The Chancheng test site is located in Foshan City, Guangdong Province, China. The soil layers consist of plain fill, silty, muddy soil, medium coarse sand, silty, and strongly weathered mudstone from top to bottom. Table 1 gives the soil properties of test site. And the results of standard penetration test (SPT) of various soil layers are depicted in Figure 1.

The test contains eight pipe piles with 400 mm or 500 mm diameter. Figure 2 depicts the ground plan of the test piles. The spacing between each pile is 5 m. High strain dynamic tests and static load tests are carried out on piles 1# to 8# piles. For the purpose of analyzing the variation law of the axial force and deformation of pile during the study of static load test, slide micrometers are embedded into the 1#, 3#, 5#, 6#, and 7# piles, and parallel embedded in symmetrical parts.

The 1# and 2# pile length is 23 m, 3# pile length is 24 m, and 4#, 5#, 6#, 7#, and 8# pile length is 25 m. The test piles were driven by D50 diesel hammer pile driver. The construction parameters are shown in Table 2. The hammering can be terminated when the final three hammer penetrations fulfill the specification of the design.

The high strain dynamic test is done several months after pile installation. The high strain dynamic test employs the eighth generation American-made Pile Driving Analyzer (PDA), free fall hammer hammering, and theodolite testing penetration (Figure 3). Each pile was hammered with an 80 kN hammer at a drop distance of 0.8 m–2.0 m, and four to five times. The hammering distance starts from 0.8 m and gradually increases until the pile end resistance is fully developed. The data sampling time is 200 m and sampling frequency is 10 kHz. According the JGJ106-2003 of Chinese technical code for testing of building foundation piles (JGJ106 –2003, 2003), two full-bridge strain sensors and two accelerometer sensors are affixed externally at diametrically opposite direction on the pile near ground surface about 1.0 m.

To perform CAPWAP automatic signal matching analysis, good quality high strain dynamic test data are required. An assumption of unknown soil parameters is made and tested by performing an analysis with one of the measured quantities as a top boundary condition. If there is disagreement between the other measurement and its calculated counterpart, the calculation is repeated with a corrected set of soil model parameters.

The static load test is conducted 30 days after the high strain dynamic test. The static load test utilizes the conventional platform stacking method and the slow maintenance load method to measure the settlement of the pile cap via the displacement sensor located on the pile top. The initial stage of loading is 800 kN, and then each subsequent load is raised by 400 kN until the termination loading condition of experiment is reached, and each unloaded stage has a force of 800 kN. After each level of load is applied, the settlement of pile top shall be measured and read according to the 5th, 10th, 20th, 35th, 50th, and 65th minutes, and then measured every 30 min.

Through the incorporated sliding micrometer on the inner side of the pile body (1#、3#、5#、6#, and 7# pile), a cone-shaped measuring ring is mounted on the HPVC measuring pipe every 1 m. The strain of the pile body under various loads of static load test is measured, and the axial force of the pile body of each test section is computed so that friction, end resistance, and pile-soil relative displacement may be determined. Figure 4 depicts the borehole instrumentation with line measurement instruments (measuring mark and casing).

The upward traveling resistance waves of pile are presented in Figure 5, and the resistance of the upward wave F _u is depicted in Eq. 1. F u 2 = 1 2 R i + R d − F d 1 + 1 2 R i (1) where R _i is shaft resistance, R _d is the resistance, F _d1 is the resistance of the traveling downward wave towards the pile toe at time t ₁. The t ₁ is designated as the time when the impact wave passes the sensor location and the time t ₂ is designated as the time when the toe reflected impact wave returns to the sensor location (t ₂ is equal to t ₁ plus 2 L/c) (Green and Kightley, 2005a; Zhao, 2005; Ding et al., 2013).

Figure 6 depicts the typical curve of HSDT in this site and the results are obtained from with CAPWAP analysis. The high strain dynamic test with CAPWAP analysis performs a wave equation form by using the velocity records as pile-top boundary conditions. The measured (Msd) force value and velocity value with CAPWAP analysis are shown in Figure 6. The cases exhibit a good proportionality between force and velocity impedance at the onset of impact. Two curves overlap well before the first peak, which shows the success of fitting between force and velocity curves. The maximum of force and velocity is 5211.43 KN and 3.63 m/s respectively. For the existence and development of shaft resistance, the two curves are seen clearly to depart sharply after the first peak. And the two curves are seen to depart sharply near the pile toe from the force time history.

Figure 7 shows the typical curves of wave up (WU) and wave down (WD) in the site. The pile top can be considered as the starting point of the rising of WD, while the pile toe represents the starting point of the falling of WU. The duration of 2 L/c is clearly illustrated in Figures 6, 7.

Figure 8 depicts the Q-s curve results of static loading The 3#、5#、6#, and 7# piles have reached the state of failure, and the curves exhibit steep drop characteristics. In addition, the curves demonstrate that the four piles are the typical friction piles. The ultimate bearing capacity of a single pile is the load value at the starting point of a steep drop section, according to the Chinese building foundation testing code (Briaud and Tucker, 1984). Taking list 3# pile as an example, the beginning of a sharp decline in settling begins below 4,800 kN. The ultimate bearing capacities of 3#、5#、6#, and 7# piles are therefore 4,800 kN, 5200 kN, 5600 kN, and 2,400 kN, respectively.

The ratio of settlement value to pile diameter is displayed in Table 3 for all test pipe piles. Extensive practical experience demonstrates that the ultimate load may occur only when the settlement reaches 10% of the pile’s diameter (Smith, 1960). The ratio of settlement to pile diameter for the majority of piles exceeds 10%, and the max ratio value is 16.99% from 7# pile. Considering the steep drop characteristics and highly weathered argillaceous siltstone of the pile tip bearing stratum, it can be concluded that the bearing capacity of the foundation was completely developed during the static load test.

Figure 9 illustrates the s-lgt curve results of 3# pile from static loading. When the load is increased from 800 kN to 4,800 kN (the twelfth load), the settlement does not decrease appreciably and a steep decline trend do not appear. However, when the load is increased to 5200 kN (the thirteenth load), the curve demonstrates a clear decreasing tendency. Consequently, the ultimate bearing capacity from the s-lgt curve results is also 4,800 kN, which is equal to the result from the Q-s curve. During the unloading test, the maximum settlement is 52.55 mm, and the ultimate residual settlement is 35.50 mm. Maximum rebound value is 17.05 mm, and the rate of rebound is 32.45%.

The strain of the pile shaft axis under various loads can be measured continuously by embedding test elements in the pile body. The internal force test of pile body is calculated by the corresponding strain. It can be seen from Figure 10 demonstrates that under the condition of small load (800 kN), the strain change from the pile body to the pile end is minimal. The pile stress is the largest at the pile depth of about 12 m, and the stress at the pile end is the smallest, but the overall stress of the pile body is relatively uniform; as the load on the top of the pile increases, the strain around the pile body tends to be unevenly distributed. When the load on the top of the pile is 5200 kN, the strain at a distance of 12 m from the top of the pile is extremely high, and drops abruptly downward. The construction record indicates that 12 m is the piles' connecting point. When it reaches approximately 18 m and 21 m from pile top, it reaches a small peak, but the measured strain is less than 12 m. It decreases sharply to the minimum at the bottom of the pile, indicating that the pile body begins to enter the plastic stage under high load.

Figure 11 demonstrates that the axial stress of the pile body decreases as the increasing of the test pile depth under the loading of the pile top. This indicates that the pile top load is mainly offset by the pile shaft resistance, which is consistent with the relevant characteristics of friction piles.

The soil strength is reflected by deformation and increases in proportion to the degree of deformation. Likewise, the friction resistance increases as the relative displacement between piles and soil. Figure 12 demonstrates that as the load increases, so does the friction resistance. At different depths of the same soil layer, the friction resistance varies, and the friction resistance of each soil layer increases to varying degrees along the pile depths. In contrast to prior research, in this test the friction resistance in the deep portion of the pile has not decreased, which is mainly due to the hard soil layers of medium coarse sand, silty sand, and severely weathered mudstone.

Before loading 4,800 kN, the load on the pile top is provided by the friction resistance of the pile body, the settlement of the pile is the compression of the pile body, and the toe resistance of the pile is not exerted as shown in Figure 13. With the increase of the load to 4,800 kN, the cumulative deformation and the settlement curves of the pile top are separated, the pile tip begins to subside, and the toe resistance gradually increases with the load. a is the settlement caused by pile subsidence.

Figure 14 depicts the curve between relative displacement of pile-soil and shaft resistance at the base of each soil layer. The pile shaft resistance is closely related to the relative displacement of the pile-soil. Regardless of the soil layer, the pile shaft resistance increases as the pile-soil relative displacement increases. When the relative displacement of pile-soil reaches a certain extent (Inflection point), the change in pile shaft resistance is negligible, indicating that the pile shaft resistance is fully mobilized. However, the shaft resistance still has an increasing trend, but the increasing trend is small, indicating that the pile shaft resistance has not yet been fully mobilized, such as in the soil layer composed of medium-coarse sand, silt, and strongly weathered mudstone. And this phenomenon is comparable to the research conclusion reached by Huang et al. (2008) (JGJ106 –2003, 2003). As a novel notion, the inflection point is crucial for evaluating the relationship between shaft resistance and displacement of pile.

Figure 15 depicts the relationship curve between unit toe resistance and toe displacement of 3#, 5# and 6# piles, derived based on the axial stress test results from static load test. In accordance with the rule depicted in Figure 14, soil resistance is proportional to displacement. Under the same displacement, when the pile toe displacement is small, the toe resistance calculated by the high strain dynamic test is less than that calculated by the static load test. However, when the displacement is large, the toe resistance calculated by the high strain dynamic test is higher than that calculated by the static load test. Due to the difference in soil resistance, the settlement calculated by dynamic test is greater than that measured by static load test when the vertical load is small (pile displacement is small), whereas the settlement calculated by dynamic test is smaller than that measured by static load test when the load is large (pile displacement is large).

Table 4 displays the average value of unit friction resistance of each soil layer from each of the five piles (1#、3#、5#、6#、and 7#). The test results of 5 test piles indicate that the unit friction resistance of the upper soil layer measured by a slide micrometer is greater than that measured by a high strain dynamic test, but those of the deeper soil layer tested are smaller than that from the high strain dynamic test; the friction resistance of the soil layer is closely related to its depth.

Figure 16 demonstrates the Q-s curve of the test pile. When the pile top load is small, the settlement calculated by high strain is close to the measured result of static load test; when the pile top load is large, especially when the pile is close to the failure condition, the settlement calculated by high strain dynamic test is obviously less than the static load test result. Table 5 compares the results of the high strain dynamic test and static load test. The errors from high strain dynamic test and static load test are within 15%.