5.1 Crossing reinforcement scheme in construction stage
The roof of the karst cave intrudes into the tunnel boundary and has local falling blocks. In consideration of collapse accidents easily occurring during roof cutting construction, it is necessary to support the roof of the cave at first. Combined with the results of geological exploration in the karst cave, the filling materials in the cave is poor. The vertical drop is large and there is a steep slope at the bottom, so the supporting wall foundation needs to be strengthened. Steel pipe pile is used to locally reinforce loose tunnel slag or soft soil under the supporting wall. In order to ensure the stability of the foundation of the supporting top wall and the steep slope at the bottom, the steel pipe pile shall be embedded into the stable bedrock. The karst cave is backfilled above the tunnel floor by block stones. During backfilling, the original karst channel shall be fully reserved to ensure the flow of groundwater. The backfill at the tunnel bottom shall be dynamically monitored. The reinforcement scheme shall be adjusted in time to meet the stability and safety requirements during the whole life cycle.
The detailed construction steps are as follows:
(1) Backfill of the karst cave: The bottom of the cave are filled by large-size block stones to form blind ditches. Groundwater can flow through these blind ditches to reduce the impact of tunnel excavation on surrounding area. On the basis of bottom block stone, the karst cave is filled to the bottom of the tunnel upper step by layer with tunnel slag. After the backfilling is completed, a settlement observation point is buried for monitoring.
(2) Cast a supporting wall foundation: Φ108 steel pipe piles are used as the foundation on the backfill slag. The reinforcement cage is set in the pipe and grouted to reinforce the surrounding loose tunnel slag and soft surrounding rocks, as shown in Fig. 10 and Fig. 11. In order to ensure the stability of the supporting wall foundation, the steel pipe pile should be embedded into the stable bedrock for at least 1 m. The reinforcement in the steel pipe pile should be exposed for 0.3 m to connect with the ground beam above the foundation. The ground beam is arranged longitudinally along the line as an expanded foundation for the supporting wall. The ground beam plays the role of connecting the supporting wall and the steel pipe pile foundation
(3) Cast a supporting wall: The section height of the supporting wall is about 3~7 m and the width is 1 m. The settlement joint is set at 5 m/track along the longitudinal direction. Cushion blocks are used at the top of the supporting wall to ensure close contact with the roof of the cave. During the tunnel construction, the monitoring and measurement of the supporting top wall should be strengthened. When the monitoring shows that the top of the supporting top wall is separated from the roof of the cave, the top gap shall be grouted in time. The supporting wall is shown in Fig. 12.
(4) Excavation and construction of supporting structure: After the completion of the supporting wall, the excavation and support of the upper step vault should be carried out. The lining parameters are type Ⅲ-a. After the treatment of the upper step of the tunnel section is completed, the upper steps of the palm face continued to excavate forward. The lining support parameters were adjusted to V-a type according to the lap 10 m principle. The dynamic design principle is adopted for the structure of the lower bench and tunnel bottom. The tunnel bottom backfilling scheme is improved after the tunnel breakthrough and the deepening geological work of the cave section are completed. According to the final detection results, the original crossing scheme should be reinforced with full consideration of the problems of tunnel floating and filling collapse in the future flood season.
(5) Drainage structure: The drainage structure consists of two concrete culverts that is buried horizontally in double rows from left to right under the tunnel. A water collection well is set up at the left inlet to ensure that the original waterway of the cave is drained smoothly. The drainage structure is shown in Fig. 13.
5.2 Drainage and anti-floating in operation stage
5.2.1 Anti-floating reinforcement of ‘plate-pile-bedrock’
On the basis of the original crossing scheme, considering the existence of mobile underground river at the bottom of the karst hall on the right side of the right line. The underground river has a small flow during the dry season of tunnel construction, but the groundwater is greatly affected by seasonal precipitation. During the future service flood period, the flow of underground river may exceed the design expectation, causing the collapse of backfill materials. A large amount of groundwater will cause the collapse of backfill materials, further leading to hazards such as tunnel floating and lining cracking. Reinforce the backfill layer at the tunnel bottom with steel pipe piles. The steel pipe pile embedded in the bedrock forms a ‘plate-pile-bedrock’ frame stress system with the tunnel floor. This system can realize the uniform transmission of upper load and ensure the stability of the tunnel while reinforcing the filling body. The structural diagram of 'plate-pile-bedrock' is shown in Fig. 14.
The detailed construction steps are as follows:
(1) Backfill of the cave: The bottom of the cave are filled by large-size block stones to form blind ditches. Groundwater can flow through these blind ditches to reduce the impact of tunnel excavation on surrounding area. On the basis of bottom block stone, the karst cave is filled to the bottom of the tunnel upper step by layer with tunnel slag. After the backfilling is completed, a settlement observation point is buried for monitoring.
(2) Construction of steel pipe pile: Φ108 steel pipe piles are used as the foundation on the backfill slag. The reinforcement cage is set in the pipe and grouted to reinforce the surrounding loose tunnel slag and soft surrounding rocks. The steel pipe piles are arranged in the shape of 1.2m × 1.2 m plum blossoms, as shown in Fig. 15. In order to ensure the stability of the tunnel foundation, the steel pipe pile should be embedded into the stable bedrock for at least 1 m. The reinforcement in the steel pipe pile should be exposed for 0.3m to connect with the reinforced concrete base plate.
(3) Construction of reinforced concrete base plate: An 80 cm thick reinforced concrete base plate is implemented on the rigid foundation reinforced by steel pipe piles. The width of the left and right floor plates is the width between the ground beams on both sides. The reinforced concrete floor plays the role of connecting the steel pipe pile with the tunnel lining. The reinforced concrete floor, steel pipe pile and bedrock together form the ‘plate-pile-bedrock’ frame stress system.
5.2.2 A reverse drainage structure
There is seasonal water in the cave, which has a certain scouring effect on tunnel lining and filling materials. During the service period of the tunnel, the filling material may flow with the groundwater, causing the blockage of the original karst pipeline. The blockage of the original drainage pipeline will lead to the increase of water pressure after the lining, resulting in the durability problems such as the collapse of the filling layer and the cracking of the lining. Considering the problem that the underground river cannot be discharged in time due to the increase of water volume affected by seasons, a new type of bottom to top central ditch drainage structure is proposed. Two drain pipes with 180° elbows are buried in the left and right central ditches respectively. Under normal conditions, the drain pipe is set together with the inspection well. The so called “bottom-to-up” refers to the process that exploiting the high pressure accumulated at the bottom of the tunnel to pump out the water automatically.
On the basis of the original drainage structure of the tunnel, a U-shaped drainage pipe with a one-way valve is set at the central drainage ditch of the inspection well to form a "bottom to up" reverse drainage structure. This structure can discharge the unexpected groundwater of the tunnel from the central drainage ditch during the high water period. The reverse drainage structure is composed of waterproof structure, water diversion structure and drainage structure. The drainage board is laid between the primary support and the secondary lining as a waterproof structure to isolate the groundwater outside the secondary lining. At the same time, it serves as a water collecting structure to collect the water invading the secondary lining into the water diversion system. The water diversion system is composed of circumferential, longitudinal and transverse drainage blind pipes. First, the water is collected into the drainage ditches on both sides of the tunnel by the circular drainage blind pipes. Then the water in the side ditches is collected into the central drainage ditch through the longitudinal and transverse drainage blind pipes for drainage. During the dry season, the above structures can basically meet the waterproof and drainage requirements of the tunnel. In consideration of the possible unexpected water scouring effect in the wet season, a U-shaped drainage pipe with one-way valve is added to form a 'bottom to up' reverse drainage structure to meet the needs of tunnel drainage and flotation resistance. Compared with the traditional waterproof and drainage structure, the 'bottom to up' reverse drainage structure refers to the process of automatically pumping water using the high pressure accumulated at the bottom of the tunnel. When there is excessive groundwater and a large water pressure is accumulated at the tunnel bottom during the wet season, the water is discharged from bottom to top through the central drainage ditch through the U-shaped drainage pipe. The working principle of the central drainage ditch with U-shaped elbow is shown in Fig. 16.