Before one can begin a discussion of soil nailing, a clear understanding of the difference between soil nails and tieback anchors is required. Many times one hears the term “Soil Nail” and “Tiebacks” used interchangeably and this demonstrates a lack of understanding of the products.
Suppose that a construction project requires an excavation where the side of a soil cut cannot be provided with a stable slope. Figure 1 illustrates the soil cut and excavation for this project. |
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| One can easily understand that without some kind of containment of the soil at the face of the cut, collapse of the soil along a failure plane is likely to occur. This failure can happen very quickly and without warning. The failure might look something like Figure 2. The unstable soil has moved to the bottom of the excavation leaving a natural and stable slope for the remaining soil. This interface between the stable and unstable soil is called a slip plane. |
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| The most common way to prevent this kind of soil failure is to provide lateral support to the
unstable soil situated in front of the slip plane. One common way to do this is with a retaining wall and tieback anchors. The tiebacks work together with the structural retaining wall to provide sufficient lateral support to retain the unstable soil mass. The retaining wall must be designed and constructed to provide rigid support for the soil mass across the distance between the tieback anchor placements. One often sees tieback anchors spaced eight to twelve feet or more apart along a retaining wall. The spacing and number of anchors depends upon the wall height, surcharge loads and properties of the retained soil. Tieback anchors must be driven to a depth that is sufficient to provide tension resistance in the anchor shaft that is equal to the soil forces pushing against the retaining wall. A typical soil cut with a retaining wall is illustrated in Figure 3. |
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IIn many construction projects soil nails are used to retain the unstable soil mass. To accomplish this, soil nails are installed in an evenly spaced close geometric pattern. When installing a soil nail stabilization project, the soil nail installations and the excavation must be accomplished in incremental depths of 4 to 6 feet until the final depth of cut is accomplished. Usually only one increment can be completed per day. Immediately following the incremental excavation of the soil and the soil nail installations, the vertical face of the soil is covered with a steel reinforced coating of shotcrete.
Soil nails are passive structural elements and are not tensioned after installation. The soil nail gains pullout resistance from within the sliding soil mass in front of the slip plane and the stable soil mass located behind the slip plane. The geometric system of soil nail placements creates an internally reinforced soil mass that is stable. Figure 4 shows a sketch of a typical soil nail installation.
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Notice that each soil nail shaft has a great number of helical plates with the same diameter. These helical plates are evenly spaced along the entire length of the shaft. By comparison, a tieback anchor has one or more helical plates situated at the tip of the tieback. These helical plates generally increase in diameter along the shaft away from the tip. Once a tieback anchor lead section is installed, extensions without helical plates are used to extend the helical plates at the tip to the target depth. This characteristic of tieback anchors is clearly shown in Figure 3. Soil nails, on the other hand, will always have identical evenly spaced, small diameter helical plates along the entire shaft from beginning to end.
Soil nails may be the product of choice in applications where the vibrations from installing sheet piling or “H” piles may cause structural distress to nearby structures. Soil nails are generally installed to a shallower depth than tiebacks, which might be an advantage if deeply installed tiebacks have to cross property lines and/or terminate under structures owned by other parties; or be otherwise obstructed.
Soil nails work very efficiently in medium dense to dense sand with Standard Penetration Test values, ”N” > 7 blows per foot. They also are suited for low plasticity cohesive soil (clays) with SPT values of “N” > 8 blows per foot, which also have soil cohesion values exceeding 1,000 psf through the entire depth of soil to be stabilized. |
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ECP Soil Nail Components |
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ECP Soil Nail products consist of a shaft fabricated from either 1-1/2 inch or 1-3/4 inch solid square steel bar. Welded along the entire length of the Soil Nail shaft are identically sized helical plates measuring six or eight inches diameter with a plate thickness of 3/8 inch. The available lead shaft lengths for ECP Soil Nails are nominally five and seven feet long; however, other lengths may be specially fabricated. Soil nail extensions are also available in nominal lengths of five and seven feet long. The extensions also contain evenly spaced helical plates. Soil nail extensions are supplied with integral couplings and hardware for attachment to already installed lead or other extensions allowing the soil nail assembly to reach the designed length requirement.
Soil nails may be terminated with a large flat wall plate or an assembly of reinforcing bars welded to a small wall plate. These wall plates will eventually be embedded into the reinforced shotcrete wall covering. |
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| Product Benefits |
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| • Quickly Installed Using Rotary Hydraulic Torque Motor |
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| • Installs With Little Or No Vibration |
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| • Installs In Areas With Limited Access |
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| • No Post-Tensioning – Immediate Support |
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| • No Need for “H” Piles, Sheet Piling, or Walers |
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| • Easily Load Tested To Verify Capacity |
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| • In Temporary Applications, Soil Nail Removal and Reuse is Possible |
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