GROUTING REINFORCEMENT TECHNOLOGY FOR OBSTACLE REMOVAL IN TBM TUNNEL CONSTRUCTION

The Most Rapid and Effective Method of Partial Grouting Reinforcement and Water Stopping

1. Project Background

Grouting Reinforcement : Thailand’s capital, Bangkok, often had severe flooding due to rainfall. The government planned construction of large drainage tunnels to alleviate the frequent urban water accumulation.In order to avoid impacts on surface structures and underground pipelines, etc. a large-bore shield tunnel design was adopted for the Bangkok drainage tunnels (diameter over 5m). 

During the construction process, tunnelling near a lake encountered unknown obstacles while advancing, which caused the project to halt, and mud and sand continued to flow from the slag outlet of the cutter head. 

In the face of imminent industrial safety accidents such as hollowing out of the ground, surface collapse, destruction of buildings and damage to underground pipelines, the contractor immediately convened a contingency meeting, resolved to stop the water as soon as possible, carry out reinforcement grouting, and worked out a plan to remove the obstacles.

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2. Solution

The cutter disk and slag outlet spiral screw of the TBM could not work normally because of unknown obstacles. The contingency plan specified the following:

(1) dismantling of part of the head assembly.

 (2) grouting to block water inrush and mud.

 (3) soft mud grouting reinforcement on the outside edge of the cutter disk and in the slag pipe.

(4) removal of solidified mud and sand from the outside of the cutter head and in the slag discharge pipe, and removal of obstacles.

(5) On-site recovery and trial operation of the shield machine.

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3. Works Design 

This grouting operation was rather complicated.

The main work objectives were:

 (1) Emergency plugging of water inrush and mud outflow.

(2) Water stoppage in tunnel layer outside the cutter disk.

(3) Reinforcement of disturbed soft mud.

(4) Filling hollowed out voids in the ground layers.

 (5) Ensure a safe restart.

It was decided to use 5 types of grouting materials in combination to perform the job, because the grouting zone was in a state of flowing water.

(1) Use PU foam grout to temporarily block the water inrush path.

(2) Use instant setting chemical grout (SS3) as water stop in the ground layers.

(3) Use chemical grout (SS5) for soft mud reinforcement

(4) Use FLW grout to fill the voided-out locations.

(5) Use JS3 grout for water stop around the cutting head periphery.

The location of the grouting borehole to be mainly based on the point of water outflow or the opening exposed by the disassembly of the cutter head or the existing grouting borehole. The distance between grouting holes to be within 0.5m – vertical holes and inclined holes to be injected with grout in combination.

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4. Works Process

As muddy water and sand were continuously flowing into the tunnel, the situation was urgent. Fortunately, the obstacles that the TBM encountered were only a distance of about 20 meters away from the launch shaft.With partial disassembly of the TBM head, and grouting equipment, material hoisting and other work lines being short, the work could be speedily completed.

However, with the equipment inside the TBM head so numerous and the space narrow, it was extremely difficult for personnel to enter and exit, and perform the grouting.

In order to avoid danger being caused by the work process, the Site Supervisor must lead the Operations Engineer to identify the equipment, the materials being used, and the steps to be executed before each procedure is performed.

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5. Results 

By studying and correlating a map of the area, the obstacles blocking the advance of the TBM were reckoned to be the steel in column piles of an old trestle. The piles themselves had been partially crushed by the cutters of the TBM, which caused the cutter disk and the spiral screw of the slag outlet to be stuck and unable to work.

The traditional excavation and removal method is extremely time-consuming for supporting work at the edge of the lake and is not suitable for emergency work.

The systematic multi-stage grouting operation completely stopped the groundwater and mud outflow. This allowed the disturbed soft mud outside the cutter disk edge and inside the slag pipe to solidify and to reach safe excavation conditions.

This made the work of removing sludge, removing obstacles, replacing damaged cutters and repairing the slag discharge spiral screw, proceed smoothly.

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