The Chinese name of CFG (Cement Fly Ash Gravel) pile is cement fly ash gravel pile, which is a high cohesive strength pile composed of cement, fly ash, gravel, stone chips or sand and water in a certain proportion, and forms a composite foundation with soil and cushion between piles. It can not only give full play to the potential of pile materials, but also make full use of the bearing capacity of natural foundation. It has the advantages of high work efficiency, low cost, small post-construction deformation and fast settlement stability.
CFG pile foundation treatment includes CFG pile body, pile cap (plate) and cushion.
Structural form: pile+slab, pile+cap+mattress (this form is adopted in this bid section).
Second, CFG pile construction technology
1, equipment selection and equipment
CFG pile can be constructed by vibrating immersed tube drilling rig or long spiral drilling rig. What kind of piling machinery and model to choose depends on the specific situation of the project. For cohesive soil, silt and muddy soil, vibration sinking pile technology is adopted. In areas with hard geological conditions of soil layer, the use of vibration sinking pipe will cause great vibration to the pile, which will lead to cracking or fracture of the pile body. For sensitive soil, vibration will destroy the structural strength and reduce the bearing capacity of soil. Spiral drilling can be used to pilot the hole, and then vibration sinking pipe pile-forming technology can be adopted. In areas where high-quality hole-forming is needed, long spiral drill pipes are used for pumping into piles. The design of this bid section adopts long spiral drilling rig for construction.
There are also two types of construction machinery for pumping concrete with long spiral drill pipes: walking type and crawler type, as shown in the following figure:
Crawler long auger drill
Walking long spiral drilling rig
According to the schedule and process test, implement the equipment configuration and timely maintenance, so that all machinery is in normal state, meet the construction needs, and do not affect the construction progress and quality.
2, choose materials and mixture ratio
The selected raw materials such as cement, fly ash, macadam and admixture shall meet the requirements and relevant standards for quality acceptance of raw materials, and shall be sampled as required.
According to the design requirements, indoor mixture ratio test is carried out to select the appropriate mixture ratio.
3. Process flow
3- 1 Level the site
Inspect, dismantle and protect the above-ground and underground pipelines on the construction site. According to the comparison between the measured original ground elevation and the designed pile top elevation, the ground vegetation will be cleared and the drilling site will be leveled. The ground elevation should be 50cm higher than the designed pile top elevation, drainage slope should be reserved, drainage ditch and other measures should be taken, and the original ground should be crushed to K30≥30MPa/m with a roller to meet the requirements of self-weight and overturning resistance of the long auger.
3-2 Construction lofting
On the pile position, drill a hole with a depth of 20cm vertically with a steel bar with a diameter of 8mm, pour lime water or lime powder into the hole, and insert markers in the hole, which can be used as disposable wooden and bamboo chopsticks to facilitate the search after the pile position is buried.
construction site
3-3 rig in place
Control of drill pipe verticality: After the drilling rig is in place, the pile driver hangs a bidirectional vertical ball, and the standing personnel can determine whether the verticality deviation of the fuselage meets the specification or design requirements by measuring the relative distance between the vertical ball and the upper and lower ends of the drill pipe, and ensure that the allowable verticality deviation of CFG pile is not more than 65438 0.0%.
Pile position control: Before each pile is constructed, mark the adjacent vertical and horizontal pile positions with a ruler to determine whether the pile machine is aimed at the pile center.
Before the construction of each pile, the standing personnel should check the verticality of the drill pipe of the pile centering machine, and the drilling can be started only after it meets the requirements.
Deviation of pile position exceeds the standard
3-4 drilling
When drilling begins, close the drill bit valve, move the drill pipe downward until the drill bit touches the ground, and start the motor drilling. Generally speaking, we should slow down first and then fast, which can not only reduce the sloshing of drill pipe, but also check the deviation of drilling so as to correct it in time. If it is found that the drill pipe is shaking or drilling is difficult during the hole-forming process, the footage should be slowed down and the verticality of the drill pipe and the pile position should be checked in time, otherwise it will easily lead to the deviation and displacement of the pile hole, and even damage the drill pipe and drilling tools.
Pile length control: clearly mark the length of the pile driver fuselage. In order to facilitate the construction control at night, reflective materials need to be used for marking; The minimum scale of recognition is generally 50cm or 25cm；; According to the footage mark on the rig tower, stop drilling when the hole reaches the design elevation. Drilling waste slag should be transported to the designated place in time.
3-5 pouring concrete
When the long auger reaches the design elevation, stop drilling and pump concrete into the drill pipe core. When the drill pipe core is full of mixed liquid, start to lift the drill pipe, inject the mixed liquid under pressure, and pump while tripping out. It is forbidden to lift the pipe first and then pump the material. The slump and mixing time of concrete shall be controlled according to the parameters determined by process test, and the mixing time shall not be less than 65438±0min. When pumping concrete, it is forbidden to pump concrete (especially end-bearing piles) after extubation. When tripping out, the drill pipe must stop rotating, and the tripping speed should be controlled according to the parameters determined by the test pile. Pull-out speed should be uniform, and it should penetrate into hard geological area, and low-grade slow drilling, soft stratum and fast drilling should be adopted to reduce reaming coefficient. Concrete pumping must also be continuous. If the concrete is not supplied in time, it will cause the situation of drilling rig and other materials, and it may cause the phenomenon of broken piles or drilling holes for a long time, especially in saturated sand and silt. The concrete shall be poured to 50cm above the top elevation of the designed CFG pile, and the pumping of concrete shall be stopped. The volume of pouring concrete should not be less than the design pile length plus the volume of over-pouring pile head.
3-6 Displacement of drilling rig
According to the specific situation of the drilling rig, the drilling rig usually moves once every 4 or 5 holes are drilled. Regardless of the distance between piles, it is not suitable to push the construction in a circle from all sides, because this will limit the lateral deformation of soil between piles, easily lead to large-scale uplift of soil, and the possibility of broken piles is greater, so the scheme of pushing from the center to the outside or from one side to the other can be adopted. When there is a weak layer on the ground, and the phenomenon of hole channeling occurs, the drilling rig should adopt hole jumping operation, and then drill the adjacent pile position after the mixture in the adjacent position is solidified.
3-7 Removing drilling mud
Drilling mud must be removed at any time during drilling, and the remaining drilling mud must be removed after the rig moves. It should be pointed out that:
① Except for 50 small excavators, no mechanical equipment is allowed to enter the CFG pile drilling and grouting site.
(2) When digging, the original soil between piles below the designed pile top elevation shall not be disturbed.
(3) The poured CFG pile shall not be excavated to avoid damaging the pile head.
3-8 Nature Protection
CFG piles are all buried underground, and the pile top is covered with wet clay, which does not need special maintenance. After the grouting of piles is completed, the excavation and cleaning of soil between piles can only be carried out when the age of mixture reaches 14 days. During natural curing, the drilling and grouting site must be protected, and no machinery is allowed to enter the drilling and grouting area.
3-9 Cut the pile head and excavate the soil between piles.
When digging between piles, the excavator is easy to collide with the pile head, resulting in shallow pile breakage. During construction, the pile head shall be cut twice to avoid artificial damage to the pile head. One is to clean the wet-cut pile with drilling mud after CFG pile pouring and before concrete solidification, leaving 30-50 cm uncut. After the strength reaches 80%, use a small excavator (bucket width 0.6m) to excavate the soil between piles, and use a modified one.
Detection of 3- 10 pile
3. 10. 1 pile integrity check
The integrity of CFG pile is tested by low strain dynamic test. Judging the integrity of the pile according to the test results.
Low strain detection of pile body
3. 10.2 CFG pile mixture strength
Each machine-team makes 1 set of standard cube specimens of mixture, and after 28d standard curing, the compressive strength of the specimens is tested. Judging its quality according to the test results.
Detection of foundation bearing capacity by overload method
3. 10.3 cfg pile composite foundation plate load test
Load tests of single pile and composite foundation are carried out respectively to calculate settlement and bearing capacity. Inspection quantity: 0.2% of the number of piles, and no less than 3 piles.
3- 1 1 quality requirements
Three, CFG pile quality control measures
1, construction shall be carried out in strict accordance with the design mixture ratio, and a group of concrete specimens shall be randomly selected from each drilling rig and each shift, and the compressive strength shall be taken as the standard for judging the strength of the mixture;
2. After the drilling rig comes into play, check the drill pipe diameter with a steel ruler. The diameter of drill pipe is not less than the designed pile diameter, and the height of main tower of drilling rig is about 5m longer than the pile length.
3. Before spudding, release the control piles and make technical disclosure to the platform personnel, who will release each pile with a steel ruler according to the control piles.
4. Before drilling, design the pile length and the thickness of the protective layer at the pile head according to the pile foundation, and clearly mark the position of the main tower of the drilling rig as the basis for controlling the drilling depth of the drilling rig.
5. After the drilling rig is in place, the commander shall command the drilling rig to adjust its position, and confirm that the verticality of the drilling rig meets the requirements by using vertical signs hanging on the frame in two directions;
6. At the beginning of 6.CFG pile construction, I was worried that pile-by-pile construction would cause string holes, so I adopted the construction method of jumping with the pile. However, when jumping with piles, the pile driver can easily crush the constructed piles for the second time, so we should choose jumping with piles according to different geological conditions.
7. When pouring concrete into CFG pile, the upper part of the pile is 1-3m. Because the pressure of concrete becomes smaller, there are small bubbles in the concrete that cannot be discharged, and the main stress part of CFG pile is in the upper part, so the upper pile body is not dense, which is easy to cause damage to the pile in the process of engineering use. Solution: First, after the construction is completed, before the concrete is solidified, the upper concrete is vibrated with a vibrating rod to strengthen the compactness of the concrete; The second is to strengthen the slump control of concrete, too small slump is easy to cause honeycomb phenomenon.
8. Control of tube drawing speed
Too fast tube drawing speed will lead to smaller pile diameter or broken pile, while too slow tube drawing speed will lead to uneven distribution of cement slurry, too much floating slurry on the top of pile, insufficient strength of pile body and segregation of mixture, which will lead to insufficient strength of pile body. Therefore, the extubation speed should be strictly controlled during construction, and it is generally appropriate to control it at 2 ~ 2.5m/min. The tube drawing speed here is linear speed, not average speed. In case of silt or muddy soil, the speed of tube drawing should be slowed down appropriately. Reverse insertion is not allowed when extubating.
9. Cause analysis and treatment of broken pile
Broken pile means that after CFG pile is completed, the concrete surface of the pile body is discontinuous, with cracks or intervals perpendicular to the central axis of the pile in the middle. Broken pile is the biggest quality accident of CFG pile. There are many reasons for broken piles, mainly including:
1) construction protection is not enough, and large construction machinery runs in the CFG pile area where the strength does not reach, resulting in pile pressing or pile head pressing;
2) The exhaust valve of the long auger rig is blocked;
3) When pouring concrete, the supply of concrete pouring is not timely;
4) Geological reasons, rich groundwater, easy to produce broken piles;
5) Disharmony between extubation and pumping concrete;
6) Improper operation causes damage to the pile head when it is cut off.
10, methods to prevent broken piles
1) For broken piles caused by insufficient construction protection, the most effective method is to strengthen the protection of the CFG pile area, enclose the site with steel pipes or other relatively solid materials, and prohibit all large machinery from entering.
2) For the solution of broken pile caused by improper setting of exhaust valve, the position of exhaust valve has been well set when the rig leaves the factory, and there is no need for improvement. Only a few rigs have this problem, so it is necessary to check the exhaust valve frequently and chisel off the concrete that wraps the exhaust valve.
3) Strengthen concrete dispatching, supply CFG pile concrete in time, ensure continuous supply of concrete, and prevent pile breakage caused by untimely supply of concrete.
4) It is difficult to control the broken pile caused by geological reasons during construction. For the geological conditions with plastic flow and abundant groundwater, the adaptability of CFG pile treatment should be demonstrated.
5) For the broken pile caused by the disharmony between pipe drawing and pumping concrete, the tacit cooperation of operators should be sought.
6) If man-made damage occurs when the pile head is cut off, it can be avoided by training the workers and improving their operation skills.
Fourth, the construction technology of pile caps
1, pile cap construction
Process comparison table of various pile cap forms
2, pile cap form
Construction technology of various forms of pile caps
Steel formwork construction
Soil formwork construction
Construction method and soil model of bored pile cap with rotary excavation and pile cutting
technical characteristics
1, simple process, high efficiency and high speed.
Adopting the soil model method of mechanical filling first and then manual excavation, there is no need to excavate and backfill the soil between CFG piles after pile formation, which simplifies the working procedure and improves the construction speed; The self-developed rotary pile cutter is used to cut off the pile head, which reduces the labor intensity of operators and greatly improves the work efficiency.
2, flexible operation, small disturbance and good quality.
Soil model method can avoid excessive disturbance of soil between piles below the design plane of pile top due to the removal and protection of soil layer and the removal of pile head, and solve the problems of broken piles and low backfill quality of soil between piles caused by mechanical excavation. The rotary cutting pile cutting mechanism is novel in design, and can be easily operated in a narrow soil mold space, which solves the problems that the common pile cutting machine can not work in the soil mold, but it is time-consuming and laborious to remove the pile head with a pneumatic pick, which is not good in impression and easy to damage the pile body.
3. Advanced technology, convenient construction and easy popularization. The construction technology of site hardening filling, pile cap soil model excavation and pile cutter rotary cutting is easy to master, and the configuration of construction organization elements is easy to realize, which is conducive to popularization.
4. Excellent equipment, low cost and remarkable benefits. The invention has the advantages of exquisite structure, simple structure, easy-to-buy accessories, low production cost, self-processing, effective reduction of equipment cost and remarkable social and economic benefits.
application area
This construction method is suitable for the construction of pile cap net (cylindrical pile cap) structure of CFG pile composite foundation, such as railway, highway, housing construction and municipal engineering. Especially, it has remarkable technical and economic benefits for projects with large quantities, concentrated tasks and tight construction period of CFG pile cap network. For the pile cap net structure with frustum shape and square column shape, this construction method still has good applicability if the jet grouting pile machine has workspace conditions.
Process principle
Different from the complexity of digging out the soil around the pile, pouring the pile cap by vertical formwork and backfilling the soil between piles after formwork removal in the previous construction of CFG pile cap net structure, this construction method replaces the working area with group A and group B fillers in advance to form a hardened layer, so that it can meet the quality requirements of backfilling and compacting the soil between piles, and then enters the site to construct CFG piles. After CFG pile is completed, remove the replacement soil and level the site. According to the size of the pile cap, the foundation groove is manually excavated in the hardened layer to make the soil model. In the soil model, the self-made rotary pile cutter is used to make the saw blade go deep into the pile cutting position below the ground, cut the pile horizontally, remove the pile head, and then pour the concrete of the bearing platform in place.
Key points of construction operation
1, site hardening layer pretreatment
After the surface cleaning operation is completed, before CFG pile construction, replace the working area with group A and group B fillers. The thickness of hardened layer is basically equal to the height of the bearing platform, and the elevation of the top of the layer is controlled according to the design elevation of the bearing platform. The layered thickness and compaction quality should meet the compaction requirements of embankment filler below the subgrade bed of passenger dedicated line, and meet the design requirements of soil backfill quality between piles of CFG pile cap net structure, so as to ensure the convenience of CFG pile machine entering the site and walking.
2. Excavate the hardened layer to make a soil model.
After the completion of CFG pile construction, before the excavation of the foundation trench of the bearing platform, the pile top elevation of each pile shall be measured first, and the technicians shall determine the excavation depth according to the measurement results and design elevation, and make disclosure to the team. The foundation pit is excavated manually, and the excavation of the soil around the pile should be carried out in strict accordance with the designed pile cap size and excavation depth, so as to ensure that the deviation of the center position of the foundation pit with pile caps is less than 15 mm, the deviation of the plane size is less than 30mm, the deviation of the height of pile caps is less than +30 and -20mm, and the bottom of the foundation pit with the center position should be smooth and free of sundries, with the elevation below the pile cutting position 10cm (design
Artificial excavation soil model
3, pile head operation
After the pile cutter rotates around the pile body once, it is necessary to place a steel wedge at the incision and hit the steel wedge with a big hammer, so that the wedge head is embedded in the pile body from the incision and pried away from the pile body, and the pile head naturally falls off; The cutting line is located below the ground, so it is necessary to dig through the soil between adjacent piles to facilitate workers to hammer the steel wedge; In order to facilitate the positioning of the steel wedge, φ8 steel bar can be welded on the steel wedge as a support rod, which is mastered by another worker to prevent the steel wedge from deviating when hammering the steel wedge; Pay attention to electricity safety when moving the machine on site, and adjust the direction and position of the power cord of the pile driver in time; Set up two working groups to carry out assembly line operation. One team is responsible for rotary sawing piles, and the other team is responsible for cutting pile heads. After cutting a pile, the rotary cutting machine can move to the next pile position to continue cutting the pile, and the other team will start cutting the pile head on the cut pile.
The pile cutter cuts the pile head.
4, concrete pouring
After the pile head is cut, the test shall be carried out. After the test is qualified, the floating soil and scum in the soil mold shall be removed, and concrete shall be poured after the acceptance of the supervisor. Pumped concrete should be used when pouring.
Covering and curing of concrete geomembrane for bearing platform
5. Level the soil between the rammed piles.
After the concrete of the bearing platform reaches the design strength, dismantle the pithead formwork. Affected by the hardening elevation control of the site and slight damage to the site during construction, the soil between piles cannot be completely flush with the top of the pile cap, and there is a certain deviation from the top elevation of the pile cap. For the soil between piles higher than the pile top, it can be manually removed to the pile top elevation; For those below the elevation of the pile cap, the soil between piles should be backfilled according to the requirements of design filler. According to the situation of the construction site, the height difference between the soil between piles under the cap top and the cap top is generally 0~ 15cm. After the concrete of the bearing platform reaches the design strength, the filler is transported to the site by dump truck, spread manually, and compacted by impact rammer. Backfill will be carried out from the edge to the center. When backfilling, attention should be paid to detecting and controlling the water content of the filler to ensure that the compaction quality of the filler meets the design standards. For the part near the edge of pile cap, in order to avoid the problem of missing corner caused by rammer collision, after the filler is 5cm higher than the top of pile cap and covers the edge of pile cap, measures are taken to tamp the part to ensure that the tamped part is in place, and finally the overfilled part is removed manually.
Artificial leveling of soil between piles
Flat ground
Tamp the soil between piles.
After the construction site is completed
Control measures in construction preparation stage
1. Before construction, on the basis of investigation, conduct material inspection on the materials to be used in Groups A and B, such as filler replacement, cement, fly ash, coarse and fine aggregate, geogrid, etc., and issue a material inspection report; Materials that meet the specification requirements with a certificate of conformity must obtain a certificate of conformity, and the materials supplied by Party A and Party A shall be determined by bidding, and unqualified materials shall not be used; The mixture ratio of concrete construction was selected in the experiment, and the construction was carried out in strict accordance with the selected mixture ratio.
2. Check and accept the tools and equipment such as molds and machinery that enter the site to ensure that their technical indicators meet the standards, operate normally and meet the construction requirements.
3, CFG pile construction, in strict accordance with the determined pile machine type, mixture ratio, protection pile length, final hole conditions, pipe drawing speed and other process parameters.
4. After the mixture pouring of 4.CFG pile is completed, the replacement soil and laitance should be dug out when the mixture is initially set. When removing and replacing soil, care should be taken not to destroy the hardened layer and the soil below, so as not to damage the pile body. If necessary, a part of the replacement soil should be reserved for removal with a small excavator.
5. Before the excavation of the bearing platform, the top of the pile should be set out to determine the excavation outline and depth, and the elevation can also be controlled by a level meter during excavation. The bottom of the soil model pit should be flat, which is convenient for the pile cutter to cut the pile head smoothly. After the pile cutting operation is completed, the pit bottom should be cleaned to ensure that there is no scum.
6. Strictly control the elevation of the bottom of the pit excavated by the soil model, and make use of the characteristic of the height difference between the saw blade of the pile cutter and the bottom of the machine 10cm to ensure the accurate position of the pile cutter. During the cutting operation, the saw blade must completely cut into the pile body to ensure the cutting depth and the quality of pile cutting. It is forbidden to cut the pile body in one week, such as three-point cutting.
7. When pouring concrete, the mixture ratio should be selected according to the mode of entering the mould, and the mixing station will concentrate on mixing to ensure the quality. In the process of concrete mixing and pouring, the weighing deviation of raw materials, the determination of water content of sand and gravel, slump and air content in the mould should be checked according to the specification requirements to ensure the pouring quality. During construction in winter, heat preservation measures should be taken to ensure that the temperature of concrete entering the mold meets the specification requirements.
8, after the completion of the concrete pouring of pile caps, artificial finishing. Cover with heat preservation materials in winter and cover with water in summer. The curing time of concrete should be determined according to the concrete material, temperature and air humidity.
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