## 3 Results and Discussion
### Pile Movement Monitoring  
[PERSON] et al. [42] state that the stress load-settlement curves reflect (1) the pile-soil interaction law, (2) the load transfer law, and (3) the pile load destruction mode. According to the previous explanations, the pile top and base settlements have been monitored for each load increment by applying the dial and strain gauges. Applied stress alteration versus total pile top and base settlement for two well-instrumented field tests (BP# 1, and BP# 2) in multilayered soils are presented in Figures 18 and 19, respectively. In the 1 st cycle, the highest sighted pile top settlement at loading 22,418 kN was 9.60 mm. During unloading to zero, the pile was rebounded to a residual settlement of 0.36 mm. On the contrary, in the 2 nd cycle, the maximum sighted pile top settlement at the peak load of 44,036 kN was 24.63 mm, so during unloading to zero, the pile was rebounded to a residual settlement of 5.34 mm. Similarly, residual settlements of 2.4 mm and 6.58 mm were recorded for the BP# 1 and BP# 2, respectively. According to [PERSON] et al. [7], the variation of \(f_{\mathrm{s}}(z)\) with depth (\(z\)) is affected by different parameters including pile-soil properties, such as(1) pile-soil interface geometry and slip properties, (2) stress performance on the pile-soil interface, (3) pile installment technique, and (4) pile load method and ratio. Figure 17: Average number of standard penetration test and the unit skin friction in BP# 1 and BP# 2. Figure 18: Different applied stresses versus total pile top settlement in (**a**) BP# 1 and (**b**) BP# 2. Different total pile settlement versus time during BP# 1 and BP# 2 is presented in Figure 20. The settlement rate in the pile's head is almost linear in the loading steps. However, when unloading begins, the settlement rate of unloading steps significantly increased depending on the loading/unloading sequences, showing that the loading time for the second cycle of BP# 1 and BP# 2 were 400 and 600 min. The measured settlement on the pile's head was rebounded to permanent vertical deformation of 5.34 mm and 8.55 mm for both tests (BP# 1 and BP# 2) after unloading to zero. Figure 19: Different applied stress versus total pile base settlement in (**a**) BP# 1 and (**b**) BP# 2. Correspondingly, the highest settlements of 24.63 mm and 19.54 mm were recorded for the pile's head vertical deformation. Some of the critical factors that were considered to be constant during pile load tests (e.g., BP# 1 and BP# 2.) were (i) piling technology, (ii) concrete maturity, (iii) the location of the groundwater, and most importantly, and (iv) the soil parameters (e.g., both mechanical and physical properties changes). Such a problem may affect \(E_{c}\) measurement during the pile servicing period. For instance, it is established that piling technology will influence soil-pile interactions [43; 44; 45]. Piling techniques  
Figure 20: Variation of the total pile settlement versus time during pile test in (**a**) BP# 1 and (**b**) BP# 2.  
that lead to changes in soil properties can affect the axial force, load-displacement response, and tip resistance of each model pile. In most cases, to assess such influences, a large number of small-scale experimental works, or real-scale numerical and analytical studies are helpful. On the one hand, the other factor that can influence the results of such investigations is concrete maturity [46; 47]. Concrete maturity factor reveals the amount of concrete strength gain during the curing period, which is typically challenging to be taken into consideration as a separate variable on the full-scale experimental programme. It is important to note that the lack of such information may cause a significant change in the load-settlement behaviors of the pile during its working lifetime. Factors such as soil properties (e.g., shear strength parameters such as soil internal friction angle, cohesion, etc.) as well as groundwater levels, are primary terms that can remarkably alter the soil-pile responses to heavy external loadings.