The application of Anchor Drill Rig──Soil Nailing

What’s Soil Nailing?
Soil nailing is a technique used to bring soil stability in the areas where landslides might be a problem. Soil nail can prevent landslides by inserting steel reinforcement bars into the soil and anchoring them to the soil strata. It is called Soil Nail, because it’s like having a nail being hammered into the soil, where the nails, are the steel bars. 

Soil nailing technique is used for slopes or excavations alongside highways, railway lines etc. Following figure shows soil nailing in railway construction:
 
Soil Nail Installation Process
1. Drilling into the soil by Anchor drill rig, where the nail or steel bar is going to be placed. 
2. After the drilling has been completed, exact depth must be provided by the geotechnical engineer, the nail must be inserted into the drilled hole. Then, it must be grouted into the soil to create a structure similar to a gravity wall. 
3. After placing the nail, a shot-Crete layer is usually placed as a facing material, to protect the exposed nail, and then other architectural options are placed over the shot-Crete, creating an aesthetic finish to the project.

Steel tendons typically used for drill-and-grout soil nails usually consist of 0.8 to 2.0 inch bars with a yield strength in the range of 60 to 72 Ksi. Drainage is a critical aspect of soil nail wall construction. Face drainage is virtually always used with permanent walls, and very commonly used with temporary walls. Face drainage usually consists of synthetic drainage elements placed between the shot-Crete and the retained soil, and may be typically 8 to 12 inches wide synthetic strips or perforated pipes. 

The grouted soil nail hole typically has aminimum diameter of 4 inches. Centralizers are placed around the soil nail to maintain an even thickness of grout around the bar. For permanent applications, nails may be epoxy-coated or provided with a protective sheath for corrosion protection. 

Soil Nailing is not recommended to use on clayey soils or clean sands where the cohesion of the soil is minimum. 

Soil Nailing Benefits and Limitations

Soil nailing has been used regularly over the last few years over traditional cut retaining walls due to several reasons:
• It is ideal for tight spaces.
• It can be used in irregular shapes.
• Less noise and fewer traffic obstructions.
• Less impact on surrounding areas.
• Minimum shoring is required.
• Lower load requirements than tieback anchors systems.
• Eliminates the time and expense of placing steel piles.
• Can be used to repair other existing wall systems.
• It can be used on new constructions, as temporary structures or on remodeling process.
• Wall height is not restricted.
• Reduced right-of-way requirements.

Some of the limitations of soil nailing could be:
• In some instances soil might be overexposed prior to the installation of the nail.
• Sand and gravels might not be compatible with soil nailing.
• Not recommended to use in areas of high water table.
• Soil nailing in very low shear strength soil may require a very high soil nail density.
• Soil nailing in sensitive soils and expansive soils for permanent long-term applications is not recommended.