1 Introduction
Small reservoirs have many problems and little funds. With limited funds, it is a great challenge for designers to complete the reinforcement design of dangerous reservoirs as much as possible, especially the construction diversion scheme. Almost every small reservoir has encountered this problem in construction organization design. In the design process, the author summarizes the characteristics of several small reservoirs. There are two main problems: first, the storage capacity curve is relatively flat,
During the construction period, the water level of the reservoir rises slightly, so it is generally economical to use cofferdam. Second, the storage capacity curve is steep, and the water level of the reservoir rises rapidly during the construction period. If the cofferdam is adopted according to the traditional diversion scheme, the total investment of small reservoirs is more than 3 million, which is unbearable. In this case, the author thinks that it is more economical and practical to adopt the method of pumping and drainage. Taking the construction organization design of Gujiagou Reservoir in Zhushan County as an example, this paper focuses on the construction diversion design in the second case.
2 Project overview
Gujiagou Reservoir is located in Xihe Village, Gu Lei Town, Zhushan County, Hubei Province. The reservoir dam intercepted the water in Gujiagou and Wuchang Miaogou in the upper reaches of Xihe River. The rain-fed area above the dam site of the reservoir is 3.2 square kilometers, and the river course is 2.9 kilometers long. The average slope of the main channel from the watershed to the dam site is 1 15.2‰, which is relatively high. The dam site is 60km away from Gulei Town10 and Zhushan County. It is a small-scale (1) reservoir hub project which mainly focuses on irrigation and has comprehensive utilization functions such as flood control and water supply. The existing buildings of the reservoir project include: clay core dam, spillway and water conveyance culvert. The total storage capacity of the reservoir is 17 1000 m3, in which the storage capacity of Li Xing is 1407000 m3 and the dead storage capacity is 63,000 m3. The reservoir provides irrigation water for 3,000 mu of farmland in the downstream irrigation area, and also undertakes flood control tasks for downstream facilities such as Gulei Town and 305 Provincial Highway, and protects 3,000 mu of farmland with a population of 654.38+0.000. Gujiagou Reservoir Hub belongs to Grade IV project, with a small scale (1), and the water retaining, discharging and conveying structures are all Grade IV buildings. The preliminary design projects of reservoir reinforcement mainly include: ① the core wall of the dam is treated by filling grouting method to solve the problem that the permeability coefficient of the core wall does not meet the requirements; Curtain grouting is used to treat the medium permeable layer of dam and spillway bedrock; (2) According to the deformation of the dam slope and the weathering and crushing of the slope protection, the upstream dam slope is renovated with 30cm thick dry block stone, and the downstream dam slope is planted with turf on the basis of leveling to improve the dam surface structure; (3) Blocking the original water conveyance culvert, and opening a new spillway tunnel on the upstream of the right bank of the dam, which can also be used for irrigation and water conveyance. Block the original spillway, dismantle and rebuild the diversion channel and the left wall of the control section, and build a new water and soil conservation dam 21m; (4) Update the opening and closing equipment, and add import access doors and export working doors.
3. Construction organization design
The valley where Gujiagou Reservoir is located belongs to subtropical continental monsoon climate, characterized by warm and humid, four distinct seasons, abundant rainfall and sunshine. The frost-free period is about 250 days, the annual average temperature is 10.2℃ ~ 15.6℃, the extreme minimum temperature is minus 9.9℃, and the extreme maximum temperature is 43.4℃. The average annual rainfall in the regional reservoir area is 849.5 mm, the average annual runoff depth is 353 mm, and the average annual inflow is 4.29 million cubic meters. Floods in the basin are mostly caused by heavy rain, which usually occurs in July ~10. Most rainstorms are formed by low vortex shear lines and frontal rain surfaces, and there are few rainstorms caused by typhoons. Heavy rain usually lasts 1 ~ 3 days, and heavy rain generally covers the whole basin. This rainfall is characterized by concentrated rainfall, high intensity, high flood peak and steep fluctuation. The annual average maximum wind speed of the dam site is 13m/s, and the wind direction is generally northeast wind. 3. 1 Construction diversion standard The permanent building of this project belongs to Grade IV building. According to the Code for Construction Organization Design of Water Conservancy and Hydropower Engineering (SL 303-2004) and the specific conditions of this project, the temporary diversion building is classified as Grade 5. In combination with the diversion requirements of this project, the diversion standard is the once-in-five-year flood in dry season. 3.2 Construction organization design According to the engineering quantity and engineering characteristics of this project, the reinforcement and management facilities of the dam hub project are completed according to 1 dry season, that is, the total construction period is 12 months. July of the first year is the construction preparation period. During the preparation period, construction roads will be built and improved, and the preparation of the yard will be done well to ensure that there are qualified soil materials and good construction roads during the construction period. In early August of the first year, the main project was fully started. First, excavation of 0+0 10 ~ 0+080 section of water conveyance tunnel, which is the key project to control the construction period. Open excavation shall be carried out at the tunnel exit first, and tunnel excavation shall be carried out immediately after open excavation. The excavation of the rock tunnel in the 0+0 10 ~ 0+080 section of the tunnel will take 3 months to complete, and then it will take 3 months to complete the concrete lining of the tunnel and backfill grouting at the top of the cave. 165438+ in the first year1early October, the construction of tunnel entrance section from 0+000 to 0+0 10 will be started immediately after the reservoir water level drops, and the flood discharge tunnel will be completed by the middle of February next year. At the same time, the construction of import and export lock chambers and outlet canals can be completed. In the first year165438+1early October, dam crest regulation, upstream and downstream dam slope leveling, masonry protection and other construction can also be carried out. At the same time, the curtain grouting construction of dam body is completed. In the middle of February of the following year, after the tunnel lining work is completed, the original low-water culvert can be blocked, and it takes 1.5 months to complete all the blocking work. At the same time, projects such as emergency reinforcement of spillway can also be carried out. By the end of April of the following year, the installation and debugging of upstream slope protection, downstream slope regulation, most main works, metal structures and opening and closing equipment of the dam will be completed; Complete all construction, dam crest road construction and dam management facilities construction in May of the following year; In June of the following year, the project was closed and cleaned up. 3.3 Construction Flood According to the analysis of rainfall characteristics in the basin, the flood season of Gujiagou Reservoir is from April to 10 every year, and the dry season is from 1 10 to March the following year. According to the construction organization design, the period that needs construction diversion is the dry season (165438+ 10 ~ March), and the flood standard of diversion buildings is once every five years. According to 1956 to 2000, the total monthly runoff at the corresponding frequency is calculated. According to the 24-hour maximum precipitation data of Zhushan Meteorological Station 1980 to 2006 in dry season, the calculation results of design flood during construction period are obtained. See table 1 for runoff calculation results in dry season, and table 2 for design flood results in construction period.
3.4 Construction diversion mode According to the design requirements of this reinforcement project, upstream dam slope renovation, tunnel entrance section and emergency spillway construction need construction diversion. The excavation, demolition, backfilling and plugging of the original culvert pipe must be carried out after the reservoir water is drained. According to the construction characteristics and actual layout of the project, the second-stage construction diversion is adopted for the construction diversion of this project. In the first stage, the original low-water culvert is directly used for drainage. In late June of 5438+the first year 10, the water level of the reservoir was reduced to the original dead water level of 549.49m through the original water conveyance culvert. In the first ten days of June of the first year 165438+ 10, the slope protection of the upstream dam slope and the construction of the entrance section of the spillway tunnel were carried out by using the low water level. In the second stage, the necessary methods are used for construction diversion, flood discharge tunnel construction, original culvert excavation, demolition and plugging, etc. According to the water intake scale and construction arrangement of this project, two sets of construction diversion schemes are compared for the incoming water during the five-month construction period of this design (from the first year 165438+ 10 to the next year's March): scheme 1: water pump drainage scheme, that is, June of the first year165438+1. According to Table 2, the peak flow of rainstorm with flood frequency p = 20% during the construction period is 5. 1m3/s, and the total flood volume is 58,000m3. At the initial stage of tunnel entrance construction, the original dead storage capacity of 58,000m3 can be used as the reserved storage capacity for flood control in the design standard (p = 20%). The total inflow is calculated according to the monthly runoff frequency (p = 20%), and the total inflow in five months is about 222,800 m3. That is, the total pumping capacity of this project is 222,800 m3. The design pump (lift 16m) pumps water at a flow rate of 600m3/h(0. 167m3/s), which can meet the requirements of construction pumping. According to the calculation, the power of each pump is 45Kw, and the electricity consumption during pumping is10530kW h, and the investment of this scheme is 9600 yuan. Scheme 2: cofferdam water retaining scheme, that is, cofferdam is used to retain water, and all incoming water is stored during construction. The design water level corresponding to the storage capacity curve is 556.3m, and the top elevation of cofferdam is 556.8m considering the safety elevation. Cofferdams are arranged around the water intake and connected with the mountains near the dam site to form a closed flood control system. Cofferdam is a cofferdam structure with stone slag mixture as weir body and clay inclined wall for seepage prevention. The crest elevation is 556.8m, the crest height 14.25m, the crest width is 3.5m, the water front slope 1:3, the water back slope 1: 1.5, and the cofferdam length is 50m. Main quantities: earth-rock cofferdam 12 150m3, clay 7 123m3, construction road 0.3km, and the investment of this scheme is 393,800 yuan. Compared with the second scheme, the first scheme makes more effective use of the existing dead storage capacity, avoids a large amount of excavation and filling work brought by cofferdam filling, reduces the construction risk and construction period, and has no construction road and is not affected by the upstream dam slope construction. Compared with the two schemes, the direct investment scheme 1 saves 384,200 yuan compared with the scheme 2, so the scheme 1 is recommended as the diversion scheme of the second phase construction. According to table 1 and table 2, a rainstorm with flood frequency p = 20% has a peak flow of 5.165438+1October-March during the construction of the whole spillway tunnel and the original water conveyance culvert, and the total flood volume during the period is 22. Considering that the upstream funds are limited and the construction of roads and cofferdams is inconvenient, the original culvert was excavated, lined, removed and blocked during the whole flood discharge tunnel construction, and the cofferdam was not built. The reservoir water was directly discharged downstream by submersible pump through the newly-built water conveyance tunnel, and the excavation, removal and blocking of the flood discharge tunnel and the original culvert were completed in five months.
4. Conclusion
Limited by its own geographical location and matching funds, the design of reinforcement of small reservoirs can not adopt the conventional cofferdam diversion method in the construction diversion design, especially in most cases of small reservoirs in mountainous areas, using pumping method can not only save investment, but also be well implemented.
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