Seismic stability analysis of reinforced slopes

In this paper, the seismic stability of slopes reinforced with geosynthetics is analysed within the framework of the pseudo-static approach. Calculations are conducted by applying the kinematic theorem of limit analysis. Different failure modes are considered, and for each analytical expressions are derived that enable one to readily calculate the reinforcement force required to prevent failure and the yield acceleration of slopes subjected to earthquake loading. Several results are presented in order to illustrate the influence of seismic forces on slope stability. Moreover, a suitable procedure based on the assessment of earthquake-induced permanent displacement is proposed for the design of reinforced slopes in seismically active areas.     

Travel distance of failed slopes during 2004 Chuetsu earthquake and its evaluation in terms of energy

A number of slope failures during the 2004 Niigataken Chuetsu earthquake were investigated, revealing that the travel distance becomes longer as the slope gets gentler and the failed soil mass gets larger. An energy-based approach, proposed in previous research to evaluate the travel distance of failed slopes, is modified by adding the model test results and introducing a simple evaluation method. The energy approach is then applied to a number of slopes failed during the earthquake to back-calculate mobilized friction coefficients, revealing their strong dependency on initial slope inclinations. The friction coefficient was found to be smaller than the initial slope inclination for gentler slopes, indicating that the failed soil mass tends to accelerate. In contrast, that for steeper slopes was higher than initial slope inclination. The friction coefficient was found to decrease with increasing volume of failed slope, which is quite consistent with previous case studies including large non-seismic landslides.     

Simplified method for generating slope seismic deformation hazard curve

A simplified method for generating slope deformation hazard curve that takes into account the variations of input parameters is presented in this paper. The main assumption in the new approach is that the occurrence of peak slope deformation is Poisson׳s process. The procedure is based on logic tree analysis, commercial software and routines programmed by the authors for generating sets of input files, and forming slope performance curve. The methodology was applied to a real landslide in order to demonstrate the advantages and limitations of the proposed approach. The results of the analysis showed the influence of the certain input factors on sliding displacement as well as the advantages of employing continuum mechanics approach.     

Bank undercutting and tension failure by groundwater seepage: predicting failure mechanisms

Groundwater seepage can lead to the erosion and failure of streambanks and hillslopes. Two groundwater instability mechanisms include (i) tension failure due to the seepage force exceeding the soil shear strength or (ii) undercutting by seepage erosion and eventual mass failure. Previous research on these mechanisms has been limited to non‐cohesive and low cohesion soils. This study utilized a constant‐head, seepage soil box packed with more cohesive (6% and 15% clay) sandy loam soils at prescribed bulk densities (1.30 to 1.70 Mg m^?3) and with a bank angle of 90° to investigate the controls on failure mechanisms due to seepage forces. A dimensionless seepage mechanism (SM) number was derived and evaluated based on the ratio of resistive cohesion forces to the driving forces leading to instability including seepage gradients with an assumed steady‐state seepage angle. Tension failures and undercutting were both observed dependent primarily on the saturated hydraulic conductivity, effective cohesion, and seepage gradient. Also, shapes of seepage undercuts for these more cohesive soils were wider and less deep compared to undercuts in sand and loamy sand soils. Direct shear tests were used to quantify the geotechnical properties of the soils packed at the various bulk densities. The SM number reasonably predicted the seepage failure mechanism (tension failure versus undercutting) based on the geotechnical properties and assumed steady‐state seepage gradients of the physical‐scale laboratory experiments, with some uncertainty due to measurement of geotechnical parameters, assumed seepage gradient direction, and the expected width of the failure block. It is hypothesized that the SM number can be used to evaluate seepage failure mechanisms when a streambank or hillslope experiences steady‐state seepage forces. When prevalent, seepage gradient forces should be considered when analyzing bank stability, and therefore should be incorporated into commonly used stability models. Copyright © 2013 John Wiley & Sons, Ltd.     

Analysis of a highway embankment failure associated with the 1999 Düzce, Turkey earthquake

An embankment slope on a major highway totally collapsed during 12 November 1999 Düzce earthquake (M_w=7.1) due to the intense near field ground motion. The slope had performed satisfactorily, without even minor deformations or cracks, during the İzmit earthquake (M_w=7.4), another major event that occurred 3 months before and was far field. Predominantly coarse-grained fill of the embankment exhibited typical non-brittle behavior during laboratory tests, implying that the prefailure shear strength would remain relatively unchanged. Stability and failure mechanism of the embankment slope were investigated through a series of static, pseudostatic and fully coupled non-linear dynamic analyses. While respective performances were correctly predicted by the pseudostatic method on a failure—no failure basis during the two earthquakes, the computed failure surface geometry did not conform to that observed. Fully coupled dynamic response analysis, on the other hand, predicted the failure mechanism and failure surface configuration in conformance with the post-failure observations. Computed displacements were generally less than those observed and critically dependent on the potential uncertainties.     

Spatial and temporal variability of pore‐water pressures in residual soil slopes in a tropical climate

It is critical to understand and quantify the temporal and spatial variability in hillslope hydrological data in order to advance hillslope hydrological studies, evaluate distributed parameter hydrological models, analyse variability in hydrological response of slopes and design efficient field data sampling networks. The spatial and temporal variability of field‐measured pore‐water pressures in three residual soil slopes in Singapore was investigated using geostatistical methods. Parameters of the semivariograms, namely the range, sill and nugget effect, revealed interesting insights into the spatial structure of the temporal situation of pore‐water pressures in the slopes. While informative, mean estimates have been shown to be inadequate for modelling purposes, indicator semivariograms together with mean prediction by kriging provide a better form of model input. Results also indicate that significant temporal and spatial variability in pore‐water pressures exists in the slope profile and thereby induces variability in hydrological response of the slope. Spatial and temporal variability in pore‐water pressure decreases with increasing soil depth. The variability decreases during wet conditions as the slope approaches near saturation and the variability increases with high matric suction development following rainfall periods. Variability in pore‐water pressures is greatest at shallow depths and near the slope crest and is strongly influenced by the combined action of microclimate, vegetation and soil properties. Copyright © 2002 John Wiley & Sons, Ltd.     

Soil slopes under combined horizontal and vertical seismic accelerations

Conventional methods of designing earth structures are based on pseudo-static stability analysis employing a horizontal seismic coefficient. This paper discusses the stability and permanent displacement of a slope subject to combined horizontal and vertical accelerations. A log-spiral failure mechanism is used. It is shown that seismic force has a significant effect on stability and permanent displacement of slopes. The parametric study reveals that vertical acceleration may play an important role on stability and permanent displacement if the corresponding horizontal acceleration is large. © 1997 John Wiley & Sons Ltd.     

Liquefaction-induced deformation of earthen embankments on non-homogeneous soil deposits under sequential ground motions

Damage of embankments during earthquakes is widely attributed to the liquefaction of foundation soil. Previous studies have investigated the dynamic response of embankments by mainly considering uniform sand foundation and a single earthquake event. However, the foundation of an embankment consists of many sublayers of soil from liquefiable sand to relatively impermeable layer, and during earthquakes a mainshock may trigger numerous aftershocks within a short time which may have the potential to cause additional damage to soil structures. Accordingly, the investigation of liquefaction-induced deformation of earthen embankments on various liquefiable foundation conditions under mainshock–aftershock sequential ground motions is carried out by a series of dynamic centrifuge tests in this study. The liquefiable foundation includes uniform sand profile, continuous layered soil profile, and non-homogeneous soil profiles. Effects of various foundation conditions on embankment deformations are compared and analyzed. From the test results, it is found that the embankment resting on non-homogeneous soil deposits suffer more damage compared to the uniform sand foundation of same relative density. The test results also suggest that the sequential ground motions have a significant effect on the accumulated deformation of embankment.     

地震作用下黄土边坡动态滑动位移研究

提出在地震作用下黄土边坡滑动的新模型,该模型由一个四边形块体和一个扇形块体构成。根据牛顿第二定律,分析该边坡在地震作用下两滑动块体的受力状态,推导任一时刻滑动块体的加速度。从而给出该边坡模型的临界地震加速度系数表达式并计算不同参数下,临界地震加速度系数的值。对已推导边坡块体加速度积分,解得任一时刻滑动块体的位移表达式,计算此时滑动块体的位移特解,并根据给定的三种实例,计算不同参数下滑动块体的位移。结果表明：块体交界面的倾角对临界地震加速度系数和块体滑动位移有显著的影响,当块体交界面倾角为正时,滑体越大,则临界地震加速度系数越小;当块体交界面的倾角为负时,滑体越小,则临界地震加速度系数越小;当块体交界面的倾角相等、持时相同时,滑块越大,位移越小。研究结果可为相关边坡的抗震设计及地震滑坡位移计算提供新的参考。     

青藏高原东北缘岩石圈变形及其机理

为了解青藏高原东北缘岩石圈变形特征,进一步研究该地区地壳运动的壳-幔耦合机理,本文通过处理分析该地区1999～2007年多期GPS观测数据、1972~2000年水准测量数据和1992年及2007年相对重力测量资料,获得了该区域地壳水平运动速度场、较长时间段的垂直形变场和相对重力变化场.分析发现青藏高原东北缘东西部的变化特征存在明显差异：西部以北东向地壳缩短运动为主,而东部以顺时针旋转为主;东部以地壳隆升为主,速率在2.1 mm/a左右,而西部隆升的速率小于1 mm/a;相对重力变化则表现为在整体增大的背景下东部升高速率较大,平均为9.0×10-8 m·s-2·a-1,而西部较小,平均值为3.1×10-8 m·s-2·a-1.我们还发现,地壳不同变形形式的转换不是渐变的,而是发生在较窄的一个转换带内.这个转换带的整体走向为NEE,北部位于金昌与武威之间,中部在祁连山东部、门源以西,南部位于德令哈以东青海湖以西.最后结合前寒武纪构造格架、重力均衡异常资料和地震SKS分裂结果对形成这种运动态势的机理进行了探讨,我们认为岩石圈物质侧向流动、岩石圈结构及壳-幔耦合方式差异可能是导致东部与西部岩石圈变形差异主要动因.     

Seismic stability analysis of fractured rock slopes by yield design theory

This paper deals with the problem of stability of fractured rock slope located in seismic area. The rock mass is crossed by two sets of fractures which are considered to be planar, parallel and persistent. The effects of both horizontal seismic coefficient and strength characteristics of fractures are addressed. The analysis is based upon the kinematic approach of the yield design theory and the pseudo-static method as well. The fundamental inequality of the kinematic approach is invoked and the failure of the fractured rock slope is considered through simple translational mechanism involving a one-partsliding block. Rigorous upper bounds of the so-called stability factor for the structure under study of given slope angle, strength properties of constituent materials and seismic loads are obtained. The results are presented in the form of stability charts relating the estimated upper bound solutions to the friction angle of fractures.The used procedure highlights the destabilizing effects of the seismic loadings and is capable of conducting parametric studies. Furthermore, the results obtained far from various sets of data show good performance and further research work is planed to extend the analysis to include a large number of sub-problems.     

土边坡地震反应及其动力稳定性分析

本文建立了计算土边坡地震反应及评价其动力稳定性的一个数值分析模型，将上边坡在动力作用下的应力状态简化为自重应力状态和附加动力状态的叠加，并对这两个应力状态分别进行研究。本文用时域集中质量显式有限元方法结合多次透射公式，研究了无限域中边坡在动力作用下的位移场和应力场，提出了土边坡动力稳定性的定量评价方法。     

岩土边坡地震稳定性分析研究评述

回顾了岩土边坡地震稳定性分析研究中的若干重要问题的研究进展，包括边坡岩土体地震反应分析方法、边坡岩土体的动力特性和强度准则及参数测试、边坡地震失稳机理与失稳位置、边坡地震稳定性评价指标与安全标准、边坡地震动输入和边坡地震稳定性评价指标的计算精度，还作了简要评述，指出存在的问题并提出今后的研究方向。     

Flow competence: A criticism of a classic concept

The largest grains found in samples of transported sediment are commonly used to estimate flow competence. With samples from a range of flows, a relationship between the flow and the largest mobile grain can be derived and used to estimate the critical shear stress for incipient motion of the different grain sizes in the bed sediment or, inversely, to estimate the magnitude of the flow from the largest grain found in a transport sample. Because these estimates are based on an extreme value of the transport grain-size distribution, however, they are subject to large errors and are sensitive to the effect of sample size, which tends to vary widely in sediment transport samples from natural flows. Furthermore, estimates of the critical shear stress based on the largest sampled moving grain cannot be scaled in a manner that permits reasonable comparison between fractions. The degree to which sample size and scaling problems make largest-grain estimates of fractional critical shear stress deviate from a true relationship cannot be predicted exactly, although the direction of such a deviation can be demonstrated. The large errors and unknown bias suggest that the largest sampled mobile grain is not a reliable predictor of either critical shear stress or flow magnitude. It is possible to define a single flow competence for the entire mixture, based on a central value of the transport grain-size distribution. Such a measure is relatively stable, does not require between-fraction scaling, and appears to be well supported by observation.     

Seismic analysis of sliding wedge: extended Francais–Culmann's analysis

Pseudo-static analysis is commonly used to design earth structures. Most pseudo-static methods of analysis require a computer program. This paper presents a simple closed form solution of seismic stability analysis by extending Francais–Culmann's analysis. The analysis is valid for a slope with the most critical planar mechanism and under the influence of horizontal and vertical earthquake accelerations. The resulting permanent displacement is discussed and demonstrated using several real earthquake records. The simplicity of extended Francais–Culmann's solution allows it to be used for preliminary design and classroom instruction.     

Post-earthquake investigation on several geosynthetic-reinforced soil retaining walls and slopes during the Ji-Ji earthquake of Taiwan

This paper gives an overview on the application of geosynthetic-reinforced soil structures in Taiwan. Taiwan has an unique topography and geotechnical conditions that rendered a less conservative and more challenging design compared to that of North America, Europe and Japan. The Ji-Ji (Chi-Chi) earthquake of 1999 gave an opportunity to examine the behavior of reinforced soil structures. The performance of several modular-block reinforced soil retaining walls and reinforced slopes at the vicinity of the fault was evaluated. Reinforced structures performed better than unreinforced soil retaining walls. The failure cases were highlighted and the cause of failure was identified. The lack of seismic design consideration could be a major cause of failure. The compound failure mode, the inertia force of the blocks, and the connection stiffness and strength relative to the large dynamic earth pressure, were among major items that would warrant further design consideration.     

Seismic stability of retaining walls with surcharge

The use of pseudo-static methods for the computation of soil thrust acting on retaining walls under seismic condition is well established in the design of such structures. Although different methods, based on the limited displacement concept, have been developed in the last 20 years, the most common design method is still the method derived from the theory developed by Mononobe and Okabe. However, the Mononobe–Okabe method presents a basic shortcoming: the solution is based on the limit equilibrium of the soil wedge without taking into account the presence of the wall. In the paper a new solution based on the pseudo-static equilibrium of the soil–wall system is presented. The developed solution takes into account the effect of the presence of the wall and it is applied to soil–wall systems with surcharged backfills. Formulas are provided to calculate directly the yield acceleration and the inclination of the failure surface. The effect of the intensity of the surcharge and of its distance from the wall is investigated and the results are compared to those obtained in the case of soil–wall systems without surcharge.     

Numerical modelling of combined erosion and weathering of slopes in weak rock

Due to various decay processes associated with weathering, the stability of artificial slopes in weak rocks may be affected well within their envisaged engineering lifetime. Conceptually, the decay following the initial stress release after excavation can be described as a process seeking equilibrium between weathering and erosion. The extent to which such an equilibrium is actually reached influences the outcome of the weathering‐erosion decay process as well as the effects that the decay has on the geotechnical properties of the exposed rock mass, and thus ultimately the stability of slopes affected by erosion and weathering. This paper combines two conceptual models for erosion and weathering, and derives a numerical model which predicts the resulting slope development. This can help to predict the development of a slope profile excavated in a weak rock in time, and can be extended with the addition of strength parameters to the weathering profile to enable prediction of slope stability as a function of time. Copyright © 2011 John Wiley & Sons, Ltd.     

Seismic Displacement of Gravity Walls by a Two-body Model

Gravity walls retaining dry soil are modeled as a system of two bodies: (a) the gravity wall that slides along the wall-foundation soil boundary and (b) the critical soil wedge in the soil behind the wall. The strength of the system is defined by both the frictional and the cohesional components of resistance. The angle of the prism of the critical soil wedge behind the wall is obtained using the limit equilibrium method. The model accounts for changes in the geometry of the backfill soil behind the wall by considering the displacements at the end of each time step under limit equilibrium. The model shows that the standard (single) block model is over-conservative for the extreme case of critical-to-applied-seismic acceleration ratios less than about 0.30, but works well for cases where this ratio ranges between 0.5 and 0.8. The model is applied to predict the seismic displacement of gravity walls (a) tested in the shaking-table and (b) studied numerically by elaborate elasto–plastic analyses.     

地震作用下双层土裂缝边坡稳定性分析

裂缝常存在于边坡表面，地震作用会大幅降低裂缝边坡稳定性。针对双层土裂缝边坡稳定性问题，定义上下土层分界处高度与边坡高度之比为深度系数以描述双层土的分布，基于极限分析上限定理，构建“点到点”离散运动学机构，并在此机构中引入一条垂直张拉裂缝，结合拟静力法和强度折减法建立能量平衡方程求解裂缝边坡临界高度和安全系数上限解。计算结果与传统上限法进行对比，验证离散运动学机构的有效性及其解的优越性，同时探究土体非均质性及深度系数对裂缝边坡稳定性以及裂缝深度和位置的影响规律。结果表明，地震作用会降低边坡稳定性；随着地震力增大，边坡临界滑动面逐渐加深，裂缝深度略微增大，裂缝位置逐渐远离坡面；对于具体的双层土边坡会存在一个特定的深度系数使边坡安全系数达到最值，同时裂缝会穿越至下层土且深度发生突增。