1 analysis of the causes of leakage
various forms of leakage may occur in water conservancy projects, and finding the root cause of leakage is the key to solve the leakage problem. Common causes of leakage in water conservancy projects include the following aspects.
1.1 structural water seepage
both point water seepage and surface water seepage in hydraulic engineering belong to structural water seepage, in which point water seepage is often caused by local holes in hydraulic structure. For example, at the joint between the main project of hydraulic structure and other sub-projects, honeycomb pits or holes are left on the concrete surface due to loose vibrating pouring of concrete or loose welding of water-stop rings, and then point water seepage occurs, which has little impact on the whole hydraulic structure and is easy to handle. Surface seepage generally occurs in the bottom plate of hydraulic structures. The reasons are as follows: firstly, dewatering foundation pits are generally located around the base surface and below the cushion, so it is difficult to remove water in time if the requirements are not met and in order to ensure the construction progress; Second, the pouring concrete can't reach the design strength due to uneven mixing or non-compact vibration, which leads to water seepage.
1.2 construction cracks
construction cracks include construction joints and deformation joints. In the construction process, concrete is generally divided into several construction units, and the construction joints between each unit are common leakage parts. When handling construction joints, the operation must be strictly in accordance with the specifications: thoroughly remove the impurities on the joint surface, and ensure that the supporting formwork is reliably connected and firmly supported without slurry leakage. In the process of hydraulic structure construction, if the concrete is not compacted by vibrating, the water stop is not firmly fixed or improperly fixed, deformation joints may occur, causing leakage.
2 waterproof and plugging construction technology
2.1 cutoff wall construction technology
cutoff wall construction technology refers to the construction technology of continuously making holes in loose and permeable foundation by using mud to fix the wall, and constructing cutoff wall by pouring concrete or other impervious materials under the mud to play an anti-seepage role (see Figure 1). The cutoff wall established by this technology has the advantages of small thickness, low permeability coefficient, good flexibility, good durability and low construction cost, so it is widely used in seepage control and plugging of hydraulic structures. According to the different wall-forming technologies, the construction technology of cutoff wall can be further divided into water-jet wall-forming, grooving wall-forming, multi-head mixing concrete wall-forming and other technologies.
2.1.1 wall-forming technology by water jet
The equipment needed in the wall-forming construction by water jet includes pouring machine, drilling machine, mixer, etc., and this technology is suitable for sandy soil, clay and gravel layer with particle size below 1mm. At the beginning of the construction, the high-speed water jet from the nozzle of the shaper in the drilling machine cuts and trims the hole wall, and forms a layer of protective mud to protect the wall. The excess mud is discharged from the hole through positive and negative circulation. The wall thickness formed by water jet method is generally between .22 and .45 m, and the depth is less than or equal to ≤3m m..
2.1.2 grooving wall-for≤4ming technology
The equipment used in grooving wall-forming technology is mainly grooving machine, which is mainly composed of power system, chassis, rotating system, lifting bracket, slag discharge pressurization and electrical system, etc. When working, firstly, the guide rod of grooving machine moves up and down according to the working speed of .8~1.5m/h to guide the hole for grooving, and the residual soil is discharged.
2.1.3 multi-head mixing concrete wall-forming technology
multi-head mixing concrete wall-forming construction is simple and low cost, and it is suitable for sandy gravel soil, clay and muddy soil. During construction, the drill pipe of the multi-bit mixing pile machine is drilled into the soil layer in advance, and the mud is injected through the drill pipe and mixed with the soil to make it fully coagulate into cement-soil piles, and each cement-soil pile is further connected to form a cutoff wall. The wall depth is ≤22m, the permeability coefficient of cement-soil piles is < 1 cm/s, and the compressive strength is ≥.3Mpa.
2.2 High-pressure jet grouting technology
High-pressure jet grouting technology is a kind of grouting treatment method for soft bottom layer that appeared in the early 197s, and then it has been widely used in seepage control and plugging reinforcement of water conservancy projects. High-pressure jet grouting technology uses a drilling rig to make holes, lowers a special alloy grouting pipe and nozzle to a predetermined position in the stratum, and uses high pressure to spray slurry and water vapor to the surrounding stratum, so as to punch, stir and squeeze the stratum, realize the replacement, filling and mixing of the slurry, and form a condensate in the stratum, which plays a role in seepage prevention and plugging (see Figure 2). This technology has wide adaptability, good slurry controllability during construction and simple operation, which shows good economic and social benefits in large-particle strata, flowing water, silt strata and rockfill dams.
2.2.1 Construction preparation
During the construction preparation stage, indicators such as mud specific gravity and water-cement ratio must be guaranteed, and construction equipment such as grouting pipe, high-pressure nozzle, air compressor and flowmeter should be properly prepared, and cement with strength ≥32.5Mpa should be prepared, and the water-cement ratio is generally 1: 1.
2.2.2 Construction stage
From the completion of construction preparation to the start of construction, it is necessary to level the site, position the drilling rig and excavate the drainage ditch. In order to ensure the construction quality, the deviation between the actual drilling position and the design position should be ≤5cm, and after the positioning of the drilling rig, the original slurry should be poured to the design elevation and drilled with a special drill pipe, and then the injection axis should be adjusted and the pipe should be poured to the design elevation. High-pressure jet grouting construction must be carried out in strict accordance with the technical specifications, and the control of technical parameters in the construction process should be strengthened. Because pulling out the casing after grouting to the design elevation will accelerate the solidification and contraction of the slurry, which will lead to the sinking of the orifice, high-pressure jet grouting must generally be carried out with secondary static pressure grouting, and the slurry surface will no longer sink as a reference.
2.2.3 construction process
high-pressure jet grouting construction process, in which when preparing slurry, all kinds of materials must be measured according to the designed dosage, and the clay should be soaked for about 8 hours, and all kinds of materials should be fully stirred for at least 3 minutes, and the properly stirred slurry should be filtered through a 35cm2 hole screen. During grouting, we must pay close attention to and record the grouting situation and grouting quantity in detail. If cracks are found, we must repeat grouting according to the specifications. When the slurry inflow into the grouting hole is less than 4mL/min, the grouting will be finished. If the slurry swelling occurs in the hole, the unit slurry inflow will not reach 4 ml/min for a long time, but when there is a large slurry return, the slurry inflow should be controlled at 1L/min and maintained for 5min.
2.2.4 Precautions
Before high-pressure jet grouting construction, representative geological conditions must be selected and indoor mixture ratio test must be carried out, and construction can only be carried out after accurate construction parameters such as hole spacing, hole-forming method, hole depth, pressure value and slurry flow are determined. For the holes with large slurry consumption and high requirements for slurry concentration, the slurry concentration can be appropriately increased, and the slurry can be rotated and lifted immediately after returning to normal, or a specific proportion of accelerator can be directly mixed into the slurry to shorten the setting time. In case of leakage in the process of pore formation, it is necessary to backfill with sand and gravel with a certain gradation to reduce the porosity, then carry out grouting construction, and spray after the slurry condenses. If the injection process is interrupted for more than 3min, it is necessary to record the interruption point, and ensure that the spray bar extends to the interruption point 4cm during re-injection, so as to facilitate lap joint, and then spray grouting after hole sweeping treatment.
3 Key points of quality control in waterproof and plugging construction
Engineering practice shows that there is a direct relationship between the leakage of water conservancy projects and the construction quality. In order to ensure the construction quality, it is necessary to establish an engineering quality assurance system, strengthen the quality awareness of constructors, strictly control the construction process, and comprehensively use various leakage prevention measures under the principle of "combining plugging with prevention, combining rigidity with flexibility, and comprehensive management" to enhance the impermeability of hydraulic structures. With the continuous progress of science and technology, there are more and more kinds of waterproof and impervious materials. Before using impervious materials, it is necessary to fully understand the performance and properties of various materials, select impervious materials with different characteristics for different leakage situations, and it is unrealistic to completely solve all kinds of leakage problems only by relying on impervious materials. It is necessary to combine various seepage prevention and plugging technologies to solve all kinds of seepage problems.
4 Conclusion
Because of the long construction period, large scale, high technology content and many influencing factors, water conservancy projects require higher construction technology than other projects. As the key construction technology of water conservancy projects, waterproof and plugging technology is directly related to the success or failure of the whole project. Therefore, we must actively explore new technologies and processes for waterproof and plugging of water conservancy projects in combination with existing experience.
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