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.     

Centrifuge model tests on the use of geosynthetic layer as an internal drain in levees

The objective of the paper is to examine the use of a geosynthetic layer as an internal drain in a levee subjected to flooding through centrifuge model tests. Three levee sections, having an upstream slope of 1V:1H and downstream slope of 1.5V:1H, were modelled at 30 gravities in a 4.5?m radius large beam centrifuge available at IIT Bombay. Out of the three levee sections modelled, one levee section was without any drainage layer (or clogged drain), while the other two had different types of horizontal drainage layers, namely, sand and nonwoven geotextile layer. The flood was induced with the help of a custom developed and calibrated in-flight flood simulator. At the onset of flood and subsequent seepage, pore water pressures within levee section, and surface settlements were measured using pore water transducers (PPTs) and linear variable differential transformer (LVDTs) respectively. Digital image analysis was employed to trace surface settlements, and downstream slope face movements at the onset of flooding during centrifuge tests. Levee section without any horizontal drain or clogged drain experienced a catastrophic failure. In comparison, the levee sections with an internal drain (sand/geotextile) remained stable at the onset of flooding. In the case of a levee with a sand drainage layer, the phreatic surface was observed to confine within the levee section itself, whereas it was found to migrate towards toe gradually in the levee section with a nonwoven geotextile layer. It is attributed to either due to suppression of drainage capacity of nonwoven geotextile layer or due to washing of fine particles into pores of nonwoven geotextile layer. Further, seepage and stability analyses were carried out numerically and compared with centrifuge test results. In order to address blocking of pores of nonwoven geotextile layer, a concept of sandwiching nonwoven geotextile layer with sand was explored. By sandwiching nonwoven geotextile layer with sand on either side, the thickness of drainage layer can be of the order of 0.05H.     

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.     

A reinforcing method for steep clay slopes using a non-woven geotextile

The effect of non-woven geotextile reinforcement on the stability and deformation of two clay test embankments is examined based on their performance for about 3 years for the first embankment and about years for the other. Horizontal planar sheets of a non-woven geotextile are expected to work in three ways: for compaction control; for drainage; for tensile reinforcement. The degree of stability of the steep slopes of the test embankments decreased during heavy rainfall. It is found that the use of non-woven geotextile reinforcement may effectively improve embankment performance. Only the stability analysis in terms of effective stresses can explain the performance of the test embankments. The horizontal creep deformation of the embankments during 2–3 years, which is partly attributed to the creep deformation of the non-woven geotextile, was found to be small. The results of both laboratory bearing capacity tests of a strip footing on a model sand ground reinforced with the non-woven geotextile and plane strain compression tests on sand specimens reinforced with the non-woven geotextile show that the non-woven geotextile gives tensile reinforcement to soils.     

浅谈土工布在南平防护工程中的应用及其效果

本文首先简要介绍了土工合成材料的性能、特点以及它的发展和应用,然后简要叙述土工布在南平防护工程中的应用情况及其使用效果。这在当时土工织物的应用日渐增多,南平防护工程适时采用新材料、新工艺、新技术用土工布代替反滤层,而且经受多次洪水考验,南平防护工程应用土工布防渗、排水、加固等经验是成功的。     

Exploring the effects of geotextiles in the performance of highway filter drains

Highway Filter Drains (HFD) are one of the most utilised drainage systems for roads, being considered as an environmental solution for sustainable drainage in transport infrastructures. However, little research has been done to understand their performance, representing a significant knowledge gap. This article therefore determines the hydraulic and clogging response of 3 different HFD designs in the laboratory; one standard design with British Standard Type B aggregate, and 2 new designs including a geotextile located at 50?mm and 500?mm depth from the surface of the HFD structure in order to assess the effect of the geotextile. The laboratory models were initially subjected to 9 rainfall scenarios with 3 rainfall intensities (2.5, 5 and 10?mm/h) and 3 storm durations (5, 10 and 15?min). Subsequently, the equivalent of 2-years’ worth of pollutants were added to test possible clogging issues under the highest intensity rainfall event, corresponding to a 1 in 1 year return period for the West Midlands, UK. No clogging issues were found in any of the models although the majority of the sediments were concentrated in the first 50?mm of the HFD profile, with higher percentages (>90% of the sediment added) in those models with an upper geotextile. Location of the geotextile significantly influenced (p-value?=?0.05) the hydraulic performance of the HFD.     

加荷条件下土工织物垂直渗透性能试验研究

作为反滤材料的土工织物尽管在岩土工程中使用越来越广泛 ,但决定其应用效果的渗透性能却在试验中往往不易准确测得。除了试验中的多种影响因素外 ,就常规渗透仪而论 ,漏水问题常常较难解决。本文使用改进的渗透仪对多种土工织物加荷条件下垂直渗透性能进行了试验研究 ,为三峡工程某排水管道中拟采用的土工积物反滤材料的设计提供了可行性对比和分析     

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.     

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.     

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

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

Modelling of hydraulic deterioration of geotextile filter in tunnel drainage system

The discharge capacity of a tunnel drainage system generally decreases with time because of the hydraulic deterioration of the geotextile filter. Hydraulic deterioration restricts groundwater flow into a tunnel and increases water pressure resulting in detrimental effects on the tunnel lining. Hydraulic deterioration of tunnel drainage system is unique in terms of clogging materials, deterioration mechanism, and flow conditions. Current studies and models investigating the clogging mechanism and hydraulic deterioration are not directly applicable to the geotextile filter of the tunnel drainage system. In this study, a theoretical model of the hydraulic deterioration of tunnel geotextile filter has been proposed considering the mechanical and hydraulic behavior of blinding, clogging and squeezing. A parametric study was carried out to evaluate the performance of the model. An experimental study has been conducted to investigate the clogging behavior of the tunnel drainage system and validate the theoretical model. Several types of clogging materials were selected: cement-leaching calcium oxide, calcium carbonate, iron oxide, and bentonite. Agglutinated clogging was mainly observed during the short-term testing. The findings suggest that the in-plane permeability of the geotextile filter decreased by approximately 90%. The proposed model corroborated the experimental results.     

Application of jute geotextiles as facilitator in drainage

For constructions on extremely soft foundation medium, the most common practice is to allow the soft soil to consolidate under the application of surcharge which generally consists of applying the necessary superimposed preload. However, because of low permeability of the in-situ soil, this often becomes a time consuming affair and also large quantities of material may have to be applied in the form of overburden. In some of the very fine grained soils encountered in practice, it may not be feasible to apply the surcharge without the danger of exceeding the bearing capacity of the existing formation soil. In such cases jute geotextiles may actually permit the construction to be carried out successfully and in a cost effective manner. A quantitative study on the efficacy of jute geotextile for consolidation purposes has been made and the outcome seems interesting. A design methodology involving selection and application of jute geotextile drains in weak foundation soil is suggested. Further, comparison of published standards with available properties of jute geotextile reveals that the fabric meets the criterion required for such purposes. Hence it may be judicious to explore the technically feasible, environmentally compatible and economically viable use of jute geotextile, as a suitable drainage medium for dealing with drainage problems encountered in the field.     

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。     

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.     

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.     

An optimal lightweight foamed mortar mix suitable for tunnel drainage carried out using the composite lining method

In this paper, an optimal lightweight foamed mortar mix suitable is proposed for facilitating tunnel drainage carried out using the composite lining method. A physical performance evaluation and pore structure analysis is provided in order to assess the performance of different lightweight foamed mortar mix proportions with various void fractions and foam solution concentrations. Furthermore, on the basis of an in-plane permeability test that simulated the permeation of fines of soil leaking with underground water, the formation and distribution of open-cell foams by measuring the outflow characteristics and outflow volume for each mix of the nonwoven geotextile and lightweight foamed mortar, which are the existing tunnel drainage materials was examined. Consequently, a mix employing a foaming agent with a dilution rate of 2% (#1) showed a better drain performance than all the other mixes, because it had the most appropriately formed and distributed open-cell foams, the key component for tunnel drainage. In other words, the mix of dilution rate of 2% was thought to have achieved stable closed-cell foams because of the decline in the surface tension of the foams, as well as the optimal formation and distribution of open-cell foams that possess excellent permeability because of the cohesiveness between the foams. Moreover, the thickness (drainage space) of it was approximately 17 times that of the nonwoven geotextile, and because the closed-cell and open-cell foams were connected like a spider web, it could be expected to reduce the blocking of drainage caused by the fines of soil.     

Investigating the influence of geotextile layers as biofilm granular filters to treat stormwater

This study investigated the application of geotextiles as sustainable urban drainage systems for degradation of organic pollutant load present in stormwater. Three experimental granular filter rigs were used, packed with alternating layers made up of gravel, pea gravel, sand and either an upper layer, an upper and lower layer or no layer of geotextile. The hydraulic loading capacity matched that commonly used on conventional sand filters. Standard water quality parameters were measured and collated data was evaluated using an ANOVA and Levine's test of homogeneity of variance procedure. It was found that the rig with both upper and lower geotextiles had a statistically significant difference in data from the rig with only a single geotextile layer. High chemical oxygen demand (58–80%) and suspended solids (88–99.99%) removal rates occurred for all rigs. However, the control rig showed increased outflow concentration of nutrients indicating the potential of geotextiles for stormwater treatment.     

土工管袋充填特性模型试验研究

为了更为深入的研究土工织物管袋的充填特性,通过开展室内土工管袋模型充填试验,对其形状、高度、所受张力、底部压力和排水速度进行研究,试验数据可为理论研究提供验证基础。由试验结果可知：土工袋高度随着充填的进行迅速增加,其它参数如土工织物所受张力、袋体底部压力和排水速度随之增加,到一定程度后袋体排水速度接近充填速度,袋体高度增加的速度明显减小,充填袋所能达到的高度由充填压力和土工织物的渗透性决定,在选定合适的土工织物后,可用充填速度来控制袋体高度;充填管袋排水主要发生在充填过程中和充填完成后的几分钟内;土工管袋横断面呈椭圆弧状,可按平面应变问题处理。     

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.     

尾矿库中新型复合排渗管排渗特征试验研究

尾矿库中排渗设施的淤堵是常见的现象,也是影响其安全的重要因素。为提高排渗管的排渗能力和抗淤堵性能,开发了一种新型复合排渗管,新型复合排渗管在土工布和排渗花管之间增加了一层格构网。通过室内模型试验,研究格构网的结构和花管开孔率对新型复合排渗管排渗特征的影响,分析了新型复合排渗管的抗淤堵性能,揭示了新型复合排渗管的排渗机理。试验结果表明：外包格构网增加了排渗管的等效开孔面积,优化了排渗管的排渗路径,提高了排渗管的排渗能力。随着花管开孔率的增加,外包格构网对排渗管排渗能力的影响减小。外包格构网提高了排渗管的抗淤堵能力,随着土工布的淤堵,传统外包土工布排渗管和新型复合排渗管的排渗量均有所降低,传统外包土工布排渗管排渗量降低的程度更大。当单向连通格构网的连通方向与排渗管的纵向方向有夹角时,排渗管的排渗能力不增反减。使用单向连通格构网时,须保证格构网的连通方向与排渗管的纵向方向一致。