How to Support the Tunnel Through the Pebble Layer?

The rock layers that the tunnel passes through may encounter complex geological conditions, and the pebble layer is one of them, which has the characteristics of broken rock and poor cohesion. The pebble layer support needs to adopt a method with strong permeability and strong cohesion. The self drilling anchor bolt can realize the function of grouting while drilling, which is more conducive to the bonding of broken rock layers and is convenient for construction.

Reasons for the Support of the Tunnel Passing Through the Pebble Layer

The pebble stratum is loose, the cohesion is small, and the pores are large. Once excavated, it is easy to have a decline in bearing capacity, poor resistance, and then some unexpected situations such as instability and disintegration. The reasons for support are as follows.

Loose Structure

The structure of the pebble layer itself is loose. If the tunnel is in the area near the river or sea water, due to the submersion of groundwater, the fine particles will be eroded, and the mechanical properties of the rock mass with poor cementation will drop sharply under the action of excavation.

Sudden Deformation

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

Face Crack

The pebble layer includes pebbles, boulders, large-diameter gravel, etc., and the mud-calcareous filler in the middle will become weak, which will lead to further loosening and cracking of the face. The phenomenon of falling blocks in the part above the arch waist of the face will lead to deformation of the surrounding rock, breaking through the reserved deformation, and squeezing the tunnel structure.

Support Method for Tunnel Crossing Pebble Layer

The geological structure of the pebble layer tunnel is loose, and the rock mass is seriously broken. It is difficult to form holes by using the general support construction method, resulting in the delay of the construction progress. The self-propelled anchor rod integrates drilling, grouting and anchoring, which can well bond the unstable rock formation and make the project achieve the expected anchoring effect.

Drilling Preparation

The drilling angle, length, mixing ratio of slurry, and selection of drill bits should be determined according to the rock formation. Most tunnel advance support will use 51, 76 self-propelled bolts. Adjust the drilling angle of the rock drill according to the terrain and geological conditions.

Drilling Grouting

When the self-propelled bolt is longer and needs to be spliced ​​together, a drill bit should be put on the front end of the first section of the bolt, and the tail end of the bolt should be connected to the grouting machine through the rotary grouting adapter. During drilling, grouting is carried out, and the slurry is pressed into the hole and into the cracks of the rock mass to consolidate the rock mass and stay in the deep stable rock mass, and bond together with the stable layer as the slurry solidifies.

Anchor Rod Extension

When the remaining 30 cm of the first section of anchor rod, stop drilling, put a connecting sleeve with artificial lubricant on the tail of the previous section of anchor rod, and then connect the latter section of anchor rod, 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 the installation of the arched backing plate and the ball nut.

Pull Test

After the self-propelled bolt is constructed, it is necessary to test the pull-out force value of the bolt to check whether it meets the design value of pull-out force. For every 300 bolts in the operation area, no less than 3 bolts should be randomly checked for pull-out force test to ensure that the construction effect meets the expected requirements.

The above is the answer to the question of how to support the tunnel crossing the pebble layer. The quality of the support affects the safety of the tunnel construction. It is necessary to adopt supporting materials with rich successful supporting cases and rich construction case verification.