What Is Backfilling?

Backfilling is the process of refilling an excavated hole.

This is a common practice in construction and—the main focus of this guide—in mining.

Backfilling of mined spaces is a basic component of underground mining. The backfill material provides support to the surrounding rock mass, mitigates the risk of surface level subsidence, reduces wasteful ore dilution, and creates a safer working area for mining activities.

Backfilling can also be a way to dispose of potentially hazardous waste and reduce the environmental impact on the surface by storing backfill waste underground.

In this short guide to backfilling, we’ll cover:

  • Types of Backfill
  • The Backfilling Process
  • Backfilling Process Hazards
  • Advantages and Disadvantages of Backfilling

Types of Backfill

Backfill typically contains waste materials left over after the target mineral is extracted, which is commonly referred to as tailings.

backfilling-mine

Mining truck hauling ore from an underground mine

The following materials are commonly used to backfill mines:

  • Cemented backfilling. Cement is poured over tailings and waste rock to help fill voids that need filling, thus binding the backfill together. This is particularly useful when low volumes of cement are needed to bind materials since cement can be very expensive in large quantities.
  • Dry rock backfilling. Dry rock consists of dried tailings, gravel, rock waste, or surface sand. This backfilling material is best used in cut and fill mining.
  • Hydraulic sand backfilling. Hydraulic sand is the result of separating out fine sand from tailings and using the coarser sand as backfill. The sands are hydraulically pumped into voids in mines. When deemed necessary, the sand can be mixed with binding agents.
  • Paste backfilling. Paste backfill is typically used in coal mines. Solid tailings are broken down and mixed with water, cementing materials, and fly ash, creating a coagulable paste that can be used for backfilling.

The Backfilling Process

The backfilling process is largely dependent on the type of mine, the tailings, and the location of the stope or void.

In most cases, heavy equipment is used to pump backfill into a stope. However, gravity can also be used to backfill a shallow mine by way of dump trucks.

backfilling-tailings

Tailings plant

Most underground mines use cemented paste for backfilling stopes.

The tailings and binders are usually processed at the surface level in large plants before backfilling a mine. Mining personnel use a vacuum disc filter to extract excess water from tailings, then add a binder to that material via weigh hoppers.

High pressure piston pumps are used to distribute the final product to the mines, and barricades or bulkheads are used at the opening of a stope to retain the backfill while it cures.

Backfilling Process Hazards

Here are the most common hazards to a mining operation associated with the backfilling process:

  • Backfill segregation. When using hydraulic sand backfill, coarse and fine particles can separate while the backfill is curing. Coarse particles fall to the bottom while fine particles settle on top, upsetting geotechnics.
  • Backfill sloughing. If the crust of a previously cured backfill sheds or shifts, fresh backfill can shift too.
  • Borehole clog. When pouring backfill down a borehole it may become clogged, requiring personnel to drill new boreholes to access the mine.
  • Bulkhead/barricade failure. Failure occurs when the bulkhead or barricade that retains the backfill breaks under pressure.
  • Pipe hammering. This can occur when shutting off the pump quickly, causing the pipe to bang around in the borehole.
  • Pipeline burst. If a pipe gets clogged or if too much pressure is used, a pipeline can burst.
  • Plugged pump. Pumps that control the flow of backfill can get clogged and wear out over time.
  • Plugging of pipeline. Pipelines can get plugged more easily when backfilling with binders.
  • Plugged sump. The sump, which is used to ventilate a mine, can get clogged by backfilling. In this case, the debris will need to be pumped out of the sump.
  • Pump failure. Because pumps typically are used to transport backfill with binders, blockages and failures can occur.

Advantages and Disadvantages of Backfilling

Backfilling has several advantages and is widely practiced in mines throughout the world—but there are also some disadvantages to backfilling that should be considered.

Advantages of Backfilling

  • It helps to support the mine structurally, making it safer.
  • It makes it possible to extract minerals left in pillars in room and pillar mining.
  • It reduces the risk of spontaneous rock bursts.
  • It minimizes groundwater contamination when used with binders.
  • It improves ventilation in the mine.
  • It can reduce acid rock drainage (ARD), which contaminates water sources.
  • It prevents roof falls when blasting underground.
  • It reduces the need for surface storage space.
  • It lessens the environmental impact of mining when the mine is closed.
  • When used in room and pillar mining, it prevents the mine from collapsing after extracting minerals from the pillars.

Disadvantages of Backfilling

  • It requires expensive pumps needed for high density tailings. 
  • It requires operating an independent tailings plant from the mine.
  • It requires extra manpower and equipment management. 
  • It is expensive when performed with binders.
  • It slows down the mining process.
  • It has increased risks of liquefaction, if saturation levels are high and a seismic event occurs.
  • Seepage from backfilling can contaminate groundwater.
  • Backfill tailings need to be dewatered, causing higher operating expenses for production and transportation.

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