Ultrasonic Removal of Clogging and Evaluation of Flow Capacity of Geotextile Drain

Clogging of geotextile drains is very common when used in fine-grained soil geo-structure such as reinforced earth retaining wall, slopes, embankments. As on date the index properties of the embedded drains are usually standardized with respect to factory-defined parameters. This paper explains the application of ultrasonic excitation for the removal of clogging of geotextiles drains extracted from two fine-grained soils, Kanto Loam, a naturally occurring soil in Japan and a commercially available clayey silt, whose drainage capacity was tested with consolidation from slurry stage. The mechanism of ultrasound for the cleaning of clogging as well as various other similar industrial applications has been explained. The amounts fine-grained soil entrapped within geotextile pores, before and after cleansing in an ultrasonic water tub was evaluated. Several terms, such as clogging index, clogging potential, cleansing efficiency, have been explained and evaluated based on the experimental data. The in-plane and cross-plane flows of clogged geotextile drains, both before and after ultrasonic cleaning, showed that cleansing efficiency was greater than 80%, which were irrespective of the type of geotextile selected and the nature of clogging by both fine-grained soils. Compared to the clogged geotextile specimens’ transmissivity and permittivity recovered nearly to 75% and above after ultrasonic cleaning. The importance of geotextile drains under confinement and in situ removal of its clogging by ultrasonics is elaborated.


Introduction
Soft ground stabilization and reinforcement application of nonwoven geotextiles are increasing steadily over the years, and stronger growth in the geosynthetics market has been surging ahead for solid waste disposal, erosion, flexible storage, rainwater harvesting and sedimentation control [1,2]. In waste barrier system, the unique pore structure of nonwoven geotextile drain provides excellent filtration and drainage properties. More so, the reinforcement and separation actions of geotextile as subgrade materials particularly in the railroad ballast foundation require clogging indexes. In all these applications, clogging has to be minimum for effective and long-term functioning of the geotextile drain. The hydraulic conductivity of a geotextile is one of the key parameters for determining the duration of its use before becoming clogged.
Drainage performance of geotextile depends on the variations in its properties in addition to interface characteristics and types of soils. As multifunctional characteristics of geosynthetics have been well recognised, it drainage and filtration efficiencies in marginal soils are still in the complexities of nature of fines entrapment and flow retention potential. Using a small size consolidometer [3], an ultra-soft soil was compressed from slurry to normal Terzaghi soil state. Pore water pressure, settlement rate, change in void ratio, change in permeability were determined and it was found that there is transitional stage at which void ratio was little smaller than the same deduced from porewater pressure (PWP) dissipation [3]. In the above experiments, all measuring devices were used at the boundary of soft slurry specimen. Using large size consolidometer [4], an arrangement was made not only to measure the PWP inside the slurry but also geotextile drain layer within (Fig. 3); so that drain layer is subjected to clogging, while consolidation from slurry stage was in progress. Thus, clogging of drains can be caused by physical, which is the case considered in the current study. However, clogging is also due to happen by biological and/ or chemical processes [5,6]. Characterizing of fine-grained soil-geotextile drain interfaces using image analysis tests is very much significant in the performance and hydraulic compatibility of geotextiles drains. Both the shape and size distribution of the pores of nonwoven geotextiles play a key role in filtration performance [7].
The geosynthetic products most commonly used for the last four decades include geotextiles and geogrids in addition to the erosion-control products, such as geocells, geonets, geocomposites and geomembranes [8,9]. Geosynthetics have been used extensively for the improvement of fine-grained soil backfilled earth-structures, such as retaining walls, slopes and embankments. When they are used directly within backfill as reinforcement layers as well as with wrap around perforated pipes for slopes or in the lining of retaining wall at the soilretained side, clogging of the same becomes an obvious. Clogging problems are widely recognized in the various water front structures, such as dams, reservoirs and also in various manufacturing industries. Mechanical as well as biological clogging of water filters and dirt or oily deposits in many components in brewing industry poses troubles. Fines entrapped within geotextile pores cause reduction in water drainage and thereby affect stability and serviceability of the structures in the long run. Using a thin sand mat around geotextile layers, named Hybrid drain [10], could prevent clogging significantly. This thin sand mat, which may be commercially produced, e.g. geosynthetic clay liner (GCL), not only prevents fines from clogging the embedded geosynthetics layer, it also ensures generating interface friction, which is otherwise absent when geotextile layer is used in direct contact with the fine-grained soils. Clogging affecting the durability of geotextile filters and its permeability often lead to hydraulic system's nonperformance [11].
The ultrasonic cleaning is one of the simple ways to remove dirt or oily deposits from different household and industrial items. There have not been many reported applications of ultrasonic waves to the cleaning of clogged geosynthetics, especially when they are clogged during projected service period. Removing or replacing them altogether is not always plausible, and therefore, it's attempted to evaluate the clogging status in a suitably customised laboratory set-up. Ultrasonic cleaning is used in narrow crevices and small holes that would not be easily accessible by turbulation, spray washing or other cleaning methods. This paper attempts explaining the principle of ultrasonic and application of this technique for the removal of clogged particles from the selected geotextile specimens exhumed from two fine-grained soils tested in the large consolidation cell.

Drainage Applications of Geosynthetics
There is extensive application of geocomposite (a combination of woven, nonwoven geotextile) wrap around the drain pipes to prevent clogging and ensuring longer service life and efficiency of drain. Even though multifunctional geosynthetics are recognised in the geo-environmental application domain, attempts to evaluate various design parameters are also substantially varied. However, the functionality of geotextiles lies in their abilities of separation, stabilization, filtration, reinforcement, as a moisture barrier or waterproofing, and drainage. So it is pertinent to study and investigate some of or all above-mentioned characteristics. There are many other important applications with special mentioning of the same such as in the encapsulation of the leachate collection systems in landfills while maintaining controlled clogging. The drain must also have the durability to survive construction and long-term drainage conditions, while its efficiency is ensured for the service life. Often the construction methods have a greater influence on drain performance than meticulous design provisions. It is also difficult and not routinely possible to check the on-site drain operability status as site conditions often go beyond specifically attributed principal drainage cum filtration.
Leachate collection systems (LCS) are designed to prevent ponding on the liner system. The flow efficiency of the LCS is a primary factor in determining the leachate collection efficiency [8]. The potential of the drainage materials to clog as a result of sedimentation, biological growth, chemical precipitation and/or biochemical precipitation, in not well addressed in the geosynthetics industry [12]. Clogging is also abundant with organic substrates when precipitating metals such as calcium, magnesium, iron and manganese are concentrated in the leachate. Aspects of -ve ion treatment to the nonwoven clogged drain to enhance the flow capacity were also tried [13].

Clogging of Geotextiles
Road constructions in hill areas are often subjected to almost vertical cutting of the slopes that leads to failure during construction phases (Fig. 1a). Appropriate stabilisation measures are not always possible and to a large extent conventional retaining walls, sometime as high as 30 m are not feasible. Therefore, many fine-grained soil backfilled earth structures are constructed with geosynthetics. These geotextiles have to function as drainage layer along with its seminal reinforcement function. In case of geosynthetic reinforced marginal soil embankment, the layers are shown in (Fig. 1b) and the ideal condition around such drain layers is taken as unit cell. Nonwoven geotextiles ( Fig. 1) often clog or blind, and woven geotextiles may allow piping. The extent of clogging depends on permeability of the soil, geotextile and soil-impregnated drain [14][15][16]. From the perspective of clogging of geotextiles in landfill clay liner system (LCS), field conditions are required to be checked [17]. The variation of physical, hydraulic and pore characteristics of some geotextiles drains and decrease in flow capacity of the same under cyclic water flow was confirmed by three-dimensional computed tomography [15,18]. Various tests on geotextile embedded with granular soils underlain by soft soil were performed [16,19]. The evaluation of the hydraulic properties of soil-geotextile drain system under confined conditions created with soft, saturated and fine-grained soils [20,21] is to be looked into. There are series of test conducted on the encapsulated geotexile sand mat [22].

Present Study
Details of the test procedures for embedded geo-drains ( Fig. 2) in both Kanto loam (KL) and clayey silt (CS) are given in [4], and they are tested under slurry stage to consolidation pressure upto 400 kPa. Geotextile drains are

(b)
Local soils being excavated to construct road inserted into the slurry after it was consolidated to 50 kPa ( Fig. 2a). In order to check the impregnation of soil particles into the drains so depicted in Fig. 2d, e, a hybrid drain consisting of thin (less than 10 mm) sand mat was used and flow tests results at different hydraulic gradients are presented in Fig. 3 [10,23]. As evident from the experiment, normalised flow capacity has reduced to as low as 20% (Fig. 3). There is hardly any effect of turbulence under low hydraulic gradient (i = 3) for the two finegrained soils chosen for this study. Therefore, the purpose of geotextile drain in fine-grained soils is overly defeated as flow reduction in in situ condition is significant. In order to check the clogged drain status after consolidation tests, Ultrasonic cleansing was tried and checked the flow enhancement at varied hydraulic gradient and normal pressure on to the exhumed drain. The aim of this paper is to check the efficacy of ultrasonic cleaning of clogged drain and thereby characterising flow capacity (transmissivity/permittivity) standards of six varieties of geosynthetic drains embedded within fine-grained soils.

Ultrasound Mechanism
Ultrasonics are used for the separation and removal of fine particles from gases and liquids in the industry [24]. Ultrasonic waves are characterized by reflection, refraction, diffraction frequency, wavelength, propagation velocity, intensity (measured in decibels) and attenuation [25]. Ultrasound cleaning is the induction of high-frequency (usually between 20 and 80 kHz) sound waves that results in the formation of cavitation within the liquid. It is created by high frequency generators that convert to sound waves through a transducer-induced vibration of the liquid. This high frequency vibration then creates microscopic bubbles form (Fig. 4) and repeatedly implodes upon a given surface. This continued excitation removes visible and even microscopic dirt particles making a dirty miniblind. The use of ultrasonic cleaning baths ranging from the small laboratory and jewellers' units of 100-200 W to large industrial cleaning tanks of several kilowatts is available. The same effect can be utilized to clean the surfaces of particles suspended in a slurry. Ultrasonic power can be applied externally as in a cleaning bath or by the insertion of an ultrasonic horn (a solid probe) into the slurry itself. The probe is particularly useful for introducing higher power (500 W) at very high intensities (0.5-2 kW/ cm 2 ) into smaller volumes (50-200 ml).

Flow Efficiency
Ultrasonic washing efficiency is defined as the amount  granular soil is not always available at site, and thereby, many reinforced geostructures are made with locally available soils. In doing so, reinforcement-soil interaction is to be looked upon by the consideration of its drainage and filtration efficiencies [26,27]. The attempt made in the current study is to evaluate the extent of clogging of various commercially available geosynthetic drains when tested from a slurry stage to soil state and subsequently finding out flow ability of the same under varying consolidation followed by ultrasonic cleaning of same and checking in-plane and cross-plane flow capacity. The images of clogging under a microscope (259) for all the six types of geotextiles tested, respectively, with two kinds of soils, Kanto loam (KL)-a natural soil collected from the Kanto plain region of Japan and an artificial clayey silt or silty clay (CS). In Fig. 5, microscopic (259) images of all the six selected geotextile consisting of one geocomposite and five nonwoven geotextile, used as embedded drain in KL (Fig. 2a, d) slurry are presented. Images after ultrasonic cleaning of the same as per Fig. 4a, b are also shown in Fig. 5a-3, f-3. It may be noted that ultrasonic cleaning for about 5 min is sufficient to de-clogg the drain specimens extracted after the consolidation tests as explained in [4]. Figures 6 and 7 show the permittivity of six types of geotextile drain tested in KL and CS. After ultrasonic cleaning, plots are shown under increasing normal pressure of Max 900 kPa. As confining pressure increases, flow capacity of all drain specimens also decreases. This is quite possible as increasing pressure not only reduces the thickness of the drains but also reduction in the pore opening area of the drain. As envisaged in the plots, trend is same for unclogged, clogged and ultrasonically cleaned drain specimen. Among all the six types of geo-drains, GCA has the lowest permittivity as it's made of combination of nonwoven layers on both sides of woven geotextile (Fig. 2d-e). After ultrasonic washing of the specimens for 5 min, the flow efficiency of the drain layers improved. With increasing confining, pressure washed specimens showed less reduction in the flow capacity. It is noted from the experiments that permittivity reduces with the increase in the normal pressure, which is a significant observation for all tests that not usually represented in the flow capacity index of drains. Out of the five nonwoven geotextiles the B-type is the thinnest one and it was severely affected by clogging. Table 1 presents the test interpretation of geocomposite (GCA) for the clogged, unclogged and after ultrasonic waterbath cleaning. Thickness of five layers of GCA, as considered in the permittivity test, has reduced to 50% at normal pressure of 900 kPa. In case of permittivity test for clogged drain and similar after ultrasonic cleaning, only the middle layer is replaced as shown in the inset diagram in Figs. 6 and 7. In the case of in-plane flow, only one layer of drain specimen was used. As the clogged particles might create a complex microstructure at the drain interface, in this study flow ability of single drain was attempted. However, more drain layers in togetherness may not be found much in practice.
Cleaning efficiency due to ultrasonics for all six types of drains remained more than 80% (Figs. 6, 7) and above. This signifies that ultrasonic washing method worked well in the cleaning of clogged geosynthetics. However, cleaning of embedded drain by the direct insertion of the ultrasonic probe has yet to be looked into. Table 2 provides summary of all tests, and some of them are being explained here under.

Clogging Potential of Drain
Clogging potential is defined as the ratio of the flow capacity of unclogged drain, and the same for clogged drain measured at normal pressure of 20 kPa. The same has KL, Kanto loam; CS, silty clay; ULT, ultrasonic washing method; CGA, composite geotextile type-A; NW-A/B/C/D/E, nonwoven geotextile types (selected based on increasing thickness); washing efficiency, ratio of clogged mass washed out by ultrasonic waves and total clogged mass; clog reduction index, ratio of clogged mass washed out by ultrasonic method and dry mass of the geosynthetic specimen; water retention index, ratio of weight of geotextile soaked in water and dry mass of specimen; clogging index by mass, ratio of dry weight of clogged particles and dry weight of geotextile specimen been evaluated for ultrasonically cleaned drain. These values for all the six types of geotextiles are shown, respectively, for KL after ultrasonic cleaning (Fig. 8), and the same for CS is plotted in Fig. 9. Clogging potential for GCA type drain is very high ([ 20 at 900 kPa) for KL with increasing normal pressure on the drain, and the same for CS type of soil is almost half. Trend is relatively lower (2 for KL and 1.2 for CS) in case of other five nonwoven geotextile drains. As evinced from the low permeability of GCA (Fig. 6a) and excessive reduction in the flow capacity due to clogging, the clogged woven layer intersticed between nonwoven geotextile (Fig. 2d-e) also caused rather complex soil-drain interaction under applied normal pressure. The current study focussed on the physical nature of clogging only. However, in actual field situation, how de-clogged soil particles from geotextile drain would be removed or cleaned would require further investigation using ultrasonic probes. In Fig. 10, the in-plane water flow capacity reduction ratio for GCA drain embedded in KL and CS as well as exhumed clogged drain and after ultrasonic cleaning of the same is shown. The flow index is defined as the ratio of in-plane flow of the unclogged drain and the corresponding value at normal pressure at 20 kPa. In order to check the effect of increasing hydraulic gradient, results were not affected much. The significant reduction of in-plane flow capacity under increasing normal pressure is noted. In the diagram, corresponding relative reduction in the drain thickness is also shown. The maximum reduction was 35% for plain GCA drain and for clogged drain specimen thickness reduced to 50%. After unloading, corresponding increase in the clogged drains was relatively more than unclogged GCA.

Effect on Transmissivity and Permittivity of Geotextile
With increasing pressure on the drain specimen, there are marked reductions in the permittivity and transmissivity. The relative rate of flow capacity reduction along and across the drain is more or less found the same in both Kanto loam and clayey silt. Evidently clogging has reduced cross-plane flow capacity more than the in-plane flow capacity. A comparison of the permittivity and transmissivity ratio as presented in Fig. 11. The beneficial effect of ultrasonic cleaning is noted. The permittivity (cross-plane hydraulic conductivity integrated over thickness) and transmissivity (in-plane hydraulic conductivity integrated over thickness) of washed specimens have been derived, and Fig. 11 presents the variation of the permittivity and transmissivity ratios corresponding to GCA. While clogged specimens showed significant reduction in the flow capacity, there was marked improvement in the flow capacity after ultrasonic cleaning. This reduction in geotextile transmissivity is having a marked effect on the discharge capacity of the geotextile drain. Therefore, due attention must be paid to situations where geotextile drains are going to be used under high normal stresses. However, tests results indicate that ultrasonic cleaning of the exhumed geotextile specimen has increased flow efficiency significantly.

Conclusions
Based on the investigation carried out on six types of geosynthetics, respectively, tested with two fine-grained soils, it is confirmed that ultrasonic waves are effective in cleaning the clogged geotextile drain. In the present case, cleaning of clogged drains is done with an ultrasonic probe in the presence of some fluid medium (water in most cases). A proper methodology has yet to be derived to use the same technique in distressed field structures such as reinforced retaining wall constructed with fine-grained soil backfills. From the study performed, it is found that (a)washing efficiency for all clogged geotextiles is more than 80%, (b) transmissivity and permittivity of washed specimen improved nearly to 75% and above and (c) among six nonwoven geotextiles, NWB, which is relatively thinnest one, was severely affected by clogging, however, ultrasonic cleaning of the same removed clogging efficiently above 80%.