What Sloping and Benching Each Require by Soil Type

Why this matters

Sloping and benching are the only protective systems that work by removing the load rather than resisting it, and they are the ones a crew can build with the machine already on site. What they cost is width, on both sides, and the width goes up with depth. That is why the decision to slope is almost never settled by the soil. It is settled by whether the corridor between the building and the curb is wide enough, and a crew that finds out at half depth that it is not will finish the cut vertical because there is nowhere left to go.

The second thing worth knowing before you plan around it: benching does not buy you width. Crews reach for it as a way to squeeze a cut into a narrow corridor and it does not do that, for a reason set out below.

What each one is

Sloping is cutting the face back to an angle at which the soil holds itself. Benching is cutting the face into a series of horizontal steps with vertical or near-vertical faces between them. Under the definitions at 29 CFR 1926.650(b), benching is a form of sloping, and Appendix B to Subpart P treats it that way: a benched cut still has to fit inside the maximum allowable slope for its soil class.

The allowable angles come from Appendix B and apply to excavations 20 ft or less in depth. Beyond 20 ft the design goes to a registered professional engineer regardless of class.

The fourth column is the only new number there, and it is the one that decides jobs. Everything else in the table is in the classification card; the run is what this card is about.

The geometry, and where the width actually goes

       spoil, kept back from the edge
        |
  ......v......
  --------------\                     /----------  grade
                 \                   /       |
       run per    \                 /        |
       side        \               /         | depth
                    \             /          |
                     \           /           |
                      -----------            |
                      |<-bottom->|
                          width

Two things the drawing carries. The run is consumed on both sides, so a corridor has to hold twice it plus the bottom width. And the spoil sits outside the top of the slope, not on it, which means the corridor a sloped trench actually needs is wider than the cut.

The arithmetic is direct. Top width equals bottom width plus twice the run per side, and the run per side equals the slope ratio times the depth. That relationship holds at a constant soil class down the full face; a cut that passes into a different stratum partway down is a layered problem, covered below.

The volume follows a different law and it surprises people. For a trench of constant bottom width, the excavated cross-sectional area is the bottom width times the depth, plus the slope ratio times the depth squared. At constant bottom width and constant soil class, the sloping portion of the excavation grows with the square of depth. Two extra feet of depth do not add two feet worth of digging; they add proportionally more, and they add it to the haul, the backfill and the compaction as well.

Why benching does not buy width

A benched cut in Type B still has to sit inside the 1 to 1 maximum allowable slope. Draw a line from the toe of the cut to the top of the outermost bench and that line has to be no steeper than the class allows. The steps are inside that envelope, not outside it.

So the top width of a benched Type B cut at a given depth is the same as the top width of a sloped Type B cut at that depth. What benching genuinely buys is different: a working surface at intermediate levels, a shape that some cohesive materials hold better than a long uninterrupted face, and easier machine reach. Those are real, and none of them is width.

Appendix B also limits the individual bench faces - the figure that appears in those configurations is a 4 ft maximum vertical bench face - and the configurations themselves are drawn in the appendix. Read the figure for the shape you are actually cutting rather than working from the ratio alone, because the ratio tells you the envelope and the figure tells you the steps.

Benching is not permitted in Type C soil at all. A crew hoping to bench its way out of a Type C width problem has nothing to reach for, and that is the situation that most often ends with a support or shield system.

Two allowances worth knowing, and their conditions

The short-term Type A allowance. Appendix B permits a steeper short-term slope in Type A soil: 1/2 to 1, which is 63 degrees, in excavations 12 ft or less in depth, with 3/4 to 1 as the short-term maximum for greater depths. Short term is defined in the appendix as an exposure of 24 hours or less. This is the allowance most often used past its own condition, because 24 hours passes and the cut is still open, and the class it was granted under is the class most easily lost to a fissure or a passing truck.

The vertically sided lower portion. Appendix B permits some configurations with a vertical lower section under a sloped upper section. The figures it carries include a 3.5 ft maximum vertical side for UNSUPPORTED vertically sided lower portions in Type A cuts up to 8 ft deep, and, for Type B and Type C alike, the requirement that the vertical portion be shielded or supported to a height at least 18 in above the top of that vertical side, with the overall cut still at 1 to 1 for Type B and 1.5 to 1 for Type C. Take the exact configuration from the appendix figure, and treat any vertical portion as the part of the cut with the highest lateral pressure, because it is.

The Type C default. 29 CFR 1926.652(b) lets you slope everything at 1.5 to 1 without classifying the soil at all. That is the Type C allowance used as a blanket. It is legitimate, it needs no data on site, and it is the widest cut on the table - which is exactly the trade: you pay in width for not having to know.

One alignment, three classes, one corridor

A sanitary service trench, 3 ft wide at the bottom, 8 ft deep, running between a building face and a curb with 24 ft of clear corridor.

Now apply the corrections the general sections impose, one visible line each.

The excavation that decision commits you to. Cross-sectional area is bottom width times depth plus slope ratio times depth squared, per foot of run:

• Type A: 3 x 8 plus 0.75 x 64, which is 24 plus 48, or 72 sq ft
• Type B: 3 x 8 plus 1.0 x 64, which is 24 plus 64, or 88 sq ft
• Type C: 3 x 8 plus 1.5 x 64, which is 24 plus 96, or 120 sq ft

Type C moves about 67% more material per foot of run than Type A on the same trench. That is the honest cost of a class you did not establish, and it is why crews reach for the blanket 1.5 to 1 on short runs and classify on long ones.

And the depth effect. Take the same Type B trench to 10 ft: 3 x 10 plus 1.0 x 100 is 130 sq ft, against 88 sq ft at 8 ft. A 25% increase in depth produced a 48% increase in material moved, because the sloping term alone went from 64 to 100 sq ft, which is the square of the depth ratio. Chasing grade another two feet is not a small change to anything.

The failure that this arithmetic predicts

A crew starts sloping at the surface, reaches about half depth, finds the slope running into the curb or the building, and finishes the last few feet vertical. It feels like a minor deviation because it is a small part of the total face.

It is the worst possible part. Lateral pressure is highest at the bottom of the cut, so the vertical section is placed exactly where the load is greatest, and it sits under the full weight of everything above it. A face that is 8 ft of slope with 2 ft of vertical at the toe is not "mostly sloped", it is an unsupported vertical cut with a surcharge sitting on top of it.

The tell is visible from the surface: a slope whose angle changes partway down, or a toe you cannot see from the edge. Both mean the cut is not the cut that was planned.

What changes the configuration

Layers. Where layer-by-layer classification is permitted - which is only where a more stable layer lies below a less stable one - each layer is sloped for its own class, which produces a cut that is flatter at the top and steeper below. Where it is not permitted, the whole face takes the weakest layer's slope. The classification card owns that rule.

Water. Seepage at a face is a Type C trigger and it also erodes the toe of a slope, so a sloped cut with water running out of the face is losing the geometry it was built to.

Time. A slope cut in Type A under the short-term allowance is a 24 hour proposition by the appendix's own definition, and a cut that stays open past it is a cut that needs re-checking against the ordinary allowance.

The cut going past 20 ft. Registered professional engineer, regardless of class or shape.

Checking a cut you did not lay out

Stand at the edge with a tape and take two measurements: the bottom width and the top width, at the same station. Subtract, halve, and divide by the depth. That gives you the run ratio the cut was actually built at, and it either sits at or flatter than the class's allowance or it does not.

Do it at more than one station. A trench cut by a machine walking along a run does not hold one geometry, and the station that fails is usually the one where the operator was working around an obstruction.

References

• Appendix B to 29 CFR 1926 Subpart P, maximum allowable slopes, benching configurations, the short-term Type A allowance, and vertically sided lower portions.
• 29 CFR 1926.652(b), the four sloping and benching design options including the blanket 1.5 to 1 that requires no classification.
• 29 CFR 1926.650(b) for the definitions of sloping and benching, and 29 CFR 1926.651(j)(2) for the spoil and equipment setback.
• See related: the library's cards on how soil is classified and what each class permits, on what a protective system is required to do, and on what a trench shield does and what it does not.