How to Apply Rock/Soil Anchor System in Deep Foundation Pit Projects?

With the continuous development of urbanization and the increasing number of deep foundation pit projects, the rock/soil anchor system has received widespread attention and has been widely used.

The rock and soil anchor system will be set at a certain depth of the ground, one end is connected to the building structure, and the other end is effectively anchored, so as to bear the structural tension and maintain the stability of the structure. Through the application of prestressed rock and soil anchor systems, the rock and soil can be made more stable, and the self-weight of the structure can be greatly reduced, which is beneficial to the saving of engineering materials and the guarantee of structural stability.

1. The structure of prestressed rock/soil anchor system

The prestressed rock/soil anchor system includes three parts:

1) Anchor head. The anchor rod is connected together by the anchor head and the enclosure structure. When the corresponding prestress is applied to the anchor rod, the prestress will be transmitted to the structure through the anchor head.

2) Rod body. The rod body connects the anchor head and the anchor body, has elastic deformation characteristics and is responsible for the transmission of prestress;

3) Anchor body. Responsible for transferring the pulling force to the formation through the rod.

2. Overview of a deep foundation pit project

A certain deep foundation pit project has a plane specification of 510m*43m and will be constructed by the cut and cover method. Based on the consideration of the depth of the foundation pit, 800mm bored piles are used for the enclosure structure, and the spacing is set to 1000mm. The rock/soil anchor system is used to realize the effective support of the foundation pit. Each anchor rod includes 4 steel strands, the horizontal spacing of the anchor rods is designed to be 2.0m, and there are 2 vertical anchor rods.

3. Construction technology of rock/soil anchor system

1) Drilling

The drilling diameter is 150mm. Before the formal drilling, the drilling location should be accurately determined and marked in accordance with the design regulations. The horizontal error of the anchor cable should be controlled within 50mm, and the vertical hole pitch error should be controlled within 100mm. The deflection size of the bottom of the borehole should be controlled within 3% of the total length of the anchor cable.

2) Assembly of rock/soil anchor system

Each anchor system is assembled by 4 strands of 15.2mm steel strands. Cut the steel strands according to the design requirements, and the error should be controlled within 50mm. Different marks shall be applied to the free section and the bond section respectively. A spacer shall be set every 1m of the anchorage cable in the bond section. In addition, a tie steel ring shall be set for the adjacent spacer. Put the steel strand of the free section in the plastic pipe and apply a certain amount of butter to the stressing end. After the setting of the guide cap is completed, it needs to be placed smoothly and reserved for use.

3) Setting of anchor cable

After confirming that the anchor cable and the hole position are consistent, the anchor cable (tied with the grouting pipe) can be inserted into the hole, so that the distance between the bottom end of the grouting pipe and the bottom of the hole is about 50mm. When placing the rod in the drill hole, attention should be paid to the coordination of the angle, while ensuring the uniformity of the force, and the grout sealing device is carefully checked to ensure the accuracy of its positioning. Once the damage is found, it should be replaced in time, and the exhaust pipe should be checked regularly to keep it in a smooth state.

4) Grouting

After the anchor cable is set up, the cement mortar with a lime-sand ratio of 1:1 is used for atmospheric grouting operation. After the initial solidification, pure cement mortar is used for high-pressure grouting operation.

5) Installation of waist beam and bearing plate

Before the anchor cable is tensioned, a steel frame for support needs to be installed at the design elevation position of the pile body. At the same time, install steel waist beams at the corresponding positions of the steel bracket and the free section. During installation, the waist beam and the pile body should be closely attached together. If this is not possible, use C20 fine-stone concrete for effective filling, firmly set the steel pad on the steel waist beam, and make the upper surface and the anchor cable in a vertical relationship.

6) Tension

When the anchor solid gradually solidifies and the strength exceeds 15.0Mpa, tensioning is allowed. Before the formal tensioning, the relevant tensioning equipment should be accurately calibrated. The tensioning should be carried out in accordance with standard procedures. If the prestress is lost to different degrees due to the loosening of the steel strand, it is necessary to make corresponding supplementary tension after the tension is completed, and finally lock it.