Application of full-face cofferdam diversion technology in water conservancy construction?

The construction of water conservancy and hydropower projects not only plays a role in protecting the surrounding water environment, but also can play its regulatory role in the face of natural disasters, thus protecting people's lives and property safety. The application of construction diversion and cofferdam technology can provide a stable environment for the construction of water conservancy and hydropower projects, and it needs to be used reasonably in specific construction to promote the orderly construction of water conservancy and hydropower projects.

1 summary of diversion technology for full-face cofferdam

The diversion technology of full-face cofferdam mainly refers to the construction of river-blocking cofferdam in the upstream and downstream of the main river bed project to ensure that the river can pass through the temporary water flow route, and then let the river pass through the original river bed when the main project is completed. There are many diversion techniques for the whole cofferdam, mainly including tunnel diversion, open channel diversion and culvert diversion. With this diversion method, when building a dam at the outlet of a large water conservancy project, only the upstream cofferdam can be built to intercept all incoming water during the construction period; In addition, if the water level at the spillway outlet is lower than the riverbed elevation at the foundation pit, it is unnecessary to build a lower cofferdam on the inclined mountain river. The application of full-face cofferdam diversion technology has a series of advantages, such as large working face, one-time cofferdam protection when building buildings on river bed; If the permanent drainage structure in the water control project is used for diversion, the project investment cost can be greatly saved.

2 Cofferdam diversion technology principles of use

2. 1 Strictly implement the technical standards for water conservancy project reconstruction.

In the actual water conservancy project construction, updating the relevant equipment and corresponding construction technology in time can improve the efficiency of the project construction and ensure the quality of the project. The following points should be done in the specific construction process change: ① Before the process and equipment change, the construction scheme should be adjusted first, and only when the requirements of the construction scheme are met can new processes and equipment be further introduced; (2) For the design scheme, it should have clear guiding significance and can promote the smooth progress of construction, thus giving play to the role of new technologies and new equipment.

2.2 Optimize the overall layout of water conservancy projects.

With the development of society and the continuous progress of science and technology, in order to keep pace with the times, water conservancy projects must keep pace with the times, focus on the global market environment, constantly draw useful elements from it, keep pace with the times, and update construction ideas and equipment. It should be pointed out that the optimization of the project layout is not simply the introduction of equipment, but a comprehensive construction plan based on the existing economic base and equipment of the construction enterprise under the premise of fully understanding the construction site. The plan should have the latest application of science and technology, construction process monitoring, construction quality management and automation and intelligence of construction process.

2.3 Develop the overall construction scheme with the overall concept.

In order to improve the efficiency and quality of construction, besides the introduction and update of new technologies and the overall layout of construction, it is also necessary to formulate an overall construction plan and do a good job in the overall construction with the whole construction environment as the center. When planning, besides consulting several documents, we should also comprehensively investigate the environmental factors such as water quality environment, water flow characteristics, terrain characteristics and so on. In addition, after the inspection, it is necessary to analyze some excellent water conservancy projects at home and abroad, analyze and summarize their successful cases, absorb their strengths, and then integrate them, so as to design a perfect water conservancy project plan.

3 Application of full-face cofferdam diversion technology in water conservancy construction

3. 1 Tunnel diversion method

Tunnel diversion is suitable for rivers in mountainous areas, because the river valleys in such areas are relatively narrow and the banks show steep terrain. Because the drainage capacity of each tunnel is very limited, the tunnel needs high cost. Therefore, if there is no large flow, tunnel diversion can be used. Generally speaking, the discharge flow of each tunnel is strictly controlled at 2000 ~ 2500m3/s. In order to reduce the diversion cost as much as possible, the combination of diversion leakage and permanent tunnel can be adopted when necessary. When arranging the diversion tunnel, it needs to be carried out in a complete and fresh rock stratum. The specific points to be noted are as follows: ① To avoid large-scale collapse along the tunnel, it is necessary to avoid being parallel to the tunnel axis, rock stratum, fault and fracture zone, strictly control the included angle between the tunnel axis and rock stratum, avoid being less than 45, and control the inclination of rock stratum above 45.

② Make full use of the favorable terrain near the dam site to ensure the straight tunnel route. If it is a curved river bank, it is necessary to arrange a tunnel on the convex bank to shorten the length of the tunnel. Pressurized tunnels and low-speed unpressurized tunnels need to turn. The turning radius should be more than 5 times of the tunnel width, and the turning angle should be controlled within 60. When straight transition sections are set at the upstream and downstream of the bend, the length of the straight transition section should be more than 5 times of the tunnel width. (3) Strictly control the intersection angle between the inlet and outlet and the mainstream direction of the riverbed, otherwise it will affect the upstream inflow conditions, and the downstream channel will produce harmful scouring flow and surge, and the intersection angle of the outlet should be controlled within 30. (4) If more than two diversion tunnels are needed, they can be arranged on both banks or one bank. The distance between the entrance and exit of the tunnel and the toe of the upstream and downstream cofferdam needs to be greater than 50m. In recent years, the distance of 10~20m ~ 20m has also appeared. If there is only a small distance, it is necessary to protect the weir slope scientifically.

3.2 Open channel diversion

Open channel diversion refers to the excavation of open channels on the river bank and the construction of cofferdams on the river bed to let the river flow through the channels. When selecting the site for open channel diversion construction, the principle of simple construction and short axis should be followed, which can effectively reduce the engineering quantity; It is also necessary to ensure that there will be no great resistance in the process of water flow, ensure construction safety, and strive to avoid potential safety hazards caused by open channels. When arranging diversion open channel, the unfavorable geological section should not pass through the canal line as far as possible, and pay full attention to landslide and collapse to ensure slope stability; If an open channel is excavated on the floodplain, it is usually necessary to set up an outer wall, which is similar to a longitudinal cofferdam.

The external wall should be laid on a reliable foundation and built directly on dry land. It is necessary to set a straight open channel axis, so that there can be a smooth and smooth water flow and S-shaped bending can be avoided. It is necessary to combine diversion open channel with permanent open channel. If the concrete buildings in the hub are arranged along the coast, the diversion open channel usually combines the tailrace channel and the power station diversion channel, so it is necessary to connect the inlet and outlet of the open channel with the upstream and downstream water flow and control the intersection angle of the main stream within 30; In order not to affect the smooth flow of water, the turning radius of open channel should be strictly controlled.

3.3 culvert diversion method

Culvert diversion method is to use concrete to construct in a fixed place. Culvert diversion method is generally used on the beach. Once the location is selected, it can be used, and then the cofferdam will be built. After the cofferdam is built, water will flow out of the culvert. The main application points of culvert diversion method are as follows: ① Choose culvert materials according to terrain conditions. For example, in rocky places, culverts are usually formed by digging a ditch in the rock and sealing it. (2) When culvert is used for diversion, the most important thing is to prevent water from leaking and affecting the construction. In this case, the seepage can be intercepted around the culvert, which can effectively reduce the influence of seepage on construction and improve the efficiency and quality of the project. With the development of science and technology, to achieve good anti-seepage effect, anti-seepage materials can be compacted, which can play a good anti-seepage effect.

4 Application example of full cofferdam diversion technology in water conservancy project

4. 1 Basic overview of the project

A water conservancy project area is densely populated and has convenient transportation. The overflow dam of this water conservancy project has a total length of 80m and a dam height of 5m (above the river bed), which is mainly used for improving the urban water environment and flood control. In the construction of water conservancy projects, its geology is residual sandstone and carbonaceous slate mixed with tuff, with great permeability, and the average critical ratio of permeability is reduced to 0.594, and the average ratio of permeability failure is reduced to 0.727. This project adopts integral cofferdam diversion technology. Culvert diversion is selected this time.

4.2 Design of diversion tunnel

In the whole cofferdam diversion technology, the cofferdam should be carried out in strict accordance with the design requirements, the slope is 1: 1, the top of the cofferdam should be 50cm above the flowing water surface, and the straw bags should be neatly and densely stacked. In case of water seepage, it must be reported in time and handled properly.

4.3 Cofferdam diversion tunnel layout

In the whole cofferdam diversion construction, according to the terrain around the dam and local strong conditions, the tunnel line can be ensured to be straight, the tunnel length can be shortened and the construction benefit can be improved. At present, the diversion methods of hydropower stations are intercepting cofferdam and tunnel diversion. The outlet elevation of the diversion tunnel is 1476.00m, the length of the diversion tunnel is about 1037.72m, the size of the diversion tunnel after lining is 12.0m× 14.5m, and the lining thickness is about 70cm. According to the topographical and geological conditions of water conservancy projects, both diversion tunnels are arranged on the left bank. The orifice size of diversion tunnel is 12m and 14m, the elevation of inlet floor is 1560m, and the elevation of outlet floor is 1476m. The first tunnel is a pressure tunnel with a diameter of 3.6m；: The cross section of the pressureless water conveyance tunnel is12.9m× 3.4m (width× height).

4.4 Main points of cofferdam diversion construction

(1) Before cofferdam filling, the center line and sideline of cofferdam can be set out with total station, and colorful flags can be inserted to control the filling direction of cofferdam. In the process of filling, field surveyors should constantly control and measure, and correct the filling direction of cofferdam in time.

(2) Strictly control the intersection angle between the inlet and outlet and the mainstream direction of the riverbed, so as to avoid affecting the upstream water intake conditions, and at the same time pay attention to avoid harmful folded water flow and swell in the downstream river course of dam water conservancy project construction; The diversion outlet of cofferdam should be controlled at 30 as far as possible, and the construction requirements at the inlet can be relaxed appropriately according to the specific situation in practice.

(3) In the whole cofferdam diversion construction, a safe distance of more than 50m is required at the entrance and exit of the tunnel and at the toe of the upstream and downstream cofferdam to ensure the construction safety.

(4) Strictly control the water content of the earth material on the upper dam. The earth material on the upper dam should be compacted by layers, with the thickness of each layer not exceeding 40cm, and compacted by excavators or bulldozers to ensure that the degree of compaction is not less than 0.85.

(5) Strengthen the inspection of diversion materials of the whole cofferdam to prevent unqualified projects from the source; In the whole cofferdam diversion, the "three inspections" system shall be strictly implemented. If the previous procedure fails to pass the acceptance, the next procedure shall not be carried out.

(6) The paving thickness and compactness must be strictly controlled. When the longitudinal cofferdam is blocked, cement can be mixed in the longitudinal cofferdam, and the remaining gravel can be used to treat the slope, and the settlement of the cofferdam should be monitored regularly. After the settlement of cofferdam is stable, cement mortar can be used for longitudinal cofferdam, which can effectively prevent cofferdam leakage and improve the safety construction quality of cofferdam.

5 conclusion

To sum up, in the management and construction of water conservancy projects, the application of full-face cofferdam diversion technology can not only meet the construction needs, but also improve the quality level of water conservancy projects, reduce the difficulty of diversion construction and optimize the diversion mode of water conservancy projects, thus ensuring the timely completion of water conservancy projects and exerting positive application value.

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