1, project overview
Bahe Bridge spans Bahe River, Xiyu Expressway and North Ring Railway. The total length of the bridge is 25 1 18.45 meters and the span is ***784. The bridge is one of the key projects of Daxi Passenger Dedicated Line, and the construction progress of bored piles in the lower part directly affects the construction period of the bridge, even the whole line.
The distribution of rock and soil layers at the bridge site is uneven, and the engineering properties of different landforms are quite different. This formation includes silty clay layer, muddy clay layer, pebble layer and moderately weathered argillaceous sandstone layer.
2, rotary drilling rig construction
In order to ensure the progress of the bridge, eight rotary drilling rigs were put into use at the same time. For places where the site and formation conditions are not suitable for rotary drilling rig construction, impact drilling rig construction is adopted.
2. 1 construction preparation, rig in place
Arrange the construction site reasonably according to the design requirements. First, complete the "three links and one leveling", and the drilling platform must be replaced and tamped to avoid accidents such as equipment tilting, displacement and even overturning caused by uneven settlement of the drilling rig. Because the track of rotary drilling rig runs frequently, especially in rainy days, the road is damaged greatly and the standard of construction access road is high, so drainage measures should be taken. Establish a temporary coordinate system, measure and set out to determine the pile position. When constructing near existing railways and highways, steel sheet piles should be used for protection.
2.2 Construction technology
2.2. 1 gasket embedding
(1) The casing is made of 8 mm thick steel plate. The length of the casing is about 1.2 ~ 2 m, and the inner diameter is 200mm larger than the pile diameter. The top of the casing is 2.0m higher than the construction water level or underground water level and 0.5m higher than the ground.
(2) Two slurry overflow holes should be opened at the top of the casing. Stiffening ribs and stiffening rings are welded at the upper and lower ends of the casing to increase the rigidity of the casing and facilitate the pulling out of the casing after the cast-in-place pile is completed.
(3) The embedding position of the liners must be accurate. Accurately determine the pile position, and after recheck, bury the casing according to the control pile to ensure the verticality and levelness of the casing. The position deviation of casing top surface is not more than 5cm, and the inclination deviation is not more than 65438 0%.
(4) artificial excavation and burying protection, the liners must pass through the silt layer and be buried in the stable soil layer for at least 0.5m
2.2.2 Transfer in place
After the casing is buried, the drilling rig is in place, the mast of the rotary drilling rig is adjusted vertically, and the center of the drill bit is aligned with the pile position. After the alignment is completed, all data are set and locked. Reinforcement and stability of cable wind rope for drilling rig.
2.2.3 pore-forming construction
After the rig is checked in place, inject mud for drilling, and use rotary drilling barrel bit for drilling. Through the rotation, cutting, lifting, soil unloading and mud hole protection of the drill bucket, the hole is repeatedly circulated until it becomes a hole. During drilling, ensure that the liquid level is always higher than the bottom surface of casing after drilling. After drilling, clean the sediment at the bottom of the hole with a bottom cleaning drill, complete the first hole cleaning and measure the hole depth. The following points should be paid attention to when using mud to protect the wall in construction.
(1) Mud is made of bentonite and water. Generally, 80kg bentonite is added per 1000k, and sodium carbonate (Na2CO3) is added to adjust the pH value of the mud to control the fluidity and suspension effect of the mud.
(2) The mud used for the first time should be fully stirred for > 2 min.
(3) When spudding, gently press and slowly advance, inject mud into the hole in time to balance the formation pressure, and form a mud layer around the hole wall to protect the hole wall.
(4) During normal drilling, the speed of footage should be controlled, so as to lift it gently and put it slowly, which shall not exceed 0.5m； at a time; If it is a sand layer, use a sand fishing bit to rotate forward and the bottom opening is closed reversely; When encountering the soil layer, use a clay bit, and inject mud with the footage to protect the hole wall.
(5) Strictly control the liquid level in the hole. The slurry storage tank is connected with the drilling hole through the mud ditch to replenish the mud in time. During construction, it is forbidden that the liquid level is lower than 30cm below the top of casing.
(6) When approaching the final hole, use the back cover sand fishing bit to drill, so as to reduce the footage (about 30cm) and gradually pull out the thick mud at the bottom of the hole. When the final hole depth is reached, stop drilling for 20 ~ 30 min, which is the precipitation of suspended solids in mud, and remove it with a drill bit.
(7) After hole cleaning, mud viscosity should be controlled at 18 ~ 20s, sand content should be less than 4%, sediment thickness should be ≤ 10cm, and mud density can be determined by trial drilling.
Quality control measures
(1) The pile position measurement should be accurate, the error and inclination of the pile casing embedding position should be within the specification requirements, the drilling rig should be leveled, the drill bit should be aimed at the center of the pile position, and the drill pipe should be vertical. If there is displacement during drilling, it should be re-centered.
(2) In the process of drilling, the drilling speed should be strictly controlled to avoid drilling accidents caused by excessive drilling scale.
(3) Control the vertical speed of the drill bit in the hole. When the vertical speed is too fast, the mud between the outer wall of the drill bucket and the hole wall washes the hole wall, and the negative pressure generated at the lower part of the drill bucket causes the hole wall to shrink or collapse. The vertical speed of the drill bucket should be controlled at 0.75 ~ 0.8m/s.
(4) To prevent the hole wall from collapsing, ensure that the water level in the hole is more than 2m higher than the underground water level.
(5) Prepare mud according to different dripping situations and replenish mud in time. According to the change of soil layer, the performance index of mud should be adjusted reasonably, and the specific gravity and viscosity of mud should be increased when the soil layer is easy to collapse.
(6) Drilling holes, placing reinforcing cages and pouring underwater concrete into downcomers should be closely coordinated, so as to shorten the time of each process between hole-forming and pouring concrete as far as possible, prevent excessive precipitation at the bottom of the hole, prevent the bearing stratum sandstone from softening due to long-term soaking, and reduce the side resistance and end resistance of the pile.
2.3 Construction in Different Strata
(1) Silty clay layer Silty clay layer exists widely at the bridge site, and it is also one of the most suitable geology for rotary drilling rig. When the rotary drilling rig is drilling, the lateral resistance is small, the speed is fast and the mud requirement is low. In some places, clean water or even dry hole drilling is used, and single-layer soil bit is used as the drill bit. The construction of silty clay layer is easy to control, the hole-forming quality is good and the drilling speed is fast, which gives full play to the advantages of rotary drilling rig.
(2) Muddy clay layer Muddy clay layer has weak strength and poor cementation, and the pressure and supporting force of mud on the hole wall within the range of orifice 10m are small, which is easy to cause shrinkage and affect the quality of pile formation. When crossing this stratum, double bottom sand fishing bit is used for drilling, and the penetration depth is strictly controlled during drilling, and the hole is repeatedly swept to prevent necking. Mud is one of the important factors for normal drilling in muddy clay layer. At the initial stage of construction, bentonite was used to make slurry, and the relative density was 1.05 ~ 1. 10. The mud pressure in the hole is low, and the hole-forming effect is not ideal. Adjust the mud mixture ratio, adjust the relative density to above 1.20, and ensure normal drilling. The small strain test shows that the pile body is complete and meets the design requirements.
(3) The gravel diameter of the pebble bed is 10 ~ 15cm, and the density is relatively high. In the early stage of construction, due to improper operation, the drill pipe was locked and broken. After analysis and improvement, the short spiral rotary head is used for vulgar and low torque drilling, floating first and then pressurizing, and replacing it with wear-resistant alloy steel bucket teeth. During drilling, check the bucket teeth in time. If any damage is found, repair them in time or replace them by welding. After drilling, the short spiral bit should be replaced with a single floor soil bit in time to catch the sediment at the bottom of the hole and prevent the bit from slipping and affecting the drilling effect.
Due to the gravel layer, the drilling tool has great loss, frequent maintenance and replacement, slow drilling speed and high cost. Therefore, in the case of thick gravel layer, impact drilling rig is selected to improve efficiency and reduce cost.
(4) The design bearing layer of moderately weathered argillaceous sandstone pile foundation is moderately weathered argillaceous sandstone with high strength and viscosity, and the standard value of rock saturated uniaxial compressive strength changes greatly. When the rock is difficult to drill or slips during drilling, the rock-socketed barrel is used to drill, and at the same time, the pressure and torque of the drilling rig are increased, and the cutting force of the alloy bucket teeth is used to cut the rock mass. When entering the rock stratum for the first time, gently press and rotate slowly, and then press and drill after the uneven rock surface at the bottom of the hole is leveled to prevent well deviation and bit damage.
Due to the high viscosity of rock strata, mud paste often appears on the drill teeth, which needs to be removed frequently with a flat shovel and washed with a high-pressure water pump to check whether there is weld crack on the drilling tool and repair or replace it in time.
3. Conclusion
Rotary drilling rig is an important guarantee to meet the progress requirements of Bahe Bridge and ensure the smooth progress of beam-laying and track-laying project. Through the construction effect, it is fully proved that the application of rotary drilling rig in the pile foundation construction of super-large bridge has the advantages of wide applicable stratum range, high hole-forming efficiency, good construction quality, energy saving and environmental protection. At the same time, a set of experience and rules are preliminarily explored, and how to adopt reasonable construction technical measures, drilling parameters, mud indexes and bit types according to the address situation to improve the working efficiency of rotary drilling rig and reduce the construction cost.
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