Optimization and Discussion on waterproof and anti

Optimization and Discussion on waterproof and anti-corrosion design of immersed tube tunnel

optimization and Discussion on waterproof and anti-corrosion design of immersed tube tunnel

November 30, 2018

the construction method of immersed tube tunnel is being paid more and more attention by the majority of engineering and technical personnel. With the increasing investment in municipal foundation projects in China, the construction method of immersed tube tunnel will have a broader application field. In addition to the existing immersed tube tunnels such as Ningbo Changhong and Shanghai Outer Ring Road, the immersed tube tunnels under construction include Guangzhou Biological Island University city tunnel, Tianjin Central Avenue Haihe tunnel and Putuo Shenjiamen subsea tunnel. It is believed that in the near future, more immersed tunnels will appear in China's rivers, lakes and coastal areas. The author has participated in the waterproof design of Ningbo Changhong immersed tube tunnel, Shanghai outer ring immersed tube tunnel, Tianjin Central Avenue Haihe immersed tube tunnel and Guangzhou Biological Island University City immersed tube tunnel. Through the design and consultation of the above projects, the author puts forward the following optimization measures for the waterproof design technology of immersed tunnel, hoping to provide some reference for the design of immersed tunnel in the future

1 pipe section is wrapped with waterproof layer

1.1 pipe section bottom plate is wrapped with waterproof layer

due to the corrosivity of a considerable part of rivers in China, the waterproof and anti-corrosion of immersed tunnel pipe section body is a key problem to be solved. If the immersed tunnel adopts the all inclusive waterproof design, the steel plate is a common waterproof measure for the bottom plate of the pipe section at present. The steel plate can be used as both the formwork for concrete pouring and the waterproof layer for the bottom plate. Moreover, the steel plate has good bonding performance with the concrete, ensuring that the two become a whole. However, the fatal weakness of the steel plate is its poor anti-corrosion durability in corrosive environment. Although the anti-corrosion performance of the steel plate surface can be improved by painting anti-corrosion coating, the welding between the steel plates will still damage the anti-corrosion coating, and the coating repair is very difficult. The steel plate is usually welded on its inner surface to closely combine with the concrete by welding the anchor bar. If the steel plate is corroded and causes perforation after several years, the corrosive medium will directly invade the interior of the concrete structure along the anchor bar, which will further cause the corrosion of the concrete reinforcement and eventually cause adverse consequences to the whole structure. In addition, considering the cost, the cost of steel plate is high, which has a great impact on the cost of the whole project. By consulting relevant foreign materials, the author believes that it is also a feasible method to use plastic PVC waterproof board as the outer waterproof layer of the bottom plate. The durability of the waterproof layer is theoretically guaranteed by using PVC waterproof board. As the PVC waterproof board is an organic material, it is not affected by corrosive media in rivers and oceans. It will only corrode under the erosion of organic solvents, but the probability of such cases is very low

the PVC waterproof board laid on the bottom plate of the pipe section is generally a flat plate with ribs. See Figure 1 for the sectional structure of the ribs. The thickness of the flat plate is about 2 mm and the height of the ribs is about 10 mm. The structural form of the rib not only makes it close to the concrete, but also has no adverse impact on the binding of the bottom plate reinforcement and the control of the concrete protective layer. The PVC waterproof board is mainly provided with longitudinal ribs, and a horizontal rib shall be set every 25 M. the spacing of longitudinal and horizontal ribs limits the possible leakage to a local range and prevents the occurrence of large-area water channeling. PVC waterproof boards are connected by hot melt welding. At the joints of vertical and horizontal waterproof boards, necessary tools can be used to cut the rib section to ensure the flatness of the rib joints and the welding quality (Fig. 2)

when the waterproof board is laid to the corner between the bottom plate of the pipe section and the side wall, it shall be turned up to the longitudinal horizontal construction joint along the side wall formwork for end treatment. In addition, an externally attached waterstop of the same material as the waterproof board shall be welded along the longitudinal direction at the end of the waterproof board to strengthen the waterstop effect at the end of the waterproof board. The end of the waterproof board to the end steel shell also needs special treatment. It is an ideal solution to fix the waterproof board with battens and bolts. There is no need to set ribs for the end waterproof board. Screw holes are preset on the end steel shell. The end waterproof board is overlapped with the end steel shell with a certain width. The corresponding screw holes are also reserved at the end of the waterproof board, and then the end of the waterproof board is compressed through the batten and bolts, that is, the sealing of the waterproof board is completed

the height difference between the waterproof board and the end steel shell shall be chamfered with prefabricated cement mortar, so that the waterproof board can be smoothly laid to the inner surface of the end steel shell. In addition, both sides of the preset screw hole of the end steel shell, the preset screw hole and the bolt surface shall be fully coated with one component polyurethane expansion sealant in advance, and then the bolts shall be tightened, which seals the water seepage channel between the end waterproof plate and the end steel shell, and between the bolts and the screw holes (Fig. 3)

while the waterproof board meets the durability of the waterproof layer wrapped on the bottom plate of the pipe section, it also has some defects, especially the material determines that it cannot withstand high temperature and sparks. Therefore, attention should be paid to the protection of the waterproof board during the construction of the pipe section. Specific protective measures can be taken as follows: 1) complete the welding and binding of reinforcement outside the waterproof board in advance, and then hoist the reinforcement cage on the waterproof board. 2) Pre lay a layer of fine sand on the waterproof board, completely cover the waterproof board, and then weld and bind the reinforcement on the fine sand. After that, wash the fine sand away from the foundation surface of the waterproof board with water

1.2 waterproof coating for the top plate and side wall of the pipe section

at present, polyurethane waterproof coating is generally used for the top plate and side wall of the pipe section. However, considering the actual situation of the construction site and the corrosive medium in the environment where the immersed tunnel is located, spray polyurea waterproof coating is recommended. Spray polyurea waterproof coating is a new type of waterproof coating gradually known to designers in recent years. This material was originally mainly used as an anti-corrosion coating on the surface of concrete and steel members in seaport engineering. Now, through the modification and optimization of the product, researchers have made it meet and exceed the performance requirements of relevant waterproof coatings, and have the following significant advantages:

1) fast curing speed. Polyurea is a solvent-free coating. When sprayed on the base surface, it can form a gel in a few seconds to ten seconds. It can be sprayed continuously at one time to make the coating reach the thickness specified in the design without falling. It can also meet the strength requirements for walking in 10 minutes, which is conducive to improving efficiency and shortening the construction period

2) the material is not sensitive to temperature and humidity, and is not prone to blistering in scientific research and industry. Therefore, the requirements for construction environmental conditions are low, which is convenient to improve the construction quality of waterproof and anti-corrosion coatings

3) spraying construction can be carried out on the base course with variable section, and a continuous, seamless, integral and dense coating can be formed, which is conducive to ensuring the quality of waterproof and anti-corrosion works

4) the coating has good stability to temperature and can be used for a long time at -50~120 ℃

5) it has the characteristics of high tensile strength, high elongation, strong adhesion, good waterproof, anti-corrosion, wear resistance and aging resistance

6) the coating has good adhesion to concrete, steel, aluminum, wood and other substrates

7) it does not contain any volatile organic compounds and has 100% solid content. It is a new environmental protection material

8) it can be constructed on the wet interface

9) good impact strength, no protective layer is required after coating construction

the performance index of spray polyurea waterproof coating shall meet the requirements of the latest national standard spray polyurea waterproof coating (gb/t 23446-2009). There are differences in performance index between type I and type II coatings in the standard, but there is no difference in application field. Considering the durability requirements of immersed tunnel, type II coating should be the first choice for waterproof coating outside the pipe section. Before coating construction, the concrete base surface shall be painted with its matching closed primer in advance. The construction thickness of spray polyurea waterproof coating is generally 1.5 mm, and it should have a certain overlap width with the waterproof layer turned up on the bottom plate. In addition, since the steel cover plate of the pipe section top plate into the hole is welded underwater, its outer surface cannot form a continuous closed waterproof layer. Therefore, when the waterproof layer is finally repaired, divers can dive to the steel cover plate and apply Epoxy Mastic materials to the steel cover plate by hand

2 Gina, Omega waterstop and their pressure parts system of pipe section joints

2.1 Gina waterstop and its pressure parts system

gina waterstop is the first and most important line of defense for pipe section joints. Its material is generally a mixture of styrene butadiene rubber and natural rubber. It can also be adjusted according to the different geological areas where the tunnel is located. For example, Gina waterstop of pipe section in earthquake prone zone, Pure natural rubber should be used to improve the physical properties of Gina waterstop

the designer shall select the corresponding Gina waterstop model according to the pressure compression deformation curve of Gina waterstop. In the process of waterstop selection, Gina waterstop model should be selected according to the principle that the water pressure borne by the joints of each pipe section and the corresponding compression amount are in the initial compression linear stage of the pressure compression deformation curve (Fig. 4), and the remaining compression value after removing the total axial deviation has a large surplus of lecture time. The above selection principles ensure that the water pressure and corresponding compression of Gina waterstop are within a controllable range

due to uneven settlement between pipe sections, Gina waterstop often produces offset compression. At this time, Gina waterstop is completely squeezed on one side of the pressing part system. According to the inspection results of the previously damaged Gina waterstop, in the case of offset compression, if there are protruding bolt heads and other components in the pressure part system, the GINA waterstop body will be damaged and its service life will be seriously affected. Therefore, in the design of Gina waterstop pressure parts in the future, the structure form that will not cause great damage to the GINA waterstop body must be adopted (Fig. 5)

2.2 Omega waterstop pressing part system

in the past, the pressing part system of Omega waterstop mainly used the combined cap nut welded on the end steel shell, and then during the installation of Omega waterstop, the side and end of Omega waterstop were pressed by means of pressing plate and bolt fixed in the nut to achieve the waterproof effect. The fixing method of welding the combined cap nut first and then tightening the bolt requires high positioning accuracy of the combined cap nut and brings inconvenience to the on-site fixing of the bolt. Therefore, it is recommended that the bolts be welded to the end steel shell first, and then the side ends of Omega waterstop shall be compacted by means of nuts, washers and pressing plates. The above improvements make the welding position of bolts clear at a glance and bring considerable convenience to the operation on the construction site (Fig. 6)

in addition, considering the complex conditions at the construction site, which may cause damage to local Omega waterstop, it is recommended to set a protective cover along the bottom plate and side wall to the outside of Omega waterstop at a certain distance above the pavement elevation

3 overlapping of embedded water stop in horizontal and vertical construction joints and steel plate water stop in longitudinal and horizontal construction joints

the waterproof measures for horizontal and vertical construction joints of pipe section are mainly to set up embedded water stop. The waterproof measures for longitudinal and horizontal construction joints are generally to set up steel plate water stop. In the past, at the joint of horizontal and vertical construction joints and longitudinal and horizontal construction joints, waterproof materials formed closed waterproof lines respectively. Now, in order to ensure the unity of the whole waterproof system, the middle buried waterstop and the steel plate waterstop are connected in the following ways: the steel edge of the middle buried waterstop is overlapped with the steel plate waterstop in a certain width,