Assessment and modeling of embankment participation in the seismic response of integral abutment bridges

The dynamic response and seismic performance of bridges may be appreciably affected by numerous contributing factors, with soil–structure interaction being the dominant exogenous influence. The most familiar form is the so-called soil–pile interaction, but embankment–abutment interaction is also documented through field observations and analytical investigations, particularly evident in integral R.C. bridges. Recent studies have shown that this form of interaction may significantly alter the bridge response and should be taken into account during design and assessment, especially in the case of typical highway overcrossings that have abutments supported on earth embankments. In light of this emerging problem and in order to facilitate quantitative estimates of the interaction effects, the question of appropriate modeling and seismic assessment of R.C. integral bridges is the main object of the present paper. Based on already established procedures to account for soil–structure interaction, a new approach is proposed to model the contribution of the embankment, the bent and the abutments to the overall bridge response. Furthermore, the capacity curve of the entire bridge system is evaluated through the implementation of Incremental Dynamic Analysis (IDA), therefore allowing for seismic assessment of the complex superstructure–foundation system with well established displacement based procedures. Using as a benchmark case two typical instrumented U.S. highway bridges located in California, the proposed method is implemented and provided results from this analysis are correlated successfully with available field data. Results obtained from the analysis indicate excessive displacement demands for the entire bridge–embankment system owing to the embankment contribution and the soil degradation under increasing shear strains. Furthermore, seismic performance is strongly related to the central bent deformation capacity, with soil–pile interaction effects being of critical importance.     

Seismic response of an urban bridge-support system in soft clay

The numerical simulation of an instrumented urban bridge support and its foundation system was conducted. The bridge works as a deck in a surface subway station and was built 12 yr ago in the so-called Lake Zone in Mexico City, where very soft clays, exhibiting low shear strength and high compressibility prevail. Since the beginning of its construction, pile loads, soil–raft contact pressures and the overall response of the foundation system have been monitored. Within this period, several earthquakes have occurred. Thus, an extensive database of accelerations, pore pressures and load histories have been recorded. In particular, this paper focuses on the bridge response observed during two moderated intensity events, the 2004, 6.3 M_w, Guerrero Coast and the 1999, 7.0 M_w, Tehuacan earthquakes. Finite element models were developed to reproduce the measured responses and to assess the soil–foundation-support performance for long-term conditions, including the effects of potential changes in the dynamic soil properties due to regional subsidence. The computed dynamic responses obtained with the simulation for the free field and structure compares fairly well with the recordings.     

Centrifuge modeling of seismic behavior of a slope in liquefiable soil

Two centrifuge tests were designed to improve the understanding the response of liquefied sandy slopes beyond initial liquefaction. A distinctive dilative behavior of the soil was observed near the slope where static shear stresses are present. The corresponding drops in the piezometric records and simultaneous negative upslope spikes in the acceleration records were measured in the transducer raw data. This dilative response became stronger as the input acceleration increased and tends to limit the downslope accumulation and thus reducing the permanent lateral displacements. Therefore, the maximum permanent displacement was smaller in the model with the larger input motion, because it developed a stronger dilative response. The dilative response was not observed away from the slope, where no static shear stresses are present.     

Centrifuge modeling of a dry sandy slope response to earthquake loading

This paper presents results of a series of centrifuge models of dry, sandy slopes excited by earthquakes and cyclic waves under 50g centrifugal acceleration to investigate the dynamic performance of slopes. Test results of four model slopes with different profiles stimulated by the adjusted El Centro earthquakes with various peak accelerations reveal the response amplification mechanism of the slope. By calculating the response spectra of recorded acceleration time histories, it was observed that the different frequency contents of the input event were amplified to different degrees. The model slope showed a completely different response under the cyclic wave with a constant frequency and amplitude in that the spectral amplification factor curves had no prominent peak values. These findings suggest that dynamic centrifuge tests excited with a real ground motion are able to better reflect the response characteristics of a slope rather than the tests with cyclic loading.     

Effect of non-linear soil–structure interaction on seismic response of tall slender structures

Some structures may be very massive and may have to be located on relatively soft soil. In such cases, the soil adjacent to the structure behaves in a non-linear fashion and affects the response of the structure to the dynamic loading. An approximate hybrid approach to analyse soil–structure systems accounting for soil non-linearities has been developed in this paper. The approach combines the consistent infinitesimal finite-element cell method (CIFECM) and the finite-element method (FEM). The CIFECM is employed to model the non-linear (near-field) zone of the soil supporting the structure as a series of bounded media. The material properties of the bounded media are selected so that they are compatible with the average effective strains over the whole bounded medium during the excitation. The linear zone of soil away from the foundation, the far-field, is modelled as an unbounded medium using the CIFECM for unbounded media. The structure itself is represented by the FEM. The proposed method is used to model the dynamic response of a one-mass structure and a TV-tower supported on a homogenous stratum and excited by an earthquake. It was found that the secondary soil non-linearity might increase or decrease the base forces of tall slender structures depending on the type of structure, frequency content of the input motion and the dynamic properties of the near-field soil.     

Soil-structure interaction effects on an instrumented building

The paper highlights the use of fem and bi-directional lumped-mass-storey-stiffness numerical models for the study of the soil–structure interaction (ssi) effects on an instrumented building. Data on the structural response have been obtained through the project for seismic instrumentation of a 16-storey r/c cast-in-place dwelling building (Chisinau, Republic of Moldova) during a series of earthquakes (Gutenberg–Richter M _GR = 5.0−6.7). The effect of soil–structure interaction is clearly observed comparing the responses recorded on foundation and free-field. ssi becomes more pronounced for higher level of ground shaking amplifying the natural period of the structure and slightly suppressing high frequences on the foundation in comparison with the free-field motion.     

Centrifuge modelling for evaluation of seismic behaviour of stone masonry structure

Many surviving ancient monuments are freestanding stone masonry structures, which appear to be vulnerable to horizontal dynamic loads such as earthquakes. However, such structures have stood for thousands of years despite numerous historic earthquakes. This study proposes a scaled-down dynamic centrifuge modelling test to study how these masonry structures resist seismic loading. The test is proposed for seismic risk assessments to evaluate risk of damage from a future seismic event. The seismic behaviour of a 3-storey, freestanding stone block structure has been modelled and tested within a centrifuge. Models were made at 3 different scales and dynamic tests were conducted using different centrifugal acceleration fields so that the behaviours could be transformed to an equivalent full-scale prototype and compared. Data from 2 earthquakes and a sweeping signal were used to simulate the effects of earthquake ground motion within the centrifuge. The acceleration and frequency responses at each storey height of the model were recorded in different centrifugal acceleration fields. Similar behaviours appeared when the results of the small-scale models were transformed to a full-size prototype scale. This confirms that the seismic behaviour of stone masonry structures can be predicted using scaled-down models.     

A centrifuge study of the influence of site response,relative stiffness,and kinematic constraints on the seismic performance of buried reservoir structures

The seismic performance of underground reservoir structures depends on the properties of the structure, soil, and ground motion as well as the kinematic constraints imposed on the structure. A series of four centrifuge experiments were performed to evaluate the influence of site response, structural stiffness, base fixity, and excitation frequency on the performance of relatively stiff reservoir structures buried in dry, medium-dense sand. The magnitude of seismic thrust increased and the distribution of seismic earth pressures changed from approximately triangular to parabolic with increasing structural stiffness. Heavier and stiffer structures also experienced increased rocking and reduced flexural deflection. Fixing the base of the structure amplified the magnitude of acceleration, seismic earth pressure, and bending strain compared to tests where the structure was free to translate laterally, settle, or rotate atop a soil layer. The frequency content of transient tilt, acceleration, dynamic thrust, and bending strain measured on the structure was strongly influenced by that of the base motion and site response, but was unaffected by the fundamental frequency of the buried structure (f_structure). None of the available simplified procedures could capture the distribution and magnitude of seismic earth pressures experienced by this class of underground structures. The insight from this experimental study is aimed to help validate analytical and numerical methods used in the seismic design of reservoir structures.     

Experimental p-y loops for estimating seismic soil-pile interaction

Seismic soil-pile interaction is evaluated in this study based on back-calculated p-y loops constructed from sampled data of pile bending moments. Fundamental properties of p-y loops are implemented to derive distributed springs and dashpots, thereby quantifying soil-pile interaction in the realm of a Beam on Dynamic Winkler Foundation modeling. The procedure is validated by means of well-documented centrifuge tests of a single pile supported structure founded on a two-layer soil profile that comprises of soft clay overlying dense sand. Two shaking levels of a real earthquake motion applied at the base of the soil profile were examined and the generated seismic p-y loops were compared to cyclic p-y curves commonly used in pile design practice. The results demonstrate the strong influence of intensity of the input motion on seismic p-y loops while cyclic p-y curves established for soft clays tend to overestimate soil stiffness under strong excitation. Typical sets of recorded and computed structural response are presented, denoting the ability of the BDWF model related to p-y loops in reproducing adequately fundamental aspects of seismic soil-pile interaction.     

Behaviour of deep immersed tunnel under combined normal fault rupture deformation and subsequent seismic shaking

Immersed tunnels are particularly sensitive to tensile and compressive deformations such as those imposed by a normal seismogenic fault rupturing underneath, and those generated by the dynamic response due to seismic waves. The paper investigates the response of a future 70 m deep immersed tunnel to the consecutive action of a major normal fault rupturing in an earthquake occurring in the basement rock underneath the tunnel, and a subsequent strong excitation from a different large-magnitude seismic event that may occur years later. Non-linear finite elements model the quasi-static fault rupture propagation through the thick soil deposit overlying the bedrock and the ensuing interaction of the rupture with the immersed tunnel. It is shown that despite imposed bedrock offset of 2 m, net tension or excessive compression between tunnel segments could be avoided with a suitable design of the joint gaskets. Then, the already deformed (“injured”) structure is subjected to strong asynchronous seismic shaking. The thick-walled tunnel is modelled as a 3-D massive flexural beam connected to the soil through properly-calibrated nonlinear interaction springs and dashpots, the supports of which are subjected to the free-field acceleration time histories. The latter, obtained with 1-D wave propagation analysis, are then modified to account for wave passage effects. The joints between tunnel segments are modeled with special non-linear hyper-elastic elements, properly accounting for their 7-bar longitudinal hydrostatic pre-stressing. Sliding is captured with special gap elements. The effect of segment length and joint properties is explored parametrically. A fascinating conclusion emerges in all analysed cases for the joints between segments that were differentially deformed after the quasi-static fault rupture: upon subsequent very strong seismic shaking, overstressed joints de-compress and understressed joints re-compress—a “healing” process that leads to a more uniform deformation profile along the tunnel. This is particularly beneficial for the precariously de-compressed joint gaskets. Hence, the safety of the immersed tunnel improves with “subsequent” strong seismic shaking!     

Centrifuge modeling of offshore wind foundations under earthquake loading

The construction of large offshore wind turbines in seismic active regions has great demand on the design of foundations. The occurrence of soil liquefaction under seismic motion will affect the stability of the foundations and consequently the operation of the turbines. In this study, a group of earthquake centrifuge tests was performed on wind turbine models with gravity and monopile foundations, respectively, to exam their seismic response. It was found that the seismic behavior of models was quite different in the dry or saturated conditions. Each type of foundation exhibited distinct response to the earthquake loading, especially in the offshore environment. In the supplementary tests, several remediation methods were evaluated in order to mitigate the relatively large lateral displacement of pile foundation (by fixed-end pile and multi-pile foundation) and excessive settlement of gravity foundation (by densification, stone column, and cementation techniques).     

Effect of soil parameter uncertainty on seismic demand of low-rise steel buildings on dense silty sand

Focusing on low-rise steel buildings supported by shallow isolated foundations on dense silty sand, this study demonstrates the effect of uncertainty in soil parameters on seismic response of structures. Considering a set of 20 ground motions representing 10% in 50 years hazard level and concentrating on peak base moment, base shear and interstory drift as the demand variables of interest, it is found that uncertainty in soil parameters may result in significant response variability of the structures, especially when vertical factor of safety is low and the structure is relatively stiff. Uncertainty in friction angle results in significant variability of the peak base moment and base shear, while peak interstory drift ratio is found to be virtually unaffected by uncertainty in soil parameters. It is also found that a linear soil–structure-interaction (SSI) model will not be able to predict such response variability under these set of ground motions.     

Three-dimensional nonlinear analysis for seismic soil–pile-structure interaction

A three-dimensional method of analysis is presented for the seismic response of structures constructed on pile foundations. An analysis is formulated in the time domain and the effects of material nonlinearity of soil on the seismic response are investigated. A subsystem model consisting of a structure subsystem and a pile-foundation subsystem is used. Seismic response of the system is found using a successive-coupling incremental solution scheme. Both subsystems are assumed to be coupled at each time step. Material nonlinearity is accounted for by incorporating an advanced plasticity-based soil model, HiSS, in the finite element formulation. Both single piles and pile groups are considered and the effects of kinematic and inertial interaction on seismic response are investigated while considering harmonic and transient excitations. It is seen that nonlinearity significantly affects seismic response of pile foundations as well as that of structures. Effects of nonlinearity on response are dependent on the frequency of excitation with nonlinearity causing an increase in response at low frequencies of excitation.     

Foundation–structure systems over a rupturing normal fault: Part II. Analysis of the Kocaeli case histories

Motivated by the observed (successful and unsuccessful) performance of numerous structures on top of, or immediately next to a normal fault that ruptured during the Kocaeli 1999 earthquake, this paper: (i) develops a two-step finite element methodology to study the propagation of a fault rupture through soil and its interplay with the foundation–structure system, denoted hereafter “Fault Rupture–Soil– Foundation–Structure Interaction” (FR–SFSI), (ii) provides validation of the developed methodology through successful Class “A” predictions of centrifuge model tests, and (iii) applies the centrifuge-validated methodology to study one-by-one the Kocaeli case histories of the first paper (Part I). It is shown that the presence of a structure on top of an outcropping fault may have a significant influence on the rupture path: with heavy structures founded on continuous and rigid foundations, the fault rupture diverts substantially and may avoid rupturing underneath the structure. The latter undergoes rigid body rotation, with its foundation sometimes loosing contact with the bearing soil, but in most cases retaining its structural integrity. In stark contrast, buildings on isolated footings and, perhaps surprisingly, piles exert a smaller diversion of the rupture which is thus likely to outcrop between the footings or pile caps; the latter may thus undergo devastating differential displacements. It is shown that structures in the vicinity of faults can be designed to survive significant dislocations. The “secret” of successful performance lies on the continuity, stiffness, and rigidity of the foundation.     

Centrifuge modeling of interaction between reverse faulting and tunnel

In this study, a series of centrifuge tests, modeling reverse fault rupture with 60° dip angle, were conducted in a dry sandy soil with a tunnel embedded in the soil layer. The test results showed that the tunnel and soil responses depended on the tunnel position, soil relative density and tunnel rigidity. Tunnels appeared be able to deviate the fault rupture path, while this deviation may be associated with significant rotation and displacement of the tunnel. However, a deeper tunnel was able to diffuse the shear deformation to a wider zone with an unsmooth surface displacement which may cause severe damage to the surface structures. Finally, the tunnel rotation, the location of the fault outcropping, the vertical displacement of the ground surface, the effect of tunnel rigidity on fault rupture path and surface displacement and the effect of soil relative density on fault–tunnel interaction were reported and discussed in this study.     

Simplified longitudinal seismic response of tunnels linings subjected to surface waves

This paper presents an engineering approach for analysing the longitudinal behaviour of tunnels subjected to earthquakes. The tunnel is modelled as a Timoshenko beam connected to the far soil by means of continuous elastic support (Winkler model). Seismic free-field inputs, such as those caused by surface waves travelling parallel to the tunnel axis, were imposed at the base of the springs of the Winkler model, generating bending moments and shear forces on the cross-sections of the tunnel. Closed-form expressions of the tunnel displacements, shear forces, and bending moments were determined at any tunnel section in terms of the seismic excitation, tunnel geometry and material properties, and subgrade reaction modulus of the soil. A dimensional analysis was carried out to ascertain directly the maximum tunnel displacement, bending moment, and shear force.     

Seismic response of retaining walls with cohesive backfill: Centrifuge model studies

Observations from recent earthquakes show that retaining structures with non-liquefiable backfills perform extremely well; in fact, damage or failures related to seismic earth pressures are rare. The seismic response of a 6-m-high braced basement and a 6-m free-standing cantilever wall retaining a compacted low plasticity clay was studied in a series of centrifuge tests. The models were built at a 1/36 scale and instrumented with accelerometers, strain gages and pressure sensors to monitor their response. The experimental data show that the seismic earth pressure on walls increases linearly with the free-field PGA and that the earth pressures increase approximately linearly with depth, where the resultant acts near 0.33 H above the footing as opposed to 0.5–0.6 H, which is suggested by most current design methods. The current data suggest that traditional limit equilibrium methods yield overly conservative earth pressures in areas with ground accelerations up to 0.4g.     

Centrifuge modeling of the behavior of caisson-type quay walls during earthquakes

A series of 2-D centrifuge modeling tests with an in-flight shaker were carried out in order to model both the deformation characteristics of backfill and the seismic responses of caisson-type walls embedded in soils with various permeabilities. The rotational and translational modes were found to be in phase or various degrees out of phase with each other for quay walls embedded in soils with varying permeabilities. The alternative pumping and suction processes in excess pore water pressure that are caused by a wall's vibrations increase the level of damage because large amounts of backfill are forcedly leaked into the sea. The test results show that the rotational mode makes the dominant contribution to the changes in excess pore water pressure and in the earth pressure in the deep layers of soil, but the translational mode makes the dominant contribution to these pressures in the shallow layers. The average shear wave velocities were found to decrease rapidly to values as low as 1/8th of the velocity measured at the beginning of shaking.     

Seismic soil–structure interaction of massive flexible strip-foundations embedded in layered soils by hybrid BEM–FEM

A study on the seismic response of massive flexible strip-foundations embedded in layered soils and subjected to seismic excitation is presented. Emphasis is placed on the investigation of the system response with the aid of a boundary element–finite element formulation proper for the treatment of such soil–structure interaction problems. In the formulation, the boundary element method (BEM) is employed to overcome the difficulties that arise from modeling the infinite soil domain, and the finite element method (FEM) is applied to model the embedded massive flexible strip-foundation. The numerical solution for the soil–foundation system is obtained by coupling the FEM with the BEM through compatibility and equilibrium conditions at the soil–foundation and soil layer interfaces. A parametric study is conducted to investigate the effects of foundation stiffness and embedment on the seismic response.     

Centrifuge modeling of load-deformation behavior of rocking shallow foundations

Shallow foundations supporting building structures might be loaded well into their nonlinear range during intense earthquake loading. The nonlinearity of the soil may act as an energy dissipation mechanism, potentially reducing shaking demands exerted on the building. This nonlinearity, however, may result in permanent deformations that also cause damage to the building. Five series of tests on a large centrifuge, including 40 models of shear wall footings, were performed to study the nonlinear load-deformation characteristics during cyclic and earthquake loading. Footing dimensions, depth of embedment, wall weight, initial static vertical factor of safety, soil density, and soil type (dry sand and saturated clay) were systematically varied. The moment capacity was not observed to degrade with cycling, but due to the deformed shape of the footing–soil interface and uplift associated with large rotations, stiffness degradation was observed. Permanent deformations beneath the footing continue to accumulate with the number of cycles of loading, though the rate of accumulation of settlement decreases as the footing embeds itself.