Excavation Shoring becomes necessary when the unsupported vertical excavation exceeds 4 feet and if the horizontal distance required for sloping the excavation per OSHA regulations exceeds the distance to property boundaries, existing structures or if the sloped excavation would present access constraints during construction.
If planned for in advance, excavation shoring systems can serve not only as temporary excavation support, but also as the permanent foundation wall. Often times design changes in the building foundation system are required to utilize the temporary excavation shoring system as the permanent foundation wall.
If not used as the permanent foundation wall, the shoring system can be design as a permanent earth retention system to reduce the lateral load on the below grade structural wall which allows the below grade wall to reduce in thickness. Anchor elements of a permanent shoring system must account for corrosion loss and be protected from corrosion.
The most appropriate excavation shoring technique and corresponding cost depends upon many variables including:
Soil nails & shotcrete are generally the most economical excavation shoring system. Soil nails are steel bars installed and grouted in a drill hole placed at regular intervals 5 ft to 6 ft horizontally and the same vertically. The system can conform to complex geometries and is appropriate in a variety of ground conditions provided the soil has adequate cohesion to maintain a 5 ft vertical unsupported face long enough for shotcrete to be applied. Soil nails are first drilled and shotcrete applied in 5 ft vertical lifts or less along the entire perimeter of the excavation. Another 5 ft lift of soil is removed, soil nails installed, then shotcrete is applied and the process repeats until the planned depth of excavation is reached.
Drilled Shaft/Beam and Lagging Walls utilize either concrete drilled shafts or steel soldier piles or ‘H’ piles drilled and installed on 8 to 10 ft centers around the perimeter of the planned excavation. The excavation proceeds in 5 ft lifts as wood or shotcrete lagging is installed atop the drilled shafts or soldier beams. If the excavation height is much higher than 12-14 ft Anchors or tiebacks are installed through the soldier beam into the soil behind and then post-tensioned to provide lateral load counter to lateral earth loads thereby reducing the net horizontal deflection.
Tangent Pile Walls are constructed as drilled shafts spaced such that there is little to no clear space between drilled shafts. Secant Pile Walls are constructed as drilled shafts with overlapped spacing, are expensive and are generally only considered if the excavation shoring system will also serve as the exterior wall or shell of the planned structure.
Reticulated Minipile Walls are expensive and are only considered when access constraints only allow smaller equipment and anchors are not feasible or allowed due to geometry or because an easement for the excavation shoring system onto the neighboring property is not attainable.
GeoCraft Builders has provided excavation shoring for projects throughout the Front Range and in remote, steep mountain terrain. Let us put our experience and expertise to work in order to help determine the safest and most economical excavation shoring system for your project.