For any trench 20 feet deep or less, OSHA’s trench sloping and benching requirements by soil type set a maximum wall angle of ¾ horizontal to 1 vertical in Type A soil, 1:1 in Type B, and 1½:1 in Type C, with stable rock allowed to stand vertical. Benching is permitted only in Type A and Type B; Type C cannot be benched at all. Anything deeper than 20 feet has to be designed by a registered professional engineer.
The Slope Ratios at a Glance
OSHA’s Appendix B to Subpart P assigns each soil category a maximum allowable slope. The ratios describe horizontal run for every unit of vertical depth.
- Stable Rock: vertical walls (90°) permitted.
- Type A: ¾ horizontal to 1 vertical, roughly 53°.
- Type B: 1 horizontal to 1 vertical, 45°.
- Type C: 1½ horizontal to 1 vertical, roughly 34°.
The footprint grows fast as the soil gets weaker. A 10-foot Type B trench needs walls cut back 10 feet on each side, opening the surface 20 feet wider than the trench floor. The same 10-foot trench in Type C needs 15 feet of setback per side, producing a 30-foot-wide opening at grade. On tight sites, that geometry alone often forces the crew away from sloping and toward shoring or a trench box.
Which Soil Type You Actually Have
The slope ratio is only as good as the classification behind it. OSHA sorts soil and rock into four categories from most stable to least stable, and a competent person on site must make the call before any worker enters the trench.
Stable Rock is natural solid mineral matter that can be excavated with vertical sides and remain standing. Excavations made entirely in stable rock are exempt from protective system requirements.
Type A covers cohesive soils with an unconfined compressive strength of 1.5 tons per square foot or greater — clay, silty clay, sandy clay. A soil that meets the strength threshold still cannot be Type A if any of these apply:
- The soil is fissured (visible cracks or fracture lines).
- It is subject to vibration from heavy traffic, pile driving, or similar activity.
- It has been previously disturbed — excavated or backfilled before.
- It sits in a layered system that dips into the excavation at four horizontal to one vertical or steeper.
- Any other factor makes it behave less predictably.
Those exclusions catch a lot of jobsites. Previously disturbed ground alone knocks out any location where earlier utility work or grading occurred, which is why many urban trenches default to Type B or C regardless of clay content.
Type B includes cohesive soils with compressive strengths between 0.5 and 1.5 tsf, plus silt, sandy loam, medium clay, and unstable rock. Soils that would otherwise qualify as Type A but are fissured or subject to vibration drop into this category.
Type C is the weakest classification: cohesive soils at or below 0.5 tsf, plus granular materials like gravel, sand, and loamy sand. Any submerged soil, or soil with water freely seeping from it, falls into Type C regardless of its dry strength.
Testing Before You Classify
The competent person must perform at least one visual test and at least one manual test before assigning a classification. Under OSHA’s definition, a competent person can identify existing and foreseeable hazards and has the authority to stop work immediately to correct them. A worker who can spot a problem but cannot shut the job down does not meet that definition.
Visual Inspection
The visual pass covers the excavation site and the spoil pile, looking for fissures, layering, water seepage, and evidence of previous disturbance. Tension cracks running along the surface near the trench edge are a strong indicator of pending wall failure. Standing water in the trench or water flowing from the walls automatically pushes the classification toward Type C.
Manual Tests
The thumb penetration test is the simplest. Press a thumb into a freshly exposed soil sample: if it barely dents, the soil has high cohesion and likely qualifies as Type A. If it sinks in easily, the soil is probably Type C. Anything in between suggests Type B.
A pocket penetrometer gives harder numbers. You press the rod perpendicular to a flat soil surface until it reaches the indicator mark, then read compressive strength directly off the scale. These instruments measure undrained shear strength up to about 5 kg/cm², covering the full range of OSHA’s classification thresholds.
The plasticity test — rolling a moist sample into a thin thread — reveals cohesion. Cohesive soils form a long, flexible ribbon; granular soils crumble before you finish rolling. The dry strength test works the other way: break a dried clump apart and note how much force it takes. A clump that shatters with little pressure points toward Type C.
These methods are spelled out in Appendix A to Subpart P. Results have to be documented before entry, and the classification is not a one-time event. If soil conditions change during the job, the competent person reassesses.
Benching Rules by Soil Type
Benching creates horizontal steps in the trench wall instead of a continuous slope. It consumes less surface area than full sloping, which makes it attractive on congested sites, but it only works in cohesive soils. Granular Type C material will not hold a vertical face long enough to form a step, so benching is prohibited in Type C entirely.
Type A
A simple bench in Type A can rise up to four feet vertically before stepping back horizontally. Multiple benches are allowed, but the overall slope from the bottom of the trench to the top of the last bench cannot exceed ¾:1, and no single bench face can exceed four feet in height. Keeping each step at four feet or less prevents large blocks of clay from shearing off in a single piece.
Type B
Type B soil supports benching at a 1:1 overall slope ratio. The total horizontal width of all benches combined must equal the total vertical depth of the trench. The bottom vertical face of any Type B benching system cannot exceed four feet. Because Type B is less cohesive than Type A, the steps need more horizontal run to distribute the load, which partially offsets the space advantage benching usually provides.
Type C
No benching. Granular soil will not hold the vertical faces a bench requires. Type C excavations that cannot be sloped at 1½:1 need shoring or shielding instead.
Layered Soils
When a trench cuts through more than one soil type, each layer is sloped according to its own classification. If Type A clay sits on top of Type C sand, the clay layer can be cut at ¾:1 but the sand layer below still needs the full 1½:1 slope. The competent person evaluates each visible layer independently rather than averaging them or applying one ratio to the whole wall.
The 20-Foot Ceiling
Every ratio and bench dimension above applies only to excavations 20 feet deep or less. Any sloping or benching system for a deeper trench must be designed by a registered professional engineer. The competent person’s authority to select ratios from OSHA’s tables stops at that depth. Engineer-designed systems account for site-specific variables the standard tables cannot capture: surcharge loads from nearby structures, groundwater pressure, and soil variability.
When Sloping and Benching Do Not Fit
Both methods consume surface area. Where adjacent buildings, roads, or utilities make it impossible to cut walls back far enough, OSHA treats shoring and shielding as equally valid protective systems under 29 CFR 1926.652(c), and both work in every soil type including Type C.
Shoring uses hydraulic jacks, timber, or aluminum rails to physically support the trench walls and prevent inward movement. It is an active system that holds the soil in place. Shielding, commonly called a trench box, does not support the walls at all. It places a rigid steel or aluminum structure inside the trench to protect workers if the walls collapse. The distinction matters for planning: shoring keeps the trench intact, while a trench box just keeps people alive if it fails. Both must follow manufacturer specifications or be designed by a registered professional engineer, and manufacturer tabulated data — load ratings, maximum depths, soil type restrictions — has to be kept on site in written form during construction.