In deep foundation pit engineering, correct waterproof construction is critical to the overall structure, and it will also have a great impact on the service life of the building. Therefore, the waterproof project occupies a very important position in the construction process of deep foundation pits. This article mainly combines the deep foundation pit construction process characteristics of Nanning Metro and Hangzhou South Railway Station Station Project to research and analyze the deep foundation pit waterproof technology, hoping to provide a certain reference value for similar projects in the future.
First, the envelope structure is waterproof
(I) Water-stop characteristics of various envelope structures
A vertical retaining structure around a deep foundation pit is generally called a retaining structure. The envelope structure is a prerequisite to ensure the safe excavation of deep foundation pits. Deep foundation pits use many structural forms, and their construction methods, processes, and construction machinery used are different. The water stopping effects obtained by various construction methods are not the same, see Table 1 for details
(Two) precautions for waterproofing of ground-to-wall construction
The foundation pit construction of Nanning Station of Nanning Metro adopts a ground-connected wall structure. The ground-connected wall has a better waterproof effect. The construction process is similar to the bored pile. The following points should be noticed.
1. The main point of the waterproof quality card control is the joint treatment between the two walls. If the main points of the joint treatment construction can be grasped, a good waterproof effect will be obtained.
2. After the groove is formed, the end surfaces of adjacent concrete should be cleaned, and the wall should be brushed to the bottom, and the number of brushes must not be less than 20, until no mud is left on the wall brush.
3. Before lowering the reinforced cage, install a small pipe at the end of the reinforced cage along the wall. During the installation, strictly control the quality of the joints to prevent the leakage of the pipe and block the pipe. During excavation of the foundation pit, if water leakage is found at the joints of the wall, grout from small pipes.
(III) Key points for waterproofing of cast-in-situ piles
Part of the envelope structure of Hangzhou South Railway Station adopts the form of bored cast-in-situ pile + high-pressure rotary jet pile curtain. Controlling the construction quality of the high-pressure rotary jet pile water-proof curtain during construction is the key point of waterproofing. During the construction of the water-stop curtain, it is necessary to strictly control the distance between the piles, the quality of the slurry and the spray pressure to ensure that a sealed waterproof belt is formed around the cast-in pile to obtain a good waterproof effect.
Control of excavation of foundation pits
During excavation of the foundation pit, the leakage of the retaining structure may be caused by improper handling of the nodes of the retaining structure. To avoid accidents caused by leakage of the retaining structure, the following points should be noted during the excavation of the foundation pit:
1. During the excavation process, blind excavation is strictly forbidden, and close attention is paid to changes in the water level around the foundation pit and the seepage of the surrounding structure. If water inrush occurs during the excavation, backfill the water in time to prevent instability due to expansion. The corresponding method shall be adopted Excavation can be continued after processing. 2. Timely treatment of small-scale leakage. Treat the concrete surface cleanly, use high-strength quick-setting cement to dilute the surface of the wall, and use a small pipe to drain, to prevent the leakage area from expanding. After the plugged cement reaches the strength, use a grouting machine with grouting pressure to place the small pipe. Blocking.
Third, the main structure is waterproof
The main structure waterproofing is the most important part of deep foundation pit waterproofing. By controlling in the following aspects, the main structure can achieve a good waterproof effect.
(I) Concrete quality control
The quality of concrete is a prerequisite to ensure the waterproofing of the structure. The designer of the choice of raw materials and the proportion of the mix guarantee the supporting conditions of the quality of concrete.
The aggregate shall be inspected and accepted in accordance with the Standards for Quality and Inspection Methods of Sand and Stone for Common Concrete, including the mud content, mud content, needle-like content, and particle gradation. On the premise of satisfying the strength and workability, the lowest sand content rate is used to ensure that there is sufficient coarse aggregate in the concrete. The mix ratio of the concrete components should meet the strength requirements of the concrete structure design, durability under various environments, and work performance such as the concrete mixture to have the flow performance adapted to the construction conditions. The concrete mixture should have uniform, easy-to-compact and segregation resistance, which is the prerequisite for improving the quality of the concrete, so it is necessary to fully ensure the workability of the concrete.
(Two) construction control
1. Concrete treatment. Construction joints are formed at the junction of new and old concrete. After chiseling, the joint area of new and old concrete is effectively increased, which not only improves the continuity of the concrete, but also helps the wall to resist bending and shearing. Before the concrete is poured, the paddles are now laid, and then the cement-based impervious crystalline material is painted. Cement-based impervious crystalline materials can well bond the gaps between concrete and prevent outside water from invading.
2. Installation of steel water stop. The water-stop steel plate should be buried in the middle of the cast concrete structure layer, and the bends at both ends should face the water-facing surface. The water-proof steel plate with construction joints in the back wall of the external wall should be placed in the middle of the concrete external wall, and it should be installed vertically and the welding circle of each horizontal water-proof steel plate should be tightly welded. After the horizontal elevation of the horizontal steel plate waterstop is determined, a line should be drawn through the upper opening of the steel plate waterstop according to the elevation control point of the building to keep its upper mouth straight.
The steel plate is fixed by welding with steel bars, and the oblique steel bars are fixed by welding on the top mold stick. Weld short steel bars under the steel plate waterstop to support the steel plate. The length should be based on the thickness of the steel mesh of the concrete slab wall and should not be too long to prevent the formation of water seepage channels along the short steel bars. Generally, the short steel bars are not more than 200mm apart, and one left and right is set. If the distance is too small, the cost and engineering amount are increased. If the distance is too large, the steel waterstop is easy to bend, and it is easily deformed by vibration when pouring concrete.
The joints of steel plates are welded. The overlap length of two steel plates is not less than 50mm, and both ends should be fully welded. The height of the weld is not less than the thickness of the steel plate. Trial welding should be performed before welding, and the current parameters should be adjusted. If the current is too large, it is easy to burn or even burn through the steel plate. If the current is too small, the arc will be difficult and the welding will not be firm.
3. Installation of water-proof swelling water stop. Before laying the water-swelling water stop, sweep the scum, dust, sundries, etc. to leak the hard base. After pouring the ground and horizontal construction joints, expand the water-swelling waterstop strips along the extension direction of the construction joints, and use its own adhesiveness to stick directly to the middle of the construction joints. The joint overlap should not be less than 5cm, and no breakpoints should be left. For the construction joints on the facade, the positioning shallow grooves should be reserved first, and the waterstop strips should be embedded in the reserved grooves. If there is no reserved grooves, it can also be fixed with high-strength steel nails, and the self-adhesion can be used to directly paste the construction joints On, encountered evenly compacted release paper. After the water stop is fixed, you can pour the concrete by tearing off the release paper.
4. Concrete vibration. The concrete vibration time and method must be correct. It must be compact and compact but not vibrating and leaking. During the vibration process, the mortar splashing should be minimized, and the mortar splashing on the inner surface of the template should be cleaned in time. The vibration points of the concrete are divided from the middle to the edge, and the rods are uniformly distributed and the buckles are layer by layer. Every part of the concrete pouring should be continuously poured. The vibration time of each vibration point should be based on the concrete surface being floating, flat, and no more air bubbles emerging, usually 20-30s, to avoid segregation caused by excessive vibration.
The concrete pouring should be carried out continuously in layers. The plug-in vibrator should be inserted quickly and slowly, and the insertion points should be evenly arranged and arranged in a plum shape. The vibrator of the upper layer of concrete should be inserted into the next layer of concrete 5-10cm, so that the two layers of concrete are firmly bonded. The direction of the vibrating sequence is as far as possible from the direction of the concrete flow, so that the vibrated concrete can no longer enter free water and air bubbles. The vibrator must not touch the embedded parts and formwork during the vibrating process.
5. Maintenance. After the concrete is poured, it should be covered and watered within 12 hours to keep the concrete in a wet state. The curing period is generally not less than 7 days. For areas that cannot be watered, the curing agent should be used for curing, or a protective film should be sprayed directly on the concrete surface after dismantling the mold, which can avoid curing and improve durability.
Fourth, waterproof layer laying
Although the waterproofing of deep foundation pits is mainly self-waterproofing of concrete, the waterproof layer laying also plays a vital role in the waterproofing engineering of deep foundation pits. Strictly controlling the quality of the waterproof layer construction is the key point for the waterproof construction.
(I) Basic surface treatment
Before laying the waterproof layer, the base surface should be effectively treated, mainly for flatness and water seepage treatment. The leakage of water on the base surface should be treated by plugging. The treated base surface must be clean and free of pollution, water drops and water.
(Two) laying quality of waterproof layer
1. The waterproof membrane must have a factory certificate, and 名媛直播 of qualified quality can be used. The foundation for waterproof construction should be flat, dry, clean, firm and sand-free. 2. Before the waterproof layer is applied, the position of the yin and yang corners of the grassroots should be treated. The position of the yin and yang corners should be rounded. The diameter of the yin corner should be greater than 50mm and the diameter of the yin corner should be greater than 100mm. 3. The construction of the waterproof layer must be performed in accordance with the specifications and design requirements. 4. Construction joint position treatment, determine the concrete pouring height, and perform waterproof reinforcement treatment on the construction joint position. 5. Protective layer should be constructed in time after laying the base waterproof layer to avoid damaging the waterproof layer when the waterproof layer is burned, punctured and vibrated during the welding of steel bars.
V. Conclusion
The penetration of underground engineering and the common problems of waterproofing have seriously affected the overall construction quality of the structure, but it is not inevitable. We mainly clarify the idea of "design is the prerequisite, materials are the foundation, construction is the key, and management is the guarantee". In the construction of waterproof projects, the construction quality of each process is strictly controlled, and targeted preventive control measures are adopted to achieve the expected goals.
