The design and construction of self-made steel sheet pile cofferdam of Tianjin Port Nanjiang Bridge Zhang Aihua (a bridge bureau, Zhengzhou 450053) is convenient, flexible, low investment, fast construction, and significant benefits. Tianjin Port Nanjiang Bridge is located at the entrance of the Haihe River and is a railway Parallel bridge, the total length of the highway bridge is 1205.08m, the total length of the railway bridge is 2873.45m, the main bridge of the highway bridge is 48m + 3x64m + 48m5 span prestressed concrete continuous box girder, the main bridge of the railway bridge is 48m + 3x64m + 48m5 The steel truss girder and the approach bridges of the two bridges are 32 m span prestressed concrete simply supported T beams.
It is known from the geological data that the soil layer in the depth of the foundation pit of the bridge pier is mainly silt and muddy clay, with strong rheology and poor permeability.
The water depth within the range of the main bridge side pier and the beach ground approach bridge is 12m. 1 Comparison of the schemes of self-made steel sheet pile cofferdam and single-wall steel box cofferdam Select the representative 0 bridge pier bearing platform for comparison.
Row, the longitudinal bridge is 2 rows, the vertical and horizontal spacing of the pile is 3.2m, the plane size of the cap is 8.7mx5.5m, the top elevation of the cap is + 1.0m, the bottom elevation is -1.5m, and the river bed elevation is + 3.58m, The water depth is ~ 2.0m. The soil layer within the excavation range of the foundation pit is silt, silty clay, and the permeability is poor.
According to the relevant technical data and the situation on site, after careful research and demonstration, it is considered that the original scheme is difficult to sink the steel box cofferdam, and the use of a single steel sheet pile with a width of about 1m is very simple to insert and pull out. In addition, the water depth is less than 2m, and the water permeability of the soil layer is poor. Therefore, it was finally decided to use self-made lock steel plate cofferdam for construction. The comparison and selection of the two schemes are shown in Table 1. 2 Design of self-made steel cofferdam 2.1 Design conditions (1) Working conditions According to the construction operation period of steel plate cofferdam, the design stress state can be analyzed according to the following working conditions: â‘ Steel plate pile insertion Hit the force check.
â‘¡ The soil and water in the cofferdam are discharged to the upper support position to â‘¢ The upper support is installed, and the soil and water in the cofferdam are discharged to 0.5-0.75m below the middle support position.
â‘£The upper and middle layers of support have been installed, and the soil and water in the cofferdam are discharged to 0.50.75m below the support position of the lower layer.
⑤The upper, middle and lower three layers of support are installed, and the soil and water in the cofferdam are discharged to the bottom of the foundation pit.
(2) Water level conditions According to the hydrological data over the years, the design water level is determined to be + 5.3m. (3) Geological conditions are known from the geological data. The soil layer above the elevation of -2.5m is silt and muddy clay, and the internal friction angle = 5. 15. Poor water permeability.
(4) Structural design conditions Synthesizing various working conditions and hydrogeological conditions to determine the design conditions of steel sheet pile cofferdam: Zhang Aihua: Design and construction of self-made steel sheet pile cofferdam of Tianjin Port Nanjiang Bridge 10kN / m2 gauge, water pressure with height According to the linear distribution 5 *; the bulk density of the silt is 7, = 16.5kN / m3; the bulk density of the muddy clay is 72. The average friction resistance of the soil around the steel sheet pile is: f = 25kPa ⑵ Calculation ①Comprehensive working condition analysis and calculation content, mainly Calculate the most unfavorable working conditions for the steel sheet pile: the steel sheet pile bears the lateral pressure and water pressure of the soil. The most unfavorable working condition is: excavating the soil in the cofferdam to the design elevation stage. The calculation of steel sheet pile includes calculation of internal force, stress and deformation of corrugated, horizontal stiffeners, face plates, locks and other components. Steel plates with unit width are used as unit bars in the calculation.
â‘¡ Internal support system calculation: It is related to the calculation of steel sheet pile, so the calculation of internal support is completed at the same time as the calculation of steel sheet pile.
â‘¢ In addition, the depth of the steel sheet pile and the stability of the soil shall be calculated.
3 Brief introduction of steel plate cofferdam structure (see) Table 1 Comparison of schemes of self-made steel sheet pile cofferdam and steel box cofferdam Advantages and disadvantages of self-made steel sheet pile cofferdam 1. Fast insertion speed, simple equipment, no backing concrete, shorten construction period and save Fund 2. Easy to pull out, can be used many times to reduce the project cost, the cofferdam needs a total of 52.1t of steel 1. Need to build an island 2. Need to form a steel template box cofferdam during the construction of the platform 1. Side plate Cap formwork 2. No need to build an island. The entire cofferdam requires a total of 51t of steel 1. Bottom concrete is required 2. There are many sinking procedures, a long period, and a large amount of on-site welding work 3. A hanger is required when sinking and cannot be completed after construction Full recovery, higher cost Net size of cofferdam plane: 9.11mx5.9m Cofferdam top design elevation: + 5.8m steel plate height: 9.3m bottom support standard board + 1.3m2.2 Design basis (1) "Tianjin Gangnan "Design Drawings of Xinjiang Bridge" (2) "General Specification for Design of Highway Bridges and Culverts KJTJ021 2.3 Design Calculation The structural design check calculation is carried out based on the analysis of the construction period of the steel sheet pile cofferdam. Here, only a brief introduction to the calculation idea is made (1) Load and main technical parameters Horizontal load : Hydrostatic pressure, main, quilt Earth pressure and other vertical loads: friction between the steel sheet pile and the soil, resistance of the steel sheet pile tip to the soil, the weight of the steel sheet pile and other related technical parameters: the hydrostatic pressure per unit area is double The wall structure is made of welded section steel and 6mm thick steel plate. The steel sheet pile is a whole piece in the length direction, and the plane is divided into four pieces: an L * type corner pile with a size of 442mmx 502mm, each weight It is 1.05t, and the width of each steel sheet pile on the long and short sides is: 17mm, which is composed of 8 and 5 steel sheet piles, each weight is about 1.5t, and the total weight of the side plates is about 43.2t. The principle of splitting the pile: easy to process and transport; easy to insert and pull out using the existing equipment, each steel plate is connected with a hollow lock, and a single steel plate pile is provided with a child and mother lock on both sides of the pile. The mouths are composed of two L75x63x8 angle steels. The vertical stiffness of steel sheet piles filled with cement, bentonite, etc. into the lock is all!〗 A, the spacing is about 500, the horizontal stiffeners are 8 = 6mm steel plates, and the spacing is 310 ~ 940mm , Changes with depth, the panel is 6mm steel plate.
The self-made steel plate cross-sectional structure shows opinions.
Steel sheet section diagram Angle pile section diagram Sheet pile section structure 3.2 The internal support system of the steel sheet pile cofferdam is composed of an inner ring beam, a horizontal support and a diagonal bar. The inner ring beam is provided with 3 layers: upper, middle and lower. The top-down elevation (Yellow Sea elevation) is * + 5.3m, + 3.3m and + 1.3m. It consists of 2, 2, and 2i, respectively. Its role is mainly to bear the load transferred from the steel sheet pile and transfer it to the horizontal braces and diagonal braces.
3.3 The upper and lower guide frames are provided with upper and lower guide frames to ensure the correct position of each board. The upper guide frame is located at the top of the bored pile guard tube, and the lower guide frame is located above the island surface. The guide is made of A-shaped steel, and the inner guide uses the inner ring beam of the cofferdam.
4 Construction sequence of steel sheet pile cofferdam (1) Fill the island with 0 pier to build island, the filling is cohesive soil with good waterproof effect.
(3) Use the Zhongyi 60 vibratory pile driver to insert single steel sheet piles. The insertion method uses two insertions. The first time all steel sheet piles are inserted and driven to a certain depth before merging, and the second replaying to the design elevation is the key to ensuring the smooth merging of steel sheet piles.
(4) Fill the lock with glue, bentonite and other materials to prevent water leakage.
(5) Excavate the soil in the cofferdam and dig the soil using a combination of manual and mud grabs. Install the ring beam and horizontal support as soon as possible after excavating to the designed internal support elevation. There is a small amount of water seepage during the excavation of the cofferdam, which can be pumped dry using a small submersible pump.
Thick gravel cushion and 100 thick M10 cement mortar, and smooth the mortar surface.
(7) Cut off the steel guard tube, clean up the pile heads, set up the cap formwork and bind the steel bars, and pour the cap concrete.
(8) Convert the internal support of the third floor to the bearing platform for the first section of pier body construction, and then convert the internal support of the upper layer part above the pier body for normal construction of the pier body.
(9) After the pier is constructed to a certain height, the steel plate is pulled out for other pier use.
5 Conclusion According to the location, depth and soil layer of the foundation pit of the Guanjin Nanjiang Bridge, the flexible use of self-made steel sheet pile construction is an innovation in bridge foundation construction, which not only saves investment, but also greatly shortens the construction period. It has achieved good economic benefits and has certain value for similar foundation pit excavation in the future. This construction method has been adopted successively in the foundation construction of the Beijing Xushui River Bridge and the Jingting Lake Bridge in Beijing-Zhanghai Expressway, and has achieved remarkable results, which is an obvious example.
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