I. Karst and its morphological characteristics
In the soluble rock area, groundwater and surface water have chemical erosion, mechanical erosion and accompanying migration and accumulation, which is always called dissolution. The special geological, geomorphological and hydrological characteristics produced in karst areas are called karst phenomena.
Karst is the result of the interaction between carbonate rocks and water, and karst only exists in the areas where carbonate rocks are distributed. Carbonate rocks are widely distributed in China, with an exposed surface area of about 654.38+0.2 million square kilometers, accounting for about 654.38+0/8 of the national area, and are buried more widely underground, mainly distributed in southern Guangxi, Guizhou and eastern Yunnan, among which Guangxi Zhuang Autonomous Region has the largest exposed area, accounting for about 65.438+0.2 million square kilometers, accounting for about 60% of the autonomous region. In addition, karst in Hunan, western Hubei, western and northern Guangdong is also well developed. Karst in North China is mainly distributed in Taihang Mountain, Taiyue Mountain, Luliang Mountain and Yanshan Mountain in Shanxi and Hebei, especially in Shanxi.
Karst is closely related to engineering construction. When building a tunnel in karst area, once the high-pressure karst pipeline water is exposed, it will cause a lot of water inrush, which will bring serious difficulties to the construction and even flood the tunnel, resulting in accidents of machine destruction and death. When huge caves are encountered in underground cavern construction, the construction of high fill in caves or bridge spans is difficult and expensive, and sometimes new roads have to be opened up, which delays the construction period. When building a highway in karst area, the underground karst water will cause water gushing from subgrade base, subgrade flooding, subgrade scouring and tunnel water gushing.
Second, the basic conditions of karst development
1, containing soluble rocks, namely limestone, dolomite and a series of transitional rocks dominated by carbonate rocks;
2. Water with solubility (including CO2) and sufficient flow;
3. There is groundwater movement, that is, the way of surface water infiltration and groundwater flow.
Three, the karst area engineering geological problems and prevention measures
1, main engineering geological problems
The engineering geological problems often encountered in engineering construction in karst areas are mainly foundation collapse, uneven settlement and water inrush from foundation pits and caves. In the excavation of building foundation pit or underground cavern, if you dig through underground river or surface water infiltration channel, it will cause sudden water inrush, which will cause major tunnels and disasters to engineering construction and use.
When building engineering buildings in karst areas: firstly, on the basis of finding out the distribution and development of karst, we should choose the location of engineering buildings and avoid the areas with serious hazards as much as possible; Secondly, due to the complexity of karst development, it is impossible to find out the distribution of underground karst, especially before construction. Once the construction is exposed, necessary measures must be taken.
2. Common preventive measures
For the karst caves in the foundation of underground buildings, grouting, pouring concrete or flaky can be used for backfilling, and reinforced with reinforced concrete cover plates when necessary, so as to improve the bearing capacity of the foundation and prevent the roof of the karst caves from collapsing.
When the tunnel passes through the karst area, corresponding measures should be taken according to the scale and location of the karst cave encountered. If the karst cave is small in scale and appears on the top or side wall of the cave, it is generally backfilled and sealed after the filler is removed; If it appears under the side wall or at the bottom of the hole, it can be reinforced or crossed; If the karst cave is large in scale, even there is a hidden river, you can bridge it in the tunnel.
Waterproof and drainage measures in karst areas should be handled well. The main principle is that it should not only be beneficial to the engineering construction and reduce the development and harm of karst, but also be beneficial to the environmental protection in this area, so as not to cause new environmental problems due to improper drainage and water diversion. In case of karst water in the tunnel, if the amount of karst water is small, grouting can be used to block the water, or side ditch or central ditch can be used to discharge the water out of the tunnel; If the amount of water is large, parallel pilot tunnels can be used as drainage tunnels.
In a word, karst should generally take comprehensive control measures combining drainage and plugging, rather than forcibly plugging, and remedial measures should be taken for ground environmental problems (such as ground collapse or surface water shortage) caused by long-term drainage.
Four, the treatment measures of tunnel karst cave
1, water treatment
The water treatment of karst caves should follow the principle of "giving priority to drainage, combining dredging with plugging, adapting measures to local conditions and comprehensive treatment". Water treatment measures can basically be divided into the following three types:
(1) drainage tunnel. When it is predicted that the karst water in the tunnel area is large and the water pressure is high, and it is impossible to avoid the tunnel, it is necessary to consider setting up a drainage tunnel to remove karst water, reduce the groundwater level, keep the tunnel dry and ensure the construction safety. The spillway tunnel should be located at the side where the groundwater comes. In order to prevent the sudden attack of karst water,
Prepare enough pumping equipment during construction. The setting of spillway tunnel may have a negative impact on the ecological environment. Whether to adopt it should be evaluated from the aspects of construction, environmental protection and safety to ensure that the scheme is comprehensive and the cost is the lowest.
(2) Culverts and inverted siphons have water absorption. When the tunnel section intersects with karst water, in order to ensure the smooth flow of karst water, a reinforced concrete circular culvert or inverted siphon is set at the bottom of the tunnel, and at the same time, the surrounding of the culvert entrance and exit to the outer edge of the tunnel side wall is backfilled and compacted with mortar rubble.
(3) Grouting to stop water. During the construction, when the karst fissure water gushes out, the "surrounding irrigation method" is adopted to stop the water. Firstly, grouting is distributed in a proper range around the water outlet to form a closed water-stop curtain. Grout fills small cracks connected with large cracks, and reduces the encirclement layer by layer from far to near and from outside to inside. Within the grouting depth, the karst fissure water turns into tubular gushing water, and finally the concentrated outlet is reversely injected, so that the slurry diffusion is more directional and spreads along the large fissure.
At the same time, the large water inflow in the tunnel mostly varies with rainfall. The hydraulic relationship between surface water and water in tunnel is determined through experiments. In addition to the necessary engineering measures, surface water should be intercepted on the surface of the tunnel to block the infiltration path of surface water, so as to reduce the influence of surface water on the tunnel.
2, small cave treatment
For small-scale karst caves with relatively stable rock walls, undeveloped and waterless caves, schemes such as plugging with mortar rubble, backfilling, arch protection, strengthening lining, riprap sand blasting grouting and setting beams at the bottom of the caves can be adopted. For small caves with no water, little groundwater and dense filling, single liquid cement slurry can be injected into the small conduit to strengthen the stratum; For small karst caves with running water and not dense filling, the stratum can be reinforced by injecting cement-sodium silicate double slurry into the small conduit.
For the small karst cave filled at the bottom of the tunnel, it can be passed by adding ground beams to strengthen the lining. The tunnel floor or inverted arch is designed as a flat plate, and the lower parts of the floor and side walls are covered with steel bars and filled with reinforced concrete, so that the side walls and inverted arch (or floor) form a reinforced concrete beam together to enhance and improve the mechanical properties.
3, large cave treatment
(1), the supporting structure is adopted to cross the cave. Supporting wall reinforcement. If the filler in the hole is loose and the thickness is small, remove the filler and reinforce the top of the hole with mortar stone wall; Water flows through the culvert to facilitate drainage. B. arch bridge top reinforcement. If the filling in the hole is loose and thick, reinforced concrete arch can be used for reinforcement. C, support and reinforce the digging pile. For deep caves, pile foundation can be used for reinforcement and manual excavation can be used to realize pile formation. The bottom of the pile is embedded in the bedrock, and the reinforcement at the top of the pile extends into the reinforced concrete floor, which is integrated with the floor to form a stable stress body. D. Bury and reinforce the supporting column. According to the different forms of karst caves, pillars are used to strengthen the roof of karst caves, and cracks are backfilled and grouting is used to strengthen the basement of karst caves.
2) Adopt the scheme of crossing the karst cave. If the karst cave is large, it is expensive to use methods such as plugging, strengthening and supporting. When the karst cave is small, it is not suitable for backfilling and plugging, and the working face is small and difficult to operate. A. the intersection of the side wall beam and the traveling beam. The construction method adopts the method of arch first and then wall, so that the side wall beam can be safely constructed under the protection of arch ring. The driving part of ballast bed adopts reinforced concrete prestressed beam. Pier and cantilever beam support the longitudinal beam to cross. Reinforced concrete plate beam is used as the bottom plate to support the side wall, joists are supported on the pier and cantilever beam respectively, and the cantilever beam is anchored in the bedrock through anchor rods. C. the side walls are arched. The arch foot of the side wall is set at the bottom of the side wall, and the vault is located at the arch foot of the lining arch ring. The lining is reinforced by arranging steel bars to form a side wall arch that passes through the cave.
3) Adopt evasive scheme. In order not to make the project come to a standstill, a circuitous pilot tunnel is used to avoid the karst cave, and then it is treated. It must be emphasized that when the detour pilot tunnel is used to avoid the karst cave, in order to avoid encountering the karst cave again, before choosing the location of the detour pilot tunnel, we should find out the karst cave situation within this range before making a decision.
Verb (abbreviation of verb) conclusion
Karst caves are very common in tunnels in limestone areas. The quality, progress and safety of karst tunnel construction can only be effectively guaranteed if the geological forecast is made in advance, the distribution of karst landform, the law of karst development and structural characteristics are correctly understood, and different treatment measures are taken according to different karst structures encountered.
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