Behavior of geogrid-reinforced ballast under various levels of fouling

This paper presents a study of how the interface between ballast and geogrid copes with fouling by coal fines. The stress-displacement behavior of fresh and fouled ballast, and geogrid reinforced ballast was investigated through a series of large-scale direct shear tests where the levels of fouling ranged from 0% to 95% Void Contamination Index (VCI), at relatively low normal stresses varying from 15 kPa to 75 kPa. The results indicated that geogrid increases the shear strength and apparent angle of shearing resistance, while only slightly reducing the vertical displacement of the composite geogrid-ballast system. However, when ballast was fouled by coal fines, the benefits of geogrid reinforcement decreased in proportion to the increasing level of fouling. A conceptual normalized shear strength model was proposed to predict this decrease in peak shear stress and peak angle of shearing resistance caused by coal fines at a given normal stress.     

Characterization of the Fouled Ballast Layer in the Substructure of a 19th Century Railway Track Under Renewal

The purpose of these field and lab studies undertaken during rehabilitation work being done on an ancient railway line was to characterize a layer of ballast fouled with soil found in the track substructure. The field studies included the characterization of the thickness, grain size distribution and void ratio of the fouled ballast layer, as well as a large number of plate load tests, both on the fouled ballast layer and on the subgrade. The resilient behaviour of the fouled ballast was evaluated in the lab by cyclic triaxial tests on large size reconstituted specimens with distinct fouling indexes (different grain size distribution) and distinct humidity states (dry or wet). The results obtained were used as support for the decision to maintain the fouled ballast layer under the new sub-ballast in a number of stretches of the renewed line.     

Mechanical characterisation of the fouled ballast in ancient railway track substructure by large-scale triaxial tests

In the track substructure of ancient railways in France, a fouled ballast layer has often been created with time. The mechanical behaviour of this coarse soil was studied in the laboratory using a large-scale triaxial cell. The soil taken from the fouled ballast layer of an ancient railway was re-compacted to a dry density of 2.01 Mg/m³ at three water contents (4, 6, and 12%) corresponding to three values of the initial degree of saturation (32, 48, and 100% respectively). Both monotonic and cyclic triaxial tests were performed under constant water content conditions. The experimental results gave the following evidence of the significant effect of the water content on the soil mechanical behaviour: (i) the lower the compaction water content, the higher the shear strength; (ii) a permanent axial strain of 0.4% was found after a large number of cycles at a water content of 4%, while it was 1.4% at the higher water content of 6%. For the saturated soil specimen, failure was even observed after a limited number of cycles. Based on the results obtained, a constitutive model for permanent deformation was elaborated, that accounts for the stress level, the number of cycles and the soil water content.     

Identification of ballast grading for rail track

Grading has long been recognised to critically influence the mechanical behaviour of ballast.To identify the ballast grading for heavy-haul rail track,monotonic and cyclic triaxial tests are conducted to assess the performances of different gradings.Permanent deformations,aggregates degradation,resilience,shear resistance,maximum and minimum densities are recorded and analysed.The grading is found to affect the behaviour of ballast in that coarser gradings exhibit relatively better strength,resilience and therefore less permanent deformation.However,ballast degradation increases with the overall aggregate size.Therefore,to identify the grading for ballast with different performance objectives,a grey relational theory is used to convert the multi-objective into single-objective,i.e.grey relational grade.A relatively optimal grading that provides the highest grey relational grade is thus suggested for the improved ballast performance.     

Drainage potential of degraded railway ballast considering initial gradation and intrusion of external fine materials

The aggregate in the ballast layer of the railway track structure degrades due to the splitting of single particles and the abrasion of the angular corners, resulting in a ballast course comprised of degraded aggregate with a lower drainage potential. In the present study, the hydraulic conductivity of clean, degraded and clay-fouled degraded ballast specimens was evaluated considering the initial particle size distribution of aggregate and the degradation mechanism to investigate the permeability of ballast fouled by both internal and external factors. First, degraded ballast aggregate was generated by using a large-scale impact loading testing apparatus. Then, a large-scale constant head permeability test was performed to estimate the hydraulic conductivity of degraded ballast aggregate contaminated with clay. Based on the obtained results, the flow regime through extremely degraded ballast remained between a completely turbulent regime (observed in the case of clean ballast) and a laminar flow regime. Darcy’s law was shown to be valid for degraded aggregate samples contaminated with clay. While the degradation of the aggregate had the expected effect of reducing hydraulic conductivity, the abrasion of asperities was shown to have an even greater impact on hydraulic conductivity. Also, the further degradation of aggregate under impact loading was shown to accelerate the reduction in the permeability of clay-fouled degraded ballast to an unacceptable extent.     

Effectiveness of geogrid reinforcement in improvement of mechanical behavior of sand-contaminated ballast

Vertical stiffness and shear strength of ballasts are significantly degraded when contaminated with sands. There is a lack of solutions/studies related to strengthening ballast against sand contamination. Addressing this limitation, a comprehensive laboratory investigation was made on effectiveness of geogrid reinforcement for improvement of mechanical properties of sand-contaminated ballast. To this end, large-scale direct shear tests as well as plate load tests were conducted on geogrid-reinforced ballast samples prepared with different levels of sand contamination. The obtained results indicate that geogrid reinforcement considerably improves shear strength and vertical stiffness of contaminated ballast. A bandwidth was obtained for contamination levels in which ballast reinforcement is effective. Through examining geogrid with different aperture sizes and locations in the ballast layer, the best performance conditions of geogrid reinforcement were derived. The results were used to propose an effective method of ballast reinforcement and an efficient ballast maintenance approach in sandy areas.     

Investigating geogrid-reinforced ballast: Experimental pull-out tests and discrete element modelling

This paper presents an evaluation of the interlocking behaviour of geogrid-reinforced railway ballast. Experimental large box pull-out tests were conducted to examine the interaction between ballast and a biaxial geogrid. The discrete element method (DEM) was then used to model the interaction between the ballast and the geogrid by simulating large box pull-out tests and comparing the findings with the experimental results. Four different shapes of clumps were used to represent each ballast particle in order to obtain an acceptable shape for modelling the railway ballast. The DEM simulation results were shown to provide good predictions of the pull-out resistance and to examine the effect of clump shape on both the pull-out resistance and the distribution of contact forces. Therefore, the calibrated geogrid model and the 8-ball tetrahedral clumps, used as ballast particles, hold much promise for investigating the interaction between geogrids and ballast, and thus, optimising performance.     

循环荷载下土工格栅加筋铁路道床累积#br# 沉降模型试验研究

散粒体道床层的循环累积变形是有砟轨道沉降的主要来源，文章采用有砟轨道路基模型试验来研究道砟层与底砟层间布置土工格栅加固道床控制有砟轨道沉降，开展了高速和重载等不同列车荷载下与不同类型土工格栅加固条件下的多组循环加载试验，加载过程中全程监测道砟层累积沉降、轨枕振动、道砟层底部土压力与土工格栅应变数据并进行分析，以研究土工格栅控制有砟轨道道床沉降的作用机理。研究结果表明：土工格栅对于控制道砟层累积沉降具有较显著的效果，但当土工格栅的刚度达到一定程度后刚度对土工格栅控制沉降效果的影响不大；土工格栅能够显著减小轨枕下方道砟层土压力峰值，从而降低道砟磨耗破碎程度及由此引起的道床沉降；道砟层累积沉降的发展意味着道砟颗粒的错动，而道砟颗粒的错动与位移将引起嵌锁于道砟层中的土工格栅发生张拉，从而土工格栅将反过来对道砟颗粒的进一步位移错动形成侧向约束，体现于宏观即表现为道砟层累积沉降得到控制。     

煤矸石在市政道路中的应用研究

将煤矸石作为市政道路路基填料的一部分,不仅节约成本,而且可以减小环境污染。首先通过室内试验得到煤矸石的物理力学特性,然后利用现场试验段试验,对不同碾压方式及碾压遍数条件下,煤矸石路基的累计沉降值及压实度进行研究,研究表明:①煤矸石的物理力学指标满足路基填料条件;②煤矸石路基随着碾压遍数的增加,累计沉降值先快速增加,然后增加缓慢,且个别观测点处累计沉降值变形规律可能会出现回弹;③“静压1遍+振压6遍”后煤矸石路基的累计沉降差及压实度均满足路基压实要求。     

Combined coagulation-disk filtration process as a pretreatment of ultrafiltration and reverse osmosis membrane for wastewater reclamation: an autopsy study of a pilot plant

The effects of the combined coagulation-disk filtration (CC-DF) process on the fouling characteristics and behavior caused by interactions between effluent organic matter (EfOM) and the membrane surfaces of the ultrafiltration (UF) and reverse osmosis (RO) membranes in a pilot plant for municipal wastewater reclamation (MWR) were investigated. The feed water from secondary effluents was treated by the CC-DF process used as a pretreatment for the UF membrane to mitigate fouling formation and the permeate from the CC-DF process was further filtered by two UF membrane units in parallel arrangement and fed into four RO modules in a series connection. The CC-DF process was not sufficient to mitigate biofouling but the UF membrane was effective in mitigating biofouling on the RO membrane surfaces. Fouling of the UF and RO membranes was dominated by hydrophilic fractions of EfOM (e.g., polysaccharide-like and protein-like substances) and inorganic scaling (e.g., aluminum, calcium and silica). The desorbed UF membrane foulants included more aluminum species and hydrophobic fractions than the desorbed RO membrane foulants, which was presumably due to the residual coagulants and aluminum-humic substance complexes. The significant change in the surface chemistry of the RO membrane (a decrease in surface charge and an increase in contact angle of the fouled RO membranes) induced by the accumulation of hydrophilic EfOM onto the negatively charged RO membrane surface intensified the fouling formation of the fouled RO membrane by hydrophobic interaction between the humic substances of EfOM with relatively high hydrophobicity and the fouled RO membranes with decreased surface charge and increased contract angle.     

理想胶结砂土力学特性及剪切带形成的离散元分析

根据近期胶结铝棒接触力学特性的实测结果,提炼出用于模拟胶结砂土粒间胶结作用的胶结接触模型,并将该模型引入二维离散元商业软件PFC^2D。通过对不同胶结强度和不同围压下胶结砂土的平面应变双轴压缩试验的离散元模拟,分析了理想胶结砂土的宏观力学特性及其剪切带的形成规律。结果表明：相比同一孔隙比的无胶结试样,胶结试样具有更高的峰值强度、显著的应变软化和剪胀现象以及明显的剪切带,宏观力学特性与其胶结接触微观力学机理密切相关,模拟结果与已有室内试验结果具有规律上的一致性;由胶结试样内部的微观信息统计可知,胶结试样剪切带的形成一般在其峰值强度之后,且剪切带的形成是试样变形、胶结破坏、孔隙比、平均纯转动率和位移场等微观参量局部化的综合表现。     

Investigating mix proportions of high strength self compacting concrete by using Taguchi method

In this study, mix proportion parameters of high strength self compacting concrete (HSSCC) are analyzed by using the Taguchi’s experiment design methodology for optimal design. For that purpose, mixtures are designed in a L₁₈ orthogonal array with six factors, namely, “water/cementitious material (W/C) ratio”, “water content (W)”, “fine aggregate to total aggregate (s/a) percent”, “fly ash content (FA)”, “air entraining agent (AE) content”, and “superplasticizer content (SP)”. The mixtures are extensively tested, both in fresh and hardened states and to meet all of the practical and technical requirements of HSSCC. The experimental results are analyzed by using the Taguchi experimental design methodology. The best possible levels for mix proportions are determined for maximization of ultrasonic pulse velocity (UPV), compressive strength, splitting tensile strength and for the minimization of air content, water permeability, and water absorption values.     

Concrete building blocks made with recycled demolition aggregate

A study undertaken at the University of Liverpool has investigated the potential for using recycled demolition aggregate in the manufacture of precast concrete building blocks. Recycled aggregates derived from construction and demolition waste (C&DW) can be used to replace quarried limestone aggregate, usually used in coarse (6 mm) and fine (4 mm-to-dust) gradings. The manufacturing process used in factories, for large-scale production, involves a “vibro-compaction” casting procedure, using a relatively dry concrete mix with low cement content (≈100 kg/m³). Trials in the laboratory successfully replicated the manufacturing process using a specially modified electric hammer drill to compact the concrete mix into oversize steel moulds to produce blocks of the same physical and mechanical properties as the commercial blocks. This enabled investigations of the effect of partially replacing newly quarried with recycled demolition aggregate on the compressive strength of building blocks to be carried out in the laboratory. Levels of replacement of newly quarried with recycled demolition aggregate have been determined that will not have significant detrimental effect on the mechanical properties. Factory trials showed that there were no practical problems with the use of recycled demolition aggregate in the manufacture of building blocks. The factory strengths obtained confirmed that the replacement levels selected, based on the laboratory work, did not cause any significant strength reduction, i.e. there was no requirement to increase the cement content to maintain the required strength, and therefore there would be no additional cost to the manufacturers if they were to use recycled demolition aggregate for their routine concrete building block production.     

An autopsy study of a fouled reverse osmosis membrane element used in a brackish water treatment plant

The fouling of a spiral wound reverse osmosis (RO) membrane after nearly 1 year of service in a brackish water treatment plant was investigated using optical and electron microscopic methods, Fourier transform infrared spectroscopy (FTIR) and inductively coupled plasma atomic emission spectrometry (ICP-AES). Both the top surface and the cross-section of the fouled membrane were analysed to monitor the development of the fouling layer. It has been found that the extent of fouling was uneven across the membrane surface with regions underneath or in the vicinity of the strands of the feed spacer being more severely affected. Fouling appeared to have developed through different stages. In particular, it consisted of an initial thin fouling layer of an amorphous matrix with embedded particulate matter. The amorphous matrix comprised organic-Al-P complexes and the particulate matter was mostly aluminium silicates. Subsequently, as the fouling layer reached a thickness of about 5-7microm, further amorphous material, which is suggested to include extracellular polymeric substances such as polysaccharides, started to deposit on top of the existing fouling layer. This secondary amorphous material did not seem to contain any particulate matter nor any inorganic elements within it, but acted as a substrate upon which aluminium silicate crystals grew exclusively in the absence of other foulants, including natural organic matter (NOM).     

Cleaning strategies for flux recovery of an ultrafiltration membrane fouled by natural organic matter

One of the most common problems encountered in water treatment applications of membranes is fouling. Natural organic matter (NOM) represents a particularly problematic foulant. Membranes may be fouled by relatively hydrophilic and/or hydrophobic NOM components, depending on NOM characteristics, membrane properties, and operating conditions. To maximize flux recovery for an NOM-fouled ultrafiltration membrane (NTR 7410), chemical cleaning and hydraulic rinsing with a relatively high cross-flow velocity were investigated as cleaning strategies. The modification of the membrane surface with either an anionic or a cationic surfactant was also evaluated to minimize membrane fouling and to enhance NOM rejection. Foulants from a hydrophobic NOM source (Orange County ground water (OC-GW)) were cleaned more effectively in terms of permeate flux by acid and caustic cleanings than foulants from a relatively hydrophilic NOM source (Horsetooth surface water (HT-SW)). An anionic surfactant (sodium dodecyl sulfate (SDS)) was not effective as a cleaning agent for foulants from either hydrophobic or hydrophilic NOM sources. High ionic strength cleaning with 0.1 M NaCl was comparatively effective in providing flux recovery for NOM-fouled membranes compared to other chemical cleaning agents. Increased cross-flow velocity and longer cleaning time influenced the efficiency of caustic cleaning, but not high ionic strength cleaning. The membrane was successfully modified only with the cationic surfactant; however, enhanced NOM rejection was accompanied by a significant flux reduction.     

厚表土薄基岩煤层开采覆岩运动规律

 厚表土薄基岩煤层采用综放开采后基本顶将难于形成稳定结构,发生滑落失稳导致工作面“压架”,在对薄基岩定义的基础上,综合采用实验室试验、理论分析、数值模拟和现场实测的方法对薄基岩煤层开采上覆岩层运动规律进行了研究。研究结果表明,若其他条件不变,“砌体梁”结构的稳定主要取决于基岩厚度和上覆表土层的力学性质及厚度,具有较大承载力的厚黏土层能与薄基岩组合形成稳定的结构,降低稳定结构所需最小基岩厚度。由此建立薄基岩工作面结构力学模型,并针对司马矿具体条件分析认为当表土为松散砂土,最小基岩厚度40 m;当黏土厚度40 m,最小基岩厚度20 m;黏土厚度30 m,最小基岩厚度30 m。据此提出首采面可安全开采,在现场工业性试验中得到成功验证。     

界面特性及填料粒径影响格栅加筋性能的离散元研究

为研究筋土界面细观结构演化及填料粒径对加筋效果的影响,采用“clump”方法开发了可模拟砂土性状的椭球形颗粒,建立三维离散元模型并结合室内试验结果验证了模型的正确性,系统分析了拉拔阻力、格栅应变、局部孔隙率等力学响应并揭示了其发展规律。拉拔试验结果显示大粒径填料置换后表观黏聚力提高显著而摩擦角变化不大,进行宏细观分析后发现,置换体系颗粒发生了更大程度的位置重排,拉拔力增量主要来源于摩擦阻力。研究成果可为从细观角度探究筋土界面机理提供新的认识。     

Field assessment of railway ballast degradation and mitigation using geotextile

Rail tracks continue to deform due to degradation of ballast under the application of heavy train traffic. The resulting track deformations often lead to drainage impairment as well as loss of resiliency. For track replenishment, deep screening of ballast is usually adopted by Indian Railway (IR) either after 10 years or passage of 500 MGT traffic, whichever is earlier. To study the effectiveness of geotextile on track stability and assess possible reductions in maintenance costs, a layer of woven geotextile was installed at the ballast-subgrade interface in Bhusawal-Akola central railway section of IR which is the present study area. The results show that the amount of degradation and fouling are different in UP and DN tracks due to inherent variation in traffic characteristics. This study also shows that the placement of geotextile in the track has led to prolonged maintenance cycle with favorable implications on cost and track shutdown periods. The findings of the present case study results will be useful for IR to reduce the ballast procurement and reuse of discarded material during deep screening in future.     

考虑填料–土工格室相互作用的加筋路堤力学响应研究

针对有砟轨道加筋路堤计算分析中难以考虑土工格室对碎石填料嵌锁及摩擦作用的现状,在大比尺直剪试验基础上建立考虑填料–格室相互作用的三维计算模型。通过室内模型试验验证该计算的可靠度和优越性,进而系统地分析有/无加筋情况下,填料强度、土工格室刚度及地基压缩性等参数对路堤力学响应的影响。结果表明:土工格室的嵌锁作用使得加筋对路堤响应的影响在其布设层发挥程度更大,该影响通过填料与格室及填料内的摩擦作用向周围传递且不断衰减;填料强度较低或地基较软时能加强土工格室对路堤响应的影响,对于填料内摩擦角为20°和地基变形模量为5 MPa的工况而言,加筋后基床侧向位移峰值分别降低60%和72%,应力峰值亦显著降低且分布区域明显扩大;土工格室弹性模量为0.2~2.5 GPa时,路堤力学响应受其影响较大。     

煤矸石细集料-水泥基材料力学性能影响的试验研究

为促进煤矸石集料在水泥混凝土中的应用,选用自燃(活性)和非自燃(非活性)煤矸石作为细集料,研究水灰比、掺量、养护制度等不同条件下两种矸石细集料-水泥基材料力学性能的发展规律,分析煤矸石的种类、活性、掺量等因素对水泥基材料力学性能的影响。试验结果表明:活性矸石作为细集料,能够在水化初期与水泥水化产物发生一定程度的二次水化反应,水化反应能提高其早期强度;非活性矸石细集料-水泥基材料的强度随水灰比的增大而减小,而活性矸石则存在合理的水灰比范围;高温养护能够促进煤矸石细集料-水泥基材料的早龄期的水化进程,提高其早期抗压强度,但28d龄期中小掺量的矸石细集料-水泥基材料的抗压强度会产生高温负效应,而对抗折强度的影响则相反。