Trench Sloping and Benching Calculator

The maximum slope for each soil type, and what it means for a 10 ft deep trench with a 4 ft bottom, 100 ft long. Angles are Table B-1’s, which it notes have been rounded off.

California’s Title 8 §1541.1 Appendix B uses the same maximum slopes.

• Simple slope: one straight face from the bottom corner to grade at the soil’s maximum slope. Every soil type has it.
• Benches: the first step at the bottom is 4 ft max; steps above it are 5 ft max in Type A and 4 ft max in Type B, to 20 ft total. Type B benches are for cohesive soil only. Appendix B has no benching configuration for Type C soil. The figures draw the maximum slope line from the bottom corner with each bench corner on it, so a benched cut has the same top width as the simple slope. Appendix B gives no bench widths; this assumes each bench corner sits on the maximum slope line, as the figures show.
• Vertical lower side, unsupported (Type A only): 3½ ft max. Up to 8 ft deep the slope above is 3/4:1; over 8 ft and up to 12 ft it is 1:1. Deeper than 12 ft, the vertical side has to be supported.
• Vertical lower side, shored or shielded (Types A, B and C, to 20 ft): the slope above is the soil’s maximum, and the support or shield has to reach at least 18 in above the top of the vertical side.
• Layered soil: Figure B-1.4 sets a slope for each layer, and Appendix A classifies a layered system by its weakest layer. The calculator does not lay out layers; use the weakest layer’s type, and your competent person classifies each layer.

The cross-section is symmetric. m is the horizontal run per foot of rise (0.75 for 3/4:1).

Worked example

A 10 ft deep trench, 4 ft across the bottom, 100 ft long, as a simple slope:

A Type A multiple bench at 10 ft has a 4 ft first step and two 3 ft steps (the 6 ft above the first step in as few equal steps as the 5 ft limit allows): 431.48 bank yd³, the same 19 ft top width as the Type A slope.

• Maximum slopes: Table B-1, stable rock vertical (90°), Type A 3/4:1 (53°), Type B 1:1 (45°), Type C 1½:1 (34°), for 20 ft deep or less; its angles are rounded off. Source: 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), Table B-1.
• Type A short term: 1/2:1 (63°) for a simple slope 12 ft deep or less, open 24 hours or less; deeper, 3/4:1. Source: 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), Table B-1 note 2 and Figure B-1.1.
• Over 20 ft: designed by a registered professional engineer, so the calculator gives no result. Source: 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), Table B-1 note 3.
• Not classified (the calculator’s starting choice): 1½:1 with the Type C layouts. Source: 29 CFR 1926.652(b)(1); OSHA 2226, Trenching and Excavation Safety (2015), p. 6.
• Benches: first step 4 ft max, upper steps 5 ft max in Type A and 4 ft max in Type B; Type B benches in cohesive soil only. Sources: 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), Figure B-1.1 (Type A benches); 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), Figure B-1.2 (Type B benches); OSHA Technical Manual, Section V, Chapter 2, Sloping and Benching.
• Type C: a simple slope or a shored or shielded vertical lower side; Appendix B has no benching configuration for Type C, and any other layout goes through tabulated data or a registered professional engineer. Sources: 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), Figure B-1.3 (Type C); 29 CFR 1926.652(b)(3) and (b)(4).
• Vertical lower sides: Type A unsupported 3½ ft max, 3/4:1 above to 8 ft deep and 1:1 from over 8 ft to 12 ft; shored or shielded in Types A, B and C to 20 ft, with the support at least 18 in above the top of the vertical side. Sources: 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), Figure B-1.1 (Type A vertically sided lower portions); 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), Figure B-1.2 (Type B vertically sided lower portions); 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), Figure B-1.3 (Type C).
• Assumption, bench widths: Appendix B gives no bench widths; this assumes each bench corner sits on the maximum slope line, as the figures show. The first step rises straight from the bottom corner; each upper step stands with its top outer corner on the line, and the treads run level between them. Upper steps are split evenly, as few as the step limit allows, unless you enter a smaller step.
• Assumption, geometry: both sides are cut the same, the slope line starts at the bottom corner (at the top of the vertical side for a vertical lower side), and the vertical-wall figure is a straight-sided dig of the same bottom width. It does not size a trench box.
• Spoil setback: materials and equipment at least 2 ft from the edge. Source: 29 CFR 1926.651(j)(2).
• Under 5 ft, ladders, and trench width: no protective system is needed in stable rock, or under 5 ft deep if a competent person finds no sign of a potential cave-in; a way out within 25 ft of lateral travel from 4 ft deep; a trench is no wider than 15 ft at the bottom. Sources: 29 CFR 1926.652(a)(1); 29 CFR 1926.651(c)(2); 29 CFR 1926.650(b), Trench.
• Soil type and layers: a competent person classifies each deposit; a layered system goes by its weakest layer. Sources: 29 CFR 1926 Subpart P, Appendix A (Soil Classification); 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), Figure B-1.4 (layered soils).
• Flatter than the maximum: with signs of distress Appendix B requires cutting back at least 1/2:1 flatter, and with surcharge loads a competent person decides how much. The calculator uses the maximum. Source: 29 CFR 1926 Subpart P, Appendix B (Sloping and Benching), (c)(3).
• California: Title 8 §1541.1 Appendix B uses the same maximum slopes. Source: Cal/OSHA Title 8 §1541.1, Appendix B.
• Metric: Appendix B sets its limits in feet, so metric entries are converted with the exact international foot (0.3048 m) and checked against the feet. Table B-1 prints 3.67 m beside 12 ft, but 12 ft is 3.658 m, so a 3.67 m cut does not get the Type A short-term slope.
• The regulation text is also on Cornell’s Legal Information Institute: Appendix B, Appendix A, 1926.651, 1926.652.
• Swell is Caterpillar’s bank weight divided by its loose weight for the same material, from Material Density Tables to Help Estimate Earthwork Volumes (archived copy): common earth (dry packed) 25%, loam 24%, topsoil 44%, sand (dry) 13%, sand (wet) 13%, gravel (dry) 12%, gravel (wet) 12%, clay (dry) 24%, clay (wet) 25%. Cat notes weight varies with moisture, grain size, and compaction.
• Rock and mixed debris are not presets: pick Other material and enter the swell from your own numbers. For reference, FHWA’s Exhibit 5.1 A (archived copy) weighs rock in place, loose and compacted in a fill, in lb per yd³: limestone 4,380, 2,690 (63% swell), 3,220 (36% bigger than in place); granite 4,540, 2,640 (72% swell), 3,170 (43% bigger than in place); riprap rock 4,500, 2,610 (72% swell), 3,150 (43% bigger than in place). FHWA: “Factors subject to ±33% variation.” Caterpillar’s Performance Handbook, Edition 49, p. 30-6 weighs broken rock loose and in the bank, in lb per yd³: limestone, broken 2,600, 4,400 (69% swell); granite, broken 2,800, 4,600 (64% swell). So rock swell runs limestone 63 to 69%, granite 64 to 72% and riprap rock 72% (Cat has no riprap row).
• 10-wheeler (tandem dump) 12 yd³. TxDOT: usually 12 or 14 yd³. Uses 12. Source: TxDOT Seal Coat and Surface Treatment Manual, Haul Trucks.
• Tri-axle dump 16 yd³. Load King: 16 to 19 yd³ for a 16 to 18 ft body. Uses 16, the low end. Source: Load King tri-axle dump body specs (Custom Truck One Source).
• Other trucks have no default; enter your bed size. For reference: belly dump 22 yd³ (Trail King bottom dump specs); belly dump 27 yd³ on a 40 ft tandem-axle trailer (MAC Bullet bottom dump flyer); semi end dump 29.0 yd³ with a 32 ft tub (Load King RH3229 end dump specs (archived)).

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