SEALING AND CONSOLIDATION GROUTING OF COLLUVIAL STRATA AT TUNNEL EXIT Exit

Dual-tube double-packer grouting reinforcement technology for colluvial rock layer

1. Project Background

At the exit of a tunnel was a thick layer of colluvial rock. In order to avoid continuous rock slides on the upper slope during excavation, it was planned to first carry out colluvium consolidation grouting to ensure the safety of subsequent construction.

Because the geological conditions of the improvement range were blocks of gneiss and quartzite mixed with weathered rock debris and sand, the traditional high-pressure jet piles would not be easy to form.

After evaluation, it was decided to use low-pressure consolidation grouting to fill and consolidate the gaps between the rocks to improve the formation uniformity in the excavation area, and at the same time increase the formation shear strength and seepage reduction capacity.

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2. Solution 

In order to achieve the goal of improving the formation uniformity of the specified range and saving engineering costs as much as possible, the proposed solution was as follows:

(1) Use dual-tube double-packer grouting technology which can repeat grouting at the same position and pour different grouts for site improvement

(2) The quick-setting cement chemical grout to be injected at the edge, bottom and excavation area of the improvement area to form a sealing curtain to prevent the loss of grout injected into the interior;

(3) Pour cement grout that is cheaper but has a long curing time in the non-excavation area within the sealing curtain;

(4) Improvement effect testing and supplemental grouting;

(5) The injection pressure cannot effectively cause lifting and increase polyurethane resin filling.

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3. Works Design

Because the improvement range was sloping terrain, the drilling depth was quite varied.

The improvement depth was 12.5m upward from the bottom of the hole, the improvement width was 29.2m extending from the arch line, and the improvement length was 29.0m.

The relevant works design was as follows:

(1) The spacing of drilling holes to be 2.0m, and the holes arranged in a plum-shaped staggered pattern;

(2) The spacing of Marshall pipe grouting holes is 1.0m, and the height of adjacent grouting pipes is staggered by 0.5m;  

(3) LW cement chemical grout to be poured into the tunnel excavation area, and the edge and bottom of the improvement area; pure cement grout to be poured into the non-tunnel excavation area;

(4) The initial setting time of LW grout should be controlled to within 95 seconds, and the initial setting time of pure cement grout should be controlled to within 6 hours;

(5) After grouting is completed, use the Split Method to test the grouting pressure;

(6) If the test results do not meet the standard, use cement or LW grout for supplementary grouting;

(7) When the gap between the rock blocks is too large and cannot be filled effectively within the excavation range, add self-drilling rock bolts with polyurethane resin grout for filling.

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4. Works Process

This improvement was above groundwater level, so there was no problem of the grout being diluted or washed away. However, due to the large spaces between the rock blocks, the CB grout had a large diffusion range when the Marshall tubes were embedded.

The grout ratio had to be adjusted appropriately according to the current situation to avoid the failure to pre-split.

In addition, due to the uneven distribution of voids in the colluvium, grouting often needed to be combined with graded grouting, adjusting the ratio, supplementary infusion, and adding self-drilling rock bolt resin grouting to achieve geological improvement and improve the uniformity of the colluvium.

The requirements for staff experience, real-time flow measure, pressure monitoring, and grout ratio management and control were high. Grouting execution time and number of people were greater than other engineering cases.

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5. Results

After grouting, testing, supplementing and localized polyurethane resin strengthening, the area of improvement was handed over to the main contractor to carry out small-diameter pre-support pipe roof excavation.

During the process of excavation, supports, and lining installation, there were no rock slides of the upper slope and top arch rock blocks, and the sealing, consolidation, and grouting work of the colluvium curtain at the tunnel exit was successfully completed.

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