New methods for interpret the ultimate axial load capacity of driven square concrete piles
Authors:   , Alselami Nimer Ali 
Published: December 2023

Piles are usually designed using analytical methods, empirical models, or dynamic formulas. Because of the uncertainties involved in the analysis and design of pile foundations, it has become customary, and in many cases mandatory, to perform full-scale pile loading tests. Often, the pile load test will not reach the ultimate load because of cost and time limitations. Therefore, the ultimate capacity must be calculated by a technique based on the load-settlement records. Numerous techniques were published to interpret pile load test curves. The capacity predicted by the interpretation techniques often varies depending on the techniques used. Using high-quality data based on pile load tests are required to resolve some uncertainties for interpreting the ultimate pile capacity. However, there is also a need to improve a method to estimate the ultimate capacity more accurately. In the study, three methods, Nimer, Cubic, and Cubic Limited, to interpret the ultimate pile capacity are proposed. The Nimer method can define the ultimate load more accurately and reliably as compared with existing methods without the need for loading the pile to a large settlement or extrapolate the load-settlement curve. In contrast, the "Cubic" technique can extrapolate the load-settlement curve beyond the maximum test load. It often gives the ultimate pile capacity higher than the actual test failure load because the maximum load can only be achieved at large displacements. Therefore, the procedure was modified and named "Cubic Limited." The modified procedure defines the pile capacity as the point where the Cubic fit curve intersects with a settlement of the pile equal to nine percent of the pile diameter. The proposed Cubic Limited extrapolation method can be used to improve the accuracy of the predicted ultimate pile capacity based on failure or proof load tests and overcome common problems faced with current extrapolation methods. The objective of the study is to establish new methods for interpreting pile load tests. The proposed methods will be compared with existing methods using the Federal Highway Administration's "FHWA Deep Foundation" database. Two different evaluations of the interpretation methods are examined through a database of fifty-five piles load-tested to failure. The first evaluation of the interpretation methods was based on failure load tests, while the second was based on assumed partially loaded pile tests, which represent the proof test condition. For failure load tests, six interpretation methods were used to calculate the piles' ultimate capacity (QC). These methods include five extrapolation methods, as well as the Davisson offset limit. The Nimer method was chosen to determine the ultimate measured capacity (QM) for each pile load test. A systematic ranking approach called Rank Index (RI) was used to evaluate the six interpretation methods based on three criteria; accuracy, variability, and reliability. RI is defined as the sum of the ranks from the three criteria, the lower the RI, the more accurate the interpretation method. Based on the three criteria mentioned above, the proposed Cubic Limited method showed the best performance according to the RI. The proposed method results in a predicted capacity that compares well with the measured ultimate load (QM). The Hansen 80 percent technique was the second-best technique, while the third-best method was the proposed Cubic method. The fourth-best method is Davisson, which has a much lower (QC/QM) ratio as compared to other methods. Finally, the least accurate performance among all methods are Chin-Kondner and Decourt methods, which have a high RI, indicating they are not suitable for interpreting load test curves when tested on the FHWA dataset. The 55 pile load tests were analyzed using the Cubic Limited method that resulted in a mean (QC/QM) ratio of 1.01 and a standard deviation (SD) of 0.05, while the Cubic method produced 1.07 and 0.06 for the mean and SD values respectively. The difference in the mean and SD values reflects the improvement due to limiting the extrapolation curve by a specific settlement obtained by the Cubic Limited method. From the 375 FHWA data, 55 pile load tests were tested to failure. Therefore, the proposed methods, along with three other extrapolation methods, were utilized for the second evaluation. The investigation of the second evaluation was conducted as follows: loading is assumed to be 90 percent, 80 percent, 70 percent, 60 percent, or 50 percent of the known ultimate capacity (QM). The curves were then extrapolated using the five extrapolation methods. The ratio of extrapolated-to-measured capacity (QE/QM) is the metric used to quantify how well an interpretation method estimates ultimate capacity. Values of QE/QM ratio close to one represent good agreement between the extrapolated and measured capacities. Values less than one indicate the ultimate capacity are conservative, whereas values greater than one indicate the capacity are unconservative. It is shown that when the assumed applied loading is close to the ultimate load, all methods estimated the ultimate capacity with the best accuracy and reliability. However, piles only loaded to a small fraction of the ultimate capacity revealed a high variability for the predicted ultimate pile load. Chin-Kondner and Decourt's methods significantly overestimate the ultimate capacity when the applied load is much lower than the ultimate load. On the other hand, the Hansen 80 percent, Cubic, and Cubic Limited methods produce conservative mean values when the applied load is much less than the ultimate capacity. However, the coefficient of variation (COV) from the proof load results increase compared to the failure load test, which indicates that even these methods have low reliability as the piles only loaded to a small fraction of the ultimate capacity. The proposed Cubic Limited method produced the best evaluation with a mean close to one and the lowest COV. The results of the analyses support the hypothesis statement regarding the high accuracy and reliability of the ultimate pile capacity prediction based on the proposed "Nimer" method. The results of Modified Davisson and Terzaghi (0.1D) methods generally agree with those from the Nimer method. The analysis shows that the capacity calculated from the Modified Davisson and Terzaghi methods is 1 percent and 3 percent, respectively, lower than the ultimate capacity obtained by the Nimer method. Therefore, the "Nimer" method is recommended as the definition of the ultimate measured capacity to be used in cases where the load test did not reach a clear ultimate load. The simplifying assumption of the extrapolation methods can lead to inaccurate determination of the ultimate pile capacity. The difficulty of obtaining the ultimate capacity from pile load tests can be overcome by using the Cubic Limited procedure and has proven to be successful in this aspect. The results of the analyses support the hypothesis statement regarding the high accuracy and reliability of the ultimate pile capacity prediction based on the proposed "Cubic Limited" method. It is shown that the accuracy of the proposed Cubic Limited method is 1.01 [plus-minus] 0.05 (mean [plus-minus] SD) 1.08 [plus-minus] 0.13, 1.06 [plus-minus] 0.15, 1.01 [plus-minus] 0.17, 0.92 [plus-minus] 0.15 and 0.82 [plus-minus] 0.18 when assuming 100 percent, 90 percent, 80 percent, 70 percent, 60 percent, and 50 percent of the ultimate capacity to be known, respectively. The adoption of the proposed Cubic Limited method, which has considerably less variability than other methods (COV [equals] 0.05), allows the designer to reduce the uncertainty for prediction of the ultimate pile capacity used in future projects.