A strip footing order starts with the footing box: plan width, plan length, and depth below formation level. The concrete is poured into the trench that safety rules required you to dig. This guide covers the OSHA excavation basics that apply to that trench and shows how the dug profile, not the footing box, controls the concrete quantity when concrete is cast directly against earth.
How do excavation safety rules change the concrete a strip footing needs?
They enlarge the trench. Sloped or benched sides add horizontal layback on each face, so the dug top is wider than the footing. When concrete is cast directly against earth, the placed concrete fills the whole dug profile, so the concrete order follows the measured trench section rather than the footing plan dimensions.
Which OSHA basics apply to a strip footing trench?
These rules come from U.S. OSHA 29 CFR Part 1926, Subpart P, which covers construction excavations in the United States. Other countries use their own workplace rules.
| Rule | OSHA source | Effect on the footing work |
|---|---|---|
| Excavations 5 ft (1.52 m) or deeper need a protective system: sloping, benching, shoring, or shielding. The two exceptions are excavations entirely in stable rock and shallower digs where a competent person finds no indication of a potential cave-in. | 1926.652(a)(1)(ii) | A deeper strip footing trench gets a sloped or benched profile. Measure the profile actually dug. |
| Maximum allowable slopes for excavations under 20 ft deep, stated horizontal to vertical: Stable Rock vertical (90 degrees); Type A 3/4:1 (53 degrees); Type B 1:1 (45 degrees); Type C 1 1/2:1 (34 degrees). | Appendix B, Table B-1 | The slope ratio sets the trench top width. Read every ratio as horizontal to vertical. |
| When there are signs of distress, the actual slope must be at least 1/2H:1V less steep than the maximum. Surcharge loads require the competent person to set the reduction. | Appendix B(c)(3) | The dug envelope can exceed the table value. Record the slope actually used. |
| Every soil and rock deposit must be classified by a competent person as Stable Rock, Type A, Type B, or Type C, using at least one visual and one manual analysis. No soil is Type A if it is fissured, subject to vibration, or previously disturbed. | Appendix A | Reworked footing ground usually classifies no better than Type B. Plan the envelope for the classification found, not the soil assumed. |
| Trench excavations 4 ft (1.22 m) or deeper need a stairway, ladder, ramp, or other safe means of egress within 25 ft (7.62 m) of lateral travel for employees. | 1926.651(c)(2) | Keep the access bay in the layout. It occupies plan space beside the pour. |
| Keep spoil, materials, and equipment at least 2 ft (0.61 m) from the edge of excavations, or hold them back with retaining devices. | 1926.651(j)(2) | Spoil piles occupy plan area beside the trench. Piling at the edge adds a surcharge load and a falling-material hazard. |
| A competent person must inspect excavations, adjacent areas, and protective systems daily: before work starts, as needed during the shift, and after every rainstorm. | 1926.651(k)(1) | Rain can change the trench profile. Re-measure the section before the pour. |
| Do not excavate below the level of the base or footing of any foundation or retaining wall that could pose a hazard, except with a support system, in stable rock, or with a registered professional engineer approval. | 1926.651(i)(2) | Applies where a strip footing runs near an existing structure. Do not undermine it. |
| Employees must not work in excavations with accumulated or accumulating water unless adequate precautions protect them against the water hazards. | 1926.651(h)(1) | Water changes the safety plan and the pour. Control or remove it before concreting. |
| Determine the estimated location of underground installations before opening the excavation. | 1926.651(b)(1) | The trench line can shift around marked utilities. Re-measure any relocated section. |
| Sloping or benching for excavations deeper than 20 ft must be designed by a registered professional engineer. | Appendix B, Table B-1 note 3 | Strip footings rarely reach this depth. The rule closes the slope table's scope. |
OSHA rules control worker protection. Project drawings, specifications, geotechnical information, contract measurement rules, and responsible qualified people control the footing dimensions, working clearance, formwork, and payable quantity. These rules are not legal advice; confirm the actual protective system through the project safety plan and the competent person.
How do you compute the concrete volume of a sloped trench?
Use the trapezoidal cross-section of the dug trench. This method assumes concrete is cast directly against earth and fills the full dug profile.
Top width = bottom width + 2 x slope ratio x depth
Cross-section = depth x (bottom width + slope ratio x depth)
Concrete volume = cross-section x run length
Worked metric example
A strip footing trench has a bottom width of 0.6 m, a depth of 0.75 m, and a run of 30 m. The competent person classifies the soil as Type B, so the maximum allowable slope is 1:1.
- Layback per side: 0.75 x 1 = 0.75 m.
- Top width: 0.6 + 2 x 0.75 = 2.1 m.
- Cross-section: 0.75 x (0.6 + 0.75) = 0.75 x 1.35 = 1.0125 m2.
- Concrete volume: 1.0125 x 30 = 30.375 m3.
Result: the dug profile needs 30.375 m3 of concrete. The footing box (0.6 x 0.75 x 30) is 13.5 m3, so ordering from the box would leave the pour 16.875 m3 short.
Worked imperial example
A strip footing trench has a bottom width of 2 ft, a depth of 2.5 ft, and a run of 40 ft. The soil classifies as Type C, so the maximum allowable slope is 1 1/2:1.
- Layback per side: 2.5 x 1.5 = 3.75 ft.
- Top width: 2 + 2 x 3.75 = 9.5 ft.
- Cross-section: 2.5 x (2 + 3.75) = 2.5 x 5.75 = 14.375 ft2.
- Concrete volume: 14.375 x 40 = 575 ft3 = 575 / 27 = 21.296 yd3.
Result: the dug profile needs 21.296 yd3 of concrete. The footing box (2 x 2.5 x 40 / 27) is 7.407 yd3, so the box understates the pour by 375 ft3.
When the trench is lined with formwork, the concrete follows the formwork dimensions instead of the dug profile. Order from the formed section, and keep the safety-driven envelope for the digging, spoil, and backfill quantities.
What conditions change the answer?
Benched sides need the stepped section actually dug instead of a smooth trapezoid. Layered soils follow the Appendix B layered-soil configurations, with each layer's controlling slope. Water, rain, and surcharge loads require flatter slopes than the table maximum or a revised plan from the competent person. Keep the trench open the shortest practical time between digging and pouring, and re-measure after any rain or visible change. The ground between the dug profile and the footing becomes backfill, a separate quantity with its own state conversions (see bank vs loose volume).
Common strip footing excavation mistakes
- Ordering concrete from the footing box while pouring into a sloped trench.
- Treating the OSHA maximum slope as the required slope instead of the steepest allowed.
- Reading slope ratios as vertical-to-horizontal.
- Using one slope ratio across a run where soil layers change.
- Piling spoil at the trench edge.
- Skipping the inspection and re-measurement after rain.
- Entering a deep trench without the required egress provision.
- Mixing the excavation quantity, the concrete quantity, and the backfill quantity in one total.
Sources and scope
- OSHA 29 CFR 1926.651, Specific Excavation Requirements, retrieved 2026-10-09: utility location, access and egress, water accumulation, adjacent structures, spoil setback, and inspection rules for covered U.S. construction excavations.
- OSHA Subpart P Appendix B, Sloping and Benching, retrieved 2026-10-09: maximum allowable slopes by soil type, distress and surcharge rules, and the 20-ft engineer-design trigger.
- OSHA Subpart P Appendix A, Soil Classification, retrieved 2026-10-08: competent-person classification requirement and Type A exclusions.
- Google Search Central, Creating helpful, reliable, people-first content, retrieved 2026-10-08: the people-first and originality standards applied during drafting.
Scope: the arithmetic computes the concrete volume of a stated dug trench profile. OSHA facts are limited to covered U.S. construction work under 29 CFR Part 1926, Subpart P. The governing drawings, survey and testing methods, geotechnical information, contract measurement provisions, safety plan, applicable law, and responsible qualified people control the actual protective system and final project quantity.
Review responsibility: Saleem Sial owns the published calculation, source, and editorial checks. Waseem Sial, External Reviewer and Engineer, remains listed for ongoing external review; no completed external-review date is claimed.
Next, use the footing excavation safety guide for the full sloped-envelope volume method, reconcile the dig with the pour in the excavation vs concrete guide, measure the footing with the strip footing measurement guide, or run the Concrete Footing Calculator: Strip, Pad and Steps.
