In this article, we will discuss soil nailing; its components, cost, types, uses, advantages and disadvantages.
Soil nailing is defined as the construction technique to reinforce and strengthen the natural or excavated soil slope by installing nails into holes made in slopes by drilling and grouting procedure.
Soil nailing can be applied for different soil types like glacial soils, sandy clays, clayey silts, gravels, etc.
But it is not suitable for loose granular soils, high corrosive soils, soft fine-grained soils, organic soils, etc.
2. History of Soil Nailing
Soil nailing technology was first introduced in France to construct a retaining wall cut in soft rock. The project, undertaken in 1961, was the first where steel nails were used to reinforce a retaining wall.
The first soil nail wall to use modern soil-nailing techniques was built near Versailles in 1972. The technique included installing high-density, grouted soil nails into a 60-ft.-high wall and facing it with reinforced concrete.
Some of the components used in soil nailing are explained below:
a. Soil Nail
This component includes tendon, grout, and corrosion-proof soil nail.
Tendon is the ground-reinforcing element like steel (hollow or solid) bar that is inserted behind a soil nail wall.
Grout is used for transferring the shear and tensile stresses from the tendon to the ground to improve stability.
Portland cement and water are used to be placed in drilled holes under gravity by using the tremie method.
iii. Corrosion proof
For long-lasting, the soil nail should be protected against corrosion.
To protect the soil nail from corrosion, it is encapsulated by a sheath of a fusion-bonded epoxy coating or galvanization.
Facing can be done by shotcreting that includes initial and final phase depending upon excavation period for temporary and permanent stability.
Reinforced shotcrete is done during initial facing with the installment of welded-wire mesh over the excavated slope of soil consisting of precast-concrete spread with the help of shotcrete machine-like Aliva-520.
The final facing gives the structurally stable aesthetic design finishing.
c. Other Components
Other components like nuts, bolts, washers, bearing plates and headed studs are embedded with soil nails. There should be a proper drainage system around soil nailing sites using ground anchors, geosynthetic materials, and so on.
The cost of soil nailing depends upon a range of soil nailing applications, materials used, manpower, complexities during construction procedures, and the effectiveness of soil nailing work.
Cost of soil nailing installation and removal, maintenance differs nationalize.
5. Machinery Used for Soil Nailing
5.1. Rotary machine with a downhole hammer.
5.2. Air compressor (of 100psi and (300-700) CFM.
5.3. Grout Mixer
5.4. Grout Pump (for shotcrete)
6. Design Considerations
a. Immediate inauguration of soil nailing work after excavation should be done to reduce the chance of collapse of soil slope.
b. According to the strength of soil surface, spacing of soil nails should be chosen wisely for better soil slope stabilization.
c. Strength limit state of soil should be analyzed carefully from potential failure.
d. Every property should be checked carefully.
e. Nail length, diameter, external loads due to wind, hydrostatic force, earthquakes, vibrations, etc. matter during designation of soil nailing process.
f. Drainage system and freeze-thaw action should be taken into consideration.
7. Types of Soil Nailing Methods
7.1. Drilled and grouted soil nailing method
In this method, in naturally or excavated slope faces, holes are made by drilling machines like Ak-10, and nails are inserted in these drilled holes with grouting materials (concrete, shotcrete) in the spacing interval of 1.5m using (100-200mm) diameter nails.
7.2. Driven soil nailing method
In this method, excavated slopes are temporarily stabilized as quickly as possible using nails of (19-25mm) diameter with a spacing of (1-1.2m). This method doesn’t assure corrosion protection to reinforcement (steels or nails).
7.3. Self-Drilling soil nailing method
Hollow bars are inserted into the slope surface with the filling of grouting materials. It is faster than drilled soil nailing method having more corrosion resistive features.
7.4. Jet Grouted soil nailing method
Less corrosive steel bars are installed in the holes made by eroding soil using jets in slope surfaces. Then, holes are grouted with concrete.
7.5. Launched soil nailing method
Steel bars are launched into the holes with a compressed mechanism using an air compressor by nails of diameter 38 mm and 6m long. It is a difficult method to penetrate the ground with nails in a single shot.
8. Failure Modes of Soil Nailing
Under considerations of design constrained conditions, there are three failure modes of soil nailing. They are as follows:
8.1. Bonded length of soil
Loose binding of soil has more possibility of failure. Although it is tried to stabilize by soil nailing technique, it won’t be effective enough.
8.2. Number of nails
Nailing in the required area is the most important thing. The coverage area of soil nailing requires more nails to be inserted into the pre-drilled holes. Thus, if a deficient number of nails are used, there is a great chance of slope failure although soil nailing.
8.3. Prestress shotcrete
In welded wire mesh nails are fitted and grouting materials are filled. After that concrete and mortar are pumped over it using a grout pump known as the shotcrete process. It is the final finishing of soil nailing.
9. Installation of Soil Nailing
After designing the techniques for soil nailing, install contractors are hired with manpower and many types of machinery for the process.
Installation processes are done based on soil nailing methods. Installation of nails is done in pre-drilled holes and grouted with an inclination of (10°-20°) from vertical.
Generally, the installation process includes the following steps
1. Excavation(Initial and Final excavation is done)
3. Installment of steel soil nails bars and grouting
4. Shotcrete facing
5. Excavating for next shotcrete process
6. Sculpted-facing process for an aesthetic finish.
10. Soil Nail Installation Considerations
1. Soil Nails should penetrate beyond the slip plane into the passive zone generally for 4 m to 5 m.
2. The spacing of soil nails in the horizontal or vertical directions must be linked to the strength of the soil. Surplus soil nails should be placed at the edge of any surface being stabilized.
3. Now, soil nailing should begin immediately after excavation. Any late may result in the collapse of the soil slope.
11. Construction Sequence
The algorithmic sequence of construction work of soil nailing is:
Step 1: Initial excavation (of depth 3-5 ft)
Step 2: Drilling of holes using drill machines(downward 15° inclination from horizontal).
Step 3: Insertion of nail reinforcement and grouting into the slope face.
Step 4: Construction of initial facing of the slope.
Step 5: Construction subsequential level.
Step 6: Repeat steps 1 and 4.
1. It is used to reinforcing and stabilizing the slope of soil and retaining walls.
2. Soil nailing is done for widening roadways, railway embankment, landslide and flood protection on the banks of rivers, highway embankment and cuttings, etc.
3. It can be used for constructing Shored Mechanically Stabilized Earth (SMSE) walls.
4. It is used for retrofitting and constructing the bridge abutments.
1. Shotcrete facing is economical.
2. Cost-effective method for durability and stability of slope soil failure.
3. It can be applied to a relatively large area.
4. It is suitable for various applications like temporary excavation shoring, tunnel portals, repairing of failures structures, etc.
5. Installation process takes less time.
6. It has less impact on the environmental system.
7. It uses fewer materials and machinery.
8. It has no height limitations.
1. Unsuitable for high water table areas.
2. For soil having low shear strength, high soil nail density is difficult to use.
3. Expert contractors and manpower are crucial.
4. 3D modeling designation is required.
5. Not suitable where strict deformation control is needed for structures.
6. Metal Soil nails can corrode easily.
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