Swell Factor in Reclamation

When mining companies reclaim a mine or quarry, they usually do it using only materials that were at the site originally. For example, when a surface mine is refilled, it is done with the same soil and rock that were removed during mining.

But how can the same soil and rock that came out of the hole refill it after huge amounts of minerals have been removed?

The reason is something called “swell factor.” Soil gets compacted when it is left in place over long periods because of the weight of dirt and rock above. The weight of glaciers can also compact soil. (Remember that soil is often in place for many thousands of years). Most of the oxygen is squeezed out of it and the soil becomes denser.

As you extract the soil, it gets broken up into particles and clumps. This creates air pockets within the soil that cause it to swell, so the extracted dirt will fill a larger volume than it did when buried and compacted.

Because of swell factor, a cubic metre of soil when buried can swell to, for example, 1.25 cubic metres, or 1.4 cubic metres, when extracted. The amount of swell depends on a site’s unique characteristics, such as the type of soil (i.e. clay, sand, loam, etc.).

The same principle applies to rock removed from a mine or quarry. Rock is denser in solid form than it is after it is blasted and extracted. As with soil, extraction creates air pockets within a pile of rock, which causes the rock to fill a larger volume than it did before extraction.

You may have experienced swell at home without realizing it. If you dig a hole in your yard and then refill it, you will usually find that some of the dirt does not fit back in and you end up with a small mound of dirt on top of the refilled hole.

Reverse order backfilling is often done to help return a site to the state it was in prior to extraction. The soil and rock are put back in reverse order so they end up at the depth they were pre-extraction. Seeds in the soil at surface before mining end up at surface afterwards, which facilitates regrowth of the species that were on the site before mining took place.

The time lapse video below shows the Stellarton surface coal mine being backfilled with the same dirt and rock that was extracted from it. The video also shows fully reclaimed parts of the mine, including fields, wetlands, Stellarton’s water tower and the Pioneer Coal Athletics Field.

The mine was about 100 metres deep in the area where the water tower is today, but it was backfilled to surface level using the dirt and rock originally extracted from the mine. The hill the water tower is on was built up an additional 35 metres to give the tower extra height, which was important because water towers rely on gravity.

Modern mining/quarrying is one of the most stringently-regulated industries in Nova Scotia, especially for environmental considerations. A mine's environmental assessment often takes 3-5 years and costs over a million dollars. It generates dozens of scientific studies.

Before getting operating permits, mining companies must get government approval of reclamation plans and post reclamation bonds (money in escrow, basically) that ensure funds are available to properly take care of sites – as the Stellarton mine is doing. In fact, reclamation is a key part of the mining process today.

HOW RECLAMATION WORKS