Part 313 min

3.2 Benching, vertical lower portions and layered soils

A slope is not the only shape Appendix B allows. Figure B-1 gives benched configurations, combination cuts with a vertical lower portion, and rules for layered ground — and each one has dimensions attached. The two numbers to carry out of this chapter are 3½ feet, the maximum unsupported vertical side in the shallow Type A configuration, and 18 inches, how far a shield or support must extend above the top of a vertical side.

1 What benching is, and where it is allowed

1926.650(b) defines benching as a method of protecting employees from cave-ins by excavating the sides to form one or a series of horizontal levels or steps, usually with vertical or near-vertical surfaces between levels. Appendix B (c)(4) then requires that configurations of sloping and benching systems be in accordance with Figure B-1.

The critical fact: Figure B-1 provides benching configurations for Type A and Type B soil only. B-1.1 (Type A) gives simple bench and multiple bench arrangements at a maximum allowable slope of ¾:1. B-1.2 (Type B) gives single and multiple bench arrangements at 1:1. B-1.3, Type C, offers no bench at all — only a simple 1½:1 slope, or a vertically sided lower portion that is shielded or supported.

Type A: bench at ¾:1

Type B: bench at 1:1

Type C: no benching

Vertical side: 18 in cover

Why benching at all, if the overall slope ratio is the same? Because a bench gives you a working surface, keeps machinery further from the edge, and can be easier to cut cleanly in cohesive soil than a long uniform face. What it does not do is let you cut steeper overall than Table B-1 allows.

2 The Type A configurations, with their dimensions

Appendix B Figure B-1.1 — excavations in Type A soil
Configuration	Limit
Simple slope, general	20 ft or less deep, maximum allowable slope ¾:1
Simple slope, short term	Open 24 hours or less and 12 ft or less deep: maximum allowable slope ½:1
Simple bench and multiple bench	20 ft or less deep, maximum allowable slope ¾:1, with the maximum bench dimensions shown in the figure
Unsupported vertically sided lower portion, up to 8 ft deep	Maximum vertical side 3½ ft
Unsupported vertically sided lower portion, more than 8 ft and up to 12 ft deep	Maximum allowable slope 1:1 and maximum vertical side 3½ ft
Supported or shielded vertically sided lower portion, 20 ft or less deep	Maximum allowable slope ¾:1; the support or shield must extend at least 18 in above the top of the vertical side
Anything else	Use one of the other options in 1926.652(b)

The combination cut — a sloped upper portion over a vertical lower portion — is how a lot of real trenches get dug: bell out the top, keep the bottom narrow where the pipe goes. Note that the unsupported version is limited to 12 feet of total depth and 3½ feet of vertical, and that at 8 to 12 feet the upper slope flattens from ¾:1 to 1:1.

Common mistake: cutting a 5-foot vertical lower portion because "it's only the bottom." Unsupported vertical sides in the Type A configurations top out at 3½ feet. Past that, the lower portion has to be supported or shielded — and then the support must come 18 inches above the top of the vertical side, so the crew never stands next to unsupported ground.

3 Type B and Type C configurations

Figure B-1.2, Type B soil: simple slope excavations 20 feet or less deep at a maximum allowable slope of 1:1; single and multiple bench excavations 20 feet or less deep at 1:1 with the maximum bench dimensions in the figure; and vertically sided lower portions that are supported or shielded, with the support or shield extending at least 18 inches above the top of the vertical side.

Figure B-1.3, Type C soil: simple slope excavations 20 feet or less deep at a maximum allowable slope of 1½:1, and excavations with vertically sided lower portions that "shall be shielded or supported to a height at least 18 inches above the top of the vertical side," with the overall maximum allowable slope still 1½:1. Everything else goes to the other options in 1926.652(b).

Compare the three and a pattern appears. As soil gets weaker, the standard removes options rather than changing numbers: Type A allows unsupported verticals up to 3½ feet, Type B allows verticals only when supported or shielded, and Type C removes benching entirely. By the time you are in Type C, you have exactly two shapes: a wide simple slope, or a box in the bottom.

4 Layered soils

Figure B-1.4 covers excavations made in layered soils, and it is the part of Appendix B most often ignored because it is drawings rather than numbers. The principle to carry: the combinations allowed depend on which material sits above which, consistent with Appendix A (c)(4) — a layered system is classified by its weakest layer unless the more stable layer lies underneath.

Practically, that means a profile like B over C — stiffer material over weaker — cannot be cut to the B slope for the full face. And where the layering does not match a configuration in the figure, Appendix B sends you back to 1926.652(b): tabulated data, or a design by a registered professional engineer. That is not a technicality; layered ground with a weak seam is exactly the profile in MIFACE 15MI117, where wet sand under wet clay took a wall down on a pipe layer.

On the job: when you see a distinct seam in the face, stop treating the wall as one thing. Estimate the dip, decide which layer is weakest, and cut to that. A seam dipping toward the excavation at 4H:1V or steeper puts the whole system in Type C — 1½:1, no benching.

5 Cutting a bench that is actually a bench

1
Establish the soil class first. Type C means no bench — stop here and slope or shield.
2
Check the overall slope: draw a line from the toe of the cut to the top of the outermost bench. That line must not be steeper than Table B-1 allows for the class.
3
Keep bench dimensions inside the maxima shown in Figure B-1 for that soil type.
4
Keep the benches clean: material left on a bench is a surcharge on the face below it and a rolling hazard on the crew.
5
Nobody works on an upper bench above people below unless the lower crew is protected — 1926.652(f).
6
Re-cut after rain. Benches collect water, and water in a bench loads the face and softens the material under it.

The most common failure of a benched excavation is not the geometry — it is housekeeping. Spoil, pipe and tools end up on the benches because they are convenient shelves. Every one of them is a load on the face and a projectile for the crew at the bottom.

6 When the shape stops fitting

Appendix B's configurations are a menu of standard answers for standard situations. The moment your excavation is deeper than 20 feet, in layered ground the figure does not show, next to a structure whose footing is at risk, or in soil nobody can classify with confidence, the honest conclusion is that you are off the menu — and 1926.652(b)(3) and (b)(4) exist precisely for that: tabulated data with an engineer's name on it, or a design by a registered professional engineer, with a copy at the jobsite while the slope is being built.

The next chapter takes the other branch: what to do when the width is simply not there.

Key takeaways

Figure B-1 gives benching configurations for Type A and Type B soil only — there is no bench for Type C.
Unsupported vertically sided lower portions in Type A are limited to 3½ feet of vertical, and to 12 feet of total depth (with a 1:1 upper slope past 8 feet).
Where a vertical lower portion is supported or shielded, the support or shield must extend at least 18 inches above the top of the vertical side — in Type A, B and C alike.
A bench does not buy a steeper overall cut: the line from toe to outer bench still has to respect Table B-1.
Layered ground follows Figure B-1.4 and the weakest-layer rule; a seam dipping into the cut at 4H:1V or steeper makes it Type C.
Material left on a bench is a surcharge on the face below and a rolling hazard on the crew.

Free educational content — not OSHA-authorized training, no certificate or card issued. Follow your employer's program and the standards cited.