Waste Dump Design: Geometry, Capacity and Checks

Every tonne of ore comes with waste: the overburden stripped to reach the orebody and the barren rock mined with it. A waste dump, also called a waste rock dump, an overburden dump or, in coal and quarrying, a spoil dump, is where that material is placed for good.

The design has to satisfy four things at once. It must hold all the waste the pit will produce over its life. It must sit close to the pit exit, because every extra metre of haul is paid for on every load. It must stand on ground that can carry it without sliding or blocking drainage. And it must end up in a shape that can be closed and reclaimed.

An external dump is built outside the pit limits on original ground. An in-pit dump backfills a mined-out part of the pit, which shortens the haul but is only possible once that part of the pit is finished and nothing of value lies below it.

Dumped rock does not hold the angle you draw; it comes to rest at its angle of repose. For dry, coarse, angular run-of-mine rock that is typically somewhere between 35° and 40°, and 37° is a common planning figure until the site has measured its own. Fine or wet material stands flatter.

A lift of height h with a face at angle α takes h / tan α of horizontal ground. At 37°, a 10 m lift runs 13.3 m. Stack n lifts with a berm of width b between each pair and the horizontal distance from the toe to the top crest is n·h / tan α + (n − 1)·b. The overall slope angle β follows from the total height over that distance: tan β = n·h / (n·h / tan α + (n − 1)·b).

The face angle is set by the material, so the berm is the only lever the designer has on the overall angle. Four 10 m lifts at 37° with 10 m berms give an overall angle of 25.7°. Widen the berms to 20 m and it drops to 19.5°.

That matters at closure. A face at the angle of repose is too steep to hold topsoil or to revegetate, and closure plans commonly call for final slopes in the range of 2H:1V (26.6°) to 3H:1V (18.4°); the figure comes from the regulator and the closure design, not from the rock. A dump built in lifts, with berms sized for that final angle, can be regraded largely within its own footprint by dozing each crest down onto the berm below.

Take a toe outline of 400 m by 300 m on level ground, four 10 m lifts, a 37° face and 10 m berms. These are the numbers the calculator above opens with.

The distance from toe to top crest is 83.1 m on every side, so the top platform that is left measures 233.8 m by 133.8 m and the overall slope angle is 25.7°. The dump holds 2.86 million m³: 1.11 million in the bottom lift, then 0.82, 0.57 and 0.36 million in the three above it. The bottom lift alone carries 39 % of the capacity.

Widen the berms to 20 m to reach a flatter final slope and the same outline holds 2.37 million m³, 17 % less, on a top platform of 173.8 m by 73.8 m. A flatter slope is paid for in capacity or in footprint; there is no third option.

That 2.86 million m³ is space, not rock in the ground. Waste swells when it is blasted and loaded, and it does not return to its bank volume in the dump. If the placed material is 25 % looser than it was in situ, 2.86 million m³ of dump space takes 2.29 million bank m³ of waste. Swell is a property of your rock and of how it is placed; measure it rather than assume it.

On level ground a dump is a stack of truncated pyramids and the arithmetic is closed-form. No real dump sits on level ground. Its capacity is the volume between two surfaces: the design surface of the dump above and the surveyed terrain below.

The difference is large. The same outline and the same top elevation hold far more in a valley than on a ridge, because the terrain falls away under the body. This is why capacity is computed against a DEM and reported lift by lift, as a table of elevation range, volume and cumulative volume. Read against the mine plan, that table answers the planning question directly: at what elevation is the dump full for the waste scheduled so far, and when does it stop being enough?

Each lift up also adds haul. Capacity high on the dump costs more per tonne than capacity low on it, so the cumulative curve is a cost curve as much as a volume curve.

Many dump failures start in the ground under the dump, not in the dumped rock. Three things about the foundation belong in the design from the first sketch.

Slope. A body placed on steep ground tends to slide along the contact. The steeper the foundation, the lower the dump that can safely be built on it, and ground above a site-specific limit is better left out of the footprint than counted as capacity.

Materials. Topsoil, organic layers and soft or wet soils make a weak base. Topsoil is stripped before dumping in any case, because it is the material the closed dump will need on its surface.

Water. A dump that buries a drainage line without a designed rock drain or a diversion channel ponds water against its upstream side and saturates its own toe. Surface water is led around the footprint, and the top of the dump is graded so that rain does not run over the crest.

Published guidance treats these together. The Guidelines for Mine Waste Dump and Stockpile Design include a stability rating and hazard classification system that weighs factors such as foundation conditions, material quality, geometry and construction method, and ties the level of investigation and analysis to the resulting hazard class.

The top platform is a workplace. It has to be wide enough for the largest truck to turn and reverse and for a dozer to work the edge; a dump whose slopes close to a narrow ridge has capacity on paper that no truck can reach.

The edge is where people are hurt. Material near a dump edge is loose and may not carry a loaded truck. US regulations require berms, bumper blocks, safety hooks or similar devices at dumping locations where there is a hazard of overtravel or overturning (30 CFR 56.9301). MSHA's inspection handbook adds the working practices: slope the top of the dumping location so that trucks back upslope to dump, dump short and push the load over where the edge is cracked, settled or undercut, and block access to the top whenever material is being loaded out from the toe.

A platform that rises towards the crest does a second job: rain drains back across the platform to a controlled outlet instead of running over the face and cutting gullies in it.

A crack or a settled strip near the crest is a warning sign, not a cosmetic defect. Covering it does not make it stable.

A dump design is ready to build when a short list of questions has numbers against it.

Is the overall slope angle within the limit set for the dump, and for the closed landform? Is every berm at least as wide as the equipment working on it needs? Is the top platform wide enough to turn a truck at the final elevation?

How much of the footprint lies on ground steeper than the foundation limit, and has that area been taken out of the capacity? How much topsoil has to be stripped, and where will it be stored?

Does the capacity, converted to bank volume with a measured swell, cover the waste in the mine plan with a margin? Is the toe clear of the pit crest, the permit boundary, watercourses and infrastructure by the required distances?

None of these is a stability analysis. Factors of safety for the dump and its foundation come from a geotechnical study with site-specific strengths and water conditions. Geometry checks tell you whether the design you are about to analyse is the design you meant to draw.

STREAM's grading module has a dump mode. Draw the toe outline on the surveyed terrain and choose the Up + inward direction: the body rises from the existing ground inward, and the panel's tabs switch to dump terms such as Dump Template, Site, Capacity and Checks.

The template takes the lift height, the berm width and the face angle, and you choose which of the three is derived from the other two. On the Site tab you set the top elevation at which the body is cut off, and a base slope limit: the share of the site steeper than that limit is reported, because the body will not sit there.

Capacity is read against the terrain: the loose volume of the body, the in-situ excavation that would fill it at the shrinkage you enter, the tonnage, the highest fill, the surface area of the body and the toe perimeter. A lift list, from the top down, gives the elevation range, the volume and the cumulative volume of each lift.

The Checks tab flags the overall slope angle against your limit, a berm narrower than your minimum, a top platform too narrow to turn a truck and the share of the site above the base slope limit, each with the value and the limit, and it reports the topsoil to strip under the body. The design can then be written into the working terrain, with drawings to DXF, quantity tables to Excel and the report to PDF.

It checks geometry, not stability: no factor of safety is computed, and the limits you enter come from your own geotechnical study. Everything runs on your machine; no survey is uploaded.