PHC prestressed pipe pile foundations have been widely used in engineering construction in Haikou and Sanya in recent years due to their advantages such as short construction period, high single pile bearing capacity, convenient monitoring, low cost, and small environmental impact. As a result, many quality issues concerning pipe piles have attracted much attention. Based on construction experience of pipe pile foundation projects in recent years, common quality problems in pipe pile foundation construction include: pile driving failing to meet design control requirements; pile position deviation and pile inclination exceeding standards; pile head crushing; pile body damage; and single pile bearing capacity failing to meet design requirements. The author will discuss the causes and countermeasures of common problems in pipe pile foundation construction, specifically the failure to meet design control requirements during pile driving and pile body damage. 1.0 Pile driving failing to meet design control requirements
1.1 The main reasons for pile driving failing to meet design control requirements are:
1.1.1 The density of survey boreholes does not meet specified requirements, and the exploration report fails to provide accurate geological data; or the design selects an inappropriate bearing stratum, or the design requirements are too high.
1.1.2 During pile driving, underground obstacles or thick hard interlayers are encountered.
1.1.3 The selected pile hammer weight is too small, or the pile hammer is worn out and does not fall freely and smoothly.
1.1.4 Dense pile arrangement or improper piling sequence makes it impossible for subsequently driven piles to reach the design depth and causes previously driven piles to heave. At a construction site in Haikou City that was just completed, Ф500 pipe piles were designed, but the pile spacing was only 1.4 meters, failing to meet the code requirement of not less than 3 times the pile diameter. During pile foundation acceptance, it was found that soil squeezing caused the pipe piles to heave, and the bearing capacity failed to meet design requirements.
1.1.5 The pile head is shattered or the pile body is broken, making continuous driving impossible;
1.1.6 When the effective pile length is relatively short (short piles less than 14m), static pressing is used and the final pressure value just meets the design requirement when pressing is stopped. When the pile tip encounters compacted silt or silty fine sand layers, a “false set” phenomenon may occur, which may also result in insufficient ultimate bearing capacity.
1.2 The main countermeasures for pile driving failing to meet design control requirements are:
1.2.1 Reasonable design should be carried out after a careful test pile process. Since the factors affecting pile bearing capacity are numerous and complex, and soil conditions vary greatly from place to place, determining the unit vertical bearing capacity through test piles followed by static load tests should be considered the most basic and reasonable approach, and is currently the most commonly used method. Test piles can verify the accuracy of geological reports, test the applicability of a certain pipe pile type under the geological conditions, and provide the most reliable basis for reasonable design. On the basis of carefully and deeply studying the geological exploration report, reasonable selection of pile diameter and reasonable arrangement of pile positions can greatly avoid the phenomenon of pile driving failing to meet design control requirements. For foundations with relatively complex geological conditions, it is recommended to use Type B pipe piles with relatively high bending resistance.
1.2.2 When abnormal geological conditions are encountered, if static pressing is used, the method of pre-boring can solve the problem; if the surrounding environment permits, hammer driving can be selected because hammer driving has stronger penetration ability and can pass through thicker hard interlayers. For example, at the Xifu Xincheng site on Haixiu Road in Haikou City, the fifth layer is a relatively hard iron concretion layer with an SPT value of more than 30. Static pressing was originally designed, but because it could not penetrate, hammer driving was later adopted, which penetrated the fifth layer and reached the design bearing stratum.
1.2.3. The energy of the selected pile driver should match the design requirements, pile diameter, pile length, and geological conditions, that is, the pile driver selection and counterweight should meet construction requirements.
1.2.4 Formulate a reasonable construction plan and adopt different construction flow lines according to different pile penetration depths. It is advisable to drive long piles first and short piles later, and high piles first and low piles later.
●If the piles are relatively dense and far from buildings with an open site, it is advisable to proceed from the center to the surroundings;
●If the piles are relatively dense and the site is narrow and long, with both ends far from buildings, it is advisable to proceed from the middle to both ends;
●If the piles are relatively dense and one side is close to a building, it is advisable to start from the side adjacent to the building and proceed from near to far;
●For pile group foundations with more than 30 piles, driving should start from the center and proceed outward;
●For piles at the edge of a pile cap, insert and drive them after the other piles in the cap have been driven and the pile positions have been re-measured;
●For pipe piles in deep foundation pits with retaining structures, it is advisable to drive the piles first and then construct the retaining structure of the foundation pit.
1.2.5 If a “false set” phenomenon occurs, the final pressure value should be greater than 1.2 to 1.5 times the design ultimate bearing capacity, and driving can continue after an interval, which will meet the requirements.
2.0 Pile body damage during hammer driving
2.1 Pile body damage during pile driving is relatively common. Existing statistical data show that its occurrence probability is generally above 5%, and it can account for more than 50% of pile driving quality accidents. The main factors for pile body damage during hammer driving are:
2.1.1 Poor pile manufacturing quality. Generally speaking, since PHC pipe piles are produced in factory assembly lines with relatively strict quality inspection, pile body quality is mostly guaranteed, but it cannot be said that pile body quality will absolutely never have problems. In fact, many piles whose ultimate bearing capacity fails to meet design requirements are caused precisely by pile body quality problems. The main manifestations are:
●Poor quality of raw materials for pile manufacturing;
●The manufacturing process does not meet code requirements, and centrifugal compaction operations are improper;
●Improper mix proportion, lax control of water-cement ratio, and concrete strength failing to meet requirements;
●Improper steam curing regime during steam curing causing brittle failure of concrete;
●During pipe pile manufacturing, serious slurry leakage at the pile head forms voids, etc.
2.1.2 During hammer driving, operations are not standardized, mainly:
●Improper selection of pile hammer, too heavy or too light, causing pile head shattering or fatigue damage;
●During driving, the axes of the pile hammer, pile cap, and pile body cannot be kept on the same centerline, causing eccentric hammer blows;
●The pile cap is too small, too large, too deep, or has too large a dimensional deviation from the pile head, so a suitable pile cap should be selected;
●The pile cap cushion material is too thin or no cushion is added, or it is not replaced in time;
●Continuing to drive vigorously when encountering boulders or hard rock surfaces;
●Restarting driving after too long an interval in thick cohesive soil layers.
2.2 The main methods to solve pile body damage during hammer driving:
●During construction, inspection and acceptance of raw materials for the pile body should be strengthened first.
●Strictly implement pile driving operation specifications and drive correctly.
●If the pile head is shattered, as long as the pile foundation bearing capacity is verified to meet design requirements, it is acceptable; otherwise, piles must be added;
●If the pile body is broken, the only remedy is to add piles.
3.0 Pile damage during static pressing
3.1 During construction of PHC piles, pile damage has two forms: pile body damage and pile end damage, or a combined form of both (including broken piles, damaged piles, crushed piles, etc.), generally manifested as bursting at the pile top end and concrete bursting of the pile body.
3.2 Analysis of damage causes.
●Bursting of the pile top end due to poor contact with the pile follower;
●Transverse shearing of the pile body caused by improper operation during pile pressing;
●Pile body fracture caused by excessive dimensional deviation and defects of the pile body.
3.3 During the pressing-in process of a clamp-type pile driver, the pile is in a three-directional stress state (both lateral compressive stress and vertical compressive stress). Pile damage mostly occurs in the final stage of pile pressing, which is not only related to the quality of the pile itself but also more related to the structure of the pile clamp and the characteristics of the jaws of the static pile driver, as well as the on-site construction method.
3.3.1 Cause analysis of end face damage. The pile end face mainly bears the reaction force during pile pressing. At the pile end face at the joint or the follower end face, if the end faces of the two pile sections are parallel and the longitudinal axes are not misaligned, then it is in a pure compressive stress state favorable to the precast concrete pile, and the stress distribution on the pile end face is uniform; if the end faces of the two pile sections are not parallel or are misaligned, stress concentration forms on the pile end face. In addition to compressive stress, shear stress also exists. Especially when the pressing force is large, the pile end face is prone to damage.
3.3.2 Methods to avoid pile end face damage are:
●Control the verticality of the pressed pile: the verticality adjustment of the first pile section before pressing is achieved by leveling the machine body, and special attention should be paid to this;
●During pile splicing, finely adjust the traveling mechanism and move the machine body slightly to align the end faces of the two pile sections;
●Add a pressure-equalizing balance pile cap on the pile end face to make the stress on the pile end face balanced during pile splicing;
●In the final stage of pile pressing, use a specially made pile follower. It is forbidden to use precast piles as followers to reduce damage to the followed pile caused by uneven pressing stress.
3.3.3 Cause analysis of pile body damage. The clamp-type pile driver relies on the static friction generated by radially clamping the pile body to overcome the pressure during pile pressing. For this reason, the pile body must withstand radial forces greater than those generated by resisting the pile pressing reaction.
●When the clamping force on the pile body is greater than the compressive strength of the precast pile, the pile body will crack under clamping;
●When the upper and lower clamping cylinders produce non-aligned clamping forces, the pile body will be damaged by shearing;
●When the pile clamp swings in the pressing platform, the pile body will also be subjected to shearing;
●When the four pressing cylinders press down asynchronously, the pile body will be damaged by flexure;
●When the complex stress state of simultaneous clamping force and friction generates a major principal stress higher than the concrete strength of the precast pile, the pile body will be damaged due to insufficient strength.
3.3.4 Methods to control pile body damage:
●Change the pairing of jaw materials with the high-strength precast pile body material to increase the friction coefficient, thereby reducing the normal pressure exerted by the pile clamp on the pile body;
●Change the stress distribution of the pile body by using a two-layer or multi-layer clamping pile clamp to make the stress uniform and avoid stress concentration;
●Increase stress points on the same clamping plane by using multiple hydraulic cylinders and multi-point clamping on the same pile section to reduce the stress of each layer of clamping jaws;
●Adopt elastic floating clamping technology on each jaw of each layer to compensate for single-point or multi-point stress on the pile body caused by diameter dimensional errors and cylindricity errors, so that each jaw has at least dozens of stress points, and the pressure of each point on the pile body is similar; increase the clamping area of each jaw on the pile body to reduce the stress at a single point on the pile body;
●Multiple jaws on the same layer adopt synchronous driving technology under the action of clamping cylinders, and fine-tune according to the force feedback of the jaws on the pile body, so that the stress on the pile body during the clamping process by the jaws on the same layer is balanced;
●Adopt matching control of clamping force and pressing force to reduce the phenomenon of excessive radial loading on the pile body. The above clamp-type pile pressing technology is more significant when applied to large-tonnage static pile drivers.
4.0 Summary
      The engineering quality of PHC pipe pile foundations mainly depends on two aspects: first, the manufacturing quality of the pipe pile itself; second, the quality of pile driving construction. The quality of pile driving construction is especially closely related to construction machinery and operating methods. As long as we strictly control the quality of pipe piles entering the site, resolutely comply with pile driving construction operation specifications, and construct carefully, we will certainly be able to reduce and avoid engineering quality problems, ensure project progress and quality, and create good benefits.