Nailing was a creative technique used by Australian engineers around the 1960s to stabilize tunnel walls.
Everything about bracing by nailing
The method of nailing is that first the walls are regulated. Then, steel meshes are installed with 50 cm nails and concrete is sprayed on it with a thickness of about 10 to 15 cm. The next step is to drill the boreholes and insert the rebar or cable. Then the boreholes are injected with cement slurry and after the slurry sets, the head of the nails is closed.
Thus, the force of soil thrust is applied to the head of the nail through mesh and shot, and the force of the head of the nail is transferred to the soil mass through the nail.
Nailing was later used by German and French engineers to stabilize trenches, tunnels, and bridge abutment. Nailing has been widely used in various construction projects such as stabilization of railway and highway trenches, construction of excavated retaining structures in urban areas for the construction of high-rise buildings consisting of several floors inside the ground, and stabilization of slopes against possible landslides. Nailing the walls means reinforcing the existing soil by installing steel bars close to each other on a sloping surface with a gentle slope (about 10 to 20 degrees below the horizon) or at the excavation site by implementation of top down.
In this method, the nails are subjected to tensile stresses during operation and provide wall stability. The main philosophy of the nailing method is based on the stabilization of the slip wedge by the nails.
Therefore, this wedge is sewn to the back volume of the soil by nails and based on this, this wedge will be stable. Rebars are usually placed inside boreholes created in the earth wall and filled with cement slurry to prevent corrosion of the rebars as well as better transfer of forces between the soil and the rebar. This operation creates a stable reinforced section that can hold the soil behind it. This reinforcement acts passively and exerts its effect through the soil-rebar interaction resulting from deformation in the soil. The nails often act in tension, but in certain conditions, their bending and shear performance are also considered. The effect of reinforcement to improve stability is achieved with the following two functions:
- Increasing vertical force and consequent shear strength at the slip surface in frictional soils.
- Reducing thrust at slip surface in frictional and cohesive soils.
After the rebars are installed, a thin surface wall, usually containing lightly reinforced shotcrete, covers the surface of the excavation wall. The thickness of the shotcrete wall will be between 10 and 15 cm. The purpose of this wall is to prevent surface erosion of the soil, to create a suitable surface for possible subsequent constructions, to create a protective and integrated layer and as a result to distribute the reaction force evenly and to increase the efficiency of the consolidated soil system, especially in the parts close to excavation wall.
Stabilization of the slip mountain by nailing
Nailing walls can be made either temporarily or permanently. Temporary nailing have a service life of less than 18 months, depending on the type of soil and design. Permanently nailed structures have a longer lifespan than this. If an excavation is reinforced with temporary nailed walls, but due to workshop delays the construction period lasts more than 18 months, executives are required to use a permanent maintenance system to stabilize the excavation. In this method, holes are drilled in the ground and then steel rebars are placed in it and the empty space of the hole is filled with slurry.
Application of nailing
Different methods of trench stabilization always face different challenges. Among these, nailing is the preferred option for most road and construction employers due to its technical and economic advantages, which provides the best reliability coefficients for excavation walls without creating conflicts within the project area. Applying up-to-date knowledge of the nailing system and post-tensioning of the nail elements made it possible to reduce relocations adjacent to dilapidated buildings and busy streets. This technology, anchorage, has become the only option for stabilizing excavations above 25 meters using multi-strand instead of rebars in boreholes.
The proposed method for nailing to stabilize the walls of the excavations usually consists of 5 main stages of cross-sectional excavation, drilling boreholes, installing reinforcement rebar, cement slurry injection, concreting, and plate and nut installation.
After completing the above steps for the first step of excavation, these steps are repeated for the next steps of excavation until reaching the desired bottom floor figure.
Primary excavation is done to a depth that the excavated wall can maintain its stability for a short period of time between 24 to 48 hours. Excavation should be done step by step and after each stage of excavation, nailing should be done in that stage and after the end of the operation, excavation will be done in the next step. The width of the excavation should be at least large enough to accommodate the necessary tools. At each stage of excavation, the depth of the created wall must have the necessary stability during the nailing, basically the depth of excavation to provide short-term stability is 2 meters. However, according to the geotechnical conditions of the construction site, the allowable excavation height is determined at each stage. The allowable excavation height without the need for bracing can be calculated through the Rankin equation as follows:
C= soil cohesion
Q= Wide overhead beside the pit
Ka= Active pressure coefficient
Hmax= Maximum excavation height without the need for a retaining structure
γ= Soil specific gravity
F.S= Factor of safety (usually between 1.5 and 2)
Since the progress of stabilization in large projects depends on the execution of drilling, the supervision of the drilling department is of special importance. The choice of machinery should be based on the length of the boreholes, soil type and groundwater conditions.
Obviously, the compressor air production capacity for boreholes above 15 meters should be higher than short boreholes. This is also important in coarse-grained soils where there is a possibility of localized boreholes and air escape. The strength and speed of the drilling machine should be selected according to the soil type to achieve the highest efficiency. The mobility of the drilling machine in saturated soils or hand soils should be considered more than natural and dry soils. The size of the hammer and the drill bit should also be selected in proportion to the diameter of the borehole specified in the executive maps. The expertise and commitment of the drilling team is the most important factor in improving the quality of drilling, which in addition to providing the desired drilling speed, facilitates the installing of rebars or strands.
Installation of reinforcement rebar and injection of cement slurry
The reinforcement rebar installation operation is performed after drilling the borehole to the desired depth. Reinforcement elements usually include steel reinforcements are often filled, but hollow rebars can also be used in accordance with the US Roads Regulations.
However, the designed rebars are placed in the borehole after drilling. To place the rebars exactly along the cylindrical axis of the drilled hole, a tool called a spacer or centralizer is used. The rebars are installed into the centralizer and then placed inside the hole. If the corrosion condition is severe, a special epoxy coating is used to protect the nails. Reinforcing steel rebars have become the most important element of nailed walls. These elements are placed in pre-drilled holes and then a grout hose goes to the end of the hole and filled the remaining empty space from the end to the beginning. As mentioned before, the slope of the drilling holes is slightly below the horizon. So, the slurry with the force of weight applied to it fits well in the holes. Such a method is called gravity grouting, which is one of the common methods in nailing. Slurry plays the main role of transferring stress from the ground to the nails.
The set of injection machine consists of three parts: primary mixer, secondary mixer and injection pump. After mixing water and cement in a certain proportion in the mixers, the cement slurry is injected into the drilled borehole through hoses. This injection can be performed at different pressures depending on the conditions. The following figure shows an example of the reinforcement installation process.
Wall shotcrete, plate and nut installation
After the injection, the wall surface is covered with a shotcrete layer to prevent erosion and also to maintain more integrity in the operation of the nails. The coating creates a kind of instrumental connection in the system. There are two types of coatings in nailing: 1- Temporary coating 2- Permanent coating. Temporary coating acts as a surface that connects the structural components of the nailing. Also, geocomposite tapes are placed on the excavation surface before the temporary coating is applied to guide the drainage water into the soil and prevent it from penetrating into the coating. Also, after temporary concreting to completely connect the nails to the wall and also to prevent the occurrence of punching shear in the wall, a metal plate is placed on the wall as a nail head and is connected to the reinforcement inside the borehole by a nut. Permanent coating is applied after the nails are installed and the temporary coating is formed. The thickness of this coating is small and is often limited to 10 to 15 cm.
Characteristics of nailing method
As mentioned earlier, the most important advantage of the nailing method is the provision of stability without disturbing the subsequent stages of the construction operation. Another advantage of nailing is to perform stabilization simultaneously with earthworks. Also, due to the concrete surface on the trench soil, it prevents weathering and localized falls. Fortunately, due to the welcome of investors in the construction sector, nailing knowledge has been established among consulting companies, contractors, suppliers of materials and machinery, and unique records in terms of depth and dimensions have been recorded in the international arena.