Drilled piles have been widely used in all kinds of housing and civil construction due to their adaptability to various geological conditions, simple and easy to operate construction, and the investment in equipment is generally not very large. Most of the construction of bored piles is carried out underwater, the construction process cannot be observed, and the excavation and acceptance cannot be carried out after the piles are formed. Problems in any part of the construction will directly affect the quality and progress of the whole project, and even cause huge economic losses and adverse social impacts to investors. Must prevent and control in the drilling process and underwater concrete filling process often construction quality problems, quality and quantity to complete the pile foundation construction task.
One, the drilling process of construction quality problems and prevention and control measures
1, the sheath water
Sheath wall water, serious will cause foundation sinking, sheath tilt and displacement, resulting in drilling skewed, and even can not be constructed.
Causes: the soil around the buried casing is not dense, or the casing water level difference is too large, or the drill bit collision when starting and stopping.
Preventive measures: when burying the cylinder, the pit and the surrounding area should be selected with the best water content of clay layered tamping. Open holes at the appropriate height of the barrel to maintain a head height of 1.0-1.5m in the barrel. When the drill bit rises and falls, collision with the shroud should be prevented. When water is found bubbling up from the casing, the drilling should be stopped immediately and the surrounding area should be filled with clay for reinforcement, and the casing should be reinstalled if the casing is seriously sinking or shifting.
2, hole wall collapse
Drilling process, such as found in the discharge of mud bubbles, or mud suddenly leakage, it is indicated that there are signs of hole wall collapse.
Causes: the main reason for the collapse of the hole wall is the loose soil, mud wall protection is not good, not tightly filled with clay around the shroud and the shroud of the water level is not high. Drilling speed is too fast, the empty drilling time is too long, too long after the hole to be filled and filling time is too long will also cause the hole wall collapse.
Preventive measures: in the loose and easy to collapse soil layer, properly buried deep shroud, with clay dense filling around the shroud, the use of high-quality mud, improve the specific gravity and viscosity of the mud, to keep the mud water level in the shroud is higher than the groundwater level. When carrying and lifting the reinforcing cage, deformation should be prevented, and the placement should be aligned with the hole position to avoid collision with the hole wall, and the welding time should be accelerated when the reinforcing cage is lengthened to shorten the sinking time as much as possible. After the hole is formed, the waiting time for filling should not be more than 3 hours in general, and the filling time of concrete should be controlled, and the filling time should be shortened as much as possible under the condition of ensuring the construction quality.
3, necking
Necking that the hole diameter is smaller than the design hole diameter.
Cause: plastic soil expansion.
Preventive measures: the use of high-quality mud, reduce water loss. Hole, should increase the pump volume, speed up the speed of holes, in the formation of holes for a period of time, the formation of the hole wall mud skin, then the hole wall will not seepage, and will not cause expansion. Or weld a certain number of alloy blades on the outer side of the guide to play a role in sweeping the hole when drilling or starting the drill. If necking occurs, the method of repeatedly sweeping the hole up and down is used to expand the hole diameter.
4, drilling skew
After the pile hole is formed, there is a large vertical deviation or bending.
Causes: poor stability of drilling rig installation, unstable installation of drilling rig or bending of drilling rods during the operation; the ground is soft or uneven hardness and softness; the soil layer is diagonally distributed or there are large isolated stones or other hard materials in the soil layer, and so on.
Preventive measures: the site will be tamped and leveled first, the track sleeper should be even on the ground; the installation of the drilling rig requires that the center of the rotary table and the drilling frame lifting pulley in the same axis, the position of the drilling rod deviation is not more than 20cm. when drilling in the uneven stratum, the use of self-supporting weight, the drilling rod stiffness of the drilling rig. When drilling in uneven stratum, inclined rock layer or encountering isolated rock, the drilling speed should be in slow gear. In addition, installing a guide device is also a simple and effective way to prevent hole skewing. When the hole is skewed, you can lift the drill bit, up and down repeatedly sweep the drill a few times, in order to cut off the hard soil, such as correcting the ineffective, should be partially backfilled with clay in the hole to the skewed hole of more than 0.5m, and then re-drill.
5, the pile bottom slag amount is too much
Causes: cleaning hole is not clean or not the second clearing; mud specific gravity is too small or mud injection is not enough and difficult to float the slag; reinforcing cage lifting and placing process, not aligned with the hole and collision with the hole wall to make the mud collapsed to the bottom of the pile; clearing the hole, waiting to be irrigated for a long time, resulting in the deposition of slurry.
Prevention measures: after the hole, the drill bit to raise the bottom of the hole 10-20cm, keep slow idling, maintain the cycle of clearing time of not less than 30 minutes. Use better performance mud, control the specific gravity and viscosity of mud, do not use water for replacement. When lifting and placing the reinforcement cage, keep the center of the cage in line with the center of the pile, and avoid collision with the hole wall. Cold compression jointing process can be used to accelerate the speed of buttressing reinforcement cage and reduce the time of empty hole, so as to reduce the slagging. After placing the cage, check the amount of slagging, if the amount of slagging exceeds the specification requirements, then the conduit should be used for the second clearing until the specific gravity of the slurry and the thickness of slagging at the mouth of the hole are in accordance with the specification requirements. Start filling concrete, the distance from the bottom of the conduit to the bottom of the hole should be 30-40mm, there should be enough concrete reserves, so that the conduit is buried below the concrete surface of more than 1.0m at a time, in order to utilize the concrete's huge impact to splash the bottom of the hole slag, to achieve the purpose of removing the bottom of the hole slag.
Two, underwater concrete filling process construction quality problems and preventive measures
1, card tube
The process of filling concrete in the water, can not continue the phenomenon.
Caused by: the initial irrigation, water plug plug tube; concrete and ease, poor fluidity caused by segregation; concrete coarse aggregate particle size is too large; a variety of mechanical failure caused by discontinuous pouring of concrete in the conduit to stay in the pipe for too long and tube; conduit into the water caused by the segregation of concrete and so on.
Prevention measures: the use of water spacer diameter should be matched with the inner diameter of the conduit, and at the same time has a good waterproof performance to ensure smooth discharge. During concrete filling, the control of concrete mixing time and concrete slump should be strengthened. Underwater concrete must have good compatibility, the proportion should be determined through the laboratory, the slump should be 18-22cm, the maximum particle size of coarse aggregate should not be larger than 1/4 of the diameter of the conduit and the minimum distance between the main bars of the cage and should be less than 40mm. in order to improve the compatibility of the concrete and retardation of the concrete, the underwater concrete is suitable for mixing with additives. The sealing of conduit connection parts should be ensured, and the conduit should be test assembled and pressure tested before use, and the water test pressure should be 0.6-1.0MPa to avoid water ingress into the conduit. In the process of concrete pouring, concrete should be poured slowly into the conduit of the funnel to avoid the formation of high-pressure air plugs in the conduit. During the construction process, mechanical equipment should be monitored at all times to ensure that the machinery operates normally and avoid mechanical accidents.
2, rebar cage floating
The phenomenon that the position of the rebar cage is higher than the design position.
Causes: reinforcing cage placed in the initial position is too high, the concrete fluidity is too small, the conduit is buried in the concrete depth is too large reinforcing cage by the concrete drag top rise; when the concrete to the reinforcing cage, if this time to raise the conduit, the bottom of the conduit from the reinforcing cage is only 1m or so, due to the pouring of concrete from the conduit out of the impact force is greater, promote the uplift of the reinforcing cage; due to concrete, the conduit is filled over the reinforcing cage and the concrete is not the same as the concrete. Pouring over the cage and conduit depth is greater, the upper layer of concrete due to pouring time is longer, has been close to the initial set, the surface of the formation of a hard shell, the concrete and the cage has a certain degree of cohesion, so that the bottom of the conduit is not mentioned in time to mention the bottom of the cage above, the concrete in the conduit after the outflow of the concrete will be at a certain speed upward to the top of the lift, and at the same time also led to the cage rise.
Prevention measures: the initial position of the cage should be accurately positioned, and fixed firmly with the hole. Speed up the concrete filling speed, shorten the filling time, or mix additives to prevent the top layer of concrete into the cage when the mobility becomes small, concrete close to the cage, control the conduit depth of 1.5-2.0 m. In the process of filling concrete, should always grasp the elevation of the concrete pouring and conduit depth, when the concrete is buried over the bottom of the reinforcing steel cage 2-3m, should be timely lifted to the catheter to the bottom of the reinforcing steel cage above. The buried depth of the conduit in the concrete surface is generally suitable to keep in 2-4m, should not be greater than 5m and less than 1m, it is strictly prohibited to put the conduit out of the concrete surface. When the cage floating, should immediately stop filling concrete, and accurately calculate the depth of the conduit and the elevation of the poured concrete surface, lift the conduit and then pouring, floating phenomenon can disappear.
3, broken piles
Concrete solidification is not continuous, the middle of the washout liquid and other loose body and soil filling the formation of interrupted piles.
Causes: because the bottom of the conduit is too far from the bottom of the hole, the concrete is diluted by the washing liquid, so that the water-cement ratio increases, resulting in the concrete does not solidify, the formation of concrete between the pile and the bedrock is filled with concrete that does not solidify; by the impact of groundwater activities or poor sealing of the conduit, washing liquid immersed in concrete water-cement ratio increases, the formation of the pile in the middle of the concrete does not solidify; due to the pouring of concrete. The conduit lifting and pulling too much, exposed concrete surface, or due to power outages, waiting for the material and other reasons caused by slag, the pile body slag deposited into layers, the concrete pile up and down the phenomenon; pouring concrete, not from the conduit filling, and the use of direct pouring from the mouth of the hole into the method of filling the concrete, resulting in concrete segregation caused by solidification is not dense and hard, and individual holes appeared loose, the phenomenon of the hole section, the hollow.
Prevention measures: after the hole, must be carefully cleaned, generally using flushing liquid cleaning, punching time should be based on the hole slagging situation, punching should be filled with concrete in a timely manner, to avoid the bottom of the hole slag more than the specification. Measure the hole diameter carefully before pouring concrete, and accurately calculate the whole hole and the first concrete pouring amount. In the process of concrete pouring, the elevation of the concrete surface and the burial depth of the conduit should be controlled at any time, and the lifting of the conduit should be accurate and reliable, and the operating procedures should be strictly observed. Strictly determine the mixing ratio of concrete, concrete should have good ease and fluidity, slump loss should meet the filling requirements. In the section with large groundwater activity, it should be treated with casing or cement beforehand, and the concrete can be filled only after the water stopping is successful. The concrete should be filled from the conduit, and the filling process is required to be continuous and fast, and the concrete ready to be filled should be in sufficient quantity, and power and water cuts should be avoided in the process of filling concrete. The wire to help tie the cement waterproof plug should be determined according to the amount of the first concrete filling, and should be strictly prevented from breaking. Ensure the sealing of the conduit, the length of the conduit removal should be based on the height of the rise of concrete inside and outside the conduit, do not rise and pull too much.
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