Far-field Soil Research Articles

동적 무한요소를 이용한3차원 지반-구조물 상호작용계의 지진응답해석

본 연구에서는 지반-구조물 상호작용을 고려한3차원 지반-구조계의 지진응답 해석을 수행하고 그 기법의 적용성과 타당성을 검토한다. 이를 위해 구조물과 구조물 주변의 근역지반을3차원 유한요소로서 모델링하고 원역지반에 대해서는 기 개발한 3차원 동적 무한요소를 적용한다. 모든 입사 성분P, SV 그리고 SH파가 고려되었을 때, 등가 지진하중은 무한요소에 의해 구해진 무한 지반의 동적 강성과 자유장 해석을 통해 구해진 지반의 응력과 변위응답을 이용하여 구해진다. 검증 및 적용 예제는 적층 자유장의 지반응답해석과 전형적 원자로 격납건물의 지반-구조물 상호작용을 고려한 층응답 스펙트럼을 구하는 것으로 하였다. 해석 결과는 다른 기법에 의해 구해진 값들과 비교하였으며, 본 기법의 정확성과 정밀성을 확인할 수 있다. This paper presents a seismic analysis technique for a 3D soil-structure interaction system in a frequency domain, based on the finite element formulation incorporating frequency-dependent infinite elements for the far field soil region. Earthquake input motions are regarded as traveling P, SV and SH waves which are incident vertically from the far-field soil region, and then equivalent earthquake forces are calculated using impedances of infinite soil by dynamic infinite elements and traction and displacement from free field response analysis. For verification and application, seismic response analyses are carried out for a multi-layered soil medium without structure and a typical nuclear power plant in consideration of soil-structure interaction. The results are compared with the free field response using a one-dimensional analytic solution, and a dynamic response of an example structure from another SSI package.

DYNAMIC INFINITE ELEMENTS FOR SOIL-STRUCTURE INTERACTION ANALYSIS IN A LAYERED SOIL MEDIUM

This paper presents the dynamic infinite element formulations that have been developed for soil-structure interaction analysis both in frequency domain and time domain by the present authors and our colleagues during the past 20 years. Axisymmetric, 2D and 3D layered half-space soil media were considered in the developments. The displacement shape functions of the infinite elements were established using approximate expressions of analytical solutions in frequency domain to represent the characteristics of multiple waves propagating into the unbounded outer domain of the media. The shape functions were determined in terms of the excitation frequency as well as the spatial and material characteristics of the far-field soil region. Thereby the element mass and stiffness matrices are frequency dependent. As for time domain analysis, the shape functions were further simplified to obtain closed-form frequency-dependent mass and stiffness matrices, which can analytically be transformed into time domain terms by the Fourier transform. The proposed infinite elements were verified using benchmark examples, which showed that the present formulations are very effective for the soil-structure interaction analysis either in frequency or in time domain. Example applications to actual soil-structure interaction problems are also given to demonstrate the capability and versatility of the present methodology.

Three-dimensional frequency-dependent infinite elements for soil–structure interaction

This paper presents three-dimensional elasto-dynamic infinite elements for the three-dimensional soil–structure-interaction problem in a multi-layered half-space. Five kinds of infinite elements are developed in Cartesian coordinates using approximate expressions of multiple wave components for the wave functions in the exterior far-field soil region. They are horizontal, horizontal-corner, vertical, vertical-corner and vertical–horizontal-corner elements. Modified wave numbers are introduced to make the wave lengths in the original radial coordinates fit into the Cartesian coordinates. The elements can be used for the multi-wave propagating problem. Numerical example analyses are presented for a rigid disk, rectangular footings, and an embedded caisson on homogeneous and layered half-spaces. The numerical results obtained show the effectiveness of the proposed three-dimensional infinite elements.

Earthquake response analysis in the time domain for 2D soil–structure systems using analytical frequency‐dependent infinite elements

AbstractThis paper presents a time domain method for soil–structure interaction analysis under seismic excitations. It is based on the finite element formulation incorporating analytical frequency‐dependent infinite elements for the far‐field soil region. Equivalent earthquake input forces are calculated based on the free‐field responses along the interface between the near‐ and far‐field soil regions using the fixed exterior boundary method in the frequency domain. Then, the input forces are transformed into the time domain by using inverse Fourier transform. The dynamic stiffness matrices of the far‐field soil region formulated using the analytical frequency‐dependent infinite elements in the frequency domain can be easily transformed into the corresponding matrices in the time domain. Hence, the response can be analytically computed in the time domain. A recursive procedure is proposed to compute the interaction forces along the interface and the responses of the soil–structure system in the time domain. Earthquake response analyses have been carried out on a multi‐layered half‐space and a tunnel embedded in a layered half‐space, and results are compared with those obtained by the conventional method in the frequency domain. Copyright © 2003 John Wiley & Sons, Ltd.

Interface Load Transfer Degradation During Cyclic Loading: A Microscale Investigation

The shaft capacity of piles in sand subjected to cyclic (wave) loading has been observed to decrease significantly with loading cycles (Poulos, 1989). A number of researchers (Boulon and Foray, 1986; Tabucanon et al., 1995; Shahrour et al., 1999) have replicated the characteristics of the load transfer degradation behavior in the laboratory through cyclic interface shear testing with a constant normal stiffness confinement condition (Vesic, 1972). However, no consensus currently exists as to the primary microscale mechanisms that govern cyclic interface shear behavior and load transfer degradation. A research program was undertaken to quantify the contribution of soil properties, cementation, confinement condition, and displacement mode, in load transfer degradation. Monotonic and cyclic interface shear tests were performed using a modified interface direct shear device with a Perspex side window. The specimen particle displacement fields were quantified during selected cycles by capturing high resolution digital images (1600 x 1200 pixels) and using Particle Image Velocimetry (White et al., 2001a). Results indicate that the confinement condition, which is intended to replicate the elastic response of the far-field soil, is of primary importance as it allows for normal stress relaxation with soil contraction adjacent to the interface. The displacement magnitude, particle characteristics, and particle-particle cementation were also observed to affect the magnitude and rate of degradation. It is anticipated that these findings will provide a fundamental rationale to identify field conditions where shear stress degradation is likely to occur and a basis from which more rigorous models may be developed.

Fluid-structure-soil interaction analysis of cylindrical liquid storage tanks subjected to horizontal earthquake loading

This paper presents a method of seismic analysis for a cylindrical liquid storage structure considering the effects of the interior fluid and exterior soil medium in the frequency domain. The horizontal and rocking motions of the structure are included in this study. The fluid motion is expressed in terms of analytical velocity potential functions, which can be obtained by solving the boundary value problem including the deformed configuration of the structure as well as the sloshing behavior of the fluid. The effect of the fluid is included in the equation of motion as the impulsive added mass and the frequency-dependent convective added mass along the nodes on the wetted boundary of the structure. The structure and the near-field soil medium are represented using the axisymmetric finite elements, while the far-field soil is modeled using dynamic infinite elements. The present method can be applied to the structure embedded in ground as well as on ground, since it models both the soil medium and the structure directly. For the purpose of verification, earthquake response analyses are performed on several cases of liquid tanks on a rigid ground and on a homogeneous elastic half-space. Comparison of the present results with those by other methods shows good agreement. Finally, an application example of a reinforced concrete tank on a horizontally layered soil with a rigid bedrock is presented to demonstrate the importance of the soil-structure interaction effects in the seismic analysis for large liquid storage tanks.

Prediction of Ground Movements due to Pile Driving in Clay

This paper evaluates theoretical predictions of ground movements caused by the installation of driven (or jacked) piles in clay. The predictions are based on an approximate analysis framework referred to as the shallow strain path method (SSPM) that simulates undrained pile penetration from the stress-free ground surface. Large strain conditions close to the pile are solved numerically, and closed-form analytical expressions are obtained from small strain approximations at points further away. These results show that, for closed-ended cylindrical piles of radius R and embedment L, the normalized displacements are functions of their dimensionless position x/L. In contrast, for a planar sheet pile or unplugged open-ended pile, the far-field soil displacements at x/L depend only on the wall thickness. The proposed analyses show favorable agreement with data from a variety of available sources including field measurements of building movements caused by installation of large pile groups; uplift of a pile caused by driving of an adjacent pile within a group; and spatial distributions of ground movements caused by installation of a single pile (both cylindrical closed-ended and sheet pile wall), including a particularly detailed set of measurements in a large laboratory calibration chamber. The comparisons show that the proposed analysis is capable of reliably predicting the deformations within the soil mass but generally underestimates the vertical heave measured at the ground surface. Further investigation suggests that this discrepancy may be related to the occurrence of radial cracks observed at the ground surface during pile installation and is consistent with tensile horizontal strains computed in the SSPM analyses.

Earthquake response analysis of the Hualien soil-structure interaction system based on updated soil properties using forced vibration test data

This paper presents results of the earthquake response analysis on a large-scale seismic test (LSST) structure which was built at Hualien in Taiwan for an international cooperative research project. The analysis is carried out using a computer program which has been developed based on axisymmetric finite element method incorporating dynamic infinite elements for far-field soil region and a substructured wave input technique. The non-linear behaviour of the soil medium is taken into account using an iterative equivalent linearization procedure. Two sets of the soil and structural properties, namely the unified and the FVT-correlated models, are utilized as the initial linear values. The unified model was provided by a group of experts in charge of the geotechnical experiments, and the correlated model was obtained through a system identification procedure using the forced vibration test (FVT) results by the present authors. Three components of ground accelerations are artificially generated through an averaging process of the Fourier amplitude spectra of the ground accelerations measured near the test structure, and they are used as the control input motions for the earthquake analysis. It has been found that the earthquake responses predicted using the generated control motions and with the FVT-correlated model as the initial linear properties in the equivalent linearization procedure compare very well with the observed responses. Copyright © 2001 John Wiley & Sons, Ltd.

A simplified procedure for formulation of soil-structure interaction problems

A soil-structure interaction system can be typically divided into regions such as the structure, the near field soil and the far-field soil. To analyze the behavior of the structure and the near-field soil, which is of most interest to practicing engineers, one has to determine beforehand the input seismic forces exerted by the source mechanism which are transmitted through the far-field soil. Traditional approaches for calculating the boundary forces often involve unnecessary tedious procedures. In this paper, it will be demonstrated that the condensation technique well known in structural mechanics can in reality be employed to formulate the soil-structure interaction problems in a rather straightforward manner. Such an approach has the advantage that the entire procedure involved is self-explanatory, which requires the minimum amount of operations.

Soil–pile–cap static interaction analysis by finite and infinite elements

A direct analysis of pile–soil static interaction by the combination of finite and infinite elements is presented. The pile and the near-field soil medium are modelled by finite elements, whereas the far-field soil medium is modelled by mapped infinite elements. Axially loaded single piles and single piles with caps subject to monotonic loading are investigated. The soil is assumed to be either elastic or elastic–perfect plastic. A weak zone is introduced around the pile to approximately account for slip between the soil and the pile. Numerical results show that this approach is quite efficient and versatile for various soil–structure interaction problems. Key words: pile, soil, cap, elastoplasticity, finite element, infinite element.