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.     

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.     

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.     

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.     

Filtration performance of non- woven geotextiles with internally-stable and -unstable soils under dynamic loading

In many applications, geotextiles are subjected to dynamic loading conditions, for example, below roads and railways, for which a Gradient Ratio (GR) test is often used to assess filtration compatibility of soil-geotextile systems. This paper presents results from GR filtration tests with internally-stable and -unstable soils under dynamic loading conditions. In the tests, four non-woven geotextiles were used with varying types of soils under a hydraulic gradient of 5. Test results were interpreted in terms of GR values, permeability values, and mass and gradation characteristics of the soil before/after testing as well as the particles passing through the geotextiles. The test results show that the dynamic loading resulted in an increase of soil migration within the soil as well as an increase in the quantity of soil passing through the geotextiles. The available criteria for evaluating the internal stability of soils are evaluated based on the experimental data. Based on the test results, improvements to filter retention design criteria are suggested which take into account the internal stability of soils under dynamic loading.     

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.     

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.     

On the hydraulic behavior of unsaturated nonwoven geotextiles

Geotextiles have been widely used in soil structures for separation, filtration, reinforcing, and drainage. They are often used to provide reinforcement and drainage for retaining walls and embankments. It has been reported, however, that geotextiles may not drain water as effectively as was initially expected. In this study, published data on the hydraulic properties of unsaturated geotextiles are compiled and analyzed in order to highlight the hydraulic characteristics of unsaturated geotextiles.

The application of the van Genuchten equations originally developed for the water characteristic curve and the hydraulic conductivity curve of unsaturated soil to unsaturated geotextiles is then examined and discussed. Finally, the drainage from a one-dimensional sand column having a horizontal geotextile layer was analyzed using the finite element method and the van Genuchten equations to assess the utility of this procedure for further study of unsaturated/saturated water flow within the soil–geotextile system.     

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.     

Design of non-woven geotextiles for coal refuse filtration

This paper presents research findings on grain size distribution changes of coal refuse affecting the design of non-woven geotextiles used as filters in rock drains at coal waste impoundments. The research involved performing hydraulic conductivity tests on refuse - geotextile filters followed by grain size distribution tests. Data was evaluated for geotextile filter retention, permittivity, and clogging potential requirements as published by the U.S. Mine Safety and Health Administration’s Second Edition, “Engineering and Design Manual Coal Refuse Disposal Facilities”.Key findings indicate that refuse particles undergo slaking and aggregation which change the initial grain size distribution. Grading envelopes were developed and indicate that particle size zones influence the geotextile design parameters for retention, filtration, and clogging. The clogging criteria do not appear to be easily satisfied by the typical ranges of coarse coal refuse, at pre- and post-compaction grain size, for compatibility with non-woven geotextiles having an AOS = 0.212 mm.Conclusions impacting the specification and field installation of geotextiles include: i) post grain size distribution tests are suggested to be performed on specimens and at all compaction levels to observe changes in key indices of D₈₅ and D₁₅ for meeting retention and clogging criteria requirements; ii) the evaluation of the initial refuse stability indicate that at the low compaction energy conditions, which have mobile fines and high Cu values, are initially unstable with regards to their internal soil gradation; and iii) construction of geotextile wrapped drains is preferred to be made in pre-compacted refuse lifts. This condition is beneficial because the filter becomes more stable for retention and permeability; however clogging is still a concern.     

Spectrophotometry as a tool for characterizing durability of woven geotextiles

Geotextiles are used in numerous applications ranging from coastal hydraulic projects to geotechnical landfill projects. Durability studies are necessary for the sizing of these structures since these projects are subject to aggressive weathering. Therefore, it is important to be able to rapidly evaluate the rate of degradation of the geotextile, without detriment to the project. For traditional tests of geosynthetics, large areas must be exhumed, therefore a test which makes use of small specimens is proposed: spectrophotometry. The procedure proposed here makes use of electromagnetic radiation to evaluate the degradation of woven geotextiles by means of analysis of ultraviolet and infrared absorption. The aim of this study is to demonstrate the analysis of geotextile materials by spectrophotometry, making comparisons between laboratory and field degradation. The analyses and correlations were demonstrated to be satisfactory for characterization of degraded geotextiles. The results, with respect to both the absorbance of ultraviolet and transmittance of infrared, yielded both qualitative and quantitative characterizations of the behavior of the studied material. Therefore, spectrophotometry may be considered viable alternative for evaluating the characterization of durability in exhumed samples.     

Analysis of geotextile filter behaviour after 21 years in Valcros dam

In 1970, nonwoven geotextiles were used for the first time in an earth dam. The geotextile acted as a filter for the toe drain and on the upstream slope below the rip-rap. In 1992, samples were taken from both locations and performance tests were conducted in the laboratory. This paper presents the main results of the hydraulic behaviour of the geotextile filter in association with the soil of the dam. Also microscopic analyses are presented and, as the filter is considered to be performing well, selected filter criteria are checked.     

Reexamination of traditional testing techniques for determining WRCs of woven geotextiles in full suction range

Woven geotextiles have been widely used in soil infrastructures for the reinforcement purpose. The hydraulic properties of a woven geotextile are not major reinforcement design parameters and the water retention capability of a woven geotextile is often ignored. The traditional testing techniques were designed for soils or nonwoven geotextiles, but not for woven geotextiles. Nowadays, a new type of woven geotextile with wicking fibers was developed which could be used for both drainage and reinforcement purposes. However, there are no proper testing techniques to determine the full-range water retention curve (WRC) for a woven geotextile, let alone for the wicking geotextile.This paper aimed at proposing a proper testing technique to determining the full-range WRC for the wicking geotextile and to compare the water retention capability of wicking and non-wicking geotextiles. Firstly, the traditional testing techniques were re-examined to check the suitability for characterizing the WRCs of woven geotextiles whose pore size distributions were anisotropic. Secondly, a proper testing technique was proposed and the WRCs of different types of woven geotextiles were determined. Thirdly, the WRCs of wicking and non-wicking geotextiles were compared to demonstrate the advantages of the wicking geotextile to hold and transport water under unsaturated conditions. Finally, the effect of wicking fiber on the water retention capability of the wicking geotextile was quantified.     

Working mechanism of a new wicking geotextile in roadway applications: A numerical study

Woven geotextiles are often to be used in roadways for reinforcement purposes due to their higher tensile strengths. In the design of a woven geotextile for practical applications, the focus is mainly put on its reinforcing effect, while its hydraulic behaviors are not major design parameters and the influence of hydraulic properties on the reinforcing effect is often ignored. However, woven geotextiles are predominantly made of polypropylene and polyester, which are hydrophobic. This characteristic can result in a capillary break effect which it is equivalent to raise the ground water table to the location where the geotextile is installed. Numerous researchers have reported that the moisture storage from a capillary break effect can be detrimental to the long-term performance of a pavement structure. Until now, no method is available to effectively resolve this issue.Recently a new type of wicking geotextile is produced which has the capability to laterally drain excess water in a roadway under both saturated and unsaturated conditions. Several field applications demonstrated its potential in improving pavement performance. This paper attempted to investigate the working mechanism of the wicking geotextile through numerical studies and quantify the benefits of the wicking geotextile in term of drainage performance in a pavement structure. A numerical model was developed and validated using column test results from existing literature. After that the drainage performance of the wicking geotextile under different working conditions was simulated and evaluated.     

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.     

A study on biological clogging of nonwoven geotextiles under leachate flow

This paper presents results of long-term permittivity tests using leachate to evaluate biological clogging of nonwoven geotextiles. Three types of geotextiles with varying masses per unit area were used in the tests. The identification and quantification of microorganisms in the geotextile were carried out as well as microscopic investigations. The accuracies of semi-empirical models to evaluate the kinetics of bacteria growth and to correlate hydraulic properties and microbiological parameters were examined. Permittivity tests under increasing water heads were also performed on the geotextile samples already subjected to long-term leachate flow in order to evaluate the values of water heads required to wash the biofilms out of the geotextile pores. The results of the tests showed the marked reduction of geotextile permeability due to biological clogging and that the results of the predictions by semi-empirical methods were consistent with the biological mechanisms observed.     

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。     

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.     

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.     

Strength enhancement of geotextile-reinforced carbonate sand

The mechanical behavior of carbonate sand reinforced with horizontal layers of geotextile is invetigated using a series of drained compression triaxial tests on unreinforced and reinforced samples. The main factors affecting the mechanical behavior such as the number of geotextile layers, their arrangement in specimens, confining pressure, particle size distribution, geotextile type and relative density of samples were examined and discussed in this research. To make a precise comparison between the behavior of reinforced siliceous and carbonate sand, triaxial tests were performed on both types of sands. Results indicate that geotextile inclusion increases the peak strength and strain at failure, and significantly reduces the post-peak strength loss of carbonate specimens. The amount of strength enhancement rises as the number of geotextile layers increases while two other parameters including confining pressure and particle size affect adversely. The strength enhancement of reinforced carbonate sand is greater than the corresponding siliceous sample at high axial strains. Reinforced and unreinforced carbonate specimens exhibit more contractive behavior than their corresponding siliceous samples and tend to dilate at higher axial strains. By increasing the relative density of the samples, the peak strength of reinforced specimens rises due to enhanced interlocking between geotextile layers and sand particles. This process continues as long as the geotextile is not ruptured. The utilization of geotextiles with high mass per unit areas was found to be uneconomical due to slight differences between the strength augmentation of geotextiles with high and low mass per unit areas. It should be noted that geotextile layers limit the lateral expansion of specimens which leads to changing the failure pattern from a shear plane to bulging between the adjacent layers of geotextile.