Soil-structure Interaction System Research Articles

Influence of local scour on the dynamic responses of OWTs under wind-wave loads

Offshore wind turbines (OWTs) often operate in complex marine environments, where they are not only subjected to wind and wave loads, but also adversely affected by scour. Therefore, it is of great significance to explore the effect of scour on the dynamic responses of OWTs under external loads to ensure structural safety, improve performance, and extend service life. In this study, a comprehensive numerical model of a 5-MW OWT, including tower, monopile, and soil-structure interaction (SSI) systems, is established by using ABAQUS platform. Aerodynamic loads is generated using blade element momentum, while wave loads is generated using the P-M spectral. The depth of scour is obtained based on on-site measured data. A comparative analysis is conducted between fixed foundations and SSI systems when conducting dynamic response analysis of OWTs under wave loads. Subsequently, the effect of scour on dynamic responses of OWTs under aerodynamic and wave loads is investigated. Results demonstrate that SSI can significantly influence the natural frequency and dynamic responses of the OWT. Therefore, it is essential to thoroughly consider SSI when evaluating the dynamic response of the OWT with local scour. The tower-top displacement and acceleration of the OWT with show a significant increasing trend compared to the non-scoured OWT. An increase in scour depth leads to higher maximum stress and stress amplitude in the steel monopile, which could potentially cause fatigue issues and should be given due attention.

The Possibility of Detrimental Effects on Soil–Structure Interaction in Seismic Design Due to a Shift in System Frequency

Soil–structure interaction (SSI) leads to a modification in the dynamic properties of structure, but due to the complexity of analysis, it is traditionally assumed in seismic designs that the structure is fixed-supported on the ground, which brings about potential risks to the seismic performances of structure. The study works on the possibility of SSI having detrimental effects by comparing the dynamic responses of the SSI system to a fixed-base structure, and presents charts for an evaluation of the system frequency of SSI for the purpose of engineering practice. In order to reveal the physical nature, the SSI model is reduced to its simplest form, consisting of a SDOF oscillator, a three-dimensional rectangular foundation, and a multi-layered half-space. The energy dissipation in the soil is achieved by foundation impedances and the substructure method. Previously, the foundation impedances are usually acquired by two-dimensional or axisymmetric three-dimensional models in uniform half-space to avoid the high cost of the more realistic, fully 3D models, while a high-precision indirect boundary element method is employed, combined with the non-singular Green’s functions of distributed loads to calculate the foundation impedances. Although SSI dampens the peak amplitude of structure response in the frequency domain, case studies on four buildings’ responses to 42 earthquakes in the time history show a possibility of 15–20% that SSI amplifies the dynamic responses of structures, such as the maximum and the mean values in the time history, depending on the properties of the structures and the site, as well as the frequency component of incident waves.

Simulation of soil-structure interaction using inelasticity-separated finite element method

The soil–structure interaction (SSI) effect is nonnegligible during the nonlinear seismic response analysis of large-scale or underground structures. However, the computation of nonlinear response of a structure considering SSI effect usually is a time-consuming process because the numerical model should involve the structure itself, a wide-range soil domain and the soil–structure interface, especially for large-scale structure. This study aims to incorporate the inelasticity-separated finite element method (IS-FEM), which is an algorithm recently developed for efficient structural nonlinear analysis, to improve the efficiency of the soil-structure interaction system. To this end, an inelasticity-separated contact element model for modeling the nonlinear behavior of soil and structure interface is developed in this study. To derive the inelasticity-separated governing equation of this model, a reference elastic stiffness is defined so that the normal and shear deformation of any point pair in the presented contact element is decomposed into reference linear and reference nonlinear parts according to this reference elastic stiffness. As a result, the problem of inconsistency between the requirement of IS-FEM for constant initial linear elastic stiffness and the existence of inflection point for the stress versus relative displacement relationship of contact behavior can be solved. Then, the Goodman element formulation for depicting the nonlinear contact behavior is introduced and an interpolation scheme to establish the reference nonlinear deformation field in an element is incorporated. By combining the proposed model with existing inelasticity-separated elements, a global analytical model of the soil–structure interaction system can be established within the framework of IS-FEM. Because the nonlinearity usually occur in some local regions of the global model and the use of the IS-FEM only require performing these operations for a small scale matrix representing local nonlinearity, the computational efficiency of nonlinear structural response analysis considering SSI effect can be improved significantly. The applicability and efficiency of the proposed method is finally verified by two numerical examples.

Multi‐mode inversion of dispersive surface‐wave free fields in layered media

AbstractInversion of surface‐wave free fields is of great importance in providing seismic excitations for the finite element (FE) analysis of soil–structure interaction (SSI) systems. In the case of layered media, the propagating surface waves are characterized by their unique dispersion property, resulting in dispersive multi‐mode surface wavefields. Available studies primarily focus on only the fundamental mode and neglect the influence of higher modes, which fail to identify the real structural response characteristics. To enhance the precision of seismic excitations for SSI systems situated in layered media, a multi‐mode inversion method for dispersive surface‐wave free fields is proposed in this study. The proposed method employs the energy flow density to characterize the participation volume of dispersive modes, and further calculates corresponding modal participation factors by combining with the dispersion curve and dynamic‐stiffness matrix (DSM) method. The modal participation factors are then utilized to assign the ground surface components of each mode and invert respective single‐mode surface‐wave free fields. Based on the mode superposition theory, all the single‐mode surface‐wave free fields are finally superimposed to construct the multi‐mode wavefields. The accuracy of the proposed method is thoroughly verified and validated across various types of layered media. Furthermore, the engineering application of the proposed method is illustrated by performing the FE analysis on an SSI example. The analysis results highlight the significant participation of surface waves' higher modes in structural seismic responses, particularly in the weak‐interbed type media.

Thin layer interface: An alternative modeling consideration in soil-structure interaction system

The numerical modeling of a soil-structure interaction (SSI) system subjected to lateral loads depends mainly upon the interface behavior. The soil stiffness in relation to the structure is quite low, and the structure generally has a rough surface in contact with the soil. As a result of the slipping and rolling of soil grains caused by friction, a thin layer of shear zone forms in soil with the application of lateral loads. A thin layer interface is represented by the shear zone. As a result, the decision on the thickness requirement is critical for appropriate modeling of the thin layer interface, which is rarely documented by researchers. Furthermore, it has been reviewed in the literature that modeling the SSI system with a thin layer interface more accurately replicates the physical system than modeling it with a zero-thickness interface. The methodology for effective usage (in terms of proper interface thickness) of the thin-layer interface in SSI system using finite element (FE) modeling is proposed in this work. The thickness of the interface has also been determined via numerical modeling of a large direct shear test (DST).

A new equivalent acceleration-based non-uniform response spectrum method for efficient seismic SSI analysis

A complex multiple-support response spectrum (CMSRS) method with a concise and universal expression is required to estimate the seismic responses of soil–structure interaction (SSI) systems under non-uniform seismic excitations to replace existing complex methods. This paper introduces and defines a new concept called equivalent acceleration of boundary load (EABL) that converts widely used equivalent loads into equivalent accelerations, and further develops an efficient CMSRS-EABL method for the seismic analysis of the SSI system. First, EABL offers three main benefits: it simplifies the expressions without sacrificing precision, extends the applicable scope of viscous-spring artificial boundary (VSAB) to all stress-type artificial boundaries, and presents dynamic equations in a familiar form. Subsequently, the CMSRS–EABL formula was successfully deduced based on the equivalent simplified dynamic equations. This formula significantly simplifies the traditional method by reducing the effective modal participation factors, mean-square-roots, and cross-correlation coefficients by 4, 5 and 15 items, respectively. Finally, a rectangular construction surrounded by a VSAB–soil combination was used as an example to illustrate the convenience of the developed CMSRS–EABL method and verify its precision. The results indicate that the proposed method is characterized by greater simplicity and ingenuity, and it is capable of high precision similar to that of the conventional methods. We believe this method will be of significant benefit in evaluating the anti-seismic performance of deeply buried and long-standing structures.

Shaking table test on seismic responses of asymmetrical underground subway stations

Currently, many subway stations are designed symmetrically and less attention has been paid to asymmetrical stations. The middle column of the asymmetric underground structure is offset to one side, which is not conducive to load bearing during strong earthquakes and is prone to damage. In this study, shaking table tests of a soil-symmetrical station and a soil-asymmetrical station were designed and conducted. A numerical model of the soil–structure interaction system was established, and the feasibility of the simulation method was verified. Then, the seismic responses of two kinds of stations were discussed and analysed. The results show that the dynamic response of both symmetrical and asymmetrical stations increases with the increase of the loading seismic intensity. Influenced by the size effect of the model test, there is less difference between the two models in their dynamic response. From the perspective of internal force and relative deformation, the response of the asymmetrical station is greater than that of the symmetrical station, which adequately reflects the disadvantage of asymmetrical structures in seismic design.

Effective-mode superposition response spectrum method for three dimensional seismic response analysis of underground structures

Response spectrum method (RSM) has been applied in seismic analysis of underground structures. However, the RSM is computationally low efficient for processing large soil domains. In this paper, the dynamic characteristics of underground structures are first investigated through three rectangular subway station structures. The results show that only a small number of effective frequencies and modes of soil-structure interaction (SSI) system significantly affect the seismic response of underground structure. These effective frequencies and modes are related to the natural vibration characteristics of site. An effective method is then developed to obtain the effective natural frequencies and modes of the SSI system. Subsequently, the effective modal superposition response spectrum method (ERSM) for seismic analysis of underground structure is developed by only combining the effective natural frequencies and modes of the SSI system to improve efficiency. Finally, the accuracy of ERSM is investigated through seismic response analysis of subway station structures. Numerical examples show that the ERSM can be used for seismic analysis of underground structures considering 3D SSI.

Comparative study on seismic performance of structure with TMD and particle TMD considering SSI effects

Tuned mass dampers (TMDs) are widely adopted to suppress structural response under dynamic loads. Some novel nonlinear TMDs have been developed to improve control performance, such as particle TMD (PTMD). The damping mechanism of TMD and PTMD are linear and nonlinear, respectively. Ignoring soil-structure interaction (SSI) effects may lead to inaccurate evaluation for the structural seismic responses and effectiveness of TMDs. In this study, the seismic control performance of the linear TMD and the nonlinear PTMD considering SSI effects are explored. Based on the shaking table test results, the mitigation effects of TMD and PTMD on floor acceleration and displacement responses are compared, in terms of average decreasing amplitude ratios and root-mean-square decreased ratios. Furthermore, to provide insights into the mitigation mechanism of TMD and PTMD including SSI effects, the recorded signals are decomposed and the energy response analysis is conducted using wavelet packet transform. The results show that when SSI effects are included, the performance of the linear TMD and the nonlinear PTMD on acceleration response is close, but the effectiveness of the linear TMD on displacement response and energy response is superior to that of PTMD. This study improves the understanding of TMDs’ effectiveness in SSI system subjected to seismic excitations.

Physical and numerical investigation of integral bridge abutment stiffness due to seasonal thermal loading

Integral Abutment Bridges (IABs) are increasingly popular due to their reduced maintenance cost compared to traditional bridges with expansion joints. However, the widespread construction of IABs is currently limited by design code prescriptions resulting from the significant uncertainties associated with how the backfill interacts with the (integral) abutment and the deck. Under cycles of seasonal thermal loading, the backfill properties change, affecting the distribution of lateral earth pressures acting on the abutment walls. Moreover, the stiffness of the abutment can significantly influence the soil-structure interaction (SSI) in IABs. This research work investigates the effect of abutment stiffness (flexural rigidity) on soil-structure interaction in IABs under seasonal thermal loading through experimental analyses and numerical modelling. To better understand this mechanism and reliably assess the performance of IABs within their life cycle, a 1 g small-scale instrumented physical model was built to simulate the backfill under accelerated seasonal expansion and contraction of the bridge deck. The experimental results were modelled numerically in PLAXIS and ABAQUS to assess the sensitivity to different flexural stiffnesses of the abutment and discuss suitable options for modelling such SSI systems through finite elements either using a geotechnical-oriented or a structural-oriented software package. It was found that flexible IABs can be more suitable for controlling earth pressure built-up within the early lifecycle of the soil-structure systems. The simplified numerical models can provide a first-order prediction of pressure distributions in the small-scale 1-g rig. This preliminary dataset informs necessary larger-scale experiments to assess the scaling and feasibility of 1-g tests.

Experimental study on soil-structure interaction of steel frame under seismic excitations

Three types of shaking table tests were carried out to investigate the soil-structure interaction (SSI) effects on the seismic performance of the steel frames resting on soft soil ground. Three different models were involved in the shaking table tests including a ten-story steel frame resting on a soft soil ground, a ten-story steel frame resting on a rigid ground, and a free field with soft soil, respectively. First, the characteristics of soils were studied to choose suitable soft soil for the tests. Then, a laminar shear soil box was developed to contain soft soil and simulate the soil in the free field. Finally, the three models were subjected to various intensity seismic excitations of the El-Centro earthquake, the Kobe earthquake, and the Taft earthquake. The results showed that SSI effects noticeably affect the propagation of seismic waves, the dynamic characteristics, and the dynamic responses of the steel frame. For the steel frame resting on the rigid ground, the fundamental frequency of the SSI system decreased, but the damping ratio increased. The peak acceleration of the steel frame decreased when rested on the soft soil ground, but the inter-story drift increased. Consequently, the seismic SSI has a complicated influence on the dynamic responses of steel frames and should be considered reasonably to provide a rational seismic design for steel frames resting on soft soils.

Quasi-static model test and numerical simulation of a single-story double-span underground subway station

The model test method is crucial for studying the seismic performance of underground structures. Presently, model tests for the seismic response of underground structures are primarily performed with 1-g and N-g centrifuge shaking table. The 1-g shaking table model test is limited by the stress similarity in the soil-structure interaction system and cannot reproduce the seismic damage process and extent of the underground structure. The small table size of the N-g centrifuge model test makes it difficult to reflect the spatial effects of the seismic response of large underground structures. Seeking a large-scale model test method that can directly simulate the seismic damage process is essential for investigating the seismic performance of underground structures. Herein, based on the seismic response characteristics of underground structures, a quasi-static pushover test device applicable to underground structures was designed, and the corresponding tests and numerical model calculations were executed using the Daikai subway station as a prototype. Based on the macroscopic phenomena of structural damage development of the subway station in the test, the damage mechanism of the station structure was analyzed and compared with the numerical model. The strain amplitudes, displacements, and digital scatter strain color maps were analyzed to verify the reliability of the cyclic pushover loading test simulating the seismic performance level and damage process of the underground structure. Simultaneously, the rationality of the proposed quantification system for classifying the seismic performance level of single-story underground structures was verified.

A direct time-domain FEM-SBFEM-SBPML method for soil-structure interaction analysis using quadtree mesh

In this paper, a novel direct time-domain numerical method, called the FEM-SBFEM-SBPML, is proposed to study the wave propagation problems in soil-structure interaction (SSI) system with complex geometry and geological condition. In the bounded domain, the structure is modeled as beam element based on the finite element method (FEM), and the surrounding truncated soil is discretized using the scaled boundary finite element method (SBFEM) combined with an efficient quadtree mesh generation technique, respectively. In addition, a solid-modified beam coupling model considering the eccentric effect of the tangential traction at the outer surface of the structures is implemented. The unbounded domain is modeled by a recently proposed scaled boundary perfectly matched layer (SBPML) method, which is a novel high-accuracy absorbing layer method satisfying the radiation condition. As a result, the combined FEM-SBFEM-SBPML approach is capable of modeling SSI with complex geometries and soil properties automatically. Some benchmark tests are conducted to illustrate the accuracy and efficiency of the proposed scheme, and a vibration analysis of a soil-tunnel system is further presented to demonstrate the applicability of the proposed method in engineering.

Multiphase wavefield inversion methodology for seismic analysis of soil-structure interaction systems

Currently available inversion methods for providing seismic excitation for soil–structure interaction (SSI) systems regard all ground motions as the body wave component and invert only the body wavefields. However, the ground motions are dominated by body waves and surface waves, and ignorance of the latter has been demonstrated to be conservative for structural responses. To provide more accurate wavefields for the seismic input of SSI systems located on the half-space of rock mediums, a multiphase wavefield inversion (MPWI) methodology considering both body waves and Rayleigh surface waves is proposed in this study. The methodology applies forward modeling and inversion theory of body waves and Snell's law for complex angles to determine the exact Rayleigh wave components and construct the Rayleigh wavefields. The residual body waves are used to invert the body wavefields, focusing on optimizing the determination of incident angles by a least-squares iteration approach. Rayleigh and body wavefields are finally combined to construct the total wavefields. The efficiency of the proposed methodology is verified and validated with analytical solutions and real downhole recordings, and the applicability is examined by response comparisons with the traditional method in the free field and an urban complex building. Simulation results demonstrate that due to the consideration of the Rayleigh waves, the MPWI methodology results in higher free-field stresses and accordingly induces more intense seismic excitation. Correspondingly, the structural responses present an amplification effect.

Prediction of the seismic behavior and backfilling scheme optimization of immersed tunnel based on the ruyifang project

The backfill soil is the first line of defense that ensures immersed tunnel's safety and stability, but its seismic performance has not been investigated in previous studies. This study employs the Ruyifang tunnel as its research object to determine the most dangerous cross-section. It establishes a two-dimensional soil-structure interaction system for complex sites to evaluate immersed tunnels' safety during earthquakes, clarify the structural failure mechanism, and optimize the backfilling scheme. In addition, this study investigates the tunnel element's response, damage, and stress under static and dynamic loads and obtains the impact of backfilling materials by considering plastic damage in structures, soil viscoelastic properties, and various backfilling schemes. The results showed that the tunnel section's stress has prominently distributed characteristics. Moreover, the structure exhibits tensile failure during ground motions, and the bottom of the side wall is the critical area. The inertial force is the primary factor affecting the seismic capacity at the overburden layer and flanks, while stiffness plays a vital role at the cushion. These dynamic properties and related backfilling technology can provide technical support for the anti-seismic design of immersed tunnels.

Determination of damage potential of seismic ground motion to underground structures at soft sites through experimental approach

The selection of a ground motion intensity measure (IM) is an essential criterion for judging the expected intensity level of ground motion in the vulnerability analysis of underground structures, and it is also part and parcel of the resilience-based seismic design of underground structures. Most of the current studies are based on a statistical analysis of numerical simulation results to obtain a specific IM measuring the damage potential of ground motions for an underground structure, and there is a lack of mechanistic studies based on experimental ways. In view of this, a series of large-scale shaking-table tests of an underground structure in soft soils were carried out in this paper to explore the appropriate IMs for measuring the damage potential of ground motions. It is found that the response characteristics of the tested soil-structure interaction (SSI) system gradually become similar to the long-period system as the ground motion intensity increases. The transient fundamental frequency of the tested SSI system moves from the acceleration-sensitive band to the velocity-sensitive band in the ground motion response spectrum. This leads to a decrease in the correlation between the response indicators (RIs) of the system and acceleration-based IMs and an increase in the correlation between RIs and velocity-based IMs as well as displacement-based IMs. Based on the test results, the site fundamental period (Tg) and the boundary periods between the sensitive bands in the bedrock response spectrum (Tc and Td) are proposed as identification indexes for selecting the appropriate IM. One of the three measures, PBA, PBV, and PBD, or a composite measure consisting of two of the three, can be adopted as an appropriate IM for an actual underground structure in soft soils in different cases.

Seismic Response Analysis of Reinforced Concrete Frame Structures Considering Slope Effects

According to the seismic damage due to past events, buildings located on slopes can present a worse seismic performance. To explore this, this study established a finite element model based on a 6-story RC frame structure and soil models based on a practical slope using OpenSees software. Combining the superstructure model with the soil model through soil spring elements, three soil-structure interaction systems with different slope rates were set up. Twenty near-field seismic actions were used as input loads for dynamic time–history analysis. The analysis shows that in the process of seismic action, the deformation tendency of the structure is affected by the slope. There is a clear tendency for lateral displacement towards the slope, and it is more obvious with a greater slope ratio. Meanwhile, the slope has no impact on the shear force at the base of the structure or at the bottom of the column. In addition, there is no correlation between the degree of impact and the slope gradient on the peak value of internal forces and deformations of structure.

Experimental and numerical investigation on seismic response of the soil-structure cluster interaction system

With the rapid advancement of urbanization in recent decades, a large number of surface dense buildings have appeared. Soil-structure cluster interaction (SSCI) has attracted widespread attention. However, most current research is based on numerical simulations, and studies using shaking table tests are rare. Here, a series of shaking table tests of soil-structure interaction and SSCI systems were designed and conducted to investigate the effect of SSCI on the response of the structure and site soil. The number of structures, seismic wave types, and amplitudes were considered. The numerical method was verified based on test data. The test data and numerical results indicate that (1) the presence of surface buildings does not always reduce the free-field soil motion. However, the soil vibration decreases as the number of surface structures increases. (2) The maximum influence of SSCI effects on the roof acceleration and displacement of the central building (CB) can be as high as –23% and −18%, respectively. (3) The relative deformation of the structure decreases with an increase in the number of structures, and the relative deformation of the middle layer is the largest. (4) Different evaluation indices can yield roughly the same results, although the focus of each evaluation index is different. (5) A denser layout enhances the SSCI effect and further reduces the response of the CB when the urban density of the structural cluster is the same.

Modal analysis and extraction method for the frame structure considering soil-structure interaction effect

For a soil-structure interaction (SSI) system, the direct method can analyze the dynamic characteristics of the arbitrary complicated soil deposits. However, to determine the radiation damping effect on the structural dynamic characteristics, the complex modal analysis using the direct method needs to calculate many complex modes for obtaining the few exact structural modes due to a large number of soil degrees of freedom. The needed structural modes among a large amount of soil modes are difficult to be extracted. In this paper, a new complex modal analysis method using sequential eigenvalue-shift is developed for the calculation of the modal parameters of the frame structure under SSI effect. Accordingly, a new index of the local modal mass ratio is proposed to quickly and accurately extract the structural modes from many combinations of modes from structures and soil. Several comprehensive examples are presented to validate the proposed method, and the influence of artificial boundary on the natural frequency, mode and modal damping ratio of the structure is investigated.

Shaking table test study of nuclear power plant model considering soil-structure interaction effect

The influence analysis of site condition is fundamental to the seismic design of nuclear power plants. In this paper, a series of shaking table tests were performed to investigate the Soil-Structure Interaction (SSI) effect on the seismic response of nuclear power structures. An AP1000 nuclear power plant was taken as the objective structure, lead rubber bearings (LRBs) were used as the isolators and Shanghai silty clay was selected as the model soil. Test results show that (1) The soil has different effects on the dynamic characteristics of isolated and non-isolated model structures. (2) During the shaking table test, the foundation edge is subject to the highest soil pressure, and a contact surface separation between the foundation and the soil begins to occur under the seismic-margin earthquake (SME) seismic input level. (3) Both for the rigid and soil foundation, seismic isolation bearings can effectively reduce the acceleration response of the superstructure. (4) For the non-isolated model, the SSI effect significantly amplifies the seismic response at the bottom floor of the structure in the low frequency range, and an obvious rocking behavior occurs at the operating basis earthquake (OBE) seismic input level. (5) For the isolated model, the SSI effect reduces the hysteresis capacity of the seismic isolation system and amplifies the seismic response at the bottom floor, especially in the vertical direction. The site condition and the use of an isolation system have a significant impact on the seismic response of a nuclear power structure. Therefore, in the design and safety assessment of nuclear power plants, the influences of soil characteristics and isolation equipment in the soil-structure interaction system should be fully considered.