2.1 Stable rock, Type A, B and C
Every slope in Table B-1 and every page of a shoring manufacturer's chart begins with one letter: A, B or C. Getting that letter wrong by one grade does not make the trench slightly less safe — it makes a 45-degree cut look legal in ground that needs 34 degrees. This chapter is the definitions, and the five conditions that quietly demote soil out of Type A.
1 Four classes, in decreasing order of stability
Subpart P Appendix A (b) defines a soil classification system as a method of categorizing soil and rock deposits in a hierarchy of Stable Rock, Type A, Type B and Type C, in decreasing order of stability, based on the properties and performance characteristics of the deposits and the environmental conditions of exposure.
That last clause is the one that surprises people. The class is not a property of the dirt alone. The same clay is Type A on a quiet lot and Type B beside a road with truck traffic, because vibration is an environmental condition of exposure. Classification describes a deposit in its situation.
2 The definitions, as written
| Class | Definition | Examples the standard gives |
|---|---|---|
| Stable rock | Natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed | — |
| Type A | Cohesive soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) 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 |
| Type B | Cohesive soil greater than 0.5 tsf but less than 1.5 tsf; granular cohesionless soils of the listed kinds; previously disturbed soils except those that would be Type C; soil meeting Type A strength but fissured or subject to vibration; dry rock that is not stable; material in a layered system dipping into the cut less steeply than 4H:1V that would otherwise be Type B | Angular gravel (similar to crushed rock), silt, silt loam, sandy loam, and in some cases silty clay loam and sandy clay loam |
| Type C | Cohesive soil of 0.5 tsf or less; granular soils; submerged soil or soil from which water is freely seeping; submerged rock that is not stable; material in a layered system where the layers dip into the excavation at 4H:1V or steeper | Gravel, sand, loamy sand |
Read the Type C list again: gravel, sand and loamy sand are Type C, full stop. There is no dry-sand exemption, no compacted-sand exemption. Granular soil is defined in the same appendix as coarse-grained soil with little or no clay content and no cohesive strength; some moist granular soils show apparent cohesion, which is precisely the illusion that gets crews killed.
And note the strength boundaries: 1.5 tsf and 0.5 tsf. Those are the numbers behind the thumb penetration test in Chapter 2.3.
3 The five things that disqualify Type A
Appendix A (b) says that no soil is Type A if any of the following is true. Any single one is enough.
- 1The soil is fissured — it tends to break along definite planes of fracture with little resistance, or shows open cracks such as tension cracks in an exposed surface.
- 2The soil is subject to vibration from heavy traffic, pile driving, or similar effects.
- 3The soil has been previously disturbed.
- 4The soil is part of a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical (4H:1V) or greater.
- 5The material is subject to other factors that would require it to be classified as a less stable material.
Now count how many ordinary jobs those five knock out of Type A. Any dig in a street or beside one: vibration. Any dig in an old utility corridor, a backfilled trench, a graded lot: previously disturbed. Any face with tension cracks along the top: fissured. In practice Type A is rare on urban and suburban work, and a competent person who reaches Type A should be able to say which of the five they ruled out and how.
Where does the demoted soil land? Type B (iv) catches it explicitly: soil that meets the unconfined compressive strength or cementation requirements for Type A, but is fissured or subject to vibration, is Type B. Previously disturbed soils are Type B under (iii), except those that would otherwise be Type C.
4 Cohesive versus granular, in the field
Everything else follows from one question: does this soil hold itself together? Appendix A's definitions give you the field version.
| Behavior | Cohesive | Granular |
|---|---|---|
| What it is | Clay, or soil with high clay content | Gravel, sand or silt, little or no clay |
| Crumbling | Does not crumble | Crumbles easily when dry |
| Vertical sides | Can be excavated with vertical sideslopes | Cannot hold them |
| When moist | Plastic — can be shaped and rolled into threads | Cannot be molded; may show apparent cohesion only |
| When dry | Hard to break up | Falls into individual grains or fine powder |
| Submerged | Exhibits significant cohesion | No cohesive strength at all |
Appendix A also gives you the moisture vocabulary, which decides a lot of borderline calls: dry soil shows no visible signs of moisture; moist soil looks and feels damp; wet soil holds significantly more moisture, in the range where cohesive material will slump or begin to flow when vibrated; saturated means the voids are filled with water; submerged means underwater or free seeping. Wet granular material that would show cohesive properties when moist loses those properties when wet.
5 What each class buys you
Classification is not an academic exercise; it converts directly into slope and into which shoring page you may use.
| Class | Maximum allowable slope | Angle | Cut-back per side at 10 ft |
|---|---|---|---|
| Stable rock | Vertical | 90° | 0 ft |
| Type A | ¾:1 | 53° | 7 ft 6 in |
| Type A, short term (≤12 ft, open ≤24 h) | ½:1 | 63° | 5 ft |
| Type B | 1:1 | 45° | 10 ft |
| Type C | 1½:1 | 34° | 15 ft |
One grade of error is 2 to 5 feet of missing cut-back per side at 10 feet of depth. That is not a rounding difference; it is the whole margin. And in the other direction, choosing Type C when you are unsure costs width and nothing else — 1926.652(b)(1)(i) lets you slope at 1½:1 with no analysis at all.
6 Who decides, and on what evidence
Appendix A (c)(1) puts the decision with the competent person: each soil and rock deposit shall be classified as Stable Rock, Type A, Type B or Type C in accordance with the definitions. And (c)(2) sets the evidence standard: the classification shall be made based on the results of at least one visual and at least one manual analysis, conducted by a competent person using the tests in (d) or other recognized methods such as ASTM's or the USDA textural classification system.
Two analyses, minimum. The next two chapters are exactly those: what the visual analysis looks at, and what the manual tests are and how to do them at the spoil pile in front of the crew.
- The hierarchy is Stable Rock, Type A, Type B, Type C, and it accounts for environmental conditions of exposure, not just the soil itself.
- Type A is cohesive soil at 1.5 tsf or more; Type C is cohesive soil at 0.5 tsf or less, all granular soils, and anything submerged or freely seeping.
- Gravel, sand and loamy sand are Type C however firm they feel — granular soil has no cohesive strength, only apparent cohesion when moist.
- Five conditions each disqualify Type A on their own: fissured, vibration, previously disturbed, layers dipping into the cut at 4H:1V or steeper, or any other factor requiring less stable classification.
- Soil that meets Type A strength but is fissured or vibrated becomes Type B under Appendix A (b) Type B (iv).
- One grade of classification error is 2 to 5 feet of missing cut-back per side at 10 feet deep.
Free educational content — not OSHA-authorized training, no certificate or card issued. Follow your employer's program and the standards cited.