Soil classification methods

The soil classification categories are Stable Rock, Type A, Type B, and Type C, listed from most to least stable. See Appendix A to Subpart P of Part 1926 (Soil Classification) for the full definitions and hierarchy.

• Use these categories when designing sloping, benching, timber shoring, or aluminum hydraulic shoring systems as allowed by 1926.652(b)(2).
• The appendix explains how to determine each category by visual and manual tests.

Type A soil is cohesive soil with an unconfined compressive strength of 1.5 tons per square foot (tsf) (144 kPa) or greater; cemented soils such as caliche and hardpan are also considered Type A. See Appendix A to Subpart P of Part 1926 (Soil Classification) for the exact definition.

• Examples include clay, silty clay, sandy clay, and clay loam.
• Note the appendix lists specific conditions that will disqualify soil from being Type A (see related question on disqualifiers).

A soil cannot be Type A if it is fissured; subject to vibration (from heavy traffic, pile driving, etc.); previously disturbed; part of a sloped layered system where layers dip into the excavation 4H:1V or greater; or subject to other factors that make it less stable. See the disqualifying list in Appendix A to Subpart P of Part 1926 (Soil Classification).

• If any of these conditions exist, reclassify the soil to a less stable category (Type B or C) as appropriate.
• When design of protection relies on these classifications, follow 1926.652(b)(2).

Type B soils include cohesive soils with unconfined compressive strength greater than 0.5 tsf (48 kPa) but less than 1.5 tsf (144 kPa), certain granular cohesionless soils (e.g., angular gravel, silt, silt loam, sandy loam), previously disturbed soils (except those that must be Type C), fissured or vibrated soils that would otherwise meet Type A strength, and some dry rock that is not stable. See Appendix A to Subpart P of Part 1926 (Soil Classification) for details.

• Type B is the intermediate category used when soils do not meet Type A strength or must be downgraded from A for other reasons.

Type C soils are the least stable category and include cohesive soils with unconfined compressive strength of 0.5 tsf (48 kPa) or less, granular soils such as gravel, sand, and loamy sand, submerged soil or free-seeping soil, submerged rock that is not stable, and layered systems where layers dip into the excavation 4H:1V or steeper. See Appendix A to Subpart P of Part 1926 (Soil Classification) for the full definition.

• Because Type C is least stable, protective systems must be designed accordingly under 1926.652.

A competent person must classify each soil and rock deposit, and the classification must be updated if site conditions change. See 1926.652(c) and Appendix A to Subpart P of Part 1926 (Soil Classification).

• The appendix requires at least one visual and one manual analysis by the competent person when classifying (see Appendix A, paragraph (c)).
• If properties, factors, or conditions affecting classification change (for example, water, vibration, or disturbance), the competent person must re-evaluate and reclassify the deposit as needed.

Acceptable visual tests include observing excavated samples and the excavation sides to estimate particle sizes and amounts, watching how soil breaks when dug (clumps vs. crumbles), checking for tension cracks or spalling (signs of fissuring), identifying previously disturbed areas and underground structures, noting layered systems and their dip toward the excavation, looking for surface water or seepage, and checking for vibration sources. See Appendix A to Subpart P of Part 1926 (Soil Classification), paragraph (d)(1).

• Visual observations provide qualitative evidence to help select the correct soil category.
• Record observations that affect stability (water, layers, fissures, vibration) because they can change classification.

Acceptable manual tests include plasticity (rolling moist samples into 1/8-inch threads), dry strength (how soil crumbles or forms clumps when dry), thumb penetration (estimating unconfined compressive strength), pocket penetrometer or shear vane measurements, and the drying test for fissuring versus granular behavior. See Appendix A to Subpart P of Part 1926 (Soil Classification), paragraph (d)(2).

• Plasticity: cohesive soils can be rolled into thin threads without crumbling.
• Dry strength: helps distinguish granular from cohesive and fissured cohesive soils.
• Thumb penetration/pocket penetrometer/shear vane: help estimate unconfined compressive strength for Type A/B/C decisions.
• Drying test: differentiates fissured cohesive, unfissured cohesive, and granular soils.

Perform the thumb penetration test on an undisturbed moist sample soon after excavation by attempting to penetrate the cohesive soil with your thumb; Type A soils (≈1.5 tsf) can be indented only with very great effort, while Type C soils (≈0.5 tsf) can be easily penetrated several inches and molded by light finger pressure. See Appendix A to Subpart P of Part 1926 (Soil Classification), paragraph (d)(2)(iii).

• This is an approximate field test; use a pocket penetrometer or laboratory testing for more precise strength estimates.
• Conduct the test on an undisturbed clump or spoil sample before drying or wetting changes its properties.

Yes — the appendix specifically allows estimating unconfined compressive strength by laboratory testing or field methods such as a pocket penetrometer, thumb penetration, and hand-operated shear vane. See Appendix A to Subpart P of Part 1926 (Soil Classification), Section (b) and paragraph (d)(2)(iv).

• Remember pocket penetrometer readings are valid only when the soil is saturated or near saturation for some instruments; check the appendix guidance on saturation conditions.
• Use these field estimates to support classification but rely on a competent person's judgment when protection systems depend on the result.

The drying test dries a roughly 1-inch thick, 6-inch diameter sample until thoroughly dry: if the sample cracks as it dries, significant fissures are indicated; if it dries without cracking and is hard to break, it is unfissured cohesive (determine unconfined compressive strength); if it breaks easily, pulverize the clumps — if they do not pulverize easily the material is fissured cohesive; if they pulverize easily into small fragments the material is granular. See the drying test procedure in Appendix A to Subpart P of Part 1926 (Soil Classification), paragraph (d)(2)(v).

• The test helps distinguish fissured cohesive soils (which behave more dangerously) from unfissured cohesive and granular soils.

You may use the soil classification method in Appendix A when designing a sloping or benching system in accordance with the requirements of 1926.652(b)(2). See Appendix A to Subpart P of Part 1926 (Soil Classification) for the tests and definitions that let a competent person classify the soil.

• Appendix A lists visual and manual testing requirements the competent person must perform to base a sloping/benching design on these classifications.
• If you use Appendix A classifications for other protective systems, ensure they are consistent with 1926.652(c).

Stable rock is natural solid mineral matter that can be excavated with vertical sides and remain intact while exposed. See the definition in Appendix A to Subpart P of Part 1926 (Soil Classification).

• When confirmed by a competent person, stable rock allows vertical sides without sloping, benching, or shoring for cave-in protection; however, other hazards (falls, water, equipment) still require controls under 1926.652.
• Always verify stability by visual/manual tests and re-evaluate if conditions change.

A layered system must be classified according to its weakest layer; however, each layer may also be classified individually when a more stable layer lies under a less stable one. See Appendix A to Subpart P of Part 1926 (Soil Classification), paragraph (c)(4).

• If the weaker layer controls stability (for example, a weak layer at the excavation face), design protection for that weakest layer.
• Pay attention to layer dip: when layers dip into the excavation at 4H:1V or steeper, that affects classification (see Type A/C rules in Appendix A).

Moisture conditions affect cohesion and stability: "dry" shows no visible moisture; "moist" allows molding and thread-rolling for cohesive soils; "wet" means the soil will slump or flow when vibrated; "submerged" or "saturated" means pores are filled with water and can change instrument behavior — saturated conditions are often required for pocket penetrometers or shear vanes. See the moisture definitions in Appendix A to Subpart P of Part 1926 (Soil Classification).

• If the excavation becomes wetter (rain, seepage), reclassification may be necessary per 1926.652(c)(5).
• Wet or submerged soils are often Type C because they lose cohesion.

When properties, factors, or conditions affecting a previously classified deposit change, the competent person must evaluate the changes and reclassify the deposit as necessary. See Appendix A to Subpart P of Part 1926 (Soil Classification), paragraph (c)(5) and 1926.652(c).

• Common triggers for reclassification include rainfall, changes in vibration, excavation of adjacent areas, dewatering, or discovery of a fissured layer.
• Document observations and any test results used to reclassify so protection decisions are defensible.

No. Appendix A specifically states that a soil is not Type A if it has been previously disturbed; previously disturbed soils are generally classified as Type B unless they meet Type C criteria. See Appendix A to Subpart P of Part 1926 (Soil Classification).

• Treat previously disturbed areas cautiously because prior excavation, backfill, or compaction often reduce soil strength and increase instability.

Visually identify fissured soil by looking for open cracks (such as tension cracks), pieces spalling off vertical faces, or fracture planes where the soil breaks along definite planes with little resistance. The appendix lists these signs in its visual test guidance. See Appendix A to Subpart P of Part 1926 (Soil Classification), paragraph (d)(1)(iii).

• Small spalls or active cracking are evidence of moving ground and indicate a potentially hazardous condition requiring reclassification and protective measures.
• Confirm visual signs with the drying test or manual tests to decide whether the soil is fissured cohesive or granular.

Saturation or near saturation is necessary for proper use of instruments such as a pocket penetrometer or shear vane, because the presence of water affects readings. See the saturation guidance in Appendix A to Subpart P of Part 1926 (Soil Classification).

• If soil is not saturated, instrument results may be misleading; use caution and supplement with other manual and visual tests.
• When relying on instrument readings for design decisions, document soil moisture conditions and test method limitations.

Appendix A permits using other recognized soil classification and testing methods such as ASTM standards (e.g., D2488) or the U.S. Department of Agriculture textural classification system, as long as a competent person performs at least one visual and one manual analysis to classify deposits. See Appendix A to Subpart P of Part 1926 (Soil Classification), paragraph (c)(2).

• Use standardized laboratory methods when a project requires precise strength data for engineered protective systems.
• If you adopt another recognized method, ensure the competent person documents how the method supports the chosen soil category and protective system design.