How to support the tunnel through the gravel layer

Time:2022-12-02From:sinorock View:

During the excavation process, the tunnel will pass through many different rock formations and will encounter some complex geological conditions. The gravel layer is one such case and has the characteristics of broken rock and poor cohesive force. Supporting the gravel layer requires the use of supporting materials with strong permeability and strong cohesive force. The self-drilling anchor bolt can realize the function of grouting while drilling, which is beneficial to the bonding of broken rock formations and is convenient for construction. This blog will introduce in detail how to support the tunnel through the gravel layer

Why does the gravel layer need support

The gravel formation is loose, with little cohesion and large pores. Once excavated, it is easy to have a decrease in bearing capacity and poor resistance, and then some unexpected situations such as instability and disintegration will occur.

The reasons why tunnels need to be supported when crossing the gravel layer are as follows.

1. Loose structure

The structure of the gravel layer itself is loose. If the tunnel is in an area close to river water or seawater, the fine particles will be eroded due to the immersion of groundwater, and the mechanical properties of the poorly cemented rock mass will be drastically reduced under the action of excavation.

2. Sudden deformation

The gravel layer is very sensitive to excavation and other actions. Once the excavation is disturbed, the stress will be redistributed and the resistance will be reduced. The load-bearing arch of the tunnel will fail in a short time, causing instability, failure, disintegration, and then collapse into the tunnel.

3. Cracking of the tunnel face

The gravel layer includes pebbles, boulders, large-sized gravels, etc., and the mud-calcareous fillers in between will become weak, which will lead to further loosening and cracking of the tunnel face. The rock falling above the arch waist of the tunnel face will lead to deformation of the surrounding rock, break through the reserved deformation amount, and cause extrusion to the tunnel structure.

Support process for tunnel crossing gravel layer

The gravel layer is characterized by a loose geological structure and severely broken rock mass. It is difficult to form a hole with the general support construction method, resulting in a delay in the construction progress. The self-drilling anchor bolt integrates drilling, grouting, and anchoring, can bond unstable rock formations well, and achieve the expected anchoring effect.

1. Drilling preparation.

The drilling angle, length, water-cement ratio, and drill bit selection of the hollow anchor bar should be determined according to the rock formation conditions. Most tunnel advance support will use self-drilling anchor bolts of Dia. 51 and 76. Adjust the drilling angle of the drill rig according to the terrain and geological conditions.

2. Drilling while grouting

Grouting is performed while drilling, and the grout is pressed into the hole and the cracks of the rock mass, and the consolidated rock mass remains inside the deep stable rock mass. As the grout solidifies the hollow anchor bar is bonded together with the stable layer

3. Hollow anchor bar extension

Stop drilling when 30 cm of the first anchor is left. Put the coupler on the tail of the previous hollow anchor bar, and then connect the latter bar, and so on until the design length is drilled. At the end of the drilling, the hollow anchor bar is exposed at least 10cm, which is convenient for installing the plates and nuts.

4. Pullout test

After the self-drilling rock bolt is constructed, it is necessary to test the pull-out force value of the anchor to check whether it meets the design requirements. The test needs to randomly check no less than 3 hollow anchor bars out of every 300 in the operation area for the pull-out force test to ensure that the construction effect meets the expected requirements.

Summary

The above is the introduction for the support of the tunnel passing through the gravel layer. The quality of support affects the safety of tunnel construction, and it is necessary to adopt support materials with rich support cases and rich construction case verification. The self-drilling anchor bolts produced by Sinorock have multiple tunnel application cases and can provide customers with a complete set of solutions. Welcome to consult sinoroco@sinorockco.com.

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