A dynamic gradient ratio test apparatus

The soil-geotextile filtration mechanism is a complex process which depends on physical compatibility between the geotextile and the soil to be retained. Several methods have been proposed by researchers for assessing the filtration behaviour of soil-geotextile composite systems under steady state conditions. The Gradient Ratio (GR) test is the most commonly used method for measuring filtration compatibility of soil-geotextile systems. This paper describes the design of a modified GR permeability test apparatus to overcome disadvantages associated with traditional GR test devices. The apparatus can perform filtration tests under static and dynamic conditions and can be used to evaluate the filtration compatibility of fine-grained soils with geotextiles. The apparatus is incorporated within a triaxial testing system, hence representative field stress conditions can be applied to test specimens. Some exemplar GR tests performed on coarse and fine-grained soils with a non-woven geotextile are presented in this paper. Unidirectional dynamic loads are applied within the filtration tests to replicate highway traffic loading. Test results show that dynamic loading affects the filtration behaviour at the soil-geotextile interface by increasing the fine particles migration towards the geotextile, but that, for the soil evaluated here, this effect was small.     

Experimental evaluation of wicking geotextile-stabilized aggregate bases over subgrade under rainfall simulation and cyclic loading

Wicking geotextile has been increasingly utilized in field projects to mitigate water-related roadway problems. The previous studies showed that the wicking geotextile could provide mechanical stabilization, serve as capillary barrier, and enhance lateral drainage. The wicking geotextile differentiates itself from non-wicking geotextiles by providing capillary or wicking drainage in unsaturated conditions, whereas non-wicking geotextiles only provide gravitational drainage under saturated or near-saturated conditions. Although the previous studies have demonstrated the benefits of soil water content reduction by the wicking drainage, it is not well understood how the wicking geotextile stabilization improves overall performance of aggregate bases over subgrade under traffic or cyclic loading. This paper presents an experimental study where large-scale cyclic plate loading tests were conducted under different conditions: (1) non-stabilized base, (2) non-wicking geotextile-stabilized base, and (3) wicking geotextile-stabilized base, over soft and moderate subgrades. Rainfall simulation was carried out for each test section. After each rainfall simulation, a drainage period was designed to allow water to drain from the section. The amounts of water applied and exiting from the test section were recorded and are compared. Cyclic loading was applied after each drainage period. The test results show that the combined hydraulic and mechanical stabilization effect by the wicking geotextile reduced the permanent deformation of the aggregate base over the subgrade as compared with the non-stabilized and non-wicking geotextile-stabilized sections.     

The role of geosynthetics in reducing the fluidisation potential of soft subgrade under cyclic loading

The instability of railway tracks including mud pumping, ballast degradation, and differential settlement on weak subgrade soils occurs due to cyclic stress from heavy haul trains. Although geotextiles are currently being used as a separator in railway and highway embankments, their ability to prevent the migration of fine particles and reduce cyclic pore pressure has to be investigated under adverse hydraulic conditions to prevent substructure failures. This study primarily focuses on using geosynthetics to mitigate the migration of fine particles and the accumulation of excess pore pressure (EPP) due to mud pumping (subgrade fluidisation) using dynamic filtration apparatus. The role that geosynthetics play in controlling and preventing mud pumping is analysed by assessing the development of EPP, the change in particle size distribution and the water content of subgrade soil. Using 3 types of geotextiles, the potential for fluidisation is assessed by analysing the time-dependent excess pore pressure gradient (EPPG) inside the subgrade. The experimental results are then used to evaluate the performance of selected geotextiles under heavy haul loading.     

交通荷载作用下软基加筋道路加筋效果分析

为了研究交通荷载作用下考虑软土软化效应的软基加筋道路加筋效果的影响因素,首先以室内动三轴试验为基础,通过回归分析得到了软土在循环荷载作用下动模量衰减的经验公式;然后编制了用户子程序将该公式导入有限元分析软件ABAQUS中,采用有限元分析了荷载形式、荷载频率、筋材模量、加筋位置、加筋层数、软土层厚度等对加筋效果的影响。结果表明,随着荷载频率的增大,加筋效果呈减小趋势。加筋效果会随着筋材模量的增大和加筋层数的增多而增大。当筋材铺设在面层和基层之间时,加筋效果最好。在软土层厚度较小时,加筋效果随软土层深度增大有明显提高;但在软土层厚度较大时,加筋效果随软土深度增加提高较少。     

Laboratory performance of unpaved roads reinforced with woven coir geotextiles

The results of an experimental study conducted to investigate the beneficial use of woven coir geotextiles as reinforcing material in a two-layer pavement section, are presented. Monotonic and repeated loads were applied on reinforced and unreinforced laboratory pavement sections through a rigid circular plate. The effects of placement position and stiffness of geotextile on the performance of reinforced sections were investigated using two base course thicknesses and two types of woven coir geotextiles. The test results indicate that the inclusion of coir geotextiles enhanced the bearing capacity of thin sections. Placement of geotextile at the interface of the subgrade and base course increased the load carrying capacity significantly at large deformations. Considerable improvement in bearing capacity was observed when coir geotextile was placed within the base course at all levels of deformations. The plastic surface deformation under repeated loading was greatly reduced by the inclusion of coir geotextiles within the base course irrespective of base course thickness. The optimum placement position of coir geotextile was found to be within the base course at a depth of one-third of the plate diameter below the surface.     

Large scale tests on geosynthetic reinforced unpaved roads subjected to surface maintenance

Geosynthetic can be effectively used as reinforcement in paved and unpaved roads. This paper presents a study on the use of geosynthetic to reinforce unpaved roads on poor subgrade. A large equipment was used to perform the tests under cyclic loading and a nonwoven geotextile and a geogrid were used as reinforcing layers installed at the fill-subgrade interface. Displacements along the fill surface and stresses and strains in the subgrade were measured during the tests. Three cyclic loading stages were applied in each test up to a rut depth at the fill surface of 25 mm be reached in each stage. At the end of a loading stage the fill surface was repaired for the following loading stage. Monotonic loading tests were also carried out for comparisons. The results obtained show the significant contribution of the presence of the reinforcement layer in increasing the number of load cycles for a given rut depth to be reached and in reducing the stresses and strains in the subgrade, particularly when geogrid reinforcement was used. It was also observed that monotonic loading tests underestimated the contribution from the reinforcement. A simple cost-effectiveness analysis showed that the reduction of maintenance works due to the use of geosynthetic reinforcement may yield to significant savings in this type of problem, seldom considered in the analysis of the economics of this type of application on a routine basis.     

交通动载下饱和软黏土累计变形的不排水循环扭剪试验

饱和软土地基上长期往复交通动载将诱发显著的路基运营沉降。交通移动荷载下场地应力路径下呈现心形旋转路径,然而现有路基长期运营沉降分析是建立在循环三轴试验基础上,忽略交通移动荷载下地基土单元主应力轴的旋转效应。利用动态空心圆柱扭剪仪模拟心形循环加载路径,并在动态空心圆柱仪上模拟了传统的循环三轴加载路径。对饱和软黏土开展试验研究,对比观察一系列心形循环加载和三轴循环的不排水累计变形行为。试验研究发现,心形循环加载将诱发更大的累计轴向应变和累计孔压,伴随着初始动应力比的增加,二者差异性更为明显。循环加载过程有效动应力比随着循环次数而增大,循环增量破坏时对应的有效动应力比近似为常数。大数目循环加载下,传统塑性安定行为可渐变为循环塑性蠕变状态。     

车辆循环荷载作用下柔性路面路基的永久变形

柔性路面路基土过量的车辙变形会造成大量的经济损失,影响到路面行车的安全性和舒适性,而且还会引起路面其它形式破坏例如反射裂缝的产生和加强.为此首先简要综述了车辆循环荷载下柔性路面路基变形的研究现状,对现有的路基土永久变形预估模型进行了简要评价并重点讨论了南非车辙预估模型;然后,基于南非重车加载试验数据建立了一个简单的力学-经验计算模型来预测柔性路面路基的永久变形量,该模型可以全面考虑路基材料特性、路基土在车辆荷载作用下的应力应变状况和荷载作用次数;最后,以一个柔性路面和半刚性路面为例,应用该模型对不同轴载下的路基变形进行了预估.该预估模型可以为以后沥青路面车辙方面的研究及其沥青路面的设计提供参考.     

Improved design criteria for nonwoven geotextile filters with internally stable and unstable soils

In geotextile filtration, the soil fines are either accumulated near the interface, clogged, or washed out, which primarily depends on the grain size distribution (GSD) of soil and the constriction size distribution (CSD) of geotextile. Also, the movement of fines significantly affects the flow capacity of geotextile. Currently, the retention requirement is satisfied based on representative grain and opening sizes, whereas the hydraulic conductivity and clogging requirements are satisfied considering the properties of virgin geotextile. This paper presents a probabilistic retention criterion considering the grain and constriction sizes as random variables. The influence of geotextile thickness is incorporated into the criterion by considering the number of geotextile constrictions in a filtration path. A theoretical approach to predict CSD is presented if the measured data is unavailable. For hydraulic conductivity and clogging requirements, a criterion is presented considering the expected partial clogging of geotextile, which is predicted based on the semi-analytical approach. The limit states for the developed criteria are evaluated based on the wide range of experimental data from the current study and published literature. The developed design criteria are applicable to internally stable and unstable soils, which offers an improvement in design compared to the existing criteria in practice.     

Effects of cross-anisotropy and stress-dependency of pavement layers on pavement responses under dynamic truck loading

Previous studies by the authors have determined pavement responses under dynamic loading considering cross-anisotropy in one layer only,either the cross-anisotropic viscoelastic asphalt concrete(AC)layer or the cross-anisotropic stress-dependent base layer,but not both.This study evaluates pavement stressestrain responses considering cross-anisotropy in all layers,i.e.AC,base and subbase,using finite element modeling(FEM) technique.An instrumented pavement section on Interstate I-40 near Albuquerque,New Mexico was used in ABAQUS framework as model geometry.Field asphalt cores were collected and tested in the laboratory to determine the cross-anisotropy(n-values) defined by horizontal to vertical modulus ratio,and other viscoelastic parameters as inputs of the model incorporated through user defined material interface(UMAT) functionality in ABAQUS.Field base and subbase materials were also collected and tested in the laboratory to determine stress-dependent nonlinear elastic model parameters,as inputs of the model,again incorporated through UMAT.The model validation task was carried out using field-measured deflections and strain values under falling weight deflectometer(FWD)loads at the instrumented section.The validated model was then subjected to an actual truck loading for studying cross-anisotropic effects.It was observed that horizontal tensile strain at the bottom of the AC layer and vertical strains in all layers decreased with an increase in n-value of the asphalt layer,from n1(anisotropy) to n=1(isotropy).This indicates that the increase in horizontal modulus caused the decrease in layer strains.It was also observed that if the base and subbase layers were considered stressdependent instead of linear elastic unbound layers,the horizontal tensile strain at the bottom of the asphalt layer increased and vertical strains on top of the base and subbase also increased.     

Investigation of geotextile–soil interaction under a cyclic vertical load using the discrete element method

Geotextiles are often used in roadway construction as separation, filtration, and reinforcement. Their performance as reinforcement in geotextile-reinforced bases depends on geotextile–soil interaction. This paper investigates the geotextile–soil interaction under a cyclic wheel load using the Discrete Element Method (DEM). In this study, soil was modeled as unbonded particles using the linear contact stiffness model, and the geotextile was modeled as bonded particles. The micro-parameters of the soil and the geotextile were determined using biaxial tests and a tensile test, respectively. The influence of the placement depth and the stiffness of the geotextile on the performance of the reinforced base was investigated. The DEM results show that the depth of the geotextile significantly affected the degree of interaction between the geotextile and the soil. Under the applied cyclic vertical load, the geotextile developed a low tensile strain. The effect of the stiffness of the geotextile on the deformation was more significant when the geotextile was placed at a shallower location than when placed at a deeper location.     

Cyclic behaviour of dry silty sand reinforced with a geotextile

Recent studies on construction material technology have indicated that soil reinforcement improves resistance of soil against compression and tension. Due to the wide use of geotextile reinforcement in road construction, the potential benefit of geotextile reinforcement in cyclic loading should be investigated. In this study we performed a series of cyclic triaxial tests to examine dry silty sand reinforced with geotextile when subjected to dynamic loading. These tests were conducted on reinforced and unreinforced dry sand and sand mixed with varying amounts of silt (0–50%). The main factors affecting the cyclic behaviour, such as the arrangement and number of geotextile layers, confined pressure and silt content are examined and discussed in this paper. The results indicate that geotextile inclusion and increased confining pressure increase the axial modulus and decreased cyclic ductility of dry sand for all silt contents examined. Also, it was found that by increasing the silt content by up to about 35 percent the axial modulus in reinforced and unreinforced sand is decreased and cyclic ductility increased. With further increases in silt content, these values are increased for cyclic axial modulus and decreased for cyclic ductility.     

哈尔滨城市道路损害调查与分析

道路病害主要表现在路面破损上,其原因是路面结构本身强度不足,在行车荷载反复作用下产生破损;另外是由于路基稳定性不足,将各种病害反射到路面结构上,使路面产生破损。本文通过对哈尔滨市区道路调查,得出了路面状况指数(PCI)值与路基土含水量,分析路基稠度与路基回弹模量之间的关系,得出影响道路状况的具体因素,为寒区城市道路病害的防治提供指导和依据。     

交通荷载作用下加筋土路基残余变形减小的机理分析

应用有限单元法对交通荷载作用下加筋土路基进行隐式动力分析,并基于累积塑性变形的计算和影响因素分析,揭示加筋减小路基土残余变形的机理。分析结果表明:对于级配碎石基层沥青混凝土路面,加筋改善路基表面的压应力分布,减小传递到路基表面的剪应力,加筋后路基土上部80cm范围内动偏应力大大减小,而动偏应力是引起路基残余变形的重要因素之一;另一方面,交通荷载作用下,路基土的累积塑性变形主要发生在路基上部100cm范围内。因此,加筋后路基土累积塑性变形的减小,主要是由于加筋减小交通荷载作用下路基土上部的动偏应力;土工格栅刚度越大,动偏应力减小越显著。     

Numerical parametric investigation of infiltration in one-dimensional sand–geotextile columns

Geotextiles are routinely used in separation and filtration applications. Design of these systems is currently based on saturated properties of the geotextiles and the surrounding soils. However, in the field, soil and geotextile can be in an unsaturated state for much of their design life during which they are essentially hydraulically non-conductive. Periodic wetting and drying cycles can result in rapid and large changes in hydraulic performance of soil–geotextile systems. The writers have reported the results from physical water infiltration tests on sand columns with and without a geotextile inclusion. The geotextile inclusions were installed in new and modified states to simulate the influence of clogging due to fines and to broaden the range of hydraulic properties of the geotextiles in the physical tests. This paper reports the results of numerical simulations that were undertaken to reproduce the physical tests and strategies adopted to adjust soil and geotextile properties from independent laboratory tests to improve the agreement between numerical and physical test results. For example the paper shows that the hydraulic conductivity function of the geotextile must be reduced by up to two orders of magnitude to give acceptable agreement. The lower hydraulic conductivity is believed to be due to soil intrusion that is not captured in conventional laboratory permeability tests. The calibrated numerical model is used to investigate the influence of geotextile and soil hydraulic conductivity and thickness as well as height of ponded water at the surface on wetting front advance below the geotextile and potential ponding of water above the geotextile due to a capillary break mechanism. A simple analytical model is also developed that predicts the maximum ponding height of water above the geotextile based on two-layer saturated media and 1-D steady state flow assumptions. The analytical model is used to generate a design chart to select geotextiles to minimize potential ponding of water above the geotextile. Ponding can lead to lateral flow of water along the geotextile in reinforced wall, slope, embankment and road base applications.     

Filtration behaviour of soil-nonwoven geotextile combinations subjected to various loads

Geotextiles are often subject to different load types in their filtration applications. The load action can cause changes in soil density, geotextile stretching and flow interaction at the soil-geotextile interface. All of these load-induced changes to a geotextile may affect the filtration behaviour of the soil-geotextile system. The impact of load type on the filtration behaviour of soil-nonwoven geotextile combinations has been studied through a series of tests using an experimental apparatus designed specially for the laboratory tests. In these tests, the soil-geotextile combination was fabricated by inserting a piece of nonwoven geotextile between a 50 mm thick soil layer and a layer of steel beads. Two chemical-bonded nonwoven geotextiles were employed in this study. One of the three load types, namely sustained, pulsatory and a combination of both was applied to the combination prior to each filtration test. The frequency of the pulsatory load was 0.1 Hz and a total of 5000 cycles of repeated load applied to the combination for each load type test. After applying this specific type of load on a soil-geotextile combination, water was allowed to flow down through the combination from the soil into a drainage layer set at various hydraulic gradients. The flow rates corresponding to elapsed times were measured and the average hydraulic conductivity value was extracted by using Darcy’s law to characterize the filtration performance of the entire soil-geotextile combination. Variations in the average hydraulic conductivity value with respect to the soil void ratio, magnitude and type of normal load were examined.The experimental results revealed that the void ratio of soil decreased with the increase of total load. Although two parent geotextiles under study, namely GT1 and GT2, have similar filtration characteristics, soil-geotextile combinations composed of these two geotextiles exhibited different filtration responses to the normal load. Soil-GT1 combinations exhibited a normal relationship between the average hydraulic conductivity and the normal load applied; the average hydraulic conductivity increased with an increase in the total load. Soil-GT2 combinations exhibited different load-dependent responses to a normal load with the average hydraulic conductivity depending on the magnitude and type of load. Such load-dependent hydraulic conductivity changes are attributed mainly to the geotextile in-plane strain and the pumping action in the combination.     

水泥路面上加铺沥青路面的施工要点

对晋城市书院街道路维修工程进行了介绍,研究了工程中水泥路面上加铺沥青路面的施工要点,分别从原水泥混凝土路面路基施工、土工布铺设及沥青混凝土路面摊铺等方面作了阐述,为道路的维修改造提供了新途径、新技术。     

Field monitoring of wicking geotextile to reduce soil moisture under a concrete pavement subjected to precipitations and temperature variations

Wicking geotextile can reduce water contents in pavement layers under unsaturated conditions due to capillary action through grooves of wicking fibers. Reduction of soil water content under the pavement can minimize pavement distresses. So far, there have been limited use and verification of the wicking geotextile in reducing water content of soil under concrete pavements in the field. In this field study, moisture sensors were installed in three test sections under a newly-built concrete pavement during its re-construction. The base course in one test section had a higher percentage of small particles than those in other two sections. The wicking geotextile was used between the base course and the subgrade in two test sections while a nonwoven geotextile was used in one test section. All test sections were subjected to precipitations and temperature variations. Field monitoring data showed that the wicking geotextile reduced the volumetric water content (VWC) of an aggregate base more than the nonwoven geotextile and its wicking ability decreased as the content of small particles increased. In addition, the wicking ability of the wicking geotextile decreased as the temperature decreased due to the reduction in the evaporation rate and the increase in the water retention capacity of the soil at low temperatures.     

Evaluation of drainage coefficients for 2D and 3D−geocomposite embedded subbase layers

Drainage is one of the primary factors considered during the design of pavements. This paper presents the data from large-scale permeameter tests conducted on pavement sections to evaluate the drainage characteristic of geocomposite (GC) embedded subbase layers with and without traffic load. Two types of GCs with 2D core and 3D core were considered to enhance the drainage properties of a subbase layer. The large-scale permeameter test results indicate that the in-plane drainage capacity of the subbase layer improved by about 12 and 22 fold, respectively, for 2D?GC and 3D?GC. The GCs can drain infiltrated water at a rate of more than 300 m/day (in-plane permeability), even if the thickness of the subbase layer is reduced by about 50%. Over the pavement's design life, long-term in-plane permeability is estimated to decrease by 65–70%. It is estimated that the GC embedded subbase layers could drain off 50% of the infiltered water from a two-lane pavement system within 2 h, even after reducing the layer thickness by 100 mm. A set of new drainage coefficients for geocomposite improved subbase layers (m_3c) were proposed based on the American Association of State Highway and Transportation Officials drainage quality guidelines.     

A theoretical and laboratory investigation of alternating flow filtration criteria for woven structures

There are two schools of thought on the sizing of geotextile filters to prevent excessive loss of fines from cohesionless soils under conditions of alternating turbulent flow. The first prescribes the positive retention of the smallest particle size by the geotextile while the second attributes the soil with a self-filtering capability. This implies that only the larger size soil particles need be retained. These concepts are explored and theoretical filtration criteria are developed. To rest the validity of these theories, laboratory testing was carried out. This indicated that large losses of fines would result, especially at high hydraulic gradients. In view of this the theory was modified to suggest a revised filtration criterion.