Numerical study on the seismic response of the underground subway station- surrounding soil mass-ground adjacent building system

Abstract

Ground buildings constructed above metro station have increased very quickly due to the limited land resources in urban areas. In this paper, the seismic response of the Underground subway station-Surrounding soil mass-Ground adjacent buildings (USG) system subjected to various seismic loading is studied through numerical analysis. The numerical model is established in terms of the calculation domain, boundary condition, and contact property between soil and structure based on the real project. The reciprocal influence between subway station and ground adjacent building, and their effects on the dynamic characteristics of surrounding soil mass are also investigated. Through the numerical study, it is found that the impact of underground structure on the dynamic characteristics of the surrounding soil mass depends on its own dimension, and the presence of underground structure has certain impact on the seismic response of ground adjacent building. Due to the presence of underground structure and ground adjacent building, the vertical acceleration generated by the USG system cannot be ignored. The outcomes of this study can provide the references for seismic design of structures in the USG system.

Keywords

underground subway station / ground adjacent building / seismic response

Introduction

Nowadays, the urban railway transportation, particularly the metro system, are developing rapidly, and the relevant structures are becoming increasingly dense due to the limited underground space resources. As a result, there appeared a large amount of cases of underground subway line crossing underneath the ground adjacent buildings. Theoretically, the presence of the underground structure, especially the huge underground structure should have certain impacts on the dynamic properties of the surrounding soil since the construction of underground structure changes the original stress state of surrounding soil and the geometry, and further influences the seismic response of ground adjacent buildings. So the underground structure-surrounding soil mass-ground adjacent buildings can be seen as an interactive system, among which the focus should be particularly put on the interactive rule of the seismic response of the underground structure and ground adjacent buildings as well as the dynamic characteristics of the surrounding soil mass.

In early times, there were not so many underground structures, most of which had small dimensions and low level of importance. Hence the impact of underground structure on the dynamic behavior of surrounding soil mass and ground adjacent buildings has not been paid enough attention to, and few researches have been focused on this topic. Earlier works mainly focused on the study of the influence of the underground circle hole on the seismic response of the surrounding soil layer via analytical solution, such as Lee [1,2], Lucco [3]and Barros [4]. Kausel [5] had first investigated the impacts of the underground structure with middle to large dimension, especially the underground subway station, on the seismic behavior of ground in the Mexican quake. The result showed that the interactive effects between underground structure and foundation of adjacent buildings were not obvious. Navarro [6] studied the effect of large size structure (i.e., nuclear power station) on the seismic response of the adjacent underground tunnel, and it was found that the location of the ground structure and clear distance between the adjacent underground tunnel and ground structure were the key factors influencing the seismic response of the underground structure. Yiouta [7] and Smerzini [8] investigated the impact of the underground circle structure on the seismic behavior of the surrounding soil mass under the SV and SH wave considering the factors such as characteristic of surrounding soil, spectrum of seismic wave, diameter and embedded depth of tunnel and the relative stiffness between tunnel and surrounding soil. It was found that: 1) when the wavelength of seismic motion is bigger than the diameter of tunnel, the influence of the existing tunnel on the seismic behavior of surrounding soil mass can be ignored; and 2) the influence zone of the existing tunnel on the dynamic behavior of the surrounding soil mass was about 3 times the diameter of tunnel. Guo [9] analyzed the effect of subway station on the storey drift of ground adjacent building, and the results demonstrated that the presence of the subway station would enhance the shear deformation of ground adjacent building with the short period, and produce larger bending moment of the structure with the long period. Wang [10] studied on the rule of seismic response of the complicated system consisting of underground structure- ground structure considering the factors such as the layout of ground building, input direction of seismic wave, clear distance between underground structure and ground structure, the shear velocity and damping ratio of soil mass and the buried depth and the span of underground structure. It was found that the seismic behavior of the ground adjacent buildings with low height was significantly influenced by the existing underground structure, and the clear spacing between underground structure and ground structure, and the frequency component of seismic wave were the main factors influencing the dynamic behavior of such complicated system. Kyrazis [11] adopted full dynamic time history analysis method to study the seismic response of the tunnel-soil mass-ground adjacent building system. In their study, the effects of the key parameters, such as the stiffness ratio of soil mass to structure, the dimension of tunnel, the embedded depth of tunnel and nonlinear properties of soil mass on the dynamic behavior of the tunnel, were analyzed. It was found that the existence of the ground adjacent building had great influence on the seismic response of the shallow embedded tunnel with large stiffness. Masoud [12] conducted a 1g shaking table test and numerical study on the impact of the circle tunnel on the acceleration amplification of the surrounding soil mass with consideration of the shear velocity of soil mass, frequency of earthquake, and embedded depth of tunnel. The results showed that the presence of tunnel had little effect on the acceleration amplification of soil mass, however, had certain influence on the seismic response of the above ground structure with low period.

In summary, there have been many studies on the seismic response of the soil-ground buildings system and soil-underground structures system in recent years, while the literature investigating the dynamic characteristic of the Underground structure-Soil-Ground adjacent building (USG) system under the seismic loading is limited. Hence, in the current context the investigation of the rule of the seismic response of the USG system subjected to various seismic loading will be carried out. The calculation model is first established in terms of the calculation domain, boundary condition, and contact property between soil and structure based on the real project. Two key factors [6,7] including clear spacing between the subway station and ground adjacent building and the types of the seismic loading, are then considered to conduct the parametric study on the dynamic behavior of the USG system. The outcomes of this study can provide the references for the seismic designs of the structure in the USG system.

Calculation model

In currrent study, the typical subway station in an east coastal city in China is selected as the basic configuration of the calcalution model for the reasons: 1) The soil straumn in that location includes very thick soft clay, which normally shows the amplification effect under the semsic wave; and 2) The ground adjacent building in this case is the center of the traffic control which has high level of importance for the city. Hence, it is meaningful to investigate the semsic response of such USG interaction system subjeced to vaious seismic loading. For the subway station, it is a three-span and two-level frame structure, and the embeded depth of the roof is 4 m. The spacing for the column in the longtitudal direction is 8 m, and the total number of column is 21, making the length of subway station to be about 160 m in longitudinal direction. For the ground adjacent building, it is a two-span and six-storey frame structure with raft foundtion. The spacing for the column was 6m both in length and width.

According to the rule of defining the calcuation domain which will be metioned later, the three diemensional model will have the diemension of 250 m in width and 360 m in length and 90 m in depth, which might induce a large amount of elements and nodals and cause low efficiency in compuation process. Hence, considering the fact that the congifration of subway station and ground adjacent building is neat and regular, in current study the plane strain model is employed to study the seismic behavior of the USG system. Figure 1 gives the plain strain calucalation model of this USG system. In the figure, it is shown that W is the width of the calculation model, H is the depth of the calculation model, and D is the clear distance between border of ground adjacent building and the side wall of the subway station.

For the convenience of building the model, the rounding method is used to simplify the diemension of the subway station and ground adjacent building. In calculation model, the subway station is a three-span and two-level frame structure, in which the spans was modeled as 7, 6 and 7 m, respectively, and the height of the storey is set to 4 and 6 m for upper level and lower level respectively. The thickness of the roof, middle slab and base slab of subway station is 0.6, 0.4 and 0.9 m, respectively. The cross section of column is 0.6 m × 0.6 m, which is cast by the concrete of C40 grade. The ground adjacent building is a two span and six-storey concrete frame structure, in which the span is 6 m and the thickness of floor slab is 0.1 m. The cross section of column in ground adjacent building is 0.4 m × 0.4 m. The column of the subway station and ground adjacent building in longitudinal direction is simplified as wall element with the unit length using the equivalent stiffness principle method.

Calculation domain

The ratio of width-to-depth (W/H) is normally used to determine the domain size of soil mass in the dynamic analysis [10], and it is found that when the W/H reaches 7, the stable response of the soil mass is achieved, in which W and H are the length (or width) and depth of the soil mass, respectively. Furthermore, in case of the presence of the underground structure, the Chinese code for the seismic design of urban mass transit structures(GB50909-2014) [13] suggests that the domain size of soil mass in the seismic design for the plane strain analysis should be decided as the following, for the depth of model, the upper boundary is the ground surface level, and the bottom boundary is taken to be the seismic datum; for the width of model, clear distance between the side wall of underground structure and the nearest vertical boundary should be at least 3 times of the total width of underground structure, which means the width of the calculation model should be at least 7 times of the total width of the underground structure. In this study, the domain size of calculation model is determined according to the above suggestions, that is, the depth of the calculation model is set to be bedrock as the seismic datum is encountered 70 m below the ground, and the width of the model is 500 m. It can be found that the dimension ratio in the current domain size isW/H = 7.14, which meets the above requirements [10].

Boundary conditions

Computational analysis in this paper has been divided into two steps, in the first step the finite element software ANSYS is used to conduct modal analysis, and in the second step, the finite difference software FLAC^3D is employed to conduct transient analysis. This is because for the ANSYS there is no suitable material model to present the nonlinear behavior of soil but it is powerful to conduct modal analysis, and for the FLAC^3D, the advanced constitutive model can be developed and implemented in the software to simulate the nonlinear behavior of soil but it cannot perform the modal analysis.

When conducting modal analysis using ANSYS, the boundary condition is set as follows: bottom side is a fixed boundary, ground surface is a free boundary, and the vertical displacement is fixed for the two side boundaries.

When using FLAC^3D to conduct transient analysis, the boundary condition is: the vertical displacement is fixed at the bottom boundary while the horizontal freedom of nodal displacement is used for input of seismic wave; ground surface is a free boundary, and the two side boundaries of the model are viscous boundaries as shown in Fig. 2. The detailed principle for this boundary can be referenced to Ref. [14],

Contact behavior

The contact behavior between the underground structure and the surrounding soil is modeled as frictional [15]. In this interfacial model, the soil material can be set apart from the structure when the normal stress at the contact surface is tensile and slip can be allowed to occur, while the normal stress is in compression state, the shear strength at interface follows Coulomb law, in which the coefficient of friction is taken as 0.3 in current study.

Calculation parameters

The grade of the concrete for both structures is C40, with Possien′s ratio of 0.2, mass density of 2600 Kg/m³, and elastic modulus of 33 GPa.

The parameters for soil stratum in numerical model are given in Table 2. In current study, in order to compare the natural frequency of soil mass from the numerical model and theoretical formula [13], the soil mass is simplified to be an equivalent single soil layer when using the theoretical formula. The velocity of the shear wave in equivalent single soil layer is equal to that of the stratum-soil layers using the method of weighted average of thickness for each soil layer. The equation can be expressed as:

(1)v_{\mathrm{s}\mathrm{e}}={\displaystyle \frac{d_{\mathrm{0}}}{t}},

(2)t={\displaystyle \sum _{i=\mathrm{1}}^n(d_i/v_{si})},

in which, v_se is equivalent shear wave velocity in equivalent single soil layer; d₀ is the depth of soil, and in current study d₀ is equal to H; t is time for the propagation of shear wave from the ground surface to the bedrock layer; d_i and v_si are thickness and shear wave velocity of i layer respectively; n is the number of soil layers. The equivalent shear wave velocity is also given in Table 2.

In the modal analysis, the elastic material model is employed to represent the soil mass and structure. In the transient analysis, for the convenience of calculation, the elastic model is used to simulate the structure, and the Davidenkov model [16] is adopted to model the nonlinear behavior of soil mass. The Davidenkov model is implemented in the FLAC^3D by author, and its formation can be expressed as:

(3){\displaystyle \frac{G_{\mathrm{d}}}{G_{\max }}}=\mathrm{1}-{\left[{\displaystyle \frac{{\left({\displaystyle \frac{{\gamma }_{\mathrm{d}}}{{\gamma }_{\mathrm{r}}}}\right)}^{\mathrm{2}B}}{\mathrm{1}+{\left({\displaystyle \frac{{\gamma }_{\mathrm{d}}}{{\gamma }_{\mathrm{r}}}}\right)}^{\mathrm{2}B}}}\right]}^A,

(4){\displaystyle \frac{\lambda }{{\lambda }_{\max }}}={\left[\mathrm{1}-{\displaystyle \frac{G_{\mathrm{d}}}{G_{\max }}}\right]}^{\beta },

where, G_max is the maximum dynamic shear modulus of soil, l_max is the maximum dynamic damping ratio, A, B, b and g_r are the regression parameters. It can be seen that when A=1.0, B=0.5, Davidenkov model would become Hardin-Drnevich model. In Davidenkov model, the regression parameter can be determined from the soil dynamic experiment, and is given in Table 2. In the table, R is the correlation coefficient for the fitting curve.

Input wave

EL-Centro and Taft wave are selected as the input seismic motion in this study, of which the amplitude is adjusted to 0.15 g (g is acceleration of gravity). Figures 3 and 4 report the acceleration-time curve and Fourier spectrum curve under the two types of wave, respectively.

Working case

The working case considered in current study is given in Table 3. In the table, it should be noted that the “all_above” working case means that the Underground subway-soil-ground frame structure (USG) system is included in the calculation model, and the parameter D is the clear distance between the side wall of the subway station and the boarder of ground frame structure. For the working case 1 to 7, the seismic response of the soil, soil-subway system, soil-ground structure system and the USG system are investigated, in which the EL wave is used as the input motion. In particular, the working cases 5 to 7 is to study the impact of clear distance D on the seismic response of the USG system. For the working case 8, the Taft wave is selected as the input motion to investigate the influence of the type of the seismic wave on the dynamic behavior of USG interaction system.

Modal analyses

By conducting the modal analysis, the first-order natural frequency of the system for different working case is obtained and given in Table 4.

For the free field case, the empirical formula f = 4H/V_{se to} can be used to calculate the first-order natural frequency of soil mass. Here, H is the calculating depth of soil layer, which is 70m in this study, V_se is the equivalent shear wave velocity of soil mass, which is 200.01 m/s (the detailed parameters is given in Table 1). Thus the first-order natural frequency of the soil mass is calculated as f = 5/7= 0.71429 Hz. It is clearly shown that numerical result of first-order natural frequency of the soil mass agrees well with that from theoretical formula, and this result further verified the rationality of the boundary condition in current calculation model.

Furthermore, the first ten-orders natural frequency of free field case is extracted from the results, and it is found that the 10th-order natural frequency is only 1.7581Hz, and first ten-order natural frequency mainly concentrated around 1.8Hz, which implies the natural frequency of the system is rather dense.

After comparing the first-order natural frequency of the system from the different working case, it can be found that: 1) The weight of the existing subway station is smaller than that of the excavated soil mass. According to the theoretical formula, it is also found that the reduction of the weight of system would enhance the system’s natural frequency. This is the reason why the natural frequency of the system (soil-subway) in working case 2 is slightly bigger than that of the free field case; 2) Due to the flexibility of the ground adjacent building, the first-order natural frequency of the system (soil-ground frame building) in working case 3 was smaller than that of the free field case; 3) In the USG system, the reduction of weight in soil mass and the flexibility of the ground building would be the main factors to influence the natural frequency of the system, however, the former factor has more significant impact on the characteristic of natural frequency than the latter one. This can explain the phenomenon that the natural frequency of the system in working cases 4 and 5 is higher than that of the free field case, but less than that of working case 2.

Based on above analysis, it is found that the presence of subway station and ground adjacent building has little impacts on the characteristics of natural frequency of the USG system. From the engineering's point of view, this influence can be ignored, however, when the huge underground structure is considered (such as large size of underground mall, underground terminal etc.) and the configuration of ground adjacent buildings is more complicated (such as skyscraper, asymmetrical buildings etc.) in this USG system, further study is needed .

Transient analyses

Horizontal acceleration of the ground surface

All above working cases are classified into two sets in this section: Set 1 includes the working cases 1 to 4, and Set 2 contains the working cases 1, 4, and 5 to 7. Figures 5 and 6 show the varying peak horizontal acceleration of ground surface along the width of calculation model for two sets. From Fig. 5, it is observed that the influence of the ground adjacent building on the acceleration response of the surrounding soil mass is little and can be ignored; while the existence of the subway station has significant impact on the acceleration behavior of the surrounding soil mass with certain influencing range, in which the influence zone in current study is about 50 m. Considering the half width of subway station is 10 m, and then the diameter of the influence range seems to be 5 times the half width of the subway station. From Fig. 6, it is shown that the clear distance D between subway station and ground adjacent building has little impact on the acceleration response of the surrounding soil mass. This might be due to the light weight of the ground frame building with regular configuration.

Story drift of ground adjacent building

The impact of the subway station on the relative horizontal displacement of the top layer in ground adjacent frame structure is first carried out. Figure 7 shows the time history curve of the relative horizontal displacement of the top layer in ground frame structure for the case of soil-ground adjacent structure system and the USG system with D = 0. From the figure, it can be seen that the trend of displacement response of top layer for two cases are almost the same, but only small diversity in amplitude in which the horizontal displacement of the top layer for the case without subway station (soil-ground adjacent structure system) shows bigger value than that of the case of the USG system (horizontal displacement in the former case is about 9.45 mm, and about 8.89 mm for the latter one, which induces the relative difference about 6.3%). This might be possible that: on one hand the existence of the subway station would obstruct the propagation of the seismic wave to some extent, and thus attenuate the seismic wave; on the other hand, the presence of the subway station weakens the stiffness of the USG system, and then enhances the response of the structure. However, these two factors seem to offset each other in the case of USG system, and hence it can be concluded that in the USG system case with D = 0 the existing subway station would mainly serve as the isolation for the seismic wave.

However, considering the height of layer in ground adjacent frame structure is 3 m, the inter-layer displacement angle can then be calculated as 9.45/3000= 1/317 for the soil-ground adjacent building case. According to Chinese code of the seismic design of building (GB50011-2010) [13], the limiting value of elastic inter-layer displacement angle for reinforced concrete frame structure is 1/550, and thus it is clearly shown that the ground frame structure sustained the plastic deformation in soil-ground adjacent building case.

The impact of the clear distance D between subway station and ground adjacent building on the seismic response of the USG system is further analyzed. Figure 8 reports the time history curve of the horizontal displacement of top layer in ground frame building for different D value. From the figure, it is found that the horizontal displacement achieves the maximum value in D = 5 m case, and gradually reduces with the increase of the D, however, comparing to that from Fig. 7, the horizontal displacement in USG system with varied D are still bigger than that of the system without the subway station. This further implied that the existence of the subway station reduces the stiffness of the system, and the seismic wave bypasses the subway station in the way of diffraction, which results in weakening the capability of mitigation and absorption for the subway structure. The phenomenon of the diffraction of seismic wave can also be envisaged from Fig. 5, in which the horizontal acceleration of ground surface reaches higher amplitude for the cases included the subway station, and then gradually reduces to the similar amplitude of the green-field case.

Horizontal acceleration response of ground adjacent building

Similarly, the impact of the existence of subway station on the acceleration behavior of top layer in ground adjacent frame structure would be first addressed. Figure 9 gives the time history curve of horizontal acceleration of top layer in ground frame structure for the cases of soil-ground adjacent building system and the USG system with D = 0. From the figure, it can be seen that the behavior of horizontal acceleration in two cases are almost the same except for small diversity in amplitude, in which the horizontal acceleration of the top layer in frame building for the case without subway station has bigger amplitude than that of the case with the subway station (horizontal acceleration in the former case is about 2.50 and 2.38 m/s² for the latter one, which induces the relative difference about 5.0%). This phenomenon also proved conclusion that the existing subway station would work as the isolation for the seismic wave in the case of the USG system with D = 0.

The impact of the clear distance D between subway station and ground adjacent building on the acceleration response of the USG system are then analyzed, and the results are summarized in Fig. 10. From the figure, the similar conclusion is achieved as that in Section 6.2, in which the existence of the subway station reduces the stiffness of the system, and the occurrence of the diffraction of the seismic wave weakens the capability of mitigation and absorption for the subway structure.

Dynamic response of subway station

In this section the dynamic behavior of the roof in the subway station structure would be analyzed and discussed.

The impact of the presence of ground adjacent building on the horizontal acceleration and displacement of the roof in the subway station is first considered. Figure 11 illustrates time history curve of the horizontal acceleration of the roof in subway station for the cases with or without ground adjacent building (soil-subway system and USG system). From the figure, it is clearly shown that the curve of acceleration response of the roof in subway station seems to overlap for the two working case. The similar phenomenon is also observed in the time history curve of the horizontal displacement of the roof in subway station as shown in Fig. 12. This might be possible since the ground adjacent frame structure with light weight has little impact on the dynamic response of the subway station as mentioned in previous section. For the subway station structure, the maximum horizontal displacement of the roof is found to be 2.6 mm from the figure, and the height of upper layer is 4 m in the calculation model, and thus the inter-layer displacement angle can be calculated as 2.6/4000= 1/1538, which is smaller than limiting value (1/550) according to the Chinese code of seismic design of building (GB50011-2010). Hence, it can be found that the subway station structure is still in the elastic stage for both cases.

Since the existence of the ground adjacent building has no significant impacts on the dynamic response of the subway station, the effect of the clear D between the subway station and ground adjacent building had not been discussed anymore.

Effect of the vertical acceleration of ground surface

For the free field case, normally the acceleration of ground surface would not induce the vertical component under the horizontal seismic wave. However, the existence of the subway station and ground adjacent building would interrupt the propagation of seismic wave, and thus result in the occurrence of the vertical acceleration along the ground surface. To investigate the influence of the vertical acceleration of ground surface on the dynamic behavior of the subway station and ground adjacent building, two working cases are considered in this section as an example, one is the case of the USG system with D = 0 m, and the other is the USG system with D = 5 m. Figures 13 and 14 show the time history curve of vertical acceleration of top layer in ground frame building and its normalized Fourier spectrum curve, respectively. Figures 15 and 16 give the time history curve of vertical acceleration of ground surface at the distance of 30 m to the origin O (the coordinated axis was given in Fig. 1) and its normalized Fourier spectrum curve, respectively. From the figure, it is observed:

1)When there is the distance between the ground adjacent building and the subway station (i.e., D = 5 m), the influence of the vertical acceleration derives from the seismic wave on the dynamic behavior of the structures cannot be ignored, in which the maximum acceleration of the top layer in ground adjacent building reached 0.25 m/s², accounting for 17% of the input ground acceleration amplitude 0.15 g; meanwhile the maximum vertical acceleration of ground surface is 0.18 m/s², accounting for 12% of the input ground acceleration amplitude 0.15 g. Based on above results, it can be concluded that the presence of underground structure and ground adjacent building would induce a large amount of the vertical seismic component which cannot be ignored in the seismic design of the structure.

2) From the normalized Fourier spectrum curve, when there is no distance between the ground adjacent building and the subway station (i.e., D = 0 m), the existing subway station partially obstructs the propagation of seismic wave; when D = 5 m the vertical acceleration response of the top layer in ground adjacent building presented a richer high-frequency composition, while frequency spectrum of soil mass shows similar frequency composition for both cases, but small diversity in amplitude.

Effect of the seismic wave

The work cases 4 and 8 are considered and analyzed in this section to study the influence of different seismic wave on the dyanmic behavior of the USG system, in which the EL-wave is used as the input wave for working case 4 and Talf wave for working case 8.

Due to different seismic input waves with the varied frequency component, the different seismic response of the USG system is observed. Figure 17 shows the time history curve of the horizontal acceleration of the ground surface at the origin of coordinate O (coordinate of axis are given in Fig. 1). From the figure, it is clearly seen that the horizontal acceleration of the ground surface subjected to the EL wave is bigger than that of Taft wave. The normalized Fourier spectrum curves of the two waves can be further analyzed to figure out the discrepancy. As shown in Fig. 18, it is found that both waves reaches the peak amplitude at the natural frequency of 0.7Hz, while the natural frequency of the free field is 0.714 Hz. It can also be seen that EL wave presents a bimodal characteristics, that is, there has another peak value at 0.82 Hz which is close to second-order natural frequency of the wave. It should be noted that although the third and forth peak value in the Talf wave is slightly bigger than that of the EL wave, the first two-order natural frequency corresponding to the first and second peak value in the spectrum has significant impact on the seismic response of the system. This is why the USG system presents the stronger seismic response under the EL wave than that under the Talf wave, and only the range of natural frequency 0~3 Hz need to be considered in the Fourier spectrum for EL and Talf wave.

Figure 19 shows the varying peak horizontal acceleration of ground surface along the width of calculation model under different seismic waves. From the figure, it can be observed that: 1) for the USG system with D = 0 case, the trend of horizontal acceleration response of ground surface under two wave input are almost the same except for small diversity in the amplitude; 2) Peak acceleration of ground surface for the free field under different input seismic wave presents large diversity, which is reconfirmed the fact that EL wave has more significant effect on seismic response of ground than that of Talf wave.

Conclusion

This paper performs the numerical study on the rule of the seismic response of the Underground structure-Soil mass- Ground adjacent building (USG) system subjected to various seismic loading, and the impact of the USG system on the seismic response of the surrounding soil mass, and the mutual influence between the existing underground structure and the ground adjacent building. Two key factors such as the clear spacing between the subway station and ground adjacent building and the types of the seismic loading are also considered to further study its impact on the dynamic response of the USG system. Some conclusions are achieved as follows:

1) The impact of underground structure on the dynamic characteristics of the surrounding soil mass depends on its own dimension; and the lighter weight of ground building has less impact on the dynamic characteristics of surround soil.

2) The presence of underground structure has certain impact on the seismic response of ground adjacent building, however, the ground adjacent building has little impact on the seismic response of underground structure.

3) Due to the presence of underground structure and ground adjacent building, the vertical acceleration generated by this USG system cannot be ignored.

4) The difference in frequency components of the seismic wave would lead to different seismic responses of the surrounding soil mass, underground structure and ground adjacent building.

From the above observation, it is found that the reciprocal influence of the seismic response between subway station and ground adjacent buildings is insignificant for the subway station with small dimension and ground adjacent building with light weight, which further results in the less influence on the seismic response of the surrounding soil mass. However, with regard to the large scale underground structure and more complexity of ground adjacent building, the impact of the USG interaction system on the seismic response of the surrounding soil mass and their mutual influences need to be further studied.

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