Guide: When the tunnel construction reaches the edge of the cave and the dark river, the tunneling should be suspended, and the cavern near the cave and the dark river should be properly strengthened. At the same time, the shape, extent, size, filling, development and groundwater of the cave should be explored. The flow (with or without long-term recharge sources, whether there is any increase in the amount of water during the rainy season), etc., is treated by measures such as introduction, blockage, over-circulation and winding.
1. When there is water flow in the dark river and the cave, it should be blocked. After identifying the flow direction of the water source and its positional relationship with the tunnel, use a dark pipe, a culvert, a small bridge and other facilities to vent the water flow, or to open a drainage hole to drain the water out of the hole. The construction method can refer to Figure (1);
2. When the position of the water flow is above or higher than the tunnel, the water diversion tunnel (or the water diversion tank) should be cut outside the appropriate distance to lower the water level below the bottom of the tunnel and then lead the row. The construction method can be referred to. figure 2).
1. For karst caves that have stopped development and have small spans and no water, they can be backfilled and sealed with concrete, masonry or dry masonry according to the location and filling of the tunnel. The construction method can refer to the map. (3) Determine whether it is necessary to deepen the foundation of the side wall according to the geological conditions;
2. The empty cavern above the arch, the rock fragmentation degree of the visible cave is strengthened by anchor spray support, or by adding arching and arch backfilling. The construction method can refer to Figure (4).
When the cave is larger and deeper; beams and arches can be used, but the beam ends or arches should be placed on the stable and reliable bedrock, and if necessary, they should be used for reinforcement. The tunnel encounters cavern crossing measures in different parts:
1. When a narrow and deep cave is encountered on one side of the tunnel, the foundation of the side wall can be deepened, and the construction method can refer to Figure (5);
2. When there is a large cave at the bottom of the tunnel and there is running water, the masonry stone wall can be built below the tunnel bottom to support the tunnel structure, and the culvert pipe is arranged in the supporting wall to drain the cave water. The construction method can be referred to Figure 6);
3. When a part of the side wall of the tunnel encounters a large and deep cave, it is not advisable to deepen the foundation of the side wall. It can be arched over the side wall or below the tunnel bottom. The construction method can refer to Figure (7);
4. When there are deep and narrow caves in the middle and bottom of the tunnel, the foundations of the two walls can be strengthened and the bridge can be set according to the situation;
5. When the karst cave is large and small, and there is part of the filling, the filling at the bottom of the tunnel can be removed, and then the reinforced concrete beam and the longitudinal beam are set below the elevation of the bottom of the tunnel. The two ends of the beam are embedded in the rock layer. The construction method can be referred to the figure. (8);
6. When the tunnel passes through a large cave, when the situation is more complicated, the frame beam can be passed through the lower part of the tunnel according to the situation.
For karst caves that are difficult to handle during construction, the sluice can be used to bypass the karst cavern, continue tunnel construction, and then treat the cave.
Karst area construction considerations
1. During the construction period, a detailed survey of the surface is carried out, and the karst state is ascertained by geological radar, and the area where the cave is encountered can be estimated.
2. Understand the situation of surface water and water discharge sites, and carry out necessary treatment on the surface to prevent underwater seepage.
3. When the construction reaches the edge of the cave, the processes should be closely linked. At the same time, we try to find out the shape, scope, size, filling and groundwater of the cave, and the construction treatment plan and safety measures.
4. When encountering a cave in a downhill section, adequate drainage should be prepared.
5. Pay attention to check the roof of the cave during construction and dispose of the dangerous stone in time. When the karst is relatively large, a construction protection frame or steel mesh should be provided.
6. In the blasting operation in the dissolution section, try to do more eye-opening, shallow-eye, and control the amount of charge.
7. Excavation in the cave filling body. If the filling is soft, it can be constructed by the advanced support method. For example, the filling is very loose gravel, stone accumulation or water, and can be pre-grouted before excavation.
8. Do not pour the slag into the cave before the karst is treated.
9. To deal with caves with complicated conditions, safety measures should be formulated according to the actual situation on site to ensure construction safety.
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