Trench Cave-Ins: Why Trench Walls Fail

What a Cave-In Is0:00

A cave-in is soil or rock suddenly falling or sliding into a trench, enough to trap or bury a worker, or injure them so they can't move. It can break from a wall, or run out from under a shield or support. Cave-ins are the main cause of trenching deaths. Trench deaths happen by suffocation or crushing when a worker is buried, and the falling soil can weigh as much as a mid-size car. Even a wall that looks stable can cave in if it isn't properly sloped, supported and regularly inspected. Most fatal cave-ins happen on small, short jobs, like service connections and digging for drains and wells. Too often, people think these jobs don't need protection against collapse. A good habit: treat even a shallow trench, about waist deep, as able to trap and seriously injure you.

What Makes Walls Fail0:47

Water is a common cause of cave-ins: rain, seepage, a high water table or a broken line. So are large loads, vibration, cracks and settling. Inadequate shoring, weather and soil that was dug before are common causes too.

Time, Vibration & Frozen Ground1:02

Soil moisture starts changing as soon as a trench is dug, and that can weaken the walls. The longer a trench is open, the greater the risk of a cave-in, and one left open a long time can suddenly collapse for no clear reason. Vibration from traffic and equipment can also contribute to a collapse. A good habit: don't trust frozen ground. In a thaw, a hard frozen surface can sit over soft, wet soil. Judge the trench by the soil under the frost, not the crust.

What Your Dig Can Undermine1:31

Ontario law covers digs that may affect the stability of a nearby building or structure. The constructor must take precautions to prevent damage to it. Where a dig may affect a nearby structure's stability, an engineer must specify those precautions in writing, and they must be taken. Best practice: before digging, look for poles, guy wires and anchors, trees, fences and anything else that could topple. Don't dig next to a pole or guy anchor until its owner has braced or held it. Stay out of the fall zone until the object is removed, secured or supported. Never cut a guy wire, and don't cut roots or anchors without clearance. Another good habit: if the ground suddenly drops, or you see signs of an old well, septic tank, cistern or other buried void, report it. Keep people and machines off the area until it has been assessed.

Recap2:22

To recap. Cave-ins are the main cause of trenching deaths. Most fatal ones happen on small, short jobs. Water, loads, vibration, time and previously dug soil can all lead to a collapse. If your dig could affect a nearby building or structure, an engineer must specify in writing how to protect it, and best practice is to check first for poles and anything else that could topple. And a good habit: treat even a waist-deep trench as able to trap you.