Safety Shares

June 2008

Highwall Hazards

Routine highwall safety instruction and best practices protect workers from ground control concerns at the quarry face.

Highwalls jut across the landscape. The soaring height of rock is breathtaking. The tall quarry walls are beautiful, showing off faults and folds of shale and rock bands. The series of layers, benches cut in the rock, resemble tiers on a cake, towering many stories, sometimes more than 100 feet in height. They are especially beautiful to those who make their living from mining operations, but they are also potentially treacherous, particularly to those who work close to them every day.

Very serious and sometimes fatal injuries occur when heavy material — rock — falls, rolls, or slides from highwalls. Workers at the crest — or the top — of a highwall may fall as the top of the face drops, while workers down the wall and in the pit may be hit and crushed. The rate of accidents and fatalities demonstrates the necessity for ongoing safety training.

The Department of Labor and the Mine Safety and Health Administration (MSHA) define a highwall “as the unexcavated face of exposed overburden and material on an open face or bank” and are conscientious about educating mining operators and workers about the potential dangers of working close to it.

The term the government uses for managing the physical aspects of surface mining is “ground control.” Ground control encompasses the management of highwalls and other potential danger areas that result from sand and gravel and crushed stone operations. The government outlines basic job steps, describes potential hazards and accidents, and recommends safe job procedures in Ground Control, module 11 of its Sand, Gravel, and Crushed Stone On-the-Job Training Modules. 

MSHA also produced a short video called Highwall Hazard Recognition, which is available on the Internet’s You Tube Web site (www.youtube.com/watch?v=HFZZF7Khksc). A scene plays out between a seasoned miner and a new miner traveling by haul truck to work in a quarry after a heavy rain. When they encounter a boulder in the middle of the road, they wonder where it came from. They call their boss who comes to the spot and sees that the rock fell from the face of the highwall and the area will need to be scaled to make it safer. The rain may have exacerbated a plane of weakness — a joint, fault, fracture, bedding plate, mud sink, or blasting damage — loosening the large rock and enabling it to fall. Other rocks may have been loosened, too. The workers were praised for noticing the rock and making a call.

In an open pit with highwalls, the first step toward achieving safety goals is to begin with a sound engineering design that helps ensure the stability of the highwall. To maintain safe control of the face as mining proceeds, the overall slope must be established and followed by operators. The engineering design, whether simply a gradual slope or a series of benches of particular widths and angles, should consider carefully the nature of the ground and the type of material mined. However, unknown structural weaknesses — the planes of weakness mentioned above — are hidden in rock, and miners should use continuous care when mining along the wall and scaling (cleaning the face of loose rock).

Once a safe plan for mining and controlling the wall has been developed and communicated, the supervisor should follow basic safety guidelines for any mining site and specific guidelines related to safe work on a highwall. For example, good housekeeping is essential at all mine sites; clutter at a highwall site exacerbates problems if rocks begin to slide.

Everyone at a site should participate in routine highwall safety instruction, including sharing tips for working at the top as well as on the bottom of a highwall. Safety belts and lines should be worn by workers who are working within 6 feet of a crest or unstable ground; hard hats should be worn by all workers.

A supervisor or other competent person must examine the working area and the working faces for unsafe conditions at the beginning of each shift and after blasting. Highwall banks, benches, and terrain sloping into the work areas should be examined after each rain, freeze, or thaw before work begins in the area. Overhanging highwalls should be taken down and other unsafe ground conditions corrected or warning signs posted.

Miners must inspect their working place before starting work and be aware of any changes in conditions. Any unsafe condition must be reported and corrected. Cracks, cavities, and other unstable areas should be identified, marked, and avoided.

Heavy equipment can cause rock falls if driven on unstable ground at the crest of a highwall or along a bench.

Miners should avoid working between mobile equipment and the highwall, where escape routes may be blocked if a rockslide begins.

To avoid accidents, MSHA recommends the following best practices:

• Examine and monitor highwall often.

• Follow ground control plan.

• Train miners to recognize hazardous highwall conditions.

• Scale down or support the hazardous highwall areas.

• Keep drill and other mobile equipment operators away from highwall face or highwall hazards by positioning them in safe locations.

• Employ mining methods that will maintain wall, bank, and slope stability in places where persons work or travel. 

Information contained in this article was provided through the Safety & Health Committee of the National Stone, Sand & Gravel Association.