First, the characteristics of the pile foundation of the project.
The pile foundation of this project is adopted.
At the same time, the use of mature static pressure construction technology, static pile driver to the construction site requirements are higher, because the pile driver and counterweight will reach more than 400 tons, in order to prevent the pile driver from sinking and causing the pile body to tilt, the impact of pile driver extrusion on the pile position, construction quality, construction safety, the construction site must be partially backfilled and leveled, and necessary measures must be taken to improve the bearing capacity of the foundation, so that it can meet the requirements of the static pile construction. The local soil quality of the project site is poor, and the clay should be replaced locally and paved with slag on it to meet the construction requirements.
Second, the quality control of static piles.
1. Pile forming process:
Measurement and pay-off, pile driver in place, review pile position, hoisting pile insertion, pile driver level, pile body alignment, static pressure pile sinking, pile connection, hydrostatic pressure pile sinking, pile delivery, termination of pile pressing, pile quality inspection.
1) Preparation before piling:
The pile driver should be equipped with sufficient weight according to the soil conditions and the design bearing capacityCarefully check the performance of each component of the pile driver to ensure normal operation and eliminate potential safety hazardsCheck the appearance quality of the square pile, pay attention to whether there is any damage during transportation, whether the square pile mark is clear, and whether the quality assurance information is complete.
2) Measurement pay-off:
The deviation of the pile position on the ground shall not be greater than 20mm;If the pile is dense, far away or the soil conditions are poor, it is not appropriate to complete all the measurements at one time, and it is necessary to check it before construction, and check one construction if necessary.
3) The pile driver is in place (the pile driver is horizontal, and the pile body is aligned and straightened):
Use the pile driver hydraulic system to adjust the level of the fuselage and basically in place after the pile is hoisted, pile insertion, pile insertion in the general situation into the soil 30-50cm is appropriate, and then adjust the vertical of the pile within the allowable range of the specification to sink the pile;The verticality deviation of the first section of the square pile hoisted in place and inserted into the ground shall not be greater than 05%, it is advisable to use a long level or theodolite for calibration, and if necessary, it needs to be pulled out and reinserted.
4) Static pressure pile sinking:
During the pile sinking process, the footage and verticality of the pile should be observed. Appropriately limit the pile pressing speed, and the pile sinking speed is generally controlled at about 1m min, so that each soil can correctly reflect its shear resistanceWhen there are obstacles such as large stones in the surface of the foundation, it is necessary to avoid pressure deviation;When the pile tip enters the hard soil layer, it is strictly forbidden to move the pile frame and other forcible pull back methods to correct the deviation;In the thicker clay and silty clay layer, each pile should be continuously driven to the end at one time, to minimize the intermediate rest time, if the rest time is too long, the soil around the pile is consolidated, and it is easy to cause difficulties in pile sinking.
5) Piling: When the pipe pile is connected, it is advisable to raise the pile head above the ground 05-1m;When connecting the pile, the upper and lower section pile sections should be kept straight, the dislocation should not be greater than 2mm, and the height of the node bending vector shall not be greater than 0 of the pile section1%。When the welding method is used to connect the pile, the joint surface and groove must be cleaned, the rust and dirt must be removed, and the cleanliness should be kept clean, the symmetrical spot welding 4 6 points should be symmetrical, and then the butt welding should be evenly layered, the weld should be full and connected, the welding slag should be cleaned in time, and the height of the surface reinforced weld should be 1 2mm;Attention should be paid to the current intensity during electric welding construction, which should match the welding machine and welding wire used;In order to reduce the welding deformation, the pile connection time should be reduced, 2 people can be set up at the same time symmetrical welding, after the welding is completed, it should be cooled naturally, it is strictly forbidden to cool with water, and then the pile is pressed after the weld is cooled, and reliable protective measures should be adopted in rain and snow days. After welding, visual inspection should be carried out, and the weld should not have surface defects such as dents, undercuts, welding nodules, slag inclusions, cracks, etc., and ultrasonic sampling inspection should be carried out on the weld.
6) Pile feeding: Because the elevation of the pile top is lower than the existing ground elevation, it is necessary to use the pile sending rod to carry out pile sending construction. Due to the problem of "sending pile" and the flatness of the pile end face, there will be a gap or difference between the "pile feeding rod" and the pile end face, so that the pile top is subjected to excessive stress, resulting in the crushing of the pile top, and even the local bending of the pile. Therefore, between the contact surface of the "pile rod" and the pile top, a padding should be added, and the contact surface between the pile rod and the pile top should be kept close to prevent the pile top from being crushedThe center line of the pile cap, pile body and pile rod should coincide, the diameter of the "pile rod" should be consistent with the pile diameter, and the verticality of the "pile rod" should be strictly controlledThe pile feeding rod should be marked with paint to measure the depth, so as to control the pile feeding depth and the elevation of the pile top. After the "pile rod" is pulled out, the pile hole should be backfilled in time.
7) Terminate pile pressing:
The project recommends the termination of the principle of pile pressing, which is mainly based on elevation control and supplemented by pressure value control: when the elevation of the pile top is consistent with the design elevation, and the pressure value reaches the design requirements, the pile pressing is terminatedWhen the pressure value far exceeds the design requirements, and the elevation of the pile top is higher than the design elevation, the pile can be terminated, and the pile cutting treatment can be adopted laterWhen the pressure value fails to meet the design requirements, but the elevation of the pile top has reached the design elevation, continue to send the pile (within 1m) until the pressure value meets the design requirements, and contact the design unit in time after the construction is completed to issue a treatment plan. The decision to terminate the conditions for pile pressing shall be jointly agreed upon by the design, supervision, etc. in the design disclosure before construction.
During the construction process, the square pile construction record should be truthful, timely and accurate to better reflect the pile sinking process.
2. The principle of piling sequence compilation (mainly considering the influence of stress diffusion and extrusion on the pile position, the construction quality, and the impact on adjacent buildings).
1) According to the density of the pile and the relationship between the surrounding buildings: if the pile is dense and far away from the building, when the site is open, it is advisable to carry out it from the middle to the surroundings;If the pile is dense, the site is narrow and long, and the two ends are far away from the building, it is advisable to carry out from the middle to both endsIf the piles are dense and one side is close to the building, it is advisable to start from one side of the adjacent building and proceed from near to far.
2) The pile foundation with more than 30 piles should be driven outward from the central positionThe pile at the edge of the cushion cap is driven after the other piles in the cushion cap are driven and the pile position is re-determined, and then the pile is inserted and driven.
3) According to the depth of the pile, it is advisable to be long first and then short, and it is advisable to be high and then low.
4) When formulating the construction sequence and process of pile foundation, it is strictly forbidden to form "closed" piles.
5) The static pressure square pile in the deep foundation pit with the enclosure structure should be pressed first and then the enclosure structure of the foundation pit should be made. Such a construction sequence can avoid the soil mass of pile pressing cannot spread due to the enclosure structure around the foundation pit. Cause the square pile of the first construction to be squeezed up by the square pile of the later construction, make the bearing capacity of the pile fail to meet the design requirements, and avoid the soil spreading and squeeze out the surrounding enclosure structure and reduce the water-stopping effect of the foundation pit enclosure structure when the pile is pressed in the foundation pit.
6) During the construction of the pile driver, attention should be paid to the group of piles in the same cushion cap, and the joints to be connected to the pile should not be at the stratum interface of "soft and hard at the top, and the soft and hard mutately", so as to avoid the cutting and damage to the overall pile body when the earth pressure and water pressure effect is largeAt the same time, the geological report should be carefully checked to understand the distribution of the soil layer, reasonably determine the combined length of the pile body, and avoid the joint being at the boundary of the soil layer and the place where the soil layer is more active, so as to prevent the damage to the pile body when the soil layer is active.
3. Test the pile before the construction of the engineering pile and the pile foundation engineering test pile after the completion of the engineering pile.
The pile test should be carried out before the construction of the static pile, and the design unit should adjust the pile length in time according to the test pile data. The pile body is required to expose the ground 20 30cm to facilitate the installation of static ballast counterpressure device;The pile head should be treated with fine sand before leveling or pressurization, otherwise the pile head is easy to crush due to stress concentration after pressurizationAfter the pile is formed, there needs to be a period of embedding period, after the combination of the pile body and the soil body basically tends to be stable, the bearing capacity of the pile has been improved to a certain extent, the embedding period is different according to different soil bodies (7 21 days), and the clay soil is best to reach 21 days.
3. Common quality problems and prevention and control measures in the construction of static piles.
1. Soil squeezing effect and vibration effect.
Cause analysis: when the pile is sunk, the soil structure around the pile is disturbed, which changes the stress state of the soil, and causes the surrounding soil to produce additional pore water pressure, resulting in soil squeezing effectThe welding time is too long during the construction of the pile driver;There are many joints of the pile and the welding quality is not good, or the pile end is stopped in the hard interlayer;The construction method is improper, the number of piles formed every day is too much, the pile pressing rate is too fast, and the piles are too dense and too dense, which aggravates the soil squeezing effect. It affects the construction quality of the pile driver and the damage to the pile body during earthwork excavation.
Prevention and control methods: 1) control the density of piles, the pile spacing can be used pre-drilled (pilot hole) pile sinking method, the hole diameter is about 100mm smaller than the pile diameter, the depth should be 1 3 1 2 pile length, and the construction should be drilled with the drilling.
2) Control the pile sinking rate, and formulate an effective pile sinking flow route to control the daily pile volume.
3) Set up bagged sand wells or plastic drainage boards to eliminate part of the additional void water pressure.
4) Excavate the ground earthquake-proof ditch to eliminate the vibration, and the ditch width is 05～0.8m。
5) In the process of pile sinking, the observation and monitoring of adjacent buildings and underground pipelines should be strengthened, and other pile types can be changed for pipelines and buildings that are close to special importance.
6) Control the stop time in the construction process to avoid the increase of friction resistance due to the stop time process, which affects the construction of the pile driver, resulting in difficulty in pile sinking. At the same time, welding in hard soil layers such as sandy silt and sand should be avoided.
2. Inclined pile. Cause Analysis:
1) The mechanical maintenance of the static pile driver is not timely, such as the oil leakage of the hydraulic system causes the pile driver support to slide;
2) The total weight of the static pile driver plus the counterweight is large, and the bearing capacity of the pile driver construction site is sufficient, and the pile driver is prone to uneven settlement in the process of pile sinking, and the pile body is very prone to deviation
3) The pile body is not vertical during construction, and the pile body is not in the same straight line;
4) The pile body and pile cap are not in the same straight line when the pile is connected;
5) Improper construction sequence, resulting in uneven stress diffusion;In particular, the adjacent pile body of the bearing cap with deep foundation pit in the basement is too close and too dense, so that the side of the first construction has a hole, and the pile body is very easy to slide when the side is constructed again.
6) In the process of pile sinking, when encountering a large piece of hard object, squeeze the pile to one side;
7) When the pre-drilling method is adopted, the vertical deviation of the borehole is large, and the pile in the pile sinking process is offset along the inclined direction of the borehole
8) The pile arrangement is too much and too dense, and the squeezing effect occurs when the pile is sunk;
9) The excavation method of the foundation pit is improper, and the one-time excavation depth is too deep, so that one side of the pile bears a lot of earth pressure, and the pile body is bent and deformed.
Prevention and control methods: 1) Pay attention to maintenance before the construction of the static pile driver to avoid affecting the construction quality;
2) The site should be flat and hard, and the slag should be properly laid in the softer site, so that the pile driver can not produce uneven settlement in the piling process.
3) During the construction process, the verticality of the pile should be controlled, and the focus should be on the first section of the pile, and the verticality deviation should not exceed 0 of the pile length5. The pile cap, pile body and pile rod should be in the same straight line, and the theodolite should be calibrated in both directions when sinking the pile.
4) Minimize pile joints, prefabricated square pile joints should not exceed 3, pile joints should be carried out after the pile tip enters the hard soil layer. The centerline deviation of the upper and lower sections of the pile should not be greater than 5mm01%。
5) Formulate a reasonable construction sequence, and the holes after pile foundation construction should be backfilled in time.
6) When encountering obstacles, the pile should be removed in time and then sunk;When sinking the pile, it is found that it is not perpendicular and should be corrected in time, if necessary, the pile should be pulled out and re-driven, and after the pile enters a certain depth, it is not appropriate to use the mobile frame for correction, so as to avoid the occurrence of pile breakage, and other measures should be taken.
7) When the pre-drilling method is adopted, the verticality of the drilling hole is strictly controlled.
8) Arrange the pile position reasonably, and the center distance between the pile and the pile should be greater than 4 times the pile diameter to control the pile sinking speed.
9) After the pile foundation construction, the foundation pit excavation construction should be carried out after the stop period, and the foundation pit excavation should be carried out evenly in layers, and the enclosure measures must be strengthened to prevent the lateral pressure of the soil on the pile to produce additional bending moments on the pile body, so as to ensure the integrity of the pile structure of the pile foundation engineering.
3. Pile body failure:
Cause analysis: 1) The phenomenon of inclined pile in the construction process leads to the concentration of stress at the pile end, which makes the pile cap slide down or the pile head bursts
2) The construction pressure value of the pile driver is super high;
3) During the construction of the pile driver, the pile driver moves the frame without authorization to correct the pile position and the verticality of the pile body, resulting in the fracture of the pile bodyAfter the end of construction, the manual pile chiseling barbaric construction and the unreasonable earthwork excavation after the pile driver construction.
4) Pile material quality;
Prevention and control methods: 1) select reasonable and effective construction methods of pile drivers to control the verticality of the pile and avoid the occurrence of inclined piles
2) Control the termination conditions of pile driver construction.
3) After the construction of the pile driver, the earthwork excavation and pile chisel construction are carried out reasonably.
4) During the construction process, the inspection and acceptance of pile raw materials should be strengthened
5) In the event of pile damage during construction, it is advisable to use small strain and other means to detect the pile situation and determine the treatment method.