## 1 Introduction  
Cross interchange stations constitute essential components of the metro system, and the expanding metro grids in modern cities lead to the growing demand of their construction. Understanding the seismic impact on such complicated underground structures and developing a rational design method is therefore of critical importance. The collapse of the Daikai Station in Kobe, Japan during the 1995 Hyogoken-Nambu earthquake [1, 2] and the failure of the Bou highway tunnel during the 1999 Duzce earthquake in Turkey [3, 4], along with the damage and collapse of the Longxi tunnel during the 2008 Wenchuan earthquake in Sichuan, China [5], provide evidence that underground structures could be vulnerable to earthquakes. As a relatively modern development, cross interchange stations have not yet been tested by a major earthquake. However, the seismic resistance of such structures may be affected by the abrupt change of stiffness at the interchange conjunction, which may generate stress concentrations [6]. Aiming to shed light on the seismic response and facilitate the design methods of underground structures, extensive research has been carried out. There are various analysis studies available in the literature, ranging from closed-form solutions [7, 8, 9, 10] to simplified pseudo-static analyses [11, 12, 13] and numerical full dynamic modeling [14, 15, 16]. The application of the first kind of solution is limited by the assumptions of linear elastic soil response (with the exception of [17]) and soil-structure interface behavior. The accuracy of the second kind of analysis is evaluated by comparing it to the numerical dynamic analysis and experimental study. The simplified pseudo-static analyses can underestimate or overestimate the full dynamic results with a difference of 20-40% [18]. Arguably, the last category of methods, numerical full-time analysis, is considered as the most accurate method for the seismic analysis of underground structures [13], provided that some of the most important aspects (e.g., soil nonlinearity, relative soil-structure stiffness and soil-structure interface) can be modeled appropriately. However, the above preconditions are not explicit if found without validation after the laboratory or field observation of the dynamic behavior of the underground structure. In fact, a recent study [19] presented several sets of numerically predicted results to a set of centrifuge campaigns of tunnels in sand subjected to earthquake loading. The numerical results were found to often not be satisfactory for predicting permanent changes in the internal force of a structure and rather dispersed, especially when high yielding was involved during strong shaking. To summarize, in all of the above analysis methods, experimental modeling has been a key factor for calibrating and validating the models and to provide evidence on the mechanisms and factors affecting the response. The above methods are mostly applied to the analysis of tunnels and underground structures with uniform cross-sections [20; 21; 22; 23; 24; 25; 26]. In the specific case of cross interchange stations, its seismic response has not been studied extensively thus far. Recent studies about the seismic response of cross interchange stations were mainly based on full-time numerical simulations [27; 28; 29; 30], while fewer proposed simplified pseudo-static analyses based on plate theory [30]. Compared to the single station with a uniform cross-section, the seismic response of the cross interchange station is affected by the interchange conjunction, which brings abrupt stiffness and a change of structure. Such influence may be more notable in a certain region close to the conjunction [30]. Thanks to the existence of the conjunction sidewall, a lower bending moment was found in the column of the interchange station, compared with the single station [29]. However, in the meantime, numerical studies predicted a stress concentration at the interchange conjunction due to the stiffness change [27].