Reinforcement Treatment of Under-Construction Tunnel Collapse


Tunnel collapse and water inrush grouting reinforcement technology

1. Project Background

During the excavation of a mountain tunnel, a fracture zone was encountered above the arch line of the workface. Rockslide, collapse and water inrush occurred. The partially completed sidewall supports were deformed by the incursion. Stepwise excavation operation was forced to stop.

In order to prevent the expansion of tunnel collapse and damage increase, the construction unit immediately carried out ballast backfilling, water diversion and shotcrete cover layer operations, and invited professional contractors to discuss solutions together.


2. Solution

The central part of the collapsed tunnel workface was metamorphic sandstone, and the left and right sides were slate. The RQD value was about 0~64% through core sampling analysis.

The metamorphic sandstone in the central position was high strength, and the collapsed position on the right was broken slate, which was also the main point of massive water inrush.

In order to keep the tunnel excavation work ongoing, it was expected that LW grout (cement + sodium silicate chemical grout) would be used for consolidation grouting, long drilling holes in the fracture zone for decompression and drainage.

Then go ahead with pipe roof pipe bracing with polyurethane resin infusion consolidation of fracture zone, damaged steel supports adjustment and replacement and shotcrete protective layer repair and other procedures for reinforcement.

3. Works Design

The length of the tunnel collapse was about 15m, and the water inrush continued to occur on the excavation surface.  

In order to integrate the manpower-equipment-materials dispatch time, after evaluation, first use the materials that can be utilized on site to backfill the collapse surface with ballast material and hang mesh to spray the grout to reduce the water inflow.

Continue removing the debris from the fracture zone, and construct a temporary concrete retaining wall in front of the collapsed zone to strengthen the stability of the collapsed surface and reduce the expansion of the emergency.

According to the engineering conditions, the grouting reinforcement design was as follows for after the collapse situation had reached transient stability:

(1) Use LW quick-setting grout to reinforce the collapse ballast material, and the reinforcement range is from the middle of the collapse area to more than 2m behind the collapse surface.

(2) Set up a long borehole about 25m deep into the collapse surface, the depth encompassing the fracture zone with large amount of confined water to reduce the water pressure in this section.

(3) Advance with set up of pipe roof bracing of the collapse surface and low-expansion rate polyurethane resin grouting, the penetration and improvement depth to be about 3m~9m behind the collapse surface, and the fragmented rock to be consolidated and excavated.

(4) Drill core sampling to confirm the improvement effect, grouting method revision and inject supplemental resin grout;

(5) Ballast excavation, supports repair and side wall hang shotcrete mesh;

(6) Stepwise excavation, drainage pipe installation, advance with pipe roof bracing installation and resin grouting reinforcement until it passes through the fractured zone.

4. Works Process

When performing on-site grouting according to the design, due to the extremely high water pressure in the fracture zone, the effect of the quick-setting LW grout was not good.

When injection pressure was strengthened, the sealing wall of the workface would be subjected to the action of water pressure and pour pressure at the same time, with a risk of damage.

After adding pressure relief holes and increasing the stiffness of the sealing wall, the pouring process was smooth, and the water inrush, the deformation of the collapse surface and tunnel side walls gradually lessened and became stable.

Afterwards, various reinforcements and stepwise operations were carried out according to the design.

5. Results

After the implementation of the adjusted geological improvement operation, the situation of the collapse location tended to stability.

During the stepwise excavation, a safe and conservative method was adopted according to the site conditions, and pressure relief drainage holes and pipe roof resin grouting were appropriately supplemented until the area behind the collapse zone was safe.

After all the work was completed and the supervision unit confirmed that it was safe, the site was returned to the contractor to carry out the tunnel excavation operation according to the original design.

The under-construction tunnel collapse and reinforcement treatment had been successfully achieved.

Reinforcement Treatment of Under-Construction Tunnel Collapse

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