Strength Loss and Localization at Silt Interlayers in Slopes of Liquefied Sand

The role of void redistribution in the liquefaction behavior of saturated sand slopes with and without silt interlayers was investigated using a series of dynamic centrifuge model tests. Twelve centrifuge model tests are described that represent four different simple slope configurations, a range of initial relative densities (DR), and three different input motions with different sequences of application. These experimental results demonstrate that the potential for void redistribution induced shear localizations and slope instability depends on the sand’s initial DR, slope geometry (silt layer shape, sand layer thickness), and shaking characteristics (duration, intensity, and history). The archived experimental data set provides a good basis for assessing the ability of numerical modeling methods to distinguish between conditions leading to localization or not. Apparent residual shear strengths mobilized in the models were backcalculated using techniques common to practice. The experimental and analytical results demonstrate that the apparent residual shear strength is unlikely to correlate closely to pre-earthquake penetration resistance alone, but rather is a function of the initial shear stresses and numerous factors affecting the process of void redistribution and localization.     

Pond Water Source for Irrigation on Steep Slopes

Existing technologies have been tailored to deliver cost-effective irrigation on a railway embankment and excavated steep slopes (referred to as batters) within a semiarid environment. Irrigation is to aid the establishment of 100% grass cover within a few weeks to mitigate soil erosion problems. It is based on water sourced from a temporary excavated pond plus the use of a solar powered pump and a drip irrigation system. Railway batter erosion remediation is timed for the wet summer season when irrigation can be used to supplement natural rainfall. For a given irrigation demand and catchment area, critical (minimum) pond volume is estimated from regional charts developed for ungauged catchments. About 20% of the critical volume is added to account for evaporation losses and dead storage. Also, seepage losses need to be considered if the soil is medium to coarse textured and if the pond is not lined with an impermeable material. Initial results are very encouraging with a cost estimate of AU$2.74/m2 of batter area treated (irrigated). Irrigation unit cost is expected to decrease with a larger scale irrigated batter area and the refinement of technologies and installation procedures. Although irrigation methodologies were developed for railway embankments and excavated slopes, they can also be used for erosion control on steep slopes such as road embankments or excavated slopes and earth dam side slopes.     

Evaluating Liquefaction Strength of Partially Saturated Sand

A method is presented for evaluating the liquefaction strength of partially saturated sand using the compression wave velocity (P-wave velocity), a new indicator of saturation. Based on laboratory test results, an empirical correlation that relates the liquefaction strength with the pore pressure coefficient B is firstly proposed. The strength is defined as the cyclic stress ratio required to cause liquefaction at a specified number of cycles. With the aid of a theoretical relation between B and the P-wave velocity, an explicit correlation of more interest is then established between the liquefaction strength of sand and its P-wave velocity. A comparison of the predictions using this explicit correlation with laboratory measurements shows a satisfactory agreement. The significance of this method lies in that it makes it possible to evaluate the liquefaction strength of sand as affected by saturation through the measurement of P-wave velocity, which can be made not only in the laboratory but particularly in the field.     

Influence of Nonplastic Fines on Shear Wave Velocity-Based Assessment of Liquefaction

Many false positives (no liquefaction detected when the normalized shear wave velocity-cyclic stress ratio (Vs1-CSR) combination indicated that it should have been) are observed in the database used in the simplified liquefaction assessment procedure based on shear wave velocity. Two possible reasons for false positives are the presence of a thick surface layer of nonliquefiable soil and the effects of fines on cyclic shear resistance (CRR) and Vs1. About 67% of the false positives that could not have been caused by an overlying thick surface layer are associated with silty sands with less than 35% fines. The effects of fines on the liquefaction resistance of silty sands and on the shear wave velocity are analyzed. Theoretical CRRfield?versus?Vs1 curves for silty sands containing 0 to 15% nonplastic fines are established. They show that the theoretical CRR-Vs1 correlations for silty sands with 5 to 15% nonplastic fines are all located to the far left of the semi-empirical curves that separate liquefaction from no-liquefaction zones in the simplified liquefaction potential assessment procedures. The results suggest the currently used shear wave velocity-based liquefaction potential curves may be overly conservative when applied to sands containing nonplastic fines.     

Bridge Embankments. I: Seismic Risk Assessment and Ranking

The objective of this study is to provide a simple methodology to conduct preliminary seismic assessment and ranking of bridge embankments in order to identify and prioritize embankments that are susceptible to failure. A ranking model that provides a priority list of embankments with the highest seismic risk of failure is generated. A step-by-step methodology is presented in a flowchart to estimate the seismic slope stability capacity/demand ratio, displacement, and liquefaction potential of bridge embankments. Three categories are presented to identify the failure risk of the embankments. The ranking model is useful for a quick sensitivity assessment of the effect of various site conditions, earthquake magnitudes, and site geometry on possible movement of designated embankments. The priority list will enable decision makers to decide on either carrying out further detailed evaluation or consider other appropriate actions for the bridge embankments with the highest seismic failure risk.     

Shear Localization Due to Liquefaction-Induced Void Redistribution in a Layered Infinite Slope

A new approach for estimating the susceptibility of a layered, liquefiable, infinite slope to shear deformations associated with void redistribution is presented. The excess pore water pressures associated with liquefaction produce upward seepage within the slope. The lower portion of a liquefied layer expels a certain volume of pore water, Vcon, as it contracts (densifies). If the liquefied layer is overlain by a lower permeability soil, then the pore water expelled from the lower contracting zones can become trapped causing void ratio increase in a dilating sublayer near the interface, reducing its undrained shear strength. The volume of water that can be absorbed by the dilating sublayer prior to slope instability is termed the dilation capacity, Vdil. The ratio Vcon/Vdil is a measure of the potential for localization due to strength losses from void redistribution. The localization potential is shown to strongly depend on relative density, slope angle, and thickness of the liquefiable layer. The thickness of the dilating sublayer, a critical parameter, is significantly greater than the thickness of a shear band (which may be only tens of grain diameters). A detailed example is presented to show how the procedure can be applied. The results of the analysis are shown to be consistent with observed deformations and localizations in centrifuge model tests.     

Bridge Embankments. II: Seismic Risk for I-24 in Kentucky

Soil types and depth of bedrock are not available for the majority of bridge embankments to allow detailed seismic analysis and risk assessment. The accompanying paper provides a quick methodology for preliminary seismic risk assessment, ranking, and prioritization of bridge embankments along priority routes. The objective of this paper is to apply the methodology to identify the seismic risk of 127 bridge embankments along I-24 in western Kentucky. The variables of each embankment including the geometry, type and properties of underlying soil, elevation of the natural ground line, and level of bedrock are estimated. The seismic slope stability capacity/demand ratio, displacement, and liquefaction potential of each bridge embankment are calculated. Three categories are presented to identify the failure risk of the embankments. A priority list of the embankments with the highest seismic risk of failure is generated for I-24 bridge embankments in western Kentucky during designated seismic events. The priority list will enable decision makers to decide on either carrying out further detailed evaluation or considering other appropriate actions for the embankments with the highest failure risk.     

Centrifuge Modeling of Rock-Fill Embankments on Deep Loose Saturated Sand Deposits Subjected to Earthquakes

Rockfill is commonly used for construction of artificial islands, breakwaters, jetties, quay walls, coastal defenses, protective barriers for reclaimed land, and even as ship impact protection structures around bridge piers. The economic construction method often involves rock dumping onto loose or liquefiable sediments with little or no ground improvement. Hence in a seismic environment, these rock-fill or rubble mound structures are potentially vulnerable to failure due to pore pressure generation effects of the underlying deposits. This paper presents experimental investigation carried out using dynamic centrifuge modeling to study the seismic performance of rock-fill or rubble mound embankment structures on liquefiable sand deposits. The centrifuge test results indicate that the rock-fill embankments suffer substantial settlement owing to rock-fill penetration into the founding sand deposit assisted by the pore pressure generation effects. This mechanism of failure was not, however, observed for a sand embankment where the particle size distribution is comparable to the foundation. This result has important implications in the design methodologies adopted for rock-fill or rubble mound structures.     

不同钒微合金化方式对钢筋强屈比的影响

通过对热轧带肋螺纹钢筋实际生产数据的回归统计和分析,发现钒微合金化降低了钢筋强屈比。其中钒铁微合金化对降低钢筋强曲比的影响最小,氮化钒铁的影响最大。     

Three-Dimensional Analysis of Nonhomogeneous Slopes

Presented is a method of three-dimensional slope stability analysis for homogeneous and nonhomogeneous symmetrical slopes based on the upper-bound theorem of the limit analysis approach. A rigid-block translational toe, above-the-toe or below-the-toe collapse mechanism is considered, with energy dissipation taking place along planar velocity discontinuities. The approach can be considered as a modification and extension of the procedure proposed by Michalowski in 1989. An effective iterative algorithm is applied to find the optimum (least) upper bound in constrained or unconstrained problems. The present procedure removes some essential limitations of limit analysis methods in two- and three-dimensional stability analysis of nonhomogeneous slopes.     

Design Parameters Analysis of Insulated Embankments in Qinghai-Tibet Plateau Permafrost Region

Based on the problems of application of embankments with insulation in permafrost regions, a finite-element method (FEM) was adopted to analyze the influence on permafrost protection effects of the parameters such as insulation materials and thickness, embedded depth, annual mean permafrost temperature, and construction season timing. The FEM calculated results have shown that, in the long-term view of Qinghai-Tibet plateau with rising temperature, all embankments with insulation can protect the permafrost underneath and delay thawing of the permafrost table. The protection effects are related to insulation materials and thickness, embedded depth, construction season timing, and annual mean permafrost temperature. For the embankment design, the influence of all parameters should be taken into account comprehensively, and the most appropriate parameters group should be adopted to protect the permafrost most effectively.     

Analysis of the Deformation of Embankments on the Qinghai-Tibet Railway

Temperature changes and deformations were monitored on various embankment types on the Qinghai-Tibet Railway. Some of these embankments utilized permafrost protection techniques such as duct ventilation, crushed-rock embankments, crushed-rock protected slopes, or thermal-insulation treatments. Some embankments were built conventionally without considering permafrost protection. It was found that the majority of the deformations on both the permafrost-protected and the conventionally built embankments were due to deformation of warm frozen layers closely related to the temperature changes in the underlying permafrost. However, it was found that building embankments with permafrost protection reduced the magnitude of the settlements. After 2–3?years, deformation of all the embankments with permafrost protection countermeasures became smaller and smaller, whereas deformation was still increasing in the conventional embankments, where the settlement in the underlying permafrost could reach a considerable level, and could be a potential trigger for embankment failure. This should be taken into consideration in the railway engineering project on the Qinghai-Tibet Plateau.     

Overtopping Breaching of Noncohesive Homogeneous Embankments

Homogeneous small-amplitude embankments were constructed in flumes from a range of uniform noncohesive materials and breached by overtopping flows under constant reservoir level conditions. Embankment erosion evolves from primarily vertical to predominantly lateral in nature. The breach channel initially erodes the downstream face of the embankment with an invert slope parallel to the face, the breach invert slope then progressively flattening to a terminal value by rotating about a fixed pivot point along the base of the embankment, the location of this pivot point being a function of the size of the embankment material. The breach channel is of a curved (hourglass) shape in plan. Below the water line, breach cross-section width B variation with elevation y above the breach invert is nondimensionally described by B? = 2k?y?0.5, where for the breach cross section at the embankment crest k? = ?2.82[ln(Hb?)]+0.351, and Hb is the centerline breach crest elevation. Breach discharge Qb can be nondimensionally expressed as a function of the head hb on the breach-crest centerline and the breach crest length in plan Lb using Qb? = 0.242?Lb?(hb?)1.5. All expressions presented are applicable to full-width breach sections (double the half-breach section tested). The present findings enable prediction of the development with time of breach cross section, breach longitudinal profile, eroded volumes, and breach flows. The findings can be utilized for predictions of erosion and flooding occurring as the result of embankment failure, although in an engineering sense the quantitative findings of the present work await confirmation for larger embankments.     

Analyzing Liquefaction-Induced Lateral Spreads Using Strength Ratios

The writers backanalyzed 39 well-documented liquefaction-induced lateral spreads in terms of a mobilized strength ratio, su(mob)/σvo′ using the Newmark sliding block method. Based on the inverse analyses results, we found that the backcalculated strength ratios mobilized during lateral spreads can be directly correlated to normalized cone penetration test tip resistance and standard penetration test blow count. Remarkably, Newmark analysis-based strength ratios mobilized during these lateral spreads essentially coincide with liquefied strength ratios backcalculated from liquefaction flow failures. The mobilized strength ratios appear to be independent of the magnitude of lateral displacement (at least for displacements greater than 15?cm) and the strength of shaking (in terms of peak ground acceleration). Furthermore, the mobilized strength ratios backcalculated from these cases appear to be consistent for a given depositional environment and do not appear to be severely impacted by potential water layer formation.     

Liquefaction Susceptibility Criteria for Silts and Clays

New liquefaction susceptibility criteria for saturated silts and clays are presented that are based on the mechanics of their stress-strain behavior and which provide improved guidance for selecting engineering procedures for estimating potential strains and strength loss during seismic loading. Monotonic and cyclic undrained loading test data for silts and clays show that they transition, over a fairly narrow range of plasticity indices (PI), from soils that behave more fundamentally like sands (sand-like behavior) to soils that behave more fundamentally like clays (clay-like behavior), with the distinction having a direct correspondence to the type of engineering procedures that are best suited to evaluating their seismic behavior. It is recommended that the term liquefaction be reserved for describing the development of significant strains or strength loss in fine-grained soils exhibiting sand-like behavior, whereas the term cyclic softening failure be used to describe similar phenomena in fine-grained soils exhibiting clay-like behavior. For practical purposes, clay-like behavior can be expected for fine-grained soils that have PI ≥ 7, although a slightly lower transition point for soils with a CL-ML classification (perhaps PI ≥ 5 or 6) would be equally consistent with the available data. Issues related to the practical application of these criteria are discussed.     

Assessment of the Liquefaction Susceptibility of Fine-Grained Soils

Observations from recent earthquakes and the results of cyclic tests indicate that the Chinese criteria are not reliable for determining the liquefaction susceptibility of fine-grained soils. Fine-grained soils that liquefied during the 1994 Northridge, 1999 Kocaeli, and 1999 Chi-Chi earthquakes often did not meet the clay-size criterion of the Chinese criteria. Cyclic testing of a wide range of soils found to liquefy in Adapazari during the Kocaeli earthquake confirmed that these fine-grained soils were susceptible to liquefaction. It is not the amount of “clay-size” particles in the soil; rather, it is the amount and type of clay minerals in the soil that best indicate liquefaction susceptibility. Thus plasticity index (PI) is a better indicator of liquefaction susceptibility. Loose soils with PI<12 and wc/LL>0.85 were susceptible to liquefaction, and loose soils with 120.8 were systematically more resistant to liquefaction. Soils with PI>18 tested at low effective confining stresses were not susceptible to liquefaction. Additionally, the results of the cyclic testing program provide insights regarding the effects of confining pressure, initial static shear stress, and stress-path on the liquefaction of fine-grained soils.     

Evaluation of Flow Liquefaction and Liquefied Strength Using the Cone Penetration Test

Flow liquefaction is a major design issue for large soil structures such as mine tailings impoundments and earth dams. If a soil is strain softening in undrained shear and, hence, susceptible to flow liquefaction, an estimate of the resulting liquefied shear strength is required for stability analyses. Many procedures have been published for estimating the residual or liquefied shear strength of cohesionless soils. This paper presents cone penetration test-based relationships to evaluate the susceptibility to strength loss and liquefied shear strength for a wide range of soils. Case-history analyses by a number of investigators are reviewed and used with some additional case histories. Extrapolations beyond the case-history data are guided by laboratory studies and theory.     

Stability Analysis of Complex Soil Slopes using Limit Analysis

The limit equilibrium method is commonly used for slope stability analysis. Limit equilibrium solutions, however, are not rigorous because neither static nor kinematic admissibility conditions are satisfied. Limit analysis takes advantage of the lower- and upper-bound theorems of plasticity theory to provide rigorous bounds on the true solution of a stability problem. In this study, finite-element models are used to construct both statically admissible stress fields for lower-bound analysis and kinematically admissible velocity fields for upper-bound analysis of soil slopes. While limit analysis of relatively simple slopes, typically homogeneous and of simple geometry, has been done previously, limit analysis of slopes with complex geometries, soil profiles, and groundwater patterns could not be effectively done in the past. In this paper, the theoretical basis and procedure for limit analysis of such slopes is presented. Various examples of slopes are selected from the literature and analyzed using both limit equilibrium and limit analysis. Factors of safety from limit equilibrium and limit analysis are compared. A comparison is also made, for each example, between the critical slip surfaces from limit equilibrium with the velocity field and plastic zone from the upper-bound solution and with the stress field from the lower-bound solution.     

Nonlinear Thermal Analysis for Qing-Tibet Railway Embankments in Cold Regions

Heat convection in ballast mass in railway embankments is a problem of heat convection in porous media. In order to calculate the temperature distribution of the Qing-Tibet railway embankment from the governing equations used to study forced convection for incompressible fluids porous media, detailed finite-element formulas for heat convection in porous media are derived using Galerkin’s method. The temperature distributions on central lines of the traditional railway embankment, the ripped-rock embankment, and the ripped-rock revetment embankment that were constructed on July 15, 2002 have been analyzed and compared on July 15, October 15 in the 24th year after construction, and January 15 in the 25th year after construction under the climatic and geological conditions on the Qing-Tibet Railway. The calculated results indicate that the traditional railway embankment will raise the permafrost temperature under the embankment base and make the permafrost embankment thermally unstable. The ripped-rock embankment and the ripped-rock revetment embankment will reduce the permafrost temperature under the embankment base in cold regions, therefore maintaining the thermal stability of permafrost. However, the ripped-rock embankment needs more rock mass while the ripped-rock revetment embankment need less rock mass, and its construction cost is lower than that of the former. Therefore, it is highly recommended that the ripped-rock revetment embankment be used for the Qing-Tibet railway embankment structure in high temperature permafrost regions so that the permafrost embankment can be protected as much as possible.