Waterproof Control Measures for Subway Construction

1.Importance of waterproofing in subway construction

In numerous influencing factors of subway construction quality, water leakage is the main factor and problem. Scientific waterproofing measures becomes the priority, but builders and researcher in subway construction cannot realize it now. Today, more attention from society are payed into the waterproofing technology in subway construction, with frequent flood damages in recent years. Meanwhile, the impacts of groundwater erosion and penetration cause the increase of maintenance costs in operation period and also reduce the life of the subway project.

Lack of understanding of waterproofing importance, difficulty in subway construction, complex waterproofing technology and the high quality requirements are longtime barriers in water leakage solution. Nowadays, leakage prevention technology got great progress under the rapid development of rail transit construction in major cities. Important significance can be drawn from the collection and sorting data of the positive role of waterproofing technology in construction accident prevention and safety performance optimization.

2.Main principles of subway waterproof construction

2.1.Overall waterproof

Engineering staff, lack of understanding and full attention, usually only do the waterproofing measures in a key place of the subway and ignore the other parts. However, the consciousness showed in which, subway waterproof is an systematic project, should be strengthened in all builders’ thoughts. Actually, overall waterproof means the whole process of waterproof construction, including the planning of owner, design of designer, construction of builders, operation maintenance of subway operation department. So, only guided by the integrated planning, high-quality management and strict examining system, the quality of subway waterproof can be guaranteed basically and completely.

2.2.Maintainable waterproof construction

Easy maintenance and repair and enduring waterproof performance is necessary for subway waterproofing construction. The whole system will be in a good situation because any part of waterproof construction built can be maintained or exchanged when there is problem in operation period. Aiming at the new idea and requirement of maintainable waterproof construction in subway raised based on current waterproof problems, the following tasks should be finished:

Carefully maintain the waterproof layer, keep the drainage system without foreign body or mud, which clog.
Scientific waterproof measures of lining joints is required for easy judgment of penetration source and convenient exchange of waterproof materials. Maintainable waterproof can be realized through a number of process technologies such as the setting controllable network drainage system, embedding grouting facilities and modern partition waterproofing technology. Under effective control the groundwater in lining back, the leakage can also easily determine, repair. Then an easy maintainable system and excellent subway waterproof function are achieved.

2.3.Dynamic waterproof

It realizes the waterproof function through material dynamic waterproof, which relies on the self-repairing capability of material, and dynamic waterproofing technology, which depends on the structural deformation conformance. According to the the groundwater conditions or structural deformation in subway project, with the usages of special technology or benefits from natural features of waterproofing materials, active waterproof comes true.

3.Main Technical Measures for Waterproof Construction

3.1.Self-waterproof of concrete structure

Its main idea is improving the compaction of concrete. It is very crucial to have a scientific construction process and to choose waterproof materials for reducing and preventing gaps in concrete structure.

The Shrinkage and cracking of concrete is usually found, so it is prior to guarantee the good impermeability, strong bleeding, high corrosion resistance, low hydration of cement for the production of waterproof concrete, which is also a strong waterproof effective protection. In addition, moderate expansion agent can be added to prevent the contraction of concrete, which can effectively prevent gaps due to the concrete converge. Moderate plasticizer and expansion agent added can enhance the workability and strength of concrete. Retarder can be added in grouting process to extend the initial condensate time, and segregation and inside joint will be reduced. Maintenance work is the second important process after the completion of the subway project. It requires the maintenance of concrete strength is reasonable, timely and scientific.

3.2.Seam waterproofing technology

1)Waterproofing technology for construction joint

The quality and service time are deeply affected by construction gaps problems which are also the key problem of self-waterproof of concrete structure. So it is required that material, performance and specification of steel side water stop is strictly based on the design shows. Water stops should be jointed smoothly and strongly with lap of splice less than 10cm, without any bubble or crack. For ensuring the smooth installation of steel side water stop which is embedded in the pre pouring pouring, post pouring concrete, each steel rims is 1/2 bandwidth; the horizontal and longitudinal construction joints are firmly fixed on the steel bar by using wire, construction joints use concrete pouring and then manually installed, at this time, the falsework also needs temporary reinforcement. It is necessary to process the circumferential and longitudinal horizontal construction joint waterstops with the required length, the cruciform joints used in the welding operation to be connected with the circumferential construction joints must be special-purpose. In order to prevent the phenomenon of concrete pouring in the template around the sealing is not tight, it is needful to ensure that the end of the construction joint is firmly fixed and do a thorough chisel, remove the debris, clean up the concrete surface of the first pouring work. In the process of pouring lining concrete operation, the concrete on both sides of the vibrating water stop can be increased by inserting the tamping device. From the above operating technique, the construction joint waterproof operation technology in the subway waterproof construction also occupies the important position.

2)Movement joint processing technology

The movement joints is a kind of structural crevice, and the optimum width is usually 20mm~30mm. The reason for setting the movement joints is that expansion and shrinkage phenomenon of the concrete structure.

Here are the common treatment measures for movement joints:

The base seal waterstop has a raised rack, which is laid on the outer part of the concrete movement joint can achieve the sealing function after injecting the concrete into a tight fit with the raised rack. The grouting pipe with a grout hole on the surface is firmly fixed on the inner side of the outermost rack of the two wings of the base seal waterstop, in this way, the gap between the rack and the concrete can be compacted and filled by using slurry to actualize the sealing function.
The buried rubber waterstop and steel side water stop both can be used in movement joint to achieve waterproof effect, in addition, we can also make use of the sealant to caulk movement joint to achieve the sealing function.

3)Waterproof of post-cast strip

Galvanized steel side water stop is installed in the middle of the two sides of the joint, that is, waterproof of post-cast strip. First, the surface debris is brushed with steel brush, and then painting the interface agent on the clean surface, finally, the concrete used for pouring must have micro expansion property. It's important to note that interface agent and concrete should be in accordance with the standard proportion, which can effectively improve the waterproof function, besides, the grouting pipe is embedded at the back surface of the parallel steel plate sealing strip with the spacing of 20 mm and the grouting mouth is arranged according to the spacing 8 m~13 m. Furthermore, the drainage effect must be paid attention to in the construction project, in order to achieve the desired waterproof function effect, it is necessary to closely combine drainage and waterproof engineering. When the shotcrete support network is used in the subway project, the drainage ditch can be set on the periphery of the structure to ensure that the construction face maintains a dry state to the greatest extent, and the influence of the ground water is reduced as much as possible.

4.Problems existing in waterproof construction of subway and suggestions

Local seepage, various forms of seepage, the construction joints and cracks of the main structure are common problems in subway waterproofing construction.

4.1.Water Percolation of construction joint and inducing joint

The common cause of the problem: the material itself such as water stop steel plate, water stop belt and so on has the quality question; the lack of the stability or positioning of the waterstop belt , bubble, gap because the contact between concrete and water seal is not good. To solve the above problems, we can take the following solutions: The side walls of the induced joints and the construction joints are carefully leveled with microcrystalline cement mortar; it is necessary to locate the water stop steel plate and the water stop firmly and accurately; and the vibration of the concrete should be enough.

4.2.Leakage of cracks in continuous concrete wall

In order to solve the leakage of cracks in diaphragm wall, first with a chisel to dig out the leaking places of concrete, cleaning up the mud around the leaking point, the grouting pipe, take the pulp tube placed in its surrounding, the final grouting plugging with double fast cement, and the entire grouting pipe plugging operation until the slurry overflow from the slurry pipe to stop the end.

4.3.Roof cracks and contraction joints

Most of the design ignores the factors that cause cracks, but only emphasizes the strength and impermeability of concrete, such as the shrinkage of the lining wall, the temperature stress and climate formed by the hydration heat of concrete, etc.. Once the roof cold cracks and contraction joints are produced, in order to avoid shrinkage cracks formed due to hydration heat of concrete, usually replaced by slag cement with low hydration heat. Closed roof openings can effectively prevent concrete from cracking due to convection of cold air, the chemical grouting method is adopted to improve the waterproof treatment of the structure, and using the low modulus polysulfide sealant.

5.Conclusion

The complex waterproof construction technology operation which is not only in the subway construction, the process involves more, but also put forward high requirements for the details, therefore, strengthening the overall control is very important in the whole subway construction. In the actual construction, even if the small parameters must be strictly controlled, in addition, the corresponding management measures must be formulated, in this way, the subway waterproofing work can be done more perfectly, and the quality of subway construction can be effectively guaranteed, which will contribute to the development of subway engineering.

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Waterproof Control Measures for Subway Construction

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