AGAPITO
ASSOCIATES, INC.Mining & Civil Engineers & Geologists
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AGAPITO
ASSOCIATES, INC. Mining & Civil Engineers & Geologists |
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Project Summaries - Mineral |
Application of Microcomputers to Geotechnical Design and Stability
Monitoring—A Case Study of Resource Recovery Optimization at the Tg Soda
Ash Trona Mine Cooperative effort between Tg (Texasgulf) Soda Ash, Inc.'s and J.F.T. Agapito & Associates, Inc.'s (AAI) ground control engineers has resulted in an innovative mine design and a continuous monitoring system at Tg's trona operation, Wyoming. Performing geotechnical analyses of mining layouts on a routine basis and monitoring design performance on real-time using microcomputers has been shown to be practical. Through such a program, significant improvements in resource recovery, productivity, and stability have been achieved. Specific layouts analyzed and implemented consist of a shortwall mining system with an advancing tailgate and a three-entry gate system using 17-ft-wide pillars at 1400-ft depth. Two computer codes, originally developed for "main frame computers," together with pre- and post-processors, have been successfully implemented on IBM-PC and compatible 386 microcomputers. These codes are routinely used for practical mine layout design and resource optimization. EXPAREA is a three-dimensional, displacement-discontinuity code, suitable for design of underground layouts. VISCOT is a two-dimensional, finite-element code with elastic, visco-elastic, and elasto-plastic material behavioral capabilities. These codes have been used together to analyze time-dependent material behavior and roof/floor stability. To collect time-dependent deformation and stresses on a continuous basis, a monitoring system was implemented at the mine. Results are monitored at the mine office using a 386 microcomputer. Such monitoring is used to verify input to the numerical models and to warn against impending stability problems. Improved mine layouts are adopted based on routine modeling and geotechnical monitoring. |
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Project Summaries - Capability |
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An Overview of the Geology of Solution Mining in Saskatchewan Presently, there is great interest in exploring for, and developing, potash deposits around the world. This interest has created record levels of interest in potash deposits within the Elk Point Basin sequence of Saskatchewan. Potash has been mined in the province since 1959 and, presently, there are 10 operating mines, 8 of which are underground extraction mines, 1 which is a flooded underground mine (PCS Patience Lake), but presently operating as a solution mine, and 1 solution mine located at Belle Plaine Saskatchewan. It is estimated that there are some 8 billion short tons of recoverable KCl reserves (5 billion short tons K2O equivalent) using conventional (underground techniques, but some 110 billion short tons of recoverable KCl reserves (65 billion short tons) solution mining techniques. Numerous permits for ground that industry considers as prospective for further potash extraction have been taken since 2005; however, the majority of these lands are along the “Conventional Mining Belt,” the trend where the beds are extractable using underground mining techniques. The trend of lands amenable to solution mining are located to the south of the “Conventional Mining Belt” further into what is termed the “Williston Basin” of Saskatchewan and the northern plains states of the United States. The potash deposit itself consists of generally flat-lying sedimentary deposits of interbedded halite, sylvite, carnallite, clay, and minor anhydrite and dolomite beds that can be mapped from central Alberta through to Manitoba, North Dakota, and Montana. In the 1960s, most of the ground in central Saskatchewan considered prospective for potash mineralization was held by a wide variety of companies experienced in the methods of conventional underground mining, such as International Minerals and Chemicals Corporation and Potash Company of America. Along the trend considered too deep for conventional mining, while a number of resource companies held permits, and several pilot solution mining projects were undertaken, such as the work of Imperial at Findlater Saskatchewan, Lumsden Potash at Bethune, and Lynbar Mining at Duval, the only company to proceed with a commercial solution mining operation was Pittsburgh Plate Glass (PPG) at Belle Plaine. This paper presents a summary resource characterization for two such abandoned pilot test sites. The mineral resource for these sites (net of areas where geophysical surveys suggest dissolution and collapse structures) is estimated at 5 to 6 million tonnes of K2O equivalent per section. This is equivalent to 2.04 tonnes of K2O per square meter or 2,040,000 tonnes per square kilometre.
Cavity Shape Characterization of a Rubble-Filled, Solution-Mined Cavity To comply with permit conditions and to provide valuable data for locating future wells, American Soda, L.L.P. (ASLLP) needed to characterize the shape of cavities developed by solution mining nahcolite. The ASLLP cavities were difficult to characterize because solution mining removed less than 25% of the cavity volume leaving the cavern filled with rubblized insolubles. After evaluating several methods, a novel downhole, seismic technique and partial fluid displacement were selected for demonstration. In early 2004, ASLLP performed a downhole seismic reflector tracing and partial fluid displacement to characterize the shape of a mature cavity. The cavity was initially estimated to be on average 95.5 ft in radius and 492 ft in height. The upper cavern volume estimated from the downhole seismic reflector tracing and the partial fluid displacement method agreed within 10%. The overall cavity shape and volume determined by the downhole seismic method compared favorably with volume estimated from the historical nahcolite production. The shape characterization indicated that the maximum cavity radius was 58% larger than the average radius.
Closure of Remote Historic Underground Mines in Desert Environments Geotechnical solutions were developed for closure of two historic, remote underground mines in the deserts of southern California. Closure criteria included access closure, minimal site impact, and optimum use of native materials. The closure designs and construction activities incorporated techniques for protecting sensitive biota, particularly bats and desert tortoise. The types of mine openings that were closed include adits, declines, shafts, daylighted stopes, and dugout dwellings. The desert locations permitted implementation of cost-effective closure techniques that relied upon the use of native materials. The strategies and techniques developed for these desert closures are adaptable to other climates.
Economic Benefits Gained by Rock Mechanics: Three Case Studies
(click to view entire paper in PDF
format) Significant economic benefits can result when rock mechanics is applied within a practical framework and integrated within the other engineering functions of the mine organization. This requires management support and understanding of rock mechanics as a practical tool to help attain high standards of safety, productivity and resource recovery. For such an approach, a long-term geotechnical program is needed in most operations to build an adequate data base and to ensure that design and ground control issues are handled in a cost-effective manner. This paper presents a summary of three case studies where significant economic benefits were realized with the help of rock mechanics. Although cost savings were not computed, the economic benefits are obvious and clearly impacted the mine operations favorably.
Evaluation of a New
Method for Work Index Estimation Using Single Particle Impact Tests
Mine plans typically consider the profits and costs associated with extracting and processing minerals into marketable products. Interrelationships between mine and mill costs are often overlooked during the process of mine plan optimization. A simple and quick method for predicting milling costs for a given block would improve the planning process, especially if the procedure could be incorporated as part of routine ore grade analysis of drill core or cuttings. Incorporating milling costs as part of the mine planning process could improve overall optimization of the mine/mill combination. A method developed by the Utah Comminution Center (UCC) at the University of Utah provides a possible answer to the need for quick and reliable grinding cost estimation (King et al., 1996). Using results from single-particle impact tests, a work index, called the Utah Load Cell Work Index (ULCWI), is calculated. The potential for routine use of the new method is explored by comparing ULCWI values with corresponding Bond Work Index (BWI) values for eight mineral materials (Free 2000).
Horizontal Stresses as Indicators of Roof Stability
(click to view entire 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.
Improvements in Resource Recovery at
Stauffer's Big Island Mine Significant increases in extraction ratio on the order of six to eight percent, equivalent to as much as 142 t/m of panel advance, have occurred at the Big Island Mine during the last five years with the help of a practical rock mechanics program. This increase in resource recovery also has contributed to improvements in productivity. Both conventional and continuous machine mining have been used to mine two flat-laying trona beds at depths of 250 to 260 m in the Green River Formation in southwestern Wyoming. Conventional room-and-pillar mining originally was conducted in panels with extraction ratios of 58 to 60 percent. The use of yield pillars has allowed the panel extraction to increase to 66 percent. Continuous machine mining was introduced recently, with a 64 percent extraction ratio using long, narrow pillars and wide rooms. Further improvements in resource recovery seem feasible in the light of present mining experience. The
rock mechanics program consisted of field instrumentation to determine the
pillar and roof response to mining, and computer modeling to evaluate and
help determine the stability of various layouts, which were then adopted for
mining.
Long-Term Stability for Two-Seam Mining at
OCI's Big Island Mine (click to view entire paper in PDF
format) Two flat-lying trona seams 3- to 3.5-m thick, approximately 10 m apart, and at depths of 250 m are mined by room-and-pillar at OCI's Big Island Mine in Wyoming. Continuous miners and a yielding pillar system have contributed to improvements in resource recovery and productivity. Long-term stability with minimal subsidence is needed for a large portion of the mine beneath the Green River channel. This was investigated by computer modeling in 1990. Although results of the study indicated good long-term stability, recommendations were made for stress determinations to verify the model. Stress determinations made in 1997 showed pillar stresses 10% to 20% higher than the model stresses, and barrier pillar stresses 10% to 15% lower. This implies that higher panel stresses will be transmitted from the upper to the lower seam. A stability evaluation of the lower seam is planned before two-seam mining to assess entry widths and support requirements.
Prefailure Pillar Yielding
(click to view Mining Engineering paper in PDF
format) Yield pillars have been used for many years to help reduce stresses near mine openings and improve roof and floor stability. A yield pillar is often defined as a pillar that fails but retains residual strength. Stress transfer occurs through the roof and floor after the peak strength of the pillar is reached. High stresses are transferred from around the openings onto abutments that can be barrier pillars or unmined ground. This mechanism, often referred to as pressure arching, is possible as long as the width of yield pillars is less than the critical width above which stresses cannot be carried by the overburden. Significant stress transfer also can occur due to small amounts of pillar and/or floor yielding before the peak strength is reached. This is accomplished in a quasi-elastic manner with little or no visible roof and pillar fracturing or floor heave. Long-term stability may be achieved when stresses and mechanical properties are favorable to pre-failure yielding. This paper gives practical examples where improvements in stability and resource recovery were achieved with this mechanism. Yielding was assessed by comparing measured and calculated vertical pillar stresses. Results indicated that calculated stresses in the pillars were 25 to 40 percent higher than the measured stresses, demonstrating significant arching load transfer to the abutments. Pre-failure yielding is probably often present but unintentional in both development and production areas. Better recognition and use of this mechanism should lead to improved designs as mines become deeper.
Rock Mechanics for Two-Seam Mining at the Big
Island Trona Mine This paper describes part of a rock mechanics program being conducted at the Big Island Mine of Stauffer Chemical Company of Wyoming. The program objective is to determine the feasibility of mining two superimposed, 10-ft (3-m) thick, trona seams. The beds are separated by 33 ft (10 m) of marlstones and shales. The lower bed is 850 ft (259 m) deep. The mining method is room-and-pillar with a 60% extraction ratio. A preliminary assessment based on a simple finite analysis indicated stress overlapping between the beds to be negligible with stability conditions similar to single-seam mining. At present, part of the test area has been mined with superimposed pillars between the seams. Test mining consists of extracting a panel over an existing mined out area. This sequence was chosen because upper bed mining on a production scale did not commence until 1976 and the finite element analysis indicated there was no appreciable difference in mining either the upper or lower bed first. Simple instrumentation showed that prior to upper seam mining the pillars in the lower bed were under full overburden load. Upper seam mining has increased pillar loading in the lower bed by 175 psi (1.2 MPa). This was accompanied by small downward movement of the strata between the seams. Observations and deformation measurements have indicated stable conditions. However, spalling of loose roof slabs and pillar corners has occurred in the lower seam as a result of upper seam blasting and residual fractures left from lower bed extraction. Two-seam test mining is still in progress; results indicate agreement between the stability analysis done ahead of mining and actual experienced conditions. Instrumentation monitoring will be carried out for months after mining the test area. Future production from two-bed mining will be delayed for two years to assess time effects on stress and deformation after completion of the test area. The program has established rock mechanics in the Big Island Mine as a practical tool to help provide maximum safety, resource recovery, and productivity. Additional stability analysis based on experience and instrumentation results will be used to evaluate future panel layouts and extraction ratios in long-range planning.
Rock Mechanics Issues in the Trona Patch The trona mines in the Green River Basin, commonly known as the "trona patch," present an interesting set of rock mechanics issues stemming from mining in a unique underground environment. Like other minerals occurring in tabular deposits, trona is mined using high-productivity room-and-pillar and longwall methods. However, trona does not behave quite like other evaporites and behaves far differently than coal. In this paper, the following key issues in trona patch rock mechanics are discussed: material properties, in situ stress field, water and gas pressure effects, creep characteristics, pillar behavior, roof span stability, floor stability, underground tailings storage, longwall mining, two-seam mining, and solution mining. Many of the existing trona producers are entering potentially more difficult mining conditions, and much consideration is being given to "new" technologies such as longwall mining and solution mining. In analyzing 20 years of experience with trona patch rock mechanics issues, the authors have identified key comparisons and contrasts with conventional tabular deposit rock mechanics that will help illuminate what has been learned as the trona patch enters the 21st century.
Site
Characterization for Planning Underground Stone Mines Underground stone mines for construction aggregates and industrial minerals are being planned in increasing numbers throughout the United States. At the middle of the last century (1950s) underground stone mines were much more common, and characteristically had no thorough site characterization other than proving reserves and trial-and-error planning. Ground control employed was minimal. Consequently, there was a tendency to conduct minimal pre-mining studies. However, the deposits that lend themselves to little site characterization and minimal ground control are few today, with the best being mined out. Failures of a number of underground stone or similar mines from that era are more common than realized. Underground stone mines almost always use some combinations of square or rectangular pillars, sized to support the overburden load, and spaced to allow the use of larger, more-efficient equipment. Heights are controlled by the available stone quality, interbeds of waste, or technological or safety considerations. Prior to committing to a specific mine plan with subsequent significant capital investment, and prior to having available the wealth of knowledge and experience gained once mining underground, a safe and workable mine layout must be achieved. Early attention to the several different rock units involved (roof, mine level, floor) and their strength as a rock mass with the inherent discontinuities, will allow first approximations of acceptable mine geometries and sequencing.
Stability Assessment of an Underground Limestone Mine--A Case Study
The stability of an underground limestone mine became a concern following numerous roof falls. Factors influencing roof behavior were identified and a program of investigation initiated to understand the causes of the roof falls and to develop recommendations for changes in mine layout and ground support. Factors of concern included high horizontal stress, the presence of water or gas in the roof, and the influence of jointing and fracturing in the roof rock. The investigations included site visits, stress measurements, and three-dimensional numerical analysis. Mine roof falls identified on the mine map were tabulated and categorized according to size and orientation and, where possible, by other distinguishing features such as jointing or water pressure. The vertical in situ component of stress was determined to be approximately 11.72 MPa, about double the magnitude from gravity loading alone. The depth of cover for the mine ranges from slightly more than 167.64 m to more than 243.84 m. The numerical simulations indicated that reduction in span widths from 15.24 to 12.19 m (E-W) and from 12.19 to 9.14 m (N-S) while maintaining center dimensions did not appreciably change the stability factors, and that the orientation of the mine layout was near optimum.
Solution Mining Cavity Stability: A Site
Investigation and Analytical Assessment
Stability assessment of large solution mining cavities near the surface was required to develop guidelines for termination of production. The stability of existing irregular-shaped cavity clusters required definition of cavity shape, characterization of salt and shale strength and mechanical properties, and analysis of stress and deformation. The most significant parameters controlling stability were the inter-cavern and the cavern fluid pressure.
Solution Mining of Nahcolite at the
American Soda Project, Piceance Creek, Colorado
American Soda, L.L.P. has commissioned the first solution mine for recovery
of naturally formed sodium bicarbonate (nahcolite) from low-grade deposits
using high-pressure, high-temperature injection fluid. This project, known
as the Yankee Gulch Project, was commissioned 33 years after discovery of
the deposit and followed an intensive four-year feasibility study that
included a full-scale pilot test. The nahcolite is co-mingled with oil shale
and is impermeable. Solution mining is achieved by a combination of
thermo-mechanical cracking and dissolution. Elevated temperatures are
required to initiate the thermally induced cracking and to enhance
solubility of the nahcolite, which is highly temperature dependent. This
paper will provide an overview of the development of the project from
discovery, characterization, pilot testing, permitting and commissioning,
with emphasis on the solution mining aspects.
Stress Issues Impacting Design and Stability at OCI Wyoming's Big Island
Trona Mine Room-and-pillar mining at the Big Island Mine uses narrow, yield (i.e., load transferring) pillars to achieve high resource recovery and productivity. Two flat-lying 3- to 3½-meter (10- to 11½-ft) seams are mined. The interburden between the seams is about 10 meters (33 ft) and the cover depth is 250 to 330 meters (820 to 1,080 ft). Most of the mining has been single-seam, but two-seam mining will begin in the near future. However, it is important that good long-term stability be maintained to prevent subsidence over a large portion of the mine beneath the Green River channel.
Three stress issues impacting stability are
discussed in this paper: (1) higher-than-gravity vertical pre-mining
stresses, (2) time-dependent stress transfer or arching, and (3) stresses
induced by strata gas. Although the stress environment and behavior is
not fully understood, steps have been taken in mine design and operations to
minimize impacts to stability. Stress determinations have been very
important to verify analytical predictions used in mine design and in
long-term stability assessment.
The History and Performance of Vertical Well
Solution Mining of Nahcolite in the Piceance Basin, Northwestern Colorado,
USA
(click to view entire paper in PDF
format) American Soda, LLP (American Soda), developed a solution mine plan for recovery of nahcolite from the vast resource in the Piceance Creek Basin using vertical wells and injection of high-temperature, pressurized water. The wellfield included 26 solution mining wells, which provided a nahcolite brine to the processing facility. The American Soda solution mining method utilizes dual, 7-inch casings cemented within a single, 19-inch borehole with 4½-inch casings being utilized as tubings within each 7-inch casing. The method injects 350°–420°F water to thermo-mechanically fracture the nahcolitic oil shale and dissolve the nahcolite. A nitrogen gas cap is maintained on the top of the solution mining cavity to limit vertical growth. The solution cavities have a productive height of approximately 500 ft. During the 3.75 years of commercial operation, American Soda mined approximately 2.6 millions tons of nahcolite with the wells producing between 75,000–150,000 tons each with cavern diameters of up to 200 ft.
Towards an Improved Stone Mine Pillar Design Methodology: Observations
from a Mistake (click to view entire paper in PDF
format) The mining engineering design professional has limited practical and reliable tools for planning successful room-and-pillar stone mines using readily-available and collectible information. Three techniques are in common use today: the hard rock CANMET method of Hedley and Grant, the hard rock method of Stacey and Page, and the oil shale method of Hardy and Agapito. Other methods have been proposed, such as the USBM method of Obert and Duvall, the CSIR/Penn State method of Bieniawski, and the soft rock confined core method of Abel, Wilson, and Ashwin. However, the latter have practical shortcomings when applied to room-and-pillar stone mines such as developed for construction aggregate production. Ideally, the use of multiple techniques resulting in the same acceptable and reliable answer is the goal. Recently, in several underground stone mines areas of undersized pillars were examined. These undersized pillars apparently resulted from non-adherence to a mine plan. These undersized pillars now exhibit strong evidence of incipient failure such as slabbing, opening of through-going fractures, and hour-glassing. This situation allows examination of pillar designs at a “safety factor” of essentially one. This rare opportunity allowed the examination of the suitability and adjustment of the first three design methods discussed, resulting in greater overall confidence in the methodologies.
Understanding and Solving
Roof Control Problems in Stone Mines
(click to view entire paper in PDF format)
Several startup or longtime underground stone mines have experienced
unanticipated roof control difficulties due to the variable geologic
character of the roof strata. Careful mapping of roof conditions, and
identification of controlling geologic and mine geometric factors, lead to
an understanding of the mechanisms causing roof problems. Factors such
as unexpected rolls of the roof strata resulting in a gradual change from
strong to weak roof; encountering open water-filled master joints; roof
strata delamination due to internal water pressures; local karst features
with mud, sand, and water hazards; and sudden loss of limestone roof due to
paleo-channel erosion are some recent problems described. Strategies
for overcoming these problems are discussed and detailed. |
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