Overburden is a natural consequence of almost all mining procedures. However, it is created in the greatest quantity by strip and surface mining operations. This is why overburden is primarily a concern and potentially useful by-product for this subset of the mining industry. In other operations, overburden materials are largely blasted into dust or removed as sludges that are harder to reuse than solid crushed material. Overburden that is produced with care can be valuable in itself, even though it won’t compare in value to the oil or ore that’s being extracted. Depending on the exact mining method and the industry, up to 5 tons of overburden can be produced for every ton of productive material extracted. There are a few different processes that can result in large quantities of overburden material in need of containment.
The Definition of Overburden
The official definition of overburden is: all of the unwanted or low value material that is located between the surface and the desired material. The material that forms the lower boundary of the overburden layer can be a seam of coal or valuable ore. If a penetrating method is used to access a liquid reservoir underground, the well runs through layers of what’s considered overburden.
The term may or may not include the first layer of soil and vegetation on the surface. For most operations, this living layer of material is called muskeg or topsoil instead. It is far more valuable than overburden due to its organic material content and is often required for habitat reclamation after the mine shuts down. Overburden generally consists of a combination of dirt, sand, and mixed rock. It can be solid rock in layers that must be drilled or blasted away, but it is usually a looser mix that only needs excavation to reveal the valuable materials below. The exact composition of the material determines how valuable it is for reuse and what it can be used for later. Almost all overburden can be used in some way to cover or refill the mining operation or a pit mine elsewhere.
In general, the biggest difference between overburden and valuable crusher run or oil sands is the content. Overburden will have 7% or less valuable ore or oil by volume, while materials above that content will go through extraction processes. The financial tipping point between the costs of containing the material and processing it for oil or ore determines whether it qualifies as overburden or not.
Surface or Strip Mining
Surface and strip mining operations handle the most overburden and therefore have developed the best containment techniques for it. With these techniques, almost all of the material lying above the valuable ore or oil deposit is gradually removed by heavy equipment or with blasting. This can create hundreds of tons of raw, crushed rock and sand that pose at least a minor risk to the environment. Almost all overburden contains a small amount of heavy metals or oil that can leach out as rain trickles through the pile. Since surface mining operations tend to produce the largest volume of by-products like overburden, they also have the greatest containment needs for it. Surface and strip mining operations vary slightly by the depth of excavation needed. Strip mining generally involves a shallow depth of overburden, while surface mining goes deeper and can include removing hundreds of feet of material.
Mountaintop Removal for Coal
Mountaintop removal is one of the most dramatic forms of surface mining and it can also generate some of the largest volumes of overburden. Since entire mountaintops are removed to depths of hundreds of feet, there can be huge amounts of blasted material left behind. Due to the sheer volume of this material, it is often used to fill in entire valleys in the surrounding area, which increases the negative environmental impact of the operation. Careful containment and more creative reuse opportunities for the overburden material can help this controversial mining process become less impactful and more accepted.
The Oil Sands Process
Oil sands require surface mining approaches since the petroleum products are mixed in with loose material like sand and rock. These unique deposits are known as soft rock mining opportunities versus the hard rock mining used for most ore. Oil sands can contain dozens to hundreds of feet of overburden over the valuable material. This material tends to be loose and broken up already because oil sands and ore-rich loose rock tend to form in a large layer that reaches to the surface. Little to no blasting is often needed, leaving the overburden material as intact as possible. Combine that with a relatively high bitumen level for non-valuable material and this kind of overburden is particularly valuable for construction use.
Other Uses for the Term
The term overburden is used in a few other industries to generally refer to any sand, rock, and soil material covering something being studied or accessed. For example, the term overburden can be found in archeological reports and geological studies alike. In these cases, the overburden will be the materials that are covering a buried feature like an impact crater or predicted fossil seam. Extensive testing is often used to determine the amount of overburden and the depth of the feature to analyze if it is feasible to remove the volume of material. It is much harder to remove this kind of thick layer of sand and rock over delicate geological or archeological features than it is to excavate and blast for mining purposes. Still, the material produced in the end requires similar containment and handling.
Overburden can vary from site to site, but it is always some kind of loose mix of various different materials. It can be nearly as rich in bitumen or ore as the targeted material, but it is usually just too expensive to process to be worth further treatment. Depending on its composition, it may have many potential valuable reuse opportunities. Containment of even the largest piles of overburden material is easy with products from BTL Liners.
