Preventing Shoring Wall Failures on Your Construction Site
Shoring walls are temporary retaining structures that prevent soil movement and cave-ins during a construction site excavation. These systems are used to minimize the excavation area, to keep the sides of deep excavations stable, and to ensure that movements will not cause damage to existing adjacent structures or to utilities in the surrounding ground.
Common Construction Shoring Methods
Excavation depths, especially those exceeding 20 feet, require specialized planning for support. The design and construction of shoring systems can be complex due to the composition of the soil, ground water, weather conditions, vibrations, and lateral loads from nearby buildings and loadings from construction equipment. The most popular construction shoring methods in urban environments, where space can be extremely limited, allow for the excavation perimeter to be cut vertical or near vertical.
Excavation support systems most frequently use tiebacks or soil nails to withstand the lateral earth pressures. In many soil conditions, tied-back sheeting systems (tiebacks, soldier piles and wood lagging) are the most economical systems for temporary support of excavations. A typical construction sequence begins with the installation of soldier piles; then the site is excavated to a depth of five-to-seven feet. Wood lagging is installed to keep the soil between the soldier beams and the tiebacks are installed to support the lateral earth pressures. This sequence is repeated until subgrade is reached. This top-down sequence provides continuous support of the cut and minimizes the disturbance behind the wall, especially where existing conditions and structures greatly limit access.
When space is really tight between new excavation and existing structures, soil nailing can be the least disruptive way to construct a shoring wall, as long as adjacent structures or utilities do not interfere with the nailing. This technique reinforces the soil while it is excavated from the top down. An array of soil “nails” (normally steel reinforcing bars) is installed in a grid that creates a stable mass of soil. This mass of reinforced soil functions to retain the less stable material behind it. A typical construction sequence begins with the excavation of a shallow cut. Then shotcrete is applied to the face of the cut and soil nails are drilled and grouted. Like tiebacks and soldier piles, this sequence is then repeated until subgrade is reached. Although soil nailing requires tedious handwork, craftsmanship and geotechnical expertise, in the right conditions, soil nailing is a rapid and economical means of constructing an excavation support system.
Shoring Wall Failures
Regardless of the shoring method that is chosen during the design phase, extreme caution must be used during any shoring construction and safety is of utmost importance. Beyond the lost material and time costs of having to replace a failed shoring structure, trench related fatalities more than doubled between 2015 and 2016 according to U.S. Department of Labor’s Occupational Safety and Health Administration (OSHA).
Shoring wall systems typically fail due to improper wall construction and/or a geotechnical failure. It is important that the excavation crew is utilizing the right shoring technique for the conditions, and takes the time to ensure that shoring is constructed in the proper sequence. Making a 20-foot cut before beginning a shoring process could create a very dangerous situation. Incrementally excavating the right amount will prevent an unbalanced soil load. Quality Assurance observations by the geotechnical engineer, or a third-party consulting firm, should be considered during construction to verify and document shoring construction in accordance with the design intent.
Geotechnical reports provide the design engineer and contractor with critical and vital information to effectively design the structural foundation systems. Adhering to the geotechnical report is also key to choosing the right shoring technique. The soil profile will determine the amount of excavation prior to constructing the wall—making sure that the sequencing is performed properly and safely.
In addition to improper construction and soil issues, unexpected subsurface or groundwater can be the catalyst for shoring failures. Sources include surface infiltration, water table fluctuation and layers of permeable soils. Ground water movement must be prevented from coming in contact with the wall structure, including the soils behind the wall. Minor water intrusion can often be addressed through temporary grading (sloped away from the shoring and excavation); more pervasive water issues may require weep holes at the bottom of the walls, or PVC drain pipes and sump systems behind the walls to discharge water and relieve the hydrostatic pressures.
Regardless of the source of a shoring wall failure, implementing rigorous quality assurance guidelines that address key issues prior to construction can mitigate a potentially dangerous situation.
What Pie Can Do for You
At Pie, whether it’s consulting on a shoring wall system for new construction or a forensic analysis of shoring failure on an existing structure, Pie’s claims consultants and forensic experts have decades of experience in construction management, project oversight and damage assessment. Contact us today for more information.
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