1. General situation of the project
Guannan Road in Kunming is flat and open, with dense water networks and numerous lakes and ditches. The total length of the road is about 4.4 kilometers, and the width of the road red line is 5 meters, which is the first-class main road of the city. The subgrade works of the whole bid section are all soft foundation treatment works. The geological conditions are mainly fluvial, lacustrine sediments, alluvial muddy clay, muddy loam and muddy clayey silt. There are 1m ~ 3m hard layers on the surface, but most of them are over-wet soil, and the soft soil layer is m-25m thick.
CFG pile+gravel cushion is adopted for culvert foundation treatment of this project. CFG piles are 4cm in diameter and arranged in plum blossom shape. The average pile length is 17m, the pile strength is C1, and 3cm thick gravel cushion is laid on the top of the pile.
2. Hydrogeological conditions
The overburden of the whole line changes greatly, and within the scope of investigation depth, groundwater mainly exists in the Quaternary overburden. During the survey, groundwater was observed, with the buried depth of .6~3.5mm and the elevation of 1,886.27 ~ 1,888.42 m, with relatively little change. According to the occurrence medium and hydraulic conditions of groundwater, groundwater belongs to upper stagnant water and micro-confined pore diving.
3. CFG pile reinforcement principle
CFG pile is a high-viscosity pile formed by mixing cement, fly ash, gravel and fine sand with water to form a composite foundation, which is connected with the foundation through the mattress, so that the soil between piles can always participate in the work regardless of whether the pile ends fall on the general soil layer or the silt soil. Because the strength and modulus of the soil between piles are larger than that of the original soil between piles, the stress at the top of the pile is larger than that of the soil between piles under load, and the load that the piles can bear is transferred to the deep soil layer and the load borne by the soil between piles is correspondingly reduced.
the pile body is bored by machinery, and the mixed concrete is pumped into the hole by a pump. In the process of pipe drawing, the gravity generated by the height difference is used to make the concrete produce self-vibrating effect, so that not only the soil between piles but also the pile body is compacted in the process of pile forming, making it hydraulic, and obviously improving the strength and deformation resistance of the treated composite foundation.
4. CFG pile construction technology
Before the construction of Guannan Road section, level the site, remove obstacles, and carry out pile-forming process test (not less than two test piles) to check whether the geological data, mechanical equipment performance, construction technology and construction sequence are appropriate, determine various process parameters such as mixture ratio, slump, mixing time and pipe drawing speed, and revise the construction technology according to the problems found in the test piles. Although there is a hard layer of 1m ~ 3m on the surface, the test pile can still pass through this layer smoothly when using the vibration sinking drill, so the vibration sinking method is adopted for construction.
specific technology: measuring and setting out the pile position → placing the pile tip → putting the pile machine in place → vibrating pipe sinking → hole forming inspection → pouring and mixing pile → vibrating pipe drawing → pile forming inspection → laying mattress.
The key working procedures are controlled as follows:
1) After the CFG pile machine is in place, check the guide rod calibration position with the vertical posts at the front, back, left and right of the drilling rig tower, so that the immersed tube is vertically aligned with the pile center, and ensure that the vertical deviation of CFG pile is not more than 1%.
2) piling sequence: alternate spacing piles are used in CFG pile construction, and the construction time interval between adjacent piles is more than 7d. Adopt the scheme of pushing outward from the center or pushing from one side to the other.
5. Construction method
5.1 Immersion pipe
Determine the height of the rack and the immersed pipe length according to the designed pile length and the depth of the immersed pipe, and assemble the equipment. Pile driver is in place, keep the pile tube vertical, and the verticality deviation is not more than 1%; If the precast reinforced concrete pile tip is used, it needs to be buried about 3mm below the surface to start pipe sinking. In order to avoid the influence on adjacent piles, the pipe sinking time should be as short as possible; The change of excitation current shall be recorded once every 1m, and the change of soil layer shall be explained.
5.2 feeding
In the process of pipe sinking, the hopper is used for air feeding (feeding can be carried out while sinking the pipe). After the immersed tube reaches the design elevation and stops, the feeding shall be completed as soon as possible until the top surface of the mixture in the tube is flush with the steel pipe mouth.
5.3 extubation
start the motor, and start extubation after the first feeding and vibration for 5 ~ 1 seconds. The tube drawing rate is controlled according to the parameters approved by the process test and supervision engineer, and generally 1.2m/min ~ 1.5m/min is appropriate. Too fast tube drawing is easy to cause local necking or broken pile; Too slow tube drawing and too long vibration time will thicken the laitance at the top of the pile and easily separate the mixture. For muddy soil, the tube drawing speed can be slowed down appropriately. It is not advisable to insert the tube backwards to keep the vibration during extubation. If the above materials are insufficient, hollow materials must be cast in the process of pipe drawing to ensure that the elevation of pile top meets the design requirements after pile completion. After completion, the elevation of pile top should be ．5rn higher than the designed pile length, and the thickness of laitance should not exceed 2cm.
5.4 sealing item
the immersed tube is pulled out of the ground, and the pile is capped with wet cohesive soil after it is confirmed that it meets the design requirements.
5.5 Move the machine
The drilling rig is moved for the construction of the next pile.
6. Quality control of CFG pile
After 28 days of CFG pile construction, CFG pile and CFG pile composite foundation are tested according to the design requirements. Use low strain to test the pile quality, and the sampling frequency is 1%. If possible, use plate load test to test the bearing capacity of composite foundation, and the sampling frequency is .5% ~ 1.%, but not less than 3 points. See table 1 for the allowable deviation of CFG pile construction for inspection items.
Table 1 Allowable Deviation Table of CFG Pile Construction
The main targets of CFG pile construction quality control on site are: pile length and strength, and the monitoring points are as follows:
1) Before measuring the pile position, the original ground elevation of the construction site shall be leveled and measured, and the accurate position of each pile shall be released after leveling and rolling with a grader, and the original ground elevation shall be controlled within 5 cm after taking into account the longitudinal slope and transverse slope of the line. The elevation of construction pile top shall be controlled at 3cm higher than the design cushion bottom elevation. Divide the construction area, number each pile, and manage it by machine and person.
2) When laying piles, the number, arrangement form and spacing of CFG piles must be strictly in accordance with the design requirements and follow the principle of pushing construction outward from the center or from one side to the other, and it is not advisable to push construction inward from four turns. The specific construction method and sequence shall be determined by the Engineering Department.
3) Assign employees who have a strong sense of responsibility, know the technology and pass the strict examination to monitor and record the CFG pile constructed by the labor service team on site.
4) During CFG pile construction, each shift must make inspection specimens and carry out 28d strength inspection. After 28d, the bearing capacity of single pile or composite foundation should be tested in time, and its bearing capacity and deformation modulus should meet the design requirements.
5) Usually, the concrete at the top of the pile has poor compactness and low strength, so vibration tamping is adopted within 2．5m below the top of the pile.
6) In order to ensure the smooth transportation of the mixture in the construction, a forced mixer is adopted in the construction.
7) The amount of fly ash added to each mixture of pile body and slump control are controlled according to the parameters determined by process test and approved by the supervision engineer according to the design and adopted construction method.
8) During soil cleaning and pile cutting, the pile body below the elevation of the pile top shall not be broken and the soil between piles shall not be disturbed.
9) During winter construction, the temperature of the mixture entering the hole shall not be lower than 5℃, and thermal insulation measures shall be taken for the pile head and soil between piles.
1) The waste soil discharged during pile formation should be removed in time during the construction of jumping, otherwise the construction progress will be affected.
11) During the whole construction process, quality inspectors shall be arranged for on-site supervision, and original records of construction shall be made, including WOB current value, hole depth, single hole mixture injection amount, pipe plugging and treatment measures, etc.
The detailed information of "Application of CFG Pile Technology in Highway Construction" can be found in Zhongda Consulting & Design Link, and all the relevant building construction information you want is available.
for more information about the engineering/service/procurement bidding, and to improve the winning rate, you can click on the bottom of official website Customer Service for free consultation:/#/? source=bdzd