Showing papers on "Soil structure interaction published in 2014"

Centrifuge Modeling of Soil-Structure Interaction in Energy Foundations

Abstract: This study presents a centrifuge modeling approach to characterize the transient thermomechanical response of energy foundations during heating-cooling cycles to provide data for calibration and validation of soil-structure interaction models. This study focuses on the response of a scale-model energy foundation installed in an unsaturated silt layer with end-bearing boundary conditions. The foundation response was assessed using embedded strain gauges and thermocouples. Other variables monitored include foundation head displacements, soil surface displacements, and changes in temperature and volumetric water content in the unsaturated silt at different depths and radial locations. Measurements during the initial heating process indicate that the thermal axial stress is greater near the toe of the foundation as a result of the restraint associated with mobilization of side shear resistance along the length of the foundation. The thermal axial strains were close to the free-expansion thermal strain...

Liquefaction-induced building movements

Abstract: Liquefaction or cyclic softening from earthquake shaking have caused significant damage of buildings with shallow foundations. In recent earthquakes, buildings have punched into, tilted excessively, and slid laterally on liquefied/softened ground. The state-of-the-practice still largely involves estimating building settlement using empirical procedures developed to calculate post-liquefaction, one-dimensional, consolidation settlement in the “free-field” away from buildings. Performance-based earthquake engineering requires improved procedures, because these free-field analyses cannot possibly capture shear-induced and localized volumetric-induced deformations in the soil underneath shallow foundations. Recent physical and numerical modeling has provided useful insights into this problem. Centrifuge tests revealed that much of the building movement occurs during earthquake strong shaking, and its rate is dependent on the shaking intensity rate. Additionally, shear strains due to shaking-induced ratcheting of the buildings into the softened soil and volumetric strains due to localized drainage in response to high transient hydraulic gradients are important effects that are not captured in current procedures. Nonlinear effective stress analyses can capture the soil and building responses reasonably well and provide valuable insights. Based on these studies, recommendations for estimating liquefaction-induced movements of buildings with shallow foundations are made.

Suction caisson foundations for offshore wind turbines subjected to wave and earthquake loading: effect of soil–foundation interface

Abstract: The response of wind turbines founded on suction caissons and subjected to lateral monotonic, cyclic and earthquake loading is studied with due consideration of the role of soil–sidewall adhesion, using non-linear three-dimensional finite-element analyses. In the case of monotonic and slow cyclic lateral loading it is shown that imperfect interface bonding could reduce the moment capacity and may lead to foundation detachment or even uplifting in the case of shallowly embedded caissons. A preliminary comparison of two caisson alternatives has shown that increasing the caisson diameter while maintaining the embedment ratio is more efficient in terms of material resources than increasing the skirt length while keeping the diameter constant. The second part of the study evaluates the response of a soil–foundation–wind turbine interacting system subjected to earthquake shaking. Contrary to an often prevailing impression that seismic effects are insignificant, apparently originating from evaluating the seismic...

Tunnelling-induced deformation and damage on historical masonry structures

Abstract: The analysis of deformation and damage mechanisms induced by shallow tunnelling on masonry structures is carried out using an integrated, geotechnical and structural, numerical approach based on two-dimensional finite-element analyses. The masonry construction, schematised as a block structure with periodic texture, is regarded at a macroscopic scale as a homogenised anisotropic medium. The overall mechanical properties display anisotropy and singularities in the yield surface, arising from the discrete nature of the block structure and the geometrical arrangement of the blocks. The soil is modelled by means of a linear elastic-perfectly plastic model. The numerical analyses are performed assuming plane strain and plane stress conditions for the soil and the masonry structure, respectively. A displacement-controlled technique is adopted to simulate the tunnel construction, which produces settlement troughs in agreement with the empirical Gaussian predictions at different volume losses under free-field con...

Centrifuge model test study on pile reinforcement behavior of cohesive soil slopes under earthquake conditions

Abstract: In this study, dynamic centrifuge model tests were conducted to investigate the dynamic response of cohesive soil slopes with the use of stabilizing piles during an earthquake. The behavior of the pile reinforcement was analyzed based on the obtained deformation over the entire slope through image-based measurement, and the behavior of the slope was compared to that of an unreinforced slope. The piles significantly increased the stability of the slope and reduced its deformation during an earthquake. The bending moment of the piles exhibited a nearly triangular distribution due to the earthquake. The acceleration response of the slope increased with increasing elevation, and the displacement accumulated apparently irreversibly over the course of the earthquake. The piles significantly affected the deformation of the slope in a certain area, the boundary of which was defined using a continuous surface. A strain analysis of the slope demonstrated that the piles had a significant effect on the reduction in the deformation of the slope in their vicinities, and this effect expanded upward along the slope and arrested the possible slip surface that would have occurred in an unreinforced slope. Several influencing factors were simulated in the tests, and observation of these factors demonstrated that the dynamic response of the pile-reinforced slope was affected by the pile spacing, pile location, slope gradient, and input earthquake to varying extent.

Fully Nonlinear versus Equivalent Linear Computation Method for Seismic Analysis of Midrise Buildings on Soft Soils

Abstract: In this study, the accuracy of a fully nonlinear method against an equivalent linear method for dynamic analysis of soil-structure interaction is investigated comparing the predicted results of both numerical procedures. Three structural models, including 5-story, 10-story, and 15-story buildings, are simulated in conjunction with two soil types with shear-wave velocities less than 600 m/s. The aforementioned frames were analyzed under three different conditions: (1) fixed-base model performing conventional time history dynamic analysis under the influence of earthquake records, (2) flexible-base model (considering full soil-structure interaction) conducting equivalent linear dynamic analysis of soil-structure interaction under seismic loads, and (3) flexible-base model performing fully nonlinear dynamic analysis of soil-structure interaction under the influence of earthquake records. The results of these three cases in terms of average lateral story deflections and interstory drifts are determine...

Centrifuge modeling of rocking‐isolated inelastic RC bridge piers

Abstract: Experimental proof is provided of an unconventional seismic design concept, which is based on deliberately underdesigning shallow foundations to promote intense rocking oscillations and thereby to dramatically improve the seismic resilience of structures. Termed rocking isolation, this new seismic design philosophy is investigated through a series of dynamic centrifuge experiments on properly scaled models of a modern reinforced concrete (RC) bridge pier. The experimental method reproduces the nonlinear and inelastic response of both the soil-footing interface and the structure. To this end, a novel scale model RC (1:50 scale) that simulates reasonably well the elastic response and the failure of prototype RC elements is utilized, along with realistic representation of the soil behavior in a geotechnical centrifuge. A variety of seismic ground motions are considered as excitations. They result in consistent demonstrably beneficial performance of the rocking-isolated pier in comparison with the one designed conventionally. Seismic demand is reduced in terms of both inertial load and deck drift. Furthermore, foundation uplifting has a self-centering potential, whereas soil yielding is shown to provide a particularly effective energy dissipation mechanism, exhibiting significant resistance to cumulative damage. Thanks to such mechanisms, the rocking pier survived, with no signs of structural distress, a deleterious sequence of seismic motions that caused collapse of the conventionally designed pier.

Centrifuge Modeling of the Seismic Performance of Pile-Reinforced Slopes

Abstract: An extensive program of centrifuge testing was conducted to quantify the improvements to seismic slope performance that can be achieved by installing a row of discretely spaced vertical precast RC piles. A key novelty of the work presented is the use of recently developed microreinforced concrete to produce realistically damageable model piles. Pile-reinforced slopes are a good example of a problem in which relative soil-pile strength is important in determining whether the soil or pile yields first, and in which the performance of a slope with structurally damaged piles may be of interest. The new model RC allows these factors to be properly accounted for in a reduced-scale physical model for the first time. Two different reinforcement layouts were considered, representing (1) a section specifically detailed to carry the bending moments induced by the slipping soil mass, and (2) a nominally reinforced section with low moment capacity. These were supported by further tests on conventional elastic ...

Numerical and Experimental Investigations on Seismic Response of Building Frames under Influence of Soil-Structure Interaction

Abstract: In this study, an enhanced numerical soil-structure model has been developed which treats the behaviour of soil and structure with equal rigour. The proposed numerical soil-structure model has been verified and validated by performing experimental shaking table tests. To achieve this goal, a series of experimental shaking table tests were performed on the physical fixed based (structure directly fixed on top of the shaking table) and flexible base (considering soil and structure) models under the influence of four scaled earthquake acceleration records and the results were measured. Comparing the experimental results with the numerical analysis predictions, it is noted that the numerical predictions and laboratory measurements are in a good agreement. Thus, the proposed numerical soil-structure model is a valid and qualified method of simulation with sufficient accuracy which can be employed for further numerical soil-structure interaction investigation studies. Based on the predicted and observed values ...

Earthquake-induced bending moment in fixed-head piles in soft clay

Abstract: This paper examines seismic effects on fixed-head, end-bearing piles installed through soft clay, using centrifuge and numerical modelling. The numerical analyses were conducted using a non-linear hysteretic, stiffness-degrading model for the clay and were validated using centrifuge data. Numerical analyses were used to extend the range of soil, pile and ground motion parameters which could not be studied in the centrifuge. Based on the centrifuge and numerical results, a framework for estimating earthquake-induced bending moments in fixed-head piles was established, which takes into account superstructural mass, ground motion and, in an approximate way, non-linear stress–strain behaviour. In this framework, pile response is sub-divided into two kinds, namely, ‘stiff' and ‘flexible', based on the notion of an active length over which significant bending moment is developed. A relationship for the active length is developed based on regression of the centrifuge and numerical data. If the pile length is sho...

Dynamic stiffness of pile in a layered elastic continuum

Abstract: A set of formulas for the dynamic stiffness of a pile (spring and dashpot coefficients) to use in inertial interaction analysis is proposed, utilising elastodynamic solutions. The method is based on solving a Lagrangian system of coupled equations for the pile and the soil motions for a range of vibration frequencies and also by considering the vertical, radial and angular stresses on the pile–soil interface. The solution extensively uses Bessel functions of the second kind and results are compared with finite-element models and field pile load tests. A dimensionless frequency related to the well-known active length of pile is proposed to separate inertial and kinematic interactions. A formula is also proposed for estimation of the active length of a pile in a two-layered soil. A specific depth is introduced beyond which soil layering does not have any appreciable effects on dynamic stiffness. It is commonly (rather arbitrarily) assumed that the first natural frequency of soil strata differentiates radiat...

Soil-structure interaction vs Site effect for seismic design of tall buildings on soft soil

Abstract: In this study, in order to evaluate adequacy of considering local site effect, excluding soil-structure interaction (SSI) effects in inelastic dynamic analysis and design of mid-rise moment resisting building frames, three structural models including 5, 10, and 15 storey buildings are simulated in conjunction with two soil types with the shear wave velocities less than 600 m/s, representing soil classes De and Ee according to the classification of AS1170.4-2007 (Earthquake actions in Australia) having 30 m bedrock depth. Structural sections of the selected frames were designed according to AS3600:2009 (Australian Standard for Concrete Structures) after undertaking inelastic dynamic analysis under the influence of four different earthquake ground motions. Then the above mentioned frames were analysed under three different boundary conditions: (i) fixed base under direct influence of earthquake records; (ii) fixed base considering local site effect modifying the earthquake record only; and (iii) flexible-base (considering full soil-structure interaction). The results of the analyses in terms of base shears and structural drifts for the above mentioned boundary conditions are compared and discussed. It is concluded that the conventional inelastic design procedure by only including the local site effect excluding SSI cannot adequately guarantee the structural safety for mid-rise moment resisting buildings higher than 5 storeys resting on soft soil deposits.

Behavior of a Pressure-Grouted Soil-Cement Interface in Direct Shear Tests

Abstract: An interface between compacted soil and a structure is commonly encountered in various geotechnical engineering projects, e.g., soil nails, retaining walls, shallow foundations, pile foundations, and so on. The interface strength depends on the way the soil-structure interface is formed. A cast-in-situ interface is very common in many geotechnical projects. This kind of interface is formed by placing concrete/cement grout over the prepared soil surface. The cement part can be formed over a prepared soil surface in two ways: (1) by normal gravity grouting and (2) by pressure grouting. In this study, a series of interface direct-shear tests was performed between compacted, completely decomposed granite (CDG) soil and cement grout under saturated conditions with different grouting pressures and normal stresses. The behaviors of the shear-stress–displacement curves of the soil-cement interface are similar to those of CDG soil. Grouting pressure and normal stress have influence on the behavior of the s...

An empirical relationship to determine lateral seismic response of mid-rise building frames under influence of soil–structure interaction

Abstract: SUMMARY In this study, to determine the elastic and inelastic structural responses of mid-rise building frames under the influence of soil–structure interaction, three types of mid-rise moment-resisting building frames, including 5-storey, 10-storey and 15-storey buildings are selected. In addition, three soil types with the shear wave velocities less than 600 m/s, representing soil classes Ce, De and Ee according to AS 1170.4–2007 (Earthquake action in Australia, Australian Standards), having three bedrock depths of 10 m, 20 m and 30 m are adopted. The structural sections are designed after conducting nonlinear time history analysis, on the basis of both elastic method and inelastic procedure considering elastic-perfectly plastic behaviour of structural elements. The frame sections are modelled and analysed, employing finite difference method adopting FLAC2D software under two different boundary conditions: (a) fixed base (no soil–structure interaction) and (b) considering soil–structure interaction. Fully nonlinear dynamic analyses under the influence of different earthquake records are conducted, and the results in terms of the maximum lateral displacements and base shears for the above mentioned boundary conditions for both elastic and inelastic behaviours of the structural models are obtained, compared and discussed. With the results, a comprehensive empirical relationship is proposed to determine the lateral displacements of the mid-rise moment-resisting building frames under earthquake and the influence of soil–structure interaction. Copyright © 2012 John Wiley & Sons, Ltd.

Influence of soil–structure interaction on seismic collapse resistance of super-tall buildings

Abstract: Numerous field tests indicate that the soil–structure interaction (SSI) has a significant impact on the dynamic characteristics of super-tall buildings, which may lead to unexpected structural seismic responses and/or failure. Taking the Shanghai Tower with a total height of 632 m as the research object, the substructure approach is used to simulate the SSI effect on the seismic responses of Shanghai Tower. The refined finite element (FE) model of the superstructure of Shanghai Tower and the simplified analytical model of the foundation and adjacent soil are established. Subsequently, the collapse process of Shanghai Tower taking into account the SSI is predicted, as well as its final collapse mechanism. The influences of the SSI on the collapse resistance capacity and failure sequences are discussed. The results indicate that, when considering the SSI, the fundamental period of Shanghai Tower has been extended significantly, and the collapse margin ratio has been improved, with a corresponding decrease of the seismic demand. In addition, the SSI has some impact on the failure sequences of Shanghai Tower subjected to extreme earthquakes, but a negligible impact on the final failure modes.

On the resonance effect by dynamic soil–structure interaction: a revelation study

Abstract: The present study makes an attempt to investigate the soil–structure resonance effects on a structure based on dynamic soil–structure interaction (SSI) methodology by direct method configuration using 2D finite element method (FEM). The investigation has been focused on the numerical application for the four soil–structure models particularly adjusted to be in resonance. These models have been established by single homogenous soil layers with alternating thicknesses of 0, 25, 50, 75 m and shear wave velocities of 300, 600, 900 m/s-a midrise reinforced concrete structure with a six-story and a three-bay that rests on the ground surface with the corresponding width of 1,400 m. The substructure has been modeled by plane strain. A common strong ground motion record, 1940 El Centro Earthquake, has been used as the dynamic excitation of time history analysis, and the amplitudes, shear forces and moments affecting on the structure have been computed under resonance. The applicability and accuracy of the FEM modeling to the fundamental period of soils have been confirmed by the site response analysis of SHAKE. The results indicate that the resonance effect on the structure becomes prominent by soil amplification with the increased soil layer thickness. Even though the soil layer has good engineering characteristics, the ground story of the structure under resonance is found to suffer from the larger soil layer thicknesses. The rate of increment in shear forces is more pronounced on midstory of the structure, which may contribute to the explanation of the heavily damage on the midrise buildings subjected to earthquake. Presumably, the estimated moment ratios could represent the factor of safeties that are excessively high due to the resonance condition. The findings obtained in this study clearly demonstrate the importance of the resonance effect of SSI on the structure and can be beneficial for gaining an insight into code provisions against resonance.

Nonlinear Soil–Foundation–Structure and Structure–Soil–Structure Interaction: Centrifuge Test Observations

Abstract: Utilizing a pair of building–foundation models placed in strategic configurations, two centrifuge tests were designed to evaluate nonlinear soil–foundation–structure interaction (SFSI) and structure–soil–structure interaction (SSSI) effects. The models were designed to represent a low-rise inelastic frame building founded on individual spread footings and a midrise elastic shear-wall building founded on a mat foundation designed to rock during seismic excitation. Four experimental cases are considered: (1) the baseline response of the low-rise frame structure; (2) the response of two structures placed adjacent to each other along the direction of earthquake shaking; (3) the response of two structures placed adjacent to each other perpendicular to the direction of earthquake shaking; and (4) the response of the frame structure when two wall structures are placed adjacent to it, such that both effects are promoted. Using spectral-analysis methods, it is demonstrated that the baseline frame structure...