© 2003 Liberty Mutual Group – All Rights Reserved 1

Construction Safety Talks

Don't Become A Trenching Statistic

No. 78

How This Talk Applies To My

Crew:

Some Things To Know Before You Give Your Talk.

When you go into a trench, you could be dead in a matter of minutes. We know what causes trenches to collapse, but we think it won’t happen to us. Discuss with your crew how to keep the odds in their favor.

A collapse of either side wall can kill within a matter of minutes after entering a trench. The separation of a mass of soil or rock from the side which suddenly moves into the excavation, either by falling or sliding, is sufficient to entrap or bury a person. Soil weighs between a 100 to130 pounds per cubic foot … or over 2,700 pounds a (cubic) yard! That’s equivalent to having your car or pick up truck fall on you!

Water from ground water or a heavy rain reduces cohesion, the ability of soil to stick together, and when it sits in the bottom of a trench, it undermines the side walls.

Vibration from traffic or nearby construction operations needs to be evaluated. Vibration tends to collapse the little air pockets found in soil. This allows nearby particles to move, which allows other particles to move. This happens without warning, resulting in a major section of the side wall falling into the trench. The side walls of an open trench will dry out when exposed to the air for more than a day. The cohesiveness of the soil will change as the moisture evaporates, most likely resulting in a less cohesive mixture.

Tips For Working In Trenches

• Remove standing water with pumps. Consider installing protection if water is seeping in from the bottom or side walls.

• Install diversion ditches, dikes, or other suitable means to divert surface water away from an open trench.

• The toe of the spoil must be at least 2 feet back from the edge; this includes a sloped trench. This reduces additional weight (surcharge) at the edge and reduces the chances of rocks and clumps from rolling into the trench.

© 2003 Liberty Mutual Group – All Rights Reserved 2

• Install protective systems from the top down. Using this method, no one has to work in an unprotected trench.

• Install protective systems whenever excavation is done under sidewalks or pavement.

• When shoring a trench, extend the protective system at least 39 inches above the surface. The system will help prevent collapse, in addition to preventing falls from the surface.

• Prohibit crossing over a trench unless a walkway with guardrails is used. • Follow the tabular data on the trench box. The bottom has to be within 2 feet of

the bottom of the trench. The access ladder must be inside the trench box. • Support any utilities that run across the trench to prevent damage.

Try This For Show And Tell! • Have the crew look around where the trench is to be dug. Are there cracks or

fissures in the surface? Any signs of previous excavation? • Ask the crew to identify sources of heavy vibration within 100 feet of the trench. • If you use a trench box, read some of the key limitations and notes from the

tabular data.

Questions You Can Use To Get Them Talking: • Have you ever seen a cave-in or heard of one happening? What do you think

caused it? • Does the soil have to cover your face for you to die in a collapse? No! Even if

you are buried up to your neck, the soil will compact every time you exhale, making it hardier to take the next breath.

• What are some of the other hazards workers are exposed to when working in a trench? Struck-by soil and materials; crushed by excavator bucket; crushed by heavy mobile equipment falling into the trench; hazardous atmospheres.

Attended By:

The illustrations, instructions and principles contained in the material are general in scope and, to the best of our knowledge, current at the time of publication. No attempt has been made to interpret any referenced codes, standards or regulations. Please refer to the appropriate code, standard or regulation making authority for interpretation or clarification.