Zhongda Consulting conducted research on the pile driving principles and characteristics of static pressure pile foundations. Through in-depth analysis of the construction process and key points of static pressure piles, we discussed common quality problems and solutions to provide It provides a reference for the application and practice of static pressure pipe pile technology in the future. The following is an introduction to the main content of the construction method of static pressure pile foundation brought by Zhongda Consulting for reference.
1 Principle of Static Pressure Pile Driving
During the pile driving construction process, the pile tip penetrates deeply into the soil, and the initial stress state of the undisturbed soil is destroyed. The soil also deformed. The soil covering the ground is squeezed out in all directions, and the pile body is subjected to the normal pressure of the soil, resulting in resistance around the pile. When the static pressure on the top of the pile > the resistance during pile sinking, the pile will continue to sink; when the top of the pile When the static pressure < the resistance during pile sinking, the pile stops sinking. When the stress at the tip of the static pressure pile is greater than the shear strength, the soil will experience squeezing and lateral movement and resistance to plastic flow. On the surface of the clay, the soil will bulge and the sand will be dragged and sink. Under strong disturbance, the balance of the soil around the pile is destroyed. The extrusion of the soil causes the pore water pressure of the soil around the pile to increase, resulting in a significant reduction in the shear strength of the soil. The compressive strength of the soil will increase when the static pore pressure and disturbance Greatly reduced under remodeling. After the construction of static pressure piles is completed, the pore water pressure in the soil begins to dissipate, the soil consolidation strength gradually recovers, the upper pile hole area is filled, the middle pile slip zone disappears, and the pressure in the lower pile squeeze zone decreases. At this time The piles obtain the ultimate bearing capacity in the engineering sense.
2 Main construction methods of static pressure piles
2.1 Main construction process
The main construction process of static pressure piles is carried out as follows: "Measurement and setting-out, The pile driver is in place, the first pile is hoisted into place, the pile position is reviewed and adjusted, the pile tip is inserted into the soil and initially pressurized, the verticality of the pile body is checked and corrected, the pile is formally sunk, the verticality of the pile body is observed and corrected, and the pile top is pressed until the distance between the top of the pile and the ground is 0.5~ 1.0m, hoist the previous pile into position, connect the pile, continue sinking the pile, record the pressure value, complete the pile, and move the machine to the next pile position" (as shown in Figure 1).
2.2 Line measurement method
Project construction must be carried out in accordance with the design requirements. According to the construction design master plan, the plane position, shape and elevation of the building should be calibrated on the ground at the construction site. Laying the foundation for subsequent in-depth construction. During the construction process, measurement work is particularly important, and it is necessary to accurately confirm the axis control points and horizontal base points.
2.3 Pipe pile stacking method
Pipe piles must pass inspection before entering the site, and the appearance, size and relevant parameters of the inspection report must be checked in accordance with regulations. The stacking location should be flat, and the pipe piles should be stacked according to different types. If the site is open enough, it can be stacked in a single layer. If the site is limited, it can be stacked in layers. The height is preferably ≤5 layers, and at least 2 pads should be installed perpendicular to the length of the pipe pile. The skids should be made of pressure-resistant long wood or sleepers and should not be replaced by angular metal components [1].
2.4 Pipe pile placement and preliminary pressurization method
After the pipe pile is in place, start the pile pressing device, first move the pile driver to the pre-marked pile position on the ground, and then use the pile The crane arm on the machine lifts the pile end into the soil. After the pile body is basically stable, the pile body is leveled and the pile is formally driven. The pile pressing speed, pile pressing time and pressure parameters are smoothly controlled.
2.5 Pile connection methods
There are two ways to connect piles: one is the mechanical joint connection method, and the other is the end plate welding connection method. Welding is the current mainstream connection. Way. The entire pile connection process must be continued without interruption. The length of each pile section should be reasonably selected, and pile connection operations must not be performed at the bearing layer position. Welding can only be started after the two pile sections are successfully connected and the verticality meets the requirements. After welding and before sinking the pile, the weld must be completely cooled before proceeding to the next step.
2.6 Pile sinking method
Pile sinking construction can only be carried out after the parameters such as depth of penetration, verticality, pile length, pile top elevation, pressure value, and penetration are confirmed to be correct. ; In order to ensure the accuracy of pile sinking, a pile feeder should be used to ensure the alignment of the center point and the qualification of the pressure parameters; after the pile sinking is in place, the part of the pile body above the ground should be cut off. After cutting off, proceed to the next pile position.
3 Key points of static pressure pile construction
3.1 Construction conditions
The prerequisite for the construction conditions is that the site conditions meet the standards. When the construction site is uneven, the site needs to be cleared Foreign objects and obstacles should be removed and compacted with sand and gravel to prevent the ground from being too soft. At the same time, a drainage system should be set up to ensure the smooth progress of subsequent construction.
3.2 Construction sequence
The principle of pile sinking sequence is "first in the middle and then around the four sides, first dense piles then sparse piles, first long piles then short piles, first large diameter piles then small diameter piles piles, first the large cap piles and then the small cap piles." In order to determine a more reasonable sequence of pile sinking, it is necessary to conduct an investigation of the surrounding and on-site engineering geological conditions, and comprehensively consider the relationship between underground pipelines, buildings and the surrounding environment.
3.3 Construction method
The design strength of pipe piles must be higher than the design value. During construction, the surface temperature of the pile concrete must be strictly controlled, and the temperature difference should be ≤ 20°C of the ambient temperature. Pipe piles are cured with pressure steam, on average once a day. Before lifting the pipe piles, ensure that the pile driver is flat and perpendicular to the ground, and is adjusted to the corresponding position smoothly and accurately. Generally, a pressure-holding type pile driver can be used to sink piles. When the pile driver lifts the pile body, the holding pressure must be < the compressive pressure of the pile body, and the weld connection part of the pile body must be avoided. If the pile body tilts during the installation process, the pile body must be pulled out as soon as possible, and adjustment cannot be made by moving the pile frame. If the pile cannot be driven or is difficult to sink during the installation process, the operation must be stopped, the geological conditions must be repeatedly confirmed, the reasons analyzed, and effective measures must be formulated before the pile sinking can be continued [2].
4 Common quality problems and solutions for static pressure piles
4.1 Pile design does not meet the requirements
1) Cause analysis: geological survey data and actual projects There are deviations in the geological conditions, and relevant parameters cannot meet the design requirements; due to emergencies, such as equipment failures, on-site abnormalities, etc., the pile driving process is interrupted, the resistance around the pile increases, and it is difficult to sink the pile to the bearing layer required by the design. 2) Solution: First, determine the geological conditions of the project. If necessary, conduct a geological survey to rationally select the bearing layer, determine pile driving parameters and pile driving equipment; if there is an abnormality in the pile sinking process, stop it immediately During construction, carefully analyze the causes and find solutions. Do not drive piles blindly to prevent pile damage.
4.2 Cracks appear at the pile joints
Cracking occurs at the pile joints of the upper and lower piles. 1) Cause analysis: The upper and lower sections of the pile are not in the same straight line, and the welding seam where the pile is connected is prone to stress concentration, resulting in cracking; during welding, the welding joint was not connected properly, the joint surface was uneven, and the surface welding slag was not cleaned. ; The welding construction was not carried out in strict accordance with the process requirements, and layered welding was not carried out, resulting in discontinuous welds, welding defects, and rapid pre-cooling time after welding, resulting in brittle cracks. 2) Solution: In order to make the upper and lower piles vertical, it is recommended to set up temporary guide hoops for connecting the piles. First, review the pile center deviation of the upper and lower pile sections to see if there is any misalignment. Before starting welding, make sure that the welding machine and related equipment are in good condition, and that the type and material of the welding rod meet the process requirements. At the same time, polish and clean the welding surface and welding joints. Welding process parameters should be strictly followed during welding, and parameters such as welding current, voltage, and welding speed should be strictly controlled. The welding joint part should be grooved first, and spot welding should be performed near the groove to fix the upper and lower piles. After fixing, the temporary guide hoops should be removed, and finally welding should be carried out in layers and evenly. Welding operators must hold Relevant qualification certificates. Use natural cooling method to cool the welded seam after welding, and it is prohibited to use water cooling method.
4.3 Pile body fracture
During the construction process, the pile body tilted and fractured. 1) Cause analysis: The quality of the concrete itself leads to insufficient strength of the column, which causes fracture during the stress process; improper protection during the storage, transportation, and lifting of the pile leads to cracks or defects on the surface. When a large external force is exerted When the force is applied, the column body is easy to break; the ground appears abnormal settlement, the pile driver and the pile body are severely tilted, the pile body receives uneven stress, is subject to eccentric load, and finally breaks. 2) Solution: Strengthen process monitoring. In the initial stage of construction, if the pile body is found to be tilted, the operation must be stopped immediately and corrected in time. Timely adjust the flatness of the upper and lower pile sections. The pile sinking speed needs to be slowed down and stabilized to facilitate timely stopping and adjustment.
If a break occurs during the construction process, do not panic. Relevant personnel should be organized to conduct on-site analysis and determine the disposal method. Those that can be reinforced should be reinforced, and those that cannot be reinforced should be replaced or repaired.
In summary, static pressure piles are a common form of pile foundation in my country's foundation projects. They have been widely used due to their many advantages such as low construction noise, low vibration, prefabrication, and fast progress. promotion and application. In the actual construction of static pressure piles, the pile forming process parameters and related equipment should be reasonably determined based on the construction site conditions, etc., each process should be carried out in a standardized manner, and quality control should be strengthened to prevent problems such as cracking at the pile connection and fracture of the pile body, and effectively ensure the project The foundation is solid and reliable, ensuring the smooth progress of subsequent construction operations.
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