Construction technology of bored piles for Yangxi Bridge?

The construction technology of bored piles for Yangxi Bridge is very important. The technology is formulated based on the actual situation and the application of professional knowledge. It is critical to use the technology rationally to achieve the expected results. Zhongda Consulting will explain to you the construction technology of bored piles for Yangxi Bridge.

1 Project Overview

The upper structure of the main bridge of Yangxi Bridge (Hangzhou-Xinjing Expressway Connection Line) is a 120m four-tower, three-span, double-cable plane self-anchored suspension bridge, and the lower structure is a bearing Desktop type, the upper structure of the approach bridge is a 30m prestressed small box girder, and the lower structure is in the form of tie beam, column and cap beam. The main approach bridge foundation uses bored piles, all of which are rock-socketed pile foundations. The pile diameters are 1.8m and 1.2m. , the pile length ranges from 16.5m to 30.5m. The geological situation is relatively complex. Due to sand mining, the upper part is basically a pebble layer, the lower part is mainly weathered dolomite and weathered carbonaceous siliceous mudstone, and part of the lower part is a stone coal layer. The construction uses impact drilling to create holes and underwater C30 concrete pouring.

2 Construction technology

2.1 Construction stakeout

The center of the bridge foundation pile is directly positioned using a coordinate method total station. During construction, a frame guide frame is laid out according to the designed pile position, a casing is installed, and the drilling deviation is checked at all times according to the distance between the casing and the drilling rig sling.

2.2 Casing installation

The Yangxi Bridge is mainly based on bored piles with a diameter of 1.8m, so the casing is rolled from 14mm steel plates with an inner diameter of 2.0m. Section 1.5m, fully dense seam welding. When burying, the guide device is made through the constructed construction platform, and then sunk into the casing under pressure using a vibrating hammer. The height of the upper surface of the casing is slightly lower than the upper surface of the construction platform, which facilitates operation and safety protection. The casing sinks through the pebble layer to reach the weathered rock layer to effectively control the occurrence of hole collapse. If necessary, follow up on the casing during drilling. The deviation of the plane position of the casing shall not be greater than 5cm, and the deflection shall not be greater than 1%.

2.3 Mud preparation

The mud used for drilling of Yangxi Bridge is composed of high-quality loess found locally as raw material. In the complex geology of Yangxi Bridge, mud is of great significance, balancing piles. External water pressure forms mud skin on the hole wall, blocking the seepage inside and outside the hole and preventing hole collapse. The pH value of the engineering mud is preferably 8 to 10, the relative density is 1.1 to 1.15, the viscosity is 20S to 35S, and the colloid rate is not less than 95%. Before impact drilling into a hole, maintain the water head in the casing and put the loess into the casing. Use the impact drill hammer to adjust the mud up and down and then start drilling. As the drilling depth increases, loess should be put in time. If necessary, the mud performance can be ensured. The indicator remains unchanged.

2.4 Drilling

This bridge uses impact drilling to drill holes. The drill hammer is made of cast iron, weighs about 8t, has a plum cone shape, and an outer diameter of 1.78m. In order to improve its ability to impact and crush stones, an alloy cone is welded around the drill hammer, and a JKL6 (A) winch is used to lift the drill hammer. Before drilling, the drilling rig must be accurately aligned to check whether the wire rope is in the pile, whether the wire rope has sufficient load-bearing capacity (whether it is damaged), and whether the steering device is intact. When starting drilling, the small stroke impact should be increased at a constant speed, generally 1.0m~1.5m. When the drilling reaches a depth of 3m, the stroke can be increased, generally 1.5m~2.0m. It is strictly prohibited to increase the stroke to prevent the upper wall of the cave. Drill stuck due to sudden rupture. When entering 1m above the boundary line of rock strata, appropriately reduce the stroke to smoothly cross the boundary line and prevent the occurrence of off-center and inclined holes. When the impact drill is just starting to drill, the drill frame is easy to sink and deviate. During construction, the hole position should be checked frequently. You can use a tape measure to measure the relative position of the wire rope and the casing for rough measurement. If adjustment is needed, it should be corrected with a jack in time. Drill stand. Impacting rock formations will cause greater wear on the drill hammer. The wear of the drill hammer needs to be checked frequently and repaired in the form of bevels in time to avoid shrinkage holes. The hammer head steering device should be lifted up to check to prevent it from malfunctioning and causing plum blossoms. hole.

2.4.1 Hole collapse

(1) Hole collapse refers to the situation where the soil or part of the rock mass on the hole wall falls off into the hole, mainly due to excessive pressure outside the hole. This results in reduced hole wall stability and hole collapse. The main reasons are as follows: 1) Mud making was not carried out in time, which caused the performance index of the mud in the hole to decline, resulting in the rock and soil collapse of the hole wall; 2) Mud leakage: During the drilling process, the mud passed through the soil layer or non-drilled holes. seepage, resulting in a decrease in mud performance indicators.

(2) Based on the causes of hole collapse, we can analyze that the remedial measures are of course to reduce the harm of the accident and prevent the accident from happening again. According to different situations, the following measures are taken: 1) After a general hole collapse occurs, water is first injected into the hole to increase the water level and balance the pressure outside the hole. Then add loess to increase the specific gravity of the mud; 2) For collapsed holes caused by karst and severe fissures, continue to lengthen the casing to plug leaks. If necessary, bagged cement can also be added to form cement slurry for leak repair.

(3) Preventive measures: 1) Monitor mud performance indicators at all times; 2) Pay great attention to the water level in the hole and keep the water level in the hole 1 to 2m higher than the outside. If the water level decreases, it should be Observe whether there is mud leakage around and determine whether there is mud leakage. If there is mud leakage, first maintain the water level in the hole and add clay or loess to maintain the mud performance indicators. Analyze the cause in time and refer to the treatment methods for remediation.

2.4.2 Drill hole deflection

(1) Cause analysis: The geological inclination at the junction of rock layers causes the ram to lose balance during drilling and cause tunnel deflection, such as Yangxi Bridge 7# and 10# pier pile foundations. (2) Treatment measures: When it is found that the hole channel is deflected, put the same stone in the hole as the rock layer to the position where the inclined hole is started, correct the hole position, drill with a small stroke, and repeat it many times to pass the inclined rock layer. If it still cannot be effectively solved, underwater concrete can be poured. After reaching a certain strength, drill through the inclined rock formation with a small stroke. (3) Preventive measures: Before constructing bored piles, carefully study the geological report, reduce the drill stroke when the drilling elevation is close to the junction of the designed rock surface, and always pay attention to the verticality of the wire rope to determine whether there is an inclined hole phenomenon. .

2.4.3 Hammer stuck

(1) Cause analysis: 1) buried hammer caused by hole collapse; 2) hammer head lowered too fast for repair welding; 3) due to oblique The hammer head rests diagonally in the hole.

(2) Treatment measures: After the hammer jams during construction, first analyze the cause of the hammer jam and try to use the hammer head of the drilling rig team to pull. If it still fails, use a crane to assist and pull the hammer at the same time. Do not use continuous force when pulling. Because lifting and lowering are repeated intermittently, excessive force may cause the drilling rig or crane to overturn. Repeated actions can loosen the soil or rock wall, and excessive pulling may also cause damage to the hole wall and make hammer jam more serious. For the phenomenon of hammer stuck due to hole collapse, generally the pressed drill hammer cannot be lifted by pulling. You can try to use a crane with a homemade small drill to bury the hammer around, and then use the drill to shake and try to remove the hammer. Sometimes it can also be combined with underwater blasting to loosen the hammer head. If you repeat this several times, you can generally successfully remove the hammer head. The 8#-7 pile foundation of Yangxi Bridge got stuck due to hole collapse. The hammer was successfully removed through the above method and the economic losses were restored. Homemade small drills can be made into long strips using some section steel. The Yangxi Bridge once used a circle shape, but the actual effect was not good. Because the weight was too light, it failed to achieve the expected effect.

(3) Preventive measures: 1) Prevent the occurrence of hole collapse and use small stroke drilling in more complex geological intervals to reduce the possibility of buried hammer; 2) Hammer heads that have just been repaired and welded with hammer teeth , it is not advisable to lower it too fast, and it is not advisable to drill, because when the hammer is dropped slowly, the lowest point should be penetrated 1m deeper from the rock surface each time, and lifted up, repeated several times, until the bottom of the hole can be reached without any obstruction, then normal drilling can be done. . 2.5 Pouring of underwater concrete

(1) The pouring of the first batch of concrete is the key to the success of the full pile concrete pouring and must be strictly controlled. (2) Based on the comparison of construction experience, the previous plug valve was changed at the junction between the lower opening of the funnel and the top opening of the conduit, and the lifting plate and soft cushion method was adopted. (3) While the concrete mixture continues to be poured, focus should be placed on controlling the maximum and minimum burial depth of the conduit. After removing the conduit and before pouring, the burial depth of the conduit should be controlled at 2 to 6m. The current burial depth during construction should be comprehensively analyzed based on the concrete supply situation, weather conditions, water temperature, etc. (4) When concrete is poured into the upper part of the pile foundation, a long-handled sampling basin can be used to determine the density of the concrete.

2.6 Grouting accident handling

(1) Short piles. It is caused by insufficient concrete pouring, mainly due to inaccurate measurements. During the underwater concrete pouring process of the pile foundation, each pouring must be measured to verify whether it is normal with data. Generally, a measuring rope is used for measurement, and the hammer head is more important. The Yangxi Bridge is equipped with a conical thick-bottomed hollow hammer head. , the lower surface is a flat surface to feel the concrete surface, and the weight of the hammer head allows people to feel the process of passing through the concrete surface to achieve accurate measurement of the surface. The hollow design allows part of the concrete stones to be retrieved to re-verify the correctness of the data.

(2) Broken piles and mud interlayers. Broken piles are caused by the interruption of the pouring due to special reasons during the pouring process. The pile heads are too far away from the designed position and cannot be connected. This situation requires us to take preventive measures and confirm that all mechanical equipment is in place before pouring. . Broken piles may also cause mud interlayers due to the deposition of mud in the mud. There are generally two reasons for the formation of mud interlayers: 1) During the pouring process, because the buried pipe is too shallow, the concrete poured later breaks through the surface of the concrete poured first and covers the mud layer deposited on the surface of the original concrete to form an interlayer broken pile. 2) The conduit is buried too shallowly or the conduit is pulled out of the concrete, causing water to enter the conduit. The above two accidents are mainly caused by inaccurate control of the minimum burial depth of the conduit during the filling process. Effective measures should be taken during construction, and careful and careful measurements should be taken to avoid accidents. (3) The catheter cannot be lifted out. The main reason is that the buried depth of the pipe is too large and exceeds the lifting capacity. When the buried depth of the pipe exceeds 8m during the pouring process, the pipe must be removed in time. Priority is given to using pipes with threaded connections instead of flange-connected pipes, because the flange will increase the resistance of the contact surface with the concrete. During the construction process, it is also possible that the conduit cannot be lifted out because the concrete is not supplied in time and the conduit is buried for too long. Therefore, while the concrete is waiting, the conduit must be allowed to sway within a certain range to ensure its normal operation.

2.7 Experience

(1) Detailed construction organization design and emergency response measures must be formulated for the construction of bored piles, and experienced technical personnel should be selected to provide guidance during the construction. Before pouring underwater concrete, the safety and working conditions of construction machines should be carefully checked, and other equipment to deal with construction accidents should be prepared to prevent accidents.

(2) When constructing bored piles in complex geological areas, read the geological histogram in detail when drilling to achieve early prevention. When a hole collapse occurs, make sure that the hammer head is not buried. In comparison, impact drills are much more flexible than rotary drills, so the use of impact drills should be given priority in construction in lava areas.

(3) Bored cast-in-place piles, as the name suggests, have two key points, drilling and pouring. During the drilling process, attention should be paid to process control, which requires specialized professional technicians to control on-site. Pay attention to the following issues from top to bottom: 1) Verticality of the wire rope (to prevent inclined holes); 2) Position of the wire rope relative to the casing (Prevent eccentric holes); 3) Relative external water level in the hole and mud performance indicators (prevent hole collapse); 4) Water quality outside the casing (observe whether there is leakage of slurry); 5) Clean the hole frequently, do a good job of mud circulation, and conduct timely analysis Rock samples, flexible control of drilling methods (selection of large and small strokes).

3 Conclusion

In the pouring process, the first pouring is particularly important and is an irreversible process. If the first pouring fails, it will inevitably cause waste of concrete and seriously affect the construction progress. The calculation of the first irrigation volume must be accurate. Only after all preparations have been made can the first irrigation work be carried out. If there is any abnormality, the irrigation should be stopped immediately to reduce unnecessary losses. The first pouring of concrete should be constructed with concrete with a relatively large slump, which can effectively avoid interlayers and broken piles caused by the solidification and cracking of the first pouring concrete. The key to the perfusion process is speed in the early stage, and on-site measurement and recording must be fast. Always pay attention to the relationship between the pouring volume and the elevation of the concrete surface to comprehensively judge the accuracy of the measurement. When the concrete surface is close to the design pile top elevation, accuracy takes the dominant position. The concrete surface should be 0.5 to 1m higher than the design pile top elevation. When necessary, use a long-handled container to judge the concrete surface. When the concrete surface is uncertain, it is better to pour more concrete to ensure the quality of the pile foundation.

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