There are many types of pile foundations with large differences in construction techniques. In addition, the ground changes are complex. During the construction process, the pile body may have defects such as shrinkage, expansion, mud inclusion, segregation, and pile breakage. Of course, after construction, mechanical excavation is required, and collisions may occur. Will cause shallow pile body defects. The existence of defects in the pile body will change the normal working characteristics of the foundation pile, thereby causing potential danger to the foundation. Therefore, pile foundation inspection needs to be strictly carried out to fundamentally control the quality of the project. At the same time, testing is not the ultimate goal. This article analyzes and summarizes the quality problems discovered after testing and their causes.
Pile Foundation Inspection
Under what circumstances should verification and expanded testing be performed?
1. During low strain detection, for rock-embedded piles, the time domain reflection signal at the bottom of the pile is a single reflection wave and is in the same direction as the hammering signal. The measured signals are complex and irregular, making it impossible to accurately evaluate them.
Concrete cast-in-place piles with gradually changing or variable pile cross-sections and large changes are always verified using the static load method or core drilling method.
2. 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 defects have an impact on the horizontal bearing capacity; the single-click penetration is large, the bottom of the pile reflects strongly in the same direction, and the reflection peak is wide, the reflection of side resistance wave and end resistance wave is weak, that is, when the waveform shows vertical bearing characteristics that are inconsistent with the geological conditions in the survey report, the static load method can be used for further verification.
3. The bottom of the rock-socketed pile has a strong reflection in the same direction, and there is no obvious end resistance reflection after time 2L/C. The core drilling method can be used to verify.
4. Shallow defects in the pile body can be verified by excavation.
5. Precast piles with cracks in the pile body or joints can be verified using the high-strain method.
6. When single-hole drilling core testing reveals quality problems with the pile body concrete, it is advisable to add drilling verification on the same foundation pile.
7. For piles whose integrity category cannot be clarified in the low strain method test or Class III piles, suitable methods such as static load method, core drilling method, high strain method, excavation, etc. can be used for verification and testing according to the actual situation.
8. When the bearing capacity of a single pile or the results of sampling inspection by core drilling method does not meet the design requirements, the reasons should be analyzed and the sampling inspection expanded after confirmation.
9. When the sum of Class III and IV piles is found to be greater than 20% of the number of piles randomly inspected by the low strain method, high strain method, and acoustic transmission method is used to randomly check the pile integrity, the original detection method should be used (the acoustic transmission method can be modified). Use core drilling method), and continue to expand random inspections in uninspected piles.
What should the foundation pile inspection report contain?
The test report should contain the following content:
1. Name of the client, project name, location, construction, survey, design, supervision and construction unit, foundation, structural type, number of layers, design requirements, testing purpose, testing basis, testing quantity, testing date;
2. Description of geological conditions;
3. The pile number, pile position, and relevant construction records of the inspected piles;
4. Testing methods, testing instruments and equipment, and description of the testing process;
5. Inspection data of the inspected piles, actual measurement and calculation analysis curves, tables, and summary results;
6. Test conclusion corresponding to the test content. To evaluate the test results of engineering pile bearing capacity, the bearing capacity test value of each tested pile should be given, and based on this, a conclusion should be drawn as to whether the characteristic value of the single pile bearing capacity under the same conditions of the unit project meets the design requirements.
What regulations should be followed to determine the characteristic value of the horizontal bearing capacity of a single pile under the same conditions as a 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 does not allow cracking, 0.8 times the statistical value of the horizontal critical load is taken as the characteristic value of the single pile horizontal bearing capacity.
3. When the horizontal 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 can be taken as the characteristic value of the horizontal bearing capacity of a single pile, but it should meet the requirements of the relevant specifications for crack resistance design.
Construction Issues
Leaking Grout
Phenomenon: Slurry leakage is called piping in hydraulics. A common phenomenon is bubbles or muddy water emerging from the water surface outside the casing. The occurrence of the above phenomena means that slurry leakage has occurred.
Treatment method: add cement or vibrate the casing to sink. The casing should not sink more than 1m. If the casing sinks more than 1m, or the casing suddenly sinks, the construction should be stopped.
Phenomenon: The amount of slag discharge increases abnormally, the load of the drilling rig suddenly increases, or a large number of fine bubbles suddenly appear on the mud surface in the casing.
Treatment method: If a hole collapses, stop drilling immediately and lift the drill bit to prevent the drill bit from being buried. Because after the hole collapses, the arching effect of the destroyed soil suddenly disappears, and the hole wall enters an extremely unstable state. At this time, backfilling the soil and letting it sit is the most effective method.
Rebar Cage Floats
Phenomenon: The steel cage cannot be lowered into place smoothly, which is the most common problem in the construction of long spiral drilled cast-in-place piles. Sometimes, because the steel cage is too long, it gets stuck in the hole halfway down, and cannot be lowered or lifted, so it is forced to Cut off from the hollow.
Approach:
1. The concrete is not poured in place, and the drill is lifted too fast or too slowly 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 may easily occur.
Within a short period, the coarse aggregate sinks and the cement slurry floats on top. The downward resistance of the steel cage increases, resulting in the steel cage not being in place; if the slump is too small, it will still increase the difficulty of lowering the cage. Therefore, the slump must be strictly controlled between 18 and 22 cm 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 in the drawing to avoid the problem of too-large dragon bars. When lowering the steel cage, the bottom of the steel cage should be made into sharp rows to avoid the phenomenon of insertion;
4. The power of the vibrating hammer selected must be appropriate. When the steel cage is lowered to 1 to 2m away from the hole, it must be lowered to the design elevation by a vibrating hammer with a sheath. Generally, for piles with a pile diameter of 400mm and a length of no more than 10m, a 1.8kW vibrating hammer can meet the requirements; For foundation piles with a pile diameter of 600mm and a pile length less than 15m, it is appropriate to use a 2.2kW vibrating hammer;
5. Check whether the drill bit and drill pipe diameters 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 installed on the outer edge to ensure that the hole diameter and protective layer meet the requirements. Make sure there is enough space between the steel cage and the hole wall so that the steel cage can be placed smoothly.
Offset hole
Phenomenon: During the construction process of punched piles, there are often problems with offset holes. When encountering larger boulders or probe stones (slope rock surface) during punching, when the rock surface is tilted at the junction of soft and hard strata, or when it is punched into a layer of sand and pebbles with widely different particle sizes, the hole expansion will be more difficult. When the hammerhead swings in one direction, when the base of the punching pile machine is not placed levelly, or when uneven subsidence or displacement occurs, an offset hole may occur.
Approach:
1. The hole and the center of the casing should be on a vertical line, and should be checked and corrected frequently;
2. When the impact of the hammerhead is found to be unstable (swinging left and right), the impact should be stopped to check whether a deviation hole has occurred. If a deviation hole occurs, deal with it in time;
3. When an offset hole is found, first find out the cause and size of the offset hole. If it is due to the impact of the hammerhead on a boulder or probe stone (slope rock surface), backfill the hole with stones in time and backfill to a higher place. Impact occurs when the rock surface is 1m.
Quality Issues
Common quality issues
1. There are errors in the measurement and setting out, which will cause errors in the entire building or cause large deviations in the pile foundation.
2. The bearing capacity of a single pile does not meet the design standards of building pile foundations.
3. The pile body is tilted too much.
4. The joints of the prefabricated piles are disconnected.
5. Pile-breaking accidents caused during pile grouting.
6. During the acceptance process, it was found that the pile position had excessive deviation.
7. Problems such as mud inclusion, segregation, and substandard strength in the pile body are caused by the quality of the concrete or its operation.
8. Insufficient elevation.
Treatment measures
1. Complementary settlement method: This method can be used when the depth of prefabricated piles in the soil is insufficient, or when the driven piles lift the piles due to soil uplift.
2. Pile patching method: This method is to carry out patching according to the pile patching plan issued by the design unit in conjunction with the opinions of the designer, supervisor, and owner. However, this method requires large investments and a long construction period and is difficult to be recognized by all parties.
3. Combined supply and supply method: When driving piles into the foundation using segmented connections, if the quality of the pile body is not up to standard, the connecting nodes may become disconnected during the sinking process. At this time, for the pile, The basic processing can be done by combining supply and delivery.
For piles with doubtful points, they should be driven again to make the pile sink, so that the disconnected pile connection can be tightened again so that the joint has vertical bearing capacity; the pile can be repaired, and some complete piles can be added appropriately. , so that the foundation can not only meet the bearing capacity requirements but also increase the seismic load of the building foundation.
4. Correction method: If it is found that the pile body is tilted during the piling process and the pile length is not long and is intact and not broken, or the pile body is tilted due to the excavation of the foundation pit but the pile body is still intact, the pile body can be corrected. The pile body is partially excavated, and then a jack is used to correct the deviation of the pile body.
Expansion of platform
1. If there is a large deviation in the pile position, and the original design size of the cap cannot meet the normative requirements of the structure, the pile position deviation can be corrected by appropriately expanding the area of the cap.
2. The pile and soil work together. If a single pile cannot meet the designed bearing capacity requirements, the cap platform needs to be expanded, and the upper load shared by the natural foundation and piles must also be taken into account.
3. Pile foundation quality issues. Uneven pile foundation quality is prone to occur during construction. To prevent cloud settlement of the cap deck layout caused by pile foundation quality problems in the later period and to improve the earthquake resistance of the building, an integral pile foundation cap can be used to improve the integrity of the foundation.
Composite foundation method
1. It uses the principle of mutual interaction between piles and soil to properly treat the foundation to improve the bearing capacity of the foundation and share the load of the pile foundation more effectively. Commonly used methods include the following.
2. Make a soil replacement foundation under the cap platform. Before the construction of the pile foundation cap, the soil to a certain depth is excavated, replaced with sand and gravel filling layer, and tamped in layers, and then the cap is constructed on the artificial foundation and pile foundation.
3. Add cement and soil piles between piles. When the pile-bearing capacity cannot meet the design requirements, the method of dry spraying cement into the soil between the piles to form cement-soil piles can be used to form a composite foundation.
Method for modifying pile type or pile driving parameters
1. Change pile type. For example, prefabricated square piles are replaced by prestressed pipe piles, etc.
2. Change the depth of pile penetration into the soil. For example, when a prefabricated pile encounters a thick layer of dense silt sand or silt during the penetration process, it becomes difficult for the pile to sink, or even a pile breakage accident occurs. In this case, the length of the pile can be shortened, the number of piles increased, and the dense silt sand layer can be used. layer (expansive soil layer) as the bearing layer.
3. Change pile position. If hard and small underground obstacles are encountered during pile sinking, causing the pile to tilt or even break, the pile position can be changed and the pile can be driven again.
4. Variable pile sinking equipment. When the pile sinking depth cannot meet the design requirements, a large-tonnage pile frame can be used and the heavy hammer low-impact method can be used to sink the pile.
Thanks
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