Discussion on the control of over grouting at the

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Preliminary study on the control of over grouting at the top of bored cast-in-place piles

bored cast-in-place piles can pass through various soil layers with complex soil or large changes in soft and hard, and strengthen the foundation. It has a wide range of adaptability to the bearing capacity (300 ~ 20000 kg), simple construction machines and tools, and the construction process has many advantages, such as low noise, little impact on adjacent buildings, good construction safety, etc., so it is widely used in foundation reinforcement projects. However, due to its many construction links, high technical requirements and complex process, it is necessary to quickly complete the pouring of underwater pouring concrete concealed works in a short time, which makes it impossible to directly control the quality. Therefore, human factors have a great impact, and a little negligence is prone to quality defects, especially the common phenomenon of pile top overfilling

I. cause analysis of over grouting

1. Cause analysis of laitance formation according to the construction specifications, the sludge at the bottom of the hole should be completely removed after drilling. However, in the actual construction process, it is difficult to completely remove the sludge. During the bottom sealing construction, the first batch of concrete rushed out of the bottom opening of the tremie and flowed and diffused around the hole bottom, and mixed with the flushing liquid in the hole to form a floating slurry dilution layer of a certain thickness. As the underwater concrete poured with the tremie can be expected to rise from bottom to top, when the tremie is buried to a certain depth, the subsequent poured concrete flows inside the poured concrete, and the first poured concrete is always at the top layer, and finally solidifies into a mixed layer of laitance, sludge and so on at the top of the pile. When pouring concrete, if the tremie is inserted too deep into the concrete, the pouring speed is fast, and it is easy to deposit more aggregate in the deep part of the hole. In addition, the segregation of concrete during vibration also easily leads to the lower strength of the upper part of the pile. The mixed layer and the inferior concrete layer under it have low strength and should be chiseled out

when the laitance layer is at the top of the pile, as long as a certain amount of over grouting is maintained, that is, to ensure the strength of the pile at the design elevation, the quality of the pile can be guaranteed. If the laitance layer is at the bottom of the pile or in the middle of the pile body, mud inclusion or pile breaking will be formed, and the quality of the pile body will not meet the requirements. Therefore, the concrete pouring technology is extremely important

2. Analysis of exposed pile head

1) limited by factors such as formation conditions, construction machines and tools, pore forming process and flushing medium, slag discharge is not complete, the effect of hole cleaning is poor, and the bottom of the hole is heavily silted. Under this condition, if the correct underwater concrete pouring process is adopted and the designed over pouring amount is not less than 50 cm, the pile body design requirements can be met and the over pouring part can be easily chiseled out

2) the hole forming process is reasonable, the hole is cleaned thoroughly, and there is almost no sediment or little sediment at the bottom of the hole. At the same time, the concrete is poured in strict accordance with the operating procedures. The resulting pile head laitance layer is very thin, generally 10 ~ 20 cm. Most of the concrete strength of the over poured pile section meets the design requirements

3) the hole cleaning is not complete, there is more sludge at the bottom of the pile, and the operation is not strictly in accordance with the underwater concrete pouring process specification. In this case, the laitance layer at the pile head is not thick, but there are hidden dangers of more sediment at the bottom of the pile, mud inclusion in the pile body and even pile breaking, and the over poured part is not easy to chisel out. In addition, the inaccurate measurement of the rising surface of concrete during the pouring process will also lead to different heights of pile heads. Some piles are over poured by about 2 meters, while others cannot reach the pile height

second, the influence of hole forming technology and grouting technology

according to the flow direction of circulating medium (mud), bored cast-in-place piles are divided into positive circulation rotary drilling and reverse circulation rotary drilling. During positive circulation drilling, the mud is pumped into the inner cavity of the drill pipe by the mud pump, suspended at the bottom of the hole and carries the drilling slag, and then returns to the ground through the annular space between the drill pipe and the hole wall to realize slag removal and wall protection. During reverse circulation drilling, the mud enters the borehole from the annular gap between the drill pipe and the hole wall, and then returns to the orifice from the drill pipe to discharge the muck. The two methods are different in terms of equipment, operation technology, adaptability to formation, hole diameter, operation depth, etc., and have different effects on the quality of hole forming and pile forming and construction speed

the drilling method of positive circulation drilling has mature technology and simple operation. It is suitable for all kinds of clay layers, sandy soil layers and bedrock, and can also be used in soil layers with gravel and pebble content less than 15%, so it has been widely used for many years

however, for large-diameter pile holes, in order to improve the ability of slurry suspension and carrying drilling slag, the method of increasing the specific gravity and viscosity of slurry is generally adopted, but the increase of slurry consistency leads to the thickness of mud skin on the hole wall and the difficulty of hole cleaning. If the reverse circulation method is adopted for the second hole cleaning, the sedimentation at the bottom of the hole will be reduced. This is a bold and successful initiative to improve and solve the needs of consumers. The laitance layer is relatively reduced

in recent years, high-rise buildings are getting higher and higher, and the diameter of engineering piles is getting larger and larger. In order to ensure the quality of piles. 2. Conscientiously implement the responsibility and bearing capacity, the construction technology of large-diameter pile hole drilling at home and abroad has been continuously improved and perfected. Taking slag removal and hole cleaning as an example, the use of reverse circulation drilling can be effectively solved

III. control of over grouting volume at the top of bored piles

from the above analysis, it can be seen that the establishment of over grouting volume should comprehensively consider various factors such as formation conditions, pore size, mud performance, pore forming technology and grouting technology, especially pore forming technology. For the cast-in-place pile drilled by positive circulation, the over grouting height of about 50 cm is more reasonable; For reverse circulation drilling, the overfilling volume should be reduced. How to solve the problem of oil trapping of rich testing machines? The professional team with rich experience, strong technical force and high operation level can meet the requirements when the overfilling volume is set at about 20 cm. It should be pointed out that no matter what kind of hole forming process is adopted, the concrete must be poured in strict accordance with the regulations, and there must be no pile quality accidents

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