Our experience with mining and mine waste management includes the following:
- DESIGN OF SLURRY IMPOUNDMENTS AND OTHER MINE
WASTE DISPOSAL FACILITIES
- CLOSURE OF SLURRY IMPOUNDMENTS
- RISK ASSESSMENT FOR TAILINGS IMPOUNDMENTS
- PREPARATION OF MINE LAND RECLAMATION PLANS
- EVALUATION OF MINE SUBSIDENCE POTENTIAL/EFFECTS
- DESIGN OF MINE INFRASTRUCTURE FACILITIES AND FOUNDATIONS
- EVALUATION OF BREAKTHROUGH POTENTIAL
- DESIGN OF DEEPLY BURIED PIPES
- MINE DRAINAGE ABATEMENT/TREATMENT
- PEER REVIEW OF DESIGNS
Brief descriptions of our capabilities in the areas listed above are provided in the following paragraphs. More detailed discussions of individual topics can be accessed by clicking on the links provided.
Design of Slurry Impoundments and Other Mine Waste Disposal Facilities
When coal or metal-containing ores are mined, the volume of material mined typically is much greater that the actual amount recovered. Various processes are then employed to separate the desired material from the material that was directly mined. In the case of coal, the processes will typically include crushing, screening and washing with the result that a large amount of residual material known as tailings is obtained. Disposing of tailings is a major component of mining. Generally, the coarse tailings are used to construct embankments that create impoundments into which the fine materials are pumped in the form of slurry. Frequently, tailings impoundments are placed in existing valleys such that existing hillsides form most of the impounding structure. The impoundment is then constructed in stages. The construction of such impoundments requires substantial engineering from both structural stability and environmental perspectives. D'Appolonia has been designing and monitoring the construction of such facilities for more than 30 years. We are experts in all aspects of staged tailings impoundment design and construction and we routinely handle all of the associated state and federal permitting issues. Our experience includes not only coal, but also engineering of facilities for the mining of limestone, silica, copper, silver, gold and uranium.
Closure of Slurry Impoundments
Closure of coal refuse slurry impoundments is accomplished by pushing out coarse coal refuse (CCR) from the sides of the slurry impoundment over the surface of the fine coal refuse (FCR). However, this activity can lead to substantial heaving of the slurry as the CCR is pushed out. The actual magnitude of the heaving is difficult to predict. Dealing with this situation may require substantial modifications to site drainage and installation of additional pumping capacity and the construction of berms to meet permit requirements relative to flooding. D'Appolonia is experienced in addressing the technical problems that may arise during slurry impoundment closure and designing measures to accommodate the heaving in the closure plan.
Risk Assessment for Tailings Impoundments
Increasingly, the construction or enlargement of a mine tailings impoundment requires a risk assessment study to determine if such construction or enlargement can be performed with acceptable levels of risk to the population in the vicinity of the facility that might be affected by a catastrophic failure. A risk analysis typically involves: (1) identification of failure modes that are potentially significant from a risk perspective, (2) assessment of the likelihood of occurrence and consequences of potentially significant failure modes, (3) consideration of potential mitigating factors, and (4) performance of a Failure Mode, Effects and Critically Analysis (FMECA).
Preparation of Mine Land Reclamation Plans
Old mine sites are frequently reclaimed so that the land can be restored to its approximate original condition or so that the site can be put back in use for other purposes. D’Appolonia provides complete engineering and design services related to mine land reclamation. A typical scope of work to reclaim a mine site might include a comprehensive site reconnaissance, hydrology and hydraulics analysis to determine potential runoff and to design surface drainage, development of erosion and sedimentation control plans, slope stability analyses, development of a site grading plan based on balanced cut and fill, off-site disposal of trash, and preparation of plans and specifications for construction. D’Appolonia is also experienced in the application of bio-solids to coal refuse to provide a growth medium for revegetation and thereby economize on soil fill requirements. We typically work with our clients to prepare bid packages, develop a list of qualified bidders, and evaluate the bids received. Also, our services typically include monitoring of construction and documentation of field construction activities.
Evaluation of Mine Subsidence Potential/Effects
Subsidence due to underground mining is a frequent occurrence in areas of the country where mining is actively occurring or where mining was performed in the past. Longwall mining, in particular, results in relatively rapid surface displacements, often over an area greater than the width mined. The extraction of coal or other mineral removes support from the overlying strata causing them to sag or subside into the mine void. The subsidence propagates upward to the surface. The surface position of the boundary between areas of subsidence and no subsidence is defined by the "angle of draw." This is the angle between a vertical line drawn upward to the surface from the edge of the underground opening (mine void) and a line drawn from the edge of the opening to the point of zero surface subsidence. D’Appolonia has extensive experience related to locating abandoned underground mines, monitoring surface movements, and predicting future subsidence effects in terms of vertical and lateral extent based upon computer modeling and empirical data. In several instances we have developed designs to mitigate the effects of mine subsidence on surface structures.
Design of Mine Infrastructure Facilities and Foundations
Mining operations typically have substantial infrastructure related to the extraction of the coal or ore, processing of the mined material, separating rock and other undesirable material from the raw coal or ore, disposing of or containing waste materials, and transporting the mined material from the site. Facilities required may include mill buildings, crusher buildings, washing facilities, conveyors, bins and silos, platforms, pump stations, piping and pipe supports, culverts, sediment control ponds, surface drainage, site access and haul roads, and office buildings. D’Appolonia is experienced in the design of slurry and refuse impoundments, all aspects of site drainage and storm water management, and the design of access and haul roads. Our experience also includes design of foundations for static and dynamic loading associated with mine equipment and structures. We routinely develop subsurface exploration and laboratory testing programs related to selection and design of cost-effective foundations, and we prepare plans and specifications for our designs and monitor construction.
Evaluation of Breakthrough Potential
The breakthrough of impoundments into active or abandoned sections of coal mines can result in the flow of large volumes of water and/or coal slurry through the mine and subsequently to surface drainage courses. The resulting contamination and environmental degradation can have a major economic impact related to cleanup and remediation activities. Breakthrough can be the result of an insufficient barrier between the slurry impoundment and the mine or a natural seam with relatively high permeability that erodes over time and eventually provides a pathway to the mine. Prevention of breakthroughs requires accurate information as to the location of mine shafts relative to the subject slurry impoundment and knowledge of the strength and permeability characteristics of the material lying between the slurry impoundment and the mine. D’Appolonia is experienced in the characterization of subsurface conditions using geophysical methods and conventional site drilling. We are also experienced in developing finite element models that can be used to assess potential seepage rates, pressures on barrier materials, and the potential for failure. We also has the technical expertise and experience to design and implement remedial measures to greatly reduce the probablility of breakthrough.
Design of Deeply Buried Pipes
D’Appolonia has advanced the use of HDPE pipe in deep burial applications through the use of flowable backfill, which preserves the advantages of flexible pipe behavior while improving installation reliability, seepage control, and conduit-backfill interaction. To alleviate problems associated with traditional backfill methods, we have increasingly specified flowable backfill in designs for conduit construction. A combination of fine-aggregate, cement and pozzolan, flowable backfill provides reliable haunch support and pipe encapsulation with low permeability and shrinkage characteristics. The controlled amount of shrinkage provides seepage protection equivalent to cast-in-place concrete construction, but at a much lower cost. Using flowable backfill is comparable to traditional trench backfill methods, but provides increased strength and uniform pipe support with tailored strengths, while also providing a high degree of seepage control. Regulatory agencies are increasingly requiring pipe designs that include seepage control in excess of that feasible with traditional backfill methods. D’Appolonia is presently accumulating data on the performance of deeply buried plastic pipes during the incremental raising of dam embankments, which will eventually result in fill depths of more than 300 feet over buried pipes at some of the monitored installations.
Mine Drainage Abatement/Treatment
During the time a mine is in active operation, dewatering and treatment is typically performed such that extraction of coal or minerals can be performed. This process must be continued after the mining is terminated. Unfortunately for many mines that were abandoned in the past, dewatering activities ceased and mine drainage then appeared as surface seeps and subsequently found its way to nearby streams. Often this water is extremely acidic, resulting in degradation of the stream and death to aquatic species. D'Appolonia has been called upon to develop schemes to control and treat acid mine drainage (AMD). We have designed pumping systems to reduce pressure head in the abandoned mine works, and we have developed and implemented schemes for grouting porous strata where seeps emanate. We are also experienced in designing systems to collect and treat AMD such that discharges to streams meet applicable criteria.
Peer Review of Designs
D'Appolonia has been retained by mine operators to perform peer reviews of mine facility designs prepared by others and to facilitate the permitting process. These reviews have covered all technical aspects of mining facility design, including embankment design, dam seepage control, hydrology/hydraulics analysis and spillway design, site grading, storm water management plans, foundation design, and retaining structure design. Our design reviews have identified significant technical deficiencies and opportunities for value-engineering of major features of the planned facilities. Our value-engineered designs have resulted in construction cost savings ranging from several hundred thousand dollars to more than $10 million on individual projects, with additional benefits accruing during long-term operation of the facilities.
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