Introduction to 0
With the rapid development of highway and railway construction, large-diameter deep-water bored piles are widely used in long-span bridge foundations. How to ensure the construction quality of bored piles in high tide deep water is very important. The construction of high-tide deep-water bored piles at piers 120 and 12 1 of the main span of the expansion project of Zhangzhou section of expressway in Xiamen-Zhangzhou is studied, so as to ensure the pile foundation.
1 project overview
The ZA4 contract section of the expansion project of Zhangzhou section of Xiamen-Zhangzhou expressway is a part of Nangang Bridge. (85+ 150+85)m prestressed concrete continuous rigid-frame box girder is used in the upper part of the main span across Nangang waters of Jiulong River, and hexagonal mass concrete cap (1 half a basketball court) is used in the lower part, and 16 bored pile is used as the foundation, with a diameter of 92.0m and the longest bored pile of 120. 12 1 is a rock-socketed pile with a pile length of 74.0m, Nangang Bridge is a tidal reach of Nangang of Jiulong River, and the tidal current is a reciprocating semidiurnal tide, with the maximum water depth of 16.0m and the designed maximum navigable water level of 7.38 mm. The construction method of bored pile is to erect steel trestle and drilling platform, bury steel liners and punch holes.
2 Construction preparation
1) After entering the site, determine the construction scheme of large-diameter bored pile in high tide and deep water according to the actual situation, and all kinds of materials will enter the site under the condition of ensuring three links and one leveling.
2) All kinds of construction machinery and equipment enter the site, and the personnel are in place.
3 Design and construction of drilling platform
3. 1 drilling platform design
The drilling platform adopts pipe pile foundation, I-beam, I-beam (Bailey beam) distribution beam and channel steel panel. In order to ensure the reasonable use of the platform in the lower bridge construction, the span of the main span drilling platform should be controlled within 6m (based on the principle of avoiding the construction of pile foundation liners and steel cofferdams), with I-beams as beams, I-beams as distribution beams (spacing of 50cm) and panels as 20cm channel steel (clear spacing of 5cm).
3.2 Construction of drilling platform
The steel pipe pile of drilling platform is driven by 50t crawler crane. During construction, use the auxiliary hook of crawler crane to lift the steel pipe horizontally, move it to a suitable position, and adjust the steel pipe pile vertically. Then fix the steel pipe on the vibrating hammer lifted by the main hook of the crawler crane with hydraulic tongs, and adjust the verticality of the steel pipe pile with hydraulic tongs. The plane position of the steel pipe pile is located under the guidance of the total station or two J2 theodolite, and the steel pipe pile is fixed by the hydraulic fixture of a 45kW vibrating hammer. After lifting, put it into the guide frame and start to vibrate. When the pile length meets the design length and the penetration is less than 5cm/5min, the steel pipe pile only needs to hold the load for 5min, and the vibration can stop without obvious subsidence. When the length of prefabricated steel pipe piles in the first section is not enough, piling while drilling can be used to make the length of steel pipe piles meet the requirements. Weld (900-900) mm2 steel cover plate on the pile top, δ = 10 mm. The steel cover plate must be completely welded with the surrounding area, and the steel cover plate must be horizontal. In order to ensure the stability of steel pipe piles, diagonal braces and horizontal braces are welded between steel pipes with channel steel. After welding the steel cover plate at the top of the steel pipe pile, hoist the I-beam to the top of the steel pipe pile with a crane. According to the design platform, I-beam or Bailey beam is placed in the center of the pile top, and the elevation is adjusted to weld with the steel cover plate on the pile top. After the steel stack bridge is installed, the longitudinal and transverse beams are installed, and the intersection of the longitudinal and transverse beams is welded or bolted. At the position of drilling platform, the longitudinal and transverse beams of I-beams near the steel casing are placed in strict accordance with the design position to prevent intrusion into the gap of the steel casing. Laying bridge deck and guardrail, using guardrail to set up night reflective paper to guide and prompt ships to pass, to ensure construction safety.
4 Burying of steel casing
The manufacture and installation of steel casing that meets the requirements is an important link in the construction of bored pile foundation in high tide deep water. How to manufacture and install steel casing in high tide deepwater drilling will directly affect whether the hole can be formed in the shortest time, and it is also the main technical problem of hole formation.
4. 1 Material selection and fabrication of steel casing
The material selection of casing is closely related to the diameter, water depth, water flow and geological conditions of bored pile. According to the site knowledge, the steel pipe with a diameter of 2.9m is made of steel coil, with a wall thickness of 14mm, a length of 9 100mm and a width of1800 mm. It is rolled into a section of 1.8m on site, which is welded firmly and watertight.
4.2 Fabrication and installation of guide frame before steel liners are buried
In order to ensure that the steel casing of high tide deepwater pile meets the requirements, a guide frame must be set. The guide frame adopts 25b I-beam, which is a double-layer H-beam frame, forming a simple guide frame.
4.3 Installation of steel lining
After the installation of the guide frame is completed, recheck the center of the pile position, and install the steel casing after confirming that the center of the pile position and the center of the guide frame are in the same position. During installation in the first section, it is 80cm~ 100cm higher than the drilling platform. Weld two support points with I-beam, fix them on the I-beam of the drilling platform, and then lift the next section for installation. In order to make the docking match, it is necessary to rotate left and right to find the best position. When the butt length of steel liners reaches the height requirement from the platform to the riverbed, lower the liners to the riverbed at low tide, and check the plane position and verticality of the liners to ensure that the steel liners are in place accurately. A vibrating hammer with a vibration force of 900kN is used to fasten the steel casing through the hydraulic tongs, and the vibrating hammer directly transmits the force to the steel casing through the hydraulic tongs.
5 bored pile construction
5. 1 rig in place
After the drilling rig is in place, the wire rope should be adjusted to coincide with the center of the pile. The base and top of the drilling rig must be smooth and stable, so as not to tilt or shake during construction. The drilling rig should be welded with the steel bars on the drilling floor.
5.2 Slurry-making of High Tidal Deepwater Bored Piles
Local red clay with plasticity index greater than 25, particle size less than 0.005 mm, and particle content accounting for more than 50% of the total is selected as slurry-making material. Before opening the hole, pour red clay from the car into the hole (the quantity is determined according to the volume of the empty hole, and the dosage is 0.4 times of the volume of the empty hole). After fully soaking for 24 hours, fully stir in the hole with a drill hammer. Assign special personnel to observe the water head during drilling to prevent possible perforation; After drilling casing 1.5m~2.0m, inject proper amount of cement, and after hitting it evenly with a drill hammer, lift the hammer up and down once every half hour or so, and drill normally after 24h to prevent the casing foot from being penetrated.
drill a well
Pile foundation is drilled by 10t impact drill. During drilling, the indicators of mud should be detected frequently, and the water head in the casing should be controlled to ensure that the pressure difference between inside and outside the hole is (0.02 0.01) MPa to prevent perforation. Measure the hole depth in time, check the original construction records, pay attention to the geological changes, take slag samples from the geological changes, make records after identification, and draw a geological histogram to compare with the design. After the drilling depth reaches the design hole depth and the geology meets the requirements, the hole depth, aperture and verticality shall be checked by the hole tester, and the hole shall be formed after meeting the requirements of the specification.
5.4 Borehole cleaning
The hole cleaning quality of large diameter bored pile in water is directly related to the quality of pile foundation. In order to ensure the mud specific gravity and indicators meet the requirements, ZX- 100B mud purifier is used for hole cleaning. Confirm by hand that the mud discharged from the hole does not contain 2mm~3mm particles, its specific gravity is not more than 1. 1, its sand content is less than 2%, and its viscosity is 17s~20s. At the same time, before underwater concrete pouring, ensure the ballast thickness at the bottom of the hole: rock-socketed piles are not more than 3cm, and friction piles are not more than 10cm. It is forbidden to use the method of deepening drilling depth instead of hole cleaning.
5.5 Installation of reinforcing cage
Because the reinforcing cage is a double-layer reinforcing cage with large mass, in order to prevent deformation during hoisting, an internal triangular brace made of φ 32 steel bar is welded on the reinforcing bar at the hoisting end, and it is cut off when the reinforcing cage sinks. At the same time, in order to ensure that the reinforcement can meet the load-bearing requirements during the hoisting process of the reinforcement cage, a certain length of short reinforcement is welded on both sides of the reinforcement to block the reinforcement. With the increase of the number and weight of reinforcing cages, the length and quantity of short reinforcing bars are increased, and double reinforcing bars are set. When installing the reinforcing cage, use 80t truck crane to lift the first reinforcing cage, aim at the center of the hole, and then lower it, and the surrounding workers will hold it to prevent deviation. Set the hook position in the reinforcing cage in advance, fix the hook on the lifting point set in the reinforcing cage by manual cooperation, and lift the next reinforcing cage. After alignment, connect the main reinforcement of reinforcing cage and tie stirrups. After the connection of the last reinforcing cage is completed, slowly lift the whole reinforcing cage with a crane and put it into the hole. After descending to the design elevation, the whole reinforcing cage is suspended on the drilling platform by setting lifting rods. The steel hanger shall be hoisted at 2 points, and the steel hanger shall be welded with 28mm round steel and the main reinforcement, with each welding length not less than 40cm. Hang the lifting ring set on the steel hanger on the hook set on the drilling platform, and weld the reinforcing cage into a triangular protective layer with steel heads in four directions, the thickness of which is the distance from the inner diameter of the steel casing to the outer diameter of the reinforcing cage, so as to ensure that the reinforcing cage is in the center of the hole pile.
5.6 Concrete pouring
In order to ensure the pouring quality, the concrete mixture ratio is optimized at first. Considering that concrete is influenced by marine environment, the cement consumption of concrete on land is relatively large. At the same time, in order to ensure the fluidity and initial setting time of cement concrete, a proper amount of fly ash and additives are added to the concrete to ensure that the initial setting time of concrete is above 10h and the slump is controlled at 18cm~22cm. Concrete is transported to the hole to be constructed by concrete truck through trestle. Considering the large amount of concrete poured into the bored pile with a diameter of 2.5m for the first time, and the hopper capacity is 10m3, it is supported on the drilling platform with four legs, and then the ball is pulled out after the whole hopper is filled with materials, so as to ensure that the buried depth of the first pouring conduit is not less than 1.0m, and the large-diameter bored pile adopts a conduit with a diameter of 35cm. During the whole grouting process, the buried depth of concrete at the bottom of the conduit should be 2m~6m, and continuous operation should be carried out, and grouting records should be made. At the same time, the mixing time of concrete is controlled, and the indexes such as fluidity and slump of concrete are detected to ensure the quality of pile foundation concrete. In order to ensure the quality of the pile head, the pile head is over-poured1m. When the concrete has not been initially set, the excess concrete will be dug out and the integrity of the pile will be tested after 12d.
6 conclusion
In the construction of large diameter bored pile in high tide deep water, whether the installation of steel casing meets the requirements is the key to directly affect the hole formation of the whole bored pile. Mud specific gravity is an important cause of well collapse and perforation. Controlling the water head in casing is an effective method to avoid perforation. Reasonable use of lifting equipment to install large-tonnage reinforcing cage is the key to ensure construction safety. Effectively controlling the initial setting time, workability and slump of concrete is the premise to ensure the quality of underwater concrete pouring.
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