Soil nailing is a way to earth retention, trenches, gables, etc. In soil nailing method, the soil is reinforced with nails with a diameter of 28 to 40 mm. Approximate distances of nails are about 2 m and the angle of the nails to the horizon is about 10 to 20 degrees. Soil nailing operations are often performed as a top down construction on a sloping or vertical surface. Nailes are placed in boreholes inside the soil wall and are filled with cement slurry to transfer forces between the soil and the nail and prevent corrosion of the nails. The nailed wall creates a reinforced section that holds the soil behind it.
Soil Nailing is an economical and relatively new method for stabilizing slopes, slopes, trenches adjacent to roads and widening roads, excavation near structures, strengthening old retaining structures, increasing bearing capacity, limiting deformation with minimal interference and so on.
History of soil nail method
The ground nail method was first used around the 1960s by Austrian engineers to stabilize rocky slopes in tunnels and has been common in Europe since the 1970s. Austrian engineers drilled a number of boreholes in the rock arches and stone walls of the tunnel and placed ground nails inside the boreholes, injecting cement slurry into the boreholes and trapping the ends of the nails in the tunnel environment with mesh and shotcrete. German and French engineers used the Austrian stone nailing method for slopes and earthen walls. They generalized the use of soil nailing technology in tunnels to stabilize excavated slopes and earthen walls and bridge abutment.
Design Basics of Soil nail walls
Unlike the soil anchorage method, the soil nail technique is passive and exerts its effect through bar-soil interaction resulting from deformation in the soil. Therefore, the use of soil nail technique in the vicinity of displacement-sensitive structures such as old structures is not recommended.
Two limit modes are considered for the analysis and design of nailed walls Soil nail wall construction:
A) Limit mode of resistance: The designed wall must maintain its stability against the following 3 failure modes:
- External failure modes
- Internal failure modes
- Facing failure modes
B) Operation limit mode: The deformation of the designed wall should be less than the allowable deformation.
The Nail Stabilization Design Regulation is the FHWA (Federal Highway Administration) Regulations prepared by the US Highway Administration. The proposal of this regulation for reliability coefficients to stabilize the excavation against failure modes is as follows:
Function of soil nailing technique
The main function of nails in the soil nailing method is tensile, and a slight bending and shear performance can be considered for them. Nails are effective in wall stability with the following two functions:
- Increasing shear strength of slip surface in frictional soils due to increased vertical force.
- Reduction of slip surface thrust in friction and cohesive soils.
Ececution of soil nail technique
The proposed method for nailing includes several main stages that are performed according to soil conditions, etc. in one of the following methods:
- The first soil nail technique:
- Excavation below the level of the first nail (cross-sectional excavation)
- Concrete shotcrete with a layer of steel mesh reinforcement (mesh) and installation of drainage
- Drilling boreholes
- Installing reinforcement nails
- Cement grout injection
- Installing the plate and nut
- The second soil nail technique method:
- Excavation below the level of the first nail (cross-sectional excavation)
- Drilling boreholes
- Installing reinforcement nails
- Cement slurry injection
- Concrete shotcrete with a layer of steel mesh reinforcement (mesh) and installation of drainage
- Installing the plate and nut
The following is a description of each of the stages:
1) Excavation:
Initial excavation is done to a maximum depth so that the soil can maintain its temporary stability to capture the cement slurry injected into the borehole. After nailing and trapping the cement, the next stage of excavation is done in the same way. For better stability and less wall deformation, excavation of each stage is done one by one or mouse teeth. The width of each part should also be suitable for the placement of nailing machines.
2) Drilling
One of the most important stabilization stages in the soil nailing method is borehole drilling because there are many variables to consider:
- Bore length: The compressor air production capacity for boreholes above 15 m should be more than short boreholes.
- Soil type: In coarse-grained soils, there is a possibility of localized boreholes and air escape.
- Power and speed of drilling machine: it should be considered appropriate to the soil material.
- Digging movement capacity: In saturated soils or hand soils should be considered more than natural and dry soils.
- Hammer and drill excavation size: it should be selected according to the diameter of the borehole.
Different methods of drilling and placing nails inside the wall of the excavation:
- Drilling and injection: This method is often used in non-fall soils, which has the following 4 stages:
- Drilling a borehole by a drilling machine
- Exit of drilling tools (drill bit or rod)
- Put the nail inside the borehole
- Injection
- Self-drilling: This method is used in fall soils or to accelerate the execution of the project. In this method, the drilling tool (self-drilling rod and drill bit) acts as a bar (nail) and remains in the wall. Meanwhile, during drilling, injection operations are performed.
- Compacting: Compacting the nail inside the wall
- Jet grouting
Drilling equipment is used for drilling in the wall. The specifications of boreholes are often as follows:
- Borehole diameter: 10 to 15 cm
- Borehole length: 4 to 15 m
- Bore angle to the horizon: 0 to 20 degrees
- Distance of boreholes with each other: 1.5 to 2 m
3) Installation of reinforcement nail and injection of cement slurry
After drilling the borehole to the length desired by the designer, it is time to install the nail. The elements of reinforcement are usually in the following 3 ways:
- Most solid nails (diameter 28 to 40 mm)
- Sometimes hollow core soil nail FHWA 2006
- Sometimes EFRI Fiberglass Nail Team2005
To place the nail in the middle of the borehole and prevent the nail from sticking to the borehole wall, a device called a spacer is used.
The nail is placed inside the spacer and then placed inside the borehole. If the soil is corrosive, epoxy coating is used to prevent corrosion of the nails. After placing the nail in the borehole, it is time to inject the cement slurry. The slurry is often poured by gravity with the same slope of 10 to 20 degrees through the tube from the end to the beginning of the borehole. By filling the gap between the nail and the borehole wall, it is possible to transfer stress to the nails.
Injection equipment has 3 parts:
- Primary mixer
- Secondary mixer
- Injection pump
Water and cement, in the proportions specified in the executive surveys, are mixed in the primary and secondary mixers and injected into the borehole through a tube. Injection pressure depends on soil conditions and so on.
4) Concrete shotcrete of walls, installation of plates and nuts
After finishing the nailing and injection operation:
1) Execution of drainage stripes: Drainage stripes (geocomposite) are installed behind the wall to reduce water pressure and conduct water on the wall surface.
2) Execution of nail steel mesh: A mesh of welded wires called mesh (WWM: Welded Wire Mesh) that run almost in the middle of the thickness of shotcrete. The length of steel mesh should be such that at least one complete source of the mesh overlaps with the next mesh.
3) Shotcrete: After installing steel nail mesh, the wall surface with a layer of 10 to 20 cm, become shotcrete to maintain greater integrity between the nails to prevent soil erosion. This coverage is implemented in 2 ways:
- Temporary cover: it connects the structural components of the nailing together.
- Permanent cover: It is applied after installing nails and temporary cover. The thickness of this cover is often limited to 10 to 15 cm.
4) Installing plate and nut due to:
- Complete connection of the nail head to the wall
- Preventing punching in the wall
- Tighten the nails if necessary
A metal plate is placed on the wall as the head of the nail and is connected to the reinforcement inside the borehole by a nut.
With the completion of these 4 stages, the first row of nails is finished and for the second row of nails to the end of the excavation, the same stages must be repeated.
Nail
Nail with a diameter of 25 to 40 mm, which are placed at an angle of 10 to 20 degrees to the horizon in the wall of the excavation
Temporary surface
It is usually applied in the form of shotcrete with light steel mesh and its thickness is 10 to 20 cm.
Permanent surface
It can be applied in the form of shotcrete with light steel mesh, in-situ concreting and precast parts (reinforced concrete, etc.) and its thickness is usually considered to be 20 cm.
Spacer
The duty of the spacers is to hold the nail along the center of the hole. The spacers can be installed every 2.4 m along the nail and at a distance of 45 cm from the head and end of the borehole.
Borehole
The diameter of the borehole is usually between 10 and 15 cm. The borehole is often drilled with a slope of 10 degrees to the horizon. The distance between the end of the nail and the end of the borehole is at least 15 cm.
Grout
The function of the slurry is to connect and trap the nail with the soil and protect the nail against corrosion. The minimum slurry coverage on the nail should be 25 mm. Usually, the ratio of water to cement in cement slurry is 0.5 to 0.4
Bearing plate
The function of the bearing plate is to distribute the force of nail on the temporary surface and the soil behind it. It is usually a square with a side of 20 to 25 cm and a thickness of 2 cm.
Headed Stud
The duty of the headed stud is to engage the permanent surface and the nail head. Usually 4 headed studs are installed on each bearing plate for each nail. The minimum coverage on the headed stud is 50 mm.
Strip Drain
Behind the temporary surface, it is placed between two or more vertical columns of the nail, and with them and with the help of drainage tubes, the water behind the wall can be directed out.
Here are some practical points to Execution soil nailing operation.
- The following methods can be used to patch the nails:
- Nail head-head welding (head-head forging)
- Mechanical connection (coupler)
- In order to avoid legal problems, before performing the soil nailing operation, a consent must be obtained from the adjacent owners in the notary public offices.
- The nails used in soil nailing operations are often of AIII type and their length is determined according to the survey.
- Soil nailing operation is not possible under the water surface. Therefore, drainage must be done first.
- Before soil nailing execution, the project soil studies should be done to determine the soil characteristics.
- Permitted excavation height depends to factors such as the shear strength of the soil, the amount of overhead, the amount of penetration of surface water, etc., which must be calculated.
- Before drilling boreholes, underground features adjacent to the excavation, such as sewage, aqueducts, underground structures, etc. are examined so that they are not encountered during drilling.
Video on how to do soil nailing
Related contents:
Federal Highway Administration (FHWA)
DEEP EXCAVATION , Soil nail wall – Soil nailing – soil nailing walls
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