OSHA Soil Types A, B and C Explained

The four categories at a glance

The classification system lives in Appendix A to Subpart P of 29 CFR Part 1926. The slopes come from Table B-1 in Appendix B, and they apply to excavations less than 20 feet deep.

Run the numbers for your own trench with the Trench Slope and Excavation Calculator — it returns the cut-back distance, the top width and the volume for each of these categories.

Type A: strong, cohesive, and easy to lose

Appendix A defines Type A as cohesive soil with an unconfined compressive strength of 1.5 tsf (144 kPa) or greater. The examples the regulation gives are clay, silty clay, sandy clay and clay loam, plus silty clay loam and sandy clay loam in some cases. Cemented soils such as caliche and hardpan also count.

The important half of the definition is the list of things that take Type A away. No soil is Type A if any one of these is true:

• The soil is fissured — it breaks along definite planes of fracture with little resistance, or it shows open cracks such as tension cracks in an exposed surface.
• The soil is subject to vibration from heavy traffic, pile driving or similar effects.
• The soil has been previously disturbed.
• The soil is part of a sloped, layered system where the layers dip into the excavation at 4H:1V or greater.
• The material is subject to other factors that would require it to be classified as a less stable material.

Read that list against a real job and Type A gets thin fast. A trench cut down a street shoulder is next to heavy traffic. A utility trench follows a corridor that was opened before, so the soil is previously disturbed. Clay that dried out over a hot week develops tension cracks along the top of the face. Any of those, and the deposit is Type B at best.

Type B: the middle ground, and the demotion bucket

Type B is defined six different ways in Appendix A, and only the first one is about strength:

1. Cohesive soil with an unconfined compressive strength greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa).
2. Granular cohesionless soils including angular gravel (similar to crushed rock), silt, silt loam, sandy loam, and in some cases silty clay loam and sandy clay loam.
3. Previously disturbed soils, except those that would otherwise be classified as Type C.
4. Soil that meets the strength or cementation requirements for Type A, but is fissured or subject to vibration.
5. Dry rock that is not stable.
6. Material in a sloped, layered system where the layers dip into the excavation on a slope less steep than 4H:1V, but only if the material would otherwise be classified as Type B.

Items 3 and 4 are the ones that matter day to day: Type B is where good soil lands after something disqualifies it. Note the guardrail in item 3 — previously disturbed soil does not get promoted to Type B if it was Type C material to begin with. Sand that was dug once is still sand.

Note also what is not in the Type B list: angular gravel is Type B, but plain gravel is Type C. The distinction is real. Crushed rock with angular faces interlocks; rounded river gravel does not.

Type C: the honest default on most jobs

Appendix A puts a deposit in Type C for any of five reasons:

• Cohesive soil with an unconfined compressive strength of 0.5 tsf (48 kPa) or less;
• Granular soils including gravel, sand and loamy sand;
• Submerged soil, or soil from which water is freely seeping;
• Submerged rock that is not stable;
• Material in a sloped, layered system where the layers dip into the excavation at 4H:1V or steeper.

This is why crews who work in Type C by default are usually not being lazy — they are being accurate. And OSHA built that shortcut into the standard: under 1926.652(b)(1)(i), an employer may simply slope at 1 1/2:1 (34 degrees) without doing any further analysis at all. Choosing Type C is the one classification you never have to defend.

How the classification is actually made

Appendix A (c)(1) puts the classification in the hands of a competent person — defined in 1926.650(b) as someone capable of identifying existing and predictable hazards and authorized to take prompt corrective measures. The authority half is not optional.

Paragraph (c)(2) sets the method: the classification must be based on the results of at least one visual and at least one manual analysis. One of each, minimum, every time.

The visual analysis

Appendix A (d)(1) lists what to look at: the particle sizes in the spoil and in the face (fine-grained means cohesive, coarse sand or gravel means granular); whether the soil comes out in clumps or breaks apart; crack-like openings or spalling on the face, which point to fissured material; evidence of existing utilities or previously disturbed ground; layers and the direction they dip; surface water, seepage or the water table; and sources of vibration near the excavation.

The manual analysis

Appendix A (d)(2) gives five accepted field tests:

Two practical notes the standard makes explicit. The thumb penetration test is run on an undisturbed sample as soon as practicable after excavation, to limit drying effects — and if the excavation is later rained on or flooded, the classification must be changed accordingly. And a pocket penetrometer or shearvane needs saturation, or near saturation, to read properly.

Layered systems, and when the answer expires

A layered system is two or more distinctly different soil or rock types arranged in layers; micaceous seams and weakened planes in rock or shale count as layered. Appendix A (c)(4) gives the rule in one sentence: the system is classified according to its weakest layer. The one exception runs the other way — each layer may be classified individually where a more stable layer lies under a less stable one.

So a trench with three feet of good clay over two feet of wet sand is a Type C excavation. Not a Type A excavation with a soft bottom.

Classification is also perishable. Under (c)(5), if the properties, factors or conditions affecting the classification change in any way, a competent person evaluates the change and reclassifies as necessary. That connects directly to the daily inspection duty in 1926.651(k)(1): before the start of work, as needed through the shift, and after every rainstorm or other hazard-increasing occurrence. A classification made on Monday morning is not evidence about Wednesday afternoon.

What the classification actually changes

The category is not paperwork. It sets three things on the ground.

• How far back you cut. At 8 feet deep, Type A means cutting back 6 feet on each side, Type B means 8 feet, Type C means 12 feet. Same trench, a 12-foot difference in top width between Type A and Type C — which is often the difference between fitting in the right of way and not.
• Which shoring table applies. Timber shoring (Appendix C) and aluminum hydraulic shoring (Appendix D) are both indexed by soil type. So is the tabulated data that comes with a trench box: a shield is rated by the manufacturer for a soil type and a depth, and using it outside that rating is using it wrong.
• Whether sloping is an option at all. Sloping and benching deeper than 20 feet must be designed by a registered professional engineer, per Table B-1 note 3, whatever the soil type is.

Write the classification down where it can be read later. The Excavation and Trenching Permit generator captures the soil type, the protective system chosen and the competent person on one printable page.

Five mistakes that show up in citations

1. Classifying from memory. "It was Type B on the last phase" is not a visual and a manual analysis of this deposit.
2. Skipping the manual test. Looking at the face is one analysis. Appendix A (c)(2) requires two, one of each kind.
3. Calling a utility corridor Type A. Soil over an existing line has been previously disturbed by definition, which rules out Type A outright.
4. Treating a trench box as a substitute for classification. The box carries a rating tied to a soil type and depth; you still need to know which one you are in.
5. Not reclassifying after rain. The standard says the classification must change accordingly when the excavation is exposed to wetting influences, and the daily inspection has to happen after every rainstorm.

The stakes are not abstract. CPWR's May 2024 Data Bulletin reports that OSHA counted 39 workers across all industries killed during trench or excavation work in 2022, up from 15 in 2021, and that construction accounted for about 85% of fatal trenching injuries from 2011 to 2021.

Bottom line

• Four categories: Stable Rock, Type A (1.5 tsf or more), Type B (above 0.5 and below 1.5 tsf), Type C (0.5 tsf or less, plus all granular and water-bearing soil).
• Fissuring, vibration, previous disturbance or layers dipping into the cut at 4H:1V or steeper all take Type A off the table.
• A competent person classifies, using at least one visual and at least one manual analysis, and reclassifies whenever conditions change.
• A layered system takes the classification of its weakest layer.
• Type C at 1 1/2:1 is always defensible: 1926.652(b)(1)(i) lets you slope at 34 degrees with no further analysis.