Dike is a water retaining structure built along the edge of rivers, canals, lakes, coastal areas or flood discharge areas, flood storage areas and reclamation areas. It is the earliest important flood control project widely used in the world. The purpose of embankment construction is to prevent floods and protect the lives and property of coastal residents and industrial and agricultural production. After the river bank restrains the flood, the flood water will be confined in the spillway, which increases the depth of the same flow and the flood discharge speed, which is beneficial to flood discharge and sediment discharge. Dikes can also be reclaimed as farmland. Increase agricultural production. Dike can also resist wind and waves, sea and lake. When building a dike, the flood retaining function should be analyzed and planned in detail to ensure that the spillway meets the design flood control requirements.
1 Main types of dike accidents in water conservancy projects
Dangers of dikes include seepage damage, landslide, cracking and uncontrollable, among which seepage damage is the main one. Seepage damage mainly includes concentrated leakage, piping, soil flow, contact erosion and contact soil flow. The danger of seepage damage of dikes can be divided into three categories: (1) The danger caused by dike body is mainly caused by uneven material composition and uneven filling density, such as silt, sandy loam or holes and cracks in some dike sections, which are mainly manifested as loose leaching, off-slope, loopholes, falling pits, etc. (2) Danger caused by the contact zone between the dike body and the dike foundation. Because the foundation is unclear when the dike is built, the materials in the contact zone between the dike body and the dike foundation are mixed; (3) There are dangers caused by sand layer and sandy loam layer with strong water permeability in the embankment foundation.
2 Selection of seepage control construction scheme for dikes of water conservancy projects
(1) The impervious body such as impervious wall, cone-probe grouting and split grouting can be used for the seepage control treatment of the embankment. When necessary, it can also help the dike to thicken the dike body or excavate and rebuild the dike body.
(2) For the cutoff wall of the dike, the key is to use thin-walled and cheap materials to effectively reduce the project cost. At present, the commonly used slotting method, deep-layer method and extrusion method can meet this requirement, among which the deep-layer method has the lowest cost and the most competitive when the wall depth is less than 20 m, and the high-pressure jet grouting method is relatively expensive, but it has better adaptability in some cases where the construction site is narrow and there are many underground obstacles.
For the stratum with high sand and gravel content and large particle size, impact drilling and other trenching methods should be considered, and of course, the cost of building walls will also be greatly increased. According to the characteristics of dike engineering, other measures such as covering counterweight, drainage and decompression, and filtration protection can also be considered for the treatment of fat leakage in such dangerous strata.
3 water conservancy project embankment seepage control construction technology
Before 1970s, the technology of dam seepage control and reinforcement in China was backward, and the method was single. Traditional measures such as clay blanket, clay cutting and grouting were mostly adopted. After the 1970s, especially since the reform and opening up, through a large number of engineering practices and scientific experiments, the anti-seepage reinforcement technology has developed rapidly, the construction equipment and methods have been continuously improved, and the construction efficiency has been continuously improved. The construction level of cutoff wall in China has entered the advanced ranks in the world. At present, the rapid development and wide application of cutoff technologies are: concrete cutoff wall, high-pressure jet cutoff wall, self-hardening mortar cutoff wall, vertical plastic paving, cement-soil mixing pile cutoff wall, curtain grouting and so on. It is of great practical significance to grasp the development trend of these technologies in time for the design and construction of water conservancy projects.
3. 1 concrete cutoff wall
Concrete cutoff wall is a vertical seepage control technology developed in the early 1960s, and now it has become the main seepage control method for granular stratum. It can be used not only for seepage prevention of permanent foundation, but also for seepage prevention of leakage dams, temporary construction cofferdams and foundation pits. Its main advantages are that the wall thickness can be effectively controlled, the wall sections are closely combined, and it is safe and reliable.
At present, the construction technology of cutoff wall has made great progress, and many new technologies for building walls and making holes have appeared. In terms of wall materials, there are reinforced concrete, ordinary concrete, plastic concrete, self-hardening mortar and solidified mortar. In terms of depth, the depth of concrete cutoff wall of Maniwagen No.3 earth dam in Canada has reached131m; The maximum depth of concrete cutoff wall of Xiaolangdi Dam in China is 8 1.9m, and the maximum depth of the second-stage cofferdam of Three Gorges Reservoir is 73.5m The depth of concrete cutoff wall of dam foundation of Xiabandi Water Control Project in Xinjiang implemented in 2003 has reached 102m. In the aspect of pore-forming machinery, there are forward and reverse circulation impact drills, grab machine, chain bucket (scraper) trencher, water jet trencher, grooving machine, vibrating sheet pile, vibrating grooving, vibrating film sinking, etc. At present, the depth of shallow cutoff wall is generally around 10~20m, with a maximum of ≤30m and a thickness of 10~25cm and a maximum of ≤30cm. This kind of cutoff wall can be used not only for seepage control of earth-rock dams with small water head and dam foundation thickness ≤30m, but also for river embankment projects. Deep cut-off walls are generally used for embankments with a water head of more than 20m and a wall depth of more than 30m and dike works in dangerous areas. Its thickness is generally 60~80cm, and the maximum is 130cm. In order to ensure the effective connection of the bottom wall section, the deeper the wall, the greater the thickness. The wall materials are mostly ordinary concrete or plastic concrete, which are determined according to the stratum and seepage control requirements. Generally speaking, the greater the water head and water permeability, the higher the required anti-seepage performance and the greater the stiffness of the wall.
3.2 High pressure jet cutoff wall
High-pressure jet cutoff wall is to disturb the dam foundation overburden by high-pressure jet impact and inject cement slurry at the same time, so that the slurry is mixed with soil particles injected into the stratum to form cutoff wall. In recent years, based on many years' practice of high-pressure jet grouting, Wang Mingsen and other scientific and technical personnel of Shandong Research Institute of Water Resources have studied the construction technology of cutoff wall in large-size stratum, and formed the construction technology of high-pressure jet grouting in large-size stratum characterized by the reasonable choice of high-pressure jet grouting and high-pressure jet grouting, which has been popularized and applied in many projects and achieved good results. In "Study on Construction Technology of High-pressure Jet Grouting Cut-off Wall in Large Strata", Wang Mingsen thinks that comprehensive technical measures must be taken in order to complete the task economically, quickly and with high quality. Using DTH hammer casing to track hole-making is an ideal technology to solve the problem of hole-making in large particle formation.
3.3 Self-setting mortar cutoff wall
Self-hardening mortar cutoff wall is developed on the basis of plastic concrete wall. "Self-setting mortar" made of cement, bentonite and a small amount of retarder can be used as wall-fixing mud for hole-making before setting, and self-setting after completion to form a wall with anti-seepage and reinforcement functions. This technology has been used in the United States, French and other countries, and China is still in its infancy.
3.4 Curtain grouting
Curtain grouting is to pump slurry with a certain proportion and fluidity into the cracks of rock stratum by drilling holes. After cementation and hardening, the strength of rock foundation is improved, and the integrity and impermeability of rock foundation are improved. In our country, the orifice sealing grouting method is often used. With the construction of Ertan and Xiaolangdi projects, some internationally effective construction methods, such as GIN grouting method and bottom-up pure pressure grouting method, were introduced to China, which promoted the development of grouting technology in China. GIN (grouting strength number) method was first put forward by Lombardi, a former chairman of the international dam conference and a Swiss scholar. Xia Kefeng of China Water Conservancy and Hydropower Foundation Engineering Bureau pointed out in the article "Application of New Foundation Treatment Technology in Water Conservancy and Hydropower Engineering": "The basic concept of g in method is that the energy consumption of grouting in any hole section is a constant value, and the value of this energy consumption is approximately equal to the product PV of the final grouting pressure p of the hole section and the volume V of injected slurry, and PV is called the grouting strength value, that is, GIN. Due to the grouting of fractured rock mass, large cracks often have a large amount of grouting and low use pressure, while small cracks often have a small amount of grouting and high use pressure. Therefore, if GIN is controlled to be a constant in the whole grouting process of each grouting section, the injection amount can be automatically limited to cracks with large opening, and the grouting pressure can be increased for dense sections with poor grouting performance. GIN grouting automatically takes into account the actual irregularity of rock geological conditions, so that the total grouting amount can be distributed reasonably along the curtain body. GIN method has been applied in some European and American countries and achieved good results.
China introduced it in 1994, and carried out grouting tests at Jiangya Water Control Project in Hunan and Three Gorges Water Control Project in the Yangtze River. On the basis of full grouting test, the Xiaolangdi Water Control Project on the Yellow River put forward the method of grafting GIN on the basis of orifice sealing method, taking the advantages of the two methods, and applied it to the anti-seepage curtain project, and achieved satisfactory results.
At present, various new seepage interception methods are constantly emerging, which are developing in the direction of greater depth, higher efficiency and wider adaptability. In the future, efficient construction methods and the combined application of different construction methods still have broad development prospects. The complex geological conditions, various dam-building materials and various operating conditions of water conservancy projects determine the different characteristics of seepage. According to the geological conditions, dam quality and design head of different projects, comprehensive analysis should be made from the aspects of safety, economy, benefit and feasibility to determine the appropriate seepage control and reinforcement scheme, and relevant construction parameters should be determined through experiments, and other projects should not be blindly copied.
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