What is anchorage?

Principles of anchorage have many similarities with nailing method. The main difference is in the application of post-tensioning force for the reinforcement element. Execution steps are similar to the nailing method and include cross-sectional excavation, drilling of boreholes, installation of reinforcing rebars, injection of cement slurry (in a part of the length of the borehole), concreting, and installation of Nile and nut top plate. Cement slurry (in part of the length of the borehole), concrete spraying and installation of nail head plate and nut. After processing the cement slurry, the post-tensioning force is applied to the reinforcement rebar and the slurry around it up to the desired amount by installing the base of the jack, tensile jack and dynamometer.

Finally, after this stage, the operation of tensioning the reinforcement element (applying post-tensioning force) is performed.

In the first stage, a retaining wall is executed. In the second stage, excavation is done up to the level of installation of the first row of anchors. In the third stage, borehole drilling is performed to perform anchoring. In the fourth stage, injection is done and in the fifth stage after setting the slurry, tensioning, excavation, and concrete coating operations are performed. In this method, it is possible to apply permanent coverage if needed.

In the anchorage method, there are various injection methods to provide resistance to the hinged part.

The main components in stabilization using anchor are as follows:

  1. Strand: The wired cable consists of steel wire strands. These steel wires have a high strength (about 1860 MPa). It is possible to use steel instead of a strand, in which case the applied pre-tension force is less due to the lower strength of the steel (about 400 MPa) compared to the anchor. The use of strand or steel is determined by the project conditions.
  2. Spacers: Spacers are used to maintain several strands in one anchor and keep them relative to each other and to observe the minimum allowable distance between strands.
  3. Injection hoses: One or more hoses with a length at least equal to the anchor length are used to ensure that the borehole is filled with grout during borehole injection,
  4. Packer: Packer is used for injection under pressure. Packer must be such that it can withstand injection pressures. The sealing of the beginning and the end of the packer should be done properly so that no leakage occurs when the pack is filled with grout or injection under the pressure.
  5. Strand sheath: To prevent the strands from sticking to the slurry and to ensure that the anchor force is not transferred to the soil, the strands are kept inside the sheath during free length of anchor.
  6. Anchor head: Anchor head is a cylinder with several conical holes in its base, which is installed on the anchor at the head of the borehole to transfer the tensile force of the anchor from the strand.
  7. Wedge: The wedge is a hollow conical steel piece that is used around each strand inside the hole of anchor head to lock the strand. It should be noted that if you use rebar instead of strand in the anchor, some of the above components are not needed.

Installation: To install the pile and anchor system, at the beginning and before starting the excavation operation, the wells required to place the steel or concrete piles are drilled to the desired level, and then the piles are installed is situ.

The excavation and installation of anchors are then done until the required level is reached.

General principles of analysis and design of braced systems

The purpose of implementing an anchorage system is to create a stable mass of soil that can withstand external instability (unstable modes that may occur in a stabilized system by anchor). Part of the rupture is occur due to the excessive tensile forces in the tendons. The initial tensile strength of the strand and mono bar for the final design should not exceed 60% of the tensile capacity of these members.

Another type of internal rupture that should be considered in designing is rupture due to the lack of sufficient bond strength. Accordingly, and considering the local soil bond strength, the length of the injected part should be considered in such a way that the total strength of the mobilized bond during the mentioned length can respond to the forces created during and after the implementation of the anchorage system.

There are a few important things to keep in mind when using this method:

  1. In some cases, it has been seen that the slope wall has collapsed along with the rebars inside the wall. This type of collapse occurs due to the placement of rebars inside the wedge. In other words, if the rebars are placed inside the ruptured wedge, this rebar will not be practically useful, and in this case, if the slope wall starts to collapse, the rebars placed in the slope wall collapse with the soil mass. Accordingly, the rebars placed inside the slope wall should continue to the back of the fracture zone so that they can transfer the force from the slip wedge to the resistant area of ​​the soil.
  2. In many cases, due to the lack of use of panels between the vertical profiles, the wall soil has fallen. To express the importance of these panels, assume that the soil of the wall is clean sandy soil. In this case, the soil between the vertical profiles is fallen and the vertical profiles do not show function. Therefore, to transfer soil pressure to vertical profiles, it is necessary to apply soil pressure on vertical profiles by an interface member. This is also true for clay soils because the stability of clay soils is temporary and these soils can function as sand after a period of time. According to the information, it becomes clear that special care must be taken for the correct execution of retaining structures.

Characteristics of Anchorage Method

The anchorage method is very similar to the nailing method. This method has more capability and ability to predict the thrust force than the nailing method due to the use of post-tensioning properties of cables and mono bars. However, its implementation costs are also higher. The anchorage method is used to ensure the stability of the excavations with a greater depth.