Wenfu Railway is a high-speed railway from Wenzhou, Zhejiang Province to Fuzhou, Fujian Province, with a design speed of 200 km/h. This test section is carried out in Lianjiang County near Fuzhou City, with a length of DK275+000 ~ DK275+400 m. The prestressed pipe pile reinforcement section is 3 10m m, and there is an irrigation culvert at DK275+270, so as to reinforce the culvert foundation. Among them, there are 46 φ400mm piles, 307 φ500mm piles 1, and the design allowable bearing capacity of a single pile is 900KN. The piles are arranged in a square, with a minimum pile spacing of 2m and a maximum pile spacing of 3m. Before the formal construction, eight process piles were tested by static pile driver and diesel hammer.
2. Overview of engineering geology and hydrology
The engineering sites are all covered by Quaternary strata, and the upper part is Holocene coastal beach-drowned valley facies deposit. The bottom clay belongs to the origin of slope accumulation, the underlying bedrock is Yanshanian tuff and intrusive rock, and groundwater is developed in underground cracks, with a buried depth of 0 ~1m. The lithology and main physical and mechanical indexes of each layer of foundation soil are as follows:
(1) Silty clay, brownish yellow ~ grayish green, hard plastic, locally mixed with a small amount of gravel. The surface layer is 0.4~0.6m planting soil, including plant roots, and the thickness of this layer is 0.4~2. 1m, which is locally distributed in the construction site.
(2) Silt, light gray, flowing plastic. It has a delicate feel, uniform distribution, a small amount of humus, 2.8~7.9% of organic matter, and locally mixed with fine sand lenses. Because it is located in the piedmont plain near the sea, its horizontal and vertical distribution changes greatly with the fluctuation of bedrock surface. It has the characteristics of high compressibility and low strength, and the sensitivity standard value is 7. 1, which belongs to sensitive cohesive soil. Layer thickness10.5 ~19.7m. 。
(3) Silty clay, brownish yellow, grayish green, light gray, etc. Hard and soft are unevenly distributed. Due to the interaction between land and sea, lens bodies such as coarse sand, gravel, gravelly soil and muddy clay are sandwiched, with a thickness of 3.6 ~ 28m
(3)- 1 ~ (3)-7 muddy clay, light gray, flowing plastic, mixed with a small amount of rotten plants and shells, after 0 ~ 6.0m; Local gravelly soil, coarse sand, silt, gravelly sand and gravelly soil lens, gray with grayish yellow, medium dense and saturated.
(4) Granite, completely weathered sandy, SPT blow count n = 25 ~ 50, thickness >; 2m；
(5) tuff, grayish green or grayish white, completely weathered sandy, SPT blow count n = 16 ~ 35, thickness >; 10m。
3, construction preparation
3. 1, construction access road
Both the mechanical approach before construction and the pipe pile approach during construction pass through the construction access road, and heavy vehicles run on it, which requires high bearing capacity of the construction access road. In this project, the bottom layer of the construction access road is rolled with flaky to remove the silt on the original ground, so that there will be no "spring soil" in the future, the middle layer is filled with tunnel construction waste soil, and the upper layer of the access road is paved with 2-4cm gravel and stone chips.
The materials used are basically coarse at the bottom and fine at the top, which can not only ensure the stability of the service road, but also make the surface of the service road smooth and easy to repair. The construction access road should be as straight as possible to reduce unnecessary bending. In the construction area, the access road should be outside the piling range, so that the construction of the pile driver will not affect the smooth access road, and the constructed pipe pile will not be squeezed by the running vehicles.
3.2, work pad
The construction site of this experimental section was originally a local paddy field, and the surface soil was perennial planting soil with poor bearing capacity. However, a single static piling opportunity with counterweight is more than 300 tons, so it is necessary to lay a working cushion in the construction area to improve the bearing capacity of the foundation and ensure that the pile driver will not sink during construction. Generally, it is required that the bearing capacity of the working cushion should not be less than 100KPa after laying. The material of the working cushion can be the spoil of tunnel blasting, but large rubble can't be used, otherwise the pile position will be easily deviated during construction. Due to the huge fuselage of the static pile driver, the laying width of the working cushion should be sufficient. In this construction, the laying width of the working cushion should be controlled by widening the outermost pile by 5 meters.
3.3, measuring lofting
According to the traverse data delivered by the design, the traverse points shall be re-measured. After review, according to the construction drawings and site construction conditions, a control point is set at 30m on the left side of Ⅰ Road at intervals of 25m along the route, which is used as the mirror mounting point and rear viewpoint of theodolite when setting out the pile position. The control point is 8.8m away from the outermost pipe pile/kloc-0, and there is still soil squeezing effect, so it is necessary to check and correct the control point once a week with total station.
According to the actual piling road map, measure and locate the control network according to the construction area. Generally, an area is set out with 10 ~ 20 piles according to the daily construction progress, and a small bamboo pile with a length of about 30 ~ 40 cm is inserted in the ground at the center of the pile and marked with red paint. Take the engineering pile position as the center, draw a circle with white ash according to the diameter, which is convenient for pile insertion and alignment. After self-inspection, the released axis and pile position shall be rechecked by the supervision engineer. In the process of pile sinking, the control points should be checked frequently, and the positioning records and technical review records should be made.
3.4. Selection of construction machinery and pipe pile materials
The construction adopts static pressure hammering technology. Static pile driver has the advantages of no noise, no vibration and no pollution. Hammer machine has the characteristics of strong rock-socketed ability.
Construction static pressure adopts YZY-750T full hydraulic side clamp pile driver. The hammer machine adopts a diesel hammer pile machine with rolling pipe, and the hammer model is HD50. All construction machines have inspection certificates. The selection of pile hammer mainly considers the diesel hammer's high energy, fast construction speed and high efficiency, and the principle of "heavy hammer and light knock".
PHC pipe piles produced by Jianhua Pipe Pile Company are adopted, and the concrete strength is C80. In strict accordance with the pretensioned prestressed concrete pipe pile (GB 13476- 1999), it is cured at high pressure and high temperature, and the vertical bearing capacity of a single pile is 2820KN.
4, the construction process
4. 1, static pile driver
4. 1. 1, piling process
After the pile driver is installed in place, move it to the position where it needs to travel, start the platform leg oil cylinder and adjust the level. To hoist the precast pile, first tie the steel wire rope and rigging for hoisting, start the machine to hoist the precast pile, so that the pile tip is vertically aligned with the center of the pile position, and slowly put it down and insert it into the soil. When the pile tip is inserted into the pile position, and the clamp clamps the pipe pile, gently start the pile pressing cylinder. When the pile is buried to 50cm, correct the verticality of the pile and the levelness of the platform again to ensure that the vertical and horizontal deviation of the pile does not exceed the standard. Then start the pile pressing cylinder, slowly press down the pile and control the pile pressing speed. Stop pressing the pile when the pile head is about one meter from the ground, and the crane lifts another pile to start pile connection. During pile extension, the axis of the new pile joint should be consistent with the axis of the original pile joint, and the dirt, oil stain and rust on the two welding surfaces should be cleaned up in advance. The gap between the upper and lower piles should be welded firmly with iron gasket, and measures should be taken to weld symmetrically during welding to reduce the joint bending caused by weld deformation. Welds should be continuous and full. The weld at the pile extension shall be naturally cooled for not less than 1 min. During pile extension, guide hoop should be installed at the next pile head to guide the new pile extension section in place. Find the square of the pile and fix it with 4-6 point symmetrical spot welding, and then remove the guide hoop. The welding construction of pipe piles shall be carried out by experienced welders in accordance with the requirements of technical regulations. During pile extension, before the new pile extension is fixed, also use theodolite to observe the verticality in two directions of 90-degree included angle, and adjust the level of pile driver.
4. 1.2, pressure stop standard
The final pressure of φ40cm pipe pile and φ50cm pipe pile in this test section is not less than 2000KN and 2500KN respectively. The bearing layer at the pile end adopts hard plastic clay layer or fully weathered bedrock, and the pressure shall be stabilized for three times after meeting the requirements, each time for no less than 2 minutes.
4.2, hammer machine
Piling process
The surveyors control the pile position according to the side piles, and after positioning and lofting with J2 theodolite, mark the length in meters on the pile position, so as to observe the buried depth of the pile and record the number of blows per meter. The verticality deviation of the first section pipe pile when it is hoisted in place and inserted into the ground shall not be greater than 0.5%, and it shall be observed and corrected simultaneously in two directions of 90 degrees; During construction, it is necessary to ensure that the center lines of the pile hammer, pile cap and pile body coincide, otherwise the pipe pile will easily bend and break under the impact of eccentric hammer. When welding pile extension, the pile head should be 0.5 ~ 1.0m higher than the ground, and the upper and lower pile sections should be kept straight, with dislocation deviation not greater than 2mm. After the welded pile head is naturally cooled, the hammering can be continued for at least 8min. It is forbidden to cool or knock with water after welding. When piling, the hammer pad is made of straight wood with the thickness of 15cm, and the pile pad is made of sacks and wood plywood. After compression, the thickness is about 12cm, so it should be checked frequently when hammering and replaced in time. The total hammering number of pipe piles should not exceed 2000, and the final hammering number of 1m should not exceed 280. Check the verticality of the pile before sending it; For construction in soft soil, each pile should be driven continuously, and the stopping time should not be too long.
4.2.2 Hammer standard
This time, only φ50cm pipe pile is constructed, and the pile end is required to enter the hard plastic clay layer or completely weathered bedrock. The penetration of the last three rows is required to be 20 ~ 40 mm, and the hammer shall be closed when the penetration of the last 10 blow is less than 20 mm.
5. Test results
After PHC pipe pile construction in this test section, Wuhan Institute of Geotechnical Mechanics, Chinese Academy of Sciences was entrusted to adopt static load and small strain detection respectively, and the detection was carried out according to the standard of JGJ 106-2003. Small strain detection 133 piles, of which 92% are Class I piles and 8% are Class II piles. No Class III or Class IV piles are found. In terms of static load, the 13 pile adopts the slow maintenance load method, with the maximum load of 1 1,800 kn (twice the design bearing capacity of a single pile), the initial load of 360KN, and then the load of each stage is 1 1,800 kn. Judging from the Q-S curve, the curve is gentle and there is no obvious steep drop. S-lgt curves are arranged smoothly and regularly, and the maximum settlement is 27.6 1 mm, and the vertical ultimate bearing capacity of single pile meets the requirements.
6. Construction experience
6. 1, selection of work pad materials
The working cushion is used to improve the bearing capacity of the original surface, so that the pile driver can walk smoothly during construction, but it cannot affect the subsequent construction. If there are many large pieces in the material of the working cushion, it will affect the pile position and verticality during pipe pile construction; However, the material with too fine particle size has no obvious effect on improving the bearing capacity of muddy soil or perennial planting soil topsoil, so the choice of working cushion material should be appropriate, and the particle size of the material can be large, but it can't be large.
Original website
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6.2. Stacking of pipe pile materials
Piling of pipe piles shall be carried out according to the specifications, length, use sequence and distance of piles; The stacking site should be in a flat and solid place, so that the piles will not cause excessive settlement after stacking, and the stow-wood in contact with the ground at the bottom should be widened and heightened. When stacking, two pads are placed under the pile, the supporting point is located on the lifting point of the two-point crane, and the top surfaces of the two pads on the same floor are kept on the same horizontal plane; When stacked overlapping, stow-wood shall be placed on each layer, and each layer shall be aligned up and down; The stacking quantity shall not exceed three; The stow-wood is made of compressive wood Fang.
6.3, extrusion effect
Prestressed pipe pile constructed by static pressure method belongs to soil squeezing type, which often disturbs the soil structure around the pile and changes the stress state of the soil, resulting in soil squeezing effect. The welding time is too long during the construction of pile driver; There are many pile joints, the welding quality is poor or the pile end is stuck in the hard interlayer; Improper construction method and sequence, too many piles per day, too fast pile pressing speed, too many piles arranged too closely, etc. , will aggravate the squeezing effect. It is also possible that a certain soil layer has low undrained shear strength and high compressibility. In the process of static pressure, when the pile is driven into the soil, it will lead to impact shear failure, at the same time, the soil around the pile will be squeezed out, and the pore water will be squeezed by this impact shear to form an uneven head, resulting in huge excess pore water pressure, while the upper miscellaneous fill layer is not cleaned up, which will make the upward stress of the soil unable to be released and increase the lateral stress of the foundation soil. This project is located in an open area, and there is a small river 50 meters to the left of the construction area, which becomes a natural anti-collapse ditch, so that the houses outside the river are not affected, while the side piles near the construction area are found to be displaced in the weekly inspection, but the displacement is not large.
The prevention and control of soil squeezing effect is of great significance, especially in areas with dense buildings around. The general practice is to clear the surface miscellaneous fill, excavate the anti-extrusion ditch, set stress relief holes and arrange the piling sequence reasonably. According to the surrounding environment and the degree of soil compaction on the construction site, quiet distance will start construction first, and then start construction from a long distance. If necessary, the number of piling should be controlled every day. If the inner cavity of the pipe pile is allowed to pour into the soil, the soil squeezing effect can also be effectively reduced by using the open pile tip.
6.4, the final pressure standard
PHC pile is an end-bearing pile, and the rationality of its compression stop standard will directly affect whether the pile can meet the design bearing capacity requirements without being crushed. Practice has proved that under the condition that the quality of pile materials is qualified and the pile sinking is normal, the damage of pile pressure to pile body is mostly related to the unreasonable control of pile pressing standard. At the same time, PHC piles have the characteristics of brittle failure and low tensile stress. When the pressure is high and the counterweight of the pile frame is not enough, the impact force generated by the lifting frame of the pile machine can easily cause PHC piles to crack or be damaged, and pile pressing also happens from time to time. Therefore, it is an important technology to control the pressure stop standard correctly and reasonably in PHC pile construction. The control of PHC pile stop pressure standard mainly considers the following two aspects: knowing the engineering geological conditions and related design requirements in detail during construction, correctly grasping the relationship between the depth of pile entering the bearing stratum and the last three penetration resistances reaching 1.8 ~ 2.0 times the design bearing capacity of single pile, and the cumulative settlement ≤10 mm; The final pressure in construction is determined according to the lateral constraint of soil under instantaneous load in construction, so the final pressure can be more than twice the vertical bearing capacity of a single pipe pile. Its stress mode is close to that of axially compressed members, and the final pressure can be approximately obtained by the following formula:
P=(0.67R-σce)A0
R-compressive strength of concrete cube
σce- effective prestress value of concrete
A0 —— Cross-sectional area of PHC pile
The bearing capacity of double single piles in this construction is 1800KN, and the pile pressing force above 2500KN is used as the pile pressing standard in actual control. According to the following empirical formula, the bearing capacity of single pile can be estimated by the final pressure of 2500KN, which is far from meeting the design requirements:
Quk=αPp
PP- final pressure (KN)
α —— The empirical coefficient obtained by analyzing the static load test values and final pile pressing values of several projects is 0.7 ~ 1.5 according to the soil conditions.
6.5, pile top displacement
In the process of construction, adjacent piles will produce lateral displacement and pile body floating. There are several reasons:
A. After the pile is buried, push the pile tip aside when encountering a large hard obstacle.
B, two or more piles construction, two piles are not on the same axis, resulting in twists and turns.
C, the number of piles is large, the soil is saturated and dense, and the pile spacing is small. When sinking piles, the soil is squeezed to the limit density and bulged upward, and the adjacent piles float up.
D. During the construction of dense pile groups on soft soil foundation, the pore water pressure caused by pile sinking pushes adjacent piles to one side or floats.
In order to effectively reduce the displacement of the pile top, the obstacles under the pile position should be cleared before construction, and the pile members should be inspected. It is found that the bending of the pile exceeds the specified value (L/ 1000 and ≤20mm) or the pile tip is not on the longitudinal axis of the pile, so it is not suitable for use. In the process of pile stabilization, if it is found that the pile is not vertical, it should be corrected in time. During pile extension, it is necessary to ensure that the upper and lower piles are on the same axis, and the joint should be implemented in strict accordance with the operation requirements. Well point dewatering, sand discharge, blind ditch and other dewatering or drainage measures should be taken. During pile sinking, the foundation pit shall not be excavated. After pile sinking is completed, excavation can be carried out at appropriate intervals. The interval should be determined according to the specific geological conditions, the depth and area of foundation pit excavation, the density of piles and the dissipation of pore water pressure. Generally speaking, it is considered to be about two weeks.
6.6, other issues
A, the geological conditions of two adjacent piles are quite different during construction, which leads to the problem of sending piles too deep. Because we have to pile caps later, it is very difficult to pile caps for pipe piles with too deep piles, which is easy to cause horizontal displacement of adjacent piles. The author thinks that the depth of pile delivery should not exceed two meters.
B, in this construction, the pipe pile of the hammer pile machine is welded by ordinary covered electrode, and the static pressure pile is welded by carbon dioxide gas shielded welding; It can be clearly seen from the small strain detection that the joints welded by ordinary covered electrode are worse than those welded by CO2 gas shielded welding. It is suggested that carbon dioxide gas shielded welding should be used in the welding of pipe pile joints in the future construction.
C, according to the relevant provisions of the safety management, it is necessary to establish and improve the relevant management system of the project, implement the responsibility system for safety management within the project, establish the assessment system, implement the reward and punishment measures, and complete the qualification of the pile driver and the work permit for special operations. In addition, the following items must be noted:
1. Before crane operation, lubricate the rotating parts and check the fastening degree of the parts to see if the wire rope is worn.
2. It is forbidden to stand under the jib, and the driver is not allowed to leave the operation room when the heavy objects are parked in the air.
3. The lifting range shall not exceed the index specified in the lifting performance. Before piling, the crane and hoisting mechanism must loosen the lifting wire rope and hook before piling to avoid breaking the wire rope and bending the boom.
4. All kinds of gas cylinders used for welding during pile extension should be marked. The distance between gas cylinders and open flame point should be greater than 10m, and the distance between gas cylinders must be greater than 5m. Gas cylinders must have shockproof rings and protective caps. When using and storing, it is forbidden to put it flat or upside down.
5. When the operation is stopped, the short shoes need to run to the middle position of the pile driver, stop on the flat ground, and the rest of the oil cylinders are retracted. Cut off the power supply, and the operator can leave the pile driver.
6. The pile head of the completed pile shall be stamped to prevent pedestrians or sundries from falling in.
7. concluding remarks
The unit bearing capacity cost of PHC pile is lower than other pile types, and the comprehensive economic benefit index is better than other pile types. Through the construction of this test section, we have a deeper understanding of the construction technology of this pile. The construction experience described in this paper is for reference only.
With the extensive application and development of PHC pipe piles, as well as the accumulation of theoretical research and engineering practice, the construction technology of PHC piles will be continuously improved.
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