A drift mine is an underground mine that enters horizontally into a hillside or mountain, following a mineral seam straight from where it’s exposed at the surface. Unlike mines that require digging a vertical shaft deep into the earth, a drift mine takes advantage of natural terrain by tunneling sideways into a slope where coal, ore, or another deposit is already visible. This makes drift mines among the simplest and least expensive underground mines to build and operate.
How a Drift Mine Works
The concept is straightforward. Where a coal seam or mineral vein is exposed on the side of a hill, mountain, or cliff face, miners drive a horizontal tunnel directly into that exposed layer. This tunnel, called a “drift,” follows the seam as it extends into the earth. Because the opening is at ground level on a slope, workers, equipment, and extracted material can move in and out without needing elevators, hoists, or ramps.
One of the biggest practical advantages is water drainage. In a vertical shaft mine, water that seeps in must be pumped out constantly, which costs energy and money. In a drift mine, water flows out naturally by gravity through the horizontal opening. Installing drainage in drift-style openings has been shown to reduce airflow resistance by about 70% while increasing air circulation by roughly 30%, making ventilation significantly easier as well.
How It Compares to Other Mine Types
Underground mines are classified by how you get into them. There are three main types:
- Drift mine: A horizontal or near-horizontal opening driven into a hillside where the deposit is exposed at the surface.
- Shaft mine: A vertical opening sunk straight down from the surface to reach deposits buried deep underground.
- Slope mine: An inclined opening that angles downward from the surface to connect with underground workings.
A tunnel, by contrast, is a horizontal opening with access at both ends, while a drift only has one entrance. The key distinction for drift mines is that they’re only possible when the geology cooperates: the mineral deposit has to be naturally exposed on a slope or hillside. Shaft mines exist because most deposits aren’t conveniently sticking out of a mountainside. They require sinking a vertical hole hundreds or even thousands of feet down, which is far more complex and expensive.
Cost Advantages Over Shaft Mines
Drift mines are consistently cheaper to develop than shaft mines, and the difference isn’t small. Cost modeling data from Montana Tech shows that across every major underground mining method, horizontal-entry (drift) mines have lower capital costs than their shaft-entry equivalents. For a room-and-pillar operation producing 1,200 to 14,000 tons per day, for example, a drift-entry mine costs roughly $46 million to $156 million to develop, while the same operation using a shaft entry runs $59 million to $185 million. Operating costs per ton are also lower for drift access, typically by a few dollars per ton.
These savings come from not having to excavate a deep vertical shaft, not needing hoisting equipment to lift material to the surface, and not requiring the constant pumping systems that shaft mines demand for water removal. The horizontal entry means you can run rail cars or conveyor belts straight out of the mine, which is far simpler than lifting material vertically.
Moving Material Out of the Mine
Because drift mines are horizontal, getting material from the working face to the surface is relatively simple. Historically, coal was loaded into small rail cars running on narrow-gauge track, each holding about 1,400 to 1,600 pounds. These cars were pulled by mules, steam power, or electric motors along entries and hauling ways inside the mine. The track itself used lightweight T-shaped rails set about two to three feet apart.
In smaller or less-equipped operations, miners used cruder methods. Some dragged coal out on wooden platforms called “buggies,” essentially flat sleds with curved runners that could be pulled in either direction without turning around. Modern drift mines use conveyor belt systems and diesel or electric vehicles, but the principle remains the same: horizontal access means you’re rolling or sliding material out rather than lifting it.
Where Drift Mines Are Most Common
Drift mining is most closely associated with the Appalachian region of the eastern United States, and the reason is pure geology. The Appalachian coalfield sits within a mountainous landscape formed from layers of sedimentary rock with coal seams sandwiched between them. As rivers and erosion carved through these mountains over millions of years, they exposed coal seams along hillsides and valley walls, creating ideal conditions for drift mining.
Early Appalachian miners would strip away the loose soil and rock covering a coal bed, then follow the exposed seam straight into the earth. This approach required minimal infrastructure and could be started with basic hand tools. Drift mines were especially common in places like West Virginia, Kentucky, Virginia, and Pennsylvania, where steep terrain and exposed coal seams made horizontal entry the most logical approach. Similar conditions exist in parts of Wales, northern England, and other mountainous coal-producing regions worldwide.
Over time, many of the easily accessible seams in Appalachia have been depleted, pushing production toward thinner and harder-to-reach deposits. This has increased costs for the region’s miners compared to those working flatter, thicker coal deposits elsewhere in the country.
Keeping Drift Mines Stable
The roof of a drift mine’s horizontal tunnel needs continuous support to prevent collapse. The standard approach uses rock bolts, steel rods drilled into the rock above the tunnel and anchored in place to hold layers of stone together. In shallow drift mines with competent rock, simple bolt patterns are often enough. In deeper or more geologically challenging conditions, standard bolts can fail as the surrounding rock shifts and deforms under pressure.
For these tougher situations, engineers have developed specialized bolts designed to flex with the rock rather than snap. Field testing of these newer support systems has shown they can reduce tunnel deformation by about 70% compared to conventional bolts. Timber supports, steel arches, and combinations of these methods are also used depending on the depth, rock type, and width of the tunnel being supported.