Overburden Removal in Opencast Mining: Stripping Ratio, BCM and Machines

Overburden removal is the digging and hauling away of the soil and rock that lie over a mineral, so that the mineral underneath can be mined. In an opencast mine it is usually the bigger part of the work, and the stripping ratio, the cubic metres of overburden that must move for every tonne of mineral, tells you how much bigger.

A coal seam 4 m thick under 20 m of cover means five cubic metres of waste dug, loaded, hauled and dumped for every cubic metre of coal. None of that waste earns money. It is the cost of reaching the part that does, which is why overburden contractors are paid by the cubic metre.

This guide covers what overburden is made of, how the stripping ratio is worked out, why a cubic metre in the ground and one in a dumper differ, and which machine chain suits which ground. It carries no rupee figures: contract rates change by mine and by year, and the arithmetic does not.

What is overburden in mining?

Overburden is everything that sits on top of the mineral you want: the topsoil, the clay and murrum (the weathered, gravelly soil common across central and eastern India), the broken rock below it and the solid rock below that. Mining engineers also call it waste or spoil. The rock band that sometimes lies between two coal seams is called interburden, and it is removed the same way.

Overburden is not the same as tailings. Tailings are what is left after a mineral has been crushed and processed. Overburden has never been processed at all; it is simply the cover that had to come off. That is also why much of it can go back into the ground later, to refill the worked-out pit.

Hardness decides whether a machine can dig overburden straight or whether it must be blasted first, and Indian earthwork already has a standard way of describing it. IS 1200 (Part 1), the Indian Standard method of measuring earthwork, sorts excavated material into classes, and the same classes describe the layers of a typical overburden bench.

The way the overburden sits over the mineral also decides which kind of mine you have in the first place, and the types of mines in India explains why a shallow, flat deposit becomes an opencast mine while a deep one goes underground.

What is the stripping ratio in mining?

The stripping ratio is the amount of overburden that has to be removed for each unit of mineral it uncovers. A ratio of 5 means five units of waste for every unit of ore or coal. The higher the number, the more digging the mine pays for before it earns anything.

The units change with the mineral. In coal mining the ratio is normally written as volume over weight, cubic metres of overburden per tonne of coal. In metal mining it is usually weight over weight, tonnes of waste per tonne of ore, so it has no unit at all. The standard definition of the stripping ratio sets out both forms. When someone quotes you a ratio, ask which one they mean before comparing two mines.

For a real block, divide the average overburden thickness from the borehole logs by the seam thickness, then convert the coal to tonnes with its measured density. Never borrow a density from another mine, because ash content moves it.

The ratio is not fixed for the life of a mine. Where the seam dips, every new strip down the dip carries more cover than the last, so the ratio rises as the pit moves on. Planners therefore speak of an average ratio for the whole deposit and a current ratio for the bench being worked this year. A contractor bidding for overburden work should ask for the second, because that is the volume the machines will actually move.

What is the break-even stripping ratio?

The break-even stripping ratio is the highest ratio at which mining a tonne of mineral still pays for the overburden above it. Above it, the mine loses money on every tonne it digs. The working is simple:

break-even ratio = (value of one tonne of mineral − cost to dig, process and sell that tonne) ÷ cost to remove one cubic metre of overburden.

Say a tonne of coal sells for 10 units of money, costs 4 units to dig and dispatch, and each cubic metre of overburden costs 2 units to remove. The margin before stripping is 6, and 6 ÷ 2 gives a break-even ratio of 3 cubic metres per tonne. Ground at a ratio of 2 is worth mining; ground at a ratio of 4 is not, unless prices rise or stripping gets cheaper. This comparison is the quick test the stripping ratio is used for: a high ratio means a large amount of waste must be moved to reach the mineral.

The cheaper a cubic metre of overburden moves, the higher the ratio a mine can tolerate and the more of its deposit is worth digging.

What is BCM in mining, and why is loose volume different?

BCM stands for bank cubic metre: one cubic metre of material as it lies undisturbed in the ground. Once it is dug it breaks into pieces with air between them and takes up more room. That larger volume is the loose cubic metre, LCM. When it is spread on a dump and rolled by machines it packs down again, into compacted cubic metres. These are three measurements of the same rock, and a contract that says only “cubic metres” has not told you which one.

Indian earthwork measurement already works in bank volume. IS 1200 (Part 1) says earthwork is measured in cubic metres, and where the ground is uneven the levels are taken before the work starts and after it ends, with the quantity computed from those levels (clauses 4.1 and 4.2). Where hard rock comes out mixed with softer material, the standard has the hard rock stacked and measured in the stack, then reduced by a pre-agreed percentage for the voids. That deduction is the standard’s way of turning a loose heap back into a bank volume.

The factor between bank and loose is the swell factor, and the excavator productivity guide explains how swell changes excavator output figures. Ordinary soil swells by roughly a quarter and blasted rock by more. Measure your own figure rather than take one from a book: survey a block before and after, count the dumper loads that carried it away, and divide. That number turns every truck count into a payable volume.

Two more IS 1200 rules matter on a long overburden haul. Lead, the distance material is carried, is measured over the shortest practicable route and not the route the trucks actually took (clause 5.1). Lift, the height the material is raised, is counted in steps of 1.5 m (clause 5.2). Read how your contract defines both before you price the haul.

How is overburden removed, step by step?

The work runs in the same order on almost every opencast mine and quarry. The guide to mining equipment in India shows how the whole pit runs as one loop; these are the steps inside the overburden part of it.

1. Clear and strip the topsoil. The top layer of fertile soil is pushed off by dozers and stored in its own heap, kept apart from the rest. It is needed again at the end, when the land is handed back.
2. Break the hard layers. Soil and murrum are dug straight. Weathered rock is ripped by a dozer or dug by a large excavator. Hard rock is drilled and blasted so that it comes out in pieces the bucket can lift.
3. Load at the face. An excavator or a rope shovel stands on a bench (a flat step cut into the pit wall) and loads dumpers. The bench height is set to suit the loading machine’s reach. The guide to bench height and width in opencast mining shows how both are worked out.
4. Haul out. Dumpers carry the overburden along haul roads kept smooth and drained by motor graders and water tankers.
5. Dump. Early in a mine’s life the overburden goes to an external dump outside the pit. Once enough mineral has been taken out, it goes back into the worked-out part of the pit instead, which shortens the haul. Dozers spread each tip into layers and keep the dump slopes safe.
6. Reclaim. The finished dump or backfilled pit is shaped, the stored topsoil is spread back on top, and the land is planted.

Steps 4 and 5 decide most of the cost. Every extra hundred metres of haul is paid on every load, which is why planners start backfilling inside the pit as early as the geometry allows.

Which machines remove overburden: shovel and dumper, dragline or dozer?

There are three main ways to move overburden, and each one suits a different kind of ground. Most Indian mines use the first; many use more than one at the same time.

The dragline method is the classic way coal is strip-mined on flat ground. As each long strip is dug, its overburden is placed into the strip mined just before it, so the waste never leaves the pit. The shovel-and-dumper method costs more per cubic metre moved but is far more flexible, which is why shovels and dumpers far outnumber draglines in Indian coal mines.

You will also hear of the surface miner. It cuts coal or soft rock in thin layers with a rotating drum and needs no blasting. In Indian coal mines it is used mainly on the coal itself rather than on the overburden above it, so it sits beside this chain rather than in it. How many shovels, dumpers, draglines and surface miners the big coal companies actually run changes every year, and the largest equipment fleets in India tracks those counts.

Draglines, rope shovels, dumpers, drill rigs and surface miners are not sold through DesiMachines; the power shovel versus excavator comparison explains how the rope shovel differs from the hydraulic machines that are. For overburden work a contractor can buy mining-class excavators, mining dozers, motor graders for the haul roads and wheel loaders for the stockpiles.

If you are pricing an overburden contract, compare live mining excavator models and speak to a dealer about the bucket sizes each one is offered with.

How do you match the excavator to the dumper?

Count passes, the number of bucket loads it takes to fill one dumper. Divide the dumper’s rated payload by the weight of one loaded bucket. The weight of one bucket is its heaped capacity, times a fill factor (how full the bucket really comes out of that material), times the loose density of the material. Mine planners usually aim for three to five clean passes per dumper.

Too few passes and the bucket is too big for the body: material spills and the dumper’s tyres and frame suffer. Too many and the dumpers queue at the face. Either way the volume moved per shift falls.

Mining excavators span a very wide range, as these listed machines show:

The bucket figure is a range on most of them because each machine is offered with buckets for different materials. Pick the bucket for the material: the large end for light, loose overburden, the small end for heavy blasted rock. Bucket sizing by class is set out in the guide to excavator bucket capacity.

Dozers do the supporting work: ripping, pushing to the loading face, and shaping dumps. In the mining class the BEML BD355 carries a 13.0 m³ semi-U blade at 48,660 kg, and the Komatsu D375A-6 an 18.5 m³ semi-U blade at 71,640 kg. Both weights are with the machine’s own listed equipment, so check what is fitted before comparing them.

What slows overburden removal down on site?

When a fleet moves less than planned, the cause is usually one of these, and most are fixed outside the excavator cab.

• Poor blasting. Big boulders left after a blast fill the bucket badly and have to be broken again. Better fragmentation (the size the rock breaks into) is often the cheapest output gain on the site.
• A long or rising haul. Each extra stretch of road and each metre of climb is paid on every trip. Backfilling inside the pit early cuts both.
• Rough haul roads. Loaded dumpers slow down on potholes and burn more diesel, and tyre damage takes trucks off the road.
• A mismatched fleet. Too few dumpers leave the excavator idle; too many leave dumpers queuing. Count passes and cycle times before adding machines.
• The monsoon. Wet clay sticks in buckets and bodies and haul roads turn slippery. Sites that keep drains, sumps and a firm road surface lose fewer days.

Where overburden fits in the mine’s budget

Overburden is the excavation a mine does not pay royalty on. Royalty is charged on the mineral that leaves the site, not on the waste moved to reach it, as the guide to royalty on minerals in India explains. A smaller quarry still has the same cover to strip before its first load of stone, and the quarry lease guide covers the permissions that come before any of it. Some overburden rock is crushed and sold rather than dumped, the route covered in the guide to quarry dust and M-sand.

For the machines, compare live mining excavators and mining dozers, and keep the haul road in mind with the range of motor graders. More mining guides sit under mining.

Prices, specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.

References: IS 1200 (Part 1) : 1992, method of measurement of building and civil engineering works, earthwork, clauses 3, 4.1, 4.2, 5.1 and 5.2. Stripping ratio definitions as given in Runge, Mining Economics and Strategy (SME, 1998) and Hustrulid, Open Pit Mine Planning and Design, as cited in the linked reference. Machine figures are taken from the specification sheets published against each machine.