The four classes
Appendix A to 29 CFR 1926 Subpart P sorts soil into Stable Rock (the category above Type A, never a class for clay, sand or gravel), Type A, Type B and Type C. For cohesive soil, unconfined compressive strength sets the class:
- Type A: 1.5 tsf (144 kPa) or greater.
- Type B: greater than 0.5 tsf and less than 1.5 tsf.
- Type C: 0.5 tsf (48 kPa) or less.
The traps: 0.5 tsf is the B/C line, 1.0 tsf sits inside Type B, and 2.0 tsf is not a threshold.
Boundary values
A cohesive soil at exactly 0.5 tsf is Type C, because Type B requires strength greater than 0.5 tsf.
Strength is not the whole test
Fissured soil is not Type A, whatever its strength; Type B expressly includes it. A stiff clay testing at 2.0 tsf that the competent person finds fissured is Type B. Calling it Type A on strength alone is the common error.
Type B also covers angular gravel, silt, silt loam and sandy loam. Type C covers:
- cohesive soil at 0.5 tsf or less;
- granular soils such as gravel, sand and loamy sand;
- submerged soil, or soil from which water freely seeps.
So sand and gravel with water freely seeping from the face is Type C, not Type B.
How the competent person classifies
Classification must rest on at least one visual analysis and at least one manual analysis. One test type alone, visual or manual, is not enough. The competent person uses field tests; no engineer's laboratory report is required.
Layered soil
A layered system is classified by its weakest layer. Type A lying over Type C is Type C. Taking the top layer's class or averaging has no basis. Each layer may be classified on its own only where a more stable layer lies under a less stable one.