Construction Sequence of a Soil Nail WallArticles > Construction Sequence of a Soil Nail Wall
The typical sequence of construction of a soil nail wall is described below and shown schematically in the figure.
- Excavation. The depth of the initial excavation lift (unsupported cut) may range between 2.5 and 7 ft, but is typically 3 to 5 ft and reaches slightly below the elevation where the first row of nails will be installed. The feasibility of this step is critical because the excavation face must have the ability to remain unsupported, until the nails and initial face are installed, typically one to two days. The type of soil that is excavated may limit the depth of the excavation lift. The excavated platform must be of sufficient width to provide safe access for the soil nail installation equipment.
- Drilling of Nail Holes. Drill holes are advanced using specialized drilling equipment operated from the excavated platform. The drill holes typically remain unsupported.
- A) Nail Installationand Grouting. Tendons are placed in the drilled hole. A tremie grout pipe is inserted in the drill hole along with the tendon; and the hole is filled with grout, placed under gravity or a nominal, low pressure (less than 5 to 10 psi). If hollow bars are used, the drilling and grouting take place in one operation.
B) Installation of Strip Drains. Strip drains are installed on the excavation face, continuously from the top of the excavation to slightly below the bottom of the excavation. The strip drains are placed between adjacent nails and are unrolled down to the next excavation lift.
- Construction of Initial Shotcrete Facing. Before the next lift of soil is excavated, an initial facing is applied to the unsupported cut. The initial facing typically consists of a lightly reinforced 4-in. thick shotcrete layer. The reinforcement includes welded-wire mesh (WWM), which is placed in the middle of the facing thickness (Figure 2.1). Horizontal and vertical bars are also placed around the nail heads for bending resistance. As the shotcrete starts to cure, a steel bearing plate is placed over the tendon that is protruding from the drill hole. The bearing plate is lightly pressed into the fresh shotcrete. Hex nuts and washers are then installed to engage the nail head against the bearing plate. The hex nut is wrench-tightened within 24 hours of the placement of the initial shotcrete. Testing of some of the installed nails to proof-load their capacity or to verify the load-specified criterion may be performed before proceeding with the next excavation lift. The shotcrete should attain its minimum specified 3-day compressive strength before proceeding with subsequent excavation lifts. For planning purposes, the curing period of the shotcrete should be considered 72 hours.
- Construction of Subsequent Levels. Steps 1 through 4 are repeated for the remaining excavation lifts. At each excavation lift, the strip drain is unrolled downward to the subsequent lift. A new panel of WWM is then placed overlapping at least one full mesh cell with the WWM panel above. The temporary shotcrete is continued with the previous shotcrete lift.
- Construction of Final Facing. After the bottom of the excavation is reached and nails are installed and tested, the final facing is constructed. The final facing may consist of CIP reinforced concrete, reinforced shotcrete, or prefabricated panels. Weepholes, a foot drain, and drainage ditches are then installed to discharge water that may collect in the continuous strip drain.
Variations of the steps described above may be necessary to accommodate specific project conditions. For example, shotcrete may be applied at each lift immediately after excavation and before drilling of the holes and nail installation, particularly where stability of the excavation face is a concern. Another variation may be grouting the drill hole before placement of the tendon in the wet grout.
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