## 3 Field Test Results Analysis  
The disturbance induced by PHC nodular pile installation was larger than that of the PHC pipe pile, and the static load tests were conducted 40 days after the installation of test PHC nodular piles. The static load test process referred to the local specification for the testing of building foundation piles [20], and the slow maintained loading method was adopted in the loading process. The photograph and schematic of the static load test of the PHC nodular pile are presented in Figure 3.  
In total, three PHC nodular piles were adopted for the field tests, and the designed ultimate bearing capacity of the test PHC nodular pile was 1800 kN. In the loading process, the pile head load was applied by two hydraulic jacks with a range of 200 tons. Three displacement transducers with a range of 50 mm were arranged at the test pile head to measure the pile head displacement. To obtain the ultimate bearing capacity of test PHC nodular piles, the static load tests were not terminated until the test pile head displacement was larger than 40 mm. The measured pile head load and displacement of three test piles were then arranged as shown in Figure 4. This figure shows that the load-displacement curve of three test piles were relatively close to each other. The pile head displacement increased linearly with pile head load when the applied load increased from 0 to 1024 kN. The pile head displacement  
Figure 3: Photograph and schematic of static load test. of three test piles were all around 6 mm when loaded to 1024 kN. The displacement then increased with a larger increasing rate when the applied load increased from 1024 kN to 2564 kN. When the applied load reached 2564 kN, the displacement of three test PHC nodular piles reached 52.2 mm, 47 mm and 39.2 mm, respectively. The pile head load is normally considered as the ultimate capacity when the pile head displacement reaches 40 mm [20]. The ultimate bearing capacity of three test piles were 2296 kN, 2401 kN and 2582 kN, respectively, and corresponding pile head displacement reached 40 mm. The recommended ultimate skin friction and unit base resistance for driven PHC pipe piles are presented in Table 1. The calculated shaft resistance of the 450 mm driven pipe pile was 1587 kN when the pile length was 40 m, and the calculated base resistance of closed-ended 450 mm driven pipe pile was 254 kN as the pile base was located in the clay layer. Hence, the calculated ultimate bearing capacity of the driven PHC pipe pile with a diameter of 450 mm and a length of 40 m was 1841 kN. The measured ultimate bearing capacity of three test piles were 1.25, 1.30 and 1.40 times of the calculated ultimate bearing capacity of the 450 mm PHC pipe pile, respectively. The previous research revealed that the shaft resistance of the 350 (400) mm PHC nodular pile was about 1.17-1.18 times of the 350 mm PHC pipe pile [1]. The present research also demonstrated that the bearing capacity of the 450 (500) mm PHC nodular pile was better than that of the 450 mm PHC pipe pile. Moreover, the static load tests were conducted 30 days after pile installation in the previous research, and the measured shaft resistance of nodular pile was about 1.17-1.18 times of the pipe pile. In this research, the field tests were conducted 40 days after pile installation, and the measured ultimate bearing capacity of the 450 (500) mm PHC nodular pile was 1.25-1.40 times that of the 450 mm PHC pipe pile. It can be considered that the increasing rate of ultimate capacity of the PHC nodular pile was larger than that of the PHC pipe pile when the curing time increased from 30 to 40 days. [PERSON] and [PERSON] [6] indicated that for the helical pile embedded in clayey soil, the diameter of helix could be considered as the diameter of the helical pile when the helix spacing/helix diameter was less than 3.0. In this research, the 450 (500) mm PHC nodular pile was adopted, and nodular spacing and nodular diameter were 1000 mm and 500 mm, respectively.