Field

What is shoring in construction?

Shoring is a temporary structure that holds something up or holds something back while work is underway: the walls of a trench, the sides of an excavation, fresh concrete in its forms, or an existing building during renovation.

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Shoring carries load temporarily until the permanent structure can, or keeps soil from collapsing on workers. The word covers three different jobs on a site: excavation shoring, concrete shoring under formwork, and structural shoring that supports an existing building while walls, beams, or foundations are altered. All three are engineered systems, not something to improvise with whatever lumber is on the truck.

Excavation and trench shoring

OSHA's excavation rules in 29 CFR 1926 Subpart P define a shoring system as a structure, such as a metal hydraulic, mechanical, or timber shoring system, that supports the sides of an excavation and is designed to prevent cave-ins. It is one of three ways to protect workers, along with sloping (cutting the sides back at a safe angle) and shielding (a trench box that protects the people inside without holding back the soil).

  • When protection is required: under 29 CFR 1926.652(a)(1), each employee in an excavation must be protected from cave-ins by an adequate protective system. The exceptions are excavations made entirely in stable rock, and excavations less than 5 feet deep where a competent person finds no indication of a potential cave-in.
  • How the system is designed: 1926.652(c) allows four options: the designs in OSHA's appendices A, C, and D (timber shoring follows appendices A and C), the manufacturer's tabulated data, other tabulated data approved by a registered professional engineer, or a design by a registered professional engineer.
  • Installing and removing: support systems must be installed and removed in a way that protects workers. Removal begins at, and progresses from, the bottom of the excavation, and backfilling progresses together with removal.

Concrete shoring and reshoring

On elevated slabs, shores hold up the formwork and wet concrete until the slab can carry itself. Under 29 CFR 1926.703, shoring must be inspected before erection and inspected immediately before, during, and after the pour. Forms and shores cannot come out until the employer determines the concrete has gained sufficient strength. As original shores are removed, reshoring goes in where the concrete would otherwise be loaded beyond its capacity, often because the floor above is being poured.

A worked example

A utility contractor is installing a 10-foot-deep sewer lateral in a street with a gas line on one side and a building foundation 6 feet away on the other. Sloping is out; there is no room. The crew's competent person classifies the soil, and the company uses aluminum hydraulic shores following the manufacturer's tabulated data, kept on site. The foundation adds surcharge load the tables do not cover, so the contractor brings in an engineer to design that section. That engineering cost, plus the slower production inside shored trench, is exactly what the estimator should have priced on bid day.

Common mistakes

  1. Pricing a trench job as if the sides can be sloped, then finding there is no room on site.
  2. Using the manufacturer's tables without having them on site, or outside their stated limits.
  3. Stripping concrete shores early to move them to the next pour before strength is confirmed.
  4. Letting someone who is not a trained competent person make daily inspection calls.
  5. Not documenting inspections. After an incident, missing records are almost as damaging as missing shoring.

Related: means and methods, backfill, site logistics plan, and the toolbox talk where trench safety belongs. For planning, see how to write a construction site safety plan. Smart Business Automator keeps daily logs and inspection notes with each job so the paper trail exists when it is needed; see construction workflow automation.

Frequently asked questions

What is the difference between shoring and shielding?

Shoring supports the sides of an excavation to prevent a cave-in. Shielding, such as a trench box, does not stop the walls from moving but protects workers inside the shield if they do.

When does OSHA require trench protection?

Each employee in an excavation must be protected by an adequate protective system unless the excavation is entirely in stable rock, or is less than 5 feet deep and a competent person finds no sign of a potential cave-in.

What is reshoring?

Reshoring is temporary support placed under a concrete slab after the original forms and shores are removed, so the slab is not overloaded before it reaches adequate strength.

Who designs shoring?

It depends. Trench shoring can follow OSHA's tables or a manufacturer's tabulated data within their limits; anything outside them needs a registered professional engineer. Concrete and structural shoring is typically designed by an engineer.

Sources

  1. Cornell LII: 29 CFR 1926.650 Scope, application, and definitions (excavations)
  2. Cornell LII: 29 CFR 1926.652 Requirements for protective systems
  3. Cornell LII: 29 CFR 1926.703 Requirements for cast-in-place concrete
Written and edited by

Maurice Francois writes and edits construction operations guides for Smart Business Automator. More from Maurice on the Scaling Legends podcast.

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