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ISSN : 2093-5145(Print)
ISSN : 2288-0232(Online)
Journal of the Korean Society for Advanced Composite Structures Vol.11 No.5 pp.1-7
DOI : https://doi.org/10.11004/kosacs.2020.11.5.001

Experimental Study on the Behavior of Hybrid Geotextile Tubes During Dredging Soil Filling

Hyeong-Joo Kim1, Hyeong-Soo Kim2, Ri Zhang3
1Professor, Department of Civil Engineering, Kunsan National University, Gunsan 54150, Korea
2Ph.D. Candidate, Department of Civil and Environmental Engineering, Kunsan National University, Gunsan 54150, Korea
3Ph.D. Candidate, Department of Civil and Environmental Engineering, Kunsan National University, Gunsan 54150, Korea

본 논문에 대한 토의를 2020년 11월 30일까지 학회로 보내주시면 2020년 12월호에 토론결과를 게재하겠습니다.


Corresponding author:Kim, Hyeong-Joo Department of Civil Engineering, Kunsan National University, Gunsan 54150, Korea Tel: +8210-5287-3395, Fax: +82-063-471-4760 E-mail: kimhj@kunsan.ac.kr
August 14, 2020 October 7, 2020 October 7, 2020

Abstract


In designing geotextile tubes, obtaining hydraulic compatibility between the geotextile and the filler material can be complex because the geotextile tubes performance is affected by many factors such as the filling pressure, soil properties, geotextile properties, foundation properties. Hybrid geotextile tubes, which are tubes composed of various geotextiles arranged circumferentially, were proposed by the authors to optimize their filling and dewatering performance. To assess the behavior of hybrid geotextile tubes during filling, a scale model test on hybrid geotextile bags during filling was conducted. Through the scale model test, the retention performance, filling time, and water pressure of four hybrid geotextile bags were evaluated. The results of the study show that the performance of geotextile tubes can be optimized in various ways by interchanging the geotextile placement, by changing the circumferential lengths of geotextiles, and using geotextiles with different properties.



준설토 충진 시 하이브리드 토목섬유 튜브의 거동에 관한 실험적 연구

김 형주1, 김 형수2, 장 르3
1군산대학교 토목공학과 교수
2군산대학교 토목환경공학부 박사수료
3군산대학교 토목환경공학부 박사수료

초록


토목섬유 튜브 설계는 토목섬유와 충진재 간의 수리학적 양립성으로 충진압과 토질특성 및 토목섬유 특성, 설치되는 지반의 기초 특성 등과 같은 많은 요소에 의해 성능이 영향을 받기 때문에 매우 복잡하다. 본 연구에서 개발된 하이브리드 토 목섬유 튜브는 원주를 다양하게 토목섬유재질로 구성하여 배수성능과 충진능력을 최적화 할 수 있었다. 실험적으로 확인하고자 제작된 복합재질의 토목섬유백에 준설토를 충진시키는 스케일모델시험을 실시하였다. 제작된 4개의 토목섬유백을 활용한 실험 을 통하여 보유성능과 충진시간 및 간극수압 등이 평가되었다. 최종적으로 토목섬유의 상호간 재질의 구성방법의 변화 및 원주 면 길이의 변화 등으로 토목섬유의 성능을 최적화한 연구결과가 제시되었다.



    National Research Foundation of Korea
    NRF-2020R1I1A3A04036506Korea Institute of Energy Technology Evaluation and Planning
    20183010025200

    1. INTRODUCTION

    The traditional practice of constructing coastal protection and hydraulic structures involves the use of conventional materials, such as rocks, aggregate, and concrete (Kim et al., 2018). However, such practice gave rise to environmental concerns and additional construction cost due to increasing transportation and material production expenses. Alternative materials in shoreline and dewatering applications have become increasingly popular in difficult and challenging conditions (Górniak et al., 2016). In recent times, geotextile tubes have been widely used since it is economical, time- efficient, and environment-friendly. Revetments, breakwaters, levees, groins, and dikes made from geotextile tubes are viable alternatives to the conventional rubble-mound structures especially when temporary protection is required or when rock is difficult to transfer to the site (Kim et al., 2016). A typical geotextile tube structure used in shoreline applications is shown in Fig. 1. The structure consists of the geotextile tube, fill material, anchor tube and scour protection.

    Shown in Fig. 2 is the geotextile tube construction processs, which includes the filling stage, dewatering stage, and consolidation stage. It is necessary that the performance of the geotextile tube is assessed during these stages because the performance of geotextile tubes is affected by many factors such as the particle size distribution of the fill material, the initial water content of the slurry, the pumping pressure, and geotextile properties, e.g., apparent opening size (AOS), percent open area (POA), and geotextile permeability. In coastal applications (Alvarez et al., 2007, Lee and Douglas, 2012), coarse materials and geotextiles with large AOS are usually utilized, and the process of draining water through the geotextile is fast. However, if the pore sizes of the geotextiles are not small enough, and if the fill material contains a considerable amount of fine particles, excessive piping could occur, whereby soil particles are able to migrate through the geotextile owing to the passage of water. In this regard, the tubes may have to be refilled several times, thereby, requiring more slurry volume to obtain the desired tube geometry. In dewatering applications (Guimarães et al., 2014, Khachan and Bhatia, 2017), using geotextiles with large AOS could result in poor filtration performance, and consequently would result in the need for post-secondary treatment of wastewater. Contrastingly, if geotextiles with small AOS are used, fine soil and small particles can be easily retained in the tube. However, the dewatering or consolidation process becomes slow because water cannot easily permeate through the geotextile due to its small AOS and due to filter cake formation (Koerner and Koerner, 2006).

    Because of the need to optimize the dewatering or consolidation performance, and the efficiency to retain fine silty soil, the hybrid geotextile tube (HGT) (see Fig. 3), which is a tube that has different types of geotextile along its circumference (Kim et al., 2019), is suggested. HGTs can compose of various combinations of geotextiles such as woven polypropylene (PP), woven polyester (PET), and composite geotextiles. The woven PP geotextile is highly permeable and is usually used in shoreline applications especially when sandy soils are used to fill the geotextile tubes. The woven PET geotextile is less permeable than the woven PP geotextile. However, it is more efficient in dewatering fine soils and wastes such as contaminated soil, sludge, etc. In some cases, composite geotextiles are used to improve the retention performance of the external woven layer by utilizing an internal nonwoven layer. The HGT mechanism is described in Fig. 4, which shows that the dissipation of water can be affected at the bottom of the tube when it is composed of a very permeable geotextile at the bottom in comparison to a geotextile tube made of a less permeable geotextile at the bottom.

    2. MATERIALS AND TEST SETUP

    The geotextiles used in the experiment are woven polypropylene (PP) and woven polyester (PET) (see Fig. 1). The properties of the geotextiles used in the experiment are shown in Table 1. Based on ASTM D4751, the apparent opening size of the woven PET and woven PP geotextiles used in the test are 315μm and 472μm, respectively. Also, based on ASTM D4491, the woven PP geotextile has a higher permeability than the woven PET. The fill material was obtained from a local dredging site in the Saemangeum river estuary. Laboratory tests such as specific gravity test, sieve test, and SEM (Scanning Electron Microscopy) were conducted. The properties of the fill material used in the scale model test are summarized in Table 2. The result of the sieve test is shown in Fig. 5a and the result of the SEM is shown in Fig 5b. Based on the results of the SEM, the particle size of the fill material ranges between 74.5 to 203μm, and that the soil sample contains smooth and rough angular particles that are either near-spherical or cylindrical. In addition, the D50 of the soil sample is 0.96mm and the percent passing #200 sieve is less than 50%, hence, the soil is classified as silty sand (SM).

    The geotextile bags used in the experiment have widths, lengths, and circumferences of 40cm, 52cm, and 80cm, respectively. Four hybrid geotextile bags were used in the experiment (see Table 3). The first geotextile bag (T-1) is composed of 15% woven PP geotextile at the top and 85% woven PET geotextile at bottom, hence, the circumferential length of the top geotextile (C1) is 12cm while the circumferential length of the bottom geotextile (C2) is 68cm. The second geotextile bag (T-2) is composed of 50% woven PET geotextile at the top and 50% woven PP geotextile at the bottom, hence, the circumferential length of the top geotextile (C1) is 40cm and the circumferential length of the bottom geotextile (C2) is 40cm. The third geotextile bag (T-3) has the same circumferential ratio as T-1 but the placement of geotextiles is interchanged with the top geotextile being woven PP and the bottom geotextile being woven PET. The fourth geotextile bag is also similar to T-1 with geotextile placement interchanged, having the woven PP geotextile at the bottom and the woven PET geotextile at the top.

    The test setup is shown in Fig 6. The bags were filled by gravity via the mixing tank. The fill material was mixed with water in the mixing tank to produce a slurry with a water content (ω) of 2000% and a unit weight (γ) of 10.11kN/m3. The mixing tank was installed with a transparent glass in order to observe the slurry discharge. An agitator was installed at the top of the mixing tank to maintain slurry consistency. The same amount of slurry volume was used to fill each bag. In the case of gravity filling, the pumping pressure is based on the hydraulic head, which is quite equivalent to the difference between the elevation of the slurry surface and the elevation of the filling port. In this experiment, the pumping pressure Pp was approximately 20kPa. To monitor the effect of the pumping pressure, pore pressure gauges were installed inside the bags in the manner as shown Fig. 6. The geotextile bags were then placed in a transparent box with dimensions of 70cm x 50cm and 21cm. Actual photo of the hybrid geotextile bag is shown in Fig. 7.

    3. RESULTS

    To obtain the slurry volume discharge from the mixing tank, the height of the slurry was measured through the transparent glass. Since the width and length of the tank does not change, the change in height of slurry was multiplied with the width and length to calculate for the water discharge from the mixing tank, as shown in Fig. 8. Results show that both T-2 and T-4 had a faster discharge at a time range of 0-15 s while both T-1 and T-3 had a slow initial discharge. The reason for this phenomenon is due to placement of the more permeable PP geotextile at the bottom. Water could easily flow out at the bottom of T-2 and T-4, which then allowed the soil particles to easily deposit at the bottom of the bags. However, as the soil particles started to deposit, the flow of water at the bottom started to slow down as some of the soil particles slightly blocked the passage of water at the pores of the PP geotextile. The increase in the discharge of T-3 showed the effect of the placement of the PP geotextile, which allowed water to easily flow out at the top of the bag. As summarized in Table 4, T-3 had the fastest filling time to reach a water discharge of 0.4m3, followed by T-4, T-2, and then T-1. The incremental change in volume was then divided with the corresponding time increment to obtain the flow rate. As shown, T-1 seems to have smallest average flow rate out of all the geotextile bags, followed by T-2, showing that geotextile composition greatly affects the flow of water since both T-1 and T-2 consists of 85% PET geotextile. In addition, placing a highly permeable geotextile at the bottom greatly helps with the flow of water during the initial stages filling. However, at the later stages, the flow of water slows down.

    The pore water pressure and the tensile forces are correlated for both filling and dewatering stages (Guo et al., 2015). The increase in the water pressure during the pumping stage will increase the tensile force and the dissipation of the water pressure during the dewatering stage will lead to a reduction in the tensile force. Shown in Figs. 9 and 10 are the variation of the pore water pressure with time at the top and middle of the bag, respectively. Results show that the pore water pressure is significant at the top when the woven PET geotextile is placed at the top (T-2 and T-4). The reason is that water could not easily dissipate because the upper geotextile has lower permeability and smaller AOS. Conversely, the pore water pressure at the middle was smaller for geotextile bags with woven PP geotextile placed at the bottom. Thus, the water pressure inside the geotextile bags is highly influenced by the geotextile’s AOS and the soil-geotextile interaction (Kim et al., 2019). Comparing the results of the soil pressure gauge, as shown in Fig. 11, it can be observed that the soil pressure was larger for T-1 and T-3. This is mainly due to the filling pressure since water could not easily dissipate at the bottom of these bags. In addition, the sudden rise in the soil pressure for T-4 at about 25 s may be caused by strong back pressure, as shown in Fig. 9. Since the flow of water was directed downward, the filling pressure, together with the effect of confining pressure, increased the density of the fill material by compressing the soil particles downward, thereby increasing the soil pressure.

    The results of the study showed that the performance of geotextile tubes can be optimized in a variety of ways for different applications by interchanging the geotextile placement, by changing the circumferential lengths, by using geotextiles with varying strengths, permeability, AOS, etc. Recommendation on the optimal circumferential ratio and HGT type depends on the application or problem. In cases where better drainage is required, placing a more permeable fabric at the bottom of the tube is suggested. However, if retention performance needs to be improved, placing a geotextile with smaller apparent opening size is more preferable.

    4. CONCLUSIONS

    The hybrid geotextile tube (HGT), which is a tube that has different types of geotextile along its circumference, was suggested for the purpose of optimizing geotextile tube performance. The HGT can be specifically applied to either reduce or increase the filling pressure, improve the retaining efficiency, or accelerate the dissipation of water. To assess the behavior of HGTs, experimentation was conducted in this study. The HGT flow rate, water pressure, and soil pressure were evaluated through a geotextile bag experiment. The results of the study showed that placing a highly permeable geotextile at the bottom greatly helps with the flow of water during the initial stages filling. However, at the later stages, the flow of water slows down. Furthermore, geotextile composition greatly affects the flow of water, since the bags consisting of 85% PET geotextile had the slowest average flow rate. Results also showed that the pore water pressure is significant at the top when a less permeable geotextile is placed at the top. Conversely, the pore water pressure at the middle is smaller when a more permeable geotextile is placed at the bottom. The results of the study showed that the performance of geotextile tubes can be optimized in a variety of ways for different applications by interchanging the geotextile placement, by changing the circumferential lengths, by using geotextiles with varying strengths, permeability, AOS, etc.

    ACKNOWLEDGMENT

    This work was supported by the National Research Foundation of Korea (NRF-2020R1I1A3A04036506), and the Korea Institute of Energy Technology Evaluation and Planning (KETEP, No. 20183010025200).

    Figure

    KOSACS-11-5-1_F1.gif
    Typical Geotextile Tube Structure Used in Shoreline Applications
    KOSACS-11-5-1_F2.gif
    Geotextile Tube Construction Process
    KOSACS-11-5-1_F3.gif
    Hybrid G eotextile Tube
    KOSACS-11-5-1_F4.gif
    Geotextile Tube Dewatering Mechanism: a) Conventional Geotextile Tube, b) Hybrid Geotextile Tube
    KOSACS-11-5-1_F5.gif
    Properties of Saemangeum Silty Sand: (a) Particle Size Distribution Curve, (b) Scanning Electron Microscopy (SEM)
    KOSACS-11-5-1_F6.gif
    Test Setup of Hybrid Geotextile Bag Experiment
    KOSACS-11-5-1_F7.gif
    Actual Photo of Hybrid Geotextile Bag
    KOSACS-11-5-1_F8.gif
    Variation of Water Discharge from Mixing Tank
    KOSACS-11-5-1_F9.gif
    Variation of Pore Water Pressure at Top Portion of the Geotextile Bags with Time
    KOSACS-11-5-1_F10.gif
    Variation of Pore Water Pressure at the Middle Portion of the Geotextile Bags with Time
    KOSACS-11-5-1_F11.gif
    Variation of Soil Pressure with Time

    Table

    Properties of the Geotextiles Used in Hybrid Geotextile Bag Experiment
    Properties of the Dredged Fill Used in Hybrid Geotextile Bag Experiment
    Properties of Geotextile Bags
    Comparison of Filling Time and Average Flow Rates

    Reference

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