In the construction of railway bridges in southwest China, it is often necessary to cross large rivers, and the geology of the riverbed is complex. The surface is mostly composed of large-grained pebbles with high strength, large quantities, and sandy clay. Since the Sichuan Basin is located in the southwest region, the annual flood season is early and lasts extremely long. At the same time, there are cascade development and construction of hydropower stations on the upper and lower reaches of the river. Therefore, the hydrological conditions are extremely complex, and the annual construction time is only a few days of the dry season. Months, the construction period is extremely stressful, and there are many unknown factors. In previous engineering cases, filling cofferdams and dams were often used to convert water construction to land construction, which required repeated changes in the river course, filling in a large amount of sand and gravel, and cleaning the river bed. However, the cofferdam damming method is difficult to implement due to factors such as time constraints, terrain conditions, inconvenient transportation, large amounts of sand and gravel, and serious environmental damage. There is an urgent need for a safe, economical, environmentally friendly and efficient water technology to solve this problem. Engineering problems.
Previous research and engineering practice on deepwater pile foundations have gained certain engineering experience. This article combines the Chengdu-Kunming Railway capacity expansion and reconstruction project with the new Qingyi River Super Bridge 37#-40# piers located in the main channel of the Qingyi River. Under complex geological conditions, it is proposed to build construction trestles and platforms in the river during the dry season, and use impact drills to construct pile foundations on the construction platform. This will minimize the impact on the navigation and riverbed of the river and create a green and environmentally friendly environment. The pile foundation construction was completed and good social and economic benefits were achieved.
1 Project Overview
The New Qingyi River Bridge is a single-lane bridge with a designed starting mileage of ZDK138 073.93 to an end point of ZDK140 176.69. The center mileage is ZDK139 409.4, and the total length of the bridge is 2102.76m. The upper bridge span structure is 7×32 3×34 16×32 (32 48 32) continuous beam 5×32 1×24 1×32 (40 4×64 40) continuous beam 18×32, of which 26#-29# are One continuous beam across the embankment (32 48 32) m; piers 36#-42# are one continuous beam across the river (40 4×64 40) m. The whole bridge has a total of 285 bored piles, 61 pier bodies and two continuous beams. This bridge is a key control project across the Qingyi River in the capacity expansion and reconstruction project of the Chengdu-Emei section of the newly built Kunming Railway.
Among them, piers 37#-40# are located in the deep water area, and the pile foundation construction is the control project of this bridge. The construction of this section is controlled by hydrological conditions. The best construction time is the dry season, which requires a short time and a large amount of work. The Qingyi River has large-scale water conservancy facilities upstream and downstream of the bridge, which will cause human intervention in the water level and flow, possibly causing unforeseen impacts.
There is a phenomenon of river bank erosion in the Qingyi River section. The river here is relatively turbulent and has a large flow. The river bed sediments are loose to slightly dense. The river channel develops meanders and swings greatly. The river banks are often washed away by floods. Farmland and houses were destroyed. The existing Jiajiang Bridge on the Chengdu-Kunming Line has been abandoned and rebuilt because its expanded foundation was exposed and moved due to river erosion. The newly built double line is close to the existing Chengdu-Kunming Line, and river bank erosion has a greater impact on the bridge foundation.
The main strata lithology along the line are: artificial fill soil, spoil soil, soft clay, loose soil, silty clay, silt soil, medium sand, landslide accumulation soil, coarse gravel soil, pebble soil, clay , mudstone, etc.; the geological conditions of the 37-40# pier in deep water of this bridge are the surface 1-3m pebble layer, and the lower 3-40m is mudstone intercalated with sandstone.
This bridge spans the Qingyi River and is designed under navigation control. Its navigation level is Level VII.
Hydrological data: Q1/100=18200m3/s; H1/100=414.35m; V1/100=3.65m/s; H highest navigable water level=410.7m; Q1/300=20300m3/s; H1/300=415.03m.
The environment category of the bridge is: carbonization environment level T2 and T3 for structures above the ground; carbonization environment level T1 for structures below the ground, and L1 level for chemical erosion environment. Water quality: Surface water and groundwater are not corrosive to concrete structures.
2 Difficulties in construction
According to the actual hydrological and geological conditions on site, taking into account the construction period, bridge structure characteristics and node construction schedule, the island cofferdam method is used for construction during the dry season. It is safe and economical, but this plan affects the waterway and brings great safety risks to navigation. Since the upstream of the bridge is the existing Qingyi River Bridge on the Chengdu-Kunming Line, which is only 30m away from the new line, the entire river channel includes shallow water areas** * Totally 600m wide, with Qianfoyan Hydropower Station in the upstream and Maotan Hydropower Station in the downstream. Taking into account the safety of existing lines, the uncertainty of river water levels caused by hydropower stations, and the need for subsequent superstructure construction, the final construction plan was to build a trestle on the water, use a construction platform, and use steel pipe piles to drill holes in the bedrock for reinforcement. Steel pipe piles for trestles and platforms. 3.5 Steel casing follow-up
According to the equipment resources and geological conditions around the construction site, the construction machine was decided to be an impact drill, and the positive circulation method was used for construction. Since the steel casing cannot pass through the pebble layer, the mud cannot be guaranteed normal cycle. Based on the on-site geology and actual construction experience, a construction process is adopted in which a steel casing is followed after drilling by a large-diameter drill bit.
Construction technology: After the steel casing is inserted once, install the casing impact drill on the platform for drilling. The diameter of the drill bit is 1.8m and the diameter of the casing is 1.9m. Continue drilling until it passes through the pebble layer. Use a crawler crane to lift a steel plate with a thickness of 12cm, hammer the steel casing until it passes through the pebble layer, then raise the steel casing, and pour 1m high C20 concrete into the casing as a sealing material. At this point, the follow-up of the casing has been completed, achieving the dual purposes of stabilization and leakage plugging. The connection of the steel pipe pile and the steel casing ensures the safety of the platform structure.
3.6 Pile foundation drilling
After the casing follow-up is completed, normal drilling of the pile foundation can begin. According to the actual construction period, a total of 9 drilling rigs will be arranged for construction. Among the 32 bored piles with a diameter of 1.5m, 3 are dedicated casing follow-up drilling rigs, and the remaining 6 are impact drilling rigs with a drill bit diameter of 1.5m.
Punching: The impact drill bit uses a cross-shaped drill bit, and the wire rope is soft, high-quality, free of dead bends and broken wires, and is firmly connected to the drill bit. The hole opening stage of impact drilling is mainly to make pulp and solidify the wall, using low-stroke punching. Normal punching can only be carried out after the drilling depth exceeds the total height of the drill bit plus the stroke.
Final hole inspection: A hole detector is used for final hole inspection. The diameter of the hole detector is the same as the designed pile diameter, and the length is 4-6 times the pile diameter. Each pile should ensure that the bottom of the pile reaches the designed pile bottom elevation, and the design and supervision site should be asked to sign for approval.
Cleaning the hole: Clean the hole after drilling to the design elevation. The holes are cleaned using the slurry replacement method, and high-quality mud is used for forward and reverse circulation slurry replacement until the specified mud and drilling quality standards are reached.
3.7 Installation of steel bars, acoustic tubes and concrete pouring
The steel cage is made in sections at the processing plant, transported to the site for assembly, and is hoisted into the hole in sections with a truck crane. There are two upper and lower sections. It should be kept straight. The steel cage should be firmly positioned after entering the hole.
When installing the acoustic detection pipe, the butt welding joint should be flat to ensure that there is no bending, that the inner diameter of the pipe is not reduced, and that there is no leakage. After all installation, clean water must be poured into the pipe to the pipe mouth to confirm that there is no leakage before use. The pipe cap can be lowered only after it is closed.
Before pouring concrete, perform secondary hole cleaning to ensure that the thickness of the sediment is no more than 5cm and the specific gravity of the mud is 1.05-1.08. The conduit method is used for concrete pouring, and the conduit method is used for pouring? Accurate 300mm quick bayonet vertical lifting guide tube. The conduit shall be assembled and numbered before use, and shall undergo tensile and water tightness tests.
4 Comparison of construction plans
Our department has carried out a study on the plan of first building a construction trestle and platform in the river during the dry season, and using impact drilling to construct the pile foundation on the construction platform. A comprehensive analysis of rationality, economy, reliability, and safety shows its advantages in Table 1.
For geological conditions where steel pipe piles cannot be driven, the steel pipe piles of trestles and platforms are reinforced by drilling holes in the pipe. Compared with the conventional construction method of punching holes first and then burying the pipe piles, the cost is saved by 1.2 million yuan. . Compared with the island cofferdam method, the construction requires filling a large amount of sand and gravel to change the river bed and flowing water of the river. At the same time, it will cause great pollution to the water environment, pose a great hidden danger to the safety of the upstream bridge, intensify the erosion of the river bed, and affect navigation.
The method of building construction trestles and platforms in the river first, and using impact drills to construct pile foundations on the construction platform allows for rapid construction, no occupation of the river, no navigation restrictions, one-time construction, no changes to the river, and a wide range of applications. Advantages, truly green, safe and environmentally friendly construction.
5 Safety Guarantee Measures
① Flatten the frame to maintain stability and avoid displacement or subsidence. The top of the drill frame is symmetrically stretched with wind ropes and the ground anchor is firm. Control the drilling speed not to be too fast or change speed suddenly.
② The drilling rig, drilling tools and wire ropes for hanging drill bits all meet the design requirements, and the safety factor of the wire rope is not less than 1.2. There are dedicated personnel to inspect and maintain the equipment during use. The working platform and drilling rig platform are covered with scaffolding boards and equipped with railings and walkways, and debris must be removed at any time. All openings that have not been constructed must be covered with protective covers.
③ When sliding the drilling rig, prevent squeezing of cables and water and air pipes.
④ Lifting operators must be certified to perform operations. It is strictly prohibited to operate with multiple people or without anyone commanding them. It is strictly prohibited to stand or walk under booms or hanging objects.
⑤All water workers must wear life jackets and safety helmets when working.
6 Construction schedule
Use to build construction trestles and platforms in the river during the dry season, use impact drills to construct pile foundations on the construction platforms, and use drilling rigs with different drill bit diameters. The construction method of rotating construction and following up the steel casing has accelerated the hole formation speed of a single pile, saving 3-5 days for a single pile, and the construction period of all 32 pile foundations has been saved by 25 calendar days compared with the original planned construction period.
7 Conclusion
During the construction of the pile foundation in the deep water area of ??the New Qingyi River Bridge, the method of erecting a steel trestle and then setting up a construction platform was used to construct the bored pile foundation, and at the same time, mature construction methods were used to drill bedrock The steel pipe pile foundation is reinforced by the anchoring method and constructed by drilling holes with drill bits of different diameters to follow up the steel casing. Through the optimization of the above series of plans to ensure the smooth construction of pile foundations, valuable experience in the construction of deep-water pile foundations in cobblestone areas was obtained through exploration and experiments, and the company's construction method "Bedrock Drilling and Anchorage Method to Reinforce Steel Pipe Pile Foundations" was implemented. A large number of results have been achieved in engineering practice. By optimizing construction methods, we have achieved three goals with one stone, achieving major economic and social benefits such as economy, construction period, energy saving, and green environmental protection.
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