DUMP DESIGN A waste dump is an area in which a surface mining operation can dispose of low grade and/or barren material that has to be removed from the pit to expose higher grade material. In some instances, material has to be removed for other indirect reasons, such as pit wall stabilization and for haul road construction. The first step in designing a dump is the selectionof a site or sites that will be suitable to handle the volume of waste rock to be removed during the mines life. Site selection will depend on a number of factors, the most important of which are: 1. Pit location and size through time. 2. Topography. 3. Waste rock volumes by time and source. 4. Property boundaries. 5. Existing drainage routes. 6. Reclamation requirements. 7. Foundation conditions. 8. Material handling equipment. All of these parameters will be considered during the site selection process. Once a site or number of alternative locations have been selected, the designing of the dumps can commence, using the same points utilized in defining the best potential dump locations. The objective of dump planning is to design a series of waste disposal phases that will minimize the horizontal and vertical distances between the source and the disposal area. Since material handling costs are usually the largest single component of the mining cost, well designed dumps play a very important andcritical role, affecting the expense of the total operation. The pit mining sequence and production schedule will be completed prior to dump design with the objective of maximizing the return on the investment. Therefore, two of the most important parameters affecting dump designhave been set before any design effort commences: the pit location and size through time and the waste production schedule and source location. These two parameters define where the dumps can start, how fast they will advance, and the ultimate volumethat the disposal area must contain. The location where dumping can commence may not necessarily be outside of the pit limits. In some instances, internal dumping may be the most economical and practical method of establishing haul roads to the disposal area or to later pit phases. Also, as an alternative, it may be wiser to dump short and rehandle the material in the future if the 664economic benefits of this can be demonstrated. This can affect the pit design in the sense that later phases adjacent to the dumps couldhave higher ratios than the original design. Therefore, these areas should be examined in more detail and the haulage cost savings gained by dumping short compared to the potential ore reserve loss. The pit mining sequence will define the rate and source of the waste rock. Generally, waste material from upper areas should be hauled todumps located at higher elevations and lower waste dumped in lower locations. This is common sense if haulage costs are to be minimized. Although this is the ideal objective, topography, property boundaries, drainage routes, dump stability, environmental considerations, and other constraints may make this objective difficult or impossible to achieve. Topography will limit the available areas and usually defines the type or shape of the waste dump. More common dump configurations are valley fills (complete or partial), hillside wedges, fan and terraced dumps, and combinations of these. If the pit mining sequence permits backfilling one area that has beendepleted while another adjacent area is still active, then this alternative can be preferable instead of extending dumps over virgin areas, depending on haulage and reclamation costs. Dump areas can also be limited by existing drainageroutes and property boundaries. In both of these cases, an economic comparison should be completed to weigh the relative cost and potential savings that would result from removing the constraint, e.g., drainage diversion or property purchase. Before commencing a dump design, two additional parameters must be determined. The material swell factor and angle of repose are very important factors in determining the dump volume needed and the overall dump slopes. In situ material, when mined, will swell from 10 to 60%, depending on the type of material and fracture frequency. In hard rock operations, the swell factor is commonly between 30 to 45%, meaning that one in situ unit will swell to a volume of 1.30 to 1.45 units. Loose density tests should be performed to determine the anticipated swell. These figures will first be used to size loading equipment buckets and haul truck box sizes. The second use is to quantify the volume of dumpingroom that will be necessary to dispose of the material from the mine. Loose material will compact to some degree after placement on the dump. This will depend on the type of material, size distribution, moisture content, disposal method, and the height of the dump. Common compaction numbers will range from 5 to 15%. Crushed and conveyed waste will not have a compaction factor as great as that of waste placed in low lifts by 154-t (170-st) haul trucks. A second parameter that must be determined is the angle of repose of the loose dump material. Dry run-of-mine rock will usually stand between 34 to 37. The lower the dump height, the more rapid the advancement and themore irregular the rock pieces, the higher the angle. For design purposes, a conservative slope of 1.5:1 (34) is recommended in order to safely project the anticipated toe position. Measurements of existing talus slopes will also give a good indication of the expected long-term dump face angle. 665The dump configuration will also be affected by the haulage method and by stability and reclamation considerations. The three methods of material handling in order of use are: truck, conveyor, and rail. Truck haulage is used in more instances because of its flexibility and lower capital cost. In particular cases, conveyors are more economical to use for waste disposal with their lower operating costs and where large tonnages have to be transported over either large horizontal or vertical distances. As in-pit mobile crusher development progresses, the tonnages handled by conveying systems will increase substantially. Rail haulage is in use at only a fewof the older surface mining operations and is not considered as an alternative for many future operations. Stability considerations will affect the design of the dumps either by lowering the ultimate height or reducing the overall slope. The slope can be reduced either by building the dump in lifts or by dozing. Sometimes,a combination of these two methods is necessary for reclamation purposes. The intermediate phases of a dump may vary to a large extent from the planned final dump configuration. For stability reasons, lower lifts or toe dumps may have to be established during the earlier stages. As the mine life progresses, additional lifts can be placed above the lower dumps, subject to future design criteria. By this, it is meant that the berm left on a lower dump must have a design width to facilitate future reclamation, overall slope reduction for reasons of stability, or leaving sufficient width for an access haul road to a future disposal area. Mining operations are conducted in many different topographic and climatic conditions. These conditions will require changing the techniques used to safely start and maintain a dump. A high wedge-type dump may be safe in a dryclimate if it progresses over a rocky and competent base. The same dump would most likely fail if it progresses over wet hillside soils or permafrost. For this reason, geotechnical studies are very important in predicting the stability of both intermediate and final dump phases. Pertaining to the same situation, dump stability monitoring is also very important in cases where failure has a high probability. The degree of monitoring will depend on the consequences and risk of failure. Continual monitoring will reduce the risk of injuryand equipment damage. Failures are acceptable if this risk can be minimized and the failure will not affect downstream facilities, equipment, and personnel. Some northern operations even use failures as a method of material transport and reclamation, sincethe failures shorten haul distances and lower the overall dump slope to facilitate reclamation. Particular emphasis should be placed on drainage in designing both intermediate and final dump phases. Dumps constructed using haul trucks have nearly an impermeable surface so that rainfall or melting snow will pond on the top of the dump or cascade over the face if care is not exercised in the dump design and construction. Dumps should, therefore, be built at a slightly adverse gradient for three reasons: 1. Carry runoff away from the crest. 2. A positive gradient means that haul trucks will have to power back to the dump crest rather than rolling back. As a safety feature, this will also reduce the chance of parked equipment accidentally rolling toward and over the crest. 6663. Most mining operations set a speed limit below what a loaded haul truck is capable of achieving. For this reason, a 1 to 2% uphill gradient will not slow haulage, but will increase dump capacity and shorten haul distances. Waste dumps that progress over or fill up drainage routes must also have special design considerations. If run-of-mine rock is end-dumped from the tip head, then given sufficient dump height, gravity will segregate the larger and smaller fragments. The larger material will roll to the bottom of the dump and will normally form a very permeable base. The finer material gathering in theupper portions of the dump will tend to form a nearly impermeable surface, especially with heavy haulage traffic. Waste dumps built with this natural segregation are free draining and offer little chance of saturation unless the base material weathers rapidly and will have reducing permeabilities through time. The high base permeability will allow the dumps to progress over small drainage routes and not block the flow. For larger streams, the shifting stresses placed on the base of the dump as it advances can jeopardize any drainage structure, such as a culvert. Therefore, adiversion tunnel is preferable where long-term drainage is critical. In the case where dump failures occur, a number of corrective procedures can be implemented. These may be as simple as rerouting surface drainage or slowing the rate of advance, or as expensive as modifying the profile and design of the dump. One of the most common methods of stabilizing dump failures and allowing use of the dump to continue is to place more material at or on the toeof the failure. If haulage access to the toe of the dump is not feasible due to elevation differences, then dozers may have to be employed to push material downslope onto the toe. This may be helpful if reclamation regulations require a 2:1 slope for topsoil and revegetation placement. As a new dump is started in a virgin area, small failures can be anticipated if the dump commences as a wedge type on relatively steep terrain. For this reason, it is better to advance a new dump slowly and not count on all the waste being disposed of at one tip point. In order to enhance the stability of the initial dump, lower benches may have to be notched into the hillside to key the base of the dump into the slope. It may also be necessary to clear off vegetation, such as trees and brush, and, in some instances, to remove soils and other unconsolidated materials that would not provide a stable dump base if the risk or magnitude of failure was unacceptable. Another component of dump design deals with operating considerations. If a side hill or contour dump is under construction and a tracked dozer is assigned to the dump, then the tracked dozer can be used to establish a pioneer road in advance of the dump. This road, established at a slightly lower elevation than the dump crest, can be used to collect drainage, act as a level control, give additional dump width, and serve such purposes as a small vehicle and lighting plant parking site. Care must be exercised so the cutting of the pioneer road does not undercut the hillside. Access to the dumps should be aligned to provide good visibility of the congested area around the dump head. In many instances, the accessroad will have to be wider than normal to allow it to be used for other purposes, such as a park-up area for mine equipment at the end of a shift, a pullout area forfueling, a truck weighing station, and 667for dump lighting. A general rule of thumb is that haul roads should normally be five times the width of the trucks using them. This width would include ditches and berms and allow sufficient room for road maintenance vehicles to work safely while trucks are using the road. Preferably, graders may be able to blade the roads while the haul trucks are using another route, but this is not always possible. A permanent lighting system can be installed along the route because of the relative long life of most dump access roads and for safety reasons. The dump width at the tip head should be sufficient to allow for a moderately sized turning circle of the haul trucks. For large trucks, this should be between 61 to 91 m (200 to 300 ft). The length of the active dumping face depends on the number of truck fleets hauling to the area. Commonly, a distance of30 m (100 ft) should be allowed per loading units truck fleet operating to that tip head. High berms should always be maintained along the total dump crest length, except at the tip head where the berm height should be equal to the radius of a haul trucks tire. A tracked dozer at the dumping point is preferred over a rubber-tired dozer for a number of reasons, including: 1. Greater traction that allows it to push more material when the ground is wet or icy. 2. The tracks crush larger rock fragments, thereby reducing truck tire damage that occurs at the dumping area. 3. A tracked dozer with a winch can free stuck equipment readily. 4. A tracked dozer can push material farther over the bank in safety, since traction is spread over a larger area and not at just four points. In most instances, dump material will not have the same supportive strength as the same material in situ, especially in wet climates. Rolling resistances may increase with traffic to a point of impassibility. Additional thinner lifts of more competent rock may have to be placed on the dump surface to maintain haul roads. The operating differences between intermediate and final dump configurations can be quite large. For example, the material handling methods may change from truck haulage to crushing and conveying as distances increase. Since the prime objective of dump design is economics, the initial dump should have the shortest haul distance. As the mine progresses, haul distances will become longer and vertical haulage more excessive. Reducing the rate of future material handling cost increases using wellthought-out alternative methods and designs is the objective of good dump planning. Accomplishing this task may mean leaving lower routes open as a future access to potential dump areas if the future discounted savings balances todays cost sacrifice. In rugged terrain, this may mean that lower lifts at the base of a high dump will have to be established early, since later access may be impossible or too costly to construct. Several dumping points of various haulage distances should be available on a daily basis so that when the operation is short of trucks, closer dump points can be utilized to maintain production and when truck availabilities are high, longer hauls can be used. Climatic conditions coupled with mine locations outside of the United States also will have some bearing on the dump design and operation. Less stringent safety and 668environmental regulations will allow more economic mine operations. Politics may also intervene and demand short-term economic savings that will be costly for the operation in the long run. In comparisons of one design to another, a method should be used that first establishes a base case. Then other designs can be completed and the economic and other advantages and disadvantages weighed. If a choice exists as to the elevation of the dump,then the preferable order of haulage gradients is level, downhill, and uphill last. If haulage costs are equated to level, -8% downhill and +8% uphill, then the cost differential for a unit of distance is approximately 1.0, 1.46, and 2.38 for a 154-t (170-st) haul truck. This means that waste dumps should be designed level from the start point and only after the dump has progressed a certain horizontal distance will an upper lift become more economic. As an example, only when the horizontal haul distance exceeds 457 m (1,500 ft) will it be more economical to lift the material 15 m (50 ft) and start another lift closer to the pit (see Fig. 5.6.1). If a dump were mistakenly designed so that all the volume was dumped from an elevation 15 m (50 ft) higher than necessary, and if 90.7 Mt (100 million st) could have been dumped at the lower elevation first, then the direct cost increase would be approximately $0.019/t ($0.017 per st) or a total of $1,700,000. Additional capital and replacement costs would also be incurred due tothe increase in the number of haul units required. Figure 5.6.1.Therefore, it is very important to recognize the best economic dumping plan and material handling method and to weigh the cost and effect of constraints such as stability, reclamation, drainage, and property boundaries. Waste dump planning is usually not as critical or as detailed as mine design. This is due to the fact that the mine is the source of the ore and revenues. However, good waste dump design can be critical in minimizing costs and increasing the value of the ore produced. Improper waste dump planning can mean thedifference between profit and loss and often should receive more attention and detail. STABILITY OF MINE WASTE DUMPS The overall stability of mine waste dumps is dependent on a number of factors such as: 1. Topography of the dump site. 2. Method of construction. 3. Geotechnical parameters of the mine waste. 4. Geotechnical parameters of the foundation materials. 5. External forces acting on the dump. 6. Rate of advance of the dump face. All of these factors combine in various ways duringthe life of a mine waste dump to aid in the stability of the dump or to contribute to its instability. The various technical analyses used to assess the stability of dumps are well documented in literature 669(Winterkorn and Fang, 1975; Anon., 1977) and will not be covered in this section. The factors affecting stability mentioned earlier will instead be discussed. The choice of dump sites and their topography usually is limited to within an economic distance from the mine and, since rearranging of site topography is rare, the topography usually becomes a fixed condition. The crucial aspect of topography is the existing slope of the natural ground upon which the dump is to be constructed. Analyses show that factors of safety begin to drop significantly above a ground surface inclination of 20, regardless of the strength parameters of either the waste or foundation material. Mine waste dumps are usually constructed by one of two common methods: in lifts or layers or by end-dumping. End-dumping is a controlled failure process where the waste material is deposited forming a slope at or close to its angle of repose and the factor of safety is accordingly close to one. Since the frontface is always advancing during the life of the dump, the slopes are not stabilized by flattening with conventional earthmoving equipment until closure of the dump. Monitoring of the livedump face is recommended to anticipate and deal with slope failures. The mine waste dumps constructed using an end-dumping technique are sometimes referred to as built from the top, whereas, layered dumps are said to be constructed from the bottom up. Layered dumps can be controlled, which adds significantly to their overall stability; however, they require a relatively gently sloping topography and usually entail a longer haul distance in the early years of the mine. Layered dumps are preferred where weak foundation conditions exist, since the load application can be controlled to allow for strength gains by consolidation and pore pressure dissipation. The geotechnical properties of the mine waste and the foundation material are major factors in determining the overall stability of a mine waste dump. Such characteristics as strength, friction angle or cohesion, and gradation are parameters determining the type of analyses that would be selected to solve ordefine the stability condition. Each mine waste dump site presents a unique set of problems and would have to be analyzed as a separate and distinct case; however, certain general conclusions may be drawn based on some simplifying assumptions. For instance, coarse frictional material on a competent foundation with a slope angle less than or equal to the materials angle of repose may be dumped to practically unlimited heights and would represent the safe sideof the problem. Cohesive wastes on a weak foundation that could fail either in the waste material itself or through the foundation by a number of mechanisms would represent the unsafe, unstable side of the problem. Between these extremes exist a large number of combinations that have different failure modes and, therefore, must be carefully analyzed. The most commonly occurring combinations are coarse frictional materials on weak shallow foundations or weak foundations extending to considerable depth. The failure modes associated with these are horizontal translation of the waste, deep seated rotational shear failure through the foundation, or a combination of the two. For cohesive wastes on shallow or deep, weak foundations, the failure modes become difficult to distinguish and the type of analyses used becomes a matter of experience and judgment. It should be remembered that regardless of the sophisticated analysis and the capacity of computers to produce numbers to severaldecimal places, the reliability of the calculated factors of safety is dependent primarily on the degree to which the input 670parameters or assumptions made are representative of the actual conditions existing in the waste material and of the foundation of the dump. This is where an engineers experience is vital to assess the stability of a mine waste dump by determining whether the assumptions and choice of analyses are reasonable. External forces, such as water and earthquakes, often play a decisive role in the stability of a mine waste dump and, therefore, should be carefully considered in the analysis. Of the two, earthquake or seismic forces are a relatively straightforward factor that can be readily accommodated in most stability analyses by determining the location of the mine waste dump in relation to seismic zones and inputting the proper seismic coefficient into the analyses as an additional horizontal force. The effect of water on the stability of mine waste dumps is more difficult to evaluate and, as a general rule of thumb, measures should be taken to prevent water from entering the dump. Water pressure buildup in a dumpwill always lower the factor of safety and, therefore, should be prevented, if possible. It is not always possible to avoid building mine waste dumps across drainage courses and, therefore, provisions should be made for unimpeded passage of flows either around the dump by ditching to divert surface waters or byproviding a coarse, filter-protected drainage layer beneath the dump. The top surface ofthe dump should be sloped away from the leading edge of the dump face to eliminateponding during periods of rainfall and snowmelt. In some instances, it may be necessary to include inclined drainage layers at the perimeter of the dump to maintain drained conditions. In summary, the stability of mine waste dumps is dependent on a combination of factors and, although sophisticated numerical analyses are available to solve stability problems, practical experience and engineering judgment are still the dominant ingredients needed to arrive at an economical, practical, and safe solution.