This paper analyzes the influencing factors of construction quality accidents of bridge bored pile foundation. According to the construction method of bored pile, it discusses from drilling, hole cleaning, fabrication and installation of reinforcing cage, underwater concrete pouring and other aspects, so as to improve the construction quality of bored pile foundation of bridge. At the same time, the solutions to reduce economic losses and speed up the construction progress of the whole project are introduced.

Keywords: bridge; Bored cast-in-place pile; Construction technology; control

Concrete pile foundation has the advantages of good quality, high bearing capacity, low cost and simple construction, and is widely used in bridge foundation engineering. Every step of pile foundation construction from drilling to concrete pouring requires careful arrangement and rigorous operation, especially construction preparation, which directly affects the engineering quality of pile foundation. Broken piles (full-face mud inclusion, holes) and defective piles (segregation, honeycomb, necking, etc.) will appear in the construction process. ), which directly affects the engineering quality, and the construction quality of bored piles directly affects the stability of the superstructure and the safety of the enclosure structure.

A, common accidents in drilling construction

The position of 1. hole is deviated.

The main cause of pile position deviation is the error of calculating pile position coordinates. Due to the calculation error of surveyors in the construction process, the cause of engineering accidents is poor audit. Therefore, surveyors and engineers and technicians must carefully check the pile position before setting out to ensure the accuracy.

2. Dislocation caused by stopping drilling and changing machines.

The engineering geological conditions of the bridge are complex, so the corresponding drilling rig should be selected according to different geology. For Quaternary river bed alluvium (sand, clay, loess, silt), rotary drilling rig is generally used, while for weathered rock and river bed gravel layer, impact drilling rig is generally used. For the sand layer with large boulders under complex geological conditions (uneven strata), percussion drill, rotary drilling rig or geological core drilling rig are used to drill holes, and the deviation of drilling position causes the deviation of hole position when changing machines.

3. The hole collapses

Hole collapse is a common phenomenon in bored pile construction. It is characterized by the sudden drop of water level in the hole, foaming at the orifice, obvious increase in slag output, difficult or no footage, and obvious increase in rig load. If not handled in time, drilling will be buried or wasted. When the regional water level changes too much, adopt appropriate methods to keep the water head relatively stable. Find the hole collapse, determine the hole collapse position, analyze the hole collapse reason, and take feasible measures according to the hole collapse situation. If the amount of collapsed water is small, continue drilling, and backfill the mixture of sand and clay to lm ~ 2m above the collapsed hole, and continue drilling. Pay attention to the change of collapse times at any time. If the hole collapse can't be controlled, the borehole casing should be removed immediately and the borehole should be backfilled, then the casing should be buried again and the borehole should be drilled again. In case of hole collapse due to hole cleaning or lifting the steel skeleton, immediately stop hole cleaning or lifting the steel skeleton, stir with a drilling rig and add mud to protect the wall, and clean up the collapsed objects at the same time. After the hole collapse is stable, clean the hole again and install the steel skeleton.

Second, the construction technology

1. drill

(1) GPS-20 drilling rig of Shanghai Prospecting Machinery Factory is adopted for positive circulation construction. Adjust the center of drill bit and the center of pile testing position to ensure that the drill pipe is vertical when the drilling rig is in place.

(2) Report to the supervisor for inspection, and start drilling only after approval. First, start the mud pump and the rotary table to idle for a period of time, and then start drilling after the mud is sent into the hole for a certain amount.

At the beginning, the footage should be properly controlled, and low-grade slow drilling should be carried out at the edge foot of the fender, so that the edge foot can be protected by a solid mud wall. After drilling to the blade root 1ITI, it can be drilled normally according to the soil conditions.

(3) During drilling, the mud index shall be detected every 1h ~ 2h, and adjusted in time to meet the specification requirements. Check the aperture and verticality every 2m ~ 3m drilling, pay attention to the change of soil layer and check with geological data. If it is not in conformity with the geological data provided by the survey and design, the supervision engineer and design representative should be consulted, and drilling should be continued after confirmation, and the drilling speed and WOB parameters should be adjusted reasonably. Using decompression drilling method, the drill pipe keeps vertical and the bit rotates smoothly, which reduces the occurrence of inclined holes, reaming and bending holes. The action of picking up and unloading drill pipe should be done quickly and safely, so as to avoid stopping drilling for too long and increasing precipitation.

(4) Maintain the mud level (head) in the hole during drilling. If the mud level suddenly drops or rises, immediately lift the drill bit out of the hole, report to the relevant personnel, find out the reason, and then continue drilling after analyzing and judging the situation.

2. Borehole cleaning

(1) After the drilling reaches the design elevation, the final hole inspection (hole depth, aperture, inclination, etc.) shall be conducted immediately. ), meet the design requirements for the first time after hole cleaning.

(2) During hole cleaning, attention must be paid to keeping the head height at 1.5m ~ 2.0m, and at the same time, the mud index must be well controlled to avoid hole collapse caused by the decrease of mud relative density. It is not allowed to deepen the drilling depth instead of hole cleaning.

(3) After the conduit is installed, check the mud performance index in the hole and the sediment thickness at the bottom of the hole again. If it does not meet the requirements, the hole should be cleaned for the second time.

Three, the problems in the construction of bored pile foundation and preventive measures

The main problems, causes and corresponding preventive measures of bored piles are as follows.

1. The bearing capacity of foundation under pile is insufficient.

Cause: The pile end is not supported on the bearing stratum.

Prevention and control measures: this situation generally occurs in complex strata, and this kind of stratum is generally best to be coring and tested. If it is impossible to drill a hole for coring, it should be comprehensively considered with reference to the adjacent coring situation, drilling speed, cuttings returned by mud, drilling conditions (generally, when drilling slightly weathered rocks, the drill bit is not stuck, the vibration of the main drill pipe is not very strong, and the drilling sound feels good) and engineering geological data.

Shrinkage (the aperture is smaller than the design aperture) Reason: Plastic soil expands.

Preventive measures: When drilling, increase the pump capacity, speed up the drilling speed, and make the hole pass quickly. After the hole is formed for a period of time, mud skin will form on the hole wall, and the hole wall will not leak water and will not cause expansion. In case of shrinkage, adopt the method of repeatedly sweeping holes up and down to enlarge the aperture.

Reasons for too much sediment at the bottom of pile: insufficient inspection, unclean hole cleaning or no secondary hole cleaning.

Precautionary measures: (1) Check carefully. After cleaning the hole once with the correct measuring rope and hammer (2), if it does not meet the requirements, take measures to improve the mud performance and prolong the hole cleaning time. (3) After the reinforcing cage is lowered, check the precipitation again. If the amount of sediment exceeds the specification requirements, the hole should be cleaned for the second time. The catheter can be reused for secondary hole cleaning, that is, a connector is prepared, one end of which is connected with the catheter and the other end is connected with the rubber hose. After running the catheter, lift it 0.4m away from the bottom of the hole, and connect the connecting pipe with a mud pump to directly circulate mud. Secondary hole cleaning can ensure timely and effective cleaning of pile bottom.

2. The reinforcing cage floats up

Reason: (1) When pouring concrete under the reinforcing cage, if the conduit is lifted at this time and the bottom of the conduit is only about 1m away from the reinforcing cage, the reinforcing cage will be pushed up after flowing out of the conduit due to the great impact of the poured concrete. (2) Due to the concrete pouring in the reinforcing cage, the conduit is buried deeply.

When it is large, due to the long pouring time, the upper concrete is close to the initial setting, and the hard shell concrete formed on the surface has a certain holding force with the steel bars. If the concrete above the bottom of the steel bar at the bottom of the conduit is not mentioned in time, it will be jacked up at a certain speed after flowing out of the conduit, which will also drive the reinforcing cage to move up.

Precautions: (1) When pouring concrete, you should always know the concrete pouring elevation and the buried depth of the conduit. When the concrete is buried 2 ~ 3m above the bottom of the reinforcing cage, the conduit shall be lifted to the bottom of the reinforcing cage in time. (2) When it is found that the reinforcing cage begins to float, the pouring shall be stopped immediately, and the buried depth of the conduit and the elevation of the poured concrete shall be accurately calculated. After the conduit is hoisted, it can be poured to eliminate the floating phenomenon.

Cause of broken pile: (1) During the process of pouring concrete, the second drop of the ball caused by water leakage or conduit leakage is also the cause of mud inclusion and broken pile. There are two reasons for conduit leakage: First, when the conduit is blocked, the concrete is forced to flow out by vibrating up and down, but when the conduit is buried deeply, it is easy to leak. Second, the mud is too thick. If the concrete surface is difficult to estimate or measure, the pouring height of concrete will be misjudged when measuring the buried depth of the conduit, but when unloading the conduit, it will lift the conduit away from the concrete surface, resulting in leakage. Broken pile: (2) The pouring time is too long, and the upper concrete is close to the initial setting, forming a hard shell. With the increase of time, the residue in the mud will continue to precipitate, thus thickening the sediment accumulated on the concrete surface and causing concrete. (3) The catheter is buried too deeply, and the bottom is close to the initial setting when it is pulled out. After the catheter is pulled out, concrete cannot be poured in time, resulting in mud pouring and broken piles.

Prevention method: (1) Carefully clean the hole wall to prevent the hole wall from collapsing; (2) Increase the concrete pouring speed as much as possible: First, pile up as much concrete as possible when pouring concrete, which will generate huge impact and overcome mud resistance. Second, rapid and continuous pouring can keep the concrete and mud flowing all the time, which can prevent the conduit from blocking: (3) the conduit is lifted accurately and reliably, and the buried depth of the conduit is measured at any time during the concrete pouring process, and the operation procedures are strictly observed; (4) Before pouring underwater concrete, check whether the conduit leaks or bends, and replace it in time if any defect is found.

Four. conclusion

Pile foundation is the lifeblood of the whole bridge and the key link of bridge construction. The construction technology of bored pile is complex, with many interference factors and great difficulty. In the construction, we should combine the actual situation of the project, choose reasonable construction technology according to the requirements of design documents and construction specifications, adhere to the principle of putting prevention first, take effective measures to control the construction, solve various problems in the construction process in time, and complete the task with good quality and quantity.

References:

Liu Ziming. Bridge engineering inspection

Handbook [M], Beijing: People's Communications Publishing House.2001

[2] The First Highway Engineering Company of the Ministry of Communications. Handbook of Highway Construction: Bridges and Culverts (Part I) [M]. People's Communications Press. 2000

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