For rotary bored piles, controlling the thickness of sediment at the bottom of the pile is the key to quality control of pile foundation construction. Excessive sediment thickness will not only cause insufficient bearing capacity of the pile foundation but also cause excessive building settlement. This article analyzes the quality problems in the pile foundation construction of a commercial and residential building project, and the causes and thickness control of the sediment at the bottom of the hole in the thick sand stratum of the rotary drilled cast-in-place pile are discussed.
Overview
With the increasing number of high-rise buildings, the requirements for the bearing capacity of pile foundations are also getting higher and higher. However, due to the particularity of the construction technology of rotary bored piles, their bearing capacity is greatly affected by factors during construction. In particular, the sediment layer at the bottom of the hole seriously affects the bearing capacity of the pile foundation. Therefore, strengthening the quality control of the sediment thickness at the bottom of the hole during the construction process is one of the key measures to ensure the construction quality of rotary bored piles and the bearing capacity of the pile foundation.
The pile foundation pile types of commercial and residential building projects are all rotary-drilled bored piles, with a total of about 805 piles. The construction workload is heavy and the construction schedule is tight. Through the analysis of quality problems in the pile foundation construction of a commercial and residential building project, the key points and countermeasures for controlling the thickness of sediment at the bottom of the hole in rotary bored piles in thick sand strata are discussed.
Analysis of the causes of excessive sedimentation at the bottom of the hole for rotary drilled cast-in-place piles in thick sandy strata
1. Insufficient setting of sedimentation tank
First of all, due to the small number of sedimentation tanks installed at the construction site, the two machines share one sedimentation tank. The low sedimentation efficiency is the main reason for the excessive sediment thickness at the bottom of the hole.
2. Long-vacant time after hole formation
There are a large number of piles in this project. Factors such as frequent use of various materials, chaotic site, untimely supply of concrete, and the long production time of steel cages affect the vacant time of the piles after they are drilled. The construction processes are not compact, and the interval between the completion of hole formation and concrete pouring is too long, resulting in the precipitation of sand in the mud in the hole, or the mud losing water and settling.
3. The sand content of the mud is too high
In thick sand formations, the control of sand content is particularly critical. Through on-site inspection of the incoming mud, it was found that the sand content of the mud did not meet the requirements.
4. Insufficient hole cleaning and slag cleaning
First, due to improper control of the mud index and insufficient hole cleaning to avoid hole collapse, the drilling slag at the bottom of the hole could not be removed. In addition, due to the shortage of water at the construction site, insufficient replacement of mud in the hole to save water also caused the hole. The main reason for excessive bottom sediment thickness.
5. The initial amount of concrete filling is insufficient and no water isolation plugs are installed.
The volume of the storage hopper at the construction site is only about 0.8m3, which cannot meet the requirement that the initial filling volume can bury the conduit more than 0.8 m below the concrete surface at one time; and the pile foundation construction unit has not set up a water isolating bolt to pass the shear ball (or twitch (partition) for pouring, but directly pour concrete into a conduit filled with mud for pouring concrete, causing the concrete at the pile end to segregate, thereby increasing the amount of sediment at the bottom of the hole.
Measures to control the thickness of sediment at the bottom of the hole in thick sandy strata for rotary boring cast-in-place piles
1. Optimize sedimentation tank settings
(1) Reasonably optimize the site layout, increase the number of sedimentation tanks and change the number of sedimentation tanks from two machines and one tank to one machine and one tank. Dedicated machines and dedicated tanks are not allowed to mix or borrow sedimentation tanks.
(2) While increasing the number of sedimentation tanks, increase the area of the sedimentation tank from the original 5x3m to 6x4m, and add partitions in the sedimentation tank.
2. Shorten the idle time after hole formation
(1) Develop a material usage plan and prepare sufficient materials;
(2) Clean up the debris on the site promptly and keep the construction roads open;
(3) Increase the number of steelworkers and increase the speed of steel cage production;
(4) Strictly control the concrete delivery time. If the concrete truck arrives early, you will be rewarded; if the concrete arrives late, you will be punished. For repeated delays, you will need to interview the general manager of the concrete supply company.
3. Control the sand content of mud
(1) Establish a system for inspecting mud every time it enters the site to control mud quality and prevent substandard mud from entering the site.
(2) During use, strictly control the sand content of the mud circulation after replacement
4. Ensure thorough and adequate hole cleaning
The purpose of hole cleaning is to remove the sediment at the bottom of the hole, and the sediment at the bottom of the hole is one of the main factors affecting the bearing capacity of cast-in-place piles. Therefore, it is necessary to ensure thorough and sufficient cleaning.
One hole cleaning
After the cast-in-place pile is drilled to the designed elevation or predetermined level, the drill pipe or slurry delivery pipe should be fully utilized to clean the hole for the first time in situ. The purpose of the first hole cleaning is to discharge the granular matter in the hole out of the hole and reduce the number of holes. Remove the sediment at the bottom and save time for secondary hole cleaning. Generally, there is no need to adjust the mud density for this hole cleaning, because if the mud density is lowered too early, the particles in the mud will settle quickly during the cage hoisting process, which will affect the effect of the secondary hole cleaning. Generally, the mud density is kept at 1.2 ~1.4, when the thickness of the sediment at the bottom of the hole is measured to be less than 50mm, seize the time to hoist the steel cage.
Second hole cleaning
The first hole cleaning is carried out after the final hole. It usually takes 4 hours after installing the steel cage, welding, lowering the conduit, etc. During this time, because the mud in the hole is in a static posture, the mud originally suspended in the mud, Gravel, and stone chips will sink to the bottom of the hole. At the same time, when installing the steel cage and conduit, they will also rub against the hole wall, causing the mud and sand to fall into the hole. For this reason, the conduit is used to clean the hole a second time before concrete pouring. During secondary hole cleaning, the sediment at the bottom of the hole should be measured while circulating the hole. When the thickness of the sediment at the bottom of the hole meets the design and specification requirements, various mud indicators will be adjusted during the cycle. When the hole cleaning is terminated, the mud indicators are generally controlled within the following range: The relative density is between 1.05 and 1.10; the viscosity is 16~20S; the sand content is <4%. When all the indicators of the mud are measured to meet the specifications, the underwater concrete pouring work should be carried out immediately. While waiting for the concrete, the hole cleaning should continue to be circulated until the concrete arrives on site and is loaded into the hopper for pouring.
5. Accurately measure sediment thickness
The sediment thickness measurement must use the correct measurement method. Improper measurement methods will cause misjudgment of the sediment thickness, and the sediment thickness cannot be effectively controlled. To accurately measure the sediment thickness at the bottom of the hole, you must first accurately measure the final hole depth of the pile, and the final diameter of the pile. The depth of the hole should be measured by measuring the length of the drill pipe or the length of the steel measuring rope. The length of the drill pipe or the length of the steel measuring rope should be checked using a standard tape measure. Sediment thickness = final hole depth – depth after hole cleaning. Clean the hole until the sediment thickness meets the specification requirements. At present, the detection methods of sediment thickness include measuring rope (weight) detection, sampling box detection, and sediment meter detection.
6. Ensure the first pouring amount of concrete
During the initial pouring of underwater concrete, to avoid and reduce the mixing of concrete pouring out from the lower port of the conduit and the mud in the hole, and to ensure the quality of the pile concrete, during the first pouring, in addition to requiring that the surface of the concrete in the storage hopper and the mud in the conduit be in contact with each other, In addition to installing water isolators at intervals, the “Technical Regulations for Construction Pile Foundations” also stipulates that “there should be sufficient concrete reserves so that the conduits can be buried more than 0.8 m below the concrete surface at one time.” The first irrigation volume should be determined by calculation based on the hole diameter, hole depth, mud concentration, conduit diameter, distance between the bottom of the conduit and the bottom of the hole, etc., using the principle of connecting pipes.
During construction, try to increase the initial pouring volume of the first plate of concrete. The first plate of concrete should ensure that the buried pipe depth is at least 0.8m. The second plate should be poured continuously to increase the impact of concrete on the residual sediment at the bottom of the hole so that the sediment at the bottom of the hole will not It is smoothly discharge from the hole under intermittent impact.
While ensuring the hole-cleaning effect, process connections should be strengthened to improve work efficiency and shorten the time between hole cleaning and concrete pouring as much as possible.
Many factors cause the sediment layer at the bottom of rotary bored pile holes to be too thick. During construction, the construction plan should be carefully formulated based on the specific machinery and equipment, construction characteristics, and geological conditions, construction should be carried out in strict accordance with relevant specifications and requirements, and detection and control should be strengthened. It was found that When the sediment is too thick, handle it in time to control the thickness of the sediment layer at the bottom of the hole within the allowable range of the specification to ensure the construction quality and bearing capacity of the rotary bored bored piles.
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