I. General situation of the project and geological conditions
The residence is located in the central area of Kunming, surrounded by dense residents, and its construction site was formed after the demolition of the old building. The residence is an eight-story frame structure with a building height of 23.7m and a building area of 4276m2, which is fortified according to the seismic intensity of octave.
According to the geological data of the site revealed by drilling, the foundation soil belongs to the lacustrine strata of ancient Dianchi Lake, and the surface layer is ① miscellaneous fill, followed by: ② silty clay (hard plastic) with a thickness of 0.5 ~ 2.2m；; ③ Silty clay (plastic) with a thickness of 0.5 ~1.3m; ④ Silty soil, 0.3 ~ 2.5m thick; ⑤ Silt thickness1.3 ~ 7.5m; ⑥ Gravel,14.4 ~ 20.4m thick. The groundwater is shallow, only 1.0m away from the surface, which belongs to the third-class construction site. In view of the fact that the gravel in the 6th layer has a large particle size of 20mm, which is sub-circular, generally 2 ~ 9 mm, and the filling sand is 10 ~ 20%, which is slightly to moderately dense, and the soil layer is thick, and the buried depth is within the range of 8.5 ~10.0m..
Second, the main design parameters of pile foundation
According to the properties of site soil and the analysis of upper load, it is determined that the pile-soil replacement rate of deep mixing cement-soil core pile is 14%. In this project, 238 engineering piles are laid, with a diameter of φ500mm, the designed pile length is 9.2m, and it is not less than 0.5m； when entering the gravel layer; The core of the pile is C20 precast reinforced concrete member, with a square cross section, an upper opening size of 220×220mm, a lower opening size of 130× 130mm, a core length of 6.0m, and four longitudinal bars of 12 and φ 6.5 @ 200 stirrups. The design requires that the unconfined compressive strength of cement-soil is 1.5MPa, and the standard value of single pile bearing capacity is not less than 450KN, and its limit value should be greater than 765KN.
Three, engineering pile construction technology and related information
According to the design requirements of unconfined compressive strength of cement-soil, the construction unit carried out the mix proportion test of cement-soil. Through experiments, it is determined that the cement dosage is 350kg and the water-cement ratio of cement slurry is 0.5 ~ 0.6 for each reinforced 1.0m3 soil. By adding phosphogypsum and 5% quicklime, the outlet pressure of grouting pump is kept at 0.8 ~ 1.2mpa. In order to mix cement-soil evenly, the construction technology of "six mixing and three spraying" was adopted. That is, stir from the surface to the design depth for the first time (or the bearing stratum is below 0.5m), press in the cement slurry, then stir and lift, and so on twice. In the process of shotcrete mixing, the vertical speed of the drill pipe of the deep mixing pile machine should be controlled not to be greater than 1.0m/min, so that the amount of cement slurry pressed in can be adapted to the lifting speed of the machine, so as to ensure the uniform and sufficient mixing of cement soil. After the last mixing and shotcreting, immediately move the drill pipe to the ground while mixing, then lift the pile core with the lifting equipment on the pile machine, correctly align the pile core of the mixing pile, slowly press the pile core into the uncured cement-soil with the microseismic static pressure device on the pile machine, and control the elevation of the top of the pile core with a level to make the deviation less than 50 mm.
* * * There are 238 engineering piles in this project, plus 3 test piles, and * * * there are 24 1 deep mixing core piles, the pile length is between 9 ~1.0m, and the total pile length is 2401m. Check the pile diameter after earthwork excavation, and all meet the design and specification requirements. After 30 days of age, three engineering piles with pile lengths of 9.3, 10. 1 and 10.5m were randomly selected for single-pile nondestructive static load test, and the measured bearing capacity of each pile has reached more than 550KN, exceeding 70% of the design bearing capacity limit.
Four. Bearing capacity test of single pile and observation results of building settlement
Judging from the results of the above three groups of pile tests, the static load of the test has reached 1.44% of the ultimate bearing capacity of 765KN required by the design, but the final settlement is less than 20mm, and the pile has not been damaged. The test data show that the mechanical properties of the pile are good and the settlement is small.
Because deep mixing core pile is a new type of pile, it is not widely used in Kunming engineering practice. In order to make a correct and objective evaluation of the settlement of the pile foundation of this project, accumulate data and experience, the owner entrusts a third party with corresponding qualifications to observe the settlement of the residence during the construction period and for a period of time after completion. The observation unit first measured four leveling points on the periphery of the building to form an annular closed leveling route, and then set up eight settlement observation points at the four corners of the building and related parts. According to the observation method and period required by the specification, the settlement of the observation points was observed at 13 (14 times). The last three observations lasted for three months. By the third observation, the final settlement value was: minimum settlement-3.43 mm, settlement-8.37 mm, and the settlement difference between two adjacent piles was only two ten thousandths. The settlement observation data show that the building settlement is uniform and the pile foundation is safe and reliable.
On-site monitoring of deep mixing cement-soil core pile
Although deep mixing cement-soil core pile is a new type of pile, it must have evolved from cement mixing pile. Therefore, in the monitoring practice, besides strictly checking and monitoring the pile position, verticality, pile length, pile diameter, performance and dosage of raw materials such as cement, grouting pump outlet pressure, vertical speed of drill pipe of pile driver and many other factors according to the quality requirements of mixing piles, we think it is necessary to carry out corresponding targeted monitoring activities on the difference between cement-soil core piles and mixing piles. To sum up, we should focus on the following aspects:
1. Carefully check the quality of the reinforced concrete pile core entering the site.
Reinforced concrete pile core is a wedge-shaped precast reinforced concrete member. In the prefabrication plant, the strength of concrete, the variety and diameter of steel bars used, and the formation of steel cages are randomly selected, and the appearance, geometric size and axial curvature of the pile cores entering the site are inspected one by one. Inspection standards shall conform to the relevant technical standards of precast piles. The prefabrication factory shall provide the material inspection report of this batch of pile cores and the product certificate of pile cores.
2. The stacking and lifting displacement of the reinforced concrete pile core at the site shall ensure that its members will not be broken or damaged.
The core site should be properly leveled and have corresponding strength. According to the length of the pile core, stow-wood should be set at both ends of the pile length 15% ~ 20%, and then the pile core should be slowly placed on the stow-wood from the suspended state. Pile cores can be stacked layer by layer, but each layer should be padded with wood, and the upper and lower positions of wood should be on the same vertical line. For the safety of stacking, lifting and shifting, the stacking height shall not exceed four floors.
Before lifting and shifting the pile core, the strength of the pile core should be checked by single-point binding and lifting. In the process of lifting, slow lifting, light lifting and light putting should be achieved; In the process of displacement, it is necessary to have manpower to pull and hold the position to avoid violent collision with other rigid objects, resulting in missing corners of components or even broken pile cores.
3. When pressing the pile core, the pile core should be pressed to the section center position and design depth of the deep mixing pile.
In order to make the mechanical properties of cement-soil core pile good, the center of pile core section should be consistent with the geometric center of deep mixing pile. However, due to mechanical and manual factors, some deviations are inevitable, and the deviation should be less than 5cm to ensure that there is enough cement-soil around the pile core. At the same time, use level to monitor the insertion depth of pile core in deep mixing pile, so that the deviation between the top elevation of pile core and its required elevation is less than 5cm.
4. The completed cement-soil core pile shall be properly protected.
After the cement-soil core pile is completed, its cement-soil is still in a plastic state, so attention should be paid to protection. When the deep mixing pile machine moves, it is necessary to avoid the collision and push-pull of the rope on the pile core. Therefore, the following route of the pile driver should be carefully designed before piling, and it should be specified in detail in the construction plan, and the construction should be carefully organized according to the plan. The supervision engineer should also control the construction plan to check the smooth progress of pile construction.
It must be pointed out that as a kind of engineering pile, the mechanical properties of cement-soil core pile should be checked randomly according to the requirements of the specification after completion to ensure that it meets the design requirements and make the project safe and reliable.
Sixth, the engineering effect of deep mixing core pile
Construction of the above part of residential pile foundation began on March 22, 2002, and all the projects were completed and accepted on February 4, 2002. The construction lasted about eight months and has been used for two years. Engineering practice has proved that the construction technology of deep mixing core pile is simple, the quality is easy to control, the noise and vibration are low, and there is basically no pollution to the environment. In addition, the bearing capacity of a single pile is high, the settlement of the building is relatively small, and the cost is only 55-65% of that of the pile foundation with vibration sinking cast-in-place pile, and the effect is very remarkable. Before the writing of this paper, the pile foundation construction unit has successively adopted this pile type in several multi-storey frame structure projects such as 26,500 square meters of commercial housing in Kanghong Community of Kunming, 4,800 square meters of library in Yunnan Orthopedic School and 6,000 square meters of commercial and residential building in Yixin Company of Kunming. The effect is very good, and it is worth popularizing.