OSHA trench sloping calculator

OSHA requires a protective system in any trench 5 feet deep or more, unless the excavation is entirely in stable rock. Below 5 feet a system is not required by the depth rule, but a competent person can still call for one if they see a cave-in hazard. The three protective options are sloping the walls back, a trench box (shield), or shoring.

For a simple slope up to 20 feet deep, the maximum allowable slopes are 3/4 to 1 for Type A (about 53 degrees from horizontal), 1 to 1 for Type B (45 degrees), and 1.5 to 1 for Type C (about 34 degrees). Stable rock can be vertical. Those ratios are horizontal run to vertical rise, so Type C lays back 1.5 feet for every foot of depth.

A competent person classifies it on site using at least one visual and one manual test. Type A is hard cohesive soil like stiff clay. Type B is medium, such as silt or clay that has been previously disturbed. Type C is the least stable: granular soils like sand and gravel, or any soil with water seeping in. When in doubt, classify to the weaker type, and remember layered or previously excavated ground is never Type A.

The top width is the trench bottom width plus the lay-back on each side, and each side lays back the slope ratio times the depth. A 6 foot deep trench with a 3 foot bottom in Type C soil lays back 9 feet per side, so it opens 21 feet wide at the surface. That is why deep sloped trenches need a lot of room and are often boxed or shored in tight corridors instead.

Once an excavation reaches 20 feet deep, the tabulated slopes no longer apply and the protective system must be designed by a registered professional engineer. Below that depth you can use OSHA's slope table, manufacturer's tabulated data for a shield or shoring, or an engineered design.