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 and collision after construction Will cause defects in shallow piles. The existence of pile defects will change the normal working behavior of the foundation pile, thereby causing potential danger to the foundation. Today we summarized the common problems of pile foundation in the project, let's take a look.
1. Pile foundation inspection
▌When should be verified and expanded testing
For low-strain testing, for a rock-embedded pile, the time-domain reflected signal at the bottom of the pile is a single reflected wave and the same direction as the hammer signal; the measured signal is complex and irregular, and it cannot be accurately evaluated; the pile body section gradually changes or changes In addition, the concrete grouting piles with large variation range are always verified by static loading method or core drilling method.
During high strain detection, there is a defect in the pile body, and the vertical bearing capacity of the pile cannot be determined; or the pile body defect has an effect on the horizontal bearing capacity; the click penetration is large, the same direction of the pile bottom reflects strong force and the reflection peak is wide, side The resistance wave and end resistance wave are weakly reflected, that is, when the waveform shows that the vertical bearing behavior is obviously inconsistent with the geological conditions in the survey report, the static load method can be used for further verification;
Rock-socketed piles have strong same-side reflections at the bottom of the piles, and there is no obvious end resistance reflection after time 2L/C, which can be verified by the core drilling method.
Defects in the shallow part of the pile can be verified by excavation.
Precast piles with cracks in the pile body or joints can be verified by the high strain method.
When the single-hole drill core inspection finds the quality problem of the concrete of the pile body, it should be added to the same foundation pile for verification.
For piles of type III or piles of which the integrity type cannot be clearly defined in the low strain method test, suitable methods such as static load method, core drilling method, high strain method, and excavation can be used to verify the test according to the actual situation.
When the test results of single pile bearing capacity or core drilling method do not meet the design requirements, the reasons should be analyzed and expanded after confirmation.
When using the low strain method, high strain method and sonic transmission method to check the integrity of the piles of class Ⅲ and Ⅳ found in the pile body is greater than 20% of the number of piles, the original detection method should be used (the sonic transmission method can be used to drill Core method), continue to expand the sampling inspection in the unchecked pile.
▌The content of the foundation pile test report should include
The test report should include the following:
Client's name, project name, location, construction, survey, design, supervision and construction unit, foundation, structure type, number of floors, design requirements, testing purpose, testing basis, testing quantity, testing date;
Description of geological conditions;
The pile number, position and relevant construction records of the inspected pile;
Test methods, test equipment, test process description;
Inspection data of the inspected pile, actual measurement and calculation analysis curves, tables and summary results;
The test conclusion corresponding to the test content. For the evaluation of the test result of the bearing capacity of engineering piles, the test value of the bearing capacity of each tested pile should be given, and the conclusion whether the characteristic value of the bearing capacity of the single pile under the same conditions per unit project meets the design requirements should be given accordingly.
▌What requirements should be met for the determination of the characteristic value of the single pile horizontal bearing capacity under the same conditions of the unit project
(1) When the horizontal bearing capacity is controlled according to the strength of the pile body, the statistical value of the horizontal critical load is taken as the characteristic value of the single pile water bearing capacity.
(2) When the pile is subjected to long-term horizontal load and the shape is not allowed to crack, the 0.8 times of the horizontal critical load statistical value is taken as the characteristic value of the single pile horizontal bearing capacity.
(3) When the horizontal load-bearing capacity is controlled according to the horizontal allowable displacement required by the design, the horizontal load corresponding to the horizontal allowable displacement required by the design may be taken as the characteristic value of the horizontal bearing capacity of a single pile, but it shall meet the requirements of the relevant standard anti-cracking design.
2. Construction issues
▌尝别补办补驳别
Phenomenon: The leakage of slurry is called piping in hydraulics. The common phenomenon is that bubbles or muddy water emerges from the outer surface of the protective cylinder. The above phenomenon means that the leakage of slurry has occurred.
Treatment method: add cement or vibrating protective tube to sink. The casing falls no more than 1m. If the casing falls more than 1m, or the casing sinks suddenly, the construction shall be stopped.
▌ collapse hole
Phenomenon: The amount of slag discharge is abnormally increased, the load of the drilling rig is suddenly increased, or a large number of fine bubbles are suddenly generated on the mud surface in the casing.
Treatment method: Stop drilling immediately when a hole collapses, lift the drill bit to prevent the drill bit from being buried. After the collapse of the hole, the ring arch of the destroyed soil suddenly disappeared, and the hole wall entered a very unstable state. At this time, the backfill soil is the most effective way.
▌Reinforcement cage floating
Phenomenon: The steel cage cannot be put in place smoothly when it is lowered, which is the most common problem in the construction of long spiral bored piles. Sometimes the steel cage is too long, and it is stuck in the hole halfway, so it is forced to neither go down nor lift up. Cut off from the air gap.
Approach:
(1) The concrete is not in place, and the drilling speed is too fast or too slow. Strictly control the time of the large concrete and its filling coefficient to ensure that the concrete filling coefficient is within the required range;
(2) The slump is too large or too small. Check whether the concrete slump meets the requirements. If the slump is too large, it is easy to produce bleeding and segregation. In a short time, the coarse aggregate sinks and the cement slurry floats. On top. Increase the downward resistance of the steel cage, resulting in the steel cage not being in place; too small slump will still increase the difficulty of lowering the cage, so the slump must be strictly controlled at 18-22cm during the construction process;
(3) The quality of the reinforced cage is too poor or the design is unreasonable. The specifications of the reinforced cage are strictly in accordance with the drawings, and there is no problem of excessively large ribs. When lowering the reinforced cage, the bottom of the reinforced cage should be made into a sharp row to avoid insertion phenomenon;
(4) The power of the selected vibratory hammer should be appropriate. When the reinforced cage is lowered to 1-2m from the hole, it should be sent to the design elevation by vibrating hammer with guard tube. Generally, the pile with a diameter of 400mm and a length of not more than 10m, the 1.8kW vibrating hammer can meet the requirements. ; For foundation piles with a diameter of 600mm and a length of less than 15m, a 2.2kW vibratory hammer is appropriate;
(5) Check whether the diameter of drill bit and drill pipe meet the requirements. If the drill bit is found to be seriously worn, it should be repaired in time, and an alloy block is inserted on the outer edge to ensure that the hole diameter and protective layer meet the requirements. There is enough space between the reinforcement cage and the hole wall to make the reinforcement cage in place smoothly.
▌Partial hole
Phenomenon: In the construction process of punching piles, there is often a problem of deviation holes. When encountering a large lone stone or probe stone (slope rock face) in the punching hole, the rock face at the junction of the inclined soft and hard formations is inclined into the sand or the sand and pebble layer with a large particle size, the hole expansion is relatively large. When the hammer head swings to one side at a large place, when the base of the punching pile machine is not level or uneven settlement or displacement occurs, deviation holes may occur.
Approach:
(1) When installing the punching pile machine, the chassis and the base should be horizontal, the lifting pulley flange, the clamping hole of the steel wire fixed to the hammer head, and the center of the protective tube should be on a vertical line, and the inspection and correction should be frequent;
(2) When it is found that the impact of the hammer head is unstable (swinging left and right), the impact should be stopped to check whether the deviation hole occurs, and if the deviation hole occurs, timely treatment;
(3) When the deviation hole is found, first find out the cause of the deviation hole and the size of the deviation hole. If it is due to the impact of the hammer head on the lone stone or the probe stone (slope rock surface), the hole should be backfilled in time and backfilled to high When the rock face is 1m, it is impacting.
▌ Plum blossom hole (or cross hole)
Phenomenon: When the punching pile impacts, the punched hole is not round.
Approach:
(1) Use mud of appropriate viscosity and relative density, and slag in time;
(2) When using low stroke, use a higher stroke impact for each impact section, and alternately modify the hole shape;
(3) After plum blossom holes appear, the holes can be backfilled with flake and pebble mixed clay and re-impacted.
3. Quality issues
▌Common quality problems
(1) An error occurs in the measurement of the pay-off. This error will cause an error in the entire building or a large deviation in the pile foundation.
(2) The bearing capacity of single pile does not meet the design standards of building pile foundation.
(3) The pile is inclined too much.
(4) The joint of the precast pile is broken.
(5) The broken pile accident caused by the grouting process.
(6) During the acceptance process, it was found that the pile position had an excessive deviation.
(7) Due to the quality of the concrete or the pile caused by the operation, there are problems such as mud inclusion, segregation, and strength failure.
(8) Insufficient elevation.
▌惭别补蝉耻谤别蝉
Compensation method
This method can be used when the depth of the precast pile is insufficient, or when the driven pile lifts the pile due to the uplift of the soil.
Piling method
It is in accordance with the design, supervision and the opinions of the owners to make up the piles according to the pile-up plan issued by the design unit. However, this method has a large investment and a long construction period, and it is difficult for all parties to jointly agree.
Supplementary delivery
When the pile is driven into the foundation by section connection, if the mass of the pile does not reach the standard, it is possible to disconnect the connecting nodes during the sinking process. At this time, the treatment of the pile foundation can be supplemented Send the combined way.
For the pile with doubtful points, it should be re-tapped to make the pile sink, which can make the connection of the disjointed pile tighten again and make the joint have vertical bearing capacity; the pile can be supplemented and some complete piles can be properly supplemented. , 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 the pile body is tilted during pile driving and the pile length is not long, and is intact and not broken, or the pile body is tilted due to excavation of the foundation pit but the pile body is still intact, the pile body can be excavated locally Open, and then use a jack to correct the pile.
Platform expansion
(1) If there is a large deviation in the pile position, then the original design size of the cap cannot meet the structural requirements of the structure, then the pile position deviation can be corrected by expanding the cap appropriately.
(2) Pile-soil interaction. If the single pile cannot meet the designed bearing capacity requirements, it is necessary to expand the bearing platform, and also take into account the upper load shared by the natural foundation and the pile.
(3) Quality problems of pile foundation. Uneven pile foundation quality is prone to occur during construction. In order to prevent cloud settlement of the bearing platform layout caused by pile foundation quality problems in the later stage and to improve the seismic resistance of the building, integral pile foundation bearing platform can be used to improve the foundation integrity.
Composite foundation method
(1) It is to use the principle of pile-soil interaction to properly deal with the foundation, improve the bearing capacity of the foundation, and share the load of the pile foundation more effectively. Common methods are as follows.
(2) Replace the foundation under the platform. Before the pile foundation cap construction, excavate a certain depth of soil, replace it with sand and gravel filling layered ramming and then construct the cap on the artificial foundation and pile foundation.
(3) Add cement and soil piles between the piles. When the bearing capacity of the pile does not meet the design requirements, the method of dry spraying cement in the soil between the piles to form a cement soil pile can be used to form a composite foundation.
Modifying pile type or pile sinking parameter method
(1) Change the pile type. For example, prefabricated square pile is changed to prestressed pipe pile.
(2) Change the depth of the pile into the soil. For example, during the penetration of prefabricated piles, a thick dense silt or silt layer is encountered, and it is difficult to sink the pile, or even a broken pile accident. At this time, the length of the pile can be shortened, the number of piles can be increased, and the dense silt can be taken. The layer (expansive soil layer) serves as a supporting layer.
(3) Change the pile position. If a hard, small underground obstacle is encountered in the pile sinking, the pile is inclined, or even broken, the pile position can be changed to re-pile the pile.
(4) Sinking pile 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 impact method can be used for pile sinking.
