Seismic Stability Analysis Research Articles

Seismic Stability Sensitivity and Uncertainty Analysis of a High Arch Dam-Foundation System

Parameter uncertainty associated with concrete arch dams always arises from modeling assumptions and the lack of knowledge or information of the engineering geological situations, especially in the seismic stability analysis of arch dams. In this research, a high arch dam is selected as a case study for probabilistic analysis of the seismic stability performance. The arch dam abutment and the dam are coupled as a system. A comprehensive approach considering contraction joints, boundaries of the probable sliding rock mass and the dam-foundation interface is presented. The contact nonlinearity is solved by using the dynamic contact model with Lagrange multiplier method. The main parameters of the probable sliding block are considered as random variables containing the friction coefficients and cohesions. Both the slippage and sliding area ratio are chosen as the engineering demand parameters (EDP). The sensitivity analysis is performed to reveal the relative influence of each parameter separately by the approximate incremental dynamic analysis (IDA) method. The friction coefficients are shown to be more crucial than the cohesions on the dam’s resistance to seismic instability. The sliding area ratio can be better used for unveiling the sliding process of the arch dam of concern, while the slippage is useful for one to judging the stability of the arch dam under seismic hazards. The Latin hypercube sampling (LHS) with approximate moment estimation is used to investigate the parameter uncertainty to the seismic stability performance of the high arch dam. The results provide a useful reference for using the median/mean-parameter model to accurately estimate the median/mean response of the dam.

Impact of Earthquake Characteristics on Seismic Slope Stability Using Modified Pseudodynamic Method

Seismic slope stability analysis is often limited by the ability to encompass specific earthquake characteristics, such as shaking amplitude, frequency, and time as well as varying soil properties, such as cohesion and friction angle across the depth with consideration of damping and amplification. In this paper, a simple procedure that combines the discretization-based kinematic analysis and modified pseudodynamic method is presented to offer a more holistic analysis considering earthquake and soil characteristics more appropriately. This is achieved by the implementation of the discretization-based kinematic analysis along with a modified pseudodynamic method. The discretization technique aids to generate a kinematically admissible failure surface with considerations to varied soil strength parameters. Its significance lies in the preprocessing to generate a unique failure mechanism dependent on the varied soil properties of the slope while also facilitating the work rate calculations. The use of the modified pseudodynamic method is adopted in this study to overcome the shortcomings of the pseudostatic and simple pseudodynamic method, enabling one to consider the complete time history of earthquake-induced ground movement, having also considered the amplification effect when shaking approaches the soil’s natural frequency. Equating the external and internal rates of work by integration of elementary rates yields the closed-form solution in yield seismic acceleration and/or slope-bearing capacity. A parametric study with a range of independent variables, accompanied with detailed discussion, is also presented to offer a more pragmatic perspective on the performance of nonuniform stratified slopes subjected to earthquakes.

Seismic stability analysis of gravity retaining wall supporting c–φ soil with cracks

This article presents an analytical approach for the seismic stability analysis of gravity retaining wall with c–φ backfill soil. Cracks which are pre-existing (open before the collapse) and are form as part of slope collapse are considered. For a translational failure mechanism assumed, formulas are provided to calculate directly the yield acceleration and the inclination of failure surface. Factors such as cracks and cracks opening, wall back inclination, soil-wall friction, backfill slope are easily to be coupled into the formulations. Both the depth and most adverse location of the crack can be determined from the optimization procedure. Comparisons are made with existing methods and the influences of cracks opening are discussed. The influence of a crack presence on seismic stability of soil-wall system is distinct and pre-existing (opened) cracks have more adverse effect.

Analysis of Seismic Stability of an Obsequent Rock Slope Using Time–Frequency Method

Obsequent rock slopes are often thought to be more stable than consequent slopes and passive sliding is unlikely to occur under earthquake loading. However, failures in obsequent rock slopes were indeed observed in recent large earthquakes. This paper presents a time–frequency solution to the seismic stability of obsequent rock slopes fully considering the time–frequency characteristics of earthquake waves. Large-scale shaking table tests were conducted to illustrate the application of the time–frequency method to an obsequent rock slope containing multiple weak layers with a small dip angle. The seismic stability of the obsequent rock slope is analyzed combining the time–frequency method, outcomes from the shaking table test, and conventional pseudo-static and dynamic numerical analyses. The results show that passive sliding can develop in the obsequent rock slope when taking the time–frequency components of the earthquake waves and the vertical seismic force into account. The middle–upper part of the obsequent rock slope is more vulnerable to seismic damage. The slope bulges under the earthquake loading; the maximum permanent surface displacement occurs at the middle–upper part of the slope, rather than the slope crest. Additionally, the response of seismic safety factor lags behind the responses of acceleration and surface displacement.

Three-dimensional seismic stability of unsaturated soil slopes using a semi-analytical method

Seismic stability analyses of soil slopes are routinely performed with dry and/or saturated assumptions under two-dimensional (2D) conditions. However, in practice, soils are often unsaturated and slopes usually fail in 3D fashion. Three-dimensional (3D) effect and suction-induced effect are essential to be considered to produce more realistic solutions in the slope stability analyses. In this regard, an analytical framework is developed in this paper for assessing the 3D seismic stability of unsaturated soil slopes. A semi-analytical method including the simplified method (SM) and the layer-wise summation method (LSM) is developed to deal with the highly variable nature of suction. The validity of the analytical framework in assessing the soil slope stability is checked with published solutions. Examples are given to illustrate the 3D effect and the effect of suction on the slope seismic stability. The described framework allows the user to quantify the effect of changes in moisture content on effective stress distribution and effective unit weight distribution in the slope and thus, changes in the slope stability.

Seismic stability analysis of a rock block using the block theory and Newmark method

SummarySeismic stability analysis of a rock block is an important issue in the field of rock mechanics and rock engineering. To solve this problem, the block theory and the Newmark method are combined, and a general method for seismic response of a rock block is introduced. First, the formation method of a three‐dimensional rock block, which includes establishing a topological relationship among the block‐polygon face‐edge‐vertex and dividing a complex block into convex subblocks, is presented, and the simplex integration method is employed to calculate the volume of a rock block and the areas of the polygonal faces. Second, the assumptions and algorithms of the general method for seismic response of a rock block are detailed. The dynamic analysis is carried out in time step. In each time step, the key technologies including analysis of the seismic force and motion mode, trial of the incremental displacement, check for the block entrance, and update of the motion parameters are performed in order. Last, two verification examples and three application examples including a wedge, a rock block of slope, and a combined rock block are used to analyze the correctness and practicality of the general method. The results show that, for a given ground motion record, the Newmark program can effectively simulate the dynamic response process of the motion mode, velocity, and safety factor of the rock block, and the permanent displacement under the earthquake action is obtained, which provides a quantitative parameter to evaluate the dynamic stability of a rock block.

Adjustment of spectral pseudo-static approach to account for soil plasticity and zone seismicity

The concept of the spectral pseudo-static procedure has been developed at the Université de Sherbrooke as an alternative to the conventional pseudo-static approach for the seismic stability analysis of clayey slopes. The destabilizing effect of an earthquake is approximated, in the new approach, by an inertial force that hyperbolically varies with depth while being proportional to the maximum acceleration of the seismic event. Its results have been rigorously verified against available static and dynamic laboratory tests, and have been extensively validated by a series of simulations performed using the computer code FLAC. Good agreements have been achieved between the results of the spectral pseudo-static procedure and complete numerical analyses, in terms of the computed safety factors and the critical slip surfaces. This implies that the spectral pseudo-static approach can be integrated into available limit equilibrium (LE) software providing a useful tool to examine the effect of dynamic soil characteristics on the proposed seismic coefficient profiles. The herein-reported study extends the previous endeavors to examine and quantify the effect of plasticity index (PI) (0%, 15%, 30%, 50%, and 100%) on the proposed formula of seismic coefficient profile using the same numerical modelling and assumptions. Original analyses were carried out considering earthquakes compatible with the seismicity of Quebec City (zone 4), and they have been repeated in the current study for earthquakes compatible with other regions having different seismicity (zones 2, 3, and 5). Based on the results of the current analyses, side-formulas were established between the spectral pseudo-static coefficient on ground surface for any value of soil plasticity index (PI) and the corresponding coefficient originally used in the main formula (PI = 30%).

A Case Study of Probabilistic Seismic Slope Stability Analysis of Rock Fill Tailing Dam

The article presents the case history of expansion of rock fill tailing dam of the Rampura-Agucha zinc mine in Rajasthan, India. Before raising the height of the dam from 27 m to 54 m, a detailed seismic stability analysis was performed considering the spatial variability of the soil. The safety values and the probability of failure were calculated using the Monte Carlo simulation method. All the analyses were carried in a 2-D limit equilibrium-based SLIDE software using Spencer's method. The cohesive strength (c), the angle of friction and the acceleration due to earthquakes were considered as the random variable. The final slope geometry was created after the seismic stability analysis of the upstream and downstream slopes. For the critical geometry of the slope, the observed factor of safety values was found to be higher than the values specified in the ANCOLD. The probability of failure value was found to be less than 8%. The newly constructed rock fill dam has already sustained three Monsoon rains and still continues to be performing at its best condition.

Behaviour of a loose silty sand under static and cyclic loading conditions

In recent years, considerable progress have been made in the study of liquefaction and flow failures with the introduction of the notion of steady-state of deformation and “collapse” envelope. These concepts are increasingly used in seismic stability analysis of slopes or embankments. However, as most of these studies were carried out on clean sands in the triaxial apparatus, one may question the general applicability of these results to practical conditions as several factors such as anisotropy, fines content, undrained strength anisotropy, boundary deformation conditions, stress history, etc. may play an important role. In this paper, the behaviour of the same soil, a loose silty sand, is compared for different loading and testing conditions: drained and undrained triaxial compression tests, drained and constant volume monotonic and cyclic direct simple shear (DSS) tests. Results allow to identify similarities and differences, in terms of undrained initial and steady-state shear strength and modes of undrained failure between triaxial and DSS test conditions and show that liquefaction in DSS cyclic tests is an instability triggered by shear failure developing once the friction corresponding to the characteristic envelope, as identified in the monotonic test, has been mobilized.

Simplified solution to analyse seismic stability of reinforced soil wall

ABSTRACT The normalized tensile reinforcement force for the seismic stability of reinforced soil walls using some rigorous methods considering the pseudo-static seismic forces is mostly used by the earlier researchers in the design of retaining walls backfilled with cohesive soil. Few studies have been presented using horizontal slice method without taking effect of the seismic acceleration in vertical direction. In the present study, a simplified solution has been presented using the pseudo-static equilibrium of the reinforced soil wall taking the effect of seismic accelerations in the horizontal and in the vertical direction. The seismic stability analysis presented here accounts the required total reinforcement strength in terms of the maximum value of reinforcement force coefficient and the critical inclination of failure plane. Taking some geometrical values of retaining wall and soil characteristics based on the literature, a detailed and comparative parametric study presented here accounts the effect of cohesion of the backfill, angle of internal friction of soil, horizontal seismic coefficient, vertical seismic coefficient and surcharge loading.

Study on dynamic response of slope under obliquely incident SV-wave

The change of the incident angle of the seismic wave will lead to the change of the mode of action of the earthquake, which will affect the dynamic response of the slope.In order to study the effect of oblique incidence of seismic SV wave on the dynamic response of rock slope, the problem of seismic fluctuation is transformed into wave source problem. The author uses the viscoelastic artificial boundary to simulate the scattering wave field and applys the equivalent loads to the boundary nodes, which realize the fluctuation input of obliquely incident seismic wave. The semi-infinite space numerical example verifies the validity and accuracy of the method. The results show that the angle of incidence and slope angle has a great impact on seismic dynamic response of slope surface. When the SV wave is incident obliquely, the peak ground acceleration of the rock slope gradually increases with the incident angle increasing, and the peak ground acceleration near the top of the slope increases with the increase of slope angle. The results of this paper can provide important reference for seismic stability analysis of slope.

An approach for the real-time slip deformation coupled with strain softening of a high rockfill dam subjected to pulse-like ground motions

The dynamic stability of a dam is a major engineering problem, especially since many high dams have been built or designed in near-fault zones. However, few studies have been focused on the dynamic stability of dams subjected to near-fault pulse-like ground motions. The stability analysis of high rockfill dam subjected to pulse-like ground motions is a meaningful topic. In the traditional computing methods for slope stability, the position of potential sliding surfaces can’t be accurate determined. In order to get more accurate results, the finite element method of sliding calculation is proposed to calculate variation process of the dynamic stress of the element. Meanwhile, the strain softening of rockfill materials under low confining pressure is obvious. Hence, the dam stability considering the strain softening of rockfill materials is an urgent problem that must be investigated. In this study, the peak strength and residual strength were obtained, and the normalized residual strength curve considering strain softening was put forward by analyzing consolidated drained triaxial test data that were derived from real projects. The soil slope stability procedure (FEMSTABLE 3.0) was developed by the first author and used to perform a seismic stability analysis of a core-wall dam in the near-fault area. In addition, the more reasonable approach was proposed to calculate the real-time slip of the dam under pulse-like ground motions, and the method was coupled real-time slip deformation and strain softening. The proposed method of sliding calculation may spend much time on computing safety factors and accumulated displacements of a large number of sliding surfaces, therefore the multicore parallel computing technique was used to improve the calculation speed. Moreover, the sliding displacement responses of the dam subjected to pulse-like and non-pulse ground motions were compared. Furthermore, the effects of pulse-like ground motions on the dynamic stability of 300 m core-wall dam considering strain softening were investigated. The results show that the pulse-like ground motion has a considerable effect on sliding displacement and strain softening of the rockfill, ignoring this factor may lead to inaccurate result.

Seismic stability analysis and charts of a 3D rock slope in Hoek–Brown media

Earthquakes act as a dominant trigger for failure of slopes in seismic active regions. In this study, a seismic stability analysis of a three-dimensional (3D) rock slope in Hoek–Brown media is conducted based on the limit analysis method. The energy balance equation is derived by equating the external work rate by rock mass weight and seismic forces to the internal energy dissipation rate, and an optimization code is then programmed to capture the optimized factor of safety (FoS) of a 3D rock slope based on the strength reduction technique. The validity of the equivalent Mohr–Coulomb parameters method and generalized tangent technique in conjunction with the limit analysis method to estimate FoS of a 3D rock slope is investigated. Subsequently, a quasi-static method is used to explore the effect of seismic forces on stability of a 3D rock slope. The results of the parametric analysis indicate that the seismic force exhibits significant negative effects on the stability of a 3D slope and that a significant difference exists between a 3D FoS solution and a solution obtained under two-dimensional (2D) plane strain. A stability chart analysis is performed to produce a set of seismic stability charts for preliminary design purposes. Finally, a case study is performed to apply stability charts in practical situations.

Seismic stability analysis of slope assuming log-spiral rupture surface using modified pseudo-dynamic method

ABSTRACT The factor of safety of slope is evaluated in the present work adopting modified pseudo-dynamic method. The failure surface is assumed to be log-spiral and using limit equilibrium approach slope safety is analysed under seismic loading condition. The modified pseudo-dynamic method used for the analysis that is based on visco-elastic behaviour of soil and satisfies zero-stress boundary condition considering soil amplification is inherent to soil properties. Under the influence of different soil and seismic conditions stability of slope in terms of factor of safety are evaluated and presented in both tabular and graphical form. The results obtained from the present analysis are compared with available results from the literature, the difference that is observed may be due to the application of modified pseudo-dynamic method and the assumption of log-spiral failure surface. Detailed studies are carried out to observe the variation of safety factor at different soil strength, seismic forces and slope inclination which can serve as very good criteria for design of slopes under seismic loading condition.

A comparative study of pseudo-static slope stability analysis using different design codes

Many researchers have developed new calculation methods to analyze seismic slope stability problems, but the conventional pseudo-static method is still widely used in engineering design due to its simplicity. Based on the Technical Code for Building Slope Engineering (GB 50330—2013) of China and the Guidelines for Evaluating and Mitigating Seismic Hazards in California (SP117), a comparative study on the pseudo-static method was performed. The results indicate that the largest difference between these two design codes lies in determination of the seismic equivalence reduction factor (feq). The GB 50330—2013 code specifies a single value for feq of 0.25. In SP117, numerous factors, such as magnitude and distance, are considered in determining feq. Two case studies show that the types of slope stability status evaluated by SP117 are in agreement with those evaluated by the seismic time-history stability analysis and Newmark displacement analysis. The factors of safety evaluated by SP117 can be used in practice for safe design. However, the factors of safety evaluated by GB 50330—2013 are risky for slope seismic design.

Dynamic stability evaluation of underground tunnels based on deformation reinforcement theory

Owing to the rapid and increasing application of underground engineering, the stability of underground tunnels under complex geological and seismic environments is receiving increasing attention. Seismic stability analysis and reinforcement design are important to guarantee engineering safety and operation. Based on the deformation reinforcement theory, a dynamic analysis method of the anti-seismic stability evaluation of underground tunnels is presented. The time history of plastic complementary energy norm is used to identify the instant when the structure is exhibiting the most devastating failure. The unbalanced force is used to represent the local failure of the structure. The reinforcement forces are derived through the distribution of unbalanced forces at the instant with the maximum PCE norm. These evaluation indices are incorporated into a three-dimensional nonlinear finite element analysis program, TFINE, which is designed by an object-oriented approach. Further, an elastoplastic integration algorithm and a comprehensive convergence rule are implemented. Examples of a single tunnel and parallel tunnels subject to earthquake are demonstrated, showing the capability and effectiveness of TFINE in evaluating the structure stability.

Experimental verification of current seismic analysis methods of reinforced soil walls

The paper investigates the accuracy of two limit equilibrium design approaches used for seismic stability analyses of reinforced soil walls. Responses from a series of reinforced soil wall models, tested on shaking table, are compared to similar responses predicted using both NCMA, and AASHTO/FHWA design guidelines. First, orientation of the soil slip surface measured during subsequent base shaking was compared to the values predicted analytically. Comparison results suggested that the current design methods tend to under predict the size of the soil failure wedge at different input base acceleration amplitudes. Therefore, the current equation proposed to predict the angle of soil slip surface is modified to match measured values. Second, the horizontal and vertical seismic coefficients used in the current seismic analysis methods are compared to values inferred from shaking table accelerometer responses. Results indicated that equations suggested to calculate horizontal acceleration coefficient for design of reinforced soil walls are non-conservative for input base acceleration larger than 0.3 g. Finally, dynamic earth force magnitudes and locations, and dynamic reinforcement force increments measured from shaking table model tests are used to identify sources of conservatism and non-conservatism of the current design methodologies. Different modifications have been suggested to reduce conservativeness or increase the safety of the design guidelines.

Seismic stability analysis of slopes with pre-existing slip surfaces

In analyzing seismic stability of a slope with upper bound limit analysis method, the slip surface is often assumed as a log-spiral or plane slip surface. However, due to the presence of a weak layer and unfavorable geological structural surface or a bedrock interface with overlying soft strata, the preexisting slip surface of the slope may be irregular and composed of a series of planes rather than strictly log-spiral or plane shape. A computational model is developed for analyzing the seismic stability of slopes with pre-existing slip surfaces. This model is based on the upper bound limit analysis method and can consider the effect of anchor bolts. The soil or rock is deemed to follow the Mohr-Coulomb yield criterion. The slope is divided into multiple block elements along the slip surface. According to the displacement compatibility and the associated flow rule, a kinematic velocity field of the slope can be obtained computationally. The proposed model allows not only calculation of the rate of external work owing to the combined effect of self-weight and seismic loading, but also that of the energy dissipation rate caused by the slip surface, interfaces of block elements and anchorage effect of the anchors. Considering a direct relationship between the rate of external work and the energy dissipation rate, the expressions of yield acceleration and permanent displacement of anchored slopes can be derived. Finally, the validity of this proposed model is illustrated by analysis on three typical slopes. The results showed that the proposed model is more easily formulated and does not need to solve complex equations or time consuming iterations compared with previous methods based on the conditions of force equilibrium.

Seismic Stability Analysis of Municipal Solid Waste Landfills Using Strain Dependent Dynamic Properties

Seismic stability analysis is an integral part of the design of municipal solid waste (MSW) landfills in the areas of high seismic activity. The conventional pseudo-static methods were used to calculate the factor of safety of MSW landfills. However, these methods didn’t consider the mode change behaviour of the landfill, amplification acceleration of the ground motion and also, neglected the damping properties of MSW. The objective of the present study is to develop an integrated methodology that gives the seismic acceleration profile in the landfill as well as the factor of safety and yield acceleration values for a typical side-hill type geometric configuration. Strain-dependent dynamic properties (shear modulus and damping ratio) of MSW are computed through an iterative scheme and the same shear modulus and damping ratio are used to calculate the seismic factor of safety of landfills. Amplification of base acceleration is observed at low-frequency input motions. The maximum shear strain generated in the landfill is significantly higher for low-frequency input motions, since, at lower frequencies, the seismic inertial forces at all depths are in phase. Whereas, at higher frequencies, those are out of phase. The factor of safety values computed using the present method are on the higher side when compared to the conventional pseudo-static method of analysis for certain combination of input parameters.

Analysis of seismic stability of large-sized tank VST-20000 with software package ANSYS

The work is devoted to the study of seismic stability of vertical steel tank VST-20000 with due consideration of the system response “foundation-tank-liquid”, conducted on the basis of the finite element method, modal analysis and linear spectral theory. The calculations are performed for the tank model with a high degree of detailing of metallic structures: shells, a fixed roof, a bottom, a reinforcing ring.