RST Environmental Solutions Ltd

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To view the ECP Helical Soil Nails brochure click Soil Nails (5.47MB)


ECP Tie Back Anchors
RST have tie back anchors available for installation. If you require information about these please ring 06 355 3625

 

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ECP Helical Soil Nails
38 mm Solid Square Bar Helical Soil Nails
44.5 mm Solid Square Bar Helical Soil Nails

Introduction
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.

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 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.

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.

In 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 1.2m to 1.8m 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. 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. 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 0.3m. They also are suited for low plasticity cohesive soil (clays) with SPT values of "N" > 8 blows per 0.3m, 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
ECP Soil Nail products consist of a shaft fabricated from either 38 mm - 44.5 mm 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 9.5mm. 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 1.5m and 2m 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.

Product Benefits

  • Quickly Installed Using Rotary Hydraulic Torque Motor
  • Installs With Little Or No Vibration
  • Installs In Areas With Limited Access
  • No Post-Tensioning - Immediate Support
  • No Need for "H" Piles, Sheet Piling, or Walers
  • Easily Load Tested To Verify Capacity
  • In Temporary Applications, Soil Nail Removal and Reuse is Possible
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Product Limitations
Soil nails are designed to attain pullout resistance from within the sliding soil mass along with the resistance from the stable soil behind the movement plane. As a result of this tensioning, one must anticipate movements horizontally and vertically at the top of the excavation on the order of 3.2mm movement for each 1.5m of excavation. These movements are normally not of concern unless a building is situated close to the proposed soil cut. Creep of the soil mass after the initial soil movement is usually not a problem; however when the soil liquidity index is > 0.2, a soil nail matrix is not recommended. Soil nails may not be suitable in situations where the soil report indicates the presence of weathered rock anywhere within the area to be stabilized. Soil nails are also not recommended in loose sand with SPT value of "N" < 7 blows per foot. Where highly plastic clays and silts are present within the soil ass, the use of soil nails must be approached with caution. Soil nails are not recommended for low plasticity clay soil having SPT value of "N" < 6 blows per foot. The practical limit for excavations using the soil nail stabilization technique is approximately 6.09m; although under ideal soil conditions, excavations as deep as 7.6m have been reported. When designing soil stabilization with surcharge loads such as buildings, roads, soil overburden, etc, near the top of the excavation; the surcharge loads must be included along with the weight of the soil mass being retained. With an expected slump of 3.2mm for each 1.5m of excavation, one should consider stabilizing the perimeter footing of nearby structures whenever the excavation exceeds 3 to 3.6 metres because lateral and vertical movements on the order of 6.35mm to 9.5mm could cause structural damage to nearby existing structures.

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