An experimental study of the liquefaction strength of silty sands in terms of the state parameter

An elaborate program of monotonic and cyclic triaxial laboratory tests on mixtures of sand and silt with fines content 0%, 15% and 25% was performed to investigate the effect of density, consolidation stress and non-plastic fines on the liquefaction strength. The monotonic tests illustrated that the critical state lines of all mixtures do not cross each other, and are, approximately, parallel to each other. The results of the cyclic tests illustrated that the relationship between the cyclic strength and the state parameter does not depend on the consolidation stress, the soil density and the silt content. Analysis in terms of the state parameter showed that: (i) as the consolidation stress increases, the cyclic strength decreases and this effect is more pronounced as the specimens become denser, especially as the fines content increases and (ii) the cyclic strength decreases as the fines content increases and this effect is more pronounced as the specimens become denser.     

Laboratory and theoretical evaluation of impact of packing density,particle shape,and uniformity coefficient on erodibility of coarse-grained soil particles

Sedimentation – including erosion, transport, and deposition of coarse-grained particles – is a primary and growing environmental, engineering, and agricultural issue around the world. Soil erosion occurs when the hydrodynamic force induced by flowing water exceeds the geotechnical resistance of soils, as measured by critical shear stress for initiation of soil-particle motion. Even though various quantitative methods have been suggested with respect to different types of soil, the most widely accepted formula to estimate critical shear stress for coarse-grained soil is a direct function of the median grain size of the soil particles; however, the erosion resistance of soils also varies with other geotechnical properties, such as packing density, particle shape, and uniformity coefficient. Thus, in this study, a combined rolling–lift model for particle detachment was derived based on theoretical analysis. A series of experimental flume tests were conducted with specimens prepared with standard soil types, as well as laboratory-prepared mixtures of coarse-grained soil to validate the theoretical model and determine the effect of other geotechnical properties on the erosion characteristics of coarse grains, coupled with the effect of median particle size. The results indicated that the median grain size is the primary variable determining the resistance of coarse grains, but the critical shear stress also varies with the packing density of the soil matrix. In addition, angular particles show more erosion resistance than rounded particles, and the erosion potential of a soil decreased when the grain is well graded (higher value of uniformity coefficient). Additionally, regression analysis was performed to quantify the effect of each parameter on the critical shear stress of coarse grains. © 2020 John Wiley & Sons, Ltd.     

Key factors affecting prediction of seismic pore water pressures in silty sands based on damage parameter

The paper provides insight into factors affecting the prediction of seismic pore-water pressure build up in clean sands and sand–silt mixtures for modeling purposes. Laboratory pore pressure measurements were conducted using stress-controlled undrained cyclic simple shear (CSS) tests carried out on both reconstituted and undisturbed specimens of silty sands under different initial conditions (density state, effective vertical stress, initial fabric and fines content). Test results were interpreted by using a damage concept-based model which is actually implemented for clean sands in non-linear time domain site response analysis codes. In the present work, such a model was properly modified for sands having fines contents higher than 35%. The general applicability of the modified procedure for predicting pore water pressure response of silty sands under irregular shear stress loading using data from stress-controlled CSS tests was also verified and all factors affecting calibration parameters of the model were throughly analyzed.     

Evaluation of static and dynamic properties of sand–fines mixtures through the state and equivalent state parameters

The results of an experimental investigation on sands with low plastic fines content are presented. Specimens with a low plastic fines content of 0%, 15%, 30%, 40%, 50% and 60% by weight were tested in drained and undrained triaxial compression tests. The soil specimens were tested under three different categories: (1) at a constant void ratio index; (2) at the same peak deviator stress in a triaxial test; and (3) at a constant relative density. By a combination with our published experimental data in recent years, the critical state line and various state parameters have been proposed and discussed for a further understanding the behavior of sand–fines mixtures. Results indicated that a unique critical line was obtained from drained and undrained triaxial compression tests for each fines content. The effects of fines content on critical state line (CSL) were recognized and discussed. In addition, the results revealed that normalized peak undrained shear stress, cyclic resistance ratio, and compression index were found to be a good correlation with state parameter Ψ as well as equivalent state parameter Ψ*. An increasing state parameter decreased the normalized peak undrained shear stress, and cyclic resistance ratio; however, the compression index increased with an increase in state parameter. Finally, there were no correlations such as the coefficient of consolidation–state parameter and maximum shear modulus–state parameter due to the different testing condition.     

Instability of sand–silt mixtures

The instability of the sand–silt mixtures with different amount of fines contents were studied in this paper. It showed that the slope of the instability lines increases with the increasing relative densities, and it approaches the slope of the failure line (steady state line) at a high relative density. The relationship between the peak stress ratio and the intergranular void ratio can be described by a common curve for all the mixtures with fines contents less than the transitional fines content. There may also exist a unique curve for the relationship between the peak stress ratio and interfine void ratio for the mixtures with fines contents higher than the transitional fines content. If the yield strength ratio is used instead of the peak stress ratio, the relationships can be characterized in the same way.     

Influence of fines content on liquefied strength of silty sands

Critical state soil mechanics is a useful framework to understand sand behavior. In this paper, a relationship is developed for estimating undrained critical shear strength of sands based on the critical state framework. The relationship is validated by comparison with laboratory test results and sand liquefied strength from field liquefaction failure case histories. Using this relationship, the influence of fines content on undrained critical shear strength is studied for different combinations of effective stress and density. The parametric study indicates that depending on soil void ratio, effective stress, and the shape and mineralogy of the fine particles, undrained critical strength may increase, remain the same, or decrease as the amount of fines content increases. Both the susceptibility to liquefaction and the severity of strain-softening are affected by adding fines. It is suggested that the critical state parameter is inadequate for describing the behavior of liquefiable sands and sand shearing-compressibility should be taken into account in liquefaction analysis.     

Effect of overconsolidation ratio on dynamic properties of binary mixtures of silica particles

Overconsolidated soils are ubiquitous in nature due to multiple mechanisms; however, the stress-history-based studies of small strain stiffness on binary mixtures, such as silty sand, are limited even though natural sand deposits are commonly mixtures of sand particles with varying amounts of fines. Consequently, this study quantified the stress-history-based dynamic properties of binary mixtures, such as sand-sand mixtures with different size small particles, and silty sand mixtures with small amounts of non-plastic fines, up to the critical fines content. By performing bender element tests on those mixtures according to fines content, size ratio, and overconsolidation ratio, the stress-history-based G_max of binary mixtures was evaluated. For the relevant data analysis, the OCR (overconsolidation ratio) exponent in the G_max formulation was expressed in terms of stress exponents during loading and unloading. It was found that the effect of OCR on the estimation of G_max increased with a decrease in size ratio (or increase in size difference), since the stress exponents during loading increased more significantly with a decrease in size ratio due to the pronounced change in interparticle coordination between large grains. However, the variation of stress exponents during unloading of different mixtures was relatively small due to the prevalent elastic deformation. It was demonstrated that the maximum stress history effect of tested mixed soils was observed at a fines content of approximately 5%, which was smaller than critical fines content of silty sand. This behavior was attributable to the delay in critical fines content observed during unloading, when compared to that observed during loading.     

Rainfall‐induced consolidation and sealing effects on soil erodibility during concentrated runoff for loess‐derived topsoils

Flume experiments simulating concentrated runoff were carried out on remolded silt loam soil samples (0·36 × 0·09 × 0·09 m³) to measure the effect of rainfall‐induced soil consolidation and soil surface sealing on soil erosion by concentrated flow for loess‐derived soils and to establish a relationship between soil erodibility and soil bulk density. Soil consolidation and sealing were simulated by successive simulated rainfall events (0–600 mm of cumulative rainfall) alternated by periods of drying. Soil detachment measurements were repeated for four different soil moisture contents (0·04, 0·14, 0·20 and 0·31 g g^?1). Whereas no effect of soil consolidation and sealing is observed for critical flow shear stress (τ_cr), soil erodibility (Kc) decreases exponentially with increasing cumulative rainfall depth. The erosion‐reducing effect of soil consolidation and sealing decreases with a decreasing soil moisture content prior to erosion due to slaking effects occurring during rapid wetting of the dry topsoil. After about 100 mm of rainfall, Kc attains its minimum value for all moisture conditions, corresponding to a reduction of about 70% compared with the initial Kc value for the moist soil samples and only a 10% reduction for the driest soil samples. The relationship estimating relative Kc values from soil moisture content and cumulative rainfall depth predicts Kc values measured on a gradually consolidating cropland field in the Belgian Loess Belt reasonably well (MEF = 0·54). Kc is also shown to decrease linearly with increasing soil bulk density for all moisture treatments, suggesting that the compaction of thalwegs where concentrated flow erosion often occurs might be an alternative soil erosion control measure in addition to grassed waterways and double drilling. Copyright © 2007 John Wiley & Sons, Ltd.     

Influence of fines content on the undrained cyclic shear strength of sand–clay mixtures

A series of undrained cyclic triaxial tests were performed on sand–clay mixtures with various sand–clay mixing ratios. Prior to the primary tests, the threshold fines content was examined by consistency tests, which was found to be approximately 20%. For sand–clay mixtures with a sand-matrix (fines content less than the threshold fines content), the cyclic shear strength of low-density mixtures increases and that of high-density mixtures decreases with increasing fines content. However, for sand–clay mixtures with a fines-matrix (fines content greater than the threshold fines content), there exists a unique correlation between the cyclic shear strength and global void ratio for different fines content. The equivalent granular void ratio is introduced in this paper to account for the contribution ratio of the fines to soil skeleton. As a result, a unique relationship between cyclic shear strength and equivalent granular void ratio was observed for pure sand and sand–clay mixtures with a sand-matrix.     

The effect of plastic fines on the pore pressure generation characteristics of saturated sands

Pore water pressure generation during earthquake shaking initiates liquefaction and affects the shear strength, shear stiffness, deformation, and settlement characteristics of soil deposits. The effect of plastic fines (kaolinite) on pore pressure generation in saturated sands was studied through strain-controlled cyclic triaxial tests. In addition to pore pressure generation, this experimental study also focused on evaluating the threshold shear strain for pore pressure generation and the volumetric compressibility of specimens during pore pressure dissipation. The results reveal that specimens having up to 20% plastic fines content generated larger values of pore water pressure than clean sand specimens. At 30% fines content, the excess pore water pressure decreased below that of clean sand. The threshold shear strain, which indicates the strain level above which pore pressures begin to generate, was assessed for different kaolinite–sand mixtures. The threshold shear strain was similar for 0–20% fines (γ_t0.006–0.008%), but increased to about 0.025% for 30% fines. The volumetric compressibility, measured after pore pressure generation, was similar for all specimens. The transition of behavior at fines contents between 20% and 30% can be attributed to a change in the soil structure from one dominated by sand grains to one dominated by fines.     

Assessment of soil factors controlling ephemeral gully erosion on agricultural fields

The soil factor is crucial in controlling and properly modeling the initiation and development of ephemeral gullies (EGs). Usually, EG initiation has been related to various soil properties (i.e. sealing, critical shear stress, moisture, texture, etc.); meanwhile, the total growth of each EG (erosion rate) has been linked with proper soil erodibility. But, despite the studies to determine the influence of soil erodibility on (ephemeral) gully erosion, a universal approach is still lacking. This is due to the complex relationship and interactions between soil properties and the erosive process. A feasible soil characterization of EG erosion prediction on a large scale should be based on simple, quick and inexpensive tests to perform. The objective of this study was to identify and assess the soil properties – easily and quickly to determine – which best reflect soil erodibility on EG erosion. Forty‐nine different physical–chemical soil properties that may participate in establishing soil erodibility were determined on agricultural soils affected by the formation of EGs in Spain and Italy. Experiments were conducted in the laboratory and in the field (in the vicinity of the erosion paths). Because of its importance in controlling EG erosion, five variables related to antecedent moisture prior to the event that generated the gullies and two properties related to landscape topography were obtained for each situation. The most relevant variables were detected using multivariate analysis. The results defined 13 key variables: water content before the initiation of EGs, organic matter content, cation exchange capacity, relative sealing index, two granulometric and organic matter indices, seal permeability, aggregates stability (three index), crust penetration resistance, shear strength and an erodibility index obtained from the Jet Test erosion apparatus. The latter is proposed as a useful technique to evaluate and predict soil loss caused by EG erosion. Copyright © 2018 John Wiley & Sons, Ltd.     

Erodibility of soil and organic matter: independence of organic matter resistance to interrill erosion

The enrichment of organic matter in interrill sediment is well documented; however, the respective roles of soil organic matter (SOM) and interrill erosion processes for the enrichment are unclear. In this study, organic matter content of sediment generated on two silts with almost identical textures, but different organic matter contents and aggregations, was tested. Artificial rainfall was applied to the soils in wet, dry and crusted initial conditions to determine the effects of soil moisture and rainfall and drying history on organic matter enrichment in interrill sediment. While erosional response of the soils varied significantly, organic matter enrichment of sediment was not sensitive to initial soil conditions. However, enrichment was higher on the silt with a lower organic matter content and lower interrill erodibility. The results show that enrichment of organic matter in interrill sediment is not directly related to either SOM content or soil interrill erodibility, but is dominated by interrill erosion processes. As a consequence of the complex interaction between soil, organic matter and interrill erosion processes, erodibility of organic matter should be treated as a separate variable in erosion models. Further research on aggregate breakdown, in particular the content and fate of the organic matter in the soil fragments, is required. Copyright © 2007 John Wiley & Sons, Ltd.     

Dynamic properties of dry sand/rubber (SRM) and gravel/rubber (GRM) mixtures in a wide range of shearing strain amplitudes

Recycled tire rubber in mixtures with granular soils has been found recently applications in many civil engineering projects. The paper presents a synthesis of the dynamic strain-dependent properties of the commonly used soil/rubber mixtures, which are necessary in any seismic design. We focus herein on high-amplitude resonant column tests on granular soil/rubber mixtures with varying percentage of rubber. The most important characteristics of the dynamic properties of the mixtures like the confining pressure, the content of rubber, the grain-size characteristics of the physical portion of the mixtures as well as the relative size of soil versus rubber solids, are thoroughly discussed. We propose generic normalized shear modulus and damping ratio versus shearing strain amplitude curves for dry mixtures of sand/rubber (SRM) and gravel/rubber (GRM) appropriate for the engineering practice. Finally, we summarize analytical expressions for small-strain shear modulus and damping ratio for SRM and GRM proposed in previous studies.     

Erosion rate of sand and mud mixtures

Erosion of mixed cohesive and noncohesive sediments is studied using the erosion test instrument SEDFlume. The sediment mixtures are composed of well-sorted quartz sand (0.25–0.5 mm) and one of the three used muds: kaolinite, kaolinite-bentonite and Mississippi River muds. The mud contents cover from 0 to 100%. The measured data of erosion rate and bed shear stress are used to examine the segmented linear, nonlinear, and exponential erosion models. The parameters of each erosion model are related to the physical properties of sediment mixtures, including clay fraction, mud fraction, mixture dry density, and mud dry density. It is found that the three models can fit well with the data, and their parameters have strong relations with the mud fraction and mud dry density, to a less extent with the clay fraction, but not with the mixture dry density.     

Understanding mass fluvial erosion along a bank profile: using PEEP technology for quantifying retreat lengths and identifying event timing

This study provides fundamental examination of mass fluvial erosion along a stream bank by identifying event timing, quantifying retreat lengths, and providing ranges of incipient shear stress for hydraulically driven erosion. Mass fluvial erosion is defined here as the detachment of thin soil layers or conglomerates from the bank face under higher hydraulic shear stresses relative to surface fluvial erosion, or the entrainment of individual grains or aggregates under lower hydraulic shear stresses. We explore the relationship between the two regimes in a representative, US Midwestern stream with semi‐cohesive bank soils, namely Clear Creek, IA. Photo‐Electronic Erosion Pins (PEEPs) provide, for the first time, in situ measurements of mass fluvial erosion retreat lengths during a season. The PEEPs were installed at identical locations where surface fluvial erosion measurements exist for identifying the transition point between the two regimes. This transition is postulated to occur when the applied shear stress surpasses a second threshold, namely the critical shear stress for mass fluvial erosion. We hypothesize that the regimes are intricately related and surface fluvial erosion can facilitate mass fluvial erosion. Selective entrainment of unbound/exposed, mostly silt‐sized particles at low shear stresses over sand‐sized sediment can armor the bank surface, limiting the removal of the underlying soil. The armoring here is enhanced by cementation from the presence of optimal levels of sand and clay. Select studies show that fluvial erosion strength can increase several‐fold when appropriate amounts of sand and clay are mixed and cement together. Hence, soil layers or conglomerates are entrained with higher flows. The critical shear stress for mass fluvial erosion was found to be an order of magnitude higher than that of surface fluvial erosion, and proceeded with higher (approximately 2–4 times) erodibility. The results were well represented by a mechanistic detachment model that captures the two regimes. Copyright © 2017 John Wiley & Sons, Ltd.     

From soil aggregates to riverine flocs: a laboratory experiment assessing the respective effects of soil type and flow shear stress on particles characteristics

Particles eroded from hillslopes and exported to rivers are recognized to be composite particles of high internal complexity. Their architecture and composition are known to influence their transport behaviour within the water column relative to discrete particles. To‐date, hillslope erosion studies consider aggregates to be stable once they are detached from the soil matrix. However, lowland rivers and estuaries studies often suggest that particle structure and dynamics are controlled by flocculation within the water column. In order to improve the understanding of particle dynamics along the continuum from hillslopes to the lowland river environment, soil particle behaviour was tested under controlled laboratory conditions. Seven flume erosion and deposition experiments, designed to simulate a natural erosive event, and five shear cell experiments were performed using three contrasting materials: two of them were poorly developed and as such can not be considered as soils, whilst the third one was a calcareous brown soil. These experiments revealed that soil aggregates were prone to disaggregation within the water column and that flocculation may affect their size distribution during transport. Large differences in effective particle size were found between soil types during the rising limb of the bed shear stress sequence. Indeed, at the maximum applied bed shear stress, the aggregated particles median diameter was found to be three times larger for the well‐developed soil than for the two others. Differences were smaller in the falling limb, suggesting that soil aggregates underwent structural changes. However, characterization of particles strength parameters showed that these changes did not fully turn soil aggregates into flocs, but rather into hybrid soil aggregate–floc particles. Copyright © 2013 John Wiley & Sons, Ltd.     

Fluvial and slope‐wash erosion of soil‐mantled landscapes: detachment‐ or transport‐limited?

Many numerical landform evolution models assume that soil erosion by flowing water is either purely detachment‐limited (i.e. erosion rate is related to the shear stress, power, or velocity of the flow) or purely transport‐limited (i.e. erosion/deposition rate is related to the divergence of shear stress, power, or velocity). This paper reviews available data on the relative importance of detachment‐limited versus transport‐limited erosion by flowing water on soil‐mantled hillslopes and low‐order valleys. Field measurements indicate that fluvial and slope‐wash modification of soil‐mantled landscapes is best represented by a combination of transport‐limited and detachment‐limited conditions with the relative importance of each approximately equal to the ratio of sand and rock fragments to silt and clay in the eroding soil. Available data also indicate that detachment/entrainment thresholds are highly variable in space and time in many landscapes, with local threshold values dependent on vegetation cover, rock‐fragment armoring, surface roughness, soil texture and cohesion. This heterogeneity is significant for determining the form of the fluvial/slope‐wash erosion or transport law because spatial and/or temporal variations in detachment/entrainment thresholds can effectively increase the nonlinearity of the relationship between sediment transport and stream power. Results from landform evolution modeling also suggest that, aside from the presence of distributary channel networks and autogenic cut‐and‐fill cycles in non‐steady‐state transport‐limited landscapes, it is difficult to infer the relative importance of transport‐limited versus detachment‐limited conditions using topography alone. Copyright © 2011 John Wiley & Sons, Ltd.     

The effect of conservation tillage on runoff erosivity and soil erodibility during concentrated flow

Crop residues in conservation tillage systems are known to cause both a reduction in the erosive runoff power and an increase in the topsoil erosion resistance. In this study, the relative importance of both mechanisms in reducing soil loss by concentrated flow erosion is examined. Therefore, a method to calculate the effective flow shear stress responsible for soil detachment in the presence of a residue cover is applied. The determination of effective flow shear stress is based on the recalculation of the hydraulic radius for residue treatments. The method was tested in a laboratory flume by comparing soil detachment rates of identical pairs of soil samples that only differ in the presence or absence of crop residues. This shear stress partitioning approach and a soil detachment correction were then applied to a dataset of soil detachment measurements on undisturbed topsoil samples from a no‐till field plot on a loess‐derived soil, sampled during one growing season. Results indicate that only a small fraction (10% on average) of the difference in soil detachment rate between conventional and conservation tillage can be attributed to the dissipation of shear forces on the residues. The remaining decrease in soil detachment during concentrated runoff after a two‐year application of conservation tillage can be explained by the increased dry bulk density and root and crop residue content in the topsoil that reduces soil erodibility. After correcting for the presence of residues, the temporal variability in soil detachment rates (D_r) during concentrated flow for a given flow shear stress (τ) for both treatments can be predicted fairly well (R² = 0·87) from dry soil bulk density (DBD, representing consolidation effects), soil moisture content (SMC, representing antecedent rainfall conditions), the dry mass of organic material (OM, representing root growth and residue decomposition) and saturated soil shear strength σ_{s, sat} using an equation of the form: This study is the first to show that the effect of conservation tillage on soil detachment rates is a result of soil property modifications affecting soil erodibility, rather than a result of the surface residue decreasing flow erosivity. Copyright © 2007 John Wiley & Sons, Ltd.     

盐分及含水率对甘肃黑方台黄土强度的影响研究

文章以甘肃黑方台灌区黄土滑坡区黄土为研究对象,开展了滑坡区盐分调查及水土化学特征分析,进行了不同易溶盐含量及不同含水率条件下的重塑黄土三轴固结不排水剪切试验,探讨盐分及含水率对黄土强度的影响。结果表明：滑坡区土样中易溶盐含量范围为0.15%～4.55%,易溶盐主要是以Na₂SO₄和NaCl为主;水化学分析显示灌溉水入渗及溶滤台塬黄土中的可溶盐,经优势通道在坡脚以泉点渗出等方式排泄并富集;试样抗剪强度随易溶盐含量的增加而提高,随含水率的增加而降低;不同易溶盐及含水率条件下,抗剪强度参数黏聚力变化范围为4.2～57.1 kPa,内摩擦角变化范围为23.1°～33.5°,黏聚力对易溶盐及含水率的变化更敏感。     

Erosion characteristics and horizontal variability for small erosion depths in the Sacramento-San Joaquin River Delta,California, USA

Erodibility of cohesive sediment in the Sacramento-San Joaquin River Delta (Delta) was investigated with an erosion microcosm. Erosion depths in the Delta and in the microcosm were estimated to be about one floc diameter over a range of shear stresses and times comparable to half of a typical tidal cycle. Using the conventional assumption of horizontally homogeneous bed sediment, data from 27 of 34 microcosm experiments indicate that the erosion rate coefficient increased as eroded mass increased, contrary to theory. We believe that small erosion depths, erosion rate coefficient deviation from theory, and visual observation of horizontally varying biota and texture at the sediment surface indicate that erosion cannot solely be a function of depth but must also vary horizontally. We test this hypothesis by developing a simple numerical model that includes horizontal heterogeneity, use it to develop an artificial time series of suspended-sediment concentration (SSC) in an erosion microcosm, then analyze that time series assuming horizontal homogeneity. A shear vane was used to estimate that the horizontal standard deviation of critical shear stress was about 30% of the mean value at a site in the Delta. The numerical model of the erosion microcosm included a normal distribution of initial critical shear stress, a linear increase in critical shear stress with eroded mass, an exponential decrease of erosion rate coefficient with eroded mass, and a stepped increase in applied shear stress. The maximum SSC for each step increased gradually, thus confounding identification of a single well-defined critical shear stress as encountered with the empirical data. Analysis of the artificial SSC time series with the assumption of a homogeneous bed reproduced the original profile of critical shear stress, but the erosion rate coefficient increased with eroded mass, similar to the empirical data. Thus, the numerical experiment confirms the small-depth erosion hypothesis. A linear model of critical shear stress and eroded mass is proposed to simulate small-depth erosion, assuming that the applied and critical shear stresses quickly reach equilibrium.