According to OSHA, excavation is one of the most hazardous construction operations. The greatest risk for worker fatalities is from cave-ins. But excavation is a necessary part of many construction projects. There are several different options for earth retention during excavation. The decision is typically made by the Project Team based on project logistics – how deep is the required excavation, how much room is available onsite between the excavation and existing underground utilities, how close are existing buildings to the excavation, and what type of soil are you working with.
Soil nailing brings soil stability in areas where landslides or slope failures might be a problem by inserting steel reinforcement bars into the soil and anchoring them to the soil strata. This technique is called soil nailing because the steel bars are hammered into the soil like nails. Soil nailing has been used to provide excavations earth retention for buildings, parking structures, tunnels, and repair to existing retaining walls.
The soil nail provides a resisting force against slope failures. The excavation is accomplished in three to six foot increments. After each depth is excavated, reinforcing rods and mesh are placed into and against the face of the slope; the soil nails are located and driven in place. Then the entire face is sprayed with concrete gunite and the nails are grouted, cured, and the tested to confirm the design strength requirement has been met. This procedure continues along the perimeter of the excavation and is repeated until the excavation is complete.
Soil nailing is ideal for tight spaces or areas where existing utilities are not in conflict with the soil nails. It eliminates time and expense of placing steel piles; wall height is not restrictive, and these systems typically can be left in place. The soil nailing system also eliminates the need for internal bracing that may not be compatible with the construction. It is not recommended, however, in high water table areas where low shear strength soil may require high soil nail density or where soil has high sand or clay content since compaction is critical for successful nailing.
The Project Team, using information from the Geotechnical Engineer, can make the best earth retention choice for each project. The project benefits when the Construction Manager has experience with all systems.