Soil Nail Mechanics

ECP Helical Soil Nails are used to stabilize slopes in a variety of shoring applications. For soil nails to be effective, they must have helix plates of equal diameters spaced evenly along the entire length of shaft.  Remember that soil nails are not tensioned to gain strength; they gain pullout resistance from within the sliding soil mass that is located in front of the slip plane.  The concept is rather simple to understand.  As the soil mass begins to slip downward and outward, it pushes against the back side of the helical plates of the helix anchor embedded within this sliding soil mass.  The force generated by the sliding soil against these helical plates are resisted equally, and in the opposite direction, on the front side of the remaining helical plates on the Soil Nail shaft that are embedded within the stable soil behind the slip plane.

Due to the mechanics of helical soil nails the forces developed within the soil nail system reduce the structural requirements for the exterior wall or slope.  In most cases the soil nails are connected directly to the wall without any need for sheet piles, “H” piles or wales.  Because the soil mass is stabilized by the matrix of helical anchors, only a thin shotcrete wall is necessary.

ECP soil nails are installed in a geometrical matrix to distribute the load more evenly; and as such, soil nails are more lightly loaded than tieback anchors. Some engineers might specify a small “seating” load be applied to the soil nail after installation to remove slack in the couplings; but in general, screw anchors are usually not tensioned after installation as this can change the balance of stresses on the helices.

 Because soil nailing is a passive system, meaning that the soil nails are not post-tensioned, the unstable soil mass has to slump slightly before the soil nail system can develop forces to resist the soil movements. Contact ECP Engineering for specifications on helical soil nails or helical anchors.

Soil nailed walls are expected to deflect both downward and outward during the slumping of the soil mass with expected movements of approximately 1/8” of vertical and horizontal movement of the top of the wall for every five feet of excavation.  These movements are normally not a concern except when an existing structure is situated near the top of the excavation.  One way to relieve some of the overburden load from a nearby structure is to provide supplemental foundation support to the perimeter beam and/or column footings of the existing structure using ECP Steel Piers.  Transferring the structural load to the deep foundation piers not only reduces the surcharge on the soil mass, but also prevents vertical settlement of the footing as the sliding soil mass moves during the tensioning of the soil nail matrix.  If there are concerns with regard to lateral movements of the building’s footings, the designer has the ability to prevent lateral footing movements of the existing structure by using ECP Helical Soil Nails along with ECP Steel Piers to provide both lateral and vertical stability.

Figure 6 shows details of a typical soil nail installation. Usually four to five feet of soil is excavated followed by the installation of the first row of soil nails. Notice that the first row has the longest shaft length because the distance to the slip plane is the greatest. The soil nail is not installed to a specified torsion requirement like with tieback anchors; rather the length of embedment, the installation angle and product spacing are the important elements in soil nail installations.

Once all of the soil nails situated within the first excavation increment are installed, one-half of the required thickness of shotcrete is placed on the wall followed immediately by the installation of the wall plates and reinforcing steel mesh. The reinforcing mesh is cut long enough to provide suitable splice overlap at the next increment of soil excavation. A surface coating of shotcrete is installed over the steel reinforcement to provide the specified final thickness. All work is then left to cure prior to the next excavated depth increment.

Prior to the beginning the next excavation increment (usually the next day), the amount of slump at the top of the excavation should be measured to insure that the wall is performing as intended. The next depth increment is then excavated followed by the installation of the next row of soil nails followed by the immediate installation of the first layer of shotcrete. The only difference between the initial and subsequent incremental excavations is that the new layers of shotcrete and steel must be interlocked to the previous work to provide continuity to the wall.