Reduction of subgrade fines migration into subbase of flexible pavement using geotextile

Pumping in pavement is defined as traffic-induced migration of saturated subgrade fines into overlying granular layers or onto the surface of the pavement, negatively impacting the performance and service life of the pavement. The objective of this study was to assess the capability of geotextile as a separation and filtration layer in reducing subgrade fines migration. A one-third scale Model Mobile Load Simulator, an accelerated pavement testing device, was used to simulate the cyclic traffic loading on a scaled model of a flexible pavement. The results from three scaled pavement tests were compared to evaluate the effectiveness of geotextile separation and filtration in reducing subgrade fines migration. The three tests had identical configurations, except that a geotextile layer was placed at the interface of subgrade and subbase in one of the tests. The lab testing revealed that, under cyclic traffic conditions, the migration of subgrade fines into subbase was significant. However, using a geotextile at the subgrade-subbase interface significantly reduced the subgrade pumping. At the end of the test, the fines that migrated to the subbase, based on % mass of subbase, were 6.39% in the tests without geotextile and 1.81% in the test with geotextile. An approximately 30% reduction was observed in the amount of pavement rutting when using geotextile at the top of the subgrade. The subgrade soil migration in mass percentage increased with the traffic loading cycles, and more migration occurred in the bottom half than in the top half of the subbase. The study concludes that geotextile can be used as an effective means to reduce pumping of subgrade fines in pavement by providing both separation and filtration.     

Effect of subgrade soil stiffness on the design of geosynthetic tube

Water or soil filled geotextile or geosynthetic tubes have been used for coastal or river protection projects in recent years. How to design and analyze geosynthetic tube is still an important research topic. Although a number of solutions for geosynthetic tube have been proposed in the past, most of these solutions assume that the geosynthetic tube is resting on a rigid foundation. In this paper, a two-dimensional analysis of geosynthetic tube resting on deformable foundation soil is presented. The deformable foundation is assumed to be an elastic Winkler type represented by the modulus of subgrade reaction, K_f. The study shows that the smaller the modulus, the smaller the height of the geosynthetic tube above the ground surface and the higher the tensile force in the geotextile or geosynthetic given the other conditions the same. When the foundation soil has a modulus higher than 1000 kPa/m which is representative of soft clay, the foundation soil can be assumed to be rigid in the analysis. The results obtained from the method proposed in this paper are compared with those from the solutions of Leshchinsky et al. and Plaut and Suherman for verification. The differences between the solutions are also discussed.     

Effect of gravel subgrade on hydraulic performance of geosynthetic clay liner

The effect of a gravel subgrade on the hydraulic performance of GCLs is investigated. Laboratory test results show that the GCL specimens exhibit significant variation in thickness when compressed against gravel. The maximum and minimum thicknesses of the specimen were about 20 and 3 mm, respectively, after consolidation by an effective stress up to 138 kPa. However, the permittivity of GCLs remained very low. The permittivity of both needle-punched and adhesive-bonded geotextile-supported GCLs decreased with increasing confining stress, regardless of the type of subgrade materials. In general, larger particles led to more significant migration of bentonite. Nevertheless, there was no significant difference in the degree of bentonite migration between the two GCLs investigated.     

Effect of geotextile reinforcement on the mechanical behavior of sand

Laboratory triaxial compression tests were carried out in order to determine the stress–strain and dilation characteristics of geotextile-reinforced dry beach sand. The mechanical behavior of the composite material was investigated through varying the number of geotextile layers, type of geotextile, confining pressure, and geotextile arrangement. In order to study the effect of sample-size on the results, tests were performed on samples with two different diameters. The results demonstrated that geotextile inclusion increases the peak strength, axial strain at failure, and ductility. However, it reduces dilation. Such improvements in the behavior of reinforced sand are more pronounced for small-size samples. Failure envelopes for reinforced sand were observed as bilinear or curved. Bulging between layers was detected in reinforced samples which failed.     

Effect of boundary conditions on the hydraulic behavior of geotextile filtration system

There are several test methods available for studying the behavior of geotextile/soil filtration systems. However, there has not been a consensus on which one of these methods should be the most appropriate. In this research, gradient ratio (GR) tests and hydraulic conductivity (HCR) tests were performed to evaluate the effects of boundary conditions on the behavior of geotextile/soil filtration systems. The test results show that the hydraulic conductivity of the filtration systems decreased as the effective stress and hydraulic gradient increased. Furthermore, the hydraulic conductivity obtained from GR tests with a hydraulic gradient of 5 could be taken as the lower bounds for HCR test results. Therefore, it is suggested that the GR test can be used to obtain reasonable and conservative design parameters of the filtration systems.     

基于路基长期性能的排水系统设计

路基修筑质量的高低和防排水设计合理与否对路面的使用性能和使用寿命有着重要的影响.为了保证路基的整体稳定和变形要求,排水系统需要合理设计.本文从保证路基长期性能的角度出发,阐述了排水系统的设计要点.     

A comprehensive method for analyzing the effect of geotextile layers on embankment stability

Commercial software is used widely in slope stability analyses of reinforced embankments. Almost all of these programs consider the tensile strength of geotextiles and soil–geotextile interface friction. However, currently available commercial software generally does not consider the drainage function of nonwoven geotextile reinforcement. In this paper, a reinforced channel embankment reinforced by a nonwoven geotextile is analyzed using two methods. The first method only considers the tensile strength and soil–geotextile interface friction. The second method also considers the drainage function. In both cases, the reinforced embankment is modeled in rapid drawdown condition since this is one of the most important conditions with regard to stability of channel embankments. It is shown that for this type of application, modeling a nonwoven geotextile reinforced embankment using commercial software which neglects the drainage function of the geotextile may be unrealistic.     

Filtration performance of geotextile encasement to minimize the clogging of stone column during soil liquefaction

Stone columns, which are frequently employed to stabilize the liquefiable soil, are susceptible to accumulation of soil particles. The progressive accumulation of the soil particles causes clogging of the stone column which decreases its drainage capacity. The stone column can be encased with geotextile to sustain its long term drainage function. The encasement prevents the movement of the soil particles into the stone pores. In the present paper, a mathematical model is presented to assess the filtration performance of the geotextile encasement to prevent the clogging. The filtration capacity of the geotextile is related to its maximum pore size, porosity and soil characteristics. It is observed that the encased stone column dissipates the excess pore pressure at a faster rate compared to the stone column without encasement. The peak maximum excess pore water pressure (U_max) is not significantly affected due to selection of the opening size of the geotextiles for single earthquake. However, the opening size can significantly affect the peak U_max value for multiple earthquakes. Depending on the capture coefficient of the stone column, the clogging can be fully prevented for higher hydraulic gradient if geotextile with maximum opening size in between D₁₀ to D₅ is used as encasement.     

Influence of geotextile arrangement on seismic performance of mid-rise buildings subjected to MCE shaking

Geotextile layers make it possible to construct mid-rise buildings sitting on shallow foundations in unfavourable soil conditions; this study investigates how the arrangement of geotextiles affects the seismic performance of mid-rise buildings under Maximum Considered Earthquake (MCE) shaking. The geotextile arrangement considered here includes the stiffness (5000?kN/m – 12000?kN/m), the length with respect to width of the foundation (B) (1B – 4B), the number of geotextile layers (1 – 7 layers), and their spacing (250?mm – 1000?mm). FLAC3D is used for the numerical simulation and to carry out nonlinear dynamic analysis in the time domain, and an inelastic constitutive model is used to simulate the behaviour of the structure and the geotextile layers under seismic loads. Variations in the shear modulus of soil and the corresponding damping ratio with cyclic shear strain are considered using a hysteretic damping algorithm to model the reasonable dissipation of energy in the soil. The interface between the foundation and ground surface, including the material and geometrical nonlinearities, are used to capture any possible slide and uplift in the foundations. The results are presented with regard to the geotextile arrangement considered, and include the tensile force mobilised in the geotextile layers, the response spectra at the bedrock and ground surface, the shear force developed in the structure, the maximum rocking angle of the foundation, permanent foundation settlement, maximum lateral displacement and the maximum and residual inter-storey drifts. The results show that the geotextile layers close to the edges of the foundation sustained most of the stress induced by foundation rocking, and the geotextile arrangement has a significant influence on the seismic response of mid-rise buildings. Thus, to satisfy the seismic performance of buildings and to optimise the design of foundations reinforced with geotextiles, the stiffness, length, number and spacing of the geotextile layers should be designed with great care.     

The efficacy and use of small centrifuge for evaluating geotextile tube dewatering performance

Geotextile tube dewatering technology has been widely used over the past two decades for dewatering high water content slurries. The dewatering process in geotextile tubes aims to decrease the volume of the dewatered slurry, which helps in the transportation, disposal, and reuse of the dewatered material. Several researchers have emphasized the effect of the retained sediment (filter cake) properties, in particular final solids content and volume (height) change, on the feasibility of geotextile tube dewatering projects. Retained sediment properties are often evaluated using small scale tests such as rapid dewatering test, falling head test, pressure filtration test (PFT), and field scale tests such as hanging bag test (HBT) and geotextile tube demonstrations test (GDT). In this study, centrifuge test is introduced as an alternative for the widely used pressure filtration and falling head tests to evaluate retained sediments properties. Centrifuge test provides a mechanism for understanding the response of slurries to externally applied pressure in geotextile tube environment. Centrifuge test was used to evaluate maximal solids content of the retained sediments and change in slurry volume of four soils that represent typical dredged soils. Tully sand, Tully fines, Elliott silt loam, and kaolin slurries were used at varying solids concentrations. Slurries were subjected to external stresses between 0.1 and 40 kPa by applying centrifugal speeds between 300 rotation per minute (rpm) and 1800 rpm. Both centrifuge test and PFT were conducted with unconditioned and cationic polyacrylamide conditioned slurries. Centrifuge tests results were compared with PFT results with respect to retained sediments final solids content and volume change. Tests results indicated that the maximal solids concentration of the retained sediments in saturated conditions is unique for each soil and is independent of the initial slurry solids concentration. Tests results also indicated that there is linear relationship between the initial concentration of the slurry and the final volume change at any externally applied stress. Finally, a relationship between the total pumped slurry volume and the final height of the dewatered sediments in a geotextile tube is presented.     

水玻璃水泥砂浆在排水管接口中的应用研究

在水泥砂浆中掺入水玻璃作为排水管的接口材料 ,能有效克服地下水影响 ,提高了接口工程质量。本文对不同水玻璃掺量的水泥砂浆进行了试验研究 ,并对水玻璃作用机理进行了探讨。研究结果已在工程中应用     

预处理对橡胶水泥混凝土的强度及抗渗性能的影响

将经过Cacl2溶液及NaoH溶液预处理的40目、60目橡胶粉以不同的掺量等体积代替河砂作为细集料制备橡胶水泥混凝土．以研究橡胶粉经过预处理后对橡胶水泥混凝土力学强度及抗渗性能的影响。试验证明：当橡胶粉掺量达到20％时．Cacl2溶液预处理的方式能使橡胶混凝土强度较未经预处理的橡胶水泥混凝土强度提高15．2％．改性效果明显。Cacl2溶液预处理的方式对提高橡胶水泥混凝土的力学性能明显优于NaoH溶液的预处理方式。     

Effect of subgrade on leakage through a defective geomembrane seam below saturated tailings

Experiments are conducted to quantify leakage through saturated tailings (with 30% fines) underlain by a geomembrane with either a 100-mm-length knife cut slit defect or a 110-mm-length defective extrusion seam. Various subgrades, a poorly graded gravel (GP) with or without nonwoven geotextile (450 or 1420 g/m²) above, a well-graded gravel (GW), and a poorly graded sand (SP) are evaluated. Test results show that leakage through the slit defect and defective seam increase with the subgrade coarseness and subgrade unevenness. The inferred upper and lower bounds opening width of a slit based on the measured leakage increase with the overburden pressure and are categorized for each subgrade. Indentations in the overlapped defective extrusion seam area arise from both the entrapped materials inside the overlap and the materials underlying/overlying the geomembrane, incrementally inducing a greater interface transmissivity with the increasing stress. Overlain by tailings at total overburden pressure of 510 kPa and water head of 35m, leakage through the slit defect is 1.1L/day for SP, 2.2L/day for GW, 2.6–3.3L/day for both GP and uneven GW; leakage drops to 0.8L/day with a geotextile layer directly above the GP; leakage through the defective extrusion seam is 0.4L/day for SP and 0.8L/day for GW.     

高填方软土路基失稳判据的研究

根据极限平衡法分析了影响边坡渐进破坏计算结果的主要因素,用该理论方法对边坡进行二维渐进破坏计算;然后围绕边坡方案的分析结果,结合边坡工程具体特点对边坡进行了安全性分析,最后给出高填方软土路基安全性指标设计标准。     

闭路水泥粉磨系统优化调节的探索

本文从理论上分析了影响闭路水泥粉磨系统的主要因素,提出优化的工艺参数和调节方法。并对Φ1.83×7.0m闭路水泥磨进行了实践探索,收到了良好的效果。     

A study on the hydration rate of natural zeolite blended cement pastes

Natural zeolite is a type of mineralogical material containing large quantities of reactive SiO₂ and Al₂O_3. It is widely used in the cement industry in China as a cement blending material. Like other pozzolanic materials such as silica fume and fly ash, zeolite contributes to concrete strength mainly through the pozzolanic reaction with Ca(OH)₂, Thus, the pozzolanic reactivity of this type of material in comparison with other pozzolans is of much interest. This paper presents experimental results on the compressive strength, degree of pozzolanic reaction, and porosity of zeolite modified cement pastes. These results are compared with those obtained from similar blended cement pastes prepared with silica fume and fly ash replacements. Based on the experimental results, it can be concluded that natural zeolite is a pozzolanic material, with a reactivity between that of silica fume and fly ash. Generally, in blended cement pastes with a lower water-to-cementitious materials ratio, the natural zeolite contributes more to the strength of the pastes. But in the pastes with a higher water to cementitious ratio and a lower cement replacement level it undergoes a higher degree of reaction.     

超塑化剂对水泥早期水化影响的灰值分析

对掺萘磺酸盐缩合物减水剂和两种聚羧酸减水剂的水泥浆体水化24h的微观形貌进行了背散射电子图像测试及灰度值分析,发现超塑化剂对水泥早期水化具有重要影响,掺不同超塑化剂的水泥早期水化产物有所区别.试验同时还得出可利用灰度值来表征掺超塑化剂水泥的早期水化产物的结论.     

Effect of temperature on porosity and on chloride diffusion in cement pastes

This paper is related to the effect of temperature upon the diffusive properties and the microstructural features of cementitious materials. Our experimental studies aim at linking the transport properties to the porosity and the cracks network of the microstructure.

The studied materials were three cement pastes having different water to cement (W/C) ratios (W/C = 0.35, 0.45 and 0.60). Rise of temperature in concrete structures was simulated by heating at different temperatures (45 °C, 80 °C and 105 °C). Evolution of microstructure was essentially characterized by mercury intrusion porosimetry (MIP). Diffusion tests in non-steady-state regime were carried out in order to characterize the diffusive properties. Apparent diffusion coefficients were determined from colorimetric method to follow the ingress of chloride and an analytical solution of Fick’s second law.

Our results showed that rise of temperature induced macroscopic cracking network and modification of pore size distribution. As a consequence, the transport properties were modified by thermal treatment. These modifications were mostly explained by the increase of capillary porosity.     

Dynamic friction and the seismic performance of geosynthetic interfaces

The stability of geotechnical structures which contain geosynthetic interfaces is closely linked to the shear strength between the geosynthetics themselves, both in static and dynamic conditions. Static friction is the maximum interface shear strength mobilised before displacement, whereas dynamic friction is related to the kinematics of the displacement itself. In polymer materials, dynamic friction may be widely variable, depending on the type, geometry and integrity of the surfaces in contact, as well as on the intensity and time-history of the seismic signal. This means that predicting interface shear strength is not simple. This paper focuses on the evaluation of dynamic interface shear strength between geosynthetics, using the results of both inclined plane tests and shaking table tests; this latter test also provided a means to analyse interface behaviour under the conditions of real seismic records. To this purpose, two common geosynthetic interfaces, which exhibit different behaviour under dynamic loading, were tested. One interface was a smooth HDPE geomembrane in contact with a nonwoven polypropylene geotextile, while the second was a textured HDPE geomembrane in contact with a different type of nonwoven polypropylene geotextile.The test results shows that dynamic friction mobilised during seismic events depends on the relative speed according to the same law outlined by the free sliding tests and by the shaking table tests carried out with sinusoidal base motions. Moreover, for the two different types of studied interfaces dynamic friction may be greater, lesser or equal to the static friction and the assumption of a constant value of dynamic friction does not lead to an accurate prediction of the seismic displacements under various earthquakes.     

某水泥混凝土公路路基边坡与排水调查

采用人工调查方法,对国内一条老旧水泥混凝土公路全线路基的边坡与排水等典型问题进行了调研,调查结果表明,右幅路基质量较差,病害形式主要表现为路肩边沟不洁、路基沉降、路肩损坏,为该道路的病害治理提供了依据。