Qujing Shuicheng Underground River Surface Underground Reservoir Project

4.4. 1. 1 project overview and benefits

Shuicheng Reservoir is located on Shuicheng River, 3 kilometers northeast of Yuezhou Town, Qujing City. Shuicheng underground river surface underground reservoir is a medium-sized reservoir formed by damming Shuicheng River downstream of the underground river outlet by using natural karst valley. The total storage capacity of the reservoir is 4927× 104m3, and the reservoir hub consists of main dam, auxiliary dam, diversion spillway tunnel, water conveyance tunnel and spillway. The main dam is a carbon shale weathered core rockfill dam with a crest length of 62 1.5m and a dam height of 59.2m The total filling amount of the dam body is 132.4× 104m3. The total filling volume of flood control dam is 36× 104m3. The investment in the main project of the reservoir is about 60 million yuan, the investment in the diversion canal is 3 1, 78 1.8 million yuan, and the total investment is about 9 1, 78 1.8 million yuan. Shuicheng Reservoir belongs to the expansion project of the old reservoir, and it is a reservoir which mainly focuses on flood control and irrigation, taking into account the comprehensive utilization of industrial water in factories and mines near the reservoir. Before the expansion, the total industrial water supply in Qujing was 346.6 1× 104m3/a, and the irrigation area was 3,500 mu. After the expansion of irrigation canal system, the annual industrial water supply can be increased by 300× 104m3. The construction of surface-underground reservoir of Shuicheng underground river provides a feasible demonstration scheme for the development and utilization of underground river on the edge of karst basin.

4.4. 1.2 Geological environment of water source

Shuicheng underground river basin is located at the source of Shuicheng River, a tributary of Nanpanjiang River, which belongs to the middle mountain of structural dissolution and is the transition zone from karst mountain to basin. The overall terrain is inclined from north to south, with an altitude of1927 ~ 2,447 m. The landform types of the recharge area are karst peak-cluster depression and Yuan Ye of stone buds, which are mainly replenished by atmospheric precipitation, with an annual average precipitation of 787.5mm and an annual average temperature of 14.5℃. According to the statistics of average precipitation for many years, surface water is directly pumped into the ground in rainy season, and about 65% of the precipitation is converted into underground runoff, with an infiltration coefficient of 0.65 and an average groundwater runoff modulus of 16.65438+.

The aquifer in the underground river basin is mainly composed of limestone, micrite, biolimestone, micrite with dolomite in Qixia Formation (P 1q) and Maokou Formation (P 1m) of Lower Permian. The aquiclude is timely sandstone and mudstone of Liangshan Formation of Lower Permian (P 1l), basalt of Middle Permian (P2β) mixed with siltstone, fine sand mixed with clay of Ciying Formation of Neogene (N2c), mudstone of Xiaotun Formation of Oligocene of Paleogene and calcareous mudstone (E3x). The structure in this area is dominated by NE-trending faults, with a large scale, followed by a small number of near-east-west small faults, and rocks are broken and fractures are developed along the fault zone. The underground river system is bounded on the north, east and west by the underground watershed or mudstone, siltstone and basalt of Liangshan Formation, and the coal-bearing clay layer of Ciying Formation with a thickness of 959.05m is deposited downstream of the outlet of the underground river in the south, forming a water-resisting boundary (Figure 4-7). The karst development in this area is obviously restricted by lithology and structure, and it is uneven. Qixia Formation and Maokou Formation are exposed in a large area in the upper reaches of the underground river. The aquifer is rich in water, and the underground karst is characterized by the development of karst pipelines, which are concentrated in cracks, gradually expanding the fracture channels and forming uneven underground corrosion pipelines. Surface karst is characterized by the development of karst depressions and sinkholes along underground rivers.

Figure 4-7 Hydrogeological Map of Shuicheng Underground River Basin

1- pure carbonate rock; 2- impure carbonate rocks; 3- clastic rock; 4— Rising spring group, flow rate (liter/second); 5— Descending spring, flow rate (liter/second); 6— outlet of underground river, flow (liter/second); 7— skylight under the river, flow (liter/second); 8- Falling Water Cave; 9- rock group type boundary; 10- normal fault; 1 1- reverse fault; 12- failure of unknown nature; 13- Surface watershed

4.4. 1.3 characteristics of karst water resources and development technical conditions

The average inflow of Shuicheng Reservoir for many years is 7600× 104m3, and the average storage capacity is 935.5× 104m3, which is unstable. The outlet of the underground river is located in the valley on the edge of the river basin, with an elevation of 1.900m, accidental discharge of 2 1.36 L/s, multi-year average discharge of 1.627.82 L/s, and dry season discharge of1.21.42.

Covered river basin covers an area of 178km2, showing a nearly north-south distribution. The total length of the underground river pipeline is about 47.2km, consisting of two main pipelines and 1 branch pipelines, which are concentrated in a tree shape from north to south along the runoff direction, with an average gradient of about 7‰, of which two main pipelines are about 43.8km long and develop along the ne-trending fault. There is a 1 branch pipe, which is only 3.4km long, nearly north-south, and extends directly to the exit. Karst water in this area occurs in carbonate cracks and tunnels, mainly in the form of pipeline flow, and the system has weak storage capacity. The water level and discharge change greatly with seasons, and the discharge increases sharply in rainy season.

The surface runoff of Shuicheng Reservoir is weak, the groundwater recharge is sufficient, and the underground river discharges centrally at the outlet of the reservoir. The dam site area is an asymmetric U-shaped valley with a river width of 60 ~ 90m and a flood thickness of 4.6 ~ 9.0m It is mainly composed of muddy clay and gravel layers, and the river flows in the north, east, south and west directions, which is basically consistent with the trend of rock strata. Karst caves, karst caves and karst caves are developed in limestone below the river bed. Groundwater in the valley is often exposed in the form of spring water. The limestone of Qixia Formation and Maokou Formation exposed at the dam site of the main dam has an exposed area of 90%, single lithology, massive structure, large spacing between structural planes, strong binding force, small deformation and good rock mass integrity, which can meet the requirements of dam construction, but the karst development intensity of rock mass is medium-strong, and there is obvious leakage problem of dam foundation, and the dam foundation is medium-serious leakage; The sandstone shale of Liangshan Formation on the left bank is weak and prone to surface sliding. The right abutment of Ciying Formation is composed of fine sand mixed with clay, with an exposed area of about 4%, high water content, medium-strong expansive soil and poor stability, all of which have adverse engineering geological problems. The right bank of the reservoir area is cut by the river to form an interriver block and a lower adjacent valley. The clastic strata on the left bank are affected by the fault zone, and the karst development on both sides of the river bed is strong. The main engineering geological problems of the main dam are lateral leakage, leakage around the dam and leakage to the lower adjacent valley after impoundment.

4.4. 1.4 exploration and development technology of karst water

The construction procedures of exploration and development projects are as follows: ① Hydrogeological mapping:1:50,000 hydrogeological mapping in the periphery of the reservoir area, cave investigation and underground river flow observation, and 1: 1:65438 hydrogeological engineering geological environmental geological mapping in the reservoir area; ② Feasibility demonstration; ③ Engineering geological exploration: drilling, water pressure test, grouting test and sampling test are carried out in the dam site hub area; ④ Engineering and construction design; ⑤ Curtain grouting for dam foundation seepage prevention; ⑥ Damming and intercepting, and storing water to form a reservoir.

Through comprehensive exploration by means of hydrogeologic mapping, dynamic observation and engineering geological exploration, we can find out the richness of water resources, recharge boundary, lithology change law, basic conditions for comprehensive utilization of water resources, stability and dynamic change of underground river flow, karst development characteristics of dam foundation, geotechnical types, geological structure, groundwater burial conditions and leakage. Curtain grouting, backfill grouting and other technologies are mainly used to build dams and store water.

(1) Curtain grouting: dam foundation leakage and leakage around the dam are the key problems of reservoir water storage. According to the seepage situation of rock stratum, grouting is adopted to prevent seepage, so as to improve the impermeability of rock mass. * * * Set 1 row of main curtain and two rows of upstream and downstream auxiliary curtains, with the distance between curtain holes of 2.5m. The main curtain is arranged at the upstream of the dam shaft 1.5m, with a total length of 675m and 27 1 hole. The distance between the secondary curtain and the main curtain is 2. 165m, and the total length of the secondary curtain in the upstream row is 470m, with 192 holes. The secondary curtain in the downstream row is divided into two sections, with a total length of 270m and 1 10 holes.

(2) Drainage of spring water: After the dam foundation excavation, there are more than 60 springs exposed in and near the riverbed in the dam site area, and the flow rate is 0.07 ~ 6.6L/s. According to the different exposed positions, the spring water in the upstream of the core wall is blocked and the spring water in the downstream of the core wall is drained.

(3) Backfilling grouting: In the stratum distribution areas of Qixia Formation and Maokou Formation, the dissolution rate of dam foundation surface after excavation is 50% ~ 90%, and the rock mass quality is poor. If the bedrock is completely cleared, the amount of excavation and backfilling will be greatly increased, which is very uneconomical. Considering various factors comprehensively, in order to make the bearing capacity and anti-sliding stability of this section of dam foundation meet the dam load requirements, after reaching the design elevation, the filler in the groove will be reduced. After the fillings in the karst cave are removed, they shall be blocked with rubble concrete and backfilled and grouted with embedded pipes. All vertical cracks are backfilled and grouted with embedded pipes, 40 embedded pipes are set on the left and right banks, and cement mortar 1, 794.045 tons are injected.

(4) Chiseling treatment: After the riverbed is cleaned, the bedrock surface is smooth. In order to improve the sliding stability between dam foundation and concrete cover plate, smooth bedrock surface is drilled with steel before pouring concrete cover plate.

(5) Expansive soil treatment: the bearing capacity and anti-sliding stability of the stratum distribution area of Ciying Formation on the right abutment meet the dam load requirements. In order to prevent the property change of expansive soil caused by moisture change, clay is used for covering protection immediately after excavation.

(6) Setting concrete cover plate: In order to make the core wall dam foundation have uniform bearing capacity and anti-sliding and anti-seepage performance, and prevent the core wall material from directly contacting the bedrock, after the surface treatment of the dam foundation, a concrete cover plate with an average thickness of about 0.5m is set on it to transition with the core wall material. At the same time, as the cover plate for consolidation grouting and curtain grouting of the dam foundation, the concrete cover plate plays a very important role in improving the rock mass quality of the dam foundation.