Karst refers to soluble rock formations, such as limestone, dolomite, dolomitic limestone, gypsum and rock salt. It is subjected to the chemical and mechanical action of water to produce grooves, cracks and cavities, as well as caves and depressions on the surface due to the collapse of the top of the cavity. Limestone is widely distributed in China, and caves are often encountered. Therefore, attention must be paid to the construction of tunnels in these areas.
(1) karst cave types and their influence on tunnel construction
Karst caves are generally dead, alive, dry, wet, large and small. Dead, dry and small caves are easier to deal with, while living, wet and large caves are more complicated to deal with.
When the tunnel passes through the soluble rock stratum, some karst caves are broken and prone to collapse. Some caves are located at the bottom of the tunnel, and the fillings are soft and deep, which makes it difficult to treat the tunnel base. Sometimes, when the tunnel is excavated to its edge, the water-containing filler keeps pouring into the tunnel, which is difficult to stop, and even causes the surface to crack and sink, and the mountain pressure increases sharply. Sometimes, large water sacs or underground rivers will be encountered, and karst water or muddy sand with water will flood into the tunnel in large quantities. Some karst caves and underground rivers are criss-crossed, with complex branches and a wide range, which is very difficult to deal with.
(2) Treatment Measures for Tunnel Encountering Karst Cave
1) When the tunnel passes through the karst area, it is necessary to find out the distribution range and types of karst caves, the integrity and stability of rock strata, the situation of fill and groundwater, and determine the construction method accordingly. For karst areas with complex geological conditions, such as developing or crossing hidden river sacs, we should find out the situation and carefully choose the construction scheme. In the case of sudden large-scale water gushing, falling rocks and landslides, measures must be taken in advance to ensure construction safety.
2) When the tunnel passes through the karst area, if the rock stratum is relatively complete and stable, the karst cave has stopped developing, the filling is relatively solid, and the underground water volume is small, the geological conditions can be ascertained by methods such as hole exploration or geophysical exploration, and corresponding measures can be taken if there are changes. If karst caves are still developing or crossing karst areas such as underground river sacs, it is necessary to find out the amount of groundwater and the direction of water flow, and solve the drainage problem in construction first. Generally, the construction scheme of parallel pilot tunnels can be adopted to advance the excavation by drilling in advance. When a large number of water gushing, rock flow, debris flow, landslide and rockfall occur, the horizontal pilot tunnel can be used as a drainage channel, and when the main tunnel is blocked, a tunneling face can also be opened in front by using the horizontal pilot tunnel, so that the main tunnel will not stop.
3) There are four common methods to treat tunnel caves in karst areas, namely "diversion, plugging, crossing and bypassing".
A. Introduction
When there is water flow in an underground river or karst cave, it is not appropriate to directly block it. We should find out the direction of the water source.
And its relationship with the location of the tunnel, drainage by underground pipes, culverts, bridges and other facilities.
Or dig a drainage hole to drain the water in the hole (Figure 10.4. 1). When the karst water flows
At the top or above the tunnel, an inclined diversion tunnel should be excavated at an appropriate distance.
(or diversion tank) lower the water level below the elevation of the bottom of the cave, and then drain it. When the tunnel is established
When there are parallel pilot holes, water can be introduced into the parallel pilot holes and discharged.
B. blocking
For the karst caves that have stopped developing and have short span and no water, the phase diagram is the same as the drainage flow chart of tunnel 10.4. 1 bridge and culvert.
The intersection position and its filling conditions shall be backfilled and sealed with concrete, mortar masonry or dry masonry; Or deepen the foundation of the side wall and add
Fix the bottom of the tunnel (figure 10.4.2). When there is a hole in the tunnel vault, depending on the degree of rock breakage at the entrance, anchor rod or shotcrete net can be used to reinforce the hole top. If necessary, grouting reinforcement can be considered, and tunnel arch protection and vault backfilling can be added for treatment (Figure 10.4.3).
Fig. 10.4.2 schematic diagram of cave plugging and filling; Figure 10.4.3 Schematic diagram of shotcrete reinforcement and arch protection.
C.yue
When one side of the tunnel encounters a long and narrow cave, the foundation of the side wall can be deepened (Figure 10.4.4). When there is a large karst cave and running water at the bottom of the tunnel, a masonry supporting wall can be built below the tunnel bottom to support the tunnel structure, and culvert pipes are set in the supporting wall to discharge the karst cave water (Figure 10.4.5). When the side wall of the tunnel encounters a large and deep cave, and it is not suitable to deepen the foundation of the side wall, an arch can be built under the side wall or at the bottom of the tunnel (Figure 10.4.6). When deep and narrow caves are encountered in the middle and bottom of the tunnel, the foundation of both side walls can be strengthened, and abutment beams can be set to pass through according to the situation (Figure 10.4.7). When the tunnel passes through a large karst cave and the situation is complicated, the side wall beam and walking beam can be used according to the situation, and the design unit will be responsible for the construction after special design.
Figure 10.4.4 schematic diagram of deepening side wall foundation 10.4.5 schematic diagram of embedded culvert pipes in the wall.
D. by-pass
In the construction in karst area, when it is time-consuming and difficult to deal with individual caves, detour pilot tunnels can be used to bypass the caves, continue construction in front of the caves, and deal with the caves at the same time, so as to save time and speed up the construction progress. When bypassing the excavation, the instability of the tunnel wall should be prevented.
(3) Matters needing attention in tunnel construction in karst cave area
1) When construction reaches the edge of the hole, each working procedure should be closely connected, and the supporting and lining should be advanced. At the same time, drilling or geophysical prospecting should be used for advance prediction to find out the shape, scope, size, filling and groundwater of the karst cave, so as to formulate construction treatment scheme and safety measures.
2) Pay attention to check the top of the karst cave during construction, and handle the dangerous stones in time. When the cave is large and high, and the top is broken, it should be reinforced by shotcrete first, and then the anchor rod should be driven near the top of the cave, and the construction protective frame or steel bar protective net should be set up.
Figure 10.4.6 Schematic Diagram of Arch Spanning 10.4.7 Schematic Diagram of Beam Spanning
3) When blasting in the solution zone, we should drill as many holes and shallow holes as possible, and control the blasting charge to reduce the disturbance to the surrounding rock. To prevent a large number of fillers in the cave from suddenly pouring into the tunnel after a blasting, or the cave water suddenly attacks the tunnel, causing serious losses.
4) Excavation in the filling body of karst cave, if the filling body is soft, advance support construction can be adopted. If the filler is extremely loose gravel, piled stones, plastic clay and sand clay, etc. Before excavation, surface grouting, in-hole grouting or the combination of surface grouting and in-hole grouting can be used for reinforcement. When it comes to flowing plastic soil with fine particles and high water content, split grouting technology can be used to reinforce it by injecting cement slurry or cement sodium silicate double slurry.
5) Before the treatment plan is formulated, the waste residue shall not be dumped into the cave at will. Because the abandoned slag covers the cave, not only can we not understand the real situation, but it will cause more difficulties.
I hope it helps you!