Soil nailing is a slope protection technique for supporting unstable natural slopes and over steeping of existing slopes. Soil nails are reinforcing passive elements drilled and grouted sub-horizontally in the ground. They are used to support excavations in soil, or soft and weathered rock and slope protection works.
Soil nail walls are used as permanent earth-retaining structures in highway projects. They can also be constructed as temporary structures in roadway work when used as shoring of deep excavations.
Components of soil nailing and its function
Components mainly constitutes installing passive reinforcement without any post tensioning in existing ground know as nails. Soil nails are later grouted if they are installed in drilled holes. Soil nails using solid bar drilling system or hollow bars (sacrificial hollow bar system which drills and grouts simultaneously ) need not need any grouting. Let us go in detail each and every component of soil nailing system.
a) Tendons used for soil nailing
They are the ground reinforcing elements and are equivalent to steel bars. Tendons can take care of tensile stress developed during the lateral movement and deformation of retained soil and also the external loads in the service stage such as surcharge loads and traffic loads.
There are two methods of fixing soil nails or reinforcement bars.
a) Holes are drilled and pressure grouted with fully threaded bars embedded inside.
b) Using sacrificial drill bits where drilling and grouting will be done simultaneously and the sacrificial drill bit is converted to rebar.
b) Grout
Normal OPC cement mixed with water is used for grouting. The function of grout are
a) Transfers shear stress between the ground and tendons and
b) Corrosion protection for rebars.
c) Installation of facing in soil nailing
Soil nail construction is done from top to bottom. Every nail is installed with anchor plates or bearing plates.
First, a single row of soil nails is installed after excavating the surface. Excavate further and install the second layer of soil nails as per design. After completing a reasonable height at which soil can free stand ( 1-2mtr) for at least 2 -3 days the next phase of the shotcreting process will start.
d) First face shotcrete on soil surface
A geotextile drain matting is placed over the soil and is followed by welded wire mesh as shown in the figure. Rebar stiffeners are sometimes provided to strengthen the shotcrete against punch shear forces. On completion of the first phase of shotcreting bearing plates with bevelled washers are installed over the shotcrete surface.
e) Second phase of reinforced concrete
If required as per design the first layer is covered with a second phase of reinforced concrete layer as shown in the figure. This concrete covers the nail head.
Applications of soil nails
Soil nails are one of the most economical and feasible tops to bottom constructed retaining walls system. They are technically feasible, fast, and reliable slope protection and earth retaining system. Soil nails offer a perfect cost-effective system for temporary retaining walls for deep excavations..
- High way cut excavation of hilly areas
- Road widening under an existing bridge end.
- Repair and reconstruction of existing retaining structures.
- Temporary or permanent deep excavations in urban areas.
Feasibility of soil nail
Before confirming the soil nail system please ensure the following parameters at the site.
- Soil should be able to free stand at a height of around 1-2 mtr unsupported for a minimum of two days.
- All soil nails within a cross-section will be above the water table.
- If the soil nails are not located above the groundwater table, the groundwater should not negatively affect the face of the excavation, the bond between the ground and the soil nail itself.
- They can be used for almost all types of soils including stiff/sandy/hard clay, dense sand and gravel areas, evenly weathered rocks.
- Avoid using soil nails in dry, poorly graded cohesion-less soils, soils with a high groundwater table, soils with cobbles and boulders, soft to very soft fine-grained soils, highly corrosive soils, weathered rock with unfavourable weakness planes, etc.
Advantages of soil nails over other retaining systems
- Soils nails require very little workspace comparing with any other retaining systems.
- The operations are mostly light and silent and there is no disturbance to the traffic and people residing nearby.
- Soil nails do not need any foundation or any structural whaler beams at the bottom like cantilever and anchored retaining walls.
- With the soil nailing method, we can reduce the duration of work, and fewer materials are consumed in this process.
- They are so flexible and easily customizable and nail location can be easily adjusted on encountering any obstructions.
- Small equipment is used for soil nailing works.
- They can accommodate differential settlements and deflection of soil nails are usually within tolerable limits.
- They are cost-effective than any other retaining wall system because shotcrete of minimal thickness is used than heavy structural walls in the case of other retaining wall systems.