Nonwoven Geotextile Filters Research Articles

Filtration compatibility of clay-nonwoven geotextiles under normal stress

Nonwoven geotextiles are commonly utilized as filter materials in drainage and filtration engineering. This study focuses on the clogging phenomenon of the nonwoven geotextile filter wrapping the blind drain in the active anti-floating technology of underground structures. In this paper, the filtration compatibility of clay-nonwoven geotextiles was evaluated using a newly developed gradient ratio test device that can apply normal stress. Two fabrication technics and four specifications of nonwoven geotextiles were used to filter clay, and the geotextiles’ clogging degree was assessed after the test. The influences of normal stresses, hydraulic gradients, and fabrication technics on nonwoven geotextile filtration performance were discussed based on results from a series of gradient ratio tests. Relevant assessment parameters (clogging coefficient, soil retention, and equivalent porosity) were also presented to quantify the clogging degree of nonwoven geotextiles after the test. Furthermore, the clogging process of nonwoven geotextiles filtering clay was initially explored, and the filtration effect of the needle-punched and heat-bonded nonwoven geotextiles was evaluated. The results showed that normal stress had a significant effect on nonwoven geotextile filtration, with needle-punched nonwoven geotextiles having better soil filtration and water permeability. Discussions on nonwoven geotextile applications in specific environments or conditions are also presented.

Effects of Filter Material on the Permeability of Sapric Peat in Flexible Wall Permeability Tests

Introduction Installation of Prefabricated Vertical Drains (PVDs) is one of the alternatives for ground improvement used in peatland deposits. However, filter paper is commonly used as standard filter material to determine the permeability of peat rather than the PVD material itself. Aims This paper presents preliminary data on the effect of using two different filter materials on the permeability behavior of Sapric peat. Methods A series of flexible wall permeability tests (FWPTs) was conducted to evaluate the peat permeability behavior under compression using two filter materials. This study compared Whatman standard filter paper and Prefabricated Vertical Drain (PVD), which is a non-woven geotextile filter material. Results The results showed that both tests using filter paper and a PVD filter exhibited a high initial coefficient of permeability, which depends on the hydraulic gradient. The coefficient of permeability significantly decreased until a certain period and then diminished with time. The coefficient of permeability from PVD filter tests was found to be approximately 2.6 times higher than that of the standard filter paper under the same compression. The duration required to reach a semi-steady state flow condition from the PVD test was 0.9 times faster than the standard filter paper. The random error of the coefficient of permeability data from the tests using the PVD filter was lower than the data of the standard filter paper. Conclusion This preliminary result suggests that standard filter material might not represent the actual coefficient of permeability of Sapric peat. The coefficient of permeability value was less consistent compared to the PVD filter. The selection of filter material plays a crucial role in ensuring accurate and reliable results, especially when dealing with PVD construction in peat. Using the PVD filter in FWPT appears to be suitable for the design of PVD in peat. The findings of this study contribute to evaluating the correct parameters for engineers to design and analyze the effectiveness of the ground treatment method using PVD in peat.

Experimental study on filtration performance of geotextile filter used in emergency rescue of dike piping

Geotextiles, as a type of common filtration material, have broad prospects in emergency rescue of dike backward erosion piping (BEP). To investigate the filtration performance of geotextile in BEP emergency rescue, several experiments were conducted using nonwoven and woven geotextiles to simulate the process of rescuing BEP with geotextiles. The influence of geotextile specifications and types on hydraulic compatibility of the filter system was analyzed, and the clogging mechanism of geotextile during dealing with BEP was revealed at a microscopic level. The results showed that the nonwoven geotextile filter with an equivalent pore size of 0.103 mm had a gradient ratio value of less than 3, and it had a highest flow rate of 260 mL/min. Increasing the thickness and pore size of nonwoven geotextiles within a certain range helped enhance their anti-clogging ability. The main mechanism of clogging in nonwoven geotextiles was the deposition of fine sand particles on their surface, forming a layer of low permeability soil. Plain woven geotextiles experienced severe clogging with a 42% reduction in flow rate, and it was not suitable for rescuing BEP. The clogging mechanism of woven geotextile involved the blocking of horizontal water passages by fine sand particles.

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

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

Filtration characteristics of composite filter of woven-nonwoven geotextile in tailings ponds

Nonwoven geotextiles are widely used in tailings ponds for filtration and drainage, but there is a risk of failure of nonwoven geotextile due to physical clogging. In this study, to reduce the clogging of nonwoven geotextile filter in tailings ponds and improve its long-term drainage performance, a layer of woven geotextile is placed between tailings and nonwoven geotextile to form a composite filter of woven-nonwoven geotextile. A series of gradient ratio tests were conducted to study the influence of the aperture of woven geotextile on the filtration characteristics of composite filter. The long-term filtration performance of composite filter was evaluated, and the filtration mechanism of composite filter was revealed. The results show that after adding a layer of woven geotextile with reasonable aperture upon the nonwoven geotextile, the gradient ratio of filter decreased and the flow rate of filter significantly increased. The filtration performance of composite filter of geotextile was better than that of nonwoven geotextile filter. The aperture of woven geotextile had an important influence on the filtration characteristics of composite filter. With the increase of the aperture of woven geotextile, the gradient ratio value of the composite filter decreased first and then increased. For relatively dense tailings, the recommended ratio of the equivalent aperture of woven geotextile (O95) to the characteristic particle diameter of tailings (d85) s is about 1.6.

Impact of the constrictions number on filtration characteristics of nonwoven geotextiles

The use of nonwoven geotextile filters is common in geoenvironmental and geotechnical engineering applications. The main requirements for successful performance of drainage and geotextile filters are soil retention, permeability and clogging resistance. In case of anti-clogging capabilities, the most popular method to evaluate the filtration behaviour of nonwoven geotextile in contact with soils is gradient ratio test. Also the number of constrictions of nonwoven geotextile should be determined on the basis of fibre diameter, thickness and porosity. The number of constrictions has been found to explain the different filtration behaviours of nonwoven geotextiles with similar or even the same opening sizes but different structures for various soil conditions. This paper presents the gradient ratio test program for internally unstable soil and nonwoven geotextiles with different structures. Test results show a significant impact of the number of constrictions on the filtration characteristics of geotextile. What is more some modifications to the filter design criteria are proposed based on interpretation of the experimental results.

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.

Influence of physical clogging on filtration performance of soil-geotextile interaction

During the past four decades the use of nonwoven geotextiles as filter materials in geotechnical works has experienced a marked increase. In this study, the water permeability coefficient of nonwoven geotextiles after 23 years of exploitation in drainage system was determined to evaluate the influence of physical clogging on filtration performance of geotextiles. The obtained results showed that the geotextiles were clogged by soil particles. What is more, the objective in this study was to establish a gradient ratio test method for estimating a clogging phenomenon of geotextile filter adequately. A series of gradient ratio tests on three kinds of needle-punched nonwoven geotextile filters whose number of constrictions differs was performed. A geotextile filter was put under a silty sand layer. Based on the test results, it was confirmed that the water permeability coefficient became small as gradient ratio increased depending on the number of constrictions of the geotextile. It was also found that the modified gradient ratio GR4 is more sensitive than GR8 and GR25. Based on interpretation of the experimental results, the authors recommend the acceptance limit of GR8 equal to 3.7 and of GR4 equal to 4.8.

The application of green roof for stormwater quantity and quality improvement

The use of green roof is becoming increasingly worldwide because of its environmental, economic, and aesthetic benefits. The ability to retain storm water and improve the quality of effluent flow are the important characteristics of green roof. It is viewed as a best technology to minimize the negative impact of urbanization. However, the application of green roof in Malaysia is less and literatures regarding these is limited. An attempt has been made to evaluate the performances of green roof in improving the water quantity and quality of stormwater runoff. A lab-scale green roof with five component layers which are vegetation layer using creeping ox-eye and beach morning glory, topsoil substrate layer, non-woven geotextile filter layer, drainage plate layer and liquid waterproofing layer was used. Results indicated that the green roof with creeping ox-eyes shows a better performance in reducing peak flow with as high as 41% percentages of reduction compared to beach morning glory (15%). On the other hand, the green roof with beach morning glory manage to improve the concentration of COD up to 99% improvement while the green roof with creeping ox-eye shows a better performance in improving BOD with 17% reduction.

Influence of polyurethane foam on chemical clogging of nonwoven geotextile and tailings caused by ferrous iron

The chemical clogging of geotextiles filters is a common issue, which threatens the safety of engineering projects. To reduce the chemical clogging of nonwoven geotextiles and enhance their drainage capability, a polyurethane foam was innovatively placed under the nonwoven geotextiles in this paper. A series of column tests were conducted to study the mechanism of the polyurethane foam to reduce the chemical clogging of the nonwoven geotextile filters in tailings caused by ferrous iron. In addition, the influence of the concentration of Fe2+, hydraulic gradient and thickness of the tailings specimen on the chemical clogging of the polyurethane foam and nonwoven geotextiles was examined. Less chemical clogging of geotextiles caused by polyurethane foam was observed and the related mechanism was firstly explained. The polyurethane foam under the geotextile reduced the contact between the geotextile and air. As a result, the chemical clogging of the geotextile was alleviated, which increased the drainage capability of the geotextiles. A high water saturation of the polyurethane foam would help to a reduce the extent of the chemical clogging of the geotextile. The chemical clogging characteristics of geotextiles and polyurethane foam under different concentrations of Fe2+ and hydraulic gradients were observed.

Compatibility of tailings–nonwoven geotextile under stress and the effect of sand filter

Geotextiles have been widely used in engineering drainage. The clogging of geotextile filters used in fine tailings dams often occurs owing to complex environmental conditions. To better understand the influence of stress and the hydraulic gradient (i) on the hydraulic compatibility of fine tailings–nonwoven geotextile filter, a series of gradient ratio (GR) tests were performed in this study. To reduce the physical clogging and enhance the drainage capability of nonwoven geotextiles, a layer of sand filter was placed between the tailings and the nonwoven geotextiles. The effects of the sand filter were explored by GR tests and the following beneficial conclusions were drawn. Compaction induced by normal stress increased the interlocking between tailings particles; consequently, the internal stability of the tailings increased, which lowered the clogging potential and volume of drainage of the system. The value of i (1–20) had only a slight effect on the clogging potential and permeability coefficient of the tailings, especially in the case of high normal stress. The sand filter prevented the formation of filter cake at the soil–geotextile boundary, and increased the volume of drainage of the system. This reduced the clogging potential and enhanced the drainage capability of the system.

A review on some factors influencing the behaviour of nonwoven geotextile filters

A review on some factors influencing the behaviour of nonwoven geotextile filters

Effect of the Number of Constrictions on the Filtration Behaviour of a Soil-Geotextile System

Nonwoven geotextile filters have been used in geo-environmental engineering for decades to prevent the movement of base soil fine particles, allowing adequate seepage to flow through the geotextile plane. Most of the design criteria developed for nonwoven geotextiles are based only on the comparison between their characteristic opening size and the indicative diameter of the soil to be filtered. In the meantime, the nonwoven geotextile fibrous structure has an influence on the filtration of the soil-geotextile system. In this paper the numbers of constrictions of nonwoven geotextile samples were determined to verify the existence of a correlation between the geotextile structure and the filtration behaviour of soil-geotextile systems. The compatibility between an internally unstable soil and a nonwoven geotextile filter was evaluated using the gradient ratio test. The results obtained can also be the basis for modifying the geotextile filter design and selection criteria.

Leachate pre-treatment using non-woven geotextile filters

Geotextiles have been used as filters in geotechnical and geoenvironmental works for decades. They are cost-effective, easy to install and they reduce or avoid the need for exploitation and utilisation of natural granular filter materials. This study investigated the filtration function of geotextiles applied to the pre-treatment of leachate from a domestic waste landfill. Equipment was assembled where vertical panels of non-woven geotextiles were used as filters for raw leachate from a landfill. Five geotextiles with masses per unit area between 134 and 1800 g/m2 were tested, and up to three filter panels were employed in the tests. Several chemical and biological parameters of the leachate were measured before and after filtration. A local sand filter was also tested for comparison. The results obtained showed that the geotextile filters reduced the values of leachate parameters, with a reduction of up to 42% in the case of chemical oxygen demand. The toxicity of the pre-treated leachate was also reduced to some extent, and the geotextile filter performed better than the sand filter tested in most of the cases. The filter arrangement employed may be particularly relevant for the pre-treatment of leachate from landfills of small- to medium-sized communities.

Evaluation of clogging effects on nonwoven geotextile filters by negative-ion treatment

Clogging effects on nonwoven geotextile filters can reduce the drainage capacity of vertical drainage materials installed in clay deposits. The preceding clogging retention improvement can be increased if the nonwoven geotextile filter is treated by negative ion material in accordance to the knowledge that fine soil particle is negatively charged material. In this study, column tests were conducted to examine the effects of negative-ion treatment by using various types of ion-treated nonwoven geotextile filters. It was found that the negative ion treatment gave apparent increase of drainage capacity after long time as describe on this paper. Clogging effects of two nonwoven geotextile filters treated by negative ion (10 %; NI-10 and 20 %; NI-20) were compared to non-ion treated one (control) by long term filtration test. Finally, the highest drainage capacity was obtained by NI-20 followed by NI-10.

The application of geotextile and granular filters for PCB remediation

The application of a surface, permeable reactive barrier has been implemented at a remote site in the Canadian Arctic for the remediation of soils and water contaminated with polychlorinated biphenyls (PCBs). The initial barrier system was installed in July 2003. Preliminary work in both the field and the laboratory suggested that geotextiles alone may not be adequate for this particular Arctic barrier system, owing to issues related to survivability (specifically the effects of high UV and freeze–thaw) and clogging. Subsequent field and laboratory work demonstrated that granular materials trapped the majority of PCB-contaminated soil without impeding hydraulic performance; however, fines were escaping. Extensive column testing in the laboratory has shown that a nonwoven geotextile filter can be applied with success with a granular permeable reactive barrier system. This paper presents the results of laboratory experiments and field research used in the design of this barrier system.

Effect of filter–separators on the clogging of leachate collection systems

This paper reports the results obtained after 6 years operation of nine mesocosm experiments that simulate the 50 cm of the drainage layer closest to the leachate collection pipe in a landfill. Five different design configurations were examined involving a 300 mm thick layer of coarse (38 mm) gravel. The designs differed in terms of the presence, nature, and location of a filter–separator layer either at the waste–gravel interface or partway through the gravel. A nonwoven geotextile filter–separator (GTF/S) is shown to reduce clogging of the gravel relative to the no filter–separator or woven GTF/S designs. Some clogging of the geotextiles is reported, with reductions in geotextile hydraulic conductivity of 23% for the woven GTF/S, 74%–89% for the nonwoven GTF/S, and 75%–94% for the nonwoven geotextile partway through the gravel. The clogged nonwoven geotextile filter–separator maintained a higher hydraulic conductivity than the extracted woven geotextile. Of the designs with a filter–separator between the waste and gravel, the granular filter–separator most effectively reduced clogging of the gravel but at the expense of leachate mounding above the sand once the sand layer clogs. The design with a nonwoven geotextile partway through the gravel (GTMF) provides better protection of the underlying gravel from clogging than other designs involving a geotextile.Key words: landfill, waste, leachate, clogging, biofilm, geotextile.

Microstructure and pore characteristics of synthetic filters under confinement

This paper presents a study on the microstructure and pore characteristics of virgin and partially clogged non-woven geotextile filters under confinement using different experimental techniques. Geotextiles commonly found in practice were tested under normal stresses up to 1 MPa and pore dimensions and constriction sizes evaluated by the bubble point test, hydrodynamic sieving test and image analyser. The evaluation of the accuracy of theoretical methods for the prediction of filtration opening sizes of geotextiles under pressure was also assessed. The results obtained show a marked influence of stress level and partial clogging on the filter pore sizes and retention capability, and extend current guidance regarding current filter criteria for geotextiles.

Microstructure and pore characteristics of synthetic filters under confinement

This paper presents a study on the microstructure and pore characteristics of virgin and partially clogged non-woven geotextile filters under confinement using different experimental techniques. Geotextiles commonly found in practice were tested under normal stresses up to 1 MPa and pore dimensions and constriction sizes evaluated by the bubble point test, hydrodynamic sieving test and image analyser. The evaluation of the accuracy of theoretical methods for the prediction of filtration opening sizes of geotextiles under pressure was also assessed. The results obtained show a marked influence of stress level and partial clogging on the filter pore sizes and retention capability, and extend current guidance regarding current filter criteria for geotextiles.

Long-term performance of nonwoven geotextile filters in five coastal and bank protection projects

This paper evaluates the long-term performance of needle punched nonwoven geotextiles used as filters in coastal and river bank applications. Filtration and mechanical properties of nonwoven geotextiles exhumed from five separate projects in Europe and the Far East sites are reviewed. Hydraulic conditions varied from wave action to stream flow, and the filtered soils ranged from sands to marine clays. Performance was based on the condition of the site and the hydraulic and mechanical condition of the exhumed geotextiles as well as a comparison of the geotextiles used with respect to current filter design criteria.