How To Support The Tunnel Pass Through Cobble Layer? Self-drilling Anchor Bolt Support Method

The rock strata that the tunnel traverses may encounter relatively complex geological conditions, and the pebble layer is one of them, which is characterized by broken rock and poor cohesive force. The support of the pebble layer needs to adopt the material of strong permeability and strong cohesion. The self-drilling anchor bolt can realize simultaneously drilling and grouting , which is more conducive to the bonding of broken rock formations and is convenient for construction. Next, we will give you detailed introduce of how to support the tunnel passing through the pebble layer.

Why has to support the tunnel in cobble layer?

The pebble stratum 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. The reason why has to support the tunnel in cobble layer are as follows.

1. Loose Structure

The structure of the pebble layer itself is loose. If the tunnel is in an area close to river or seathe 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 pebble 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. The Tunnel face Cracked

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

 drill bit in tunnel

How To Support The Tunnel?

The geological structure of the tunnel is loose, and the rock mass is severely broken. 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 functions together, and can bond unstable rock formations well, so that the project can achieve the expected anchoring effect.


1. Drilling Preparation

The selection of angle, length, cement grout ratio, and drill bit should be determined according to the rock formation conditions. Most pipe roofing in tunnel will use self-drilling anchor bolts, with diameter of 51mm and 76mm. Adjust the drilling angle of the rock drill according to the terrain and geological conditions.


2. Drilling and Grouting

The drill bit should be put on the front end of the first anchor rod, and the tail end of rock bolt should be connected to the grouting machine through the rotary grouting adapter. Grouting is performed while drilling, and the grout is pressed into the hole and cracks of the rock mass to consolidate the rock mass, stay inside the deep stable rock mass, and bond together with the stable layer as the grout solidifies.


3. Coupling Anchor Rod

Stop drilling when 30 cm of the first anchor rod remains, put a coupler with artificially applied lubricant on the tail of the previous rock bolt, and then connect the next section of the bolt, and so on until drilling to the design length. At the end of the drilling, the anchor rod is exposed at least 10 cm, which is convenient for installing the arch backing plate and the nut.


4. Pull-out Test

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


The above is the answer to the question of how to support the tunnel passing through the pebble layer. The quality of the support affects the safety of the tunnel construction. It is necessary to adopt support materials that are rich in successful support cases and verified by rich construction cases. Sinorock is a professional manufacturer of self-drilling bolts. As a leading expert in the ground support industry, the Sinorock team has many years of experience in providing complete geotechnical engineering services. If you have any needs or questions about self-drilling anchor bolt systems and their applications, please feel free to contact us at

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