Analysis of quality problems of rotary drilling bored pile
Common quality problems of rotary drilling bored piles
When the rotary drill bored pile is subjected to vertical load pressure, the pressure on the top of the pile is the largest, and the pressure on the lower part is relatively small. However, the pile forming process of the rotary drilling bored pile is contrary to the actual stress condition. The strength of the upper concrete is often low, and the strength of the middle and lower concrete is high. If not strictly controlled, the strength of the upper section of the pile may not meet the quality requirements. Happening. In addition, it is also prone to quality defects such as necking, hole wall collapse, hole bottom sedimentation, pile body cavity, honeycomb, mud inclusion, etc., resulting in a decrease in the bearing capacity of the pile foundation and affecting the safety of the engineering structure.
Analysis of the reasons affecting the strength of the pile body
1. According to the regulations of the construction specification, the silt at the bottom of the hole must be completely removed after drilling, but in the actual construction process, it is difficult to completely remove the silt, so when the first bucket of concrete is poured for the bottom sealing construction, the silt deposited at the bottom of the hole It must be mixed into concrete. Since the underwater concrete poured by the pipe is lifted from bottom to top, the concrete poured first rises above the hole, so that the upper section of the pile tends to have lower strength.
2. When pouring concrete, if the pipe is inserted too deep into the concrete and the pouring speed is faster, it is easy to deposit more aggregates in the deep part of the hole, and the segregation of the concrete caused by the vibrating process will also easily lead to piles. The quality problem of lower body upper strength.
1. In the construction of concrete pouring, if the pipe is inserted into the concrete too shallowly <1.5m, the rise of the concrete during the pile formation process is not a jacking type, but a paving type. At this time, mud and mud blocks are easily mixed in In the concrete, it affects the quality of the pile. In addition, if the designed pile diameter is too small, the concrete will be restricted by the hole wall when the concrete is turned up, which will cause the pile body to produce voids and honeycomb defects.
2. The bearing capacity of bored piles is mainly manifested as the friction resistance around the pile, and the friction resistance around the pile is closely related to the shape of the hole wall and the quality of the wall. During the construction process, the shape of the hole wall is determined by factors such as the rotation speed of the drill bit, the descent speed of the drill pipe and the soil quality. The mud properties including bulk density, viscosity, colloid rate, sand ratio and other indicators are better and higher, the better the protection For wall protection, the greater the friction resistance around the pile, the greater the difficulty of construction and the corresponding increase in costs.
3. During the whole process of drilling and forming holes, removing the drill pipe mud, stopping the circulation to the lifting of the steel cage, and pouring the underwater concrete, there are many construction links and long time, which will cause thicker silt at the bottom of the hole to affect pile formation. quality. The longer the standing time, the more silt will be deposited.
4. When concrete is poured underwater, its fluidity, initial setting time, and cohesive performance will become worse. If you neglect a little, it is easy to produce voids, honeycombs, segregation, mud and even broken piles. .
Measures to ensure the strength of the upper section of the pile
1. According to the pile diameter and the concentration of the pile bottom, the volume of the first bucket of concrete is correctly determined. Generally, it can be 1.5～2.0m, and it can also be controlled by 10% of the design volume of the pile.
2. The pile quality is related to the pouring height of the pile body. Generally, the pile height is controlled to be 0.5~1.0m higher than the designed pile top elevation. After cutting off the higher part of the concrete, the remaining part should not have laitance and mud, and the concrete number should meet the design requirements, otherwise it will be reworked and repoured.
3. The length of the pipe inserted into the concrete should be appropriate, generally 2-6m, and the long pile can be increased accordingly.
Acceptance of pile quality
The inspection content and method of the quality of the cast-in-place pile should meet the requirements of the specification. Usually, the pile load test or the large-strain measurement method is used to detect the bearing capacity. Less than 10% of the total number of piles. If required by the project, the number of inspections can be increased accordingly until the pile is inspected one by one.