LABORATORY TEST ON SELF HEALING CAPACITY OF GCL

Transcription

1 LABORATORY TEST ON SELF HEALING CAPACITY OF GCL K. Sari 1*, J.-C. Chai 2, T. Hino 3, and M. Mizuno 4 1 PhD Student, Graduate School of Science and Engineering, Department of science and Advance Technology, Saga University Japan; Telp: ; @edu.cc.saga-u.ac.jp 2 Professor, Graduate School of Science and Engineering, Department of Science and Advance Technology, Saga University, Japan; Telp: ; chai@cc.saga-u.ac.jp 3 Associate Professor, Institute of Lowland Technology, Saga University, Japan, hino@ilt.saga-u.ac.jp 4 Member, Hojun Co.,Ltd, Haraichi, Annaka-shi, Gunma, Japan ; Telp: , mmizuno@hojun.co.jp ABSTRACT Laboratory flow rate tests of geosynthetic clay liner (GCL) with defects were conducted to investigate the self-healing capacity of GCLs. The defects considered were circular holes 3 25 mm in diameter. The liquids used for the tests were tap water and salty water, and the overburden pressures applied were kpa. The test results show that with elapsed time the flow rate was significantly reduced which indicates the self-healing capacity of the GCLs tested. Also the flow rate reduced with the increase of the overburden pressure. The mechanisms considered for the decrease of flow rate are: (1) expansion of bentonite into the defect (hole) due to hydration and (2) squeezing of the hydrated bentonite into the hole by the overburden pressure. The results also show that the flow rate of using salty water is much higher than that of using the tap water, which indicates that the self-healing capacity is a function of the type of fluid used to hydrate the bentonite. Keyword: geosynthetic clay liner, defect, self healing, leakage rate INTRODUCTION Because of its low hydraulic conductivity and easy installation, geosynthetic clay liners (GCLs) have been widely used in geotechnical and geoenvironmental engineering as liners. However, GCLs can be easily damaged during the installation process. Some of the primary concerns related to incorrectly installation techniques are tearing and puncturing, hydration prior to placement of cover materials and stress concentration from construction equipment (Fox et al, 1998). Due to the expansion capacity of bentonite, which forms part of a GCL, it is generally believed that GCLs have self-healing ability. Many studies have been conducted to investigate the self-healing ability of GCLs. Babu et al. (2001) reported that the GCLs tested had a good selfhealing capacity for the case of desiccation cracks or punctures, but they indicated that moisture absorption capacity and effective pressure affected self-healing capacity of GCL. Egloffstein (2001) reported that the self-healing of a calcium bentonite GCLs took place by swelling and plastification of bentonite if a soil cover of more than 0.75 m thick was provided. Mazzieri and Pasqualini (2000) reported that for the case of damages with bentonite loss, holes up to 30 mm in diameter were self-healed with only slight increase in hydraulic conductivity compared to the intact GCL. They concluded that the hole was filled by the swollen bentonite. Since the self-healing relies on the expansion of bentonite, it may be affected by the type of fluids as well as overburden pressures. This experimental program was designed to further investigate the mechanism of self-healing of GCLs. The effect of the overburden pressure and the size of the hole on the self-healing capacity of two types of GCLs was investigated by using both tap water and salty water as fluids. LABORATORY TESTS Equipment Leakage rate of GCLs with defect was investigated by falling head and constant head flow rate tests. Falling head flow rate test The equipment of falling head flow rate test is shown in Figs. 1 and 2. It can test a GCL sample 150 mm in diameter with a defect at the center. The overburden pressure can be applied is up to 200 kpa. Constant head flow rate test The picture and the sketch of the device are shown in Figs. 3 and 4, respectively. The GCL sample can be tested is also 150 mm in diameter. 531

2 The cylinder is made of acrylic resin and piston of perforated stainless steel with a ceramics porous stone (140 mm in diameter) insert into the bottom of the piston. A O ring 4 mm in diameter was used as a sealing between the cylinder and the piston. With this device, the overburden pressure can be applied is up to 200 kpa by Bellofram cylinder system using air pressure. Material GCLs Fig. 4 Sketch of Constant head test Fig.1 Picture of falling head test Fig. 2 Sketch of falling head test Two types of GCL, designated as GCL-1 and GCL-2 were tested. GCL-1 is a bentonite high density polyethylene geomembrane (HDPE) type one, and has a thickness of 3.5 mm (0.5 mm of geomembrane and 3 mm of bentonite). GCL-2 is also a bentonite-hdpe geomembrane type, and has a thickness of 4.5 mm (0.5 mm of geomembrane and 4mm of bentonite). The photos of specimen of GCL- 1 and GCL-2 are given in Figs. 5a to d, and the illustration of the cross-section of GCL-1 and GCL- 2 are given in Fig. 5e. Some of the basic properties of the GCLs tested and the fluids used are given in Table 1. Test Procedure Preparation of GCL specimen a) Cut GCL specimen with a diameter of 150 mm. b) Made a hole 3 to 25 mm in diameter in the center of the specimen by a driller. c) Glue the specimen onto the piston in case of the constant head test, and the upper pedestal in case of the falling head test along the outer periphery of the piston or the pedestal. Procedure for the falling head test Fig. 3 Picture of constant head test a) Set up the test and apply a pressure of 25 kpa on the sample to ensure a firm contact between the specimen and the loading system for 1 hour before the flow rate test. b) Apply the desired load (25, 50 and 100 kpa) and the water head of about 1.0 m on the top of the specimen. 532

3 c) Open the valve for inlet flow and start the test. d) Measure the flow rate until it become stable. Procedure for the constant head test a) Make a soil layer inside the cylinder of about 90 mm thick. The soil is compacted in 3 layers. Decomposed granite soil passed 2 mm sieve was used. The soil was compacted to a density of about 14.5 kn/m 3, with a coefficient of permeability of about m/sec. b) Install the piston with GCL specimen attached into the cylinder until it reaches the surface of the soil layer. After that, install the loading system and applied 50 kpa to ensure a firm contact between the specimen and the soil layer for about 1 hour. c) Apply desired load (0, 50, 100 and 200 kpa) and put fluid into the cylinder with a water head of 210 mm above the GCL and start the test. d) Measure the flow rate until it is stable. The fluid is added periodically to maintain a constant water head. (a) bentonite of GCL-1 (b) geomembrane of CCL-1 (c) bentonite of GCL-2 (d) geomembrane of GCL-2 HDPE HDPE geomembrane Bentonite (e) cross section of GCL-1 and GCL-2 Fig. 5 Specimen of GCL-1 and GCL-2 No 1 2 Table 1 Properties of GCL Type of Bentonite GCL-1 GCL-2 Swelling index - ASTM D (ml/2gr) Table 2 Properties of hydration liquid Density (gr/cm 2 ) No Properties Tap water Salty water (10 gr/l) Test Results ph Ec NaCl Method for comparing the results of the constant head and the falling head tests Comparing the results of the constant head and the falling head test provides a check on the reliability of the results, and identification of the effect of the soil layer used in the constant head test. From the falling head test, the apparent hydraulic conductivity (k) of GCL can be determined by using the following equation. 2.3a.L h k = log 1 A. t h2 (1) where, h 1 = initial water level, h 2 = final water level, A = cross section area of GCL specimen, a = cross section area of stand pipe, t = duration of observation, L = thickness of bentonite of GCL specimen. While from the constant head test, leakage through the defect of GCL (Q) can be measured and the flow rate (q) can be determined as follow: q = Q t (2) To make a direct comparison, a flow rate is calculated for the falling head test as follows. q = k i A (3) FH FH CH where, k FH = hydraulic conductivity obtained from the falling head test, A ch = cross sectional area of GCL specimen, i CH = hydraulic gradient, the same value as for the constant head test. Variation of flow rate with time Figures 6 and 7 present the typical results of flow 533

4 rate using GCL-2 with a defect hole (φ) of 3 mm and 25 mm using tap water as fluid. The overburden pressures used were 25 kpa to 200 kpa. As can be seen from these figures, the flow rate significantly decreased with the elapsed time in the first 5 days. The post-test inspection indicates that an initial defect hole up to 10 mm in diameter (φ) was almost filled by the hydrated bentonite. However for φ = 25 mm, the hole still remained but the size was reduced a lot. space for hydrated bentonite to expand and under a higher p value, expansion in the vertical direction is limited. φ = 3 mm Fig. 8 Moisture distribution of bentonite after Falling head test Fig. 6 Variation of flow rate with elapsed time of GCL-2 φ = 25 mm To further confirm the mechanism of self-healing due to bentonite expansion, a simple free expansion test was conducted using the same bentonite as used in the GCL. Bentonite was compacted inside of 70 mm-diameter of PVC container. Water was added into the PVC container to increase the water content of the bentonite to 20, 40, 60 and 80%. Then the increment in the height of the bentonite layer with time was recorded. The results are shown in Figs. 9 and 10. It can be seen that for tap water as liquid and water content increased from 7 to 80%, the volume increased about 80% after 24 hours. While for salty water as liquid and water content increased from 11 to 20%, the volume increased about 56%. Effect of overburden pressure (p ) Fig. 7 Variation of flow rate with elapsed time of GCL-2 (Falling head test) The mechanism considered for the flow rate reduction with time is the expansion of bentonite due to hydration. After the flow rate test, the bentonite on the specimen was carefully collected and the variation of water content in the radial direction was measured. Figure 8 shows the results of moisture distribution of GCL-1 and GCL-2 with 3 mm hole. It can be seen that near the holes, the water content was higher. Also the larger the hole, the higher the water content; and the higher the over burden pressure, p, the lower the water content. This is because for a small hole, there is not much Figures 11 and 12 show the stabilized flow rate (q) versus overburden pressure (p ) plots of GCL-1 and GCL-2 respectively. The tests were conducted using the tap water. As can be seen from Figs. 11 and 12 that q tends to decrease as p increased. Fig. 11 shows for GCL-1 and φ = 3 mm, q value of p = 50, 100 and 200 kpa are 50%, 30% and 10% of the value at p = 0 kpa respectively. For φ = 5 mm, q value of p = 50, 100 and 200 kpa are 60%, 43% and 33% of the value at p = 0 kpa respectively. Figure 12 indicates that for GCL-2 and φ = 3 mm, the q values for p = 50, 100 and 200 kpa are 50%, 30%, 10% of the value at p = 0 respectively. For φ = 10 mm, at p = 100kPa, q are about 35% of value at p = 25 kpa. While for φ = 25 mm, q value of p = 50 and 100 kpa are about 25% and 18% of value at p = 25 kpa respectively. The mechanism considered for the reduction of q with p is squeezing of hydrated bentonite into the holes. Hydration will reduce the shear strength of the bentonite, which will make the squeezing easier. 534

5 Fig. 9 Volume expansion correspond to moisture content with tap water as liquid Fig. 12 q versus p (GCL-2) Effect of the size of the defect (φ) Figures 13 and 14 show the relationships between the q value and the size of the defect under p values of 25 to 100 kpa and 0 to 200 kpa from the falling head and constant head tests respectively. Both the figures indicate that q values increased with the increase of the size of the defects. Fig. 10 Volume expansion correspond to moisture content with salty water as liquid φ (mm) Fig. 13 q versus φ of GCL-2 (falling head test) Fig. 11 q versus p from constant head test (GCL-1) φ (mm) Fig. 14 q versus φ of GCL-2 (constant head test) 535

6 The relationship between the rate of leakage flow across the defect area (q/a defect ) and the size of defect is plotted in Figs. 15 and 16. These figures indicates that value of q/a defect tend to decrease as the size of defect increase under p values of 25 to 100 kpa and 0 to 200 kpa from falling head and constant head test respectively. φ = 3 mm Effect of the type of fluid The stabilized flow rates of using the tap water and the salty water as the fluids are compared in Figs. 17 and 18 for GCL-1 and GCL-2 respectively. For both GCLs, q value of using salty water is higher than that of using the tap water. The expansion of bentonite during hydration process is due to increase of diffusive double layer. The thickness of the double layer is influenced by the cation concentration of the fluid. The higher cation concentration tends to reduce the thickness of the double layer, and result in less expansion of the bentonite. Therefore, the self-healing capacity of GCLs is influenced by the type of fluids. Fig. 17 Comparison of q values of GCL-1 (constant head test) φ = 5 mm Fig. 18 Comparison of q values of GCL-2 (constant head test) Fig. 15 q/a defect of GCL-2 (Falling head test) Fig. 16 q/a defect of GCL-2 (Constant head test) CONCLUSIONS Self-healing capacity of defected GCLs has been investigated by laboratory constant head and falling head tests. The influencing factors considered are overburden pressure (p ), size of the defect (hole) and the types of liquid (tap water and salty water). From these experiment tests, the following conclusions can be drawn: (1) The flow rate significantly decreased with the elapsed time in the first 5 days. The mechanism considered is the expansion of bentonite due to hydration. The results of moisture distribution in the sample and the free expansion test of the bentonite verified the mechanism. (2) The flow rate decreased as the over burden pressure increased. The hydrated bentonite squeezed into the hole is considered as the primary mechanism of the flow rate reduction. (3) As the size of the defects increased, the flow rate increased but the rate of leakage flow through the defect decreased (4) Comparing with the result of using the tap water, 536

7 the flow rate of using the salty water is higher. The expansion of bentonite during hydration process is due to increase of diffusive double layer. The higher Na + concentration in the liquid will tend to reduce the thickness of the double layer, and result in less expansion of the bentonite. ACKNOWLEDGEMENTS The writers would like to thank Mr.Aoki for conducting some tests for collecting data which used in this paper. REFERENCES Babu, G.L.S. and Sporer, H. (2001). Self healing properties of Geosynthetic clay liners. Geosynthetic International, Vol. 8, No. 5, pp Egloffstein, T. (2001). Natural bentonites-influence of the ion exchange and partial dessication on permeability and self-healing capacity of bentonites used in GCLs. Journal of Geotextiles and Geomembranes 19, Fox,P.J, et al. (1996). Bearing capacity of Geosynthetic clay liners for cover soils of varying particle size. Geosynthetic International, Vol.3, No.4,pp Mazzieri F and Pasqualini, E. (2000). Permeability of damage Geosynthetic clay liner. Geosynthetic International, Vol.7, No.2, pp Shan, H.-Y, and Lai, Y.-J. (2002). Effect of hydrating liquid on the hydraulic properties of geosynthetic clay liners. Journal of Geotextiles and Geomembranes 20, Touze-Foltz, N, et al. (2006). Hydraulic and mechanical behavior of GCLs in contact with leachate as part of a composite liner. Journal of Geotextiles and Geomembranes 24,

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