3.3 Shoring: four design paths, and installing it safely

1 What shoring is, and its parts

1926.650(b) defines shoring as "a structure such as a metal hydraulic, mechanical or timber shoring system that supports the sides of an excavation and which is designed to prevent cave-ins." The same section names the components, and using the right words prevents a lot of confusion on site:

In practice most crews meet two kinds. Aluminum hydraulic shoring — cylinders with rails or plates, pumped from the surface — is fast, light and installable from outside the trench, which is why it dominates utility work. Timber shoring is still in the standard and still used where geometry is odd, and it is slower and more exposed to build.

2 The four design routes in 1926.652(c)

Support systems, shield systems and other protective systems are designed under 1926.652(c), which offers four routes. You pick one, and you can defend which one you are on.

Appendices C and D are public and free, and they are real design paths — Appendix C for timber shoring in trenches, Appendix D for aluminum hydraulic shoring. Both work from the soil classification you made in Part 2 and from the trench depth and width. This course does not reproduce their tables, and neither should a toolbox talk: sizing a member is done from the actual table, with the actual dimensions, by someone who knows how to read it.

3 The paperwork is part of the system

Three of the four routes require documents at the jobsite while the protective system is being built — (c)(2)(iii), (c)(3)(iii) and (c)(4)(iii). After construction the data may be stored off site, but a copy must be made available to the Secretary on request.

This is not a filing requirement dressed up as a safety rule. The data on site is what lets the person installing the shoring check the spacing, the depth limit and the soil class. "It's back at the shop" means nobody in the trench can verify that what they are building matches what was designed — which is exactly the situation the paragraph exists to prevent.

1926.652(d) covers the hardware itself, and it is short and strict:

• (d)(1) — materials and equipment used for protective systems shall be free from damage or defects that might impair their proper function.
• (d)(2) — manufactured materials shall be used and maintained consistently with the manufacturer's recommendations, and in a manner that prevents employee exposure to hazards.
• (d)(3) — when material or equipment is damaged, a competent person examines it and evaluates its suitability for continued use. If they cannot assure it will support the intended loads or is otherwise suitable, it is removed from service and must be evaluated and approved by a registered professional engineer before returning.

Read (d)(3) once more: a damaged cylinder or a bent rail does not go back in the hole on a competent person's optimism. Either they can assure it, or an engineer signs it off.

4 Installing and removing support: 1926.652(e)

This is the paragraph that keeps people alive while the system is being built and dismantled, and it is worth knowing line by line.

1. 1
   (e)(1)(i) Members of support systems shall be securely connected together to prevent sliding, falling, kickouts, or other predictable failure.
2. 2
   (e)(1)(ii) Support systems shall be installed and removed in a manner that protects employees from cave-ins, structural collapses, or from being struck by members of the system.
3. 3
   (e)(1)(iii) Individual members shall not be subjected to loads exceeding what they were designed to withstand.
4. 4
   (e)(1)(iv) Before temporary removal of individual members begins, additional precautions shall be taken — such as installing other structural members to carry the loads.
5. 5
   (e)(1)(v) Removal shall begin at, and progress from, the bottom of the excavation. Members shall be released slowly so as to note any indication of possible failure of the remaining structure or of a cave-in.
6. 6
   (e)(1)(vi) Backfilling shall progress together with the removal of support systems.

Two more rules are specific to trenches. (e)(2)(i): excavation to a level no greater than 2 feet below the bottom of the members of a support system is permitted, but only if the system is designed to resist the forces calculated for the full depth of the trench and there is no indication, while the trench is open, of possible loss of soil from behind or below the bottom of the system. (e)(2)(ii): installation of a support system shall be closely coordinated with the excavation of trenches.

"Closely coordinated" is the sentence that decides how a job runs. Dig 60 feet and then start shoring, and someone will be working in unsupported trench for hours. Dig a section, shore it, work it, advance — and nobody is ever in an unsupported hole.

5 Shoring against something else: 1926.651(i)

Support systems also protect what is next to the trench. 1926.651(i)(1): where the stability of adjoining buildings, walls or other structures is endangered by excavation operations, support systems such as shoring, bracing or underpinning shall be provided.

(i)(2) is stricter still. Excavation below the level of the base or footing of any foundation or retaining wall that could reasonably be expected to pose a hazard to employees is not permitted except when: a support system such as underpinning is provided; or the excavation is in stable rock; or a registered professional engineer has approved the determination that the structure is sufficiently removed from the excavation to be unaffected; or a registered professional engineer has approved the determination that the work will not pose a hazard.

And (i)(3): sidewalks, pavements and appurtenant structures shall not be undermined unless a support system or another method of protection is provided. That covers the driveway you are digging along and the sidewalk you are working under — the exact geometry of the fatality in MIFACE 05MI084, where the wall and part of the sidewalk came in together.

6 Choosing shoring honestly

Shoring is the right answer when the trench must stay narrow, when there is a structure or pavement to protect, or when the excavation has to stay open long enough for the weather to change the soil. It is the wrong answer when nobody on site has the tabulated data, when the components are mismatched or damaged, or when the plan requires an unprotected entry to build it.

The next chapter covers the alternative that most production pipe crews reach for first — the trench box — and the specific things it does not do.