OSHA Soil Type Classification Helper (A, B, C)

Q: How does OSHA classify soil into Type A, B and C?

29 CFR 1926 Subpart P, Appendix A sorts deposits into Stable Rock, Type A, Type B and Type C, in decreasing order of stability. Type A is cohesive soil with an unconfined compressive strength of 1.5 tsf (144 kPa) or greater; Type B is cohesive soil greater than 0.5 tsf but less than 1.5 tsf, plus granular cohesionless soils such as angular gravel, silt, silt loam and sandy loam; Type C is cohesive soil of 0.5 tsf or less, granular soils such as gravel, sand and loamy sand, and any submerged soil or soil from which water is freely seeping.

Q: What disqualifies a soil from being Type A?

Appendix A lists five conditions. No soil is Type A if it is fissured; if it is subject to vibration from heavy traffic, pile driving or similar effects; if it has been previously disturbed; if it is part of a sloped, layered system where the layers dip into the excavation at four horizontal to one vertical (4H:1V) or greater; or if it is subject to other factors that would require a less stable classification. Material that meets the Type A strength or cementation criteria but is fissured or vibrated becomes Type B.

Q: How do you do the thumb penetration test?

Press your thumb into an undisturbed sample, such as a large clump of spoil, as soon as practicable after excavation. Type A soils, at 1.5 tsf, can be readily indented by the thumb but penetrated only with very great effort. Type C soils, at 0.5 tsf, can be easily penetrated several inches by the thumb and can be molded by light finger pressure. Appendix A (d)(2)(iii) points to ASTM D2488 as the basis for the test, and a pocket penetrometer or hand-operated shearvane gives the same estimate more precisely.

Q: How many tests does OSHA require before a soil is classified?

At least one visual and at least one manual analysis, conducted by a competent person using the tests described in Appendix A (d) or other recognized methods such as ASTM or the USDA textural classification system. That is paragraph (c)(2). A visual observation on its own is not a classification, and neither is a manual test on its own.

Q: Is previously disturbed soil always Type C?

No. Previously disturbed soils are Type B under Appendix A Type B (iii), except those which would otherwise be classified as Type C soil — for example a backfilled trench in gravel or sand, or one where water is freely seeping, stays Type C. Disturbance always removes Type A.

Q: What happens when the soil is layered?

In a layered system the system is classified in accordance with its weakest layer; each layer may be classified individually only where a more stable layer lies under a less stable layer (Appendix A (c)(4)). Separately, material in a sloped, layered system whose layers dip into the excavation at 4H:1V or steeper is Type C, and that same geometry removes Type A.

Q: Does the classification hold for the whole job?

No. Appendix A (c)(5) requires the competent person to evaluate any change in the properties, factors or conditions that affect the classification, and to reclassify the deposit as necessary. Rain, flooding, a new source of vibration, deepening the cut or water showing up in the face all trigger that re-evaluation — the thumb penetration paragraph says so explicitly for wetting.

Q: Can this page classify the soil for me?

No, and nothing on this site can. Appendix A (c)(1) puts the classification on a competent person who is on site and has run the analyses. This helper reproduces the published criteria, shows which clause drives each downgrade, and falls back to the least stable outcome whenever an observation is missing, so you can document your reasoning — the reasoning still has to be yours.

This helper walks the soil classification criteria of 29 CFR 1926 Subpart P, Appendix A: what the deposit is, its estimated unconfined compressive strength, fissures, vibration, previous disturbance, water and layering. It shows the clause behind each downgrade and the maximum allowable slope that would follow. It does not classify soil for you — only a competent person does that, on site, after at least one visual and one manual analysis.

What is the deposit?

Appendix A (b) definitions and the (d)(1) visual tests: cohesive soil stays in clumps and is plastic when moist; granular soil crumbles and has no cohesive strength.

Estimated unconfined compressive strength (cohesive or cemented soil only)

Thumb penetration, pocket penetrometer or hand-operated shearvane — Appendix A (d)(2)(iii) and (iv). 1 tsf = 96 kPa.

Is the material fissured?

Open cracks or tension cracks in an exposed surface, chunks spalling off a vertical face, or a drying test sample that cracks — Appendix A (b), (d)(1)(iii) and (d)(2)(v)(A).

Is the soil subject to vibration?

Heavy traffic, pile driving or similar effects, observed next to the excavation — Appendix A (d)(1)(vii).

Has the soil been previously disturbed?

Backfill over a utility, a previous excavation, a structure — Appendix A (d)(1)(iv).

Water conditions

Appendix A (d)(1)(vi). Submerged soil means soil which is underwater or is free seeping.

Layered system

Two or more distinctly different soil or rock types in layers; estimate the degree of slope of the layers — Appendix A (b) and (d)(1)(v).

Analyses completed by the competent person

Appendix A (c)(2) requires at least one visual AND at least one manual analysis before a deposit is classified.

Results are estimates for planning purposes and do not replace a competent person's evaluation or an engineer's design. Follow the OSHA standards that apply to your work and your employer's written program.

Who classifies the soil, and on what basis

Appendix A (c)(1) puts the classification on a competent person, on site: each soil and rock deposit is classified as Stable Rock, Type A, Type B or Type C using the definitions of paragraph (b). Paragraph (c)(2) sets the basis — the classification is made on the results of at least one visual and at least one manual analysis, using the tests of paragraph (d) or other recognized methods such as ASTM or the USDA textural classification system. One observation is not a classification.

Two more rules decide most real jobs. In a layered system, the system is classified by its weakest layer; each layer may be classified individually only where a more stable layer lies under a less stable layer (c)(4). And when the properties, factors or conditions affecting the classification change in any way, the competent person evaluates the change and reclassifies as necessary (c)(5) — rain, flooding, new vibration, a deeper cut, water in the face.

Type A, Type B and Type C in the words of Appendix A

Soil classification criteria — 29 CFR 1926 Subpart P, Appendix A (b)
Class	Cohesive soil	Granular and rock	What removes it
Stable Rock	—	Natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed	Rock that does not stay intact is not stable rock: Type B when dry, Type C when submerged
Type A	Unconfined compressive strength of 1.5 tsf (144 kPa) or greater — clay, silty clay, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam	Cemented soils such as caliche and hardpan	Fissured; subject to vibration from heavy traffic, pile driving or similar effects; previously disturbed; part of a sloped, layered system where the layers dip into the excavation at 4H:1V or greater; or subject to other factors requiring a less stable classification
Type B	Greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa)	Granular cohesionless soils: angular gravel (similar to crushed rock), silt, silt loam, sandy loam and, in some cases, silty clay loam and sandy clay loam; dry rock that is not stable	Previously disturbed soils are Type B except those which would otherwise be Type C; Type A material that is fissured or vibrated lands here
Type C	Unconfined compressive strength of 0.5 tsf (48 kPa) or less	Gravel, sand and loamy sand; submerged rock that is not stable	Submerged soil or soil from which water is freely seeping; material in a sloped, layered system where the layers dip into the excavation at 4H:1V or steeper

Cohesive soil is clay or soil with a high clay content: it does not crumble, can be excavated with vertical sideslopes, is plastic when moist and hard to break up when dry. Granular soil — gravel, sand or silt with little or no clay content — has no cohesive strength, cannot be molded when moist and crumbles easily when dry.

The tests Appendix A accepts

Visual analysis — paragraph (d)(1)

Estimate particle sizes in the spoil and in the face: mostly fine-grained is cohesive material, mostly coarse sand or gravel is granular.
Watch the soil as it is excavated: what stays in clumps is cohesive, what breaks up and does not hold is granular.
Look at the opened face and the ground next to it for crack-like openings and tension cracks — possible fissured material. Chunks spalling off a vertical side point the same way; small spalls are evidence of moving ground.
Look for existing utilities and other underground structures — the sign of previously disturbed soil.
Identify layered systems in the face and estimate whether, and how steeply, the layers dip toward the excavation.
Look for surface water, water seeping from the sides, and the level of the water table.
Look for sources of vibration near the excavation.

Manual analysis — paragraph (d)(2)

Acceptable manual tests and what each one tells you
Test	Procedure	Reading
Plasticity	Mold a moist or wet sample into a ball, then roll it into threads as thin as 1/8 inch	If at least a 2 inch (50 mm) length of 1/8 inch thread can be held on one end without tearing, the soil is cohesive
Dry strength	Crumble a dry sample by hand	Crumbles into individual grains or fine powder → granular. Falls into clumps that break into smaller clumps only with difficulty → clay in some combination. Clumps that do not break into smaller clumps and break only with difficulty, with no visual sign of fissures → may be considered unfissured
Thumb penetration	Press your thumb into an undisturbed sample — a large clump of spoil — as soon as practicable after excavation	Type A (1.5 tsf): readily indented by the thumb, but penetrated only with very great effort. Type C (0.5 tsf): easily penetrated several inches, and molds under light finger pressure
Other strength tests	Pocket penetrometer or hand-operated shearvane	A direct estimate of unconfined compressive strength; the instruments need saturated or near-saturated soil to read correctly
Drying test	Dry a sample about 1 inch thick and 6 inches across until thoroughly dry, then break it by hand	Cracks as it dries → significant fissures. Dries without cracking and takes considerable force to break → unfissured cohesive material, so determine its strength. Breaks easily → fissured cohesive or granular: pulverize the clumps, hard to pulverize means cohesive with fissures, easy means granular

The thumb penetration paragraph adds a rule people forget: run it on an undisturbed sample as soon as practicable after excavation, to keep the effects of drying to a minimum — and if the excavation is later exposed to rain or flooding, the classification must be changed accordingly.

What the class changes on the ground

Maximum allowable slopes for excavations less than 20 feet deep — Appendix B, Table B-1
Class	Maximum allowable slope (H:V)	Angle from horizontal	Cut back per foot of depth
Stable rock	Vertical	90°	0 ft
Type A	3/4:1	53°	0.75 ft
Type A — short term (24 h or less, 12 ft or less deep)	1/2:1	63°	0.5 ft
Type B	1:1	45°	1 ft
Type C	1 1/2:1	34°	1.5 ft

One class down costs real width: an 8 ft deep trench cut back at Type B slopes is 16 ft wider at the top than at the bottom, and 24 ft wider at Type C. Run the numbers with the Trench Slope & Excavation Calculator, and remember that sloping is only one of the options — shoring and shielding take the same soil type as an input, from the manufacturer's tabulated data or Appendices C and D.

Table B-1 applies below 20 feet. Sloping and benching for excavations deeper than 20 feet must be designed by a registered professional engineer. Employees must be protected from cave-ins by an adequate protective system except when the excavation is made entirely in stable rock, or is less than 5 feet deep and a competent person's examination of the ground finds no indication of a potential cave-in — 29 CFR 1926.652(a)(1).

How it's calculated

Starting point — what the deposit is: stable rock → Stable Rock | unstable rock → Type B if dry, Type C if submerged | cohesive or cemented → by unconfined compressive strength | angular gravel, silt, silt loam, sandy loam → Type B | gravel, sand, loamy sand → Type C
Appendix A (b): Type B (ii) and (v), Type C (ii) and (iv). Cemented soils such as caliche and hardpan are considered Type A before the exclusions are applied.
Cohesive soil — unconfined compressive strength: qu >= 1.5 tsf (144 kPa) → Type A | 0.5 tsf < qu < 1.5 tsf → Type B | qu <= 0.5 tsf (48 kPa) → Type C
Appendix A (b), Type A, Type B (i) and Type C (i). With no estimate of strength the helper falls back to Type C, the least stable outcome.
Downgrades that remove Type A: fissured, or subject to vibration, or previously disturbed, or layers dipping into the excavation at 4H:1V or steeper, or any other factor requiring a less stable classification
Appendix A Type A (i) to (v). Fissured or vibrated Type A material becomes Type B under Type B (iv); previously disturbed soil is Type B under Type B (iii), except soil that would otherwise be Type C.
Downgrades to Type C: submerged soil or soil from which water is freely seeping, submerged rock that is not stable, or layers dipping into the excavation at 4H:1V or steeper
Appendix A Type C (iii), (iv) and (v).
Layered systems: classification = weakest layer (each layer may be classified individually where a more stable layer lies under a less stable layer)
Appendix A (c)(4). Answer for the layer you are judging, then repeat for the others.
Resulting maximum allowable slope (Table B-1): Stable Rock vertical (90°) | Type A 3/4:1 (53°) | Type B 1:1 (45°) | Type C 1 1/2:1 (34°)
Appendix B, Table B-1, for excavations less than 20 ft deep. A short-term 1/2:1 (63°) slope is allowed in Type A soil in excavations 12 ft or less deep. Deeper than 20 ft, sloping and benching must be designed by a registered professional engineer.

Frequently asked questions