Support of Excavation

A Support of Excavation provides vertical support and stability for an excavation. Also referred to as an “Earth Support System” (ESS), an SOE system is utilized when open cut excavations cannot be performed due to any number of site constraints, including the presence of nearby structures, underground utilities, property lines, easement issues, and/or soil and groundwater conditions.

Atlas Drilling specializes in support of excavation systems consisting of pre-drilled Soldier Piles as the main structural component and utilizes either timber lagging or shotcrete facing to retain the soil. A Soil Nailing System is another effective method of excavation support performed by our team. An ESS, if shallow, can be designed and installed as a cantilevered system, requiring no additional horizontal support. As the proposed excavation depths increase, however, horizontal support is eventually required, and can consist of either internal or external bracing. The depth of the excavation dictates how many tiers of lateral support are required, in addition to the overall load that must be restrained by the bracing.

Internal bracing begins with steel members known as walers that are installed along the perimeter of the ESS, and can either be segmental or continuous. Depending upon the footprint and depth of the excavation, the horizontal components of support can be provided by a combination of cross-lot braces (“struts”), corner braces, and/or rakers. Struts and corner braces are installed along the same plane as the walkers, while rakers are connected to the walers, installed downward at an angle, and anchored against “heel blocks”. Internal braces can be pre-loaded if desired, or can simply be loaded as the excavation progresses.

External bracing consists primarily of tiebacks (also known as soil or rock anchors). These anchors are drilled through the face of the ESS and develop their capacity through a frictional bond between the soil and the drill hole. This drill hole, after being extended to its required depth, is then filled with a neat cement grout, as well as a steel bar or tendon. After the cement grout has achieved its design strength (generally within five to seven days), the anchor can be tensioned and locked off against the earth support system. Depending upon the type of system utilized, the tiebacks may require walkers to transfer load, or may be locked off directly against the ESS.

The use of internal and external bracing depends on several factors. External bracing is often more desirable from a construction standpoint, as it enables more freedom for excavation and construction within the SOE system. However, issues of easements, utilities, and nearby underground structures must be carefully examined and evaluated before selecting tiebacks for a project. Internal bracing presents its own set of concerns, including a greater impact to related trades, and it can add considerably to the overall duration of a project. In addition to Atlas’s vast experience with both types of bracing, a thorough review of site-specific constraints and needs enables our team to select the most appropriate and cost-effective solution for your project.