OSHA explains it best, saying that shoring is a “support system” installed to eliminate the risk of movement of soil, roadways, foundations, or underground utilities. Often called shielding, as well, it is put in place when the depth or the location of the excavation prevents sloping back to the maximum allowable slope.
Installed in a trench facing, shoring in excavation 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.
At what depth does OSHA require shoring? As stated above, should trenches or excavations exceed four feet, support is required. However, OSHA requires proper soil type determination and evaluation, as well as the use of test equipment for conducting evaluations. Based on those findings, OSHA institutes slope and shield configurations.
Described as sloping and benching, they address the maximum allowable slopes for excavations under 20 feet and take soil type and angle to the horizontal into the equation. The use of benching, which steps the slope, is another factor where shoring in excavation is concerned.
There can be simple and multiple benches, and again the soil type determines the horizontal to vertical ratio of the benched side. Generally, the bottom vertical height must not exceed four feet for the first bench, but subsequent benches may be up to five feet in what is known as Type A soil, but in Type B the total depth cannot exceed 20 feet, with any subsequent benches below the maximum allowable slope for the type of soil in question.
There are several kinds of shoring in use. The following are commonly used in trench shoring (for below ground utility construction):
In addition to the TYPES of shoring, there are also many EXCAVATION SHORING TECHNIQUES and primarily include the following:
Permanent or Temporary Excavation Shoring?
If planned for in advance, excavation shoring systems can serve not only as temporary excavation support, but also as the permanent foundation wall. Often 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 designed 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.
Selection of Appropriate Excavation Shoring System
The most appropriate excavation shoring technique and corresponding cost depends upon many variables including:
Excavation Shoring System Descriptions
With those basics, it is a good idea to take time to review the more specific information about the types of excavation shoring. Below are full definitions and explanations of the types of excavation shoring, the relative economy, and the conditions to which they are best suited:
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. This excavation shoring system can be cantilevered, meaning that no anchors or tiebacks are needed. However, 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 to counter the lateral earth loads thereby reducing the net horizontal deflection.
TangentPile 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.
What is the most suitable excavation shoring system for your project? Contact Geocraft Builders, we can evaluate a site, review the best approaches given the project context and soil type, and offer the offer the most economic shoring system.
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.