There are many types of pile foundations, large differences in construction technology, and complex stratum changes. During the construction process, the pile may have defects such as diameter reduction, diameter expansion, mud inclusion, segregation, and broken piles. Of course, mechanical excavation after construction may cause collisions. It will cause shallow pile defects. The existence of pile body defects will change the normal working behavior of the foundation pile, thereby causing potential danger to the foundation. Today we have summarized the common problems of pile foundation in engineering, let's take a look.
1. Pile foundation inspection
▌Under what circumstances should be verified and expanded testing
For low-strain detection, for rock-socketed piles, the time-domain reflected signal at the bottom of the pile is a single reflected wave and is in the same direction as the hammering signal; the measured signal is complex and irregular, and it is impossible to accurately evaluate it; the pile section is gradual or variable , And the concrete cast-in-place piles with large changes are always verified by static load method or core drilling method.
During high-strain testing, there are defects in the pile body, and the vertical bearing capacity of the pile cannot be determined; or the pile body defect has an influence on the horizontal bearing capacity; the single-click penetration is large, the pile bottom is strongly reflected in the same direction and the reflection peak is wider, The resistance wave and end resistance wave are weakly reflected, that is, when the waveform shows a vertical load-bearing behavior and does not meet the geological conditions in the survey report, the static load method can be used for further verification;
The rock-socketed pile has strong reflection in the same direction at the bottom of the pile, and there is no obvious end resistance reflection after time 2L/C. Core drilling method can be used for verification.
Excavation verification can be used for shallow defects of the pile body.
Precast piles with cracks in the pile body or joints can be verified by the high strain method.
When single-hole drilling core testing finds a pile body concrete quality problem, it is advisable to add drilling verification on the same foundation pile.
For the piles whose integrity category cannot be clearly defined in the low-strain method testing or type III piles, suitable methods such as static load method, core drilling method, high-strain method, and excavation can be used to verify the inspection according to the actual situation.
When the bearing capacity of a single pile or the result of sampling by core drilling does not meet the design requirements, the reasons should be analyzed and the sampling should be expanded after confirmation.
When using low strain method, high strain method and acoustic transmission method to check the pile body integrity, the sum of the piles of type III and IV found to be greater than 20% of the number of the selected piles, the original detection method should be used (the acoustic transmission method can be replaced by a drill Core method), continue to expand random inspections among uninspected piles.
▌What should be included in the pile inspection report
The test report should include the following:
The name of the client, project name, location, construction, survey, design, supervision and construction unit, foundation, structure type, number of floors, design requirements, inspection purpose, inspection basis, inspection quantity, inspection date;
Description of geological conditions;
The pile number, pile position and related construction records of the pile to be inspected;
Testing methods, testing equipment, testing process description;
The inspection data, actual measurement and calculation analysis curves, tables and summary results of the inspected piles;
The test conclusion corresponding to the test content. The evaluation of the test results of the bearing capacity of engineering piles should give the test value of the bearing capacity of each inspected pile, and based on this, give a conclusion whether the characteristic value of the single pile bearing capacity under the same condition of the unit project meets the design requirements.
▌What regulations should be met in determining the characteristic value of the horizontal bearing capacity of a single pile under the same conditions in a unit project
(1) When the horizontal bearing capacity is controlled by the strength of the pile body, the statistical value of the horizontal critical load is taken as the characteristic value of the water bearing capacity of a single pile.
(2) When the pile is subjected to a long-term horizontal load and cracks are not allowed, 0.8 times the statistical value of the horizontal critical load is taken as the characteristic value of the horizontal bearing capacity of a single pile.
(3) When the horizontal bearing capacity is controlled according to the allowable horizontal displacement required by the design, the horizontal load corresponding to the allowable horizontal displacement required by the design may be taken as the characteristic value of the horizontal bearing capacity of a single pile, but it should meet the requirements of the relevant code for anti-cracking design.
2. Construction issues
▌尝别补办补驳别
Phenomenon: Slurry leakage is called piping in hydraulics. A common phenomenon is bubbles or muddy water on the surface of the protective tube. The occurrence of the above phenomenon means that slurry leakage has occurred.
Treatment method: add cement or vibrate the protective tube to sink. The sinking of the protective tube should not exceed 1m. If the protective tube sinks more than 1m, or the protective tube sinks suddenly, the construction should be stopped.
▌ collapsed hole
Phenomenon: The amount of slag discharge increases abnormally, the load of the drilling rig suddenly increases, or a large number of fine air bubbles are suddenly generated on the mud surface in the casing.
Treatment method: When a hole collapses, stop drilling immediately and put up the drill bit to prevent the drill bit from being buried. After the hole collapsed, the ring-arching effect of the damaged soil suddenly disappeared, and the hole wall entered an extremely unstable state. At this time, the backfilling soil is the most effective method.
▌Floating steel cage
Phenomenon: The steel cage cannot be placed in place smoothly when it is lowered. This is the most common problem in the construction of long spiral bored piles. Sometimes the steel cage is too long and the lower half is stuck in the hole. Cut off from the gap.
Approach:
(1) The concrete is not in place, and the drilling is too fast or too slow in the middle. Strictly control the time of large concrete and its filling coefficient to ensure that the concrete filling coefficient is within the required range;
(2) If the slump is too large or too small, check whether the concrete slump meets the requirements. If the slump is too large, bleeding and segregation are likely to occur. The coarse aggregate sinks and the cement slurry floats in a short time. On top. Increase the downward resistance of the steel cage, causing the steel cage to not be in place; if the slump is too small, it will still increase the difficulty of the cage. Therefore, the slump must be strictly controlled at 18-22cm during the construction process;
(3) The quality of the steel cage is too poor or the design is unreasonable. Strictly follow the specifications of the steel cage on the drawing, and there is no problem of excessive dragon reinforcement. When lowering the steel cage, the bottom of the steel cage should be made into a sharp row to avoid the phenomenon of inserting;
(4) The power of the selected vibration hammer should be appropriate. When the steel cage is lowered to 1-2m away from the opening, it must be delivered to the design elevation by a vibrating hammer with protective tube. Generally, for piles with a pile diameter of 400mm and a pile length of no more than 10m, the 1.8kW vibration hammer can meet the requirements. ; For foundation piles with a diameter of 600mm and a length of less than 15m, a 2.2kW vibration hammer is appropriate;
(5) Check whether the drill bit and drill pipe diameter meet the requirements. If the drill bit is found to be severely worn, it should be repaired in time and an alloy block should be inserted on the outer edge to ensure that the hole diameter and protective layer meet the requirements. There is enough space between the steel cage and the hole wall, so that the steel cage is in place smoothly.
▌Partial hole
Phenomenon: In the process of punching pile construction, there are often partial holes. When encountering larger boulders or probe rocks (slope rock face) in the punching hole, when the rock face is slanted in at the junction of the soft and hard strata, or in the sand and gravel layer with different particle sizes, the hole expansion is more difficult. When the hammer head swings to one side at a large place, and the base of the punching pile machine is not placed horizontally or uneven settlement or displacement occurs, partial holes may occur.
Approach:
(1) When installing the punching pile machine, the chassis and base should be level. The edge of the lifting pulley, the hole of the wire rope for the fixed hammer and the center of the barrel should be in a vertical line, and check and correct frequently;
(2) When the impact of the hammer is found to be unstable (swinging from side to side), the impact should be stopped to check whether there is a deviation hole, and if a deviation hole occurs, deal with it in time;
(3) When a partial hole is found, first find out the reason for the partial hole and the size of the partial hole. If the hammer hits the boulder or probe rock (slope rock surface), backfill the hole with stones in time, and backfill to the height. The impact occurred at 1m from the rock surface.
▌Plum hole (or cross hole)
Phenomenon: When the punched pile impacts, the punched hole is not round.
Approach:
(1) Choose mud with proper viscosity and relative density, and dig out the slag in time;
(2) When using a low stroke, change to a higher stroke impact for each impact segment, and alternate impact to modify the hole shape;
(3) After the plum blossom hole appears, the drilled hole can be backfilled with flakes and pebbles mixed clay and re-impact.
3. Quality issues
▌Common quality problems
(1) There is an error in the measurement and laying out, which will cause errors in the entire building or large deviations in the pile foundation.
(2) The bearing capacity of a single pile does not meet the design standards for building pile foundations.
(3) The pile body is inclined too large.
(4) The joint of the precast pile is disconnected.
(5) The pile-breaking accident caused by the process of grouting the pile.
(6) It is found that the pile position has excessive deviation during the acceptance process.
(7) Problems such as mud inclusion, segregation, and substandard strength of the pile due to the quality of concrete or operation.
(8) Insufficient elevation.
▌惭别补蝉耻谤别蝉
Replenishment method
This method can be used when the depth of precast piles is insufficient, or when the driven piles are lifted up due to the uplift of the soil.
Mending pile method
It is in conjunction with the opinions of the design, supervision, and owners to carry out supplementary drilling based on the pile supplement plan issued by the design unit. However, this method requires a large investment and a long construction period, and it is difficult to be recognized by all parties.
Supplementary combined method
When the pile is driven into the foundation by means of joint connection, if the quality of the pile is not up to the standard, then the joint may be disconnected during the sinking process. At this time, the pile foundation can be treated with compensation. Send the combined way.
For suspected piles, re-driving should be carried out to make the pile sink, which can make the disconnected pile connection tighten again, so that the joint has vertical bearing capacity; piles can be supplemented, and some complete piles can be supplemented appropriately. , So that the foundation can not only meet the bearing capacity requirements, but also increase the seismic load of the building foundation.
Correction method
If it is found that the pile body is inclined and the pile length is not long and is not broken during the piling process, or the pile body is inclined due to the excavation of the foundation pit but the pile body is still intact, partial excavation of the pile body can be carried out Open, and then use a jack to correct the pile.
Platform expansion
(1) If the pile position has a large deviation, the original design size of the pile cap cannot meet the structural normative requirements, then the pile position deviation can be corrected by appropriately expanding the area of the pile cap.
(2) Pile-soil interaction. If the single pile cannot meet the design bearing capacity requirements, it is necessary to expand the cap and take into account the upper load shared by the natural foundation and the pile.
(3) The quality of pile foundation. Uneven pile foundation quality is prone to occur during construction. In order to prevent the pile cap layout cloud settlement caused by pile foundation quality problems in the later period and to improve the seismic resistance of the building, integral pile caps can be used to improve the integrity of the foundation.
Composite foundation method
(1) It uses the principle of pile-soil interaction to properly treat the foundation to increase the bearing capacity of the foundation and more effectively share the load of the pile foundation. There are several commonly used methods.
(2) Replace the foundation under the cap. Before the construction of the pile foundation, the soil of a certain depth is excavated, replaced with a sand-gravel layer and rammed in layers, and then the artificial foundation and pile foundation are constructed on the foundation.
(3) Add cement-soil piles between piles. When the pile bearing capacity does not meet the design requirements, dry spray cement in the soil between the piles can be used to form cement-soil piles to form a composite foundation.
Modify pile type or pile sinking parameter method
(1) Change the pile type. For example, prefabricated square piles are changed to prestressed pipe piles.
(2) Change the depth of the pile into the soil. For example, the precast pile encounters a thicker dense silt or silt layer during the penetration process, and it is difficult for the pile to sink, or even a broken pile. In this case, shorten the length of the pile, increase the number of piles, and take the dense silt The layer (expansive soil layer) serves as the supporting layer.
(3) Change the pile position. If the pile sinking encounters a hard, small underground obstacle, which causes the pile to tilt or even break, the pile position can be changed to re-sink the pile.
(4) Variable pile sinking equipment. When the sinking depth of the pile does not meet the design requirements, a large tonnage pile frame can be used, and the heavy hammer low-strike method can be used to sink the pile.
