Diversion In the process of building water conservancy projects in water areas (mostly flowing rivers), in order to create dry land construction conditions, cofferdams are used to protect the foundation pit in the early stage, and the river flow direction is guided to the downstream by bypassing the construction site in a predetermined way. Construction diversion is a very important engineering measure unique to hydraulic engineering construction, especially dam construction. The selection of diversion scheme is related to the construction period, quality, cost and safety of the whole project, and should be carefully designed in advance.
The design contents mainly include: 1, mastering and analyzing the hydrological characteristics of the river and the basic data such as meteorology, topography and geology of the project site; 2. Select diversion period, design standard, diversion flow, diversion mode and diversion building type; 3. Draw up the construction sequence of diversion structures, the construction methods of removing cofferdam and plugging diversion structures; 4. Formulate flood control and foundation pit drainage measures; 5. Determine the comprehensive utilization measures such as navigation, water crossing and water supply during the construction period. Construction diversion measures are restricted by many factors. A complete scheme needs technical and economic comparison. If necessary, do model experiments, demonstrate repeatedly, and then make a final decision.
The diversion mode is 1. According to the position of river bed, it can be divided into external diversion and internal diversion. Diversion outside the river bed: the cofferdam is used to block the whole river bed at one time, so that the river water can be diverted downstream through diversion and drainage structures outside the river bed; River bed diversion: cofferdam is used to enclose part of the river bed in stages, and the river water is taken away through the narrowed part of the river bed, that is, staged diversion. 2. According to the types of drainage structures, it can be divided into open channel diversion, tunnel diversion, culvert, dam bottom hole, comb teeth and gap overflow, culvert diversion and other diversion methods. Culvert diversion is generally used in small and medium-sized sluices, earth-rock dams and other projects. The bottom hole diversion is adopted in the construction of concrete dam, and all or part of the water flow is led to the downstream through temporary or permanent drainage holes arranged on the dam body. Comb-tooth diversion is to reserve a comb-tooth gap for water to flow through during the construction of concrete dam, and to block the gap in turn with the rise of the dam. Culvert diversion is a construction method using culvert for diversion, which is suitable for rivers with small water diversion or only used for diversion in dry season. Generally used in the construction of earth dams, rockfill dams and other projects. Because too many culvert pipes are unfavorable to the dam body and interfere with the dam construction, it is not advisable to bury too many pipes under the dam and the size of a single pipe should not be too large. Culverts are built on dry land, which is easy to be arranged on the floodplain, and the elevation of the floodplain is above the low water level.
In general, the selection of diversion mode must consider: ① hydrological conditions. River flow, hydrograph characteristics, flood and low water conditions, water level fluctuation and ice flow all directly affect the scheme selection. For rivers with large fluctuation of water level, overflow cofferdams should be adopted in time, and the water retaining height of cofferdams and water diversion and drainage structures only considers the flow in dry season. ② Terrain conditions. If the riverbed is wide and navigation is needed during the construction period, diversion by stages can be adopted; If the river is narrow, open channel or tunnel should be used for diversion according to the terrain and geological conditions. (3) If conditions permit, the drainage structure of permanent hydraulic structures should be fully utilized and combined with construction diversion. For example, diversion tunnel can be combined with flood discharge tunnel, and cofferdam can be combined with earth-rock dam. ④ Meet the comprehensive utilization requirements of navigation, tree crossing, water supply and irrigation during construction.
Design standard According to the construction progress and drainage conditions in each period, the construction diversion can be divided into three stages: ① initial diversion, that is, cofferdam water retaining stage, from river bed closure to dam construction to cofferdam elevation. ② Mid-term diversion, that is, the water retaining stage of dam body. At this time, the diversion and drainage structures have not been blocked, and the dam will block water in flood season. With the elevation of the dam, the reservoir capacity increases and the flood control capacity gradually increases. (3) In the later stage of diversion, that is, from diversion and closure of drainage structures to comprehensive restoration of the dam to design elevation, permanent drainage structures have been put into operation. In the construction diversion design, it is necessary to choose the diversion period, that is, whether to retain water in dry season or all year round during the working duration of the water retaining cofferdam. The diversion period should be reasonably divided and selected according to the hydrological characteristics of the river and the construction characteristics and progress of the main project. The maximum discharge and flood volume of design frequency during diversion should be adopted in diversion design. Small projects should be completed in dry season to simplify diversion facilities; Large and medium-sized projects are generally difficult to build in dry season, so diversion can be considered in different years, and the design flow of diversion is based on the flood flow with a certain frequency throughout the year. The dam body of local material dam is generally not allowed to pass water. When the dam construction is difficult to reach the flood retaining elevation in flood season, the cofferdam elevation and the scale of diversion buildings should be considered according to the annual diversion standard. Concrete dams are usually allowed to pass water, and annual diversion standards or dry season diversion can be considered. When the monthly design frequency is used to arrange the construction progress, it is necessary to fully demonstrate the hydrological characteristics of the river and treat it with caution. The diversion design standard is the regulation of design flow frequency adopted in diversion design. For different diversion stages and different buildings, the specified frequency is also different. The general requirement is that the flood standard in the initial diversion stage can be lower, and the flood standard in the middle and late diversion stage will be gradually improved. When the project is required to play its role in advance (such as power generation in advance), the flood control standard at the corresponding diversion stage should be higher. For concrete and masonry buildings, the flood standard is lower, while for earth-rock dams, the flood standard is higher. On the other hand, the design standard of diversion varies with the level of permanent buildings.
Full-face cofferdam method is full-face cofferdam method, also known as one-time closure method or river bed external diversion. The main river channel is blocked by the whole cofferdam at one time, and the water flow is guided to the drainage structure on the side. It is suitable for projects with narrow riverbed, small working face of foundation pit, acute water depth and thick overburden, and it is difficult to build longitudinal cofferdam and realize diversion by stages. Type I tunnel diversion: suitable for mountain rivers with steep banks, hard rocks, thin weathered layer and narrow valley, or permanent tunnels. Open channel diversion: Open channel diversion is suitable for plain rivers with gentle bank slopes or wide beaches. In mountainous rivers, such as rivers, the shape is obviously asymmetric. Culvert diversion: culvert diversion is mostly used for small and medium-sized earth-rock dams, and the diversion flow does not exceed1000/s. Aqueduct diversion: Aqueduct diversion is generally suitable for small projects in dry season, and the diversion flow does not exceed 20 ~ 30/s, and in some cases it can reach100/s.
Segmented cofferdam defines segmented cofferdam method, also known as staged cofferdam method or river bed diversion. Staging is to enclose the riverbed into several dry construction foundation pits and construct them in sections. Staging is to divide the shunt process into stages in time. Staging is in time, and segmentation is in space. In engineering practice, two-stage and two-stage diversion are the most widely used. Applicability: Under the conditions of wide riverbed, large flow and long construction period, it is easy to meet the requirements of navigation, tree crossing and ice release. type
① Narrow riverbed diversion, bottom hole diversion, notch diversion, comb diversion and workshop diversion.
On the basis of analyzing the subjective and objective conditions related to construction diversion, diversion design divides diversion period, selects diversion standard and diversion design flow, designs diversion closure scheme, determines the type, structure, size and layout of diversion buildings, formulates construction sequence and construction method, dismantles and blocks diversion buildings, formulates flood control scheme, foundation pit drainage scheme and comprehensive river utilization measures during construction, and formulates construction control schedule.
Diversion period, also known as water retaining period, refers to the duration of diversion by relying on cofferdam to retain water in the whole process of water conservancy project construction. Rivers are divided into dry season, intermediate period and flood season according to hydrological characteristics. X small water conservancy project cofferdam only needs to be closed in a dry season, so the cofferdam is low and the engineering quantity is small; Large and medium-sized water conservancy projects are usually difficult to build to the flood retaining height in dry season, especially earth-rock dams are not allowed to overflow, so the diversion period should be considered as a whole year Cofferdams have to be cut off in flood season, so the cofferdam is high and the engineering quantity is large. The concrete dam can overflow the dam body, and the overflow cofferdam can be used when the foundation pit is flooded. When choosing diversion period, we should carefully study the hydrological characteristics of the river channel and make technical and economic comparison according to the characteristics of the main project and the requirements of construction progress. Generally speaking, it is not appropriate to divide the diversion period too finely, and the monthly design frequency flow is only used as a reference when arranging the construction progress plan, which is not suitable as the basis of diversion design.
Determine the flood standard of diversion design flow. Diversion design flow is the main basis for selecting diversion scheme and determining the scale of diversion buildings. During the construction process, the river flow often changes. In order to make the designed diversion flow conform to the actual flow during the construction period as much as possible, countries choose the diversion standard according to the characteristics of rivers, buildings, construction experience, safety and economy. Diversion standards vary greatly in different countries. Considering economy and safety in a unified way, many countries have formulated that diversion buildings with different importance should bear different risks (that is, the probability of an emergency) for reference in determining diversion design flow.
Selection of diversion scheme A complete diversion scheme is restricted by many factors, and it needs technical and economic comparison and determination after repeated argumentation. These factors mainly include: ① hydrological conditions. The size of river flow, the characteristics of flow hydrograph, the range of water level change, the duration of flood and low water, ice flow and freezing in winter are all important factors that directly affect the selection of diversion scheme. For example, in rivers with large fluctuation of water level, overflow cofferdams are sometimes needed to flood the foundation pit for diversion in a short time. Rivers with ice flow in winter should fully consider the problem of ice flow discharge. ② Terrain conditions. The riverbed and topography on both sides of the river in the construction area have great influence on the diversion scheme. For example, the river channel is wide, and there are navigation requirements during the construction period, so it is appropriate to adopt staged diversion; Tunnel diversion is suitable for narrow riverbed, steep bank wall and deep river valley. ③ Engineering geological and hydrogeological conditions. When choosing the construction diversion scheme, the economic rationality of tunnel or open channel diversion, the possible narrowing degree of river bed, the structure and construction method of cofferdam, and the drainage measures of foundation pit should all consider the engineering geological and hydrogeological conditions. For example, in the riverbed with deep overburden and high permeability, the problem of erosion resistance and seepage prevention of cofferdam foundation should be properly solved, and the beam width of rock foundation riverbed is allowed to be larger under the condition of satisfying navigation conditions. ④ Layout and building type of water control project. Considering the construction diversion combined with the layout of the hub, make full use of the structural characteristics of the building. For example, the combination of flood discharge, power generation, ventilation holes and temporary diversion holes, and the combination of cofferdam and flood control. As for the dam structure, it is more likely that the concrete dam will adopt staged diversion. ⑤ Comprehensive utilization of rivers. During the construction period, in order to meet the requirements of navigation, tree crossing, water supply, drainage and irrigation, the construction diversion problem is complicated. For example, when diverting water by stages, the narrow channel should meet the requirements of navigable velocity, gradient (longitudinal slope, transverse slope, local gradient), flow pattern, water flow connection, etc., and sometimes it is necessary to set up a temporary lock; When sealing the hole to store water, attention should be paid to the downstream navigable water level and the needs of normal water use such as irrigation, water supply and hydropower station; For fishery production, fishing facilities should be considered. ⑥ Construction progress, construction methods and site layout. When selecting the diversion scheme, we should fully consider the social impact, the supply of construction equipment and building materials and the construction experience, and strive to simplify the construction diversion project, reduce the cost and shorten the construction period on the premise of ensuring safety. For important construction diversion projects, necessary model test or computer simulation calculation should be carried out, and full comparison and demonstration should be made.
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