Pile foundation bearing capacity testing

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With economic development, various high-rise buildings have sprung up in cities, and the foundation of such high-rises often accounts for a large proportion of the total investment in the building. Pile foundations are often used for high-rise structures; therefore, choosing the appropriate type of pile foundation is crucial for ensuring safety, saving investment, and reducing costs. This requires us, as designers, to carefully analyze the inspection reports for each building in order to select an optimized foundation plan. The author discusses the issues worthy of attention in pile foundation design from the following aspects. I. The importance of static load tests in pile foundation design. In the current process of pile foundation design, time constraints often play a role: first, the design value of the bearing capacity of individual piles is determined based on the parameters provided in the geological reports. Using this estimated bearing capacity, the pile foundation is designed and constructed; only after the construction of the engineering piles is complete are test piles selected for static load tests. This process is rather unscientific; if the results meet the estimated requirements, everything is fine, but otherwise it becomes very difficult to add additional piles once the construction is already complete. Moreover, discrepancies in the geological reports can sometimes cause considerable difficulties during construction. There are mainly two issues here, illustrated with examples below. First is the standard value of the bearing capacity of a single pile in the site, which is calculated according to the standard values of soil friction force around the pile and the standard value of soil bearing capacity at the pile tip provided in the geological report, using the code JGJ94-94. This is an empirical value and should not be used directly. Over the past few years, through the testing of pilot piles and operational piles in various pile foundation projects, the author has found that the actual bearing capacity of the vast majority of piles is greater than the calculated values; in some cases, the difference is quite significant. Therefore, designing the foundations based on the actual bearing capacity determined from the pilot piles can result in substantial economic benefits compared to using the bearing capacity estimates provided in the survey reports. For example, the author has designed Nandu in Suzhou Industrial Park. The Linglong Bay Garden residential complex is a high-rise building with one basement floor and 18 above-ground floors. According to the geological survey reports, D500 prestressed pipe piles with a length of 20 meters are planned to be used. Using formula 5.2.8 from JGJ94-94, the designed bearing capacity per pile is estimated to be around 1400 kN. However, the three destructive tests conducted showed that the actual bearing capacity of each pile can reach 1850 kN, which is approximately 30% higher than the estimated value. The actual design for the engineering piles was based on these test results, thereby saving costs for the client. Secondly, when the site is uneven or the values in the geological report are inaccurate, proceeding with the construction of engineering piles based solely on that geological report without conducting pile tests will pose significant difficulties and lead to unnecessary waste. For example, in a five-story commercial and residential building in Weiting, precast square piles 10 meters long were used based on the geological reports; the diameter of these piles was 400x400. The ultimate bearing capacity of each pile was approximately 1350 kN. Static pile driving was employed, but in practice almost every pile was driven to a load of 2000 kN before reaching the intended depth. By that point, the strength of the precast piles had already been reached, so splitting the piles was necessary during construction, which resulted in significant waste of time and money. The engineering piles that did not meet the design specifications after static load tests all achieved the designed bearing capacity; in other words, if pile testing is carried out first as part of the design process, the pile length can be reduced by at least 1.5 meters, without any decrease in the pile’s bearing capacity and without the need for pile splitting. As can be seen from the above, static load testing is a very important step in the design of pile foundations. This is because the quality of the subsequent work directly affects the determination of the pile foundation type, pile specifications, and pile penetration depth, and it also has a significant impact on the difficulty of construction. By obtaining accurate data through scientific experiments, design plans can be made more rational, feasible, and cost-effective, yielding benefits that far exceed those obtained from shortening the project timeline. II. The importance of pile type and pile length design in pile foundation design. In the design of pile foundations, making reasonable choices regarding pile type and pile length has a significant impact on the overall design; appropriate selections of these parameters can yield substantial economic benefits. In the residential design for \"Kunshan Huadi\", initially, due to time constraints – as D400 prestressed pipe piles were already available – the client requested the use of such piles. Based on the geological report, the pile length was set at L=16m, with a maximum allowable bearing capacity per pile of 850 kN. The cost of the foundation portion was approximately 160 yuan per square meter, accounting for a significant proportion of the total cost of the residence. In the subsequent design, the author kept the pile length unchanged; drawing on local design experience, the pile type was changed to prefabricated reinforced concrete square piles with dimensions of 250x250, and the ultimate bearing capacity of each pile was approximately 600 kN. The construction cost of these prefabricated square piles in the area is only about 50 yuan per meter, whereas the cost of prestressed pipe piles is around 100 yuan per meter. By using these square piles, the estimated cost comes to about 90 yuan per square meter, resulting in a clear economic advantage. It can be seen that choosing an appropriate pile type will have a significant impact on the project cost. In pile foundation design, the selection of pile length is also crucial. In the design of a pile raft foundation for a high-rise residential building, D500 prestressed tubular piles were used based on the survey reports; the available pile lengths were 25 meters, with the characteristic value of the bearing capacity per pile being Ra=900 kN ； The pile length is 34 m, and the characteristic value of the single-pile bearing capacity Ra = 1300 kN. Using 25-m piles, approximately 290 piles are required ； When using 34m piles, 200 engineering piles are required. In terms of the piles themselves, the total number of engineering piles per meter length is similar for both options. However, when considering the corresponding raft design, using 25m piles results in a raft thickness of around 1200mm, whereas using 34m piles for placement beneath the walls allows the raft thickness to be reduced to 900mm, offering significant economic benefits. Therefore, we designers must employ multiple option comparisons in the design of pile foundations in order to select appropriate pile types and lengths; these choices will have a significant impact on the rationality and cost-effectiveness of the entire foundation design. Of course, we should also take into account various factors such as construction feasibility. III. Control and treatment of pile deviation: During pile foundation construction, strict control over pile deviation is necessary, especially for cap piles and strip piles; deviations in their positions can generate significant additional internal forces, thereby putting the foundation design at risk. Regarding pile position deviations, we focus on controlling two aspects. The first is the vertical deviation; in accordance with Article 7.4.12 of JGJ94-94, the allowable deviation for the elevation of the pile top is set at -50 to +100 mm. However, in actual construction, such large deviations can result in extensive construction tasks and losses. When the elevation of the pile top is higher than the designed elevation, pile splitting is required. This is especially true for hollow piles such as prestressed pipe piles, as it is both difficult and uneconomical to perform pile splitting at the pile top where a pile cap is present ； When the elevation of the pile top is lower than the designed elevation, it is necessary to add pile caps, which not only affects the project timeline but also results in cost wastage. This requires the construction party to strictly control the elevation of the pile tops during construction, striving to keep the elevation of the engineering piles in line with the design specifications. In particular, it is necessary to take into account the settlement of the piles after unloading; otherwise, without such consideration, each pile will end up being higher than the designed elevation. Moreover, our designers should also take construction errors into account during the design process. The author suggests that, given the current quality of construction, a tolerance of around 2 mm can be considered in the design; this would eliminate the need to split numerous piles due to minor deviations. This approach is quite practical in real-world projects and helps avoid a great deal of unnecessary work. The second is the horizontal deviation of the pile position. According to Article 7.4.11 of JGJ94-94, the deviation at each pile location shall be controlled; if a significant deviation is detected during construction, additional piles shall be installed promptly. For pile foundations with 4 to 16 caps, Article 7.4.11 of the JGJ94-94 standard specifies that the allowable deviation is 1/3 of the pile diameter or 1/3 of the side length, whereas Article 5.1.3 of the GB50202-2002 standard stipulates that the allowable deviation is 1/2 of the pile diameter or side length. This is clearly contradictory, and in practice it is easy to lead to different understandings with the construction inspection party; therefore, the author emphasizes that the standards governing the allowable deviation values for pile positions should be specified during the design phase. Furthermore, regarding small-diameter piles (D≤250), the author emphasizes that their displacement must be strictly controlled and should not follow the aforementioned standards; the author suggests that for cap piles, a displacement of 70 mm can be tolerated ； For strip footings, the distance is 50 mm perpendicular to the direction of the footing and 70 mm parallel to that direction; of course, these requirements must be clarified prior to construction. Of course, the fact that the pile position deviation meets the specification or design requirements only indicates that the pile itself has passed acceptance; as for the resulting overall eccentricity of the cap or reduction in foundation height, we must address those issues separately. For pile eccentricity, we can address it by increasing the stiffness of the cap or the stiffness of the tie beams, as well as by adding reinforcement; in practical engineering projects, appropriate measures need to be taken based on the specific circumstances. IV. Handling of Special Circumstances During Construction: Due to the uncertainty of soil conditions, many abnormal situations arise during pile foundation construction. This requires us to conduct a careful analysis of each specific situation and employ appropriate methods to resolve various issues. 1) The pile foundation reaches its ultimate bearing capacity and cannot be pressed to the designed elevation. There are two possible scenarios here: first, the geological report is incorrect, and the actual bearing capacity of the pile is greater than the calculated value; in such a case, pilot piles must be driven first to determine the appropriate length and bearing capacity of the pile. The second reason could be due to characteristics of the soil layer itself; for example, the pore water pressure generated by saturated sand prevents the pile foundation from being driven in, and this requires us to find solutions through construction measures. Firstly, it is necessary to establish a reasonable construction sequence, such as skipping certain steps, so that the water pressure generated by the piles constructed earlier can dissipate before moving on to construct the next pile ； Secondly, for static pile driving, it is necessary to choose construction machinery with sufficient pile-driving force in order to avoid phenomena such as pile lifting ； Additionally, measures such as drilling holes and installing drainage holes can be taken to minimize the pore water pressure as much as possible. Of course, when driving piles, it is necessary to ensure that the driving force remains within the range of the pile’s ultimate strength, and attention must also be paid to the impact of soil displacement caused by pile driving on surrounding buildings. 2) The pile-driving force during pile foundation construction is much lower than the designed bearing capacity. The mid-rise residential buildings in the Changxu Apartments in Suzhou use prestressed pipe piles with a length of 18m and a diameter of D400. According to the geological survey report, the designed bearing capacity per pile is 650 kN. However, during the trial driving of these engineering piles, the maximum pressing force applied to four consecutive piles was only 300 kN, which is far below the designed bearing capacity. We conducted a thorough analysis of the survey report and found that the characteristics of various soil layers described in it are generally accurate. Geological reports from other projects in the vicinity also confirm the accuracy of this survey report. Therefore, we believe that the high driving speed of the pile-driving machinery, combined with the low cohesion of the soil layers, may have caused the piles to shear through the soil directly during driving, resulting in lower pile-driving forces. Over time, the soil is able to regain its stability. The pile test conducted after 15 days proved that our judgment was correct, and the test bearing capacity met the design requirements. This also indirectly emphasizes the importance of conducting static load tests on piles first. 3) The static load test of the pile foundation failed. Due to time constraints in a certain project, the client requested that the pilot piles be tested simultaneously with the structural piles. Static load tests were conducted once the pilot piles met the requirements specified in Article C.0.6 of JGJ94-94; as a result, one set of pilot piles satisfied the design requirements, while the other two sets failed under loads that were lower than the designed bearing capacity. This allows us to analyze these two sets of test piles from various aspects such as design, construction, and testing. However, after comparing them with surrounding projects and analyzing the records of on-site construction tests, no special issues were found; in other words, there were no mistakes in the construction or testing process. The author compared the conditions of the first group of qualified test piles and finally found that the pause time required for the latter two groups of test piles was sufficient; however, the construction of the remaining engineering piles in the surrounding area was completed just 2 days before the tests. It is entirely reasonable to assume that the soil around the test piles was damaged during the construction of those engineering piles and failed to consolidate, which affected the load-bearing capacity of the test piles. Therefore, when the downtime for the engineering piles met the requirements specified in Article C.0.6 of Appendix JGJ94-94, static load tests were conducted on the two test piles once again. The results were in complete agreement with our assessments, and both test piles met the design requirements. This example shows us that there are many factors that affect the results of pile tests. In engineering practice, we must analyze various situations carefully to identify the underlying problems, rather than taking actions blindly and causing unnecessary losses and waste. 4) Treatment of pipe pile cracks. Prestressed pipe piles are widely used in engineering design due to their high strength, short construction time, and material savings compared to precast piles; however, they have the drawback of poor shear strength. In engineering practice, factors such as verticality deviations or soil displacement often cause cracks in the pipe walls, affecting its quality. In a project in Kunshan, due to the low natural ground level of the site, approximately 2 meters of soil was filled in before pile construction. Appropriate measures were not taken to address this situation during the construction process, which resulted in uneven lateral pressure being applied to the piles by the pile-driving machinery. After completion of the construction, it was found that some piles had shifted sideways; small-strain tests revealed that these pipe piles all had cracks to varying degrees, and determining how to deal with this issue was quite critical. After analyzing and categorizing the deviant data, for pipe piles with a verticality deviation of less than 0.5% and virtually no cracks in their walls, we believe that their bearing capacity should not be affected. Therefore, after conducting an additional set of tests to confirm that the bearing capacity met the design requirements, no further action was taken. For pipe piles with a verticality deviation greater than 0.5%, it can be assumed that cracks have formed in their walls, affecting their load-bearing capacity. For such piles, we first corrected the deviation and then carried out core filling to allow the force to be transmitted through the concrete in the filled area; final static load tests have confirmed that this approach is effective. Therefore, in the actual construction of pipe piles, it is essential to pay close attention to maintaining verticality, as pipe piles have poor shear strength and are prone to damage, resulting in unnecessary economic losses. Pile foundation construction is a demanding and complex process; we, as designers, must take every aspect into consideration, plan everything carefully, and ensure that everything is rationalized from all angles. A good design not only ensures the safety of the building but also makes it economically reasonable.