Generally speaking, it is necessary to take construction measures such as "guiding, intercepting, blocking, storing and discharging" to guide all or part of the river flow to the downstream safely, or to block it to ensure that the main building of the hub can be constructed on dry land. This is the water flow control in the construction process, which is customarily called construction diversion. Construction diversion runs through the whole process of engineering construction. It not only affects the general construction organization, but also affects the layout of the hub and the selection of permanent building types to a great extent, and becomes an important part of the overall design of water conservancy and hydropower hub. In engineering practice at home and abroad, the role of construction diversion planning is highly valued. Correct and reasonable construction diversion planning can reduce the project cost and speed up the construction progress. Otherwise, it will bring unexpected difficulties to engineering construction and even cause engineering accidents.
2. Construction diversion mode
Construction diversion method, also known as construction diversion method, can be generally divided into two basic methods, namely full-face cofferdam method and segmented cofferdam method. Full-face cofferdam diversion, also known as one-time diversion or river bed diversion. Its basic characteristics are: 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. According to the different drainage structures, the one-time interception method can be further divided into tunnel diversion, open channel diversion, culvert diversion and other segmental cofferdam diversion methods, also known as staged cofferdam method or river bed diversion. However, it is customarily called phased diversion. In engineering practice, two-stage and two-stage diversion are the most commonly used. According to the characteristics of drainage channels in different periods, the diversion methods by stages include narrow riverbed diversion and diversion through completed or unfinished permanent buildings (mainly bottom hole diversion, notch diversion, comb hole diversion, factory building diversion, etc.). ).
3. Key and difficult contents
Understand the special position and function of construction diversion in the construction of water conservancy and hydropower projects, focus on learning the basic methods of construction diversion, understand the main factors affecting diversion methods, understand the division of diversion periods, understand the determination of diversion design standards and diversion design flow, understand the layout and hydraulic calculation of diversion projects, and understand the comparison and selection of diversion schemes. Understand the method of cofferdam plane layout and weir crest elevation determination, and master the basic application types of cofferdam; There are many forms of cofferdams, so it is necessary to master the applicable conditions, structure and layout of earth-rock cofferdams and concrete cofferdams. River closure is closely related to cofferdam, but because river closure plays an important role in construction diversion and is one of the main control items in the overall construction progress, we should mainly master the main problems of vertical river closure method and design. After the cofferdam is closed, it is necessary to start foundation pit drainage and do well in foundation pit drainage, which can create dry land construction conditions. It is mainly to master the arrangement and calculation of initial drainage and regular drainage, as well as the method of open drainage.
What is "segmentation" when cofferdam method is used for diversion? What do you mean by "by stages"? What are the similarities and differences between them?
The so-called segmentation means that the riverbed is surrounded by several dry foundation pits and constructed in segments. The so-called staging is to divide the shunt process into several stages in time. Subdivision is in space, and staging is in time. The diversion series and the number of cofferdam sections are not necessarily the same. The more segments, the more complicated the construction. The more assigned duration, the longer the duration will drag on. Therefore, in engineering practice, two-stage and two-stage diversion are often used.
5. Definition of diversion standard, diversion procedure, diversion period, cofferdam and river closure.
6. What is the diversion design flow? How to determine the design flow of watertight cofferdam? How to determine the design flow of overflow cofferdam in water retaining period and overflow period?
The design discharge of diversion depends on the flood frequency standard and diversion period of diversion design. In the same diversion period, the higher the diversion design standard, the greater the diversion design flow. Under the same diversion design standard, the flow in flood season is greater than that in dry season, so the maximum value in diversion period is usually taken as the diversion design flow. For overflow cofferdam, if the cofferdam needs to retain water all the year round, as long as the return period standard of diversion flood is selected according to the specification, the corresponding design flow can be determined. The design flow of diversion and retaining structures is the same as that of drainage structures. For the low-flow cofferdam, the flood with the same frequency in the water retaining period is taken as the design flow of the cofferdam and diversion and discharge structure in this period. For overflow cofferdams, the characteristics of overflow cofferdams are both water retaining and overflow, and their working conditions include water retaining and overflow (similar to overflow dams). Obviously, the design standard of overflow case should be the same as that of general overflow cofferdam for flood control all the year round. This standard is mainly used for stability analysis and structural calculation of weirs, and also for checking the water capacity of all diversion channels. The design standard of water retaining is generally based on the principle of not exceeding water in dry season. However, the specific choice of water retaining flow should be determined by technical and economic comparison.
7. How to determine the position of the longitudinal cofferdam when the sectional cofferdam method is used for diversion?
Choosing the position of longitudinal cofferdam is actually to determine the appropriate river bed bundle narrowness. The suitable position of longitudinal cofferdam is related to the following main factors. (1) Topographic and geological conditions (2) Hydraulic layout (3) Narrow beam allowed in riverbed (4) Diversion and water flow requirements (5) Rationality of construction layout. The above five aspects are only the main problems that should be considered when selecting the position of longitudinal cofferdam. If the natural river course is symmetrical and there are no obvious favorable topographic and geological conditions available, the economic comparison method can be used to select the appropriate longitudinal cofferdam position to minimize the total cost of the first and second stages of diversion.
8. What are the factors that affect the choice of diversion mode?
(1) Hydrological conditions. The hydrological characteristics of rivers have great influence on the selection of diversion modes. Each diversion mode has a suitable flow range. Besides discharge, discharge hydrograph, ice condition and sediment characteristics also affect the selection of diversion mode. For example, in rivers with short flood peaks and sharp peaks, foundation pits may be flooded in flood season; Rivers with high sediment concentration are generally not allowed to flood the foundation pit. Narrow riverbed and wide river channel are conducive to ice discharge; Tunnels, culverts and bottom holes are not conducive to ice discharge. If it is used for ice discharge, it should be open flow during the ice flow period, with enough clearance and the orifice size should not be too small. (2) Topographic and geological conditions. For wide plain rivers, staged diversion or open channel diversion should be adopted. Tunnel diversion is often used in mountainous rivers with narrow valleys. The topographic and geological conditions suitable for each diversion mode have been discussed before, so I won't repeat them here. (3) Hub type and layout. Staged diversion should be adopted for concrete dam hub. Because the earth dam should not be built by stages, and the dam body is generally not allowed to pass water, the earth dam hub hardly needs to divert water by stages, but adopts a one-time interception method. The late diversion of high-head water control project often needs the combination of various diversion methods, and the diversion procedure is complicated. For example, in a concrete dam in a canyon, tunnel diversion can be used in the early stage, but drainage holes arranged at different elevations of the dam are often used in the later stage (completion period). In the early and late stages of high-head earth-rock dams, multi-layer diversion tunnels are generally arranged at different elevations on both banks. If there are permanent drainage structures in the hub, such as tunnels, culverts, bottom holes, diversion canals and sluices, they should be used as much as possible. (4) Requirements for comprehensive utilization of rivers. Staged diversion and open channel diversion can easily meet the requirements of navigation, crossing trees, releasing ice, crossing fish and supplying water. In order to meet the navigation requirements, some rivers should be divided into three, four or even eight grades, not just two grades. In some canyon areas in China, tunnel diversion was originally designed, but in order to meet the requirements of crossing trees, open channel diversion was used instead of tunnel diversion. In this way, not only the problem of deep excavation of high slope is encountered, but also the diversion procedure is complicated and the construction period is greatly extended. It can be seen that it is not easy to solve the problem of comprehensive utilization of rivers when choosing diversion mode.
9. How to compare the construction diversion schemes?
(1) Before the comparison of construction diversion schemes, several basically feasible diversion schemes should be drawn up. When drawing up the scheme, first consider whether the possible diversion mode is phased diversion or one-time interception. How many stages, how many segments and which bank to encircle first should be studied in staged diversion. It is also necessary to study the way to complete the diversion in the later stage, whether it is to use bottom holes, combs, gaps or unfinished factories; Whether tunnels, open channels, culverts or aqueducts are used for primary interception, and on which river bank the tunnels or open channels are arranged. In addition, whether the foundation pit is allowed to be submerged, whether to use overflow cofferdam, and whether to block it in stages or once. On the basis of comprehensive analysis, the obviously unreasonable schemes are eliminated, and several feasible schemes or possible combination schemes are retained. When the diversion mode or large scheme is basically determined, the basic scheme should be further refined. (2) When comparing the schemes, the technical and economic indexes of the schemes are analyzed and calculated. We should focus on the following aspects: the cost and economy of water diversion project; Rationality of construction intensity; Balance of labor, equipment and construction load; Construction period guarantee, especially the time for river closure, installation, water storage, power generation or other benefits; Feasibility of comprehensive utilization of rivers during construction; Reliability of construction diversion scheme implementation, etc. Therefore, when comparing the schemes, the following work should be carried out, including hydraulic calculation, engineering quantity calculation and cost calculation, preparation of construction progress plan, calculation and analysis of construction intensity index, possibility and effect analysis of comprehensive utilization of rivers, etc. (3) Scheme comparison According to the above technical and economic indicators, comprehensively consider various factors, weigh the advantages and disadvantages, and prioritize. Qualitative analysis and quantitative comparison are carried out, and finally the technically reliable, economical and reasonable implementation scheme is selected. In the process of comparison and selection, it is forbidden to make subjective assumptions and rashly determine the scheme. In the comparison of diversion schemes, the specified completion period should be taken as the unified benchmark. On this basis, a technical and economic comparison is made. We should not only pay attention to economic rationality, but also pay attention to technical feasibility and schedule reliability. Otherwise, there is no economic rationality. In a word, we should adopt the principle of the best overall economic benefit. If conditions permit, modern system analysis methods should be adopted.
10. What problems should be paid attention to in the plane layout of cofferdam? What problems should be paid attention to in determining the elevation of cofferdam crest?
The plane layout of cofferdam is a very important problem. If the plane layout is improper and the scope of the retaining foundation pit is too large, it will not only increase the amount of cofferdam work, but also increase the capacity of drainage equipment and drainage costs; If the scope is too small, it will hinder the construction of the main project and affect the construction period; If the outline of the cofferdam with staged diversion is improper, it will also cause poor drainage, scour the cofferdam and its foundation, and affect the safe construction of the main project. The plane layout of cofferdam mainly includes two problems: outline and space in weir. The contour is not only related to the layout of diversion and discharge structures, but also depends on the type of cofferdam, geological conditions and the consideration of anti-scour measures. The space in the weir, that is, the size of the foundation pit, mainly depends on the outline of the main project and the corresponding construction method. The upstream and downstream cofferdams are generally not perpendicular to the center line of the riverbed when diversion is conducted by segmented cofferdams. The plane layout of cofferdam is often trapezoidal, which can not only make the water flow smooth, but also facilitate the layout and connection of transportation roads. When the one-off diversion method is adopted, there is no prominent water flow problem in the upstream and downstream cofferdams. In order to reduce the engineering quantity, cofferdams are mostly perpendicular to the main river channel. The elevation of the crest of the longitudinal cofferdam should be adapted to the water surface curve of the weir side. Usually, the top surface of longitudinal cofferdam is stepped or inclined, and the elevation of its upstream and downstream is the same as that of the corresponding transverse cofferdam. The height of weir crest on icy river should still consider the backwater height caused by ice jam and ice dam. When earth-rock cofferdam is used, the top of impervious body of cofferdam should be slightly higher than the static water level. The superelevation value of inclined wall is 0.8 ~ 0.6m；; The core wall type is 0.6 ~ 0.3m 。
1 1. What are the main problems in river closure design?
The main problems of river closure design are: the determination of river closure period and design flow; Selection of axis and intercepting position of intercepting dike; Selection of interception methods; Hydraulic calculation of river closure; Determination of material type, size and quantity; River closure construction planning; Hold your breath and stop water, etc.
12. Discuss the relationship between diversion, closure, cofferdam and foundation pit drainage.
Construction diversion runs through the whole process of engineering construction. Construction diversion planning is mainly to divide diversion periods, select diversion standards and determine diversion design flow. Select diversion scheme and types of diversion water retaining and discharging buildings, and determine the layout, structure and size of diversion buildings; Draw up the construction and demolition of diversion and water retaining structures and the plugging method of drainage structures. Cofferdam is the main diversion and water retaining structure, and there are many forms of cofferdam, among which earth-rock cofferdam and concrete cofferdam are the most commonly used structural types. River closure is closely related to cofferdam. Usually, the closure dike is a part of the cofferdam. River closure is an important part of construction diversion and one of the main control items in the total construction progress. After the cofferdam is closed, raised and thickened to form a cofferdam, the foundation pit drainage begins. Doing a good job of foundation pit drainage can create conditions for dry land construction. It can be seen that the construction diversion runs through, and there is an obvious obedience relationship among river closure, cofferdam and foundation pit drainage, that is, the logical sequence relationship determined by the construction process.
I believe that after the above introduction, everyone has a certain understanding of the diversion technology of the twelfth construction. Welcome to Zhong Da for more information.
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