Temporary Shoring Walls for Building ExcavationsNov 16 2017 · 0 comments · NISHKIAN DEAN, Technical notes ·0
By Robert A. Aman, PE, SE
Many buildings constructed today extend one or more stories below grade level to maximize building area and provide additional spaces for parking, storage, mechanical/electrical rooms, and sometimes even office space or living units. In many cases the soil excavations required for these below-grade structures are constrained by the site’s property lines and surrounding buildings, and therefore require that a vertical excavation cut is made within the property lines and construction boundaries. Since the soil excavations are vertical, temporary excavation shoring must be installed to prevent the soil from caving. In cases where there is available space adjacent to it, the excavation typically will be laid back on a slope at a 1 ½ horizontal to 1 vertical to prevent caving and to eliminate the need for excavation shoring. In situations in which this is not possible, shoring provides a means to safely accomplish the site excavation and greatly improve the utilization of the site development.
The design of excavation shoring can be complex and must take into account many factors, including depth of cuts, groundwater elevations, horizontal loads imposed by soil, resistance loads provided by soil at embedded shoring, and superimposed vertical loads from traffic, construction equipment, construction material storage, and adjacent structures. The vertical loads that occur adjacent to shoring walls usually result in additional horizontal loads applied to the shoring. The geotechnical engineer is responsible for providing all soil design parameters for the temporary shoring design, and works together with the foundation and structural engineer to accomplish this.
Excavation shoring, on most projects, is delegated to a specialty contractor who is also typically responsible for its design. In many jurisdictions, including the City of Portland, excavation shoring design drawings and calculations must be approved as part of the building permit submittal process. Site construction cannot begin without this information in hand, so it is important to understand the jurisdictional requirements before getting to this point so that the start of the project is not impacted.
When needed, there are several different shoring systems that can be utilized. The selection of the best system for each particular project considers the site soil conditions, adjacency with other elements, depth of excavation, and the experience of the specialty excavation contractor. Two common systems are soldier pile walls, and soil nail walls, which we go more in depth about below (pun intended):
Soldier Pile Walls
This method is fast to construct and typically consists of structural steel H-shaped piles that are inserted into a deep round hole filled with concrete that is spaced at regular intervals, usually in the 6- to 12-foot range. The concrete hole is typically 24” in diameter and the H-Pile is 10” to 14” wide/deep. Alternatively, the steel piles may be driven or vibrated into the ground without the use of any concrete. Where the excavation is located adjacent public property, the temporary shoring wall is typically located directly outside the property line, where permitted on a public right-of-way, and is used to apply a reinforced shotcrete wall that then serves as the permanent structural wall. Where shoring walls are located adjacent to a private property it can only be located over the property line with an easement from the property owner.
Between the soldier piles, 4×12 horizontal wood lagging is installed to retain the soil behind the wall. The lagging is installed in 3- to 4-foot increments as the vertical excavation cut proceeds downward. The soldier pile is typically embedded 10 to 12 feet below the bottom of the final excavation, and is designed to cantilever out of the ground. Where the excavation exceeds a range of 10 to 12 feet, the soldier piles may require a soil anchor/tieback or internal diagonal brace near the top of the wall for additional support. For deeper excavations, additional tiebacks are required as the depth increases further.
Soil anchor (or tiebacks) usually consist of a steel tendon or rod encased in a hole filled with a concrete grout mixture. The anchors are tensioned after installation is complete to fully engage the soil. Soil anchor tiebacks are installed at a downward angle and typically extend into the ground a minimum of 25 feet and may therefore encroach into the adjacent properties or the city’s right-of-way (in which case, permission from the property owner is required.) Encroachment into a public right-of-way is typically allowed for temporary construction.
Location of all existing utilities must be confirmed prior to the installation of the anchors. The City of Portland requires an additional encroachment permit, and states that the anchors are de-tensioned after the temporary wall is no longer needed. Additionally, the City requires that any shoring elements in the public right-of-way that are located within 5-feet below grade must be removed, including soldier piles and tiebacks. The use of internal shoring bracing eliminates encroachments, but the shoring is located where the building construction occurs and must not conflict with those activities.
Pros: fast to construct, can be used in deep excavations, flexible layout geometry, can be designed for large surcharge loads
Cons: in certain cases, may require additional soil anchors/tiebacks or support, tiebacks in adjacent property will require an easement, internal braced configurations can be obstructing.
Soil Nail Walls
Another common excavation shoring wall solution is a soil nail wall, which is purely an anchor tieback wall. A soil nail wall consists of steel bars installed in a drilled hole filled with a concrete grout mix spaced at approximately 5 feet on center. The rods are typically installed at a 15-degree downward angle and embedded in the 15-foot range. The nails are then covered with a 4”-thick reinforced shotcrete facing wall that retains the soil behind the wall. The walls are constructed from the top down in 3- to 6-feet-tall sections, depending on the soil type and its ability to withstand caving.
Soil nail walls may be advantageous where overhead construction requirements are tighter because they do not require drilling equipment or the installation of soldier piles. Smaller equipment is generally needed with this method, and no additional embedment of a vertical structural element is required. Embedment of soil nails is also much less than with tieback walls, which may reduce conflicts with adjacent underground obstructions or utilities. A soil nail wall will require a more specialized and experienced contractor, however. Like a traditional soldier pile wall, the structural wall for the building would be a shotcrete wall installed on the face of the soil nail wall. The design of the soil nails is typically provided by the geotechnical engineer, while the shotcrete facing wall is designed by a structural engineer.
Pros: do not require drilling equipment or the installation of soldier piles, needs smaller equipment, no additional embedment of a vertical structural element is required
Cons: limited depth of excavation, limited wall surcharge loads can be accommodated, requires a more specialized and experienced contractor
Basement excavations are a common necessity of many projects, and as you can see above, there are many options and avenues for selecting the right system for each individual project. Developing a strategy to address the excavation is an important part of the early design effort – it helps with creating a scheme that is readily buildable for a specific site and ensures that everything is in place for the permit application. The Nishkian firms are regularly involved in the design of many projects that incorporate basement excavations and are available to consult on your project needs.
Robert A. Aman, PE, SE is an Associate with Nishkian Dean a structural engineering consulting firm in Portland, Oregon.