Building pile foundation projects have many construction procedures and high process requirements. Many factors affect the quality of pile foundations. Generally, they include insufficient detailed and accurate engineering geological survey reports, reasonable design values, and various reasons during construction. The analysis and treatment of quality problems and hidden dangers during pile foundation construction will affect the structural safety of the building. This article focuses on the categories, cause analysis and common treatment methods of common quality problems in pile driving (pressure) construction.
Analysis of the Categories and Causes of Common Quality Problems in Construction Pile Foundation Projects
Common quality problems in pile driving (pressure) projects include five major categories: the bearing capacity of a single pile is lower than the design value, the pile tilt is too large, the pile is broken, the pile joint is disconnected, and the pile position deviation is too large.
Reasons for the above problems:
1. Common reasons why the bearing capacity of a single pile is lower than the design requirements are:
1. The pile sinking depth is insufficient;
2. The pile end has not entered the bearing layer specified in the design, but the pile depth has reached the design value;
3. The final penetration is too large;
4. Other reasons such as excessive inclination and fracture of the pile lead to a decrease in the bearing capacity of a single pile;
5. The stratigraphic profile, foundation-bearing capacity, and other relevant data provided in the survey report are inconsistent with the actual situation.
2. Common reasons for excessive pile tilt:
1. The quality of prefabricated piles is poor. The tilt of the top surface of the pile and the misalignment or deformation of the pile tip are most likely to cause the pile to tilt;
2. The pile driver is not installed correctly, and the pile frame is not vertical to the ground;
3. The center lines of the pile hammer, pile cap, and pile body do not coincide, resulting in hammering eccentricity; 2.4 The pile end encounters stones or hard obstacles;
4. The pile spacing is too small and the piling sequence is improper, resulting in a strong soil-squeezing effect;
5. Improper excavation of foundation pit.
3. Common reasons for broken piles:
In addition to excessive pile inclination that may cause pile fracture, there are three other reasons:
1. Improper position of the fulcrum or lifting point for stacking, lifting, and transporting piles;
2. During the pile sinking process, the pile body bent too much and broke. Such as the bending caused by the quality of the pile manufacturing, or the bending caused by hammering when the pile is slender and encounters a harder soil layer;
3. Too many hammer blows. If the piles required by some designs are hammered too hard and the designed penetration is too small, excessive hammering may cause the piles to break during construction.
4. Common reasons for pile joint disconnection:
When the designed pile is long, due to the needs of construction technology, the pile is prefabricated and sunk in sections. Steel welded connectors are often used as pile joints between each section. This disconnection phenomenon of pile joints is also common. The reasons include, in addition to 2.1-2.5 in Section 2, the center lines of the piles in the upper and lower sections do not coincide with each other; the construction quality of the pile joints is poor, such as insufficient weld size.
5. Common reasons for excessive pile position deviation:
Errors in measurement and setting out; poor pile sinking technology, such as tilting of the pile body, resulting in large deviations in the completed pile position
Commonly used methods to deal with quality problems in construction pile foundation projects
During the piling process, if quality problems are discovered, the construction unit must not deal with them on its own. It must report them to the supervisor and owner, and then analyze and study them together with design, survey, and other relevant departments to make a correct solution. The design department will issue a design modification notice. General treatment methods include: the supplementary settlement method, supplementary pile method, combination of supply and supplementation method, deviation correction method, expanded capping method, composite foundation method, etc., which are briefly introduced below:
1. Supplementary sinking method
This method can be used when the depth of the prefabricated piles in the soil is insufficient, or when the driven piles are lifted due to soil uplift.
2. Pile patching method
Either of the following two options can be used:
1. Patch piles in front of the pile foundation cap. When the distance between piles is small, the method of drilling first, then planting piles, and then sinking piles can be used.
2. After the pile foundation cap or basement is completed, static pressure piles will be added. The advantage of this method is that the cap platform or basement structure can be used to withstand the construction reaction force of static pressure piles. The facilities are simple and easy to operate, and the construction period is not extended.
3. Combined supply and delivery method
When driven piles are connected in sections and driven one by one, the connection nodes of poorly connected piles may become disconnected. In this case, the method of combining supply and repair can be used. The first is to drive the doubtful piles again to make them sink, and to tighten the loosened joints so that they have a certain vertical bearing capacity; secondly, to make up for the piles with full length and complete, on the one hand, to make up for the entire foundation. Insufficient vertical bearing capacity, on the other hand, the entire pile added can withstand seismic loads.
4. Correction method
If the pile body is tilted but not broken, and the length of the pile is short, or the pile body is tilted but not broken due to excavation of the foundation pit, the method can be used to correct and reset the pile using a jack after partial excavation.
5. Expand the platform support method
Due to the following three reasons, the original plane dimensions of the pile foundation cap cannot meet the structural requirements or foundation-bearing capacity requirements, and the area of the pile foundation cap needs to be expanded.
1. The pile position deviation is large. If the plane dimensions of the originally designed cap platform cannot meet the structural requirements stipulated in the specification, the enlarged cap platform method can be used.
2. Consider the interaction between pile and soil. When the bearing capacity of a single pile cannot meet the design requirements, it is necessary to expand the cap platform and consider the piles and natural foundation to share the superstructure load.
3. The quality of the pile foundation is uneven. To prevent uneven settlement of the independent pile foundation, or to improve the earthquake resistance, the independent pile foundation foundation can be connected into a whole block to improve the integrity of the foundation, or to install earthquake-resistant ground beams.
6. Composite foundation method
This method uses the principle of pile-soil interaction to properly treat the foundation, improve the bearing capacity of the foundation, and more effectively share the load of the pile foundation. Commonly used methods include the following.
1. Make a soil replacement foundation under the platform. Before the construction of the pile foundation cap, the soil to a certain depth should be excavated and replaced with sand and gravel filling layers and tamped in layers, and then the cap should be constructed on the artificial foundation and pile foundation.
2. 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.
7. Modify pile type or pile sinking parameters
1. Change the pile type. For example, prefabricated square piles are replaced by prestressed pipe piles, etc.
2. Change the depth of the pile into the soil. For example, when prefabricating piles, a thick layer of dense silt sand or silt soil is encountered, which makes it difficult for the pile to sink, or even a pile breakage accident. In this case, the length of the pile can be shortened, the number of piles can be increased, and the dense silt sand layer can be used as the holding force. force layer.
3. Change the 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. Change 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.
8. Other methods
1. The bottom plate is elevated.
The ground floor was changed to an elevated floor to reduce the dead weight of the filling soil and reduce the load on the cap.
2. Unload the upper structure.
Some major pile foundation accidents are difficult to deal with, cost a lot, and take too much time. The only way to reduce the pile foundation load is to reduce the number of superstructure layers. It is also possible to use lightweight and high-strength partition walls or other materials to replace the heavy structure of the original design to reduce the weight of the superstructure.
3. Structural verification.
However, if there are accidents such as insufficient concrete strength in the pile body and low bearing capacity of the single pile, solutions can be found through structural verification and other methods. If the verification results still meet the requirements of the specification, you can negotiate with the design unit and no special treatment will be performed. However, this method taps design potential and must be approved by the design department. It must not be used immediately and should be used with caution.
4. Comprehensive treatment method.
Selecting several comprehensive applications of the aforementioned methods can often achieve ideal results.
5. Use peripheral patching piles to increase peripheral embedding and prevent or reduce pile position lateral movement.
In short, the quality of pile foundation construction is related to the engineering quality of the entire building. During the pile foundation construction process, if various unexpected situations are encountered, the owner, supervision, and design department should be contacted in time, and the design modification notice or meeting minutes of the design department should be followed Undertake construction.
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