BACKFILL GROUTING BETWEEN OLD TUNNEL INVERTS AND ROCK MASS
Filling and reinforcement of voids behind tunnel lining
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1. Project Background
To accommodate the electrification of the Taiwan Round-Island Railway, comprehensive safety inspections and maintenance were required for sections of old tunnels before their final opening to traffic.
These tunnels had been in use for over thirty years, and factors such as seismic activity, erosion of the roadbed, and natural degradation of concrete had led to various types of repair work being necessary. Some tunnels exhibited voids of varying sizes behind the concrete linings of their arches.
To prevent damage to the concrete linings from falling rocks, cracking, and water infiltration, the construction unit commissioned contractors with tunnel maintenance experience to conduct on-site surveys. They classified defects, compiled statistics, proposed solutions, and carried out the necessary repairs after approval.
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2. Solution
Due to the initial construction of the tunnels using drilling and blasting methods, the outer rock layers adjacent to the concrete linings showed irregular patterns. When grouting is applied to fill voids outside the arches on the outer side, it is often challenging to ensure complete filling due to the influence of gravity.
Over time, some rock masses on the external layers had become loose, coupled with the effects of water seepage through rock fissures, leading to localized water seepage and cracking. Before grouting, additional engineering techniques were required to reinforce these areas to ensure effective filling.
The relevant operation plan is outlined as follows:
1. Utilize self-drilling rock bolts to reinforce the rock strata and fill the cracks in the lining with ultrafine cement.
2. Seal leakage points with polyurethane resin, and reinforce the concrete lining with carbon fiber cladding after leakage ceases.
3. Drill penetrating grouting holes along the upper end of the arch in the direction of tunnel advancement.
4. Embed grouting pipes with valves into the grouting holes and use quick-setting cement to fix and seal the pipe wall gaps.
5. Inject low water-cement ratio cement grout at low flow rates, and after reaching the predetermined grouting pressure and ensuring that the flow rate per unit time is less than the design value, close the grouting pipe valves for cement grout curing.
6. After curing is complete, remove the valves and outer grouting pipes, fill the grouting holes, and restore the arch to its original condition.
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3. Works Design
Due to the uncertainty in the scale and continuity of voids between the lining and the rock masses, after completing the leakage reinforcement of the concrete lining, grouting was to be carried out according to the following design approach:
1. Drill grouting holes with a diameter of at least 50mm at the center of the tunnel arch at intervals of 3.0m. The drilling depth should penetrate the lining, and make a record of the thickness of the lining.
2. Use PVC pipes with a diameter of 1 inch as grouting pipes, and fix them with quick-setting cement inside the grouting holes. Adjust the burial depth according to the depth of the rock mass.
3. To prevent damage to the lining, the maximum grouting pressure should not exceed 200kPa.
4. Considering the presence of fine cracks in the tunnel lining and rock layers, use cement grout with a water-cement ratio of 0.6 for grouting initially, and adjust moderately between water-cement ratios of 0.4 to 2.0 based on actual injection pressure and crack leakage conditions.
5. Stop grouting and close the grouting pipe valves for pressure maintenance when maintaining a grouting pressure of 200kPa and the grouting volume is less than 5 liters per minute or as approved by the Engineering Officer.
6. After 28 days of maintenance, remove the exposed grouting pipes and valves, fill the grouting pipes with non-shrinkage cement, and restore the arch surface.
7. If the grouting process encounters situations where the pressure cannot be increased or abnormal leakage occurs in the lining, immediately suspend grouting, report to the Engineering Officer, clean the grouting pipes, and resume grouting the next day after remedial actions.
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4. Works Process & Results
During the maintenance process of existing tunnels, the transportation of various materials and equipment heavily relied on rail trolleys. Before commencing construction, detailed coordination of interface usage was necessary. Since the grouting points were mainly located near the arch, operations had to be carried out with the assistance of scaffolding.
To avoid setting up a large number of scaffolding inside the tunnel, which could potentially obstruct the entry and exit of other workers, special scaffolding fixed on rail carts and platforms for various construction machinery were designed. These scaffolding and equipment platforms were transported in and out by locomotives during construction, facilitating work progress and equipment maintenance.
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5. RESULTS
After completing the arch grouting for all the old tunnels on the line, the voids behind the linings had been effectively filled, and most of the leakage issues have been resolved. The remaining few leaking points have been properly sealed after injection with polyurethane resin.
Following an on-site inspection by the Engineering Official, it was been confirmed that the project met the contractual requirements, achieving the successful completion of the grouting work between the arches and the rock mass in the old tunnels.
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