STABILITY ANALYSIS OF TWO-DIMENSIONAL LANDSLIDES SUBJECTED TO SEISMIC LOADS

In the past, the pseudo-static method used to be the most common for evaluating the stability of landslides under seismic loads, in which static forces acting on the center of gravity of a sliding body are used to consider the effects of earthquake. However, seismic loads of rock and soil differ from time and location to time and location. Obviously, it is irrational for the pseudo-static approach to be applied to solving dynamic problems. In this paper, a displacement- pseudo-dynamic model is proposed to assess seismic stability of landslides, in which the sinusoidal- cosinusoidal wave is applied to simulating earthquake displacement, and an ＂amplification factor＂ of peak seismic displacement is referred to as the amplification of seismic wave when it propagates from the bottom to the top of the landslide. The effects of physico-mechanical behaviors of a sliding body on seismic stability of soil slopes are taken into account as well as inertia forces and damping forces. The sensitivity analyses on the permanent displacement and the dynamic factor of safety of landslides are studied in detail. Moreover, a large-scale Tangjiashan landslide that occurred in Wenchuan earthquake is investigated to verify the robustness and precision of the present method. It is found that the results from the present method is in good agreement with those from the previous method.

Generalized response displacement methods for seismic analysis of underground structures with complex cross section

The response displacement method(RDM)is recommended for the seismic analysis of underground structures in the transverse direction for many codes,including bases for design of structures-seismic actions for designing geotechnical works(ISO 23469)and code for seismic design of urban rail transit structures(GB 50909-2014).However,there are some obvious limitations in the application of RDM.Springs and the shear stress of the soil could be approximately evaluated for the structures having a simple cross section,such as rectangular and circular structures.It is necessary to propose simplified seismic analysis methods for structures with complex cross sections.This paper refers to the idea of RDM and proposes three generalized response displacement methods(GRDM).In GRDM1,a part of the soil surrounding a structure is selected to generate a generalized underground structure with a rectangular cross section,and the same analysis model as RDM is applied to analyze the responses of the structure.In GRDM2,a hollow soil model without a generalized structure is used to compute the equivalent load caused by the relative displacement of the soil,and the soil-structure interaction model is applied to calculate the responses of the structure.In GRDM3,a continuous soil model is applied to compute the equivalent load caused by the relative displacement and shear stress of the soil,and the soil-structure interaction model is applied to analyze the responses of the structure,which is the same as the model used in GRDM2.The time-history analysis method(THAM)is used to evaluate the accuracy of the proposed simplified methods.Results show that the error of GRDM1 is about 20%,while the error is only 5%for GRDM2 and GRDM3.Among the three proposed methods,GRDM3 has obvious advantages regarding calculation efficiency and accuracy.Therefore,it is recommended to use GRDM3 for the seismic response analysis of underground structures that have conventional simple or complex cross sections.

Comparison of seismic stability for slopes with tensile strength cut-off and cracks

The strength of geomaterials is typically predicted by the Mohr-Coulomb yield criterion in slope stability analysis.The tensile strength of soils in this yield criterion,which is an extrapolation of the triaxial compression test results,is usually overestimated.Generally,the influences of tensile stresses in slopes are evaluated by two approaches:1)introducing cracks to eliminate the tensile stresses in slopes,and 2)truncating the strength envelope to reduce the tensile strength of the soils.However,comparative analyses of the two approaches have not been fully implemented,especially under dynamic conditions.In this study,three slope failure mechanisms corresponding to the predictions of slope stability by the mentioned two approaches subjected to seismic loadings are systematically evaluated.The stability factor considering the pre-existing crack,crack formation,and tension cut-off are compared one another.The most unfavorable crack locations corresponding to the different mechanisms are evaluated.The influence of tensile strength on the factor of safety of slopes is estimated as well.Further,the critical acceleration and the permanent displacement of slopes with pre-existing crack,and tension cut-off are derived in the framework of the kinematic theorem of limit analysis.The vertical effects of seismic coefficient on critical acceleration and permanent displacement are discussed.Conclusions can be drawn that the consideration of tension zone effects can sharply reduce the stability factor of slopes,especially for steep slopes and large horizontal seismic loads;different from the static condition,the slope with pre-existing crack is not always the most vulnerable to collapse,the tension cut-off mechanism in steep slopes may predict the lowest stability factor.In addition,the calculation shows that an evidently increase in the slope displacement is induced by the increasing downward vertical loads,while a decrease occurs if the vertical loads are upward.

Topology optimization of shear wall structures under seismic loading

A topology optimization formulation is developed to find the stiffest structure with desirable material distribution subjected to seismic loads. Finite element models of the structures are generated and the optimality criteria method is modified using a simple penalty approach and introducing fictitious strain energy to simultaneously consider both material volume and displacement constraints. Different types of shear walls with/without opening are investigated. Additionally, the effects of shear wall-frame interaction for single and coupled shear walls are studied. Gravity and seismic loads are applied to the shear walls so that the definitions provide a practical approach for locating the critical parts of these structures. The results suggest new viewpoints for architectural and structural engineering for placement of openings.

The Effect of Core Eccentricity on the Structural Behavior of Concrete Tall Buildings

The main purpose of this paper is to investigate the effect of core eccentricity on the structural behavior of concrete tall buildings.Concrete buildings of 55 floors with plan dimensions 48.0×48.0 m2 were investigated.Three cases of main core locations are studied:centric(A),eccentric by one sixth(B)and one third(C)of building width.The three-dimensional finite element method has been used in conducting structural analysis through ETABS software.Gravity and lateral(wind and seismic)loadings are applied to all building cases.It has been concluded that the core location is the prime parameter governing the structural behavior of tall buildings.Although the first two cases(A,B)have acceptable and similar structural behaviors conforming to code limits,in the third case(C),the building behavior came beyond code limits.The author introduced remedial action by adding two secondary cores in the opposite direction of the main core(C-R)to restore the building behavior to the code limits.The results of this action were satisfactory.

Slope stability analysis under seismic load by vector sum analysis method

The vibration characteristics and dynamic responses of rock and soil under seismic load can be estimated with dynamic finite element method （DFEM）. Combining with the DFEM, the vector sum analysis method （VSAM） is employed in seismic stability analysis of a slope in this paper. Different from other conventional methods, the VSAM is proposed based on the vector characteristic of force and current stress state of the slope. The dynamic stress state of the slope at any moment under seismic load can he obtained by the DFEM, thus the factor of safety of the slope at any moment during earthquake can be easily obtained with the VSAM in consideration of the DFEM. Then, the global stability of the slope can be estimated on the basis of time-history curve of factor of safety and reliability theory. The VSAM is applied to a homogeneous slope under seismic load. The factor of safety of the slope is 1.30 under gravity only and the dynamic factor of safety under seismic load is 1.21. The calculating results show that the dynamic characteristics and stability state of the slope with input ground motion can be actually analyzed. It is believed that the VSAM is a feasible and practical approach to estimate the dynamic stability of slopes under seismic load.

Contribution of tuned liquid column gas dampers to the performance of offshore wind turbines under wind, wave, and seismic excitations

The main intention of the present study is to reduce wind, wave, and seismic induced vibrations of jacket- type offshore wind turbines （JOWTs） through a newly developed vibration absorber, called tuned liquid column gas damper （TLCGD）. Using a Simulink-based model, an analytical model is developed to simulate global behavior of JOWTs under different dynamic excitations. The study is followed by a parametric study to explore efficiency of the TLCGD in terms of nacelle acceleration reduction under wind, wave, and earthquake loads. Study results indicate that optimum frequency of the TLCGD is rather insensitive to excitation type. In addition, while the gain in vibration control from TLCGDs with higher mass ratios is generally more pronounced, heavy TLCGDs are more sensitive to their tuned frequency such that ill-regulated TLCGD with high mass ratio can lead to destructive results. It is revealed that a well regulated TLCGD has noticeable contribution to the dynamic response of the JOWT under any excitation.

Complex of general seismic zoning maps OSR-2017 of Uzbekistan

The quantitative assessment of seismic hazard of Uzbekistan has been examined,and new maps of seismic zoning has been developed.Quasihomogeneous seismological areas and seism-generating zones are considered as seismic sources,based on the analysis of seismotectonic data.The seismological parametrization of seismic sources has been carried out,including determination of parameters of earthquake’s reoccurrence for different power levels,seismic potential and a predominant type motion in the source of occurring earthquakes.The reoccurrence parameters of seismic sources were both determined by directly from Gutenberg-Richter dependence law and using summation and distribution methods.Setting were conducted separately for a sample of strong(M≥5)and weak(M<5)earthquakes.The seismic potential Mmax of seismic active zones was determined by seismological and seismotectonic methods.The predominant type of movement in the faulting for each seismic source is reverse fault.Regional regularities of seismic intensity attenuation with distance of different power levels are determined.The seismic hazard is expressed in points of a macro seismic scale,in velocities and accelerations of ground motions,and it is characterized by calculated seismic intensity with the set probability(P=0.9,P=0.95,P=0.98 and P=0.99)not exceed within 50 years in the constructed maps.In seismic zoning map,the seismic intensity was in average soil conditions.The developed complex of maps considers a number of uncertainties of the input parameters in relation to both incompleteness of initial seismological and seismotectonic data,as well the probabilistic nature of seismic process and seismic intensity.The factors of uncertainty of the input parameters are taken into account by constructing a logic tree.The constructed maps of the general seismic zoning are intended for imple menting antiseismic actions in Uzbekistan.

Seismic Performance Assessment of Reinforced Concrete Box Culverts under Near and Far Fault Seismic Ground Motion Records

Studying the critical response characteristics of reinforced concrete box culverts with diverse geometrical configurations under seismic excitations is a necessary step to develop a reasonable design method. In this work, numerical analysis and assessment of reinforced concrete box culverts for seismic loading in addition to standard static loading from dead and live loads is conducted, aiming to highlight the critical difference in the seismic performances between two and three cell box culverts under near and far-fault ground motion. The results show how and where the seismic loading alters the responses of seismic loading of the models including the effect on safety and failure. The geometrical configurations of the culvert combined with the loading scenarios also significantly influence the magnitude and distribution of the seismic responses. The findings of this work shed light on the critical role of the geometrical configurations and shaking event in the seismic responses of reinforced concrete box culverts and this procedure can be applied as seismic assessment method to any culvert shape, size, and material.

Reliability analysis of reinforced concrete columns under combined seismic and blast loads

It is possible for certain building structures to encounter both the seismic load and blast load during their service life.With the development of the economy and the increase of security demand,the need for design of building structures against multi-hazard is becoming more and more obvious.Therefore,the damage analysis of building structures under the combined action of multiple hazards has become a very urgent requirement for disaster prevention and reduction.In this paper,the refined finite element model of reinforced concrete(RC)columns is established by using the explicit dynamic analysis software LS-DYNA.Combined with the Monte Carlo method,the damage law of RC columns under the combined action of random single earthquake or explosion disaster and multi-hazard is studied,and the damage groups are distinguished according to the damage index.Based on the support vector machine(SVM)algorithm,the dividing line between different damage degree groups is determined,and a rapid method for determining the damage degree of RC columns under the combined seismic and blast loads is proposed.Finally,suggestions for the design of RC column against multi-disaster are put forward.

Pseudo-static stability analysis of rock slopes reinforced by passive bolts using the generalized Hoek-Brown criterion

The stability analysis of passive bolt-reinforced rock slopes under seismic loads is investigated within the framework of the kinematic approach of limit analysis theory.A pseudo-static method is adopted to account for the inertial forces induced in the rock mass by seismic events.The strength properties of the rock material are described by a modified Hoek-Brown strength criterion,whereas the passive bolts are modeled as bar-like inclusions that exhibit only resistance to tensile-compressive forces.Taking advantage of the ability to compute closed-form expressions for the support functions associated with the modified Hoek-Brown strength criterion,a rotational failure mechanism is implemented to derive rigorous lower bound estimates for the amount of reinforcement strength to prevent slope failure.The approach is then applied to investigating the effects of relevant geometry,strength and loading parameters in light of a preliminary parametric study.The accuracy of the approach is assessed by comparison of the lower bound estimates with finite element limit analysis solutions,thus emphasizing the ability of the approach to properly predict the stability conditions and to capture the essential features of deformation localization pattern.Finally,the extension of the approach to account for slipping at the interface between reinforcements and surrounding rock mass is outlined.

Ground vibration analysis under combined seismic and high-speed train loads

The rise of high-speed railway induces an increased probability of serious derailment accidents of operating high-speed trains during earthquakes.A two-and-half-dimensional finite element model(2.5D FEM)was developed to investigate the ground vibration under combined seismic and high-speed train loads.Numerical examples were demonstrated and the proposed method was turned out to provide an effective means for estimating ground vibration caused by high-speed train load during earthquakes.The dynamic ground displacement caused by combined seismic and high-speed train loads increases with the increase of the train speed,and decreases with the increase of the stiffness of ground soil.Compared with the seismic load alone,the coupling effect of the seismic and high-speed train loads results in the low-frequency amplification of ground vibration.The moving train load dominants the medium–high frequency contents of the ground vibration induced by combined loads.It is observed that the coupling effects are significant as the train speed is higher than a critical speed.The critical train speed increases with the increase of the ground stiffness and the intensity of the input earthquake motion.

A comprehensive review on coupling vibrations of train-bridge systems under external excitations

In recent years,high-speed railways in China have developed very rapidly,and the number and span of the railway bridges are keeping increasing.Meanwhile,frequent extreme disasters,such as strong winds,earthquakes and floods,pose a significant threat to the safety of the train–bridge systems.Therefore,it is of paramount importance to evaluate the safety and comfort of trains when crossing a bridge under external excitations.In these aspects,there is abundant research but lacks a literature review.Therefore,this paper provides a comprehensive state-of-the-art review of research works on train–bridge systems under external excitations,which includes crosswinds,waves,collision loads and seismic loads.The characteristics of external excitations,the models of the train–bridge systems under external excitations,and the representative research results are summarized and analyzed.Finally,some suggestions for further research of the coupling vibration of train–bridge system under external excitations are presented.

Three-Dimensional Seismic Response in Complex Site Conditions: A New Approach Based on an Auxiliary-Model Method

In this paper, an auxiliary-model method is proposed for calculating equivalent input seismic loads in research of ground motions. This method can be used to investigate the local effect of 3 D complex sites subjected to obliquely incident SV and P waves. Using this method, we build a fictitious auxiliary model along the normal direction of the boundary of the area of interest, with the model’s localized geological features remaining the same along a vector normal to this boundary. This model is divided into five independent auxiliary models, which are then dynamically analyzed to obtain the equivalent input seismic loads. Unlike traditional methods, in this new technique, the mechanical behavior of the auxiliary model can be nonlinear, and its geometry can be arbitrary. In addition, a detailed description of the steps to calculate the equivalent input seismic loads is given. Numerical examples of incident plane-wave propagation at uniform sites with local features validate the effectiveness of this method. It is also applicable to elastic and non-elastic problems.

Seismic behavior of cantilever wall embedded in dry and saturated sand

The embedded cantilever retaining walls are often required for excavation to construct the undergroundfacilities. Significant numbers of numerical and experimental studies have been performed to understand the behavior ofembedded cantilever retaining walls under static condition. However, very limited studies have been conducted on thebehavior of embedded retaining walls under seismic condition. In this paper, the behavior of a small scale modelembedded cantilever retaining wall in dry and saturated sand under seismic loading condition is investigated by shaketable tests in the laboratory and numerically using software FLAC2D. The embedded cantilever walls are subjected tosinusoidal dynamic motions. The behaviors of the cantilever walls in terms of lateral displacement and bending momentare studied with the variation of the two important design parameters, peak amplitude of the base motions and excavationdepth. The variation of the pore water pressures within the sand is also observed in the cases of saturated sand. Themaximum lateral displacement of a cantilever wall due to seismic loading is below 1% of the total height of the wall in drysand, but in case of saturated sand, it can go up to 12.75% of the total height of the wall.

Hysteretic Model for Corroded Rectangular Reinforced Concrete Bridge Column Under Seismic Loading

Additional hysteretic experiments for corroded rectangular reinforced concrete(RC)columns with an axial load ratio of 0.27 were implemented.A quasi-static cyclic lateral loading with constant axial force was subjected to tests.Herein,a modified ductility factor model for corroded RC column is developed on the basis of the previous work and additional experiments.The model involves the influence of both the corrosion ratio of longitudinal rebar and the axial load ratio.A four-linear envelope curve model concerning lateral load and displacement is proposed in a combination of determination rules of the peak point and the failure strength point.The hysteretic model of corroded RC columns is developed by considering both degraded unloading stiffness and reloading stiffness on the history peak point.The hysteretic model can predict the residual life of corroded RC columns under seismic loading.

Numerical modeling of a tunnel in jointed rocks subjected to seismic loading

The studies on the performance of tunnels under static loads are reported extensively in the literature but their performances under dynamic loads are limited.The present study highlights some of the important aspects of jointed rock tunnels during seismic loading.The literature review provides a shake table experimental study of a jointed rock tunnel.A Universal Distinct Element Code(UDEC)model is developed from this shake table experiment.The model tunnel is subjected to a scaled input motion of the 1985 Mexico earthquake.The numerical results are validated systematically with the findings of the shake table experiment.Further,the developed numerical model is used to perform parametric studies to understand the effect of in-situ stress,joint angles,joint stiffness,and joint friction angle on the deformation and stability of the tunnel under the same earthquake input motion.It is observed that some joint angle combinations form a wedge that yields excessive deformation and subsequently a complete failure.An exponential decrease in deformation occurred in the tunnel as the joint stiffness increases.It is found that the shallow tunnels are more susceptible to damage under the action of earthquake loads.

Collapse Patterns and Their Discrimination of Reinforced Concrete Shear Wall Under Combined Stresses of Compression and Shear due to Strong Earthquake

The collapse patterns of reinforced concrete(RC)shear walls under seismic load are proposed.The crack distribution and propagation of shear walls are specifically based on the failure criterions of Mohr-Coulomb with tension cutoff.Three zones and five different corresponding failure modes of RC shear walls are determined according to the transfer path of shear stress in shear wall.These failure modes of shear walls under seismic load are verified by many experimental results and can be utilized in collapsing analysis for frame-shear wall structure.

Geotechnical forensic investigation of a slope failure on silty clay soil--A case study

Qila Bala Hisar is one of the noteworthy places of Peshawar,Khyber Pakhtunkhwa.The fort was constructed on a flled ground during the 18th century and it was renovated several times by the occupants ever since.Recently,due to an earthquake of magnitude 7.3,the upper part of the south-western wall of the fort collapsed.The collapse of the wall was attributed to the failure of the retained slope.This research was undertaken to characterize the slope material,study causal factors of failure and evaluate remedial strategy.The investigation involved conventional field and laboratory testing and geophysical investigation using electrical resistivity technique to evaluate the nature of stratum.Also,X-ray Diffraction and Scanning Electron Microscopy was used to study the slope material at a molecular level to evaluate the existence of swelling potential.The analysis has shown that excessive seepage of water caused by the poor maintenance of runoff and sewage drains is the causal factor triggered by the seismic event.A remedial strategy involving soil nails,micro piles and improvement of the surface drainage is recommended.