Describe Representative aspects of soil nail wall construction.
Describe Representative aspects of soil nail wall construction.
Soil nails are reinforcing, passive components that are sub-horizontally drilled and grouted in the ground to support soil excavations in soft and weathered rock that contribute to the stability of structures that resist earth. They carry tensile loads to the surrounding ground through shear stresses and act as a barrier to protect design procedures. In comparison to traditional hardened soil where the elements are mounted horizontally that would impact their contact with the surface so soil nails are positioned at an inclination to the horizontal.
Components of Soil Nail Wall
- Soil Nails
- Tendons: - Tendons are the components that stabilize the ground behind a wall of soil nails which is equivalent to steel bars. Tensile stress in each tendon mobilizes in reply to lateral movement and deformation of the retained soil. The tendons may be bars which are solid or hollow. Solid bars are inserted in stable drill holes and grouted in place whereas hollow bars are fitted with a sacrificial drill bit and are used to fill the hole and then remain in place as permanent soil nail reinforcement.
- Grout: - Grout used for soil nails is generally made from Portland cement and water. The main functions of grout are to (i) transfer shear stresses between the ground and tendon. (ii) transfer tensile stresses from tendon to the soil.(iii) provide protection from development of corrosion in tendons
- Corrosion Protection: - Soil nails used in permanent application must be protected from corrosion in order to withstand for longer period of time so soils nails required some physical and chemical against corrosion.
B. Facing
Facing can be done in two types i.e. initial facing and final facing. After excavation, initial facing should be done to provide temporary stability and protection. Bearing of soil nails are done in initial facing. The final facing is designed over the initial facing and provides structural consistency over the entire design life.
C. Drainage system
A drainage system is established behind soil nail walls to: (i) collect perched groundwater or infiltrated surface water which is present behind the facing; and (ii) direct the collected groundwater away from the wall.
Construction Sequence
Step 1 Excavation: - Excavation process is done till the required depth so that a sufficient platform will be created for smooth and safe soil nail installation process.
Step 2 Drilling of Nail Holes: - Specialized drilling equipment operated from the excavated platform is used to advance the drill holes. Typically the drill holes stay unsupported.
Step 3 Tendons and Strip Drain Installation
- Nail Installation and Grouting: - Tendons are inserted into the drill holes and a grout pipe is also inserted into the drill hole along with tendons so that hole should be filled with grout.
- Installation of Strip Drains: - Strip drains are also inserted from top to bottom of the excavation simultaneously with tendon insertion.
Step 4 Construction of Initial Shotcrete facing: - Before the next lift of soil is excavated, the unsupported cut is applied with an Initial facing. Some horizontal and vertical bars are also inserted in order to provide bending resistance. As the shotcrete begins to heal, a plate of steel bearing is placed over the tendons which get stick out from the drill hole. The bearing plate is pressed gently into the new shotcrete. Before continuing with the next excavation lift, testing of the mounted nails is to be completed to prove their ability or to verify the load specific criterion.
Step 5 Construction of different levels: - Steps 1 to 4 are repeated for different subsequent levels of excavation lifts. At each level, vertical drainage strip is placed downward until the subsequent level and at the end the drainage strip is tied to a collecting toe drain.
Step 6 Construction of Final facing: - After reaching to the bottom of the excavation lift, final facing is to be constructed after testing various load specific criterion.
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