Table B-1 slopes
Appendix B, Table B-1, gives the maximum allowable slopes for excavations less than 20 ft deep:
- Stable rock: vertical (90°)
- Type A: 3/4:1 (53°)
- Type B: 1:1 (45°)
- Type C: 1½:1 (34°)
Angles are measured from horizontal. The trap: 56° is 90° − 34°, the angle from vertical, a listed common error.
Read the ratio as H:V
Every ratio is horizontal to vertical. 1½:1 means 1½ ft horizontal for every 1 ft of depth. It is not a multiplier on 45°, and it relates run to depth, not to trench width or setback.
Horizontal run = ratio × depth. In Type C soil 12 ft deep, the top of slope sits 1.5 × 12 = 18 ft back from the toe. The trap: 12 ÷ 1.5 = 8 ft reads the ratio as V:H.
The short-term 1/2:1 slope
Table B-1 allows a short-term slope of 1/2:1 (63°) only when all three hold:
- Type A soil
- 12 ft deep or less
- open 24 hours or less
The trap: Type B never qualifies (its maximum stays 1:1), and 14 ft deep or 36 hours open each fails.
Top width
Top width = bottom width + run on each side. Type B, 8 ft deep, 3 ft bottom: each side runs 1 × 8 = 8 ft, so the top is 3 + 8 + 8 = 19 ft. The trap: 3 + 8 = 11 ft slopes only one side.
Volume in cubic yards
Volume uses the average width. Type A, 6 ft deep, 2 ft bottom, 100 ft long, sloped at 3/4:1:
- each side runs 0.75 × 6 = 4.5 ft, so the top is 2 + 4.5 + 4.5 = 11 ft
- cross-section = (2 + 11) ÷ 2 × 6 = 39 ft²
- volume = 39 × 100 = 3,900 ft³ ÷ 27 = 144.4 yd³
The traps: bottom width alone (44.4 yd³), full top width (244.4 yd³), and dividing by 9 instead of 27 (433.3 yd³).