# Seismic Performance of Modal Transfer Stations on Soft Clays
## 1 Introduction  
Densely populated urban areas with increasing service demands require the development of proper strategies to enhance the resilience of urban infrastructure and lifeline systems to natural hazards [1, 2, 3, 4]. To achieve this in earthquake-prone regions, a proper assessment of the seismic demand expected to occur in the infrastructure systems is required. This need has led to a growing interest in the research community for assessing the vulnerability of transportation systems, following a holistic approach, where each component affects the behavior of the whole system [3, 4]. Regarding seismic performance, important interaction effects have been observed to occur in structures that are closer to each other, considerably modifying their seismic demands. Thus, more robust methodologies and analysis models should be implemented to capture such effects, considering the structure proximity of each element of the transportation system [5, 6, 7]. Multiple research groups have studied the interaction between surface and underground structures, implementing several methodologies. These include advanced 3D numerical modeling [8, 9, 10], experimental small-scale testing using shaking tables [5, 11], or centrifuge testing [12], as well as large-scale research involving real instrumented structures [13]. Overall, these studies have provided significant contributions, establishing that structural proximity, soil characteristics, input motion frequency content, fundamentalvibration modes of on-ground structures, foundation type, tunnel diameter, and burial depth are the key variables that define the cases where interaction is beneficial or detrimental for the structural seismic performance [14]. Regarding the interaction among surface structures, significant advances have been made through recent studies [15; 16; 17; 18]. This effect has been called site-city or structure-soil-structure interaction (SSSI). In the particular case of the Mexico City Basin (MCB), some anomalous beating cycles have been identified in the recorded seismic \"free-field motions\" of recent earthquakes. Some researchers have associated this signal pollution with SSSI effects [19; 20; 21], which are exacerbated by the very low soil shear stiffness and damping of the shallow clay layer typically found in the MCB, and the large impedance ratio of such a layer with respect to the underlying stiff soils. Furthermore, building-soil deposit resonance and the seismic motion frequency content in which the energy of the excitation is concentrated also play a major role in the SSSI. Under those characteristics, wave fields radiating from nearby buildings are trapped in shallower layers and are then propagated as surface waves, affecting neighboring structures. Considering both phenomena, there is a complex interaction between the incident, reflected, and diffracted seismic waves by underground structures, and those generated by the swaying of the surface structures during earthquakes, as Figure 1 depicts. Although such contributions have provided important insights, they considered idealized scenarios, such as plane-strain conditions, one-degree-of-freedom oscillator representations of surface structures, and harmonic time history excitations, and, more importantly, they neglected the interaction effects among on- and underground structures. These limitations can potentially lead to expensive and, in some cases, unsafe structure designs. In the worst-case scenario, interaction effects can increase the seismic demand of the structures in terms of internal forces, story drifts, or bridge decks' relative displacements, reaching or exceeding a particular damage level, which can be directly associated with potential economic losses, and reductions in seismic resilience. This paper presents a parametric study, using numerical three-dimensional finite-difference models, on the seismic building-tunnel-bridge interaction effects commonly encountered in urban environments.