Fine fraction filtration test to assess geotextile filter performance

The proper design of the openings of a geotextile filter requires a balance between providing upstream soil particle retention and avoiding excessive geotextile clogging. While this balance can be reasonably achieved quite well for most soil types and hydraulic conditions, it is different when the flowing liquid is turbid (containing a large amount of suspended particles) and/or under high, or dynamic, hydraulic gradients. This paper presents a test method to assess the behavior of individual soil particles in a slurry form as they approach, encounter and interact with a geotextile filter.

The paper describes the concept and details of the test method, called the fine fraction filtration (F³) test, and presents data on five different geotextiles which were evaluated using three different soil types. It was seen that soils with particle sizes larger than the geotextile's opening structure can build a stable upstream network; soil with particle sizes smaller than the geotextile's opening structure can pass through the geotextile; and intermediate particle size conditions can give rise to excessive clogging. An additional series of tests were conducted using the same five geotextile filters but now using a pre-placed upstream soil filter above the geotextiles. Clogging conditions generally occurred albeit within the soil column rather than within the geotextile.

The F³ test is felt to be a meaningful test for those conditions where the upstream soil particles are not in intimate contact with the geotextile filter. In such cases, the test method can probably be considered to be a performance test. For other, more typical soil placement conditions, the test method can be considered to be a very challenging indext test.     

Novel experimental techniques to assess the time-dependent deformations of geosynthetics under soil confinement

A new experimental approach to assess the impact of soil confinement on the long-term behavior of geosynthetics is presented in this paper.The experimental technique described herein includes a novel laboratory apparatus and the use of different types of tests that allow generation of experimental data suitable for evaluation of the time-dependent behavior of geosynthetics under soil confinement.The soil-geosynthetic interaction equipment involves a rigid box capable of accommodating a cubic soil mass under plane strain conditions.A geosynthetic specimen placed horizontally at the mid-height of the soil mass is subjected to sustained vertical pressures that,in turn,induce reinforcement axial loads applied from the soil to the geosynthetic.Unlike previously reported studies on geosynthetic behavior under soil confinement,the equipment was found to be particularly versatile.With minor setup modifications,not only interaction tests but also in-isolation geosynthetic stress relaxation tests and soil-only tests under a constant strain rate can be conducted using the same device.Also,the time histories of the reinforcement loads and corresponding strains are generated throughout the test.Results from typical tests conducted using sand and a polypropylene woven geotextile are presented to illustrate the proposed experimental approach.The testing procedure was found to provide adequate measurements during tests,including good repeatability of test results.The soilegeosynthetic interaction tests were found to lead to increasing geotextile strains with time and decreasing reinforcement tension with time.The test results highlighted the importance of measuring not only the time history of displacements but also that of reinforcement loads during testing.The approach of using different types of tests to analyze the soilegeosynthetic interaction behavior is an innovation that provides relevant insight into the impact of soil confinement on the time-dependent deformations of geosynthetics.     

Geotextile revetment filters

The use of geotextiles as revetment filters comprised one of the earliest applications for these materials. In performing its role within the revetment structure the geotextile must meet specific mechanical and hydraulic criteria. The mechanical criteria presented in the paper are based on empirical relationships derived by correlating analysed in-situ performance with appropriate laboratory mechanical test methods. The hydraulic criteria presented in the paper rationalizes the plethora of published geotextile filter criteria, and consider such variables as geometric and hydrodynamic stability, mode of water movement through the geotextile filter, and type of foundation soil to be protected.     

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.     

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.     

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.     

Deformation analysis of geotextiles in soils using the finite element method

This paper introduces a no-tension FEM analysis of the deformations of geotextile-reinforced embankments. The influence of the discontinuous plane between the geotextile and the soil must be taken into consideration in such an analysis. Goodman et al.¹ suggested the introduction of joint elements in order to analyze the influence of the discontinuous plane in the application of the finite element method. In the analysis presented, the above suggestion by Goodman et al. was extended so that the joint elements with a thickness of t were introduced between the geotextile and the soil. The geotextile was converted to plane truss elements which would resist tensile stress but not flexural stress. Pull-out testing was then carried out to study the stress and deformation of the geotextile in the soil. Large model tests were also conducted to study the effect of geotextiles in preventing the formation of bumps at bridge approach banks. The correlation of test results and analytical results obtained by the finite element method was confirmed.     

Geotextile filter design guide

This paper provides the practicing designer with a comprehensive, systematic approach to solving common filtration design problems. Current filter design procedures often include only permeability and retention criteria. Several other concepts should be incorporated into the design process, such as: (i) consideration of whether permeability or retention is the primary function of a filter within the given application; (ii) quantifying the internal stability of soil; and (iii) addressing survivability and durability issues.

This paper incorporates several recently-developed design concepts, together with currently-used geotextile filter design criteria, into a comprehensive nine-step design methodology. Each step is discussed, specific numerical criteria are given, and information is provided for determining the parameters needed to satisfy the design criteria. The result is a user-oriented document for designing geotextile filters.     

反滤机制下土工布孔隙结构变化研究

阐述在过滤排水行为中,土壤颗粒受水流冲刷对土工布孔径造成的影响。说明在不同土壤级配的条件下,过滤排水行为中,土体内部的颗粒行为与机制。同时提供试验方法的应用与限制条件。     

Geotextile strain in a full scale reinforced test embankment

A geotextile reinforced test embankment was constructed on a soft organic clayey silt deposit at Sackville, New Brunswick, Canada in September/ October 1989. A relatively high-strength polyester woven geotextile (ultimate strength of 216 kNm⁻¹) was used as reinforcement. The reinforcement was instrumented with a number of electrical resistance, electromechanical and mechanical gauges. The details of this instrumentation and field performance of the geotextile reinforcement during the construction of this test embankment are described in this paper.

The field data indicated that the strain in the geotextile was comparatively small (typically less than about 0·7%) up to an embankment thickness of 3·4 m. The strain increased to a maximum of about 2% when the embankment thickness was increased above 4·1 m, suggesting the initiation of movement (or yielding) of the foundation soil. A large increase of strain was evident for thicknesses exceeding 5·7 m and the available evidence indicates that the soil approached failure at a fill thickness of about 5·7 m. The strain increased to over 8·5% when the embankment was first constructed to 8·2 m thickness and then failed as the soil continued to deform at constant fill thickness and the geotextile strain increased until failure (inferred tearing) of the geotextile occurred. After the embankment failed at a thickness of 8.2 m, the addition of more fill did not result in any     

Drainage performance of geotextiles

It should be noted that the drainage conditions and mechanisms are somewhat different when geotextiles are used as back fill material behind retaining walls. One of the major differences is that the soil installed by the geotextile may not necessaroly be saturated. Generally, the drainage performance of geotextiles can be evaluated by examining combined behavior of geotextiles, soil particles and water. However, in addition to the above materials, in investigating the drainage performance of geotextiles as back fill material behind retaining walls, the effect of air should be taken into account. Therefore, this study has concentrated on investigating the effect of drainage performance of an initially dry geotextile. A further long-term test was carried out primarily to examine the mechanism and development of self-induced filters, which is believed to determine the drainage performance of the geotextile.     

The effect of thickness reduction on the hydraulic transmissivity of geonet drains using rigid and non-rigid flow boundaries

In this paper, relationships between in-plane flow capacity reduction and thickness reduction are presented in tri-planar and bi-planar geonets for rigid and non-rigid flow boundaries. Using these equations, the long-term flow capacity of geonets can be determined using creep test results. To validate these relationships, geonet thickness was measured under different conditions and the theoretical values of the transmissivity reduction ratios were calculated by substituting the results in the equations. Transmissivity tests were then performed under the same conditions to obtain experimental values of the reduction ratios. A comparison showed that the theoretical and experimental values of the transmissivity reduction ratios were in agreement, and the relationships provide a useful tool to predict the drainage capacity of both tri-planar and bi-planar geonets influenced by loading pressure. However, special precautions must be taken when applying the equations to investigate the hydraulic capacity of other types of geosynthetic drains as well as when the geonet is covered by geotextile material acting as a filter between the geonet and adjacent soil, is overlain by geosynthetic clay liner material where the swelling potential of the bentonite in the geonet exists, is placed in inclined positions or is subjected to complex combinations of load.     

Geotextile filtration opening size under tension and confinement

Nonwoven geotextiles have been used as filters in geotechnical and geoenvironmental works for half a century. They are easy to install and can be specified to meet the requirements for proper filter performance. There are situations where a geotextile filter may be subjected to tensile loads, which may alter relevant filter properties, such as its filtration opening size. Examples of such situations are silty fence applications, geotextile separators, geotextile tubes and geotextiles under embankments on soft soils. This paper investigates the effects of tensile strains on geotextile pore dimensions. A special equipment and testing technique allowed tests to be carried out on geotextile specimens subjected to tension and confinement. The results obtained showed that the variation in filtration opening size depends on the type of strain state the geotextile is subjected, under which the geotextile pore diameter may remain rather constant or increase significantly. However, confinement reduces the geotextile filtration opening size independent on the strain mobilised. An upper bound for the filtration opening size of strained nonwoven geotextiles is introduced and was satisfactory for the geotextile products tested.     

An analytical solution for geotextile-wrapped soil based on insights from DEM analysis

This paper presents a novel analytical solution for geotextile-wrapped soil based on a comprehensive numerical analysis conducted using the discrete element method (DEM). By examining the soil–geotextile interface friction, principal stress distribution, and stress–strain relations of the constituent soil and geotextile in the DEM analysis, a complete picture of the mechanical characterization of geotextile-wrapped soil under uniaxial compression is first provided. With these new insights, key assumptions are verified and developed for the proposed analytical solution. In the DEM analysis, a near-failure state line that predicts stress ratios relative to the maximums at failure with respect to deviatoric strain is uniquely identified; dilation rates are found to be related to stress ratios via a single linear correlation regardless of the tensile stiffness of the geotextile. From these new findings, the assumptions on the stress-state evolution and the stress–dilatancy relation are developed accordingly, and the wrapped granular soil can therefore be modeled as a Mohr–Coulomb elastoplastic solid with evolving stress ratio and dilation rate. The development of the proposed analytical model also demonstrates an innovative approach to take advantage of multiscale insights for the analytical modeling of complex geomaterials. The analytical model is validated with the DEM simulation results of geotextile-wrapped soil under uniaxial compression, considering a wide range of geotextile tensile stiffnesses. To further examine the predictive capacity of the analytical model, the stress–strain response under triaxial compression conditions is solved analytically, taking both different confining pressures and geotextile tensile stiffnesses into account. Good agreement is obtained between the analytical and DEM solutions, which suggests that the key assumptions developed in the uniaxial compression conditions also remain valid for triaxial compression conditions.     

有纺土工织物覆土条件下的渗透特性试验研究

分别配制不同孔隙比的粉砂、标准砂和黏土试样,采用自主研制的一套多功能渗透试验装置,开展了一系列纯土和有纺土工织物覆土条件下的渗透试验,对比分析了这两种条件下渗透系数的差异,并探讨了有纺土工织物与土共同作用下的渗透机理。结果表明,有纺土工织物对于土体渗流略有一定的抑制作用,表现为覆土条件下的渗透系数略小于纯土的渗透系数,但是对于粉砂,当其孔隙比比较大、细砂颗粒的含量相对较多时,细砂颗粒则有可能在渗流作用下通过有纺土工织物孔隙而产生流失,使得覆粉砂条件下的渗透系数略大于纯粉砂的渗透系数。     

The effect of submersion in the ochre formation in geotextile filters

This analyzes the effect of submersion in the formation of ochre biofilm in geotextile filters used in drainage systems. The chemical microbiological aspects involved in ochre formation and clogging of drainage systems are discussed. Clogging by ochre may be considered a major threat in the performance of filters and drainage systems. This process has been observed in the field and demonstrated in laboratory tests under well-controlled conditions. Oxygen is needed for ochre formation and is available at the water–air interface of the filters. If the filters are submerged, oxygen may also be available dissolved in the water, with higher concentrations close to the surface due to the diffusion process. Column filter tests with the introduction of iron bacteria under three different filter submersion conditions were carried out. Woven geotextile filters were used in all tests. Biofilm formation on the geotextile filters were evaluated through the analysis of EDS (Energy Dispersive X-ray detector) and scanning electron microscopy. Ochre formation was verified in all tests, confirming that ochre formation can occur even under submerged conditions. The formation of ochre biofilm decreased with the depth of the geotextile filter in relation to the water surface, following the expected reduction of available oxygen below the water surface.     

土工布加固软土地基的机理试验研究

土工布与土体的界面摩擦力是衡量其对土体加固效果的关键因素,本文首先根据土工布与土体拉拔摩擦试验的机理,设计并制作了界面剪切测试仪,该试验测试仪具有试验精度高、应用广泛和操作步骤简单的特征,然后采用此测试仪对土工布与砂土的界面摩擦力进行了试验研究,试验参数包括不同的竖向压力、土工布层数和砂土含水率。试验结果表明:增加竖向压力能够显著提高土工布与土体的界面摩擦力;相同竖向压力作用下,土工布层数超过2层时,对其界面摩擦力提高并不显著;相同条件下,当砂土含水率超过5%时,提高砂土含水率能够显著降低土工布与土体界面摩擦力。这些研究结果可为土工布加固土体的设计和施工提供依据。     

Influence of particle shape on the shear strength and dilation of sand-woven geotextile interfaces

The influence of particle shape on the mechanical behavior of sand-woven geotextile interfaces over a wide domain of soil density and normal stress is studied. A uniformly graded angular fine sand, and a blend of well rounded glass beads with identical particle size distributions, were selected as granular material. Experiments revealed the impact of particle shape on peak and residual friction angles as well as the maximum dilation angle of interfaces between both granular media and woven geotextile. It was observed that the residual friction angles of interfaces between angular sand/glass-beads and woven geotextile are very similar to the residual friction angles of angular sand and glass-beads in soil–soil direct shear test. It is understood that the peak friction angle and maximum dilation angle of angular sand-woven geotextile were slightly lower than corresponding values for angular sand in soil–soil direct shear test. While the peak friction angle and maximum dilation angle of angular sand-woven geotextile interface decrease with the increase in normal stress, experiments showed that these factors are insensitive to normal stress for glass beads-woven geotextile interfaces, at least for the range studied herein. All interfaces with woven geotextile as the contact surface exhibit an abrupt loss of shear strength in the post-peak regime of behavior. Finally, a unified stress-dilation law for the angular sand-woven geotextile, glass beads-woven geotextile, and angular sand-roughened steel interfaces is obtained.     

Hydraulic conductivity requirement of granular and geotextile filter for internally stable soils

A filter media satisfying the hydraulic conductivity requirements allows unimpeded seepage without generation of surplus pressure head and decrease of flow rate. This paper proposes design criteria for the hydraulic conductivity requirements of a filter based on governing flow equations. The results have shown that the hydraulic conductivity requirements of pressure head and flow rate are satisfied with a single condition of hydraulic conductivity of filter greater than or equal to the hydraulic conductivity of soil times the hydraulic gradient in soil. The proposed model is developed for saturated conditions and is also applicable for partially saturated conditions. The developed model is validated based on the experimental evaluations of sandy soil with three granular filters and two needle punched non-woven geotextile filters. The developed design criterion applies to internally stable soils with granular and geotextiles filters and offers an improvement in the standards and current design guidelines for protective filters.     

深圳河反滤土工布试验研究

对深圳河治理工程边坡防护反滤运行期的土工布进行了综合试验研究 ,分析了土工布长期运行过程中的渗透性、保土性和淤堵性能 ,从强度损失角度分析了土工布长期运行的强度衰减情况 ,并对土工布的耐久性进行了试验分析。由试验知 ,土工布渗透系数降低约 10 0倍左右 ,而土工布强度降低了 5 0 %左右 ,强度衰减速率小于 0 .2 6% /月。试验表明 ,土工布已进入稳定渗透期和强度的稳定衰减期 ,土工布满足防护反滤的要求