## 3 Seismic Behavior of Micropiles
### Factors of Influence
#### 3.2.4 MP Seismic Interaction Factors  
To further expand the discussion regarding MP role in the seismic behavior, it is important to demonstrate the mechanism through which soil-foundation-superstructure are all interacting together when excited by a seismic motion; this mechanism is influenced by all the factors and subfactors previously mentioned. The free-field motion applied to a building and its foundation are transferred into a foundation-input motion due to the kinematic interaction. This latest is the one that determines the excitation in the structure [17,19]. Furthermore, the structure, its foundation, and the surrounding soil all interact due to this foundation-input motion; this type of interaction is called inertial interaction [17,19]. The kinematic interaction is the product of wave incoherence, which can be due to the foundation shape (footprint) and embedment depth, as the medium in which the waves are transferred through starts to differ when arriving to the foundation. Usually this causes a decrement of the foundation input motion, while the inertial interaction is due to the flexibility of the system and damping (both hysteretic and radiation damping) [19]. MPs have an important influence on the kinematic interaction, as the embedment depth is relatively high, it can cause incoherence in the seismic waves, also due to the footprint area, the angle of inclination, and the installation method (technological effect). This influence is generally assumed to be positive, but also minor or even negative influence can be potentially obtained, as the outcome of the interaction is not only a function of the soil and MPs, but also depends on the seismic wave progression and diffraction. Furthermore, MPs can also affect the inertial interaction through both stiffness and damping. This is expected (pile effect due to the embedment depth) as it was noted with other type of piles, but this is a sensitive topic; for example, increasing the stiffness of the soil leads to an increase in the stiffness of the system (soil-MPs-structure), and depending on the excitation source, this can move the seismic response away or closer to resonance [6,17,20] (Figure 7). Figure 7: MPs possible influence on the kinematic interaction due to the embedment depth (after [17,19,20]). Complex SSI (soil-structure interaction) analysis might be a good option to manage seismic wave propagation issues, similar, for example, to the recent work of [PERSON] et al. [21], which compared the structural response of a bridge pier using analyses with different levels of SSI complexity. The authors also compared the deterministic approach and simple design methods with a probabilistic approach. The latter allows for more accuracy in modeling the soil dynamic properties, which can also be very helpful in studying the seismic behavior of MP and MPed structures, as a probabilistic approach can capture more accurately and in detail the local behavior of the soil that is influencing wave propagation. Referring to seismic waves, they are divided into two types, body and surface waves; in many cases the focus is on body waves, as in most of the discussed papers. Even so, the foundation is excited by both types and not considering these two types of seismic waves in the studies may limit the ability of reaching rigorous conclusions. Two possibilities exist: body waves arrive before surface waves or intersecting of body and surface waves. From the point of view of the authors, the role of MPs can be useful in decreasing the potential hazard of the \"softening\" phenomenon, which is related to the difference in the arrival of the seismic waves; this can be potentially achieved again by influencing the damping and stiffness value (inertial interaction) or by considering that the surface waves are also subjected to the kinematic interaction as well as for the body waves [17; 19].