SOIL NAILING

Soil Nailing is a construction technique used to reinforce soil to make it more stable. Soil nailing is used for slopes, excavations, retaining walls etc. to make it more stable. In this technique, soil is reinforced with slender elements such as reinforcing bars which are called as nails. These reinforcing bars are installed into pre-drilled holes and then grouted. Soil nailing is used to stabilize the slopes or excavations where required slopes for excavation cannot be provided due to space constraints and construction of retaining wall is not feasible. It is just an alternate to retaining wall structures. As the excavation proceeds, shotcreting or other grouting materials are applied on the excavation face to grout the reinforcing steel or nails. These provide stability to the steep soil slope. Soil nailing technique is used for slopes or excavations alongside highways, railway lines etc.

Principle theory of Soil Nailing:

According to Abramson (2002), the soil mass behind the soil slope is divided into an active zone and a passive zone which are separated by a shear face called slip surface. The stabilizing manner depends on the soil frictional force between the soil nail surface and soil which is generated by the surrounding soil mass in passive zone. The soil nail must penetrate beyond the slip surface into the passive zone.

Applications of Soil Nailing:

Soil Nailing has been used for both temporary and permanent works. The main applications of soil nailing are summarized as under: 

1. It can also be used for natural hillsides stabilization and disturbed terrain. 
2. Roadway cut excavations. 
3. It can be used to stabilize the man made soil cut slopes. 
4. Road widening under an existing bridge end. 
5. Repair and reconstruction of existing retaining structures. 
6. Providing an earth retention system for deep excavations. 
7. Supporting and strengthening ground around tunnel excavations.

Advantages of Soil Nailing: 

1. Soil Nailing results in saving in cost when compared to other methods/ cost effective technique. The equipments required for execution of soil nailing are relatively small scale, easily movable and produce little noise/ simple and equipments. 
2. Soil nail installation is relatively rapid and uses typically less construction materials. 
3. The maximum lateral displacement of the soil nailed cut at the time of excavation was generally not more than 0.3 % of excavation depth. 
4. Soil Nailing provides an obstruction free working environment. The technique also requires lesser working space for the construction of soil nail wall. 
5. Soil Nailing performs well even in seismically active regions/ suitability during earthquakes. 
6. Soil nail walls are relatively flexible and can accommodate relatively large total and differential settlements.
7. Shotcrete facing is typically less costly than the structural facing required for other wall systems.

Limitations of Soil Nailing:

1. Unsuitable soil: Cohesionless soil slopes are not suitable for soil nails for increasing slope stability. This is because during the drilling of the hole, the un-grouted hole may collapse. Usually, casing drilling may be applied during the drilling process. 
2. Groundwater: Soil nailing has to occur above groundwater level. When soil nail holes are drilled, the drilled hole may collapse because hole surfacing soil is saturated or is filled with water. Therefore, a drilled hole cannot support itself and in result the hole will collapse. Furthermore, when the soil nails are being grouted, groundwater inside the drilled hole may affect the water/cement ratio of the cement grout. This may affect the grout quality and reduce the cement grout strain capabilities. 
3. Utilities: Soil nails are drilled inside the slope. Behind of slope may contain utilities such as buried water pipes, underground cables and drainage systems. There are some limitations that state that soil nails must have a safe distance between soil nails and these utilities. Therefore, a soil nail must change its inclination or length or spacing to achieve this distance.
4. Vibration sensitive structure: During the drilling procedure, vibration may occur and cannot be avoided. Some building structures are vibration sensitive such as Historical Buildings. Therefore, soil nailing is not the suitable method for slope improvement in these cases. 
5. Rock base slope: Some cut slope contain only few meters of top soil. During site investigation the deep layer soil type or a large boulder may be undetected (which would be possible with ground investigation, indicating it’s importance). When drilling the soil nail holes and the rock layer is reached, dust and stone powder may affect the environment and public health.