3.1 Table B-1 and the geometry of a slope
Sloping is the only protective system that needs no equipment, no rental and no manufacturer's chart. What it needs is ground, and the amount is not negotiable: Table B-1 converts your soil class into an angle, and geometry converts that angle into feet of cut-back per side. This chapter is that arithmetic, plus the two adjustments — distress and surcharge — that make the legal slope flatter than the table says.
1 Table B-1, and the three notes that come with it
Appendix B (c)(2) is one line: "The maximum allowable slope for a soil or rock deposit shall be determined from Table B-1 of this appendix." The table itself is titled for excavations less than 20 feet deep.
| Soil or rock type | Maximum allowable slope (H:V) | Angle from horizontal |
|---|---|---|
| Stable rock | Vertical | 90° |
| Type A | ¾:1 | 53° |
| Type B | 1:1 | 45° |
| Type C | 1½:1 | 34° |
The three notes are as binding as the table:
- Note 1: the angles in parentheses are degrees from the horizontal, rounded off.
- Note 2: a short-term maximum allowable slope of ½H:1V (63°) is allowed in Type A excavations 12 feet or less in depth. Deeper than 12 feet, the short-term maximum is ¾H:1V (53°). Appendix B defines short term exposure as a period less than or equal to 24 hours that an excavation is open.
- Note 3: sloping or benching for excavations greater than 20 feet deep shall be designed by a registered professional engineer.
2 From ratio to feet
H:V means horizontal run per unit of vertical rise. Cut-back per side = depth × H. Total top width = bottom width + (2 × cut-back). That is all the math there is, and it is worth doing out loud before the machine starts because the answer often does not fit the site.
| Depth | Type A (¾:1) | Type B (1:1) | Type C (1½:1) |
|---|---|---|---|
| 5 ft | 10 ft 6 in | 13 ft | 18 ft |
| 6 ft | 12 ft | 15 ft | 21 ft |
| 8 ft | 15 ft | 19 ft | 27 ft |
| 10 ft | 18 ft | 23 ft | 33 ft |
| 12 ft | 21 ft | 27 ft | 39 ft |
| 16 ft | 27 ft | 35 ft | 51 ft |
| 20 ft | 33 ft | 43 ft | 63 ft |
A residential lot is 50 to 70 feet wide. A city street between curbs is 26 to 40 feet. Read the Type C column against those numbers and the reason trench boxes exist becomes obvious: at 10 feet in Type C soil you need 33 feet of open ground, and a street with utilities on both sides does not have it. Run your own numbers with the trench slope calculator before committing a crew to a slope that will not fit.
3 Actual slope versus maximum allowable slope
Appendix B keeps two ideas separate, and the difference is where crews get caught. Maximum allowable slope is "the steepest incline of an excavation face that is acceptable for the most favorable site conditions." Actual slope is what you actually cut. Then (c)(3) sets three rules:
- 1(i) The actual slope shall not be steeper than the maximum allowable slope.
- 2(ii) Where there are signs of distress, the actual slope shall be less steep than the maximum allowable slope — cut back to at least ½ horizontal to one vertical (½H:1V) less steep than the maximum.
- 3(iii) Where surcharge loads from stored material or equipment, operating equipment or traffic are present, a competent person shall determine the degree to which the actual slope must be reduced below the maximum allowable slope, and shall assure that such reduction is achieved.
Note the phrase "most favorable site conditions" in the definition. Table B-1 is a ceiling for good conditions, not a target. Distress and surcharge both push the legal slope flatter, and (iii) makes surcharge a judgment the competent person has to make and enforce, not a factor to note and ignore.
Worked example: Type B soil, 8 feet deep, maximum allowable 1:1, so 8 feet of cut-back per side. Tension cracks appear along the top of the north face — distress. Under (c)(3)(ii) the actual slope must be at least ½H:1V less steep, so 1½:1, which is 12 feet of cut-back. You just lost 4 feet per side, and the excavator has to come back.
Second example: the same trench with the spoil pile and the excavator both on the south side. (c)(3)(iii) does not give a formula; it gives a duty. Someone has to decide how much flatter, and that decision is far easier to defend if the spoil is hauled off or the machine is moved instead.
4 Route (b)(1): the slope you can use with no analysis
1926.652(b)(1)(i) is the pragmatic option: excavations shall be sloped no steeper than 1½ horizontal to 1 vertical (34 degrees) unless the employer uses one of the other options. (b)(1)(ii) adds that slopes under this option must form the configurations shown for Type C soil in Appendix B.
Two consequences worth being clear about. First, this route requires no soil classification at all — it is the honest answer when nobody has classified the ground or when the classification is contested. Second, it is the Type C geometry, so it is the widest and most expensive slope in the table. It buys simplicity with width.
| Route | What it requires | Paperwork on site |
|---|---|---|
| (b)(1) Allowable configurations and slopes | Slope no steeper than 1½:1, Type C configurations | None required by this paragraph |
| (b)(2) Appendices A and B | Classify the soil, then use Table B-1 and Figure B-1 | None required, but the classification should be recorded |
| (b)(3) Other tabulated data | Written data identifying parameters, limits of use and explanatory information | A copy identifying the approving registered professional engineer, at the jobsite while the system is built |
| (b)(4) Design by a registered professional engineer | Written design giving safe slopes, configurations and the engineer's identity | A copy at the jobsite while the slope is being constructed |
5 Working on and around a sloped face
1926.652(f) adds a rule specific to sloped and benched excavations: employees shall not be permitted to work on the faces of sloped or benched excavations at levels above other employees, except when the employees at the lower levels are adequately protected from the hazard of falling, rolling or sliding material or equipment.
On a benched excavation that is a real scheduling constraint: a crew trimming an upper bench above a pipe crew is prohibited unless the lower crew is protected. Everything that comes off the upper level — soil, tools, a length of pipe — lands on the people below.
Also remember what sloping does not solve. A properly cut slope does nothing about the spoil pile at the lip (651(j)(2)), the ladder (651(c)(2)), the air at the bottom (651(g)), water (651(h)), or the loose rock hanging in the face (651(j)(1)). Part 4 covers all of it.
6 Deciding sloping honestly
Sloping is the right answer when three things are true: you own the width, the soil class supports the angle you need, and the excavation is not going to sit open through weather that changes the class. Miss any of the three and the honest conclusion is that this trench needs shoring or a shield — the subject of the next two chapters.
The cheapest failure mode in this whole subject is a slope cut to a class nobody established, on a lot that was never measured, with the spoil at the lip. It is also, by the citation data, the most common.
- Table B-1: stable rock vertical, Type A ¾:1 (53°), Type B 1:1 (45°), Type C 1½:1 (34°) — for excavations less than 20 feet deep.
- The Type A short-term ½:1 (63°) slope applies only at 12 feet or less and only when the excavation is open 24 hours or less.
- Cut-back per side = depth × H. A 3-ft trench 10 ft deep in Type C needs 33 feet of total top width.
- Signs of distress force the actual slope at least ½H:1V flatter than the maximum allowable — Appendix B (c)(3)(ii).
- Surcharge loads from spoil, equipment or traffic require the competent person to reduce the slope further and to make sure it happens — (c)(3)(iii).
- 1926.652(b)(1)(i) allows 1½:1 with no soil analysis at all; it costs width and nothing else.
Free educational content — not OSHA-authorized training, no certificate or card issued. Follow your employer's program and the standards cited.