Trench Slope, Spoil & Shoring
OSHA sloping ratios by soil type and depth, the spoil the trench throws out in yards and tons, shoring sets, and the 5 ft trigger for a protective system.
Type C slops at 1.5 horizontal to 1 vertical, Type B at 1:1, Type A at 0.75:1, and 0.5:1 in Type A only for a short-term opening 12 ft deep or less. Above 20 ft, a licensed engineer signs the slope.
How the math works
- Maximum allowable slopes (OSHA 1926 Subpart P, Table B-1, excavations under 20 ft): stable rock vertical; Type A 3/4:1 (53°), short-term 1/2:1 (63°) when 12 ft deep or less; Type B 1:1 (45°); Type C 1½:1 (34°). Over 20 ft, a registered professional engineer designs the slope.
- Spoil volume: the sloped sides are triangles: extra area per cross-section = slope ratio × depth², so section area = width × depth + ratio × depth². Multiply by length, divide by 27, then ×1.35 to bulk the spoil.
- Spoil weight: 1.35 tons per loose cubic yard for A and B, 1.15 for granular C, and 1.40 for saturated clay. Trench spoil is the heaviest dirt a small contractor moves.
- Shoring: hydraulic shields are set to the manufacturer tabulated data with a 10 ft maximum use depth, boxes on 4 ft spacing for timber slide rails, and the shield must extend 18 in above the top of the vertical side when the sides are vertically sided and supported.
- Site rules: a protective system is required at 5 ft unless the excavation is entirely in stable rock or a competent person finds no cave-in hazard; spoil and equipment stay 2 ft back from the edge; a ladder within 25 ft of any worker; daily inspection by a competent person and after every rain.
How steep can I slope a 5 foot trench in Type C soil?
1.5 horizontal to 1 vertical, which is 34 degrees from horizontal, and it takes 7.5 ft of width at the top for every 5 ft of depth on each side. A 2 ft bottom trench is 17 ft wide at the top, so the spoil is the real job.
Do I need shoring for a 5 foot trench?
Yes: at 5 ft and deeper, OSHA requires a protective system unless the excavation is entirely in stable rock, or a competent person examines the ground and finds no cave-in indication. Sloping the sides to Table B-1 ratios is a protective system and is the cheapest one.
How far back must spoil be from the trench?
Two feet minimum from the edge of the excavation, and that is the same rule that keeps surcharge load off the shoring. Keep spoil 2 ft back, 20 ft from a slope crest when you can, and no equipment at the edge.