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Section 4: Excavation Support

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Overview

An excavation is any human-made cut, cavity, trench, or depression in an earth surface formed by earth removal. A protection system for an excavation includes support systems, sloping and benching systems, shield systems, and other systems that provide protection. The two main types of excavation protection are trench excavation protection (see standard specification Item 402) and temporary special shoring (see standard specification Item 403).

For either protection system, the Contractor must be compensated for the method of choice. For example, for temporary special shoring when excavation techniques such as sloped cuts or benching are used to provide the necessary protection, the surface area of payment is calculated based on the area described by a vertical plane adjacent to the structure.

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Trench Excavation Protection

Trench excavation protection is used for the installation of linear drainage or electrical features that will result in trenches deeper than 5 ft. It provides vertical or sloped cuts, benches, shields, support systems, or other systems providing the necessary protection in accordance with Occupational and Safety Health Administration (OSHA) Standards and Interpretations, 29 CFR 1926, Subpart P, Excavations.

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Temporary Special Shoring

Temporary special shoring is used for installations of walls, footings, and other structures that require excavations deeper than 5 ft. Temporary special shoring is designed and constructed to hold the surrounding earth, water, or both out of a work area. It provides vertical or sloped cuts, benches, shields, support systems, or other systems to provide the necessary protection in accordance with the approved design. Unless complete details are included in the plans, the Contractor is responsible for the design of the temporary special shoring. The Contractor must submit details and design calculations bearing the seal of a licensed professional engineer for approval before constructing the shoring. The design of the shoring must comply with OSHA Standards and Interpretations, 29 CFR 1926, Subpart P, Excavations. Design structural systems to comply with AASHTO Standard Specifications for Highway Bridges or AASHTO LRFD Bridge Design Specifications. Design shoring subject to railroad loading to comply with railroad Guidelines for Temporary Shoring and any additional requirements of the railway being supported.

Standard specification Item 403 can be used for both cut and fill shoring. When temporary MSE walls are used for fill situations, construct these walls in accordance with the requirements of standard specification Item 423, Retaining Walls, and include the standard sheet RW(TEW). For cut situations where soil or rock nail walls may be used, include special specifications for the appropriate nailing method and for Prefabricated Soil Drainage Mat. Amend special specifications to remove pay item reference for the soil/rock nail anchors, making them subsidiary to Item 403.

Consider temporary shoring concurrently with the permanent wall layout and design or grade change requirements of any given project. The best wall design or project geometry is difficult to execute and may put both workers and the traveling public at risk if proper shoring requirements are not addressed. In extreme cases, the cost of temporary shoring required to construct a wall can exceed the cost of the permanent wall. Avoid this and reduce negative effects with proper planning and proper wall selection.

Design temporary shoring like a permanent retaining wall. Determine the proper design loading that will act on the shoring wall. Consider the effect of surcharges or slopes behind the shoring wall. Due to the impermeable nature of some shoring types such as sheet piling, you may also need to consider water pressure or additional drainage details in design.

Consider temporary shoring for the following conditions:

  • At the back of fill-type retaining structures in cut situations
  • In front of existing structures such as retaining walls, bridge supports, header banks
  • On projects with staged construction
  • Near railroads
  • For bridge footings
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