Dynamic Stress Analysis of a Ballasted Railway Track Bed during Train Passage

Scientific design of a railway track formation requires an understanding of the subgrade behavior and the factors affecting it. These include the effective resilient stiffness during train passage, which is likely to depend on the stress history and the stress state of the ground, and the stress path followed during loading. This study investigates the last of these, by means of a two-dimensional dynamic finite-element analysis. The effects of train speed, acceleration/braking, geometric variation in rail head level, and a single unsupported sleeper are considered. Results indicate that dynamic effects start to become apparent when the train speed is greater than 10% of the Rayleigh wave speed, vc, of the subgrade. At a train speed of 0.5vc, the shear stresses will be underestimated by 30% in a static analysis, and at train speeds greater than vc the stresses due to dynamic effects increase dramatically. Train acceleration/braking may increase shear stresses and horizontal displacements in the soil, and hence the requirement for track maintenance at locations where trains routinely brake or accelerate. For heavy haul freight trains, long wavelength variations in rail head level may lead to significantly increased stresses at passing frequencies (defined as the train speed divided by the wavelength of the variation in level) greater than 15, and short wavelength variations at passing frequencies of 60–70. Stress increases adjacent to an unsupported sleeper occur in the ballast and subballast layers, but rapidly become insignificant with increasing depth.     

砂卵石土的剪切流变特性研究

应用RYL-600岩石剪切流变仪对长沙市某边坡砂卵石土试样进行剪切流变试验,分析砂卵石土的剪切流变特性。试验表明正应力越高时,能够引起砂卵石土试件发生剪切流变破坏的剪切应力也随之增高。砂卵石土剪切流变在低于长期抗剪切强度的应力作用下,表现出黏弹特性;在高于长期强度的应力作用下,表现出黏弹塑性。应用五元黏弹性模型与VR黏塑性模型串联得到的黏弹塑性模型对砂卵石土全程流变曲线进行模拟,将拟合结果与试验数据进行分析比较,验证了新模型具有正确性和合理性,这对砂卵石土工程具有重要的指导意义和参考价值。     

Experimental and Modeling Studies of Permanent Strains of Subgrade Soils

Mechanistic-empirical pavement design guide for flexible pavements as per the AASHTO design guide requires characterization of subgrade soils using the resilient modulus (MR) property. This property, however, does not fully account for the plastic or permanent strain or rutting of subgrade soils, which often distress the overlying pavements. Soils such as silts exhibit moderate to high resilient moduli properties but they still undergo large permanent deformations under repeated loading. This explains the fallacy in the current pavement material characterization practice. A comprehensive research study was performed to measure permanent deformation properties of subgrade soils by subjecting various soils under repeated cycles of deviatoric loads. This paper describes test procedure followed and results obtained on three soils including clay, silt, and sandy soils. The influence of compaction moisture content, confining pressure, and deviatoric stresses applied on the measured permanent deformations of all three soils are addressed. A four-parameter permanent strain model formulation as a function of stress states in soils and the number of loading cycles was used to model and analyze the present test results. The model constants of all three soils were first determined and these results were used to explain the effects of various soil properties on permanent deformations of soils. Validation studies were performed to address the adequacy of the formulated model to predict rutting or permanent strains in soils.     

不同影响因素下路用黄河泥沙动剪切模量和阻尼比试验及理论模型研究

沿黄河高速公路建设过程中,黄河泥沙作为路基填料的可行性已经得到验证和重视,然而目前有关黄河泥沙作为路基填料的动力特性的研究较少.本文利用英国GDS动态三轴试验系统,对取自黄河中下游郑州段的泥沙进行应力控制的动三轴试验,探究了围压、相对密实度和试验频率对黄河泥沙动剪应力–动剪应变关系、动剪切模量G和阻尼比D的影响,绘制了动剪应力–动剪应变关系骨干曲线和滞回曲线.结果表明,黄河泥沙的动剪切模量、阻尼比与剪应变关系可以用Hardin双曲线模型描述,围压对G和D的影响较大、试验频率对G和D的影响较小.综合与其他土体的动力特性对比表明,黄河泥沙动剪切模量折减曲线规律以及阻尼比D曲线规律和其他土体相符,其动力特性更接近于粉土和砂土,但与其他土体并不完全一致,具有一定的特殊性.最后,本文考虑了围压、相对密实度的影响,并结合现有经验公式,建立可以较好描述黄河泥沙最大动剪切模量G_max与围压、孔隙比关系的经验公式,同时建立了动剪切模量比G/G_max和D的数学模型,拟合结果显示,建立的模型能较好地描述黄河泥沙的G/G_max和D随剪应变的变化...     

Shear Strength and Pore-Water Pressure Characteristics during Constant Water Content Triaxial Tests

Shear strength parameters used in geotechnical design are obtained mainly from the consolidated drained (CD) or consolidated undrained (CU) triaxial tests. However in many field situations, soils are compacted for construction purposes and may not follow the stress paths in CD or CU triaxial tests. In these cases, the excess pore-air pressure during compaction will dissipate instantaneously, but the excess pore-water pressure will dissipate with time. Under this condition, it can be considered that the air phase is drained and the water phase is undrained. This condition can be simulated in a constant water content (CW) triaxial test. The purpose of this paper is to present the characteristics of the shear strength, volume change, and pore-water pressure of a compacted silt during shearing under the constant water content condition. A series of CW triaxial tests was carried out on statically compacted silt specimens. The experimental results showed that initial matric suction and net confining stress play an important role in affecting the characteristics of the shear strength, pore-water pressure, and volume change of a compacted soil during shearing under the constant water content condition. The failure envelope of the compacted silt exhibited nonlinearity with respect to matric suction.     

The influence of moisture content on the consolidation properties of hydroxypropylmethylcellulose K4M (HPMC 2208)

The effect of moisture content, compression speed and compression force on the compaction properties of HPMC K4M has been evaluated. As the moisture content increased from 0 to 14.9% w/w, the thickness of HPMC K4M compacts increased at constant compression force and speed. This increase in moisture content also resulted in a marked increase in the tensile strength of the tablets. At a speed of 15 mm s-1 and force of 10 kN, as the moisture content increased from 0 to 14.9% w/w, the tensile strengths increased from 1.34 to 8.54 Mpa. Equivalent tensile strengths could be obtained with less compression force as the moisture content in the polymer was increased. Increasing the compression speed generally decreased the tensile strength of HPMC K4M tablets. The dependence of tablet porosity and tensile strength on compression speeds showed that HPMC K4M is consolidated by plastic deformation. At all compression speeds, an increase in moisture content reduced the percentage elastic recovery of HPMC compacts due to greater tablet consolidation. The lowest elastic recovery (1.18%) was found for tablets made at 15 mm s-1 and 5 kN, containing 14.9% w/w moisture content.     

不同张开度裂隙类岩体循环加卸载下滞回环特征与损伤变形分析

为探究张开度和不同应力等级循环加卸载复合影响下裂隙岩体的损伤特性和裂纹演化规律,利用水泥砂浆制备不同张开度单裂隙类岩石材料,基于RMT-150B岩石力学试验机对类岩石材料开展了3种等级的应力循环加载试验,分析了裂隙类岩石材料的应力—应变曲线特征、滞回环面积变化规律、动弹性模量变化规律及损伤特性.试验结果表明:类岩石材料...     

Laboratory Evaluation of the Briaud Compaction Device

Soil compaction quality control plays an important role in earthwork construction. Compacted dry density is only loosely related to the actual deformation of the compacted soil. Rather than using dry density as the controlling factor for compacted fills, it would be better to measure properties more closely related to soil compressibility. The Briaud compaction device (BCD) is a simple, small-strain, nondestructive testing apparatus that can be used to evaluate the modulus of compacted soils. The use of the BCD as a field testing device for compacted soil quality control may be more beneficial than the current practice of measuring in situ dry density. In this study, the laboratory procedures of the BCD were evaluated for compacted silt. The modulus determined by the BCD was compared to the dynamic elastic moduli (Young’s and shear moduli) determined from ultrasonic pulse velocity testing on the same compacted silt samples. The BCD modulus correlated well with the ultrasonic pulse velocity results with R2 value of 0.8 or better. Finally, a repeatability and reproducibility study conducted on the BCD showed a variation of 4% from the mean when only the soil properties were altered.     

Compaction and elastic relaxation of fullerite powder

Structural changes are considered that arise during loading of fullerite powder under free settlement conditions at room temperature. The shear modulus is evaluated and the relaxation of elastic distortion is demonstrated. Considerable residual distortion during compaction of fullerite and a decrease in lattice parameter are caused by residual macrostresses that reach 850 MPa. Disintegration of a fullerite compact into powder leads to the total removal of macrostresses and consequently the disappearance of the corresponding lattice deformation. This indicates the elastic reversibility of deformation for fullerite under pressure.     

渗透剪切作用下黄土的力学特征

黄土高原地区黄土在灌溉作用下,逐渐达到饱和状态,饱和中,陡坡类黄土坡体自重增加引起下滑力增加. 该过程持续进行后,坡体内部同时发生渗流和剪切过程,导致坡体的变形不断增大,直至破坏后形成滑坡. 本文选取黑方台4.29滑坡为研究对象,在现场调查的基础上,利用滑坡后壁原状黄土试样,基于三轴试验设置10组共60个原状样对饱和黄土的渗透剪切行为进行模拟. 试验中设置了0.5、0.1和0.05 mm·min-1三个不同的加载速率对黄土试样进行剪切,为比较分析,对0.1 mm·min-1剪切速率试样设置了0、1、2和5 m几个不同水头进行了试验. 试验结果表明:饱和黄土在渗流与剪切耦合作用下,表现出应变硬化特征,渗透作用明显降低了黄土的强度,尤其是黄土黏聚力降低,其降幅达5.24%~63.35%. 对已有强度指标拟合后获得黄土在渗透剪切工况下的强度修正公式.      

含水量对压实粘土抗剪强度的影响

通过室内直剪试验,研究了含水量对压实粘土的抗剪强度的影响,并从土体结构与土中基质吸力变化两个方面分析了作用机理。试验表明,随压实含水量增大,粘土的抗剪强度降低,粘聚力随压实含水量增加并非单调变化,其曲线型式类似于“”型,内摩擦角随压实含水量增加大体上是减小的;压实粘土浸水饱和后抗剪强度和粘聚力则显著降低,且压实含水量越小的土体在其它条件相同的条件下因饱和引起的抗剪强度和粘聚力损失越大,内摩擦角受浸水饱和的影响较小。     

Modulus-Suction-Moisture Relationship for Compacted Soils in Postcompaction State

Despite clear evidence, changes in mechanical properties (i.e., stiffness or modulus) of compacted subgrades in response to subgrade moisture regime changes after construction have rarely been investigated in the geotechnical profession. In particular, when in-service assessment of pavement subgrade is made, the modulus-moisture variation should be addressed on the basis of unsaturated soil mechanics. This study presents the unsaturated small-strain modulus behavior of five predominately fine-grained compacted subgrade soils. The small-strain shear modulus (Go) of saturated compacted specimens subjected to a desorption soil-water characteristic curve (SWCC) was evaluated using bender elements. A test apparatus was designed to apply two stress state variables, the net confining pressure and matric suction, during the Go measurements. The relationship between Go and the SWCC under a constant mean net stress was developed. Additionally, the effect of compaction moisture content, compaction energy, and soil type on the Go-SWCC relationship was investigated. Finally, a relationship describing the small-strain modulus behavior of unsaturated compacted soils is proposed.     

Effective Stress Behavior of Quasi-Saturated Compacted Cohesive Soils

Important geotechnical structures constructed on compacted cohesive soils often involve compaction either around or on the wet side of optimum water content. In general, at these water content values, water voids are continuous and air voids are occluded, and the soil may be assumed to be in a state termed as “quasi-saturated.” This paper evaluates the effective stress behavior of such quasi-saturated compacted specimens of Gangetic silt and Canyon dam clay in the broad framework of the conventional modified Cam-clay model. The initial state of quasi-saturated compacted specimens is shown to lie on the recompression line in w versus ln(p′) space. The actual recompression line on which the specimen state would lie, and the corresponding equivalent past maximum pressure, are found to depend only on the amount of compaction energy and the soil structure, and are independent of the molding water content or initial dry density. It is observed that, at low effective confining stresses, quasi-saturated compacted soils behave like overconsolidated soils and the effective stress paths during undrained shear lie on the Hvorslev surface. However, at confining stresses greater than the past maximum pressure, these soils behave like normally consolidated soils and the effective stress paths move practically along the Roscoe surface toward the critical state line.     

Vehicle Induced Dynamic Behavior of Short-Span Slab Bridges Considering Effect of Approach Slab Condition

In this paper the vehicle induced dynamic bridge responses are calculated by modeling the bridge and vehicle as one coupled system. The dynamic behavior of short slab bridges with different span lengths induced by the AASHTO HS20 truck is investigated. A parametric study is conducted to analyze the effects of different truck speeds and different road surface conditions. Critical truck speeds that result in peaks of dynamic response are found to follow the rule that describes the resonant vibration of bridges due to train loading. The approach slab condition that consists of faulting at the ends and deformation along the span is considered in the analysis. Although the effect of the along-span deformation on the dynamic response of bridges is trivial, the faulting condition of the approach slab is found to cause significantly large dynamic responses in short-span slab bridges. Impact factors obtained from numerical analyses are compared with those values specified in the AASHTO codes.     

Postcyclic Degradation of Strength and Stiffness for Low Plasticity Silt

Stress-controlled undrained cyclic triaxial tests followed by strain-controlled monotonic compressive shear tests were carried out on normally consolidated and overconsolidated reconstituted Keuper Marl silt to investigate the strength and stiffness degradation characteristics of a low plasticity silt. Special attention was paid to the changes in undrained strength and deformation modulus after undrained cyclic loading. It was observed that cyclic degradation in stiffness for low plasticity silt is more marked than that of strength, and this tendency increases with increasing overconsolidation ratio. It was found that a previously proposed model for predicting postcyclic degradation in strength and stiffness of normally consolidated fine-grained soils could be applied to that of overconsolidated silt but not however to the postcyclic degradation in Young’s modulus. Thus, an attempt was made to correlate postcyclic degradation of overconsolidated silt to the equivalent cyclic shear strain instead of the normalized excess pore pressure. It was concluded that cyclic shear strain was a better parameter than cyclic-induced excess pore pressure for correlating the postcyclic stiffness degradation not only for normally consolidated but also for overconsolidated silt.     

Effects of High Strain Rate on Properties and Microstructure Evolution of TWIP Steel Subjected to Impact Loading

 The mechanical properties of the TWIP steel subjected to impact loading at various strain rates were analyzed by the Split Pressure Hopkinson Bar. Meanwhile the microstructure of the TWIP steel fore-and-after the dynamic deformation were characterized and analyzed by optical microscopy (OM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The result shows that when the TWIP steel was deformed under dynamic station, the stress, microshardness and work hardening rate increase with the increment of strain and strain rate; there exist stress fluctuation and decline of work hardening rate for adiabatic temperature rising softening. There exist many pin-like deformation twins in the microstructure of the TWIP steel subjected to impact loading, the grain size after deformation is bigger than that before; the interaction of twins with dislocation and twins with twins, especial emergence of high order deformation twins are the main strengthening mechanisms of the TWIP steel. The nucleation mechanism of deformation twins will be “rebound mechanism”; the incomplete deformation twins can be observed when the strain rate is low; when strain rate raises, deformation twins unite together; furthermore, deformation twins become denser because the nucleation rating enhancing with strain rate increasing.     

含水率对尾矿抗剪强度影响的试验研究

尾矿含水率的变化对尾矿的抗剪强度有很大的影响,研究它们之间的变化规律对工程实践有着重要意义。该文以云南某铜矿的尾矿为研究对象,在分析其基本物理特性的基础上,通过直剪试验研究了在相同压实度条件下含水率对尾矿抗剪强度特性的影响。其结果表明:尾矿的抗剪强度与初始含水率存在相关关系;当尾矿含水率从6.30%增加到18.31%时尾矿的黏聚力随着含水率的增加而增大,当尾矿含水率从18.31%增加到24.14%时尾矿的黏聚力随着含水率的增加而减小;而在尾矿含水率从6.30%到24.14%变化过程中,内摩擦角总体上先增加后减小,但仅在24.64°~26.24°变化,有明显的峰值。通过回归分析,得出了尾矿黏聚力随着含水率变化的拟合公式。该研究成果对尾矿库的稳定性分析具有重要的指导意义。     

Conditions for Compaction Band Formation in Porous Rock Using a Two-Yield Surface Model

Compaction bands, a form of localized deformation found in field and laboratory specimens of high porosity rock, consist of planar zones of pure compressional deformation that form perpendicular to maximum compression. Experimentalists report compaction bands and/or shear bands (angled to maximum compression) in high porosity sandstone during a transitional loading regime with multiple active deformation mechanisms. Conditions for localized deformation are determined using a two-yield surface constitutive model and bifurcation theory. The shear yield surface corresponds to a dilatant, frictional mechanism while the cap corresponds to a compactant mechanism. Unlike a single yield surface model, the two-yield surface model predicts both experimentally observed band types for reported values of key material parameters. Observed and predicted shear band angles generally agree. Theory suggests that shear band formation may coincide with activation of the shear yield surface by a previously active cap. If the bulk hardening modulus, k, equals zero (corresponding to localization on the peak or plateau of the mean stress–volume strain curve) compaction band conditions are more favorable than for small positive values of k.     

Defining Yield from Bender Element Measurements in Triaxial Stress Probe Experiments

This paper describes the elastic response of a block sample of compressible Chicago glacial clay under a variety of stresses and its relationship with the deformation characteristics at relatively large strains. The elastic shear stiffness was obtained from bender element tests during consolidation and shearing in drained triaxial stress probe tests. An empirical correlation was established based on the elastic shear stiffness in a preyield condition. By comparing the empirical correlation with the measured elastic shear stiffness in the stress region during probing, the changes of elastic shear stiffness were investigated. The departure of elastic shear stiffness from values computed by the empirical relation based on K0 loading directly relates to the yielding characteristics of the clay. The large-scale change of soil structure at yielding alters the well-established relationship between the elastic shear stiffness and stresses in the preyield condition. The mechanical yielding response of clays can be detected based on the systematic analysis of the elastic shear wave velocities.     

砂岩单轴压缩变形规律及声发射特性

为准确确定岩石岩爆倾向性,对砂岩进行单轴压缩条件下声发射检测试验,研究岩样单轴压缩变形规律与声发射特征.对荷载-变形曲线、AE参数进行统计分析,结果表明,压密阶段和线弹性阶段声发射振铃计数较少,累积能量曲线十分平缓,释放弹性应变能较低,从塑性变形阶段开始,AE活动频繁,累积能量呈指数级释放,损伤加快,声发射类型由之前的突发型变为连续型,应变曲线拐点处,往往出现AE参数峰值.分别对AE振铃计数、累积能量和岩石损伤进行监测分析,能较准确判断砂岩岩爆倾向性.