil. That suggests that a national energy policy can be effective and support an expansive economy. Given our current military confrontation in the Persian Gulf, our growing imports of crude oil, and our declining dominance in the world economy, continuation of the status quo could be risky business. . .
Rock Mechanics Applications to the
Design of Oil Shale Pillars The mining engineer must be able to predict the structural behavior of the rock around underground openings so that he can answer questions relating to safety and economics. With the present status of rock mechanics knowledge, exact predictions cannot be attained and approximate idealizations are made. As a first approximation, structural design makes use of theory, laboratory testing of rock properties, and available experience from excavations in similar rock formations. However, realistic design can usually be obtained only after in situ observations and measurements are made. After all the information is collected, judgement and experience play a most important role in the final formulation of the design. This paper describes part of a geotechnical program which was instrumental in obtaining information for the design of large oil shale pillars. The work was carried out during 1971 and 1972 in the experimental mine of the Colony Development Operation. Colony is an oil shale venture formed between Atlantic Richfield Company, operator, The Oil Shale Corporation, Cleveland-Cliffs Iron Company, and Sohio Petroleum Company to conduct research and development in mining and retorting of oil shale.
Roof Design Considerations in
Underground Oil Shale Mining The results of overcore stress determinations and stratascope observations in the roof of the Colony Pilot Mine are presented and compared with results from numerical calculations. These results show compressive stresses in all horizontal directions and imply that movement has occurred on one or more bedding separations. Numerical modeling was used to simulate the roof behavior at the Colony Mine; the primary modeling technique used was a displacement discontinuity model developed by Crouch. To match the analytical results with the field measurements, the structural model of the roof was perfected by varying the location and properties of the bedding separations as well as the regional stress field. Reasonable agreement between field and analytical results were obtained, using five bedding separations. A comparison with elemental beam theory is made, and the implications of applying the numerical analysis with the selected structural model to the design of roof spans in a prototype oil shale mine are discussed.
Saline Dissolution Collapse in the
Piceance Creek Basin Dissolution of salts in the Piceance Creek Basin oil shales has lead to considerable collapse. Fracture systems affecting dissolution at depth show visible lineaments. Remote sensing, hand in hand with drill hole data, allows interpretation of solution collapse and modeling of rock quality and hydrology in the leached zone and overlying strata. In this study, video-enhanced Landsat imagery is used to relate relic salt bulges in a single zone to lineaments in the central basin. Fractures indicated by linear gulches and lineaments bounding salt in bulges are suggested to have localized solution collapse. |
||
Top |
Back
|
Next |
Home
Contact Us |
Profile |
Client List |
Personnel
| News |
Jobs |
Publications |
Links
Mining Engineering |
Mine Ventilation Engineering |
Highwall Mining
|
Blasting & Explosives Engineering
|
Solution Mining
Rock Mechanics |
Rock Testing
|
Stress Measurements |
Forensic Engineering |
Geology & Drilling Management