Trench Slope Calculator: OSHA Sloping, Benching and Box Depth
Depth, bottom width, length and soil type, and it returns how far each side has to be cut back, how wide the trench will be at the top, how much ground to keep clear either side, the spoil volume, and how many ladders the trench needs. The defaults are an unclassified trench 8 feet deep, 3 feet wide at the bottom and 100 feet long. Unclassified is Type C, so that is 12 feet of cut back on each side, 27 feet across at the top, 31 feet of ground to keep clear, and 444.4 cubic yards to move.
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Worked example
These are the figures the calculator opens with and the answer it gives. Change anything above and every number below moves with it.
- Trench depth8 ft
- Width at the bottom3 ft
- Trench length100 ft
- Soil typeNot classified yet
- How the sides are heldSloped back in one simple slope
- How long it stays openMore than 24 hours
Cut back each side12 ft
These are the slopes in Option 1 of 1926.652(b), which is Appendices A and B. It is the option available with no engineer and no manufacturer's data on site, and it is deliberately the conservative one. Options 2, 3 and 4 can buy a steeper cut, and every one of them puts written data in somebody's hands at the trench.
1926.651(k)(1) puts a competent person at the trench before the start of work and again as needed through the shift, looking for cave-in evidence, protective systems that are failing, and hazardous atmospheres. It is a daily job, not a one off at the start of the dig, and it is the first thing an inspector asks to see.
Nothing here classifies your soil, and the page will not pretend otherwise. Appendix A puts that on a competent person running at least one visual and one manual analysis, redone whenever conditions change. Unclassified ground comes out as Type C because 1926.652(b)(1)(i) makes one and one-half horizontal to one vertical the slope you may use with no classification at all.
| Soil class used | Type C, unclassifiednothing classified falls to Type C |
|---|---|
| Maximum allowable slope | 1 1/2:1, 34 degreesTable B-1 |
| Cut back each side | 12 ftdepth times the horizontal figure |
| Width at the top | 27 ftbottom width plus both cut backs |
| Keep clear, edge to edge | 31 ft2 ft of bare ground each side |
| Soil to move | 444.4 cu ydbank measure, no swell |
| More than a vertical cut | 355.6 cu ydwhat the slope costs you |
| Ladders or ramps | 2 ladders25 ft of lateral travel, so one per 50 ft |
| Spoil and gear set back | 2 ft minimum1926.651(j)(2) |
The material list this produces
| Ladders or ramps, each | 2 |
|---|---|
| Soil to move, cu yd | 444 |
| Width to keep clear at the top, ft | 31 |
| Spoil setback from the edge, ft | 2 |
| Competent person on site, each | 1 |
| Utility locate request, each | 1 |
| Daily inspection record, each | 1 |
What this calculator assumes
- The slopes are Option 1 out of 1926.652(b), the allowable configurations set out in Appendices A and B. That is the option an employer can use with nothing on site but a competent person, which is why it is the one a free calculator should compute.
- Unclassified ground is treated as Type C, because 1926.652(b)(1)(i) makes one and one-half horizontal to one vertical the slope available with no soil classification at all.
- Volumes are neat bank measure: the trapezoid times the length, divided by 27. No swell on the way out and no shrink on the way back in, so the truck count is higher than the number here and the backfill is short of it.
- The ladder count comes from 1926.651(c)(2), which puts a stairway, ladder or ramp in any trench 4 feet or more deep so that nobody is more than 25 feet of lateral travel from one. One ladder therefore covers 50 feet of trench.
- Nothing on this page classifies soil. Appendix A puts that on a competent person using at least one visual and one manual analysis, and it has to be done again when conditions change.
Five feet is the line, and the exception under it is a person, not a depth
1926.652(a)(1) requires a protective system in every excavation, with exactly two exceptions. The first is an excavation made entirely in stable rock. The second is an excavation less than 5 feet in depth where examination of the ground by a competent person provides no indication of a potential cave-in.
Read the second one again, because it is not a depth rule. It is a finding made by a specific person. 1926.650(b) defines a competent person as one who is capable of identifying existing and predictable hazards in the surroundings and who has authorization to take prompt corrective measures to eliminate them. If nobody on site holds that authority, the exception is not available at four feet either.
At 5 feet the discretion is gone entirely. The trench gets sloped, benched, shored or boxed, and there is no examination that changes it. This calculator tells you which one you asked for and what it costs in ground.
Table B-1 is the whole answer, and it stops at twenty feet
Appendix B to Subpart P has four rows. Stable rock is vertical, 90 degrees. Type A is 3/4:1, 53 degrees. Type B is 1:1, 45 degrees. Type C is 1 1/2:1, 34 degrees. The ratio is horizontal run per one foot of vertical rise, so the cut back is the depth times the first number, on each side. Footnote 1 says the angles have been rounded off, and 1926.652(b)(1)(i) independently calls one and one-half horizontal to one vertical 34 degrees measured from the horizontal, which fixes the convention.
Soil that nobody has classified is Type C here. Not because a sentence anywhere says so, but because 1926.652(b)(1)(i) makes sloping at no steeper than 1 1/2:1 the option that is available with no classification at all. That is why the default in the box above is the safest row in the table, and why an 8 foot trench 3 feet wide at the bottom opens out to 27 feet across the top.
The table is headed for excavations less than 20 feet deep, and footnote 3 sends anything greater than 20 feet to a registered professional engineer. 1926.652(b)(4) says the same thing from the other side. So this calculator stops computing a slope at 20 feet and says so on the page, because a number printed past 20 feet is a number somebody will dig to.
A trench box is not a slope, and it carries two numbers of its own
Choose the shield option and the sides stay vertical, which is the entire point of hiring the box. 1926.652(g)(1) then applies: the shield must not be subjected to loads exceeding those it was designed to withstand, it has to be installed so it cannot move sideways, and workers have to be protected from cave-in while entering and leaving the protected area.
The box has to be rated for the full depth of the trench, not for the height of the box. 1926.652(g)(2) permits digging to a level not greater than 2 feet below the bottom of a shield, but only if the shield is designed to resist the forces calculated for the full depth of the trench, and only while there is no sign of soil loss from behind or below it. That is the number this calculator prints as the depth the box must be rated for.
The other number is 18 inches. Figure B-1 in Appendix B requires the support or shield system to extend at least 18 inches above the top of the vertical side, and it says so in all three soil types. A box that finishes level with the ground is a box that is 18 inches short.
Questions people ask
How far back do I have to slope a trench?
The depth times the horizontal figure for the soil, on each side. Type C and unclassified soil is 1 1/2:1, so an 8 foot trench is cut back 12 feet each side and finishes 27 feet across the top on a 3 foot bottom. Type A is 3/4:1 and Type B is 1:1. Put your own depth and soil in above.
What is the OSHA maximum slope for Type C soil?
1 1/2:1, which Table B-1 prints as 34 degrees from the horizontal, for any excavation less than 20 feet deep. Past 20 feet the table does not apply and the sloping has to be designed by a registered professional engineer.
Can I use the short term 1/2:1 slope in Type A soil?
Only when both conditions hold. Footnote 2 to Table B-1 allows 1/2:1, 63 degrees, in Type A, and Figure B-1 adds the other half: the excavation has to be open 24 hours or less and 12 feet or less in depth. Over 12 feet the short term slope goes back to 3/4:1, 53 degrees.
Does OSHA allow benching in Type C soil?
Appendix B publishes benched configurations for Type A in B-1.1 and for Type B in B-1.2. B-1.3, which is Type C, has none: it gives simple sloping at 1 1/2:1, vertically sided lower portions that are shielded or supported, and otherwise the other options in 1926.652(b). With no benched configuration published for Type C, this calculator will not put one on screen.
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