What is Soil Nailing in Construction? Principle and Methods

Soil nailing is a soil or earth retention technique to help soil remain flat and stable without collapsing during an excavation or construction at a site. These are also mentioned as soil nail wall construction which involves the insertion of reinforcement bars into the soils and anchoring them to the soil strata. This can be used either to provide temporary to permanent support.

Figure-1: Soil Nailing

As shown in the figure-1, it looks like a nail hammered into the soil, these nails are steel bars.

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Principle and Installation of Soil Nail System

The process of soil nailing can be done in various methods. This choice is completely dependent on the cost, the accessibility to the site, the working space, the ground water level and the conditions. The common methods are:

1. Drill and Grout Soil Nailing Method
2. Self-Drilling Soil Nailing Method
3. Driven Soil Nailing Method

1. Drilling and Grout Soil Nailing Method

In this method, the soil nail reinforcement is inserted into a hole that is already pre drilled. After insertion of reinforcement, it is filled using a cement grout either by gravity or low pressure method.

The holes are drilled using a down-the-hole hammer or a rotary percussion or a rotary. The technique is selected based on the soil and the ground conditions. 

Advantages

• Underground obstructions can be overcome
• Information about the ground is obtained
• Long soil nails can be used in this method
• The drill holes can be checked before inserting the reinforcement

Disadvantages

• Drilling holes can sometimes result in collapse
• Critical site areas require casing before drilling holes.

2. Self Drilling Soil Nailing Method

In this method, a drill bit is used to directly insert the reinforcement bar into the soil. The rebar act as a drill rod and a grout pipe. This method involves simultaneous drilling and grouting, hence it is rapid. The flushing medium used here is cement grout instead of air or water which helps to maintain the stability of hole. 

Advantages

• Do not require casing , centralisers or grout pipes.
• Fast operation

Disadvantages

• Penetration into rock is difficult
• Concern  of corrosion and durability issues
• Required grout cover may not be obtained in the absence of centralizers

3. Driver Soil Nailing Method

Here, the soil-nail reinforcement is driven into the soil by ballistic method using compressed air launcher. This can be performed either using hammering equipment or by vibratory methods using a vibrator. 

In this process, while driving the nail, the soil around the reinforcement will be displaced and compressed. Hence, the method create less soil disruption. 

Advantages

• Rapid
• Less disruption to soil

Disadvantages

• Short length penetration is only possible by this method as limited power is involved.
• Penetration result in excessive buckling of the nail reinforcement
• Not suitable for stiff and cores tone site
• Susceptible to corrosion, so non-corrodible reinforcement is used.

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