Strength sets the starting class
Appendix A to Subpart P classes cohesive soil by unconfined compressive strength:
- Type A: 1.5 tsf (144 kPa) or more
- Type B: greater than 0.5 tsf and less than 1.5 tsf
- Type C: 0.5 tsf (48 kPa) or less
So 1.0 tsf is Type B. The trap is the boundaries: exactly 1.5 tsf is Type A, not Type B, and exactly 0.5 tsf is Type C. Classifying needs no engineer; the engineer rule is Appendix B sloping deeper than 20 ft.
Type A and what knocks soil out of it
Cemented soils such as caliche and hardpan are Type A. They are named as soils, not as stable rock.
No soil is Type A if it is fissured, subject to vibration from heavy traffic or pile driving, previously disturbed, or part of a sloped layered system dipping into the excavation at 4H:1V or greater. Type B expressly includes soil that meets Type A strength but is fissured or subject to vibration, so vibrated 2.0 tsf clay is Type B. The trap: vibration drops soil out of Type A but does not by itself make it Type C. Depth plays no part in classification; the 12 ft figure belongs to the short-term Type A slope.
Granular soils: B or C
- Type B granular cohesionless soils: angular gravel, silt, silt loam, sandy loam
- Type C granular soils: gravel, sand, loamy sand
Type C also takes submerged or seeping soil. The trap: angular gravel and silt loam are Type B, while loamy sand is Type C.
Layered systems and unstable rock
A sloped layered system dipping into the excavation at 4H:1V or steeper is Type C; one dipping less steeply than 4H:1V is Type B. A 3H:1V dip is steeper than 4H:1V, so it is Type C. The trap: the top or strongest layer does not set the class.
Rock that is not stable is classed with the soils:
- dry rock that is not stable: Type B
- submerged rock that is not stable: Type C