## 3 Results and Discussion
### Underpinned Foundation Behavior
#### 3.2.2 Axial Load Transfer Mechanism in Micropiles  
Strain measurement results provided the axial force at certain depths in each micropile under additional loading, \(Q_{a}\), from 294 to 1176 N. Figure 8 shows the results, revealing that the axial force of the micropile increased with the applied load and decreased with depth owing to the initiation of skin friction. The reduction of axial force along the micropile was more prominent in the dense soil layer than in the loose soil layer. Comparing piles CMP1 and WMP1 (with the same length but different shapes) shows that WMP1 (with shear keys) carried a greater load than CMP1 under a given applied load. This is attributed to its high axial stiffness. It also showed a more obvious increase in shaft resistance than CMP1, especially in the dense soil layer from 350 to 500 mm. This finding indicates that shear keys along the pile increased its shaft resistance. This effect was more significant in the soil region with a higher confining pressure, which is in good agreement with the results of [PERSON] and [PERSON] [36; 37]. Interestingly, WMP2 provided a similar shaft resistance to CMP1 despite being only about 80% of its length. This also confirmed that shear keys significantly influenced shaft resistance. This result also implies that under the additional loading to the underpinned foundation, the load carrying capacity and bearing resistance of the micropile increased with its increasing axial stiffness.  
Figure 9 depicts the relation between the total shaft resistance of the underpinning micropiles and the applied additional loads. The shaft resistance was evaluated as the difference between the vertical load of the pile head and the axial force at the pile tip evaluated by the strain data. The shaft resistance of WMP1 increased sharply by increasing the applied load and at the final loading level, the mobilized shaft resistance was about 2.5 times that of CMP1 and WMP2. The shaft resistance of WMP2 and CMP1  
Figure 8: Load transfer behavior of different types of micropile subjected to additional loads. (**a**) CMP1; (**b**) WMP1; (**c**) CMP2; and (**d**) WMP2.  
increased slowly with the increasing load, and both showed almost identical behavior, indicating that the shear keys along the pile's shaft significantly increased the shaft resistance, as explained by [PERSON] and [PERSON] [37]. The shaft resistance of CMP2 also increased slowly with the applied load and the increase is the smallest measured here for these micropiles. After the shaft resistance reached to a certain value of 8 N, which occurred under an applied load of 735 N, the line flattened, implying that the shaft resistance was fully mobilized.  
The tip resistance was determined from the strain gauge near the tip of each micropile. It is plotted for the underpinning micropiles under additional loading in Figure 10. With the exception of WMP1, the tip resistance of all the piles increased steadily with the increase of the applied load. Moreover, the increasing rate of tip resistance significantly increased for micropiles as the applied load exceeded 441 N. This result implies that the skin friction was mobilized prior to the tip resistance. The tip resistances of CMP1 and WMP2 were the highest measured here and both showed similar progressions. In the case of WMP1, the tip resistance was not mobilized at loading below 800 N, which suggests that up to that point, the load was not transferred to the tip owing to the effect of high shaft resistance. This indicates that the effect of shear keys along the pile's shaft increased the shaft resistance, especially in dense sand.  
Figure 10: Tip resistance of the underpinning micropiles with respect to the additional load applied to the foundation.  
Figure 9: Shaft resistance of the underpinning micropiles with respect to the additional load applied to the foundation.