What are the construction technologies for piles in the middle of the super-large foundation pit at Heping Road Station of Tianjin Metro Line 3? Below, Zhongda Consulting will introduce relevant content for your reference.
1 Project overview: Tianjin Metro Line 3 Heping Road Station has 3 underground floors and the property development part has 4 underground floors. The main body is a multi-span rectangular frame structure. The foundation pit is about 150m long, 140m wide, with an excavation depth of 21.46m and a shield tunnel depth of 22.61m. The enclosure structure is an underground continuous wall with a wall thickness of 1m and a wall depth of 40.05m. The main body of the station runs from northeast to southwest, and is connected to the planned Heping Road Station of Metro Line 4. The station plan is shown in Figure 1. The foundation pit is constructed using the cover-and-excavation reverse method. Each layer of structural slabs adopts permanent steel columns based on bored piles as the support system. Bored piles and steel columns are constructed before the construction of the structural roof. The construction of the structural slabs is completed. Finally, the temporary columns also serve as part of the permanent columns. Reinforcement bars, formwork, and concrete are poured around the steel columns according to the size of the structural columns to form permanent columns. The cross section of the station foundation pit is shown in Figure 2. 2 Engineering and Hydrogeological Conditions The strata in the station area are mainly artificial fill layers of the Fourth System and Holocene, the first continental layer, the first marine layer, the second continental layer, the third continental layer, and the second marine layer. , the IV continental layer. The groundwater is Quaternary pore water and exists in the artificial fill layer ①, the first continental layer ③ and the first marine layer ④. The second continental layer ⑤1 silty clay and ⑥1 silty clay It is a water-proof floor. The phreatic groundwater level is relatively shallow, approximately 1.03-1.93m (elevation 0.68-1.52m) during the survey period, and has obvious changes between high and low water periods. The slightly confined water uses the ⑤1 silty clay and ⑥1 silty clay of the II continental phase layer as the water-isolating roof. ⑥2 silt, ⑥4 silt, ⑦2 silt, and ⑦4 silt are the main water-bearing strata, with large thickness and relatively stable distribution. Each aquifer is partially sandwiched with lens-shaped silty clay. The water level of slightly confined water is not greatly affected by the seasons, and the water level changes slightly. 3. The construction plan of the intermediate pile column determines that the intermediate pile column is a load-bearing component that bears the weight of the underground structure, the superstructure and the construction load together with the underground diaphragm wall before the bottom plate is sealed and stressed during the inverse construction method. It is also the key to the inverse method. step. There are many methods of erecting columns. The most commonly used ones are steel columns with cast-in-place piles inserted at the bottom and concrete-filled steel tube intermediate support columns. After research and comparison, it is proposed to use the construction method of inserting steel columns first. 4 Construction methods and control points 4.1 Technical requirements The pile foundation of this project is divided into two types: 1 400mm and 1 000mm. The deepest drilling depth of 1 400mm pile is 83.65m, and the drilling depth of 1 000mm pile is 36.85m. The verticality of the pile is required. <0.3%. The steel columns on the piles have 3 cross-sectional sizes of 650mm-650mm, 800mm-600mm and 800mm-800mm. They are connected and welded by 4 └200-24 decorative plates. The lattice columns are inserted into the pile concrete for 2m, and the positioning deviation is ≤2cm. . The key to construction lies in how to ensure the positioning accuracy of the lattice columns, thereby ensuring the stability of the support system and the safety of foundation pit excavation. 4.2 Construction technology According to the technical requirements of the piles and the engineering geological conditions of the site, the bored piles are drilled using a positive circulation rotary drilling rig. The construction process of bored piles and steel columns is shown in Figure 3. 4.3 Construction control points 4.3.1 Ensure the verticality of the holes. To ensure the installation accuracy of the steel structure columns, the verticality of the drilling should be ensured first, especially in the section from the opening to the depth of 30m. Control from the following aspects: ① The casing is processed uniformly, with a thickness of ≥ 6mm to ensure that it will not deform after multiple uses; ② The pile center is positioned using the cross method, and the pile center is extended to fixed points on the surrounding ground for inspection and verification at any time; ③ During the drilling process, check the horizontality of the drilling rig and the verticality of the drill pipe every time a drill pipe is drilled. At the same time, closely monitor the inclinometer and make adjustments at any time; ④ When the deviation is serious, backfill sand and clay to more than 0.5m of the offset hole. After the sedimentation is dense, drill again. 4.3.2 Mud performance control During construction, mud quality is a very important factor, both in terms of hole-forming quality and ultimately the pile bearing capacity. The mud performance control indicators and test methods are shown in Table 1. 4.3.3 Reinforcement cage processing Due to the high bearing capacity requirements of the pile foundation, the designed reinforcement is 40?28, which is arranged in two layers, inside and outside, which reduces the clearance inside the pile and increases the difficulty of rebar cage processing and steel column positioning.
Therefore, the processing of the steel cage requires that it be made in sections, assembled as a whole, and hoisted into the holes in sections; the main bars are connected with mechanical sleeves to facilitate docking at the hole; the main bars and stirrups are spot welded to ensure rigidity and safety during hoisting. 4.3.4 Processing of Steel Structure Columns Steel structure columns are processed on a specially made platform. First, two angle steels are lengthened and fastened tightly on the inner mold of the processing platform. The two angle steels are welded and formed into a shape using a patch plate. On one side of the steel structure column, two pieces are welded and formed into a steel structure column. In addition to the steel structure processing specifications and standards, the welding of steel structure columns must be carried out in the steps of spot welding → interval welding (1~2cm) → full welding. During the processing, if there is local deformation of the lattice column, tools such as oil top or chain hoist can be used to adjust the shape. Specific accuracy requirements: ① The steel structure columns are made of Q345B steel; ② The steel structure columns are extended by single-sided groove welding, and the welding seam is two-level; ③ The bending sagittal height of the processed column body is ≤ H/12 (H is the column length), and ≤ 12mm; ④ Column body distortion ≤ h/250 (h is the longer side of the section), and ≤ 5mm; ⑤ The diagonal error of column section size is controlled within ± 4mm. 4.3.5 Installation and positioning of steel structure columns 1) The positioning frame uses the existing slideway on the drilling rig tower to make a tic-shaped positioning frame at the top and bottom (wellhead of the drilling rig platform). The upper part of the positioning frame is fixed, and the lower part can be adjusted with bolts in both horizontal directions. At the same time, a 3m-long fixed steel structure column is processed as a guide column, which is connected to the steel structure column after entering the hole. The positioning of the column is adjusted by the positioning frame bolts. Accuracy. 2) The steel cage is hoisted into the hole in sections. After the whole connection is completed, it is fixed on the hole frame with a steel wire rope. The truck-type crane lifts the upper end of the steel structure column so that it naturally droops, and aligns the lower end with the center of the hole. and the center of the steel cage, use its own weight to slowly lower it to the hole, and after connecting with the guide steel column bolts on the positioning frame, continue to lower the steel cage to the design height and fix it. 3) For positioning, first loosen the well-shaped bolts at the lower part of the positioning frame to allow the guide column and steel column to sag naturally, and then use a level and adjusting bolts to accurately position the steel column. After the installation and positioning of the lattice columns is completed, use the center line of the casing to check that the center of the lattice column coincides with the center of the pile position, then fix the lattice column on the wellhead platform, remove the guide column, and provide a location for concrete pouring (see Figure 4). 4.3.6 After clearing the holes for concrete pouring and lowering the steel cages and steel columns and accurately positioning them, start lowering the conduits and pouring underwater concrete. Since the conduits are installed in the steel columns and the space is small, several aspects should be controlled to ensure that the piles are Positioning of the column: ① Select a conduit with a smooth outer wall to connect the hoop, and the axis of the connection is in a straight line; ② Be careful when lowering and removing the tube, it must be vertical up and down, and the bottom ends of the four angle steels of the steel column are cut into wedge shapes to prevent The pipe hoop scratches the steel column, causing it to deflect; ③ The first batch of concrete pouring should ensure that the bottom of the conduit is buried in the concrete ≥ 1.0m. During the pouring process, the concrete surface should be 2.0m higher than the bottom of the conduit. The conduit should be removed each time Its lower end is buried to a depth of ≤6.0m; the pouring must be continuous to prevent pile breakage; ④ As the concrete in the hole rises, the conduit must be quickly removed section by section, and the time should not exceed 15 minutes; ⑤ During the pouring process, when the concrete in the conduit is not full , when air is contained, the subsequent concrete should be slowly poured into the funnel and conduit. The entire concrete bucket should not be poured into the pipe from top to bottom to avoid forming a high-pressure air bag in the pipe and squeezing out the rubber sealing gasket of the pipe section. 4.3.7 After the upper backfill underwater concrete is finally set for 12 hours, the mud between the upper steel column and the pile hole shall be removed and backfilled with sand to prevent displacement and deformation, and then the constraints on the upper end of the steel column by the hole positioning platform shall be released. 5 Conclusion Judging from the excavation of the foundation pit, the intermediate piles in the form of steel columns can meet the safety requirements of foundation pit construction, and the positioning accuracy can also meet the requirements of permanent structural column construction. Only two of them need to cut off part of the angle steel, which is generally feasible.
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