Search Documents

In the search for significant new economic base-metal resources within Australia during the early 1970s BHP's Exploration Department had observed that the majority of lead and zinc con- centrate production in Europe and the USA came from carbonate- hosted Mississippi-Valley-type (MVT) zinc-lead deposits, but Aus- tralia had no history of production from this class of deposit despite several very favourable geological environments, par- ticularly in the large Palaeozoic sedimentary basins. An exploration-research team was established in 1975 and the Lennard Shelf region, near Fitzroy Crossing, was selected as a most promising geological environment. Previous exploration for this class of deposit by other companies in Australia had largely used conventional exploration philosophies (geological mapping, geophysics and geochemistry to identify specific drill targets) but BHP sought specialist advice from a North American expert and adopted a grid-drilling philosophy, which involved wide spaced relatively deep diamond drill holes on a regular grid pattern, and statistical evaluation of the results. This technique resulted in the discovery of the Blendevale Deposit in 1978 in an area which had previously been explored by other companies using conventional methodologies. The deposit was delineated and a pre-feasibility study carried out in 1985 indicated that the resource of 20 Mt @ 8.3% Zn, 2.5% Pb was a marginal proposition.

Jan 1, 1988

By Rowena C. Duckworth

The Ocean Drilling Program (ODP) is the largest international geoscience project in the world, and the state-of-the-art drill ship, the JOIDES Resolution, is permanently at sea. Each year there are six scientific cruises (legs) which each last approximately two months. These legs are designed by geoscientists from the nineteen countries that subscribe to ODP. Australia and Canada presently share ODP membership under the banner of the AUSCAN Consortium. The benefits of Australia's involvement in ODP include internationalisation of Australian geoscience, advanced graduate training, and unique scientific investigations of the ocean floors in the Australasian region. In addition, visits to Australian ports by the ODP drill ship, the JOIDES Resolution, contribute up to $2 million per visit to the local economy. Amongst the many other aspects of ten years of scientific drilling, ODP has significantly contributed to our understanding of hydrothermal processes on the seafloor. To date, three hydrothermal sites have been drilled to date by the ODP, Snakepit (MAR), Middle Valley (Juan de Fuca Ridge) and TAG (MAR). Two more hydrothermal sites are scheduled for 1996.

Jan 1, 1995

By Cutmore NG

The moisture content of iron ore is important in handling, stockpiling, transport and further processing operations. Typically these ores contain both `free' and `bound' moisture, the latter being moisture that is not removed in a conventional laboratory determination of moisture by air drying at 105¦C, and it is the free moisture that is important in dust control and ore handling. The CSIRO Division of Mineral and Process Engineering has previously developed and commercialised gauges, based on capacitance and microwave techniques, for the on-belt determination of moisture in coal and minerals. However, these gauges determine total moisture, and have a different sensitivity to free and bound moisture in the material analysed. Consequently, the accuracy of determining total or free moisture alone may be significantly affected by variations in the bound moisture content. An investigation has been carried out on the simultaneous determination of free and bound moisture in iron ore fines (

Jan 1, 1993

By Tagawa H

In Japan, geological condition do not only vary in a wide range of strata but are also extremely complex with the existence of faults and aquifers. With respect to rocks, in particular, it is very rare that homogeneous rocks are distributed uniformly. In general, geological conditions are distributed in a wide variety, ranging from earth, sand, soft and medium rocks to hard rocks. Under these geological conditions, when planning of the tunnel construction is to be made out, either blasting method or tunnel boring machines must be selected taking into consideration the geological structures along with the designed tunnel route,' possibility of encountering aquifers, hardness of rocks anticipated and also, the adjacent environ- mental conditions. In the process of select- ing an excavation method,, it is important to make a comparative investigation between the above two methods considering the advantages and disadvantages with regard to the respective operating efficiency, economical efficiency and the conditions and scope of application as well. Furthermore, in addition to these considerations, the recent environmental restriction, which does not allow the blasting in the surroundings of residential areas, must also be taken into account. Because of this, in some cases there is no alternative but to use a static excavation method or a tunneling machine even when these are less efficient than blasting.

Jan 1, 1987

There is no other effective way for study ing firedamp and coaldust explosions than to produce controlled explosions in a pre-monitor- ed environment - either mine openings kept for the purpose, or special surface galleries. Such explosion testing facilities have been employed historically at Buxton in U.K., with lesser known installations at Bruceton, U.S.A., Tremonia, West Germany, Katowice-Mikolow, Poland and elsewhere. Both firedamp and coal dust explosions are studied. Normally coal dust explosions are initiated by somewhat standardised methane air mixtures, by explosives, or purely by coal dust. Various structures and suppression installations are installed in roadways and galleries prior to explosion to simulate destruction and suppression, and the results of such tests form the basis of suppression installations, especially stone dust barriers and water barriers. Other data collected from these experiments include rev- ersals of explosion waves, pressures and temperatures and duration of explosion waves, analyses of aftergases etc. Other uses of explosion testing galleries include testing of explosives, electrical equipment and frictional . sparking.

Jan 1, 1981

By Hannington MD

On the southern flank of Lihir Island, a group of three volcanic cones were discovered at water depths from 1000-1500 m. The volcanoes are located in a narrow zone of recent seismic activity and elevated heat flow (up to 100 mW/m2). The recovered rocks consist of fresh alkali-olivine basalts, clinopyroxene-rich basalts, and porphyritic phlogopite basalts. One of the volcanoes has a recently erupted ejecta blanket (recovery of warm ash!), containing a suite of abundant mafic to ultramafic xenoliths. These xenoliths appear to represent a unique cross-section of subduction- modified mantle material sampled by volatile-rich primary fore-arc magma during its ascent to the seafloor. The most spectacular finding was the discovery of an active hydrothermal system with associated vent fauna and mineralization at Edison Seamount, and the recovery of unusually gold-rich (up to 43 ppm Au) hydrothermal precipitates in 1050 m water depth at the top of Conical Seamount, only 25 km south of the world-class epithermal Ladolam gold deposit on Lihir. The discovery of submarine hydrothermal vents associated with highly alkaline, quartz-undersaturated fore-arc volcanics represents a significant departure from similar hydrothermal systems on the mid-ocean ridges and in back-arc basins. The high gold grades in hydrothermal precipitates at Conical Seamount may indicate the first documented example of a shallow-marine epithermal system analogous to those known on land, and has important implications for the gold potential of submarine arc environments.

Jan 1, 1995

By E Nik, A Manzoori, H Namavari, P Witt

The smelter building at Sarcheshmeh Copper Complex, Iran, houses two reverberatory furnaces and five Pierce-Smith converters. Under normal operating conditions SO2 emissions fill the building, posing a major health problem for all workers in the building, and reducing their ability to maintain the plant. The converter fume extraction system was analysed using plant measurements, engineering modelling of the entire system and CFD modelling of the extraction hoods. By applying fundamental engineering concepts, a user-interactive computer program was developed that enabled plant engineers and operators to gain a detailed understanding of the system. In conjunction with plant measurements, the model quantified the amount of inleakage throughout the system, identifying it as a significant contributor to the poor extraction performance. Furthermore, it enabled investigation of the effects of hood gates, converters on line, hood dampers, flue insulation, electrostatic precipitator (ESP) operation, and inclusion of an acid plant. The model gave unprecedented insight into the operation of a counterintuitive buoyancy-driven extraction system.

Jan 1, 2004

By D Xiong, G Xu, L Yao, F Wang, Y Duan

The goal of this project has been to digitise and use drill parameter data from rotary drilling (measurement while drilling; MWD) for a more effective planning of the blasting process in surface mines. A more accurate and reliable digital drilling and automatic hardness identification technology was developed, which includes both a hardware and software system. In the aspect of hardware, we designed a positioning, navigation and hardness perception terminal system which consists of a global positioning system original equipment manufacturer board, programmable logic controller and sensors of measuring major parameters, and a wireless data transmission system which combines digital blasting platform and wireless transmission facilities. In the aspect of software, we developed the navigation software of locating blastholes using LabVIEW and C++, and the blasthole design software using AutoCAD ActiveX (C#) technologies. This new technology is applied at Jiangxi Dexing Copper Mine, and initially realising the digitisation of drilling and hardness identification in blasting zone.CITATION:Duan, Y, Xiong, D, Yao, L, Wang, F and Xu, G, 2015. Advanced technology for setting out of blastholes and measurement while drilling, in Proceedings 11th International Symposium on Rock Fragmentation by Blasting, pp 593–598 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Aug 24, 2015

Project-finance risk assessment is mature and approached systematically. Mining industry due diligence remains fragmented with specialists addressing different aspects of the project such as reserves/JORC, environmental impact, commodity market, even financial analysis/ modelling. Each professional has a natural tendency to believe that the risk factors they have examined are the key ones necessary to be reviewed, as if their review will stand in isolation from others in the way a project materialises. By first laying out the project risk categories and risk matrices, this paper examines the due diligence process with a keen eye to the risk assessment and overlap/interaction. Some cross fertilisation of due diligence techniques from other industry sectors is placed into the mining context. Six case studies are then presented with regards to due diligence features in each projectÆs feasibility review or financial information/ offering memorandum. A power case first shows the differences between the due diligence expectations of banks versus bond financings. Then some Australian and Indonesian mining development cases are analysed to see the due diligence learning points. Some of these were project/due diligence failures. The conclusion focuses on the value of a systematic risk assessment for project evaluation, all from the project-finance perspective.

Jan 1, 2007

Total extraction is a misnomer and invariably some coal losses occur in the mining of pillars. "Fulls" pillar extraction is by the pocket and fender method, the splitting method or the open end method, with extraction in the pillar, and in rows of pillars in planned sequence, using combinations of coal and timber and roof-bolts for support. Where circumstances dictate, partial extraction is used, leaving behind portions of pillars in large blocks for support. The use of such extraction plans and supports as minimum guidelines has contributed greatly to safety in United States Coal Mines.

Jan 1, 1974

By J Player, R de Vries, E Villaescusa, R Hassell

Concerns regarding the uncertainty of stope dilution at MMG Dugald River Underground Mine led to a trial stoping program to acquire full-scale comprehensive geotechnical information and to test the validity of the geotechnical and mining parameters. Given that stope production drilling and blasting is a key parameter that influences dilution a detailed stope blast vibration project was implemented. The stoping trial consisted of 19 stopes, a combination of both sublevel open stoping and longitudinal and transverse stopes, of which five were monitored for blast vibrations.The blast vibration project utilised 20 triaxial geophones and five triaxial accelerometers installed in the hanging wall of five monitored stopes. For each stope fired, full blast waveforms were recorded along with detailed information of the blast design. Analysis of the blast vibrations used the Holmberg-Persson approach, with potential damage determined by the relative (Holmberg-Persson) K values.The project identified the benefit of parallel, breakthrough slot holes in reducing blast vibration damage. The results show that radial rings toeing or stabbing into the stope wall lead to higher vibrational energy yield which contributed to increased overbreak. Hanging wall overbreak, by comparison, with parallel, breakthrough blastholes with 1 m standoff, combined with lower density explosive charging (0.8 EP) and increased delays limited blast vibration damage. A number of ring burdens from 1.4 m to 2.5 m were monitored with no difference in terms of energy yield being identified. Changes to the stope drill and blast design are suggested to better control the large blast vibrations and recommendations from this project have been implemented into future mine design and planning.CITATION:Hassell, R, Villaescusa, E, de Vries, R and Player, J, 2015. Stope blast vibration analysis at Dugald River Underground Mine, in Proceedings 11th International Symposium on Rock Fragmentation by Blasting, pp 221–232 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Aug 24, 2015

By J J. L du Plessis

Escalating energy and electricity costs have become one of the biggest expenditure drivers in mining operations. During the last eight years, energy costs have trebled when expressed as a percentage of total running cost in South African mines. Power consumption on mines is controlled by three main strategies, namely load clipping where energy use is reduced for parts of the day, load shifting where energy use is shifted to other parts of the day, and energy efficiency where energy use is reduced permanently.To reduce the operating costs of ventilating and cooling underground mines continuously, system optimisation studies must be completed. A part of this paper describes methods that can be used to reduce energy usage by optimising cooling and ventilation systems during planning and in existing operations using network simulation models that accurately reflect the current and planned ventilation conditions. These models are used to examine various options for improving the overall ventilation and cooling efficiency. Furthermore, an overview of refrigeration and cooling practices, and ventilation strategies is presented.Ventilation-on-demand and cooling-on-demand strategies have the potential to reduce both the capital and operating costs associated with mine ventilation and cooling systems. In addition, it has been proven that modern strategies and energy-efficient tactics are technically feasible.It is also important that the correct equipment selection is made. This must be based on the best technology with the highest energy efficiency. Where equipment replacement is being considered, energy efficiency must be one of the key decision drivers.In this paper a number of studies were presented and from the experience and the data presented it is clear that a large financial benefit is possible through optimising ventilation and cooling during the planning of mines or within an existing operating mine.CITATION:du Plessis, J J L, 2015. Strategy and tactics implemented to achieve energy-efficient ventilation and cooling of mines, in Proceedings The Australian Mine Ventilation Conference, pp 9–18 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Aug 31, 2015

By J Masliyah, Z Xu

The largest oil sand deposit in the world is located in Alberta, Canada, with a total proven reserve of 2.5 trillion barrels of oil. The water based extraction process is currently being used in commercial operations to recover bitumen by flotation from oil sand ores. A recognised challenge in bitumen extraction is the reduced bitumen recovery when processing ores containing high fines content or æweatheredÆ oil sand ores, collectively known as poor processing ores. Inspired by demonstrated success of using kerosene as collectors in coal flotation, an innovative idea of using oily bubbles (air bubbles coated with a thin layer of oil or solvent) was tested to improve bitumen flotation recovery from poor processing ores. In this study, dynamic contact angle of air bubbles or oily bubbles on bitumen and induction time of bitumen-oily bubble attachment in water were measured to justify the use of oily bubbles in bitumen recovery from poor processing ores. The type of oil or solvents, including kerosene, toluene and naphtha, used in generating oily bubbles showed a marginal effect on the contact angle values, similar to the values measured with air bubbles. However, the spreading of an air bubble was found to be much slower than that of oily bubbles on a bitumen surface. The induction time for oily bubble-bitumen attachment was found to be much shorter than that for corresponding air bubble-bitumen attachment in the process water from poor processing ores. The reduced induction time for kerosene-coated air bubble-bitumen attachment correlates well with the improved bitumen recovery from poor processing ores using kerosene-coated oily bubble flotation technology.

Jan 1, 2005

By D Connelly

Quartz deposits are common in nature but the presence of quartz suitable for yielding high purity quartz is exceedingly rare. This paper describes a test work study on producing high purity quartz (HPQ) product to meet the tight product specifications of greater than 99.97per cent, including four and even five nines, for uses including, lighting and fibre optics and modern-day high efficiency solar panels. Due to demand in renewable energy applications the market for high purity silica solar panels known for their increasing efficiency and durability is rapidly growing. Demand continues to rise for the use of high purity quartz in semiconductors, electronics, imaging and sensor markets. Microelectronic components are created by chemically fabricating wafers of semiconductors such as silicon to obtain the desired transport of electrical charge and control of the current. The production of silicon metal from high purity quartz has been designated a Strategic Mineral by the European Commission and by the US Department of Justice due to the growing significance of this metal in various industries. The subsequent processing of quartz is the key step to adding value and aiding technological advances. Overall, the high purity quartz market is a highly secretive, specialised market with increasing opportunities for stakeholders. The attempt to produce HPQ in this work was carried out using an Australian quartz ore following a typical HPQ process flow sheet. The unit processes included primary crushing, scrubbing, magnetic separation, flotation, acid leaching followed by hot chlorination. The results indicate that grinding of quartz requires specialist equipment due to the hardness and extreme abrasiveness of the ore leading to additional iron contamination in the ground ore. The critical impurities were aluminium, iron, sodium, potassium, lithium, titanium, zircon, calcium and magnesium. The chemical refining was found effective. The hot chlorination process needed to be done at 1200°C in a chlorine – hydrogen chloride gas atmosphere. The risk of any gas being released during this stage is very serious and only specialist companies should and can carry out this step. More recent studies reported many new options that appear to facilitate the production of HPQ with the requisite purities more easily, but costly equipment along with highly trained operators are required and hence unlikely to become available in standard metallurgical laboratories any time soon.

Sep 1, 2024

By A Guillen, J-P ChilFs

Most 3D geological modelling tools were designed for the needs of the oil industry or detailed mine planning and are not suited to the variety of situations encountered in other application domains. Moreover, the usual modelling tools are not able to quantify the uncertainty of the geometric models generated. The potential field method was designed to build 3D geological models from data available in geology and mineral exploration, namely the geological map and a digital terrain model (DTM), structural data, borehole data and interpretations of the geologist. This method considers a geological interface as a particular isosurface of a scalar field defined in the 3D space, called a potential field. The interpolation of that field, based on cokriging, provides surfaces that honour all the data. The 3D model and its parts are always consistent with the observations. New developments allow the covariance of the potential field to be identified from the structural data. This makes it possible to associate sensible cokriging standard deviations to the potential field estimates and to express the uncertainty of the geometric model. It also, for the first time, gives a statistically optimal, geologically sound way of interpolating geology, other than directly joining the dots as you do with CAD. Practical implementation issues for producing 3D geological models are presented: how to handle faults, how to honour borehole ends, how to take relationships between several interfaces into account, how to model thin beds over many kilometres, how to optimise lithological properties and how to integrate gravimetric and magnetic data. We describe all geology surfaces and volumes using implicit functions. These are then rendered onto the required sections, plans, etc. The estimation of orebody grades and tonnes, using an unbiased and optimal geostatistical technique, makes use of the stratigraphically bound 3D geology model. An application to the geological modelling of the Hamersley iron ore district, Australia, is briefly presented.

Jan 1, 2009

By Abdulhadi H. M, Soliman S. A

The Western Desert of Egypt is a part of the arid Great Sahara of Africa which extends from the Nile Valley in the east to the Atlantic Ocean in the west. It is covered by a sedimentary sequence of Mezozoic and Teriary rocks. Recently this part of the country (about 65% of its surface area) has attracted an increasing attention in the plan of finding new agricultural and industrial communities outside the crowded Nile Valley. The plan aims to reduce the high population density in the narrow Nile Valley and to cover a considerable part of the increasing food demands in the country of Egypt. Accordingly, the groundwater exploration work in the Western Desert has been remarkably increased in the last decade, however it is still limited compared with the vast area of the Desert. The present work is a part of the groundwater exploration in the 6th October City, a new community located at 40kms west of Cairo. The rock exposures belong to a sedimentary stratigraphic section starting from the Cretaceous in the east to the Pliocene in the west. The Cretaceous outcrops due east are attributed to an almost N-S uplift and forms a thick massive limestone plateau which separates the present area from the Nile Valley. The Teriary section in the west have alternations of different porous formations such as the sands and sandstones which may acquire a good potentiality of groundwater resources. The previous groundwater exploration work included combined geological and geophysical investigations followed by exploration drilling. Although the area lies directly to the west of the Nile Valley, the discovered groundwater is saline and located at deeper reserviores compared with the groundwater in the Nile Valley. This groundwater was described as marine fossile water by the oil companies working in the Western Desert. From the geological point of view, it is believed that the Cretacious massive limestone plateau, in the east, roots to a depth which far exceeds the groundwater levels in the Nile Valley and acts as an impervious barrier between the aquifers in the Nile Valley and the area under investigations.

Jan 1, 1988

By H Mansouri, M A. Ebrahimi Farsangi, F Faramarzi

A blast fragmentation model is developed based on the gamma function to describe the run-of-mine fragmentation distribution. The model presented is aimed to benefit from simplicity in application and also to be efficient in description of the fine to coarse part of the blast rock fragments. In fact, the gamma based model (KC-KUM), gives a unit distribution curve, which is made up of two separate equations, covering different fragment sizes in the muck pile. The generated values of the fines to central part of the equation and the central to coarse part of the equation are connected at the 40 per cent grafting point. For validation, eight production blasts, which were specially designed for this purpose, were carried out at an iron ore mine (Jalal-Abad iron ore mine/Iran). Results achieved by KC-KUM and two of the most well-known models. Kuz-Ram and Swebrec, are compared with measured data. Moreover, to assess the models’ efficiency in presenting the best distribution curve, a Logarithmic error method is chosen. Based on the achievements. KC-KUM has the least deviations by the least absolute log error of 0.52, whereas the values obtained for the Swebrec and Kuz-Ram models were 0.79 and 1.34 respectively. Furthermore, the models were applied to predict X80 for 12 production blasts carried out at Gol-e-Gohar iron ore mine/Iran. These predictions were compared with the results from these trials. The coefficient of determination (R2) and root mean square error (RMSE) are calculated. Consequently, the highest R2 was obtained for KC-KUM at 0.837 with the least RMSE of 11.31. On the other hand, R2 and RMSE are equal to 0.832 and 20.19 for Swebrec; and 0.813 and 19.75 for Kuz-Ram respectively. The results obtained suggest the superiority of the KC-KUM model over the other two models in assessment and prediction of blast fragmentation in a reliable way.CITATION:Faramarzi, F, Ebrahimi Farsangi, M A and Mansouri, H, 2015. Prediction of rock fragmentation using a gamma-based blast fragmentation distribution model, in Proceedings 11th International Symposium on Rock Fragmentation by Blasting, pp 685–692 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Aug 24, 2015

The difficulty in screening of ores when the moisture contents reach certain critical levels has resulted in mining circuits using wet processes for screening. While this has been acceptable in past years it is no longer acceptable in certain countries and where the mining company is looking to reduce its impact on the surrounding environment.   Various mining companies have been looking at ways to remove the need of using water, with all its inherent drawbacks and to be able to produce its desired products in a dry process.   This would have the benefits of less environmental impact and cost, (due to not requiring wet processing equipment), less power consumption (reduced equipment needed), no corrosion issues, no contaminated waste waters and reduced maintenance activities.   Until recently this has proven elusive or with technical restrictions with current available acceptable technologies for high capacity plants.   One major mining company began seriously exploring ways to overcome this dilemma so that they did not replicate their current ways of operating for their new expansion plans.   They partnered with Metso Minerals to undertake extensive groundbreaking laboratory test work and provide potential solutions as to how to solve this. The laboratory test work proved interesting enough to design, manufacture and test on the actual mining site a prototype screening machine in the circuit.   This break through in screening technology reduces the environmental impact on the area in which the mine is located by: eliminating the storage and use of water, reduction in energy consumption due to having no pumps in the plant, reduction in plant costs and the upstream affects on the environment process used to manufacture and ship the materials the plant structure is built from, and no large tailings dams to take up land space and affect the environment through groundwater/ water table contamination and the negative effects on animals and ecosystems Laboratory test were conducted and a full size machine was installed at the plant site along side conventional machines. The screen size (2.4 m wide x 6.1 long double deck) was determined by the size of screens currently in operation at the mine. The parameters that were explored were: types of screen surface (polyurethane, woven wire etc), types of screen motion (linear, circular or elliptical), angle of motion ( 40,45,55,60 degrees etc), and throw (amplitude), speed and 'g' force.

Jun 1, 2010

The sixth international symposium on mining with fill nearly coincides with the 25th anniversary of the Jubilee Symposium on Mine Filling. That symposium, held in August 1973 in Mount Isa, in part to commemorate the 80th anniversary of The Australasian Institute of Mining and Metallurgy, was the first in what has turned out to be a very successful series of symposia. Subsequent symposia were held in Sudbury, Canada (1978), Lulea, Sweden (1983), Montreal, Canada (1989) and Johannesburg, South Africa (1993). The series of symposia has now become an established part of the mining industry calendar, and it will be interesting to see which country is given the honour, and accepts the challenge, to organise the next symposium. When the chairman of the conference organising committee invited (or was it instructed?) me to prepare and deliver one of the keynote addresses, his request was for me to be in parts historical, controversial, predictive and entertaining. The mix was left to me, with one proviso - that an early pioneer of mine filling in Australia be given due recognition. The chairman of the organising committee alone will be able to determine if I fulfilled the last objective. My success, or otherwise, with the other objectives will have to be judged by my fellow delegates. One way of predicting the future, or some small part of it, is to revisit where we have already been with a view to determining the existence of any trends that may impact on the future. An historical review of filling practice and research, with special emphasis on Australian experience in particular and worldwide in general over the previous 25 years, will enable me to address the historical and predictive objectives.

Jan 1, 1998

By Follington I. L

Various methods for stress measurement in the coal industry are briefly reviewed. Methods for presenting and interpretation of measured stress and displacement data are discussed with a view to assessing the mechanisms of failure. Arguments are presented for reviewing the current practices in this regard with reference to coal pillar and strata stability analyses.

Jan 1, 1991