The waterproof design principle of general railway tunnels is "combination of prevention, interception, drainage and blockage, comprehensive treatment according to local conditions", and underground excavation method is generally adopted in subway construction. The waterproof design level of underground excavation subway project is relatively high, with the station being Grade I and the interval being Grade II. The underground station has a large passenger flow and a variety of metal equipment, which requires absolute impermeability and leakage. Its waterproofing is mainly based on "prevention" and "blocking", combined with other measures. "Prevention" means strengthening the waterproof performance of the secondary lining, and setting a flexible waterproof layer between the secondary lining and the primary support to prevent the communication between the station and groundwater; "Blocking" means grouting and filling in the stratum and lining to prevent the flow path of groundwater during tunnel construction, thus playing a waterproof role.
The design lining of Dongdan subway station is a composite lining, and a fully enclosed flexible waterproof layer is set between the primary support and the secondary lining. C20 early strength shotcrete is used for initial support, C30 waterproof concrete is used for secondary lining, and the impermeability grade is S8. The flexible waterproof layer adopts two kinds of waterproof boards, the main body of the station adopts ECB waterproof board with a thickness of 1.2mm, the entrance, exit and air duct adopt EVA waterproof board with a thickness of 0.8mm, and the cushion adopts polyethylene (PE) with a thickness of 4 mm.
1, waterproof concrete lining structure includes primary support and secondary lining, the primary support generally adopts shotcrete, and the secondary lining generally adopts shotcrete.
The waterproof performance of sprayed concrete is low, so it is generally not considered in areas with less water seepage, and the focus of waterproof is on secondary lining. Secondary lining is generally designed as impervious concrete, which is an important part of self-waterproofing of structures. Strictly control the material inspection, mix design, transportation and pouring, form removal and maintenance to ensure the waterproof performance of concrete.
1. 1 material requirements
(1) The cement is ordinary portland cement ≥425, with alkali content ≤ 0.6%;
(2) Medium sand should be used for sand, with silt content ≤ 3%;
(3) The maximum particle size of stone is ≤40mm, which meets the requirements of pumping and net spacing of reinforcement, with silt content ≤ 1% and water absorption ≤1.5%;
(4) The admixture is UEA expansion agent, and the dosage is determined by the limit expansion μ=0.5%;
(5) Water is non-corrosive and clean.
1.2 mixed design
(1) The mixture ratio of waterproof concrete must be determined by experiments, and its impermeability grade should be 0.2MPa higher than the design requirements;
(2) The cement dosage per cubic meter of concrete is ≥320kg (including UEA expansion agent); The appropriate water-cement ratio is 0.55;
(3) The sand ratio is 35% ~ 40%, and the ratio of lime to sand is (1∶ 2.5);
(4) the slump is180 ~ 210 mm;
(5) In order to improve the workability of concrete and reduce the water-cement ratio, the superplasticizer is appropriately added, and the dosage is controlled at 0.8% of the cement dosage; In order to reduce the hydration heat of concrete, fly ash can be properly mixed.
The concrete of subway tunnel structure is transported by 1.3 commercial concrete, transported by concrete truck, pumped and poured. Due to the influence of traffic control, the subway concrete pouring construction is generally carried out at night, and the one-time pouring quantity is large. Pump pipes are transported vertically through vertical shafts, with many elbows and generally long transportation distance. Therefore, the concrete delivery pump pipe should adopt high-pressure pipe, and its joint should be tight and airtight. Before pumping, first hit a plate of mortar with the same cement-sand ratio as pouring concrete, one wet pipe, and two check whether the pipe joints are leaking.
Generally, the distance from the mixing station to the construction site is far, and the transportation time is long, so the workability of concrete will change greatly. Before unloading from the tank car to the pump house, the tank car should be backed up for 3 ~ 5 minutes, so that the concrete in the tank can be mixed evenly again. If the workability still cannot meet the pumping requirements, the same superplasticizer solution can be added appropriately, but it is forbidden to add water.
1.4 the pouring height of the concrete on the side wall is >: 2m, and the string tube and chute shall be used, and the plug-in vibrator shall be used. The vibrating time shall be 10~20s ~ 20s, which shall be subject to the fact that the concrete begins to pan-grout or has no bubbles. The pouring thickness of each layer is ≤300mm, the vibrator is inserted into the lower concrete ≥50mm, and the displacement distance is not greater than 1 times the action radius. When pouring arch concrete, pouring windows should be reserved on the formwork on both sides, and the concrete should be poured horizontally and evenly on both sides. Using surface vibrator, the thickness of each pouring is 200mm, and the displacement overlaps with the poured concrete 100 mm.
1.5 Formwork removal and maintenance of waterproof concrete should strictly control the time of formwork removal, so as to ensure the strength of concrete and the temperature difference between concrete surface temperature and surrounding air ≤20℃. This is very important, and it is often overlooked because it is eager to build. Early formwork removal exposes concrete to the atmosphere, and internal stress causes concrete to crack due to excessive temperature difference with its surroundings. After form removal, water shall be sprayed in time to maintain the surface moist, and the maintenance period shall be ≥ 14 days.
2. Flexible waterproof layer The flexible waterproof layer is divided into two layers. The cushion layer is placed in contact with the surface of the initial support to protect the side of the waterproof board in contact with the initial support, and then the waterproof board is hung on its surface.
The flexible waterproof layer not only plays a waterproof role, but also separates the primary support from the secondary lining due to the smoothness of the waterproof board, and basically does not transmit shear force between them, but only transmits pressure, so that the secondary lining is uniformly bent under pressure and cracks caused by local stress are reduced. Therefore, the waterproof layer has high requirements on the base surface, and there can be no protrusions or clear water, so the base surface should be treated before the waterproof layer construction.
2. 1 After the initial support of basement treatment is completed, the basement surface should be treated and the rebar head and anchor head should be removed. The internal angle of the cut part, section change or corner shall be painted as an arc with r ≥ 10 mm, and the obviously uneven part shall be leveled by spraying in layers. If there is open water on the base surface, measures should be taken to plug or drain it with grouting to facilitate the construction of waterproof layer.
2.2 Pave the cushion with 75 thermoplastic circular washers and nails, and fix polyethylene (PE) on the primary support. Nails are 50cm away from the arch, and the side walls 100cm are arranged in a plum blossom shape. The nail spacing at the changing section and corner should be properly encrypted. Paving from the vault to both sides shall be smooth, and the overlapping length of long and short sides shall be more than ≥50mm, so as to be smooth, free from bulge and firm in weld.
2.3 Laying waterproof boards ECB and EVA boards are laid from the vault to both sides in the circumferential direction, and they are hot-melt welded with annular gaskets. Waterproof board should not be stretched too tightly when laying, and appropriate lap allowance should be left. The flashing and the circular gasket are welded by voltage welder or hot air plastic welding torch. Waterproof board shall be welded by double-seam self-climbing heat sealing welder, with overlapping width of long and short sides 10cm, and weld width ≥ 10mm, which shall be uniform and continuous, without false welding, missing welding and penetration.
2.4 During quality inspection, connect the 5# injection needle with the pressure gauge, and inflate it between the double welds with a pump. When the pressure reaches 0. 1 ~ 0. 15 MPa and the holding time is ≥ 1min, the weld is qualified. If the pressure drops, it proves that something is not welded well, and repair welding should be implemented.
3. Waterproofing of special parts Special parts, including construction joints, deformation joints, expansion joints and through-wall pipes, are the weak links of structural waterproofing, and should be operated in strict accordance with the construction technology. If improper operation often leaks water, it is very troublesome to rectify it.
3. 1 Construction joints shall be set between two pieces of concrete. Before pouring, the construction joints should be treated. Firstly, remove the cement slurry film, loose stones or weak concrete on the surface, and then set a water-swellable rubber strip in the middle of the second lining. The base surface of the construction joint must be chiseled, so that the two concrete pouring contacts are good and possible cracks are reduced.
3.2 There is no deformation joint within the whole length of the main body of the station structure, and it is only set at the junction between the station and the section, entrance and exit, air duct junction, open excavation and underground excavation junction and other main bodies and ancillary structures. Rubber waterstop is set in the middle of deformation joint lining, polystyrene board is filled between joints, and the mouth is blocked with two-component polysulfide rubber and waterproof mortar. The bridge rubber waterstop is used as the waterstop, with the thickness of 10mm, and it is buried in the middle of the lining 200mm away from the inner edge of the second lining. The water stop adopts lap joint, and the stubble pressing direction is consistent with the water flow direction, that is, the upper water stop is close to the initial support and the lower water stop is close to the inner edge of the lining. The surface of the water stop lap must be brushed with a file. The lap length is 10cm, and it is completely bonded with adhesive.
3.3 Expansion joint subway runs at constant temperature and is less affected by temperature changes. Expansion joints are only set at the mouth of the cave and places where it is easy to freeze.
3.4 The wall-penetrating pipe shall be coated with R 1 material and polyurethane coating, and bonded with ECB waterproof board through SBS coiled material, or welded with water-stop steel plate ring on the wall-penetrating pipe, and the surface shall be treated with anti-corrosion. When wall bushing is used, waterproof packing should be embedded between the two pipes, flange should be pressed on the inside, and waterproof layer should be laid on the outside.
4, filling grouting
If there are holes in the waterproof board, groundwater will penetrate into the secondary lining, and the concrete itself is not dense, so water will penetrate into the surface of the secondary lining. Filling and grouting behind the tunnel can not only fill the remaining space caused by insufficient concrete pouring, so that the primary support and the secondary lining can be closely attached and stressed together, but also fill the gap caused by the incompact or cracking of concrete and block the flow channel of groundwater, thus playing a waterproof role. Filling and grouting behind the hole can avoid the appearance defects formed when the inner surface of concrete is treated. Before pouring concrete in the arch, the grouting pipe is embedded in the vault. Grouting pipe adopts 42 seamless steel pipe, with the upper end 2cm away from the lower waterproof board of initial support and the lower end passing through the formwork 10cm. The steel pipe is straight, and there is no need to drill holes in the pipe, and it is arranged in the middle of the tunnel arch of each lining cycle and where water seepage may occur. When the concrete strength reaches 70% and forms a completely closed circle, it can be started.
Grouting slurry can be cement mortar and cement slurry, and a certain proportion of waterproof agent and fly ash can be mixed when necessary. Generally, cement mortar is injected first, and the consistency is thick first and then thin. The mixture ratio and water cement ratio should be adjusted according to the grouting speed to control its diffusion within a certain range. If the slurry spreads quickly, the consistency of the slurry can be increased, or it can be carried out after stopping spraying for 8 ~ 12h. If the gap is small, direct grouting can be used, but the stone rate of cement slurry is lower than that of cement mortar, and the grouting amount is large but the effect is not obvious. If only there are cracks on the lining surface, but grouting after the hole still can't solve the leakage, we can directly drill holes and inject chemical grout into the lining surface at the leakage place. However, the timeliness of chemical slurry is not as good as that of cement slurry, so it is not recommended to use it in large quantities in lining. Grouting sequence should be from bottom to top, from water-free and water-deficient area to water-rich area, from downhill direction to uphill direction, and from both ends to the middle. The grouting length of each section is generally ≤20m. Filling grouting can make up for the defects of waterproof board and secondary lining, and it has been widely used in Dongdan subway station and achieved good results. It is an indispensable step in fully enclosed waterproof construction.
5. Key points of seepage control technology in subway engineering
(1) Base surface is the basis and key to ensure the quality of waterproof board, and there can be no dew and protrusions.
(2) Butt rubber water stop in construction joints is difficult to attach to concrete in actual operation, and cracks are easy to form when pouring and vibrating. It is suggested to install an ordinary non-expansive rubber strip slightly wider than the water stop on the end formwork before pouring, pull out the rubber belt from the concrete after formwork removal, and install an expandable water stop in the formed groove.
(3) Strengthen the tensile strength and elongation of the cushion, and choose nonwoven or composite geotextile, which not only has the functions of filtration, isolation, drainage, acid and alkali resistance, but also can effectively protect the waterproof board.
(4) In addition to meeting the strength index, the waterproof board should also focus on testing its acid and alkali resistance and ultraviolet resistance, and strengthen the protection of finished products, especially to prevent the damage of welding slag to the waterproof board. Due to the erosion of groundwater and the addition of alkaline accelerator, the surface of sprayed concrete often oozes alkaline water to form crystals, which is very corrosive to human skin. When waterproof board is constructed, joints are often reserved, and some of them are exposed to underground humid air, resulting in corrosion. After a period of time, the solderability is greatly reduced. In addition, the waterproof board exposed to the air should be protected from sunlight and covered with shading before sealing.
(5) Concrete pouring should be as full and dense as possible, and filling grouting is an indispensable remedial measure for water plugging, which cannot be omitted or ignored.
(6) Waterproof concrete shall strictly implement the precondition of form removal, and the influence of temperature on concrete cracks shall be considered.
6. Concluding remarks
In Dongdan subway project, secondary lining, flexible waterproof board, special parts and filling grouting form a comprehensive waterproof system. The combination of "prevention" and "blocking" is used to control groundwater, and satisfactory results have been achieved. After repeated inspection and acceptance by the construction unit, no leakage was found. The waterproof construction technology of subway underground excavation project has certain reference value for the totally enclosed waterproof construction of other underground excavation projects.
For more information about project/service/procurement bidding, and to improve the winning rate, please click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd
Model essay on personal summary of annual assessment of public security civil servants 1
Time flies, th