## 3 Seismic Behavior of Micropiles
### Factors of Influence
#### 3.2.1 MP-Related Factors  
Noted that it can be misleading, to a certain degree, to manage MPs as one type of pile that exhibits different behavior in function of some factors, and it may be more appropriate to manage MPs as a family of piles. For example, when studying the seismic behavior of type-A MPs (grouting under gravitational head) and comparing it with the seismic behavior of type-D MPs (multiple-steps post-grouting), one should expect that the behavior of the whole system will be different, starting from the MP itself, to the soil, and the superstructure. Hence, special attention should be given when interpolating MP studies/tests results as they may not be appropriate for a global consideration on all MPs [6; 14]. As well, as stated before, the technology highly influences the behavior; presented below are some examples of technological influence on the behavior of MPs. As for example, [PERSON] et al. [6] performed a field test on two MPs with two different grouting methods (gravitational and pressurized) to study their dynamic response. The test was performed in an alluvial silty deposit, with two MPs 8 m long for each. As stated above, one was grouted under gravity head only and a hollow-core steel bar was installed, while the second was initially installed in the same way but had an additional grouting step. MPs were instrumented for monitoring with sensors, and the tests performed were ambient vibration tests and impact load tests. Not only was the response different between these two MPs, but there were also differences on the level of the pressurized MP itself, as the behavior was different in the two orthogonal horizontal directions. According to [6], the pressurized grouting modified the surrounding soil, which led to a modification in the soil stiffness and resulted in the response differences between these two MPs. Additionally, since the direction of the pressurizing process was different (one direction was pressurized directly), it led to another difference in the stiffness level of the soil and the dynamic response as function of the pressurizing direction [6; 14]. In a similar manner, [PERSON] et al. [7] performed a shaking table test to study the dynamic behavior of MPs in loose sand; the outcome of the test can be beneficial in terms of kinematic interaction. A densification phenomenon of the surrounding soil around the MPs took place according to the author; this densification can be due to the excitation source and/or influence of the MPs. From the point of view of enhancing the seismic behavior of the superstructure, MPs provide wide possibilities for modifying the soil, in small or relatively large ranges, as also noted by other authors [14]. The pile cap itself or the MPs were out of phase with the base shaking; theoretically, this results from the stiffness and damping differences between the soil and the MPs/MPs cap. [PERSON] et al. [15] confirmed this phenomenon through a comparative study on three MPs: one was also a gravitational MP, while the other two were pressurized. The difference among them is the type of packer used during the grouting process; furthermore, the differences in the measured \"resulting\" stiffness after MP installation were extremely high between the three cases. The predicted (based on the cross-section properties, same for all three cases) and measured elastic modulus of the first MP (gravitational MP) were very close to each other, as the measured stiffness had a value almost 10% higher than the predicted one, while the first pressurized MP with a geotextile packer (the common practice) had a measured stiffness value of almost 40% higher than the predicted one. Interestingly, the second measured MP that used a rubber packer reached a value almost 120% higher than the predicted one, similar to using a rebar element of 147.6 mm diameter instead of 81.3 mm, which was the one used in the test.