Choosing Trench Protection: Sloping, Shoring and Trench Boxes

Intro0:00

Before anyone steps into a trench, ask one question: what protection does this trench need? The answer depends on how deep it is, and on the soil you are digging in.

When Walls Must Be Protected0:10

If anyone works in a trench 5 feet deep or more, the law requires a protective system to protect them from a cave-in, unless it's dug entirely in stable rock. Under 5 feet, it can be left out only if a competent person examines the trench and finds no sign of a possible cave-in. There are three main ways to protect a trench: slope or bench the walls, shore them, or use a trench box. Rock counts as stable only if it stays intact with vertical sides, or is secured with rock bolts or another system a registered professional engineer designs.

Sloping0:44

Sloping: in a trench 20 feet deep or less, the soil type sets the steepest slope allowed. In Type A, the strongest soil, the walls can be no steeper than 3/4 horizontal to 1 vertical, or 1/2 to 1 in a trench no deeper than 12 feet that is open for 24 hours or less. In Type B, no steeper than 1 to 1. In Type C, the weakest, no steeper than 1 1/2 to 1. Walls can be benched within the same slope: in Type A, the bottom bench is no more than 4 feet high, and any bench above it no more than 5 feet. In Type B, benching is allowed only in cohesive soil, with each bench no more than 4 feet high; the standard layouts give no benching for Type C. In Type A, the bottom 3 1/2 feet can stay vertical in a trench up to 8 feet deep. These limits are for the best conditions: if the walls crack, slump or bulge, cut the slope back at least another 1/2 to 1, and with loads near the edge, a competent person decides how much flatter it must be.

Shoring1:48

Shoring: a timber, hydraulic or mechanical structure that supports the trench walls to prevent a cave-in. Timber shoring is sized from tables by soil type, depth, width and brace spacing, within the tables' limits. Hydraulic and pneumatic shoring that isn't timber must be designed under the rules for protective systems. It's wise to pick one with a safeguard that keeps it holding if pressure is lost, and to check its pressure at each inspection.

Trench Boxes2:15

A trench box, also called a shield, can withstand the forces of a cave-in, and so protects the workers inside it. It must be installed so it can't move dangerously if a sudden sideways load hits it. It must never be loaded beyond what it was designed for.

When an Engineer Must Sign Off2:31

Some protection needs an engineer. A trench more than 20 feet deep must have its protective system designed by a registered professional engineer, unless it's a manufacturer's shoring or box rated for that depth and used the way the manufacturer says. A box or shoring built from a manufacturer's data must follow all of the manufacturer's specifications, recommendations and limits; any change needs the manufacturer's specific written approval. These, and any approval to change them, must be kept in writing at the jobsite while it's being built.

Recap3:03

To recap. Anyone working in a trench 5 feet deep or more must be protected from a cave-in, unless it's dug entirely in stable rock. Protect them by sloping or benching the walls, shoring them, or using a trench box. Slope the walls no steeper than the soil type allows: 3/4 to 1 in Type A, 1 to 1 in Type B, and 1 1/2 to 1 in Type C. Cut them flatter if the walls show distress or carry loads at the edge. Deeper than 20 feet, a registered professional engineer designs the protective system, unless it's a manufacturer's shoring or box rated for that depth and used as the manufacturer says. And for a box or shoring built from a manufacturer's data, any change needs the manufacturer's written approval.