Analysis and treatment of common quality problems in pile foundation construction

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Pile driving (pressing) construction involves many procedures and high technical requirements, with numerous factors that can affect the quality of the pile foundation. These generally include: 1. Insufficiently detailed and accurate engineering geological survey reports； 　　2. Reasonable values for the design ； 　　3. Various reasons during construction. 　　The analysis and handling of quality issues and potential hazards during pile foundation construction will affect the structural safety of the building. This article focuses on the types of common quality issues in pile driving (piling) construction, their cause analysis, and common treatment methods. Common quality issue categories and cause analysis: The common quality problems in pile driving (compression) projects include five main categories: single-pile bearing capacity lower than the design value, excessive pile inclination, broken piles, separation at pile joints, and excessive deviation in pile placement. Reasons for the above problems: 1.1 Common reasons for the single pile’s bearing capacity being lower than the design requirements are: 1.1.1 Insufficient pile penetration depth ； 1.1.2 The pile tip has not reached the bearing stratum specified in the design, but the pile depth has reached the designed value ； 1.1.3 Excessive final penetration ； 1.1.4 Others, such as a decrease in the bearing capacity of a single pile due to excessive pile inclination or fracture ； 1.1.5 The data provided in the survey report, such as the stratigraphic profile and foundation bearing capacity, do not correspond to the actual conditions. 1.2 Common causes of excessive inclination of piles: 1.2.1 Poor quality of precast piles, where an inclined pile top surface and an incorrect or deformed pile tip are the most likely to cause pile inclination ； 1.2.2 The pile driver is not installed properly; the pile frame is not perpendicular to the ground ； 1.2.3 The center lines of the pile hammer, pile cap, and pile shaft do not align, resulting in eccentric impact ； 1.2.4 Pile tip encounters stones or hard obstacles ； 1.2.5 Excessive pile spacing and improper piling sequence result in a strong soil displacement effect ； 1.2.6 Improper excavation of foundation pit soil. 1.3 Common causes of broken piles: In addition to pile breakage that may occur due to excessive inclination, there are three other reasons: 1.3.1 Improper placement of the support points or lifting points during pile stacking, lifting, and transportation ； 1.3.2 During pile driving, the pile shaft bends excessively and breaks. Such as bending caused by the quality of pile fabrication, or bending resulting from hammering when the pile is slender and encounters a harder soil layer ； 1.3.3 Excessive number of hammer strikes. In some designs, the hammer force required is too high and the designed penetration depth is too low; as a result, during construction, excessive hammering causes the pile to break. 1. Common causes of joint separation in 4-pile structures: When the piles are designed to be long, construction techniques require that they be prefabricated in sections and sunk one after another; steel welded connectors are commonly used to form joints between these sections. The separation of such pile joints is also relatively common. The reasons, in addition to 1.2.1–1.2.5 in Section 1.2, are also the fact that the center lines of the piles in the upper and lower sections do not align ； The construction quality of pile joints is poor, due to reasons such as insufficient weld dimensions. 1. 5 Common causes of excessive pile position deviation: errors in surveying and setting out ； Poor pile-driving techniques, such as the tilting of the pile shaft, can result in significant deviations from the intended position of the completed piles. Common solutions: When quality issues are detected during pile driving, the construction party must not attempt to handle them on its own; instead, it should report them to the supervisor and the client, and then work together with relevant parties such as the design team and surveyors to analyze the situation and develop an appropriate solution. A notice of design modification is issued by the design department. Common treatment methods include: settlement compensation method, pile reinforcement method, combination of reinforcement and settlement adjustment method, deviation correction method, enlarged cap method, composite foundation method, etc. The following is a brief introduction to each of them: 2.1 Settlement compensation method. This method can be used when the depth of the precast pile in the soil is insufficient, or when the driven pile is lifted upward due to soil heaving. 2.2 Pile reinforcement method. Either of the following two methods can be used: 2.1.1 Adding piles in front of the pile cap. When the pile spacing is small, the method of drilling first, then installing the piles, and finally driving them down can be adopted. 2.1.2 Static pile pressing shall be carried out after the pile cap or basement is completed. The advantage of this method is that it allows the cap or basement structure to bear the construction reaction forces of the static pressure piles; it is simple in design, easy to operate, and does not prolong the project timeline. 2.3 Supplementary delivery combined method. When driven piles are connected in sections and driven in one by one, poor connections between the piles may result in the separation of the connection joints; in such cases, the supplementary insertion method can be employed. First, the suspicious piles are driven again to sink them, so that the loose joints can be tightened again, thereby granting them a certain vertical bearing capacity ； Secondly, it is appropriate to add some piles with a full length; this helps to compensate for the insufficient vertical bearing capacity of the entire foundation, and the newly added piles can also withstand seismic loads. 2.4 Correction method. If the pile is tilted but not broken, and its length is relatively short, or if the tilting has occurred due to excavation of the foundation pit without causing breakage, it can be corrected by carrying out local excavation followed by using jacks to realign the pile. 2.5 Method of enlarging the cap: For the following three reasons, the planar dimensions of the existing pile cap fail to meet the structural requirements or the requirements regarding foundation bearing capacity, hence it is necessary to increase the area of the pile cap. 2.5.1 Large deviation of pile position. The planar dimensions of the originally designed cap slab do not meet the structural requirements specified in the codes, and this issue can be addressed by using an enlarged cap slab. 2.5.2 Consider the interaction between the pile and soil. When the bearing capacity of a single pile does not meet the design requirements, it is necessary to enlarge the cap and consider having the pile and the natural foundation share the load of the superstructure together. 2. 5.3 To address the uneven quality of pile foundations and prevent uneven settlement of individual pile caps, or to enhance seismic resistance, it is possible to connect these individual pile cap structures into a single unit in order to improve the integrity of the foundation, or to install seismic beams. 2.6 Composite foundation method: This method makes use of the principle of interaction between piles and soil to properly treat the foundation, thereby increasing its bearing capacity and enabling a more effective distribution of the loads carried by the pile foundations. The common methods are as follows. 2.6.1 Replace the soil foundation under the cap slab. Before constructing the pile cap, soil to a certain depth is removed and replaced with a sand and gravel fill, which is tamped in layers; thereafter, the cap is built on top of the artificial foundation and piles. 2.6.2 Add cement-soil piles between the piles. When the bearing capacity of the piles does not meet the design requirements, the method of dry spraying cement into the soil between the piles to form cement-soil piles can be employed to create a composite foundation. 2.7 Modifying pile type or piling parameters 2.7.1 Changing the pile type. For example, replacing precast square piles with prestressed pipe piles, etc. 2.7.2 Changing the pile-soil depth. For example, during the installation of precast piles, if thick layers of compact silt or silty soil are encountered, it can lead to difficulties in driving the piles downward, or even pile breakage. In such cases, it is possible to reduce the length of the piles, increase the number of piles, and use the compact silt layer as the bearing stratum. 2.7.3 Changing the pile position. If hard, small underground obstacles are encountered during pile driving, causing the pile to tilt or even break, it is possible to drive the pile again by changing its position. 2.7.4 Changing the pile-driving equipment. When the penetration depth of the pile does not meet the design requirements, a high-tonnage pile driver can be used, along with a low-impact hammering method for driving the pile. 2.8 Other methods 2.8.1 Elevating the base plate. The underlying floor has been changed to a raised floor slab in order to reduce the weight of the fill soil and lower the load on the foundation slabs. 2.8.2 Unloading of the superstructure. Some major pile foundation accidents are difficult to handle, requiring substantial costs and a lot of time; the only solution is to reduce the number of floors in the superstructure in order to lessen the load on the piles. Lightweight and high-strength partition walls or other materials are also used to replace the originally designed heavy structures, thereby reducing the weight of the superstructure. 2.8.3 Structural verification. However, in cases of issues such as insufficient concrete strength in the pile shaft or low bearing capacity of individual piles, solutions can be found through methods like structural analysis. If the verification results still meet the requirements of the codes, consultation with the design unit can be carried out to decide against taking any special action. However, this method taps into the design’s potential; it must be approved by the design department. It should only be used as a last resort, and with extreme caution. 2.8.4 Comprehensive treatment method. A combination of several of the aforementioned methods often yields relatively satisfactory results. 2.8.5 Peripheral supplementary piles are used to enhance the fixation around the area, thereby preventing or reducing lateral displacement of the piles. In summary, the quality of pile foundation construction is related to the overall quality of the building. During the pile foundation construction process, if various unexpected situations arise, it is necessary to promptly contact the owner, the supervision team, and the design department, and proceed with the construction in accordance with the design modifications specified by the design department or the minutes of relevant meetings. 　　Of course, it is also possible for the same property owner to fall into several different categories at the same time; in such cases, an analysis can be conducted based on the specific circumstances.