Occurrence and mining of coal and sand deposits in the Middle Eocene Domengine Formation of the Mount Diablo Coalfield, California

ABSTRACT Mount Diablo Coalfield was the largest producer of coal in California from the 1860s to 1906. The now-depleted coalfield is located on the northeast limb of the Mount Diablo anticline. The mineable coal seams occur in the Middle Eocene Domengine Formation, which is predominantly composed of quartz-rich sandstone with several thin coal seams. As many as 26 mine operations were established to mine the coal, and it has been estimated that the total production exceeded 4 million tons. The coal fueled the industrial growth of the major cities of northern California. The mines closed at the turn of the nineteenth century as competition from better coals from Washington Territory and overseas entered the market. After coal mining was abandoned, sand operations were established in the early and mid-twentieth century to mine the silica-rich sandstone. The extraction methods used for sand were underground room-and-pillar mining and surface open-pit mining. The high-quality sand was used widely in the production of pottery and glass, and in foundries. Previous studies have interpreted the environment of deposition of these quartz-rich sandstone and coal deposits as barrier island with tidal channels or delta, tidal shelf, and marsh complexes along a north-south–trending shoreline. However, the excellent exposures in the sand mines display abundant evidence for their deposition in a fluvial/estuarine system. Their regional distribution indicates that they were deposited in a northeast-southwest–trending incised-valley system formed by fluvial incision during a lowstand. The incised valley was filled with fluvial and estuarine deposits made up of quartz-rich sand brought in by streams that flowed westward from the Sierra Nevada.

Application of Hydraulic Backfill for Rockburst Prevention in the Mining Field with Remnant in the Polish Underground Copper Mines

In the polish underground copper mines owned by KGHM Polska Miedz S.A, various types of room and pillar mining systems are used, mainly with roof deflection, but also with dry and hydraulic backfill. One of the basic problems associated with the exploitation of copper deposits is rockburst hazard. Aa high level of rockburst hazard is caused by mining the ore at great depth in difficult geological and mining conditions, among others, in the vicinity of remnants. The main goal of this study is to investigate how hydraulic backfill improves the geomechanical situation in the mining filed and reduce rockburst risk in the vicinity of remnants. Numerical modeling was conducted for the case study of a mining field where undisturbed ore remnant, 40 m in width, was left behind. To compare the results, simulations were performed for a room and pillar mining system with roof deflection and for a room and pillar mining system with hydraulic backfill. Results of numerical analysis demonstrate that hydraulic backfill can limit rock mass deformation and disintegration in the mining field where remnants have been left. It may also reduce stress concentration inside or in the vicinity of a remnant, increase its stability, as well as prevent and reduce seismic and rockburst hazards. Hydraulic backfill as a local support stabilizes the geomechanical situation in the mining field.

DETERMINATION OF DEFORMATION PROPERTIES AND NATURAL STRESSES IN ROCK MASS BY UNDERGROUND GEODESY DATA

The authors have developed and implemented a 3D geomechanical model using the finite element method for a typical configuration of an underground space during room-and-pillar mining. The authors formulate and solve an inverse problem on determination of values and orientation of external horizontal stresses and deformation characteristics of structural elements of the geotechnology by the measurement data of sidewall convergence in rooms in the course of mining. The level curves of different objective functions are analyzed, the mixed inverse problem resolvability is demonstrated, and the equivalence domain size is correlated with the relative error of input data.

EVALUATION OF THE STABILITY OF A TEMPORARY PILLAR WHEN MIINING ORE HORIZONS WITH BACKFILLING AND CAVING

A method is proposed for mining thick and flat ore deposits at great depths in conditions of a decline in the value of extracted mineral raw materials. It is found that safe mining with solidifying backfill and caving is achieved by determining the parameters of stable spans of rooms, in place of which artificial supports and temporary ore pillars are formed. These pillars are recovered with a lag behind room-and-pillar mining by caving of ore and enclosing rocks. It is shown that, depending on the type of geomechanical model of geomedium and orientation of the initial natural stresses acting in the rock mass relative to the mining front, the field of application of the mining system is limited by the depth and parameters of excavation. Predictive areas of possible rock failure are determined applicably to rock masses with different degree of disturbance.

Similar Physical Modeling of Roof Stress and Subsidence in Room and Pillar Mining of a Gently Inclined Medium-Thick Phosphate Rock

Gently inclined medium-thick orebodies are generally recognized as the most difficult type of orebody to mine, using current available strategies (i.e., the room and pillar method). In the present study, a similar physical model was used to investigate the roof stress and subsidence for mining gently inclined medium-thick phosphate rock from the Jinning Phosphate Mine, Yunnan Province, China. The stress field, displacement field, and roof failure evolution characteristics of the surrounding rock with stope structures of 3 m, 5 m, or 8 m ore pillars were considered. The results showed that, after mining stopped, obvious pressure relief areas formed above the three stope structures, and pressure-bearing areas formed at the front of the roof. With extending the mining in the working face, the stress relief boundary also gradually increased, and the top of the roof tended to sink with a maximum subsidence of –14.58 mm, –4.67 mm, and –3.48 mm. Due to the mining activity, the overlying strata bent and subsided from top to bottom, creating bending subsidence, fracture, and caving zones.

Laboratory Determination of Coal Dust Cleaning Efficacy of a Fibrous Filter for Flooded-Bed Dust Scrubber

Fibrous-type dust filters are used in flooded-bed dust scrubbers to capture dust from underground room and pillar mining atmospheres. They have 10–30 layers of finely woven strands that trap particles through the interception and impaction process. A full-cone water spray is installed upstream of the filter, which floods the screen. A scrubber’s efficacy is usually measured and reported in terms of reduction in gravimetric dust concentration at a known location in a mine. This paper reports the particle-size-dependent dust removal efficiency of a fibrous filter obtained from an instrumented test-set up. A variable frequency drive and an inline flow control knob were used to control the airflow through the filter and water flow onto the filter. Optical particle counting of coal dust particles upstream and downstream was carried out to determine the cleaning efficacy. Experiments showed an increase in cleaning efficiency for all the dust particles with their size. A progressive decrease in dust concentration downstream of the filter with an increase in water flow through the nozzle affirmed the capture of dust particles by water sprays.

Abandoned mine after-use as a museum and research site

<p><strong>Abstract. </strong>Abandoned mines are unique underground facilities due to their unique conditions. Mined-out mine can be used as a museum or scientific and technical research site. Depending on specific mine conditions the after-use purposes can vary. The former Kohtla oil-shale mine, located in the eastern part of Estonia, was closed 10 years before the idea to re-open it as a mining museum. Now old Kohtla mine is used as an underground museum to present for local people and tourists how mining works were carried out in the past (Estonia has 100-year-old experience in oil shale mining) and which methods are still in use. Besides mining, it also shows ventilation and water barrier solutions in the mine. </p><p> </p><p>We present an overview of abandoned mine new challenges to be a safe environment for tourists and as a future research center. The project team had a challenge do design: </p><ul><li>Renovate underground railway, walker’s platform’s, design and establish new roof supports </li> <li>New railway platform, stations for tourists, water barriers (3)</li> <li>Ventilation duct and walls (8), new ventilator;</li> <li>Closing the workings which are not needed.</li> </ul><p>Designing was challenging but not the most difficult part of the project. More complicated was to find a competent builder for the underground museum. Renovated and re-ventilated mine was opened in 2012 and today museum is one of the most visited places by the tourists in the eastern part of Estonia, because of its uniqueness. </p><p> </p><p>Besides as a museum, it can be used as a testing site for researchers, because its former infrastructure and facilities have remained. For example, 4 years ago Tallinn University of Technology used the museum area for the backfilling testing because the temperature and other underground conditions were suitable for room-and-pillar mining method backfilling tests. By using backfilling technology, environmental problems such as ground collapses can be avoided and production residues can be reused. As a result of the research, it became clear which ashes of Estonian power plants and the oil industry are suitable for the backfilling technology in terms of physical-mechanical and chemical properties.</p><p> </p><p>In addition, we will highlight the best design practices and experiences that have implemented in order to improve old mine everyday working conditions as a museum. These best practices are usually more than the national laws and regulations have requested and they are deeply connected with practical experiences. It meant a lot of collaboration in bringing the best know-how together by different stakeholders. </p>

Classification of protective pillars toward higher safety and innovation in room-and-pillar coal mining

During room-and-pillar mining, protective pillars of coal are usually left standing. The protective pillars differ in shape, size, process properties and extractability ratios of coal. The authors propose a classification of the protective pillars with respect to their structure and shape in order to estimate and predict their properties. The extractability ratios of coal are calculated for the initial rib pillars and panels at different final shapes of protective pillars. The protective pillars are ranked with respects to the coal extractability ratios. Such systematization of pillars enables selecting the best engineering solutions and innovative improvement of room-and-pillar mining in specific geological conditions toward enhanced safety and efficiency of underground coal production. The article is prepared in the framework of the Competitiveness Enhancement Program of the Tomsk Polytechnic University.