Trench Slope, Benching & Spoil Calculator (OSHA Soil Types)

How it works

Top width = bottom + 2 × depth × slope ratio (A ¾:1, B 1:1, C 1½:1); section area = (bottom + top) ÷ 2 × depth; bank yd³ = area × length ÷ 27; loose = bank × (1 + swell)

Trench collapses kill workers every year, and OSHA's Subpart P requires every trench 5 ft or deeper to be protected by sloping, benching, shoring or a shield. The allowable slope depends on soil classification by a competent person: cohesive Type A clay can stand at ¾ horizontal to 1 vertical, typical Type B at 1:1, and granular, wet or previously disturbed Type C — the most common on utility work — only at 1½:1, which makes an 8 ft deep trench 27 ft wide at the top. That geometry drives the excavation volume, the spoil to be stockpiled at least 2 ft back from the edge, the truck count and the working corridor, which is why trench boxes dominate in tight streets even though they cost more per day. Anything deeper than 20 ft needs an engineer's design.

Worked example

An 8 ft deep, 3 ft wide, 100 ft long trench in Type B soil sloped at 1:1 is 19 ft wide at the top: 88 ft² of cross-section, 326 yd³ of bank excavation and about 407 yd³ loose at 25% swell — 34 twelve-yard truckloads if hauled. A trench box would cut the dig to 89 yd³ and the top width to 3 ft.

Frequently asked questions