Common construction difficulties
Due to the fast construction speed, relatively stable quality and little impact of climate factors, underwater bored pile foundations have been widely adopted. The basic construction process of bored pile foundations: construction layout, laying casing, drilling rig in place, clearing the bottom hole, impregnating steel cage ballast, secondary retention catheter, underwater concrete pouring and clearing the hole, pile. Due to the complexity of the factors affecting the quality of underwater concrete pouring, the construction quality control link often becomes a difficult point in the quality control of underwater bored pile foundations.
Common problems in underwater concrete pouring construction include: serious air and water leakage in the catheter, and pile breakage. The concrete, mud or capsule that forms a loose layered structure has a floating slurry interlayer, which directly causes the pile to break, affecting the quality of the concrete and causing the pile to be abandoned and redone; the length of the conduit buried in the concrete is too deep, which increases the friction around it and makes it impossible to pull the conduit out, resulting in the pile breaking phenomenon, which makes the pouring not smooth, causing the concrete outside the conduit to lose fluidity over time and deteriorate; the workability and slump of concrete with a low sand content and other factors may cause the conduit to be blocked, resulting in broken casting strips. When pouring again, the position deviation is not handled in time, and a floating slurry interlayer will appear in the concrete, causing Pile breakage; due to the increase in concrete waiting time, the fluidity of concrete inside the pipe becomes worse, so that the mixed concrete cannot be poured normally; the casing and foundation are not good, which will cause water in the casing wall, causing the surrounding ground to sink and the pile quality cannot be guaranteed; due to actual geological reasons and incorrect drilling, it is possible to cause the hole wall to collapse; due to the error of the final hole test or the serious hole collapse during the process, the subsequent precipitation under the steel cage is too thick, or the pouring height is not in place, resulting in a long pile; due to the staff’s carelessness or the wrong operation, the acoustic detection tube cannot work normally, resulting in the ultrasonic detection of the pile foundation cannot be carried out normally.
“The mix ratio of concrete should be accurate
1. Cement selection
Under normal circumstances. Most of the cement used in our general construction is ordinary silicate and silicate cement. Generally, the initial setting time should not be earlier than two and a half hours, and its strength should be higher than 42.5 degrees. The cement used in the construction should pass the physical property test in the laboratory to meet the requirements of the actual construction, and the actual amount of cement in the concrete should not exceed 500 kilograms per cubic meter, and it should be used strictly in accordance with the specified standards.
2. Aggregate selection
There are two actual choices of aggregates. There are two types of aggregates, one is pebble gravel and the other is crushed stone. In the actual construction process, pebble gravel should be the first choice. The actual particle size of the aggregate should be between 0.1667 and 0.125 of the conduit, and the minimum distance from the steel bar should be 0.25, and the particle size should be guaranteed to be within 40 mm. The actual grade ratio of coarse aggregate should ensure that the concrete has good workability, and fine aggregate is preferably medium and coarse gravel. The actual probability of sand content in concrete should be between 9/20 and 1/2. The ratio of water to ash should be between 1/2 and 3/5.
3. Improve workability
In order to increase the workability of concrete, Do not add other admixtures to the concrete. The concrete admixtures used in underwater construction include water reducing, slow-release and drought-strengthening agents. If you want to add admixtures to concrete, you must conduct experiments to determine the type, amount and procedure of adding.
In short, the concrete mix ratio must be suitable for underwater pouring in the conduit. The concrete mix ratio should be suitable so that it has sufficient plasticity and cohesion, good fluidity in the conduit during the pouring process and is not prone to segregation. Generally speaking, when the underwater concrete strength is high, the durability of the concrete will also be good. So from the strength of cement The concrete quality should be ensured by considering the concrete grade, the total ratio of the actual amount of cement and water, the performance of various doping additives, etc. And ensure that the concrete grade ratio strength grade should be higher than the designed strength. The concrete mixing time should be appropriate and the mixing should be uniform. If the mixing is uneven or water seepage occurs during the concrete mixing and transportation, the concrete fluidity is poor and it cannot be used.
“First pouring quantity requirements
The first pouring quantity of concrete should ensure that the depth of the conduit buried in the concrete after the concrete is poured is not less than 1.0m, so that the concrete column in the conduit and the mud pressure outside the pipe are balanced. The first pouring quantity of concrete should be determined by calculation according to the following formula.
V=π/4（d 2h1+kD 2h2）
Where V is the initial concrete pouring volume, m3;
h1 is the height required for the concrete column in the conduit to balance the pressure with the mud outside the conduit:
h1=（h-h2）γw /γc, m;
h is the drilling depth, m;
h2 is the height of the concrete surface outside the conduit after the initial concrete pouring, which is 1.3～1.8m;
γw is the mud density, which is 11～12kN/m3;
γc is the concrete density, which is 23～24kN/m3;
d is the inner diameter of the conduit, m;
D is the pile hole diameter, m;
k is the concrete filling coefficient, which is k =1.1～1.3.
The initial pouring volume is extremely important to the quality of the cast-in-place pile. A reasonable first pouring volume can not only ensure smooth construction, but also ensure that the depth of the concrete buried pipe meets the requirements after the funnel is filled. At the same time, the first pouring can effectively improve the bearing capacity of the pile foundation by flushing the sediment at the bottom of the hole again, so the first pouring volume must be strictly required.
“Pouring speed control
First, analyze the conversion mechanism of the pile body’s deadweight transmitting force to the soil layer. The pile-soil interaction of bored piles begins to form when the pile body concrete is poured. The first poured concrete gradually becomes dense, compressed, and settles under the pressure of the later poured concrete. This displacement relative to the soil is subject to the upward resistance of the surrounding soil layer, and the weight of the pile body is gradually transferred to the soil layer through this resistance. For piles with fast pouring, when all the concrete is poured, although the concrete has not yet initially set, it is continuously impacted and compacted during pouring and penetrates into the surrounding soil layers. At this time, the concrete is different from ordinary fluids, and the adhesion to the soil and its own shear resistance have formed resistance; while for piles with slow pouring, since the concrete is close to initial setting, the resistance between it and the soil wall will be greater.
The proportion of the deadweight of bored piles transferred to the surrounding soil layer is directly related to the pouring speed. The faster the pouring speed, the smaller the proportion of the weight transferred to the soil layer around the pile; the slower the pouring speed, the larger the proportion of the weight transferred to the soil layer around the pile. Therefore, increasing the pouring speed not only plays a good role in ensuring the homogeneity of the concrete of the pile body, but also allows the weight of the pile body to be stored more at the bottom of the pile, reducing the burden of friction resistance around the pile, and the reaction force at the bottom of the pile is rarely exerted in the future use, which plays a certain role in improving the stress condition of the pile foundation and improving the use effect.
Practice has proved that the faster and smoother the pouring work of a pile, the better the quality of the pile; the more delays, the more likely accidents will occur, so it is necessary to achieve rapid and continuous pouring.
The pouring time of each pile is controlled according to the initial setting time of the initial concrete, and a retarder can be added in an appropriate amount if necessary.
“Control the buried depth of the conduit
During the underwater concrete pouring process, if the depth of the conduit buried in the concrete is moderate, the concrete will spread evenly, have good density, and its surface will be relatively flat; on the contrary, if the concrete spreads unevenly, the surface slope is large, it is easy to disperse and segregate, affecting the quality, so the reasonable buried depth of the conduit must be controlled to ensure the quality of the pile body.
The buried depth of the conduit is too large or too small, which will affect the quality of the pile. When the buried depth is too small, the concrete will easily overturn the concrete surface in the hole and roll in the sediment, causing mud or even broken piles. It is also easy to pull the conduit out of the concrete surface during operation; when the buried depth is too large, the concrete lifting resistance is very large, and the concrete is unable to push up in parallel, but only pushes up along the outer wall of the conduit to the vicinity of the top surface and then moves to the four sides. This eddy current is also easy to roll the sediment around the pile body, producing a circle of inferior concrete, which affects the strength of the pile body. In addition, when the buried depth is large, the upper concrete does not move for a long time, the slump loss is large, and it is easy to cause pile breakage accidents caused by pipe blocking. Therefore, the buried depth of the conduit is generally controlled within 2 to 6 meters, and for large-diameter and extra-long piles, it can be controlled within the range of 3 to 8 meters. The pouring process should be frequently lifted and removed, and the elevation of the concrete surface in the hole should be accurately measured before removing the conduit.
“Control the hole cleaning time
After the hole is completed, the next process should be carried out in time. After the second hole cleaning is accepted, concrete pouring should be carried out as soon as possible, and the stagnation time should not be too long. If the stagnation time is too long, the solid particles in the mud will adhere to the hole wall to form a thick mud skin due to the certain permeability of the hole wall soil layer. The mud skin is sandwiched between the concrete and the soil wall during concrete pouring, which has a lubricating effect and reduces the friction between the concrete and the soil wall. In addition, if the soil wall is soaked in mud for a long time, some properties of the soil will also change. Some soil layers may swell and the strength will decrease, which will also affect the bearing capacity of the pile. Therefore, during construction, the requirements of the specifications should be strictly followed, and the time from hole formation to concrete pouring should be shortened as much as possible. After the hole is cleaned and qualified, concrete should be poured as soon as possible within 30 minutes.
“Control the quality of concrete at the top of the pile
Since the upper load is transmitted through the top of the pile, the strength of the concrete at the top of the pile must meet the design requirements. When pouring close to the elevation of the pile top, the last pouring amount should be controlled, and the slump of the concrete can be appropriately reduced so that the over-pouring of the concrete at the top of the pile is higher than the designed elevation of the pile top by one pile diameter, so that the requirements of the design elevation can be met after the floating slurry layer at the top of the pile is removed, and the strength of the concrete at the top of the pile must meet the design requirements. The over-pouring height of large-diameter and extra-long piles should be considered comprehensively based on the pile length and pile diameter, and should be larger than that of general cast-in-place piles, because large-diameter and extra-long piles take a long time to pour, and the sediment and floating slurry accumulate thickly, which prevents the measuring rope from being difficult to accurately judge the surface of thick mud or concrete and causing mismeasurement. When pulling out the last section of the guide tube, the pulling speed should be slow to prevent the thick mud precipitated on the top of the pile from squeezing in and forming a “mud core”.
During the process of underwater concrete pouring, there are many links that deserve attention in order to ensure the quality of piles. During the secondary hole cleaning, the performance indicators of the mud should be controlled. The mud density should be between 1.15 and 1.25 according to the different soil layers, the sand content should be ≤8%, and the viscosity should be ≤28s; the thickness of the sediment at the bottom of the hole should be accurately measured before pouring, and pouring can only be done when it meets the design requirements; the connection of the conduit should be straight and sealed, and the conduit should be pressure tested before and after use for a period of time. The pressure used for the pressure test is based on the maximum pressure that may occur during construction, and the pressure resistance should reach 0.6-0.9MPa; before pouring, in order to allow the water stopper to be discharged smoothly, the distance between the bottom of the conduit and the bottom of the hole should be controlled at 0. 3～0.5m. For piles with a standard diameter of less than 600, the distance between the bottom of the conduit and the bottom of the hole can be appropriately increased; before pouring concrete, 0.1～0.2m3 of 1:1.5 cement mortar should be poured into the funnel first, and then concrete should be poured.
In addition, during the pouring process, when the concrete in the conduit is not full and air enters, the subsequent concrete should be slowly injected into the funnel and conduit through the chute. Concrete should not be poured into the conduit from above to avoid forming a high-pressure air bag in the conduit, squeezing out the rubber pads between the pipe sections and causing the conduit to leak. During the pouring process, a dedicated person should measure the rising height of the concrete surface in the hole, fill in the underwater concrete pouring record, and record all faults during the pouring process.
“Common problems and solutions
1. Mud and water in the conduit
Mud and water in the conduit used for pouring underwater concrete is also a common construction quality problem in the construction of cast-in-place piles. The main phenomenon is that when pouring concrete, mud gushes in the conduit, the concrete is polluted, the strength is reduced, and interlayers are formed, causing leakage. It is mainly caused by the following reasons.
1) The reserve of the first batch of concrete is insufficient, or although the reserve of concrete is sufficient, the distance between the bottom of the conduit and the bottom of the hole is too large, and the bottom of the conduit cannot be buried after the concrete falls, so that mud and water enter from the bottom.
2) The depth of the conduit inserted into the concrete is not enough, so that the mud is mixed into the conduit.
3) The conduit joint is not tight, the rubber pad between the joints is squeezed open by the high-pressure airbag of the conduit, or the weld is broken, and water flows into the joint or weld. The conduit is pulled out too much, and the mud is squeezed into the pipe.
In order to avoid mud and water entering the conduit, corresponding measures should be taken in advance to prevent it. The main preventive measures are as follows.
1) The amount of the first batch of concrete should be determined by calculation, and sufficient quantity and downward force should be maintained to discharge the mud out of the conduit.
2) The conduit mouth should be kept at a distance of not less than 300 mm to 500 mm from the bottom of the groove.
3) The depth of the conduit inserted into the concrete should be kept at not less than 2.0 m.
4) Pay attention to controlling the pouring speed during pouring, and often use a hammer (clock) to measure the concrete rising surface. According to the measured height, determine the speed and height of pulling out the guide tube.
If water (mud) enters the guide tube during construction, the cause of the accident should be immediately found out and the following treatment methods should be adopted.
1) If it is caused by the first or second reasons mentioned above, if the depth of the concrete at the bottom of the trench is less than 0.5 m, the water stopper can be re-placed to pour concrete. Otherwise, the guide tube should be pulled out, the concrete at the bottom of the trench should be cleared out with an air suction machine, and the concrete should be re-poured; or a guide tube with a movable bottom cover should be inserted into the concrete and the concrete should be re-poured.
2) If it is caused by the third reason, the slurry guide tube should be pulled out and re-inserted into the concrete about 1 m, and the mud and water in the slurry guide tube should be sucked out and drained with a mud suction pump, and then the waterproof plug should be added to re-pour the concrete. For the re-poured concrete, the cement dosage should be increased in the first two plates. After the concrete is poured into the guide tube, the guide tube should be slightly lifted, and the bottom plug should be pressed out by the deadweight of the new concrete, and then the pouring should continue.
2. Pipe blocking
During the pouring process, if the concrete cannot go down in the conduit, it is called pipe blocking. There are two cases of pipe blocking.
1) When the concrete starts to be poured, the water stopper is stuck in the conduit, causing a temporary interruption of pouring. The reasons are: the water stopper (ball) is not made and processed in regular sizes, the size deviation is too large, and it is stuck in the conduit and cannot be flushed out; before the conduit is lowered, the concrete slurry residue on the inner wall is not completely cleaned; the concrete slump is too large, the workability is poor, and the sand is squeezed between the water stopper (ball) and the conduit, so that the water stopper cannot go down.
2) The concrete conduit is blocked by concrete, the concrete cannot go down, and it is difficult to pour smoothly. The reasons are: the distance between the conduit mouth and the bottom of the hole is too small or it is inserted into the sediment at the bottom of the hole, making it difficult for concrete to be squeezed out from the bottom of the pipe; the concrete downward impact is insufficient or the concrete slump is too small, the stone particle size is too large, the sand ratio is too small, the fluidity is poor, and the concrete is difficult to fall; the interval between pouring and feeding is too long, the concrete becomes thicker, the fluidity decreases, or it has solidified.
For the above two situations, analyze the causes of their occurrence and take favorable preventive measures, such as the processing and manufacturing size of the water stopper must meet the requirements, the conduit must be cleaned before pouring concrete, the mixing quality and pouring time of the concrete must be strictly controlled, the distance between the conduit and the bottom of the hole must be calculated, and the amount of initial concrete must be accurately calculated.
If a pipe blockage occurs, analyze the cause of the problem and find out which type of pipe blockage it belongs to. The following two methods can be used to deal with the type of pipe blockage: if it is the first type mentioned above, it can be dealt with by tamping (upper blockage), upsetting, and dismantling (middle and lower blockage). If it is the second type, long steel bars can be welded to ram the concrete in the pipe to make the concrete fall. For minor pipe blockage, the crane can be used to shake the pipe rope and install an attached vibrator at the pipe mouth to make the concrete fall. If it still cannot fall, the pipe should be immediately pulled out and dismantled section by section, and the concrete in the pipe should be cleaned up. The pouring work should be re-performed according to the method caused by the third reason of water inflow into the pipe.
3. Buried pipe
The pipe cannot be pulled out during the pouring process or the pipe cannot be pulled out after the pouring is completed. It is generally called buried pipe, which is often caused by the deep burial of the pipe. However, the pouring time is too long, the pipe is not moved in time, or the steel bars on the steel cage are not welded firmly, and the pipe is collided and scattered during the hanging and pouring of concrete, and the pipe is stuck, which is also the reason for the buried pipe.
Preventive measures: When pouring underwater concrete, a special person should be assigned to regularly measure the buried depth of the conduit in the concrete. Generally, it should be controlled within 2 m~6 m. When pouring concrete, the conduit should be shaken slightly to prevent the conduit from sticking to the concrete. The pouring time of concrete should be shortened as much as possible. If it is necessary to intermittently, the conduit should be pulled to the minimum buried depth. Before lowering the steel cage, check that the welding is firm and there should be no open welding. When the steel cage is found to be loose during the lowering of the conduit, it should be corrected and welded firmly in time.
If the buried pipe accident has occurred, the conduit should be immediately lifted by a large-tonnage crane. If the conduit still cannot be pulled out, measures should be taken to forcefully pull the conduit off, and then deal with it in the same way as the broken pile. If the concrete has not initially solidified and the fluidity has not decreased when the conduit is buried, the mud residue on the surface of the concrete can be sucked out with a mud suction pump, and then the conduit can be re-lowered and re-poured with concrete. The treatment method during pouring is similar to the third reason of water in the conduit.
4. Insufficient pouring
Insufficient pouring is also called short pile. The reason is: after the pouring is completed, due to the collapse of the hole mouth or the excessive weight of the mud on the lower top, the slurry residue is too thick. The construction personnel did not measure the concrete surface with the hammer, but mistakenly thought that the concrete had been poured to the designed elevation of the pile top, resulting in an accident caused by the short pile pouring.
The prevention measures include the following aspects.
1) The hole mouth casing must be buried in strict accordance with the requirements of the specification to prevent the hole mouth from collapsing, and the hole mouth collapse phenomenon must be dealt with in time during the drilling process.
2) After the pile is bored, the sediment must be cleared in time to ensure that the sediment thickness meets the requirements of the specification.
3) Strictly control the mud weight of the drilling wall protection so that the mud weight is controlled between 1.1 and 1.15, and the mud weight within 500 mm of the bottom of the hole before pouring concrete should be less than 1.25, the sand content ≤8%, and the viscosity ≤28s.
The treatment method depends on the specific situation. If there is no groundwater, the pile head can be dug out, the pile head floating slurry and soil can be manually chiseled off to expose the new concrete joint, and then the formwork can be supported for pile connection; if it is in groundwater, the casing can be extended and buried 50 cm below the original concrete surface, and the mud pump can be used to drain the mud, remove the debris, and then clean the pile head for pile connection.
5. Broken piles
Most of them are secondary results caused by the above problems. In addition, due to incomplete hole cleaning or too long pouring time, the first batch of concrete has been initially set and the fluidity has decreased, and the continued concrete breaks through the top layer and rises, so there will be mud and slag in the two layers of concrete, and even the whole pile will be sandwiched with mud and slag to form a broken pile. For the prevention and control of broken piles, it is mainly necessary to do a good job in the prevention and control of the above problems. For broken piles that have occurred, they should be studied together with the competent department, design unit, engineering supervision and the superior leadership unit of the construction unit to propose practical and feasible treatment methods.
According to past experience, the following treatment methods can be adopted if broken piles occur.
1) After the pile is broken, if the steel cage can be taken out, it should be taken out quickly, and then the hole should be re-drilled with an impact drill. After the hole is cleaned, the steel cage should be lowered and the concrete should be re-poured.
2) If the pile is broken due to pipe blockage and the poured concrete has not initially solidified, after the conduit is taken out and cleaned, the top surface position of the poured concrete is measured with a hammer, and the volume of the funnel and conduit is accurately calculated. The conduit is lowered to a position 10 cm above the top surface of the poured concrete and a ball bladder is added. Continue to pour concrete. When the concrete in the funnel fills the conduit, press the conduit below the top surface of the poured concrete, and the wet joint pile is completed.
3) If the pile is broken due to collapse or the conduit cannot be pulled out, a pile supplement plan can be proposed in conjunction with the design unit in combination with the quality accident handling report, and the piles can be supplemented on both sides of the original pile.
4) If a broken pile is found during the pile body inspection, the pile has been formed at this time, and the unit can be consulted to study the treatment method of grouting reinforcement. For details, please refer to the relevant pile foundation reinforcement information.