Large-scale Shaking Table Tests Research Articles

Nonlinear dynamic behaviour and seismic fragility analysis of irregular multi-span RC bridges

This paper investigates the nonlinear dynamic behaviour and failure probability of multi-span Reinforced Concrete (RC) bridges supported on piers of unequal heights. To this end, a three-dimensional nonlinear finite element model of RC bridges with substructure irregularity is developed. The model is verified against an available experimental data of a large-scale shake table test results of a benchmark irregular two-span RC bridge. Six hypothetical two-span irregular RC bridges with piers varied in height, and various superstructure mass-distribution conditions (equal and unequal) and a regular bridge layout (as a reference) are considered. Through Incremental Dynamic Analyses (IDAs), the seismic performance of selected bridge layouts is investigated at both local and global scales. Finally, the influence of bridge layout on seismic vulnerability of piers of varying heights is analysed. Results show that the unbalanced seismic displacement demand and failure probability of different bents of a multi-span irregular RC bridge significantly depend on the height of piers and their arrangement. Therefore, the typical presumption of shorter piers having a higher failure probability due to their higher seismic force absorption is not always the most possible failure mechanism.

Fragility Analysis of Helical Piles Supporting Bridge in Different Ground Conditions

This paper examines the seismic performance of a bridge–helical pile foundation based on the seismic fragility analysis, considering the element fragility of a coupled bridge–soil–foundation system. Nonlinear time history analyses were conducted using a finite-element modeling scheme that was validated using the results of large-scale shaking table tests of soil–piles–structure systems involving both liquefiable and nonliquefiable soils. Incremental dynamic analysis was conducted to generate the fragility curves for a two-span bridge supported on helical piles in nonliquefiable and liquefiable sites considering a suite of ground motions. The damage limit states were defined to describe the capacity of the bridge components. In total, 440 nonlinear time-history analyses were performed to evaluate the seismic demand of the helical piles and the bridge-reinforced concrete pier components, and the results were used to establish their fragility curves. The results revealed that the helical piles were the most fragile component in the nonliquefiable and liquefiable tests. The liquefiable soil could decrease the seismic demand on the column lateral deformation and increase the demand dispersion. On the other hand, the reinforced concrete pier exhibited a large drift response in the nonliquefiable soil, causing it to be more vulnerable to seismic hazards than in the case of liquefiable soil.

Experimental investigation of seismic performance of shield tunnel under near-field ground motion

The near-field ground motion is one of the challenges to underground structure in soft soil site. In order to study seismic performance of shield tunnel under near-field ground motion, a series of large-scale shaking table tests are conducted. A refined tunnel lining is developed to capture the extension/closure response of joints. The difference of seismic response of shield tunnel under near field ground and far field ground motion is compared and analyzed. The findings show that compared with far-field ground motion, the acceleration response of segmental lining, cross-sectional deformation, extension/closure of longitudinal joints, dynamic normal earth pressure, and strain on segmental lining are increased obviously under the action of near-field ground motion. The near-field ground motion significantly stimulates the response of the tunnel structure in a wider low frequency range. The adverse effect of near-field ground motion on shield tunnel in soft soil site is significantly greater than that of far-field ground motion.

Shaking table test and numerical research on seismic response of a CFST arch bridge for different soil grounds

Severe damages or failures of arch bridges on different soil grounds have been reported during previous strong earthquakes. The influence of different soil grounds on the seismic response of arch bridges is unclear. In this study, three large-scale shaking table tests were conducted to investigate the seismic response of a model bridge abutment of a concrete-filled steel tubular (CFST) arch bridge, which was set inside a model soil box at a reduced scale of 1/90. Grounds with three types of soil were simulated: silty clay, clay-gravel layered, and bare gravel soil. The accelerations and displacements of the model soils and the sliding displacement and settlement of the model bridge abutment were evaluated through the tests. The test results showed that the soil types significantly affected the seismic response of the soil and bridge abutment system, and the spectral characteristics of input waves significantly affected the acceleration amplitude ratio (AAR) of the bridge abutment; however, the changing trend of the AAR was usually related to the corresponding soil types. The soil grounds exhibited a remarkable filtering effect along the soil heights under seismic excitations, and the filtrations of the frequency components of the input waves for different soil grounds shows significant difference. Furthermore, the dynamic shear stress–strain curves showed that the silty clay soil remained in the nonlinear behavior development stage under strong seismic excitations, while the bare gravel soil remained in the linear behavior development stage. The energy dissipation capacity of the bare gravel soil ground could not be utilized. The test results in this study can be used for seismic risk assessment of different soil grounds, to guide the optimal seismic design of arch bridges for different soil grounds.

Experimental and analytical study on ground motion characteristics under structure cluster disturbance

AbstractSoil‐structure cluster dynamic interaction (SSCI) has a significant effect on structural responses as well as ground motions for soft soil, while investigations on the latter remain limited. In this paper, four test models, including the free field model, soil‐single structure model, soil‐equal height structure cluster model, and soil‐multiple height structure cluster model, were manufactured and investigated through large scale shaking table tests. The effect of structure numbers, layouts, structural dynamic characteristics and seismic spectrum characteristics on the ground motions was discussed. On this basis, finite element analysis and wavelet packet decomposition were employed to further investigate the evolution rule of cluster effect. The results manifested that, due to the structure cluster, the predominant period of the original free field could be elongated notably and earthquake response spectrum in the long period domain was amplified, especially under the excitation of seismic waves with abundant low‐frequency components. The influence of SSCI on ground motions is chiefly reflected in the variation of site predominant period and the soil damping during the vibration. In addition, the cluster effect on ground motion presents a monotonous degressive trend with the increase of the structural spacing, and its attenuation rate is negatively correlated with structural height, while the difference in ground motion due to different structure numbers becomes insignificant with the increase of structural spacing.

Seismic behaviour of granular slope under railway embankment in large-scale shaking table test

This study investigates shear zone development in a coarse-grained soil slope beneath a railway embankment under seismic action. Clarifying the dynamic stress and residual strain responses under different seismic excitations reveals the slope instability mechanism. A typical coarse-grained soil slope under construction of the Sichuan-Tibet Railway Railway was selected for large-scale shaking table model testing. The evolution of slope response parameters (dynamic displacement, acceleration, dynamic strain and residual strain, and dynamic earth pressure) was monitored. The seismic damage modes and resulting deformations of the embankment and slope were analysed. The evolution characteristics of a shear zone were described; the relationship among acceleration, dynamic stress, and dynamic strain was clarified; and the critical state of earthquake-induced slope instability was defined. The main conclusions are as follows: under seismic excitation, the slope slip surface forms mostly near the interface between the sand and breccia layers, and a weakness zone exists between the subgrade and slope surface. Under high-energy seismic excitation (i.e., PGA ≥ 0.4 g), positive peak acceleration is more suitable for assessing the elevation amplification effect. Considering the cumulative strain distribution contour maps, the growth characteristics in the dynamic stress peak area can describe the development of a shear slip zone. Acceleration amplification factors can reflect slope responsiveness, but the energy contained in the seismic waves determines deformation. When the ratio of dynamic to shear stress exceeds 2, the dynamic strain in the slope slip zone increases considerably with shear zone extension and slope instability. With increasing dynamic stress, the growth characteristics of residual strain exhibit stability, transition and failure states.

Shaking table tests on fault-crossing tunnels and aseismic effect of grouting

Tunnels crossing inactive faults may experience large differential deformations along their lengths during earthquakes, resulting in damage. Faults always have inferior material properties for historical geological reasons. Grouting is an auxiliary construction measure in fault-crossing tunnels and has the potential to be an economical and effective aseismic measure. However, the feasibility and seismic mitigation mechanisms of grouting in fault-crossing tunnels have not been sufficiently studied. In this study, large-scale shaking table tests were conducted to verify the aseismic effect of grouting in a tunnel crossing an inactive fault and to explore the seismic mitigation mechanism behind it. On this basis, verified numerical models were developed to determine the optimized design parameters. The test results indicate that grouting reduces the acceleration response of the tunnel within the fault and reduces the differences in the acceleration responses in the longitudinal direction of the tunnel induced by the fault. As grouting increases the shear strength and stiffness of the strata around the tunnel within the fault, the strain of the tunnel invert is effectively reduced by over 80%, and the potential damage of the invert uplift is reduced simultaneously. Meanwhile, grouting effectively reduces the diameter deformation of the tunnel within the fault and reduces the differences in tunnel deformation in different areas of the fault site. By reducing the resonance caused by the frequency components of the input seismic wave that is close to the natural frequency of the strata, which contributes most to the tunnel response, grouting effectively reduces the seismic responses of the fault-crossing tunnels. Increasing the stiffness of the grouting zone is an effective approach to improve its seismic mitigation performance. In the example of this study, adjusting the relative stiffness ratio to approximately 100 between the grouting zone and the fault resulted in a favorable condition for aseismic effects, which could be deemed to be a suitable parameter for grouting. The above results can provide a reference for the seismic design of mountain tunnels crossing inactive faults.

Shaking table test of the effect of an enclosure structure on the seismic performance of a prefabricated subway station

The enclosure structure is an indispensable part of the construction system of the prefabricated subway station structure (PSSS), and has a significant impact on the overall mechanics and deformation performance, soil-structure interaction, seismic performance and damage characteristics of the PSSS. In order to study the effect of the enclosure structure on the seismic response of the PSSS, a series of large-scale shaking table tests of a PSSS with and without an enclosure structure were carried out to study in detail the seismic response characteristics of the model system, such as macroscopic phenomena, acceleration, strain, and joint deformation. Then the mechanism of the enclosure structure in strengthening the seismic performance of the PSSS was revealed. Finally, the seismic damage evolution mechanism of the PSSS with the enclosure structure was explained systematically. The results showed that the PSSS with the enclosure structure had good seismic performance, and the collaborative seismic design of the enclosure structure and PSSS should be paid attention to. The acceleration response of the foundation soil and PSSS increased due to the existence of the enclosure structure, but the enclosure structure effectively improved the deformation resistance and joint stiffness of the PSSS and reduced the local stress concentration of the structure. The enclosure structure and the prefabricated structure together constituted a complete structural system to resist earthquake action, and could effectively reduce the seismic damage to the PSSS. Under an extreme earthquake, the PSSS degenerated into a three-hinged arch structure, and still had a certain self-balancing performance, which made it less prone to serious damage such as overall collapse. The research conclusions could provide a scientific reference for the design and construction of PSSS.

Failure Mechanism and Dynamic Response Characteristics of Loess Slopes Under the Effects of Earthquake and Groundwater

A large-scale shaking table test was conducted with simulated earthquakes and groundwater to study the dynamic response and instability mechanism of a loess slope with a saturated bottom. By examining the acceleration, dynamic pore water pressure, corresponding soil pressure and observed macroscopic failure behaviour of the slope under different shaking intensities, the dynamic responses of acceleration and pore water pressure of the loess slope were analysed. By comparing the time history curve of acceleration with the time history curves of pore water pressure and surrounding soil pressure, we found that the instability failure of the slope was first due to the continuous vibration that caused the local structure of the slope soil to be damaged, forming locally saturated soil and the rise in the groundwater table during shaking. Then, the saturated soil liquefied under the action of strong vibration, and the liquefied soil mass slid under the action of a high-intensity earthquake force. Finally, the acceleration and spectral characteristics of the pore water pressure changed sharply with a shift in the predominant frequency at the liquefied locations under a high shaking intensity. The significant changes in dynamic pore water pressure mainly occurred when the shaking components in the low-frequency bands below 1 Hz. This implied that the shift in the frequency mainly resulted from the damage in the structure of the loess slope after high-intensity shaking. These results help us better understand the triggering mechanism and failure mechanism of saturated loess slopes during the Minxian-Zhangxian earthquake.

Failure mechanism investigation on loess–mudstone landslides based on the Hilbert–Huang transform method using a large-scale shaking table test

Loess–mudstone landslides occur easily because of earthquakes or rainfall; they account for a large number of landslides in the loess area of China. The Buzi landslide was one of the two largest landslides triggered by the 2013 Minxian–Zhangxian Ms. 6.6 earthquake and resulted in two deaths. Field investigations showed that it was a loess–mudstone landslide with a short sliding distance (approximately 20 m). By simplifying the original site conditions of the landslide, a model slope was built to perform a large-scale shaking table test. The dynamic response of the model was investigated in the time–frequency–energy space using the Hilbert–Huang transform method, and the failure mechanism was clarified from an energy perspective. The Hilbert spectrum indicates that the distribution characteristics of the instantaneous frequency energy are completely different between the loess layer and mudstone layer. Under 0.06 g, large energy is mainly concentrated in a frequency range 10–40 Hz in the loess layer, whereas the energy is mainly concentrated in the 5–10 Hz range in the mudstone layer. This implies that complex reflection and refraction waves occurred in the loess layer. With the increase in intensity, the energy in the loess layer gradually concentrates in the 5–10 Hz range, suggesting that the absorption capacity of the loess layer for high-frequency energy from the relative movement of soil particles increased. The marginal spectra changed from single peak to multipeak with increase in elevation under 0.06 g and changed from multipeak to single peak (with a predominant frequency of approximately 5.6 Hz) with increase in intensity, demonstrating that elevation (>1/2H) can amplify the high-frequency cumulative energy under 0.06 g, showing that input seismic energy gradually takes dominance. Large cumulative energy (10–40 Hz) and instantaneous energy are mainly distributed at the slope shoulder and loess layer. This causes the formation of cracks and a sliding surface in the loess layer. The failure phenomenon of the model slope is close to that of the Buzi landslide, in which sliding occurred at the middle–upper part of the slope within the loess layer.

Seismic Performance of Small and Medium-Sized Homogeneous Earthen Dams Considering Valley Site Effects in Large-Scale Shaking Table Tests

Seismic Performance of Small and Medium-Sized Homogeneous Earthen Dams Considering Valley Site Effects in Large-Scale Shaking Table Tests

Assessment of the Seismic Response of Shallow Buried Elliptical Tunnels

ABSTRACT Investigating dynamic responses and damage modes of tunnels constructed in different geological conditions is of great significance to understand the damage mechanism and put forward mitigation measures. In this paper, large-scale shaking table tests with the similarity ratio of 1:12 were used to study the seismic response (acceleration and spectrum characteristics, hoop strain, dynamic earth pressure, and longitudinal strain) of shallow buried elliptical tunnels motivated by different dynamic loads. Three different conditions of tunnel section were discussed: with shock-absorbing layer (SAL), with fault, and without SAL and fault. Then the slipping and expanding displacement, dynamic stress concentration, and damage modes of the tunnel across the fault fracture zone were studied based on numerical simulations. The research results illustrate that the presence of SAL has slight influences on amplifying the peak acceleration of the excitation motions and controlling the longitudinal strain, but it could effectively mitigate the motion energy and dynamic earth pressure around tunnels; it also demonstrates that the SAL can be utilized as a damping layer to weaken the seismic waves. Additionally, tunnels in the fault fracture zone perform remarkable dynamic response than those in the normal surrounding rock; the largest slipping and expanding displacement of fault often occur at the moment when the acceleration of the excitation motion reaches its high value.

Seismic Responses of a Large Unequal-span Underground Subway Station in Liquefiable Soil Using Shaking Table Test

ABSTRACT Sand liquefaction is considered to be one of the main causes of severe earthquake damage. The seismic response of large-scale underground structures is mainly controlled by the deformation of surrounding soils, and the large liquefaction-induced lateral displacement poses a serious threat to large underground structures. In this paper, a large-scale shaking table test is performed to simulate the dynamic interaction between liquefiable foundation, diaphragm wall and underground subway station. The results indicate that the interaction mode between soil and underground structure changes significantly with the different liquefaction states of surrounding soil. The underground structure improves the liquefaction resistance of adjacent lateral soils, but significantly reduces the liquefaction resistance of soils located directly below the structure. The development of dynamic pore pressure is related to the seismic intensity, and a significant dissipation of dynamic pore pressure usually occurs after the end of a strong seismic excitation. Moreover, the acceleration response law along the vertical direction has changed significantly within the buried depth of the underground structure. Specifically, the acceleration response of the lateral foundation is inhibited in the absence of liquefaction, but is enhanced in the complete liquefaction. In addition, the liquefaction states of model site also affect the spatial strain response of the underground structure.

Effectiveness of a vertical micropile system for mitigation of liquefaction induced lateral spreading effects on pile foundations: 1g large scale shake table tests

Liquefaction-induced lateral spreading has caused severe damages to pile foundations during past earthquakes. Micropiles can be used as a mitigation strategy against lateral spreading effects on pile foundations. However, the available knowledge regarding the possible efficiency of such a strategy is quite limited. In this paper, the effectiveness of a vertical micropile system as a lateral spreading countermeasure was evaluated using large scale 1g shake table tests on 3×3 pile groups. The results showed that the micropile system was not able to effectively reduce bending moments in piles while it reduced lateral soil pressures exerted on the upslope piles of the group by the upper non-liquefiable layer. The employed micropiles restricted lateral displacement of the upper non-liquefiable layer and partially that of the liquefiable layer, especially at upper depths. Solutions such as increasing the number of micropiles with a tighter pattern, using stiffer micropiles or fixing them in the underlying non-liquefiable layer can enhance their performance.

Seismic response of helical pile groups from shake table experiments

This paper presents and discusses the measured responses of single and grouped helical piles to strong ground motions during a large-scale shake table testing program. The effects of earthquake characteristics (i.e., intensity and frequency content) on the seismic performance of single and grouped helical piles are evaluated from the measured responses. In addition, the performance characteristics of helical pile groups are discussed in terms of the interaction between piles within a group and the contributions of vertical and lateral stiffness of individual piles to the rocking stiffness and the overall capacity of the pile group. Furthermore, the effect of pile head connection to the pile cap (fixed or pinned) on the pile group's response is evaluated and the responses of fixed pile groups (2-bolts connection) and pinned pile groups (1-bolt connection) are compared. Finally, the behaviour of a single pile and a pile within a group is compared in terms of their normalized responses.

Seismic behaviour of piles in non-liquefiable and liquefiable soil

This paper investigates the nonlinear soil–pile–structure interaction employing three-dimensional nonlinear finite element models verified with the results of large-scale shaking table tests of model pile groups-superstructure systems. The responses of piles in both liquefiable and non-liquefiable soil sites to ground motion with varying intensities were evaluated considering both kinematic and inertial interaction. The calculated piles and soil responses agreed well with the responses measured during the shaking events. The numerical models correctly predicted the different pile deformation modes that were exhibited in the experiments. The finite element analysis was then employed to perform a parametric study to evaluate the kinematic and inertial effects on the piles' response, considering different ground motion Intensity and piles characteristics. It was found that the bending moment of piles in the liquefiable site increases significantly, compared to the non-liquefiable site, due to the loss of lateral support of the liquified soil, and the maximum bending moment occurs at the interface between the loose and dense sand layers. The inertial interaction contributes the most to the bending moments at the pile top and the interface between the top clay and liquefied loose sand layers. For piles with a larger diameter, the bending moment due to kinematic interaction increases significantly, and the bending moment distribution corresponds to short (rigid) pile behaviour. In addition, the piles at the saturated site displace laterally as a rigid body during strong ground motions because the pile base loses the lateral support due to the soil liquefaction. Finally, the kinematic interaction effect becomes more significant for piles with higher elastic modulus.

An experimental evaluation of helical piles as a liquefaction-induced building settlement mitigation measure

This study evaluates the application of helical piles to reduce liquefaction-induced foundation settlement and investigates their seismic performance in liquefiable grounds. Two large-scale shake table test series, one without mitigation and one using helical piles, were conducted using the shake table facility at the University of California, San Diego. Each model was extensively instrumented and subjected to two consistently applied shaking sequences. The experimental results indicated reduced excess pore-water pressure generation around the helical pile group, attributed mainly to the densification around the piles during installation. The foundation supported on helical piles underwent almost no foundation differential settlement and tilt. The post-shaking liquefaction-induced settlement mechanisms did not affect the helical pile foundation settlement. Although this study introduced helical piles as a reliable and highly efficient measure to mitigate liquefaction-induced foundation tilt and settlement, the proper design and application of helical piles still need thorough investigation due to possible amplified superstructure response.

Investigation of seismic behavior of slope reinforced by anchored pile structures using shaking table tests

An anchored pile structure is a type of retaining structure that is widely used in excavation support systems, water-front structures, slope stabilization, and landslide prevention. This structure has good performance because the anchor can limit the displacement and control the deflection of the pile head. Field observations of the performance of various retaining structures during many recent earthquakes have revealed that these structures behave particularly well. However, the dynamic responses and seismic performance of this type of retaining structure in slope stabilization have not been elucidated. In this study, a large-scale shaking table test was conducted to investigate the seismic behavior of a landslide reinforced by anchored pile structures. The acceleration field, anchor force, bending moment, lateral earth pressure along the pile, and permanent displacement of the slope and pile are presented and discussed to systematically determine the seismic responses, reinforcement, and failure mechanism of this retaining structure. The tests results reveal that a landslide reinforced by anchored pile structures achieved good seismic performance even under strong earthquakes. Additionally, some reasonable measures are proposed to improve the seismic performance of a slope reinforced by anchored pile structures. The results obtained by this study can provide a theoretical basis for the seismic performance assessment and design of anchored pile structures.

Evaluation of seismic mechanical response of tunnel linings using shaking table tests and numerical analyses

This paper investigates seismically induced circumferential forces and bending moment of tunnel linings using a series of large shaking table tests and numerical analyses. In this regard, a scaled-down tunnel, embedded in two different depths of sandy soil, was tested under different frequencies of input motion. Then, dynamic finite difference analyses, first verified using the results of the performed large scale shaking table tests, were conducted to study the effects of depth ratio (h/a; h: depth of tunnel centerline and a = radius of the tunnel), flexibility ratio (1.2, 10 and 110) and different frequencies of input motion, corresponding to the dimensionless periods (λ/D), on the mechanical behavior of the tunnel lining. The results indicated that, low frequencies of input motions for the tunnel with low flexibility ratio resulted in greater circumferential bending moment values of the tunnel lining. Furthermore, the tunnel depth and adjacent surface foundation intensified the bending moment, shear, and axial forces of the shallow tunnel by about 43 and 17% and did not affect the maximum bending moment and forces locations in the tunnel lining. It is, therefore, necessary to consider the effect of a surface foundation on tunnel lining when constructing shallow subway tunnels in urban areas.

Seismic response characteristics and deformation evolution of the bedding rock slope using a large-scale shaking table

Qinghai-Tibet Plateau is one of the most seismically active aeras of China and rock mass structures or geological structure are also complex here. Earthquake-induced geological disasters are occurred frequently in this region. Among them, plane failure is often generated on bedding rock slope especially in destructive area of the Wenchuan earthquake which caused numerous casualties and serious economic loss. To reveal failure mechanism of bedding rock slope, this paper studied the seismic response and investigated progressive failure characteristics of the bedding rock slope through a large-scale shaking table test. A bedding rock slope of 45° contained an unfilled joint set with spacing of 0.1 m that dipped at an angle of 34° out of the surface, was conducted in a rigid model box, with a length of 3.47 m, width of 0.68 m, and height of 1.2 m, respectively. A series of tests results show that acceleration amplification factor in horizontal direction (AAF-X) increases with the increase of the slope elevation and acceleration amplification factor in vertical direction (AAF-Z) amplifies at lower part of the slope. When the shaking intensities are over 0.2 g, the slope crest and its vicinity start to show nonlinear dynamic response. Existence of bedding planes let the isoline morphology of AAF-X redistribute and dominate the seismic amplification at the crest. The progressive failure progress of the model under earthquake can be divided into four stages. This novel experiment offers some important insights to mechanism of bedding plane landslides triggered by earthquakes, evaluation stability of slopes under earthquake, and disaster prevention and mitigation.