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.

Baseline correction for near-fault ground motion recordings of the 2008 Wenchuan Ms8.0 earthquake

In this study, both records of a digital accelerometer and a seismograph at a far-field station for the 2008 Ms8.0 Wenchuan earthquake were analyzed, and a pulsive noise model for acceleration record was found. By comparing with the result of a rotary-table tilt test, we concluded that the noises in the acceleration records were caused by ground tilt as a result of rotational ground motion. We analyzed the key noises that may cause baseline offset, and proposed a baseline-correction scheme for preserving the long-period ground motion in accordance with specific pulse positions. We then applied this correction method to some near-field strongmotion acceleration records. The result shows that this method can obtain near-field ground displacements, including permanent displacements, in agreement with GPS data, and that this method is more stable than other methods.

Seismic analysis of cantilever earth retaining walls embedded in dry sand by simplified approaches and finite element method

In engineering practice simplified methods are essential to the seismic design of embedded earth retaining walls,as fullydynamic numerical analyses are costly,time-consuming and require specific expertise.Recently developed pseudostatic methods provide earth stresses and internal forces,even in those cases in which the strength of the soil surrounding the structure is not entirely mobilised.Semiempirical correlations or Newmark sliding block method provide an estimate of earthquake-induced permanent displacements.However,the use of these methods is hindered by uncertainties in the evaluation of a few input parameters,affecting the reliability of the methods.This study uses 1 D site response analyses and 2 D fully-dynamic finite element analyses to show that simplified methods can provide a reasonable estimate of the maximum bending moment and permanent displacements for stiff cantilever walls embedded in uniform sand,providing that a few input parameters are evaluated through semiempirical correlations and a simple 1 D site response analysis.

Measuring ground deformations caused by 2015 Mw7.8 Nepal earthquake using high-rate GPS data

The April 25, 2015 Mw7.8 Nepal earthquake was successfully recorded by Crustal Movement Observation Network of China （CMONOC） and Nepal Geodetic Array （NGA）. We processed the high-rate GPS data （1 Hz and 5 Hz） by using relative kinematic positioning and derived dynamic ground motions caused by this large earthquake. The dynamic displacements time series clearly indicated the displacement amplitude of each station was related to the rupture directivity. The stations which located in the di- rection of rupture propagation had larger displacement amplitudes than others. Also dynamic ground displacement exceeding 5 cm was detected by the GPS station that was 2000 km away from the epicenter. Permanent coseismic displacements were resolved from the near-field high-rate GPS stations with wavelet decomposition-reconstruction method and P-wave arrivals were also detected with S transform method. The results of this study can be used for earthquake rupture process and Earthquake Early Warning studies.

A Comparative Study of the Seismic Performances and Failure Mechanisms of Slopes Using Dynamic Centrifuge Modeling

In this study, dynamic centrifuge model tests were performed for sand slopes under different earthquake ground motions and slope angle to characterize the seismic performance of slopes. Four groups of tests under varying seismic input amplitude were conducted. Under the action of increasing earthquake intensity, the rigidity of the soil decreases and the damping ratio increases, both of the dynamic response and the predominant period of slopes are increased. Three types of seismic waves with the same seismic intensity were applied in the model tests. It shows that the variability in the ground motion leads to the acceleration response spectra of the slopes being completely different and the Northridge seismic wave with low-frequency component is closest to the predominant period of the slope model. In addition, the effect of slope angle on the seismic performance of slopes were also clarified. The results reveal how the slope angle affects the acceleration recorded on the ground surface of the slope, both in terms of the peak ground-motion acceleration(PGA) amplification factor and the predominant period. Finally, the permanent displacement of the model slopes under different earthquake intensities were further analyzed. Based on the nonlinear growth of the permanent displacement of the slope, the test results demonstrated the failure process of the slope, which can further provide a basis for theperformance-based seismic design of slopes.