Manufacturing effects on the hydraulic properties of needlepunched nonwoven geotextiles

Needlepunched nonwoven geotextiles are entangled to form a complex three-dimensional structure by random fibers, accounting for its bulky nature, wide range of pore size distribution, and good drainage. With needlepunched nonwoven geotextiles, water can move in both the vertical and horizontal directions. This paper examines two types of needlepunched nonwovens: one produced from polyester staple fiber and the other made from polyester spunbond continuous filaments. Experimental results indicate that the permittivity of staple needlepunched nonwoven geotextiles varies from 1.77-4.51 s⁻¹; the permeability coefficient varies from 0.63-2.87 × 10⁻² m/s. The permittivity of spunbond needlepunched nonwoven geotextiles varies from 1.13-1.97 s⁻¹; the permeability coefficient varies from 0.48-1.09 × 10⁻² m/s. In addition, the transmissivity of needlepunched nonwoven geotextiles decrease to an essentially constant value as the normal stress is increases. The transmissivity of needlepunched nonwoven geotextiles examined varies from 155-2.75 × 10⁻⁶ m²/s over the normal stress range examined (5-200 kN/m²). The AOS value of 3 denier staple fiber needlepunched nonwovens is less than 0.074 mm, the AOS value of spunbonded 7 denier and, 15 d and 20 d needlepunched nonwovens are 0.21 mm, 0.25 mm and 0.30 mm, respectively.     

Pore size distribution of hybrid nonwoven geotextiles

Pore size distribution has become a prerequisite in determining the performance of geotextiles for various functions including filtration, separation and reinforcement. The pore structure and morphology in a nonwoven geotextile are known to be complex and it becomes further complicated in hybrid nonwoven geotextiles consisting of two types of fibers. In this study, a modified model of pore size distribution of hybrid nonwoven geotextiles has been proposed based on sieving-percolation pore network theory. A comparison has been made between theoretical and experimental pore size distributions of hybrid needlepunched nonwoven geotextiles consisting of predefined weight proportions of viscose and polyester fibers. The weight proportions of the constituent fibers have been theoretically analysed for obtaining the desired pore size distributions of hybrid nonwoven geotextiles.     

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.     

Microstructure characteristics of nonwoven geotextiles using SEM and CT methods

Two digital image methods based on scanning electron microscope (SEM) and computed tomography (CT) were proposed to study the microstructural characteristics of staple fibers and continuous filament geotextiles. Two-dimensional (2D) image analysis was developed for SEM images using a machine-learning-based segmentation algorithm. Three-dimensional (3D) image analysis of the CT images was based on 3D reconstruction and a pore network model. The fiber orientation distribution, porosity, pore size distribution (PSD), and characteristic pore size O₉₅ determined from image analysis were compared with the theoretical equation and bubble point test (BBP) results. It is shown that 2D and 3D image analyses can accurately measure the fiber orientation distribution of the geotextiles. The porosity values obtained using 3D imaging were comparable to theoretical values. The PSD curves obtained in the BBP tests were in good agreement with those obtained using the 3D image method. O₉₅ sizes of continuous filament geotextiles estimated by 2D image analysis compared well with O₉₅ sizes obtained by BBP tests, whereas this was not the case for staple fiber geotextiles. The O₉₅ pore throat sizes of the two nonwoven geotextiles determined by 3D image analysis were comparable to the BBP test-based values and 2D image analysis-based values.     

Evaluating wettability of geotextiles with contact angles

Geotextiles have been used for drainage purposes in pavements for many years. To drain water out of road sections, the geotextiles need to get wet first. In this study, the wettability of three different types of geotextiles, namely wicking woven (WW) geotextile, non-wicking woven (NWW) geotextile, and nonwoven (NW) geotextile, was investigated in terms of their contact angles dependent on water-geotextile interaction. Contact angle was observed by the VCA Optima XE tensiometer for up to 12 s after a water droplet was dropped at the center of a geotextile's surface. Water droplets of two different sizes (2 μL and 5 μL) were used to demonstrate the droplet size effect on the contact angles of water on undisturbed geotextiles. Test results show that the contact angle decreased to smaller than 90° and the droplet disappeared on the wicking woven geotextile within a few seconds after water dropping, while the contact angle remained larger than or approximately equal to 90° on the other two types of geotextiles within the observation period. This comparison indicates that water penetrated faster into the wicking woven geotextile than other geotextiles. Furthermore, this study investigated the effects of soil particle intrusion and geotextile or fiber deep groove flattening associated with compaction on the wettability of geotextiles.     

Biaxial tensile behavior of spunbonded nonwoven geotextiles

Geotextiles can be successfully employed for any geotechnical application when they are able to sustain pre-defined levels of tensile stresses. The biaxial tensile test has an advantage over other tensile test methods in that it does not allow “necking” during deformation which simulates the operational conditions of geotextiles under confined stresses. In this study, the model for uniaxial tensile behavior of nonwovens has been modified to investigate the biaxial tensile behavior of spunbonded geotextiles. The model has included the effect of fiber re-orientation, stress-strain behavior of constituent fibers, and physical characteristics of nonwovens when the geotextile specimen is laterally constrained. A comparison is made between predicted and experimental stress-strain curves obtained from previous work (Bais-Singh and Goswami, 1998). Theoretical findings of biaxial tensile behavior obtained using the layer theory are also critically discussed. In addition, it has been revealed that fiber re-orientation is a key factor in translating the random spunbonded nonwoven geotextiles to anisotropic structures under defined biaxial tensile stresses.     

Nonwoven geotextiles from nettle and poly(lactic acid) fibers for slope stabilization using bioengineering approach

This article deals with needle-punched nonwoven geotextiles prepared from nettle and poly(lactic acid) fibers in different weight proportions for potential slope stabilization application using bioengineering approach. The geotextiles were tested for tensile strength, biodegradability, and enhancement of soil fertility. The tensile strength of the geotextiles was found to decrease with addition of stronger nettle fibers. This apparently surprising behavior was explained in the light of theoretical tensile mechanics of nonwovens. Further, the nettle fibers displayed higher biodegradability than the poly(lactic acid) fibers, and when buried under soil, all the geotextiles exhibited a loss in tensile strength. Interestingly, the fertility of the soil was remarkably improved after biodegradation of poly(lactic acid) fibers. Overall, the nonwoven geotextiles prepared in this work were found to be promising for slope stabilization application.     

A wave flume experiment for studying erosion mechanism of revetments using geotextiles

Unfavorable erosion to a revetment can affect the stability of the bank and may jeopardize the safety of adjacent structures, thus improvement work is needed to increase the stability of the revetment as well as reducing the possibility of failure. The use of geotextiles as a protection material for banks is not only environmentally friendly, but also stable in the long run. However, improper design of geotextiles may cause considerable loss of soil, which might result in failure. The actual flow behavior in revetments using geotextiles is rather complicated and can be categorized into three zones, namely, the uni-directional flow zone, the cyclic flow zone, and the tangential flow zone. In this study, a wave flume experiment was performed on model revetments using two kinds of geotextiles as the filter material to prevent erosion induced by cyclic flows. Soil migration behaviors were monitored. Furthermore, two kinds of cover blocks, riprap and concrete blocks, were carefully placed on the revetments in order to avoid puncture and abrasion of geotextiles during construction of revetments. The main purpose of this study is to elucidate the erosion control and filtration performance of soil-geotextile filtration systems under wave action. Two nonwoven needle punched geotextiles were tested. The geotextiles both have the same characteristic opening size, but have a different number of constrictions and different structures. One is a thin double-layer nonwoven material consisting of continuous filaments and the other is a thick one-layer nonwoven material consisting of short fibers.     

Interface shear characteristics of jute/polypropylene hybrid nonwoven geotextiles and sand using large size direct shear test

In this study, large-size direct shear tests were conducted to determine the interfacial shear characteristics of sand–geotextile under three different normal stresses. The geotextiles used in the present study were hybrid needlepunched nonwovens containing defined weight proportions of jute and polypropylene fibers. Subsequently, the interfacial shear characteristics of hybrid and that of a nonwoven geotextile consisting of solely polypropylene fibers with sand were compared and analyzed under different normal stresses. Initial higher shear stiffness of sand-polypropylene geotextiles was observed corresponding to sand-hybrid geotextiles specifically under higher normal stresses. Nevertheless, the contact efficiency of sand-hybrid nonwovens was similar to that of sand-polypropylene geotextiles. The surface morphology of sand particles has been investigated based on the images obtained from scanning electron microscopy (SEM) and quantitatively analyzed by means of Wadell roundness and degree of angularity methods.     

单向拉伸对土工织物反滤性能影响的试验研究

为研究单向拉伸对土工织物反滤性能的影响，选取两种条膜机织有纺织物和两种短纤针刺无纺织物，将不同拉应变下的织物与非连续级配土组成反滤系统，利用梯度比渗透仪测试系统反滤参数随拉应变的变化。根据反滤设计的透水、保土和防淤堵3个准则，分析拉应变对透水率、漏土量、梯度比等各参数的影响。试验结果表明：随着拉应变增加，有纺织物透水及防淤堵性能增强，保土性能减弱；无纺织物则相反，透水及防淤堵性能减弱，保土性能增强；同种土工织物厚度越大，拉应变对其反滤性能影响越大。     

The influence of uniaxial tensile strain on the pore size and filtration characteristics of geotextiles

The influence of uniaxial tensile strain on the pore size distribution and filtration characteristics of geotextiles is studied. An experimental apparatus was designed and used to conduct tests for pore size distribution, flow rate through the geotextiles and the gradient ratio. Four geotextiles made of polypropylene (two heat-bonded nonwoven and two slit film woven) were studied. Throughout the test series, the geotextiles were stretched to maintain 5%, 10% and 20% in-plane uniaxial strains. The strained specimen test results were compared with those from unstrained specimen. The experimental results illustrate the pore size and the mean flow rate through the plain geotextiles increase with the increase in tensile strain. The differences in changed percentages for apparent opening size and flow rate between the two nonwoven geotextiles are much higher than those between the two woven geotextiles. The increase in tensile strain results in reduction in the gradient ratio for the soil–geotextile system. This effect is more pronounced for nonwoven geotextiles. More testing is recommended to gain a deeper understanding into tensile strain effect on various geotextiles.     

Bioengineering of river earth embankment using natural fibre-based composite-structured geotextiles

A Jute-HDPE composite structured geotextile was developed to improve the performance of earthen structure of river embankment. The optimized geotextiles (430 g/m²) containing 86% natural component (on weight) having better physical, mechanical (tensile strength, 10 kN/m (machine direction) and 18 kN/m (cross direction), index puncture (163 kN) and CBR (1.5 kN)), hydraulic (AOS 178 μ) and endurance properties than 100% HDPE geotextiles. A coconut fibre geotextile net was placed over jute-polyolefin geotextiles to resist washing-off of loose cover soil until the establishment of vegetation. Placing of continuous seamless geotextile tube (weight 196.2 kg/m) filled with moist river sand at the anchor trench-cum-toe guard assisted in safeguarding from eddies. It was observed that initially closed structure of the geotextile assisted in efficient filtration leading to soil stabilization through compactness of soil layer (14 cm thick). The uniqueness of work lies in conversion of closed structure of geotextiles to open-mesh of HDPE slit film on degradation of jute, remained beneath the cover-soil, through which grass root penetrated the geotextiles sheet and riveted both the layers of soil, the cover and the compacted back layers. The remnant synthetic part thus acts as durable reinforcing element and its increased porosity provides breathability for growth of soil flora and fauna. Bermuda grass turf provided very high nailing strength (658.8 kN/m²) with the soil through intertwining of grass roots with durable synthetic network.     

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.     

Comparison of the behaviour of various geotextiles used in the filtration of clayey sludge: An experimental study

This paper presents the results of an experimental study of various geotextiles used to filter clayey sludge. The use of geotextiles to filter clayey sludge or suspensions of fine particles in water is more complex than that for filtering suspensions of granular soils. In practice, such applications generally use flocculants to postpone the formation of a low-permeability filter cake. The objective of the present study, which does not use flocculants, is to determine how geotextile characteristics affect the capacity of the geotextile to filter clayey sludge. Three key questions are addressed: (1) What are the main differences between vertical and horizontal filtration? (2) How do geotextile characteristics (nature, opening size, permeability, etc.) affect its capacity to filter clayey sludge (3) How do clayey sludge characteristics (i.e., grain size distribution and concentration)? and the type of flow (i.e., constant head or constant flow) affect the filtering capacity of geotextiles? To evaluate the capacity of a geotextile to filter clayey sludge, we propose three relevant criteria and analyse two filtration phases induced by different cake-formation processes (controlled by the geotextile and controlled by the filter cake). To determine the main differences between vertical and horizontal filtration, the settling of fines in the testing device and its influence on the results are analysed and discussed. This study shows that, for the various clayey sludge tested, the geotextiles (needle-punched nonwoven and thermally bonded nonwoven) with the smallest opening sizes (O₉₀?≤?60?μm) give the most promising results for filtering fines without the use of flocculants. Of these geotextiles, the thermally bonded nonwoven structure seems to offer the best filtration performance for the largest range of fines concentration in the sludge.     

Damage to geomembrane liners from tire derived aggregate

Tire derived aggregate (TDA) is currently being used as a cost-effective substitute for gravel in landfill leachate collection systems. TDA is composed of tires that have been shredded into pieces. However, the particles often contain a small portion of high risk protruding wires that may puncture a geomembrane placed below the TDA. The chance that these wires puncture is a function of the number of wires present in a sample, how the particles land, and the efficiency of the protection layer to prevent punctures. Using heavier nonwoven geotextile protection layers with larger size TDA and thicker geomembranes, the number of punctures may be expected to be fewer than 20 per hectare for the materials tested in this study. A second component to long term geomembrane performance is the presence of localised zones of high strain resulting from the point loading on the geomembrane. The strain resulting from TDA was found to be less than the strain which occurred from gravel using the same protection layers. It is concluded that, for different reasons, gravel and TDA both require effective protection layers of soil or heavy nonwoven geotextile to ensure long-term performance.     

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.     

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.     

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     

无纺织物单向受拉时孔径变化研究

现有反滤设计中保土准则使用土工织物未受拉时的等效孔径,但平面单向拉伸会导致该值变化,影响土工织物反滤性能。采用动力水筛法对三种无纺土工织物单向受拉时等效孔径变化进行测定。无纺织物被单向张拉至3%、5%和10%的平面应变,随着拉应变的增加,三种针刺无纺土工织物等效孔径减小。推求了无纺织物单向张拉时的等效孔径计算公式,对于较厚无纺织物,公式计算值和测试值较吻合,但对于较薄无纺织物,二值有一定差异。     

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.