Contact Erosion at the Interface between Granular Coarse Soil and Various Base Soils under Tangential Flow Condition

Under embankment dams and dykes, horizontal groundwater seepage prevails. If the subsoil is layered, and if some coarse layers are not appropriate filters for finer layers, there can be contact erosion at the interface between fine and coarse soils. In order to study contact erosion threshold, some base-soil and coarse-soil combinations were submitted to a flow parallel to the interface between the coarse soil and the base soil. Critical velocities and critical hydraulic gradients were measured for various base soils. Using effective base-soil grain diameter, an empirical expression for critical velocity was proposed that is well adapted for silts or sand/clay mixtures as well as for sands. The mass of eroded soil was measured relative to the flow velocity for each base-soil/coarse-soil setup. The shear stress applied to the interface between base soil and coarse soil was derived from the hydraulic gradient. Using an empirical relationship between applied shear stress and measured eroded mass, erosion rate was estimated for each base-soil/coarse-soil setup.     

Experimental Parametric Study of Suffusion and Backward Erosion

Within hydraulic earth structures (dikes, levees, or dams), internal seepage flows can generate the entrainment of the soil grains. Grain transportation affects both particle size distributions and porosity, and changes the mechanical and hydraulic characteristics of the earth’s structure. The occurrence of failures in new earth structures due to internal erosion demonstrates the urgency of improving our knowledge of these phenomena of erosion. With this intention, a new experimental device has been developed that can apply hydraulic stresses to reconstituted consolidated cohesive soils without cracks in order to characterize the erosion evolution processes that might be present. A parametric study was conducted to examine the influence of three critical parameters on clay and sand erosion mechanisms. When the hydraulic gradient was low, it was concluded that the erosion of the structure’s clay fraction was due to suffusion. When the hydraulic gradient increased, it was concluded that the sand fraction erosion initiation was due to backward erosion. The extent of the erosion was dependent on the clay content. The study underlines the complexity of confinement stress effects on both erosion phenomena.     

Erosion Function Apparatus for Scour Rate Predictions

Scour is the number one cause of bridge failures. Scour in coarse grained soils (sand, gravel) is relatively well known, but scour in fine grained soils (silt, clay) and weak rock is not. In coarse grained soils, scour takes place very rapidly and the scour rate is rarely an issue because one flood is likely to create the maximum scour depth. In fine grained soils, the scour process is much slower; as a result, even after a hundred years, a bridge may not experience the maximum depth of scour. Therefore, in fine grained soils it becomes necessary to predict the rate at which scour takes place. A new apparatus called the EFA (Erosion Function Apparatus; 〈http:∕/tti.tamu.edu∕geotech∕scour〉) has been built and tested to measure the erosion rate of fine grained soils; the EFA can also be used to measure the erosion rate of coarse grained soils if necessary. The end of a Shelby tube sample from the bridge site is fitted through a tight opening at the bottom of a pipe with a rectangular cross section. Water flows through the pipe and erodes the soil sample, which protrudes 1 mm above the bottom of the pipe. The rate at which the sample erodes is measured, and the shear stress imposed by the water on the soil is calculated. The plot of erosion rate versus shear stress is the result of an EFA test. It indicates the critical shear stress at which erosion starts and the rate of erosion beyond that shear stress. This function can be used to predict the rate of scour at a bridge.     

Experimental Investigation of the Hydraulic Erosion of Noncohesive Compacted Soils

This paper presents the results of a laboratory investigation whose purpose was to evaluate the effects of compaction on the erodibility of cohesionless soils. By means of a recently developed flume experiment, sediment erosion rates and incipient motion, as a function of shear stress, average velocity, and dry density, have been determined for three compacted sand and gravel mixtures. A preliminary comparison of the incipient motion values shows that granular soils compacted at the Proctor optimum have a higher resistance to free surface flow erosion than those compacted at lower and higher densities. This leads one to infer that the Proctor optimum, generally used as a standard for construction, might also be an optimum for hydraulic resistance and stability. Additional comparison of the experimental data with two commonly used incipient motion criteria also suggests that Yang’s criterion is a better predictor of soil detachment than the Shields-Yalin criterion.     

Durability of Cement Stabilized Low Plasticity Soils

Three testing methods for predicting the durability of cement-stabilized soils—the tube suction (TS), 7-day unconfined compression strength (UCS), and wetting–drying durability tests—were tested and compared for their correlations and influence factors using a problematic low plastic silt clay from subgrade commonly encountered in Louisiana. A series of samples was molded at six different cement dosages (2.5, 4.5, 6.5, 8.5, 10.5, and 12.5% by dry weight of the soil) and four different molding moisture contents (15.5, 18.5, 21.5, and 24.5%). The test results indicate that the water–cement ratio of cement-stabilized soil had the dominant influence on the maximum dielectric value (DV), 7-day UCS, and durability of stabilized samples tested, although the dry unit weight of cement-stabilized soil could cause the variation of the results. This study confirms that TS, 7-day UCS, and wetting-drying durability tests are equivalent in predicting durability, and tentative charts to ensuring the durability of cement-stabilized low plasticity soils are developed using their 7-day UCS or the maximum DV values.     

Dynamic Properties of Chemically Stabilized Sulfate Rich Clay

A series of resonant column tests was conducted on chemically stabilized specimens of sulfate-rich expansive clay from southeast Arlington, Tex. Specimens were tested for different stabilizer types, stabilizer dosages, compaction moisture contents, and confining pressures. Three chemical stabilization methods were used: sulfate resistant type V cement, low calcium class F fly ash, and lime mixed with polypropylene fibers. Results in the small-shear strain amplitude range (<0.0001%) were analyzed to assess the influence of compaction moisture content and confining pressure on the linear shear modulus Gmax and material damping Dmin of stabilized soil. Tests were also conducted at small- to mid-shear strain amplitude levels (0.0001–0.01%) to assess the threshold strain limit γth for each treatment method, and to study the effects of torsional shearing on the rate of degradation of normalized modulus G/Gmax of treated soil. A 10%-by-weight dosage of sulfate resistant type V cement was found to give the highest modulus and lowest damping when compacted at 95% of maximum dry unit weight γd-max on the wet side of Proctor optimum.     

Levee Erosion by Overtopping in New Orleans during the Katrina Hurricane

Erodibility of a soil is defined here as the relationship between the erosion rate of a soil dz/dt and the velocity v of the water flowing over it, or the relationship between the erosion rate of a soil dz/dt and the shear stress developed by the water at the water-soil interface. This is called the erosion function. The test used to measure the erosion function of the levee soils is the erosion function apparatus test. The test consists of eroding a soil sample by pushing it out of a thin wall steel tube and recording the erosion rate for a given velocity of the water flowing over it. Several velocities are used and the erosion function is defined. A new erosion category chart is proposed to reduce the erodibility of a soil or rock to a single category number. Twenty three samples were retrieved from 11 locations at the surface of the levees around New Orleans. Thirteen were samples from Shelby tubes while ten were bag samples. The results obtained show a large variation of erosion resistance among the soils tested. Some of the levees associated with the location of the samples resisted the overtopping erosion very well; others eroded completely. On the basis of the erosion test results and of the observed behavior of the levees during the hurricane, a chart is presented which can be used to select soils for overtopping resistance. Numerical simulations were performed using the program CHEN 3D to obtain the distribution of velocity vectors in the overtopping flow and of shear stresses at the interface between the water and the levee surface. The comparison of the numerical simulation results and of the erosion function gives added credibility to the proposed levee overtopping erosion chart.     

Addressing Sulfate-Induced Heave in Lime Treated Soils

Civil engineers are at times required to stabilize sulfate-bearing clay soils with calcium-based stabilizers. Deleterious heaving in these stabilized soils may result over time. This paper addresses critical questions regarding the consequences of treating sulfate laden soils with calcium-based stabilizers. The authors describe the nature (chemistry and structure) of the minerals (ettringite/thaumasite) blamed for deleterious reactions and explain why these structures may lead to damage. The writers also describe the mechanisms of the mineral growth, and the extent of mineral growth based on the amount of sulfate minerals present in the soil. The writers explain why the rate of ettringite growth in treated soils should not be expected to follow a controlled rate of ettringite development such as that which normally occurs in portland cement concrete. The writers compare the rate and degree of ettringite development in soils to the classical model of nucleation and growth typical of most crystal structures. Finally, the writers evaluate the role of soil mineralogy in controlling soil behavior at varying sulfate contents and verify the existence of a threshold level of soluble sulfates in soils that can trigger substantial ettringite growth.     

Lessons Learned for Ground Movements and Soil Stabilization from the Boston Central Artery

This paper summarizes lessons learned about soil stabilization with the deep mixing method (DMM) as it was developed and applied over 10 years during construction of the Boston Central Artery and Tunnel (CA/T). It also summarizes lessons about the control of excavation-induced ground movements and their characteristics. Deep deposits of marine clay were stabilized with DMM for large open cuts at Bird Island Flats and Fort Point Channel, both of which are described with respect to site conditions, soil properties, DMM installation and characteristics, and measured field performance. Topics addressed in this paper include water pressure distribution behind DMM walls, statistical characterization of soil cement properties, quality control/quality assurance procedures, comparison of measured and numerically simulated deformation in clay stabilized with various configurations of soil cement elements, shear modulus degradation characteristics of in situ soil cement, and ground movement patterns. Recommendations are made for soil cement properties, installation procedures, analytical modeling, design, and inspection.     

Responses of Excess Pore Water Pressure in Soft Marine Clay around a Soil–Cement Column

The soil ground treated by deep cement mixing (DCM) in the field normally consists of cement–soil mixed columns and untreated soils. Although many attempts have been made, research on the consolidation behavior of the treated soil ground has been limited. To better understand the consolidation process of the DCM treated ground, in this study, an axisymmetric physical model test with full instrumentation was carried out. The physical model ground consisted of a central cement–soil column and surrounding soft soil. Excess pore water pressures in the soil and vertical pressures carried by the DCM column and the untreated soil were recorded throughout the test. Responses of excess pore pressure under loading and unloading stages are highlighted. Based on the data analysis, it is revealed that the improved ground consolidates faster than the pure soil ground. The major reason is considered to be that the DCM column reduces the vertical stress increment in the soil and results in a lower value of excess pore pressure. The decrease of excess pore water pressure in the middle of soil seems to be controlled by the reduction of total stress in the soil. Besides, a delayed pore water pressure increase was observed in the early period of the loading stage. During the unloading stages, the stress on the DCM column was found to be reduced by approximately the same magnitude as the decrease in the vertical pressure on the model ground. In addition, a small residual pore pressure in the soil was also found at the end of the unloading stages.     

Automated Sediment Erosion Testing System Using Digital Imaging

Measurement of vertical profiles of the critical shear stress, τc, and the erosion rate, E, from the same undisturbed sediment core is crucial for modeling the resuspension of fine-grained natural sediments. The automated sediment erosion testing system (ASETS) was developed to determine profiles of τc and E with centimeter spatial (vertical) resolution in an undisturbed (Shelby tube) sediment core, whose surface was eroded by steady turbulent flow through a flume. The unique feature of ASETS is that it is a real-time imaging method that accurately determines the position of the core surface during erosion for both calculating the vertical profile of E and controlling a motor-driver system that automatically pushes up the core to maintain its surface flush with the flume bottom. Undisturbed, field cores were tested over a range of flow (average bed shear stress, τb) conditions. The amount of eroded sediment from both optical backscattering measurements and the imaging method were in good agreement, which validated ASETS. Measured vertical profiles of τc and E were similar to those reported in literature. E correlated well with (τb?τc)2, which agrees with previous results in literature.     

Determining Erodibility, Critical Shear Stress, and Allowable Discharge Estimates for Cohesive Channels: Case Study in the Powder River Basin of Wyoming

The continuous discharge of coalbed natural gas-produced (CBNG-produced) water within ephemeral, cohesive channels in the Powder River Basin (PRB) of Wyoming can result in significant erosion. A study was completed to investigate channel stability in an attempt to correlate cohesive soil properties to critical shear stress. An in situ jet device was used to determine critical shear stress (τc) and erodibility (kd); cohesive soil properties were determined following ASTM procedures for 25 reaches. The study sites were comprised of erodible to moderately resistant clays with τc ranging from 0.11?to?15.35?Pa and kd ranging from 0.27?to?2.38?cm3/N?s. A relationship between five cohesive soil characteristics and τc was developed and presented for use in deriving τc for similar sites. Allowable discharges for CBNG-produced water were also derived using τc and the tractive force method. An increase in the allowable discharge was found for channels in which vegetation was maintained. The information from this case study is critical to the development of a conservative methodology to establish allowable discharges while minimizing flow-induced instability.     

管道内气固两相流冲刷磨损特性数值模拟

针对有色冶金中低温烟气对管道材料的冲刷磨损,利用计算流体力学软件Fluent对气固两相流开展数值模拟研究,讨论了冲刷磨损量和切应力在不同烟气速度、颗粒直径、颗粒含量时的变化情况。结果表明：气固两相流冲刷集中磨损区位于管道前部约1/5处,最大磨损发生在管道入口后部;烟气速度增大时,磨损量与切应力都增大;在一定颗粒直径范围内,颗粒直径增大时,磨损量减小,切应力几乎不变;随着颗粒含量增加,磨损量增大,切应力基本不变。     

加档坝后高炉炉缸内剪应力对侵蚀的影响

炉缸内铁水流动产生的剪应力对炉缸内衬的侵蚀有重要影响。为此,以流体力学有关理论为基础,建立了炉缸炉底三维流体数学模型,应用CFX软件,研究了不同时期的炉缸剪应力的变化;由于铁水环流对炉缸的侧壁以及炉缸侧壁与炉底交界部位的冲刷作用较强,因此在炉缸侧壁和炉底位置修砌一道环形档坝,观察其对炉底剪应力的影响。结果表明,炉底出铁口近端受到的剪应力较大,而在出铁口远端炉底剪切应力最小;炉底剪应力随着死铁层深度的增大而减小;增加档坝可以有效地减轻炉底受到的剪应力,炉底剪应力越大,增加档坝后减轻的炉底受到的剪应力值越大。     

Anisotropic Strength Evaluation of Clay Reinforced with Grout Piles

Grout piles are often used to reinforce the base soil against base heave when carrying out deep excavations in soft clay. However, there is still a lack of an adequate criterion to describe the shear strength of clay reinforced with grout piles. In general, the anisotropic strength characteristic of clay reinforced with grout piles is more significant than that of clay. The objective of this work is to develop an anisotropic strength criterion for the reinforced soil mass. Only four parameters are needed in this anisotropic strength criterion: two are the strength properties of the in situ clay, namely, the axial compressive and axial extensive undrained shear strengths; another is the undrained shear strength of treated soil; and the final is the improvement ratio which is related to the spacing and layout pattern of the grout piles. To be used in two-dimensional undrained stability analysis, the suitability of this anisotropic strength criterion under plane strain conditions is verified by comparing the results with true triaxial test. The maximum difference between the calculated and laboratory measured shear strengths is less than 8%. The results of this study indicate that the anisotropic undrained shear strength of clay reinforced with grout piles under plane strain condition decreases with an increase in the angle between the vertical direction and the major principal stress and decreases with a decrease in the strength anisotropy ratio of clay reinforced with grout piles. However, there will be a greater improvement in the effect if the grout piles are installed in the active zone rather than in the passive zone. This is because the shear strength of a grout pile mobilized in the active zone is close to its maximum level.     

Constitutive Model of Soil Based on a Dynamical Systems Approach

A soil when sheared ultimately reaches a steady-state condition at which it deforms at a constant shear stress, effective normal stress, and void ratio. Various systems in nature dynamically evolve similarly from some initial condition, to a final steady-state condition. Such systems have been studied using dynamical systems theory. This technical note uses this theory to model monotonic shear of soil as a dynamical system. The principle proposed is simple—the rates of change of the shear stress, effective normal stress, and void ratio are proportional to the applied values of the shear and effective normal stress with the proportionality values decaying with strain until ultimately these proportionality values become zero at the steady-state condition. It provides a well-formed qualitative principle that fits closely the stress-strain-void ratio curves of undrained shear tests on uncemented, resedimented clays at various over consolidated ratios (OCRs), and drained shear tests on sands and silts at various relative densities, for various stress paths including compression, extension from standard triaxial, and true-triaxial tests. For the undrained shear of resedimented clay, these proportionalities and their decay rates vary smoothly with OCR. For drained shear of sand and silt, the model parameters show orderly variation with relative density. Its value lies in that a well-formed qualitative principle derived from the steady-state condition provides an alternate approach to current complex elastoplastic models based on critical state theory.     

State Boundary Surfaces for an Aged Compacted Clay

The yielding and the peak strength of an aged compacted clay were studied by conducting a series of suction-controlled triaxial tests. The test results were interpreted using the framework of intrinsic properties of reconstituted soil. The peak strength envelopes of undisturbed samples lie above those of reconstituted samples. The suction provides additional attractive forces to stabilize the soil structure, which result in the augmentation of the yield stress and peak strength envelope. The shear strength is normalized by the equivalent preconsolidation pressure (pe′) and Hvorslev surfaces are identified from undisturbed samples which expand with suction. A single peak strength envelope and Hvorslev surface will be emerged from the saturated and unsaturated (degree of saturation >80%) samples if the shear strength data are presented in terms of the average skeleton stress. The influence of the soil structure on the shear strength of the aged compacted clay may be measured by the ratio of normalized strengths at the intrinsic critical state which is about 1.26     

Scour of Cohesive Soil by Submerged Circular Turbulent Impinging Jets

This paper introduces a method for estimating the scour in cohesive soils produced by a submerged vertical circular turbulent impinging jet. Determining scour in cohesive soils is a complex problem, partly because the clay particles within the soil are held together by electrochemical forces that are not easily quantifiable. As well, erosion occurs in many forms, such as the removal of individual particles or as large chunks of soil. Results of a laboratory study of scour by a circular impinging jet of a cohesive soil, consisting of 40% clay, 53% silt, and 7% fine sand, are presented. Analysis based on the mechanics of the impinging jets shows that the dimensions of the scour hole at an equilibrium state of scour are a function of the momentum flux from the jet, the impingement height (for “large” impingement heights), the viscosity and density of the eroding fluid, and the critical shear stress of the soil. Mass erosion was the predominant type of erosion observed.     

Erosion of the Upper Layer of Cohesive Sediments: Characterization of Some Properties

A recent companion paper reported an experimental protocol used to analyze sediment properties. This protocol identified for both freshwater and marine sediments a surface layer with specific dynamic properties (critical erosion shear stresses in the range 0.025–0.05?N?m?2) and a second layer with critical erosion shear stresses about ten times larger. The present study compares these former results with recent work which extended the applicability domain of the Shields diagram to very fine particles. The surface layer is shown to consist in fine and unconsolidated sediments that behave like noncohesive material whereas the second layer is characterized as being cohesive. The surface layer is mainly representative of recent deposits of suspended particles. This points out the existence of a fluffy layer of fine sized particles resting near the bed, with specific erosion characteristics, which has to be considered separately when studying sediment properties.     

Measurements of Sediment Erosion and Transport with the Adjustable Shear Stress Erosion and Transport Flume

Soil and sediments play an important role in water management and water quality. Issues such as water turbidity, associated contaminants, reservoir sedimentation, undesirable erosion and scour, and aquatic habitat are all linked to sediment properties and behaviors. In situ analysis is necessary to develop an understanding of the erosion and transport of sediments. Sandia National Laboratories has recently patented the Adjustable Shear Stress Erosion and Transport (ASSET) Flume that quantifies in situ erosion of a sediment core with depth while affording simultaneous examination of transport modes (bedload versus suspended load) of the eroded material. Core erosion rates and ratios of bedload to suspended load transport of quartz sediments were studied with the ASSET Flume. The erosion and transport of a fine-grained natural cohesive sediment were also observed. Experiments using quartz sands revealed that the ratio of suspended load to bedload sediment transport is a function of grain diameter and shear stress at the sediment surface. Data collected from the ASSET Flume were used to formulate a novel empirical relation for predicting the ratio of bedload to suspended load as a function of shear stress and grain diameter for noncohesive sediments.