Construction engineering; Filling pile; Drilling construction; Grouting construction
I. Introduction
Piles formed by mechanical or artificial methods to form holes and pouring concrete into the holes are called bored piles, which are referred to as bored piles for short. At first, bored piles were mainly used in foundation underpinning projects in Europe. However, with the increase of building reconstruction and expansion projects worldwide and the development of urban municipal engineering, cast-in-place piles have been rapidly popularized and applied worldwide. In recent years, their application scope has been extended to slope stability and reinforcement, retaining wall of underground engineering, alternating load foundation of high-rise buildings and structures, industrial alternating load equipment foundation, anti-floating foundation of pool structure, open control support of deep foundation pit and foundation reinforcement of urban reconstruction and expansion projects. Compared with other pile types, the advantages of bored pile are obvious:
(1) construction produces little noise and vibration, which is suitable for urban operation with strict pollution control, especially for urban construction with dense buildings or many underground obstacles.
(2) If pressure grouting is adopted in the construction of cast-in-place pile, the pile-soil combination is close, the surface friction of pile-soil is large and the bearing capacity is high. From the mechanical mechanism of cast-in-place pile, it can be seen that the volume bearing capacity of unit pile is much greater than other pile types.
(3) Because the diameter of the pile can be designed to be very small, the setting of the pile in the wall and foundation soil will hardly produce any stress and will not cause great disturbance to the wall and foundation soil, which can effectively meet the requirement that the construction will not affect the use and original appearance of the building.
(4) The settlement of the pile under the design load is very small, which can meet the engineering requirements of buildings with strict restrictions on settlement. At the same time, it is suitable for all kinds of soil layers except the pebble layer with larger particle size. It can not only carry out dry-binding operation above the water surface, but also carry out drilling and piling in the presence of groundwater.
(5) Less steel, simple process and less machines.
Second, the pile foundation mechanism of cast-in-place pile
When the bearing capacity of the building foundation can't meet the current load, or the upper load needs to be increased in reconstruction and expansion, or the deformation of the foundation needs to be controlled within a certain range, the foundation is underpinning and reinforced with cast-in-place piles, and part of the load of the upper structure is transferred to the deep part of the foundation through piles, so as to reduce the additional stress acting on the basement soil layer, reduce the settlement and improve the bearing capacity of the foundation. In order to keep the deformation coordination between pile and soil, even if the building only produces the minimum settlement, the pile will react quickly and bear part of the load of the building, thus reducing the soil reaction under the foundation. Because the modulus of the pile is much greater than that of the soil between piles, the stress in the soil will be concentrated on the pile, so that the vertical stress borne by the pile is much greater than that in the soil. With the increase of settlement, the proportion of the load shared by the pile in the total load increases until the load borne by the pile reaches its ultimate bearing capacity.
Third, the construction technology of bored pile
3. 1 Selection of pile construction machinery
There are many methods to form holes for piles, such as rotary drilling rig, impact drilling rig, mud wall casing follow-up, manual Luoyang shovel, etc. The commonly used system in China is mechanical pore-forming. For cohesive soil, silt, muddy soil, sandy soil and other strata, submersible drilling rig can be used to form holes; For gravel soil, sandy soil, cohesive soil, strongly weathered rock and other strata, rotary drilling rig can be used to form holes; For gravel soil, sandy pebbles, strongly weathered rocks and other strata, punching or impact drilling machine can be used to form holes.
3.2 Selection of pore-forming construction technology
There are two kinds of bored piles: dry drilling and wet drilling. Dry drilling generally adopts spiral drilling machine or artificial hole-forming. At present, wet drilling method is mainly used to form holes, that is, water or mud is circulated during drilling, and water and clay are mixed into mud to protect the wall. The diameter of cast-in-place pile is generally small, and the positive circulation drilling method is often used in practical engineering.
In engineering, the construction technology of straight piles and inclined piles is slightly different. The conventional straight pile construction technology is drilling, lowering the reinforcing cage, lowering the conduit and pouring concrete. The conventional construction technology of inclined piles is as follows: with the deepening of drilling, the outer wall casing will deepen the wall protection, lower the steel bar or cage, lower the grouting pipe, carry out grouting and pull out the casing.
3.3 Determination of inclination angle after drilling rig is in place
After the drilling rig is in place, the drilling rig direction and vertical shaft angle should be adjusted according to the drilling inclination and orientation designed by the construction. The deviation of pile position is not more than 20mm, and the vertical deviation of straight pile is not more than 65438 0%. The inclination of the inclined pile should be adjusted according to the design requirements.
3.4 Drilling Technology
When drilling, the hydraulic pressure of the drilling rig should be controlled at 1.5-2.5MPa, the supporting water supply pressure is 0. 1-0.3MPa, the ROP is generally controlled at 220 r/min, and the maximum advancing distance is generally 500mm// time. When drilling a vertical pile hole, except for the thick miscellaneous fill layer on the surface, it is generally not necessary to set a hole protector, or only set a hole protector casing at the orifice, but it should be 10cm higher than the ground to prevent the drilling tool from collapsing when it hits the orifice. When crossing the miscellaneous fill stratum, the wall protection casing should be set, and when drilling the inclined hole, the outer wall wall protection casing should be deepened with the drilling. When drilling through silt and muddy soil, the drilling speed should be slowed down to cover the hole wall with a layer of mud to protect the pile hole.
At the same time, when drilling, the drill bit should be the same as the design diameter of the pile, and the drilling depth should be 0.5- 1.0m longer than the design pile. In order to improve the bearing capacity of piles, reaming drill can be used to reach the design elevation. When drilling into pebble layer or other easily collapsed soil layer, cement slurry can be injected into the hollow drill pipe, and the slurry is discharged from the drill bit to implement pile hole wall protection.
3.5 hole cleaning treatment
Mud or clear water can be used to protect the borehole wall when drilling. When cleaning holes, observe the overflow of mud at any time and control the water supply pressure. Mud should be changed continuously until concrete pouring is completed. When the soil of the hole wall is good and it is not easy to collapse, a suction fan can be used to clean the hole, and the original soil should be pulped. The relative density of mud after cleaning should be controlled at around 1. 1; When the soil quality of the hole wall is poor, it is easy to clean the hole by circulating mud, and the relative density of mud is controlled at 10 15 ~ 1.25.
3.6 Hoisting reinforcing cage and burying conduit
Reinforcing cages are generally hoisted by cranes, and sometimes they can be lowered by the rig's own derrick. In order to ensure that the hoisting reinforcing cage does not bend and deform, the top of the reinforcing cage should be provided with a cross beam. When hoisting the reinforcing cage, the hole wall should be prevented from being scratched, and it should be slowly lifted and gradually lowered. After the reinforcing cage is in place, check whether the top surface reaches the design elevation. And temporarily fix the reinforcing cage to prevent it from moving up and down.
The main equipment for concrete pouring of conduit stone requires that the conduit can pass through the concrete to meet the construction requirements, the connection is straight, the joint is well sealed, the wall thickness of the conduit is not less than 3mm, the diameter is 200 ~ 300 mm, the length of the bottom tube is not less than 4 meters, and the bottom of the conduit is 300 ~ 500 mm away from the bottom of the hole, and pouring is started.
3.7 pouring concrete
Pour the concrete into the funnel. At the beginning of pouring, there should be enough concrete reserves to make the conduit buried more than 0.8 meters below the concrete surface at a time. At the same time, concrete is poured continuously, and the buried depth of the conduit is calculated to ensure that the buried depth of the conduit is more than 2m. Remove the redundant conduit. In the subsequent grouting, the rising height of the concrete surface should be measured regularly with a hammer, and the grouting amount and buried depth should be accurately recorded to ensure that the buried depth of the conduit is not less than 2m. After the last concrete pouring is completed, test whether the concrete surface reaches the pouring height with a hammer, take out the concrete sample, and finish pouring.
label
Through analyzing the application mechanism of cast-in-place pile in the foundation of building engineering, it puts forward the concrete application technology and matters needing attention of cast-in-place pile in the foundation, and summarizes the corresponding construction parameters, which can effectively guide similar projects.
refer to
[1] Sun You. On the construction technology of bored pile foundation [J]. China new technology and new products, 2012,28 (03):118 ~119.
Ren Weiliang is from Liu Zhigang. Construction technology and quality control of bored pile [J]. Heilongjiang Science and Technology Information, 2008,27 (04): 53 ~ 55.
It's cloudy Study on construction technology and quality control of bored pile [D]. Master's degree thesis of Huazhong University of Science and Technology, 2007.