Coal is removed from a mine and a bulldozer piles it above a fast-moving conveyor system that deposits it into rail cars. This loose coal on this surge pile can suddenly engulf and suffocate a worker, catch on fire, or the dust explode.

The bulldozer operator must wear a seat belt, because it will save his life if the dozer tips over. Without the seatbelt, the bulldozer operator can suffer fatal injuries inside the cab.

This cab has super reinforced windows, two air packs, two radio communication systems, fire extinguisher, and an operator’s manual.

The bulldozer often drops below sight of other workers. Night time operations require properly placed floodlights to indicate the pit location.

Two orange balls are positioned directly above the pit.

Coal constantly slides down the surge pile into the pit and onto the conveyor under the coal pile. Does that make sense? The coal is carried away underground.

Look for the two orange balls.

Density, surface area, and coefficient of friction of the material are constantly being changed by the additional coal being dumped and other coal being conveyed away.

The bulldozer operator must constantly be aware of the orange balls in relationship to his bulldozer’s position.

In this incident, coal has bridged over the pit and coal has stopped falling onto the conveyor.

A coal bridge will not support the bulldozer.

Coal’s angle of repose constantly changes on a moving surge pile. This bulldozer, backed onto a coal bridge, and it giving way, both fell into the pit. Coal continued to fall and covered the bulldozer cab.

Should the bulldozer operator attempt an escape and climb out while he has a chance?

NO! This loose coal can suddenly engulf and suffocate a worker. The conveyor system has a radio controlled emergency shut off. Everyone working with this operation carries a conveyor emergency stop transmitter.

In this incident, the bulldozer operator calmly shut off the conveyor.

How would you extricate the bulldozer operator?

Everyone heard the conveyor stop and heard its alarm.

The bulldozer operator described his predicament using one of the two communication radios, and the other workers followed a well laid out extrication plan.

The bulldozer operator wanted to climb out, but forced himself to stay inside the buried cab.

He overcame his anxiety.

His coworkers fastened a fall protection harness to a rope ladder and lowered it into the cab.

The bulldozer operator donned the harness then climbed the rope ladder out of the pit.

This bulldozer operator’s life was saved because his company’s management placed a price on safety and implemented a number of brilliant ideas e.g. reinforcing the cab windows, having an extrication plan in place, two communication radios, the conveyor emergency radio shut-off system, two air packs.

Question stem: Why does the angle of repose vary in a moving surge pile?

Answer: different coal is constantly being added and other coal is being conveyed away

Distractor: each color of coal indicates its coefficient of space and where it will clump

Distractor: temperature of the sun changes the shape of coal on windy days

Distractor: angle of internal friction is mostly determined by the friction of the conveyor belt

This sequence took me three hours to prepare using U.S. Government photos.

Another three hours to proofread, spellcheck, locate references, develop one question…

Total development time is six hours.

jeffrey.meade.osha@gmail.com