Hydraulic Conductivity Testing of Geosynthetic Clay Liners (GCLs) Using the Constant Volume Method

Transcription

1 Xiaodong Wang 1 and Craig H. Benson 2 Hydraulic Conductivity Testing of Geosynthetic Clay Liners (GCLs) Using the Constant Volume Method REFERENCE: Wang, X. and Benson, C. H., Hydraulic Conductivity Testing of Geosynthetic Clay Liners (GCLs) Using the Constant Volume Method, Geotechnical Testing Journal, GTJODJ, Vol. 22, No. 4, December 1999, pp ABSTRACT: Hydraulic conductivity tests were conducted using open and constant-volume permeation systems on specimens from a geosynthetic clay liner (GCL). Two constant volume (CV) systems were employed: the falling-head constant-volume (FHCV) system and the constant-head constant-volume (CHCV) system. A conventional burette system using pressurized air was employed for the open system (OS) tests. The test results show that hydraulic conductivity tests can be conducted 30 or more times faster with the FHCV and CHCV systems than with an open system. Typically the permeation portion of the FHCV and CHCV tests can be conducted in one-half day. Slightly lower hydraulic conductivities are measured with the CV systems due to the slightly higher effective stress applied during testing with these systems. The CHCV system has several advantages over the FHCV system, including minimizing initial transient behavior, constant applied effective stress during testing, and simpler calculations. KEYWORDS: geosynthetic clay liner, GCL, hydraulic conductivity, open system, closed system, constant volume, FHCV, CHCV 1 Geotechnical Laboratory Manager, Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, WI Associate Professor, Department of Civil and Environmental Engineering, University of Wisconsin-Madison, Madison, WI by the American Society for Testing and Materials 277 Introduction Hydraulic conductivity testing is nearly always required for quality control and assurance assessment during construction of barrier layers in waste containment systems. Laboratory tests conducted for quality control are normally performed in flexible-wall permeameters using an open system following methods described in ASTM D 5084 (Standard Test Method for Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible-Wall Permeameter) or Geosynthetic Research Institute (GRI) test method GCL-2 (Standard Test Method for Permeability of Geosynthetic Clay Liners) for GCLs. While these open system methods have proven to be a reliable technique to measure hydraulic conductivity, reaching the termination criteria often can require up to a week or more even when conditions appear steady (i.e., the hydraulic conductivity is no longer changing with time). This limitation is particularly true for geosynthetic clay liners (GCLs) that have very low hydraulic conductivity (10 10 to m/s). In the study described in this paper, an alternative technique called the constant-volume method was used to measure the hydraulic conductivity of GCLs. This method, which was originally developed for testing clays (Bjerrum and Huder 1957, WCC 1991), employs a closed-loop system that forces inflow to equal outflow and a precision burette for measuring flow rates. As a result, measurements can be made accurately in a shorter period of time, which permits earlier termination. Tests were conducted using the falling-head constant-volume (FHCV) procedure (Trautwein Soil Testing 1995) and a newly developed constant-head constant-volume (CHCV) procedure. Richard Ladd and Stephen Trautwein developed the FHCV device used in this study. The authors developed the CHCV device. For comparison, hydraulic conductivity tests were also conducted on GCL specimens using an open system. Background Geosynthetic Clay Liners (GCLs) GCLs are thin pre-fabricated clay liners that consist of a layer of sodium bentonite clay approximately 5 mm thick (when dry) sandwiched between two geotextiles or glued to a geomembrane. The GCL used in this study contained granular bentonite (median aggregate size 0.6 mm) sandwiched between two geotextiles (woven slit-film monofilament geotextile and non-woven staple-fiber geotextile) joined together by needle punching (Fig. 1). The mass per unit area (ASTM Test Method for Measuring Mass per Unit Area of the Textiles, D 5261) of the GCL was 6.5 kg/m 2 under room conditions (~60% relative humidity). The dry mass per area of bentonite was 6.1 kg/m 2 and, under room conditions, the gravimetric water content of the bentonite was 7.2%. X-ray diffraction testing of the bentonite showed that its mineralogical composition consisted of 67% sodium Montmorillonite, 10% quartz, 7% cristobalite, 11% plagioclase feldspar, 5% potassium feldspar, and a trace of illite and mica. Open and Closed Permeation Systems Permeation systems used for hydraulic conductivity testing can be classified as open or closed systems. An open system implies that the influent and/or effluent ends of the system are open to the atmosphere or an applied compressible fluid pressure source (e.g., pressurized air). Burette systems operating with compressed or atmospheric air are open systems (Fig. 2). Some flow pump systems can also be classified as open systems. In an open system, a saturated specimen can change volume by expelling or absorbing water from the influent and effluent burettes during permeation.

2 278 GEOTECHNICAL TESTING JOURNAL FIG. 1 Schematic of needle-punched GCL used in study. FIG. 2 Open system for hydraulic conductivity testing. In contrast, a closed system employs a closed loop of liquid to permeate the soil (Figs. 3 and 4). A closed loop is achieved by connecting the inflow and outflow drainage lines of the permeameter in-line with a mercury-filled capillary tube. Displacing the mercury in the tube induces a hydraulic gradient, which drives flow through the soil until the mercury level returns to equilibrium. Provided the soil and the tubing are saturated and the tubing is stiff, inflow and outflow from the specimen are forced to be equal and the specimen cannot change volume, i.e., the specimen is maintained at constant volume. Thus, closed system tests are usually referred to as constant-volume (CV) tests. The CV systems used in this study are shown schematically in Figs. 3 and 4. 3 The specimen is initially consolidated and backpressure saturated with a traditional open-system panel board 3 U.S. patent pending on constant-head, constant-volume hydraulic conductivity system (Fig. 4).

3 WANG AND BENSON ON HYDRAULIC CONDUCTIVITY TESTING 279 FIG. 3 Schematic of falling-head constant-volume (FHCV) apparatus. FIG. 4 Schematic of constant-head constant-volume (CHCV) apparatus.

4 280 GEOTECHNICAL TESTING JOURNAL through tubing connecting the flexible-wall permeameter to the panel board. An additional device called a permometer (Trautwein Soil Testing 1995) is attached to the flushing lines of the permeameter and forms the closed-loop constant-volume permeation system. During saturation and consolidation, valves connecting the permeameter drainage lines to the permometer are closed. After saturation and consolidation are complete, the valves to the permometer are opened and the tailwater bleed valve on the permometer is opened slightly to displace the mercury in the capillary tube. In the FHCV system the mercury is displaced vertically up the capillary tube (Fig. 3). In the CHCV system, the mercury is displaced vertically up a tube and then horizontally through the capillary tube (Fig. 4). The headwater drain valve is then closed along with the valves connecting the porewater pressure burettes to the flexible-wall permeameter. In both systems, a closed loop exists with a raised mercury column that applies a hydraulic gradient across the specimen. In the FHCV device, the mercury column falls back to equilibrium at a rate proportional to the flow rate through the specimen. As the mercury falls, the space above it fills with water from the effluent line because the system is closed. Because the head is falling throughout the test, the hydraulic gradient, applied pore water pressures, and applied effective stresses also change throughout the test. When the mercury column reaches its equilibrium position, the hydraulic gradient and the flow rate through the specimen are zero. To continue testing, the mercury level is raised again. Each time the mercury is reset, a trial is complete. Often times only a single trial is necessary. Multiple trials can be conducted to assess repeatability. In the CHCV device (Fig. 4), the mercury travels horizontally in the capillary tube at a rate proportional to the flow rate through the specimen until the meniscus reaches the end of the capillary tube, at which point it is reset using the same procedure followed to initiate the test. The head remains constant throughout the test. As a result, the hydraulic gradient, applied pore water pressures, and applied effective stresses also remain constant. The FHCV and CHCV devices employ a relatively large-diameter mercury-water reservoir at the effluent end of the mercury column. In the FHCV device the mercury level in the mercury-water reservoir rises slightly throughout the test (Fig. 3). Thus, the FHCV test is actually a falling-head rising-head test. In the CHCV device, mercury flows out of the effluent tube and spills into a mercury storage annulus (Fig. 4). Thus, the mercury effluent head remains constant because the water-mercury interface remains at constant elevation. Methods Trimming GCL Specimens GCL specimens were trimmed from a large roll (1.5 by 3 m) supplied by the manufacturer using the following procedure. A circular steel trimming ring (100 mm in diameter) with a sharp edge was pushed down onto the GCL and slowly rotated so that it pressed into the upper and lower geotextiles without grabbing or twisting them during rotation. The geotextiles were then cut along the outer edge of the trimming ring using a utility knife with a new blade. A thin stream of de-ionized water was then sprayed along the interface of the interior of the trimming ring and the GCL. The moistened GCL was allowed to hydrate for 5 min. Subsequently, the trimming ring was gently lifted away while pressing down lightly on the GCL specimen. This procedure nearly always resulted in a clean uniform edge. The hydrated bentonite near the edge smears slightly when the ring is removed, and the smeared surface prevents drier interior particles from falling out from between the geotextiles. The edge of each specimen was inspected carefully after the trimming ring was removed, and any loose fibers were gently cut away with a scissors. Any gaps or non-uniformities in the edge were sealed with a light coating of bentonite paste prepared from de-ionized water and bentonite from the waste portion of the GCL obtained from outside the trimming ring. Hydraulic Conductivity Testing Methods described in GRI test method GCL-2 were followed for testing the GCL specimens. ASTM Method D 5887 (Standard Test Method for Measurement of Index Flux Through Saturated Geosynthetic Clay Liner Specimens Using a Flexible-Wall Permeameter) was not used because the intent was to measure hydraulic conductivity, whereas D 5887 is for measuring index flux. Moreover, D 5887 was not an approved standard when the testing was initiated. ASTM D 5084 could have been used in place of GCL-2, but the testing described here was initiated as part of the round robin study reported by Daniel et al. (1997), which stipulated using GCL-2. GCL-2 is very similar to ASTM D 5084, except the termination criteria are slightly different. The termination criteria in GCL-2 are as follows: (1) at least three hydraulic conductivity measurements must be determined, (2) the ratio of outflow to inflow must fall between 0.7 and 1.3 for the last three hydraulic conductivity measurements, (3) no upward or downward trend may exist in the last three hydraulic conductivity measurements, and (4) the last three consecutive hydraulic conductivity measurements must fall within 0.75 to 1.25 times the average of the last three measurements. The average of the last three measurements is reported as the hydraulic conductivity. GCL-2 also does not require that saturation be verified. The only requirement is that the backpressure must be applied for at least 48 h. However, the authors have found that GCLs are saturated by the time testing under GCL-2 is complete. The specimens were saturated and consolidated using a backpressure of 266 kpa and an effective confining pressure of 44 kpa for a 48-h period. De-ionized water was used as the permeant. Tests were conducted using a traditional open system (OS) using the falling-head rising-head method as well as the FHCV and CHCV methods. Burette readings were typically made after the meniscus moved at least ten times the smallest division on the burette scale, which is the practice employed at the University of Wisconsin laboratories. The permometer used for measuring hydraulic conductivity using the FHCV method had a capillary tube with a cross-sectional area of mm 2 and a mercury-water reservoir with a crosssectional area of mm 2. For the CHCV tests, the capillary tube had a cross-sectional area of mm 2 and the mercury-water reservoir had a cross-sectional area of mm 2. The effluent tube in the mercury-water reservoir of the CHCV device also had a cross-sectional area of mm 2. The mercury in the FHCV test was raised approximately 280 mm above the equilibrium level to initiate permeation, which induced an initial hydraulic gradient of about 440. For the CHCV tests, the capillary tube was raised mm, which yielded a hydraulic gradient of about 175. A lower gradient was used in the CHCV tests to better simulate the open-system tests. The net capillary rise (R p ) of the mercury in the FHCV device (i.e., between the capillary tube and the mercury-water reservoir tube in the FHCV device) was measured as 3 mm of mercury at equilibrium. There is no capillary rise in the CHCV test

5 WANG AND BENSON ON HYDRAULIC CONDUCTIVITY TESTING 281 because the capillary tube has the same diameter at the influent and effluent mercury-water interfaces. For the FHCV tests, hydraulic conductivity was computed using the falling-head rising-head equation in D 5084 with a correction for capillarity and the net unit weight of mercury. The equation is: K aea i L (a e a i )( GHg 1) ln A t Z 1 Rp Z2 Rp (1) where a e is the cross-sectional area of the mercury-water reservoir in FHCV test, a i is the cross-sectional area of the influent capillary tube, G Hg is the specific gravity of mercury (13.54 at 23 C), L is the thickness of the specimen, A is the cross-sectional area of the specimen being permeated, t is the length of the time step, and Z 1 and Z 2 are the differences in elevation between the mercury-water interfaces at the influent and effluent ends of the mercury column (Fig. 3) at times t 1 and t 2. The constant head formulation was used for the CHCV tests. If head loss within the tubing is negligible, fluid mechanics show that the difference between the influent and effluent pore water pressures (u i and u e ) measured at the specimen is: u i u e Z(G Hg 1) w L (2) where Z is the difference in elevation between the two ends of the mercury (Fig. 4) and w is the unit weight of water. Pore water pressures measured during testing were identical to those predicted using Eq 2, indicating that the head losses in the tubing are negligible. Inspection shows that the drop in elevation head across the specimen cancels when the drop in total head is calculated. Thus, by applying Darcy s law, the hydraulic conductivity is computed as: acl x K AZ(G Hg 1) (3) t where x is the horizontal displacement of the mercury-water meniscus during a time step t, and a c is the cross-sectional area of the capillary tube (Fig. 4). FIG. 5 Results of hydraulic conductivity test on GCL specimen using FHCV method. Results FHCV Testing Graphs showing typical results of the FHCV tests are shown in Fig. 5. There is a transient period during the early part of the test (~60 min) during which the hydraulic conductivity drops rapidly. Subsequent to this period, the hydraulic conductivity remains fairly steady. The tests are also very reproducible, as shown in Fig. 5. Essentially the same behavior was recorded during six FHCV trials using the same GCL specimen. Termination of this test in accord with GCL-2 could have been achieved during the first trial after about 95 min of permeation. The basis for the transient portion of the FHCV test is not clear, but it appears to be related to transient pore water pressures (Fig. 6). During the first 60 min the pore water pressures change rapidly. This time corresponds almost exactly to the transient period in the hydraulic conductivities, as shown in Fig. 5. Subsequently the pore water pressures change gradually and the hydraulic conductivity measurements remain steady, as shown in Fig. 5. After this specimen was tested using the FHCV device, it was tested again with a conventional open system using the falling-head rising-head procedure. In this procedure, the influent porewater pressure is increased by 34 kpa to induce the hydraulic gradient as per GCL-2. In contrast, when testing using the FHCV device the FIG. 6 Pore water pressures at influent and effluent ends of GCL specimen during FHCV testing. effluent porewater pressure is lowered to raise the mercury column and induce the hydraulic gradient. Consequently, the specimens are tested under slightly different effective stress. Results of the open system tests are shown in Fig. 7. The hydraulic conductivity ceases to change significantly after the second measurement (i.e., the data appear steady ), but the outflow/inflow criterion is not satisfied until at least 70 h have elapsed, and more realistically termination would probably not have been considered to have been achieved until 145 h because the outflow-inflow ratio increased significantly between the third and fourth measurements. Thus, the test in the open system required about 68 to 143 h longer than the FHCV test. Moreover, because the specimen was essentially saturated after FHCV testing, the hydraulic conductivity became steady faster than usual. Had an untested specimen been permeated, the open-system testing time would have been longer.

6 282 GEOTECHNICAL TESTING JOURNAL FIG. 7 Results of hydraulic conductivity test on GCL specimen using open system after FHCV method. FIG. 9 Hydraulic conductivity of GCL specimen measured using CHCV method and subsequently an open system. FIG. 8 Hydraulic conductivity of GCL specimen at three mean effective stresses (69, 72, and 76 kpa) using an open system. Both methods yielded comparable hydraulic conductivity, but the hydraulic conductivity measured with the FHCV system is slightly lower (OS m/s; FHCV m/s). The lower hydraulic conductivity can be attributed primarily to two factors: (1) lowering the backpressure in the FHCV test, and (2) the slightly higher effective stress applied at the effluent end of the specimen when permeating using the FHCV system. Lowering the backpressure is believed to be less significant because measurements made at the end of testing showed that the specimens were saturated. To assess the importance of the higher effective stress, additional tests were conducted at three mean effective stresses (69, 72, and 76 kpa) on another GCL specimen. An open system was employed for these tests using the same pressures described previously. The mean effective stress was increased by increasing the cell pressure and permitting the specimen to consolidate at a higher effective stress. Results of these tests are shown in Fig. 8. The hydraulic conductivity at a mean effective stress of 76 kpa is about 50% lower than the hydraulic conductivity at 69 kpa, which is comparable to FIG. 10 Applied effective stresses at influent and effluent ends of GCL during CV testing: (a) FHCV and (b) CHCV.

7 WANG AND BENSON ON HYDRAULIC CONDUCTIVITY TESTING 283 TABLE 1 Summary of Hydraulic Conductivity Test Results on GCLs. Hydraulic Conductivity, cm/s Permeation Time to Termination, h K cv /K os t os /t cv Specimen OS FHCV CHCV OS FHCV CHCV FHCV CHCV FHCV CHCV VE VE VE UW UW UW UW NOTE: (1) K cv hydraulic conductivity from CV test, (2) K os hydraulic conductivity from open system test, (3) t cv termination time for CV test, and (4) t os termination time for open system test. the differences in hydraulic conductivity obtained with the FHCV system (33% difference, Figs. 6 and 7). CHCV Testing The initial transient behavior observed when testing GCLs with the FHCV method (Fig. 5) can be minimized by using the CHCV method. Results from another GCL specimen tested using the CHCV method are shown in Fig. 9. The specimen was first permeated with the CHCV method and then with an open system. The termination criteria were met after 40 min of permeation with the CHCV test, whereas 140 h of permeation was required for the open system test. As with the FHCV test, a slightly lower hydraulic conductivity was obtained using the CHCV method relative to the open system method (OS m/s; CHCV m/s). A key advantage of the CHCV test is that the effective stress remains constant during permeation because the applied pore water pressures do not change as water flows through the specimen (Fig. 10a). In contrast, the effective stress is continually changing during a FHCV test (Fig. 10b) because the influent pressure drops and the effluent pressure increases as the head falls. This is particularly important for GCLs, which are very soft when hydrated, particularly under low effective stresses. As shown in Fig. 8, the hydraulic conductivity of GCLs is sensitive to small changes in effective stress. Other Tests Results of additional tests along with those shown in Figs. 5 to 9 are summarized in Table 1. The data labeled with the prefix VE were obtained from Kenneth Criley of Vector Engineering, Grass Valley, CA, who performed FHCV tests on the same specimens he tested for the round-robin study reported by Daniel et al. (1997). Table 1 includes the permeation time required to meet the termination criteria in GCL-2 as well as the hydraulic conductivities measured using open and constant volume systems. The data from Vector Engineering and those from the University of Wisconsin ( UW ) are similar. From the data the following conclusions can be drawn: (1) hydraulic conductivity tests on GCLs can be run in significantly less time using the constant volume method (less than one day after backpressuring is complete), and (2) the constant volume method yields slightly lower hydraulic conductivity than the open system method (~80% of the open system value) because the effective stress is higher in the constant volume test. Summary and Conclusions Hydraulic conductivity tests conducted on GCLs using conventional open systems and two constant-volume systems have been described in this paper. The results show that hydraulic conductivity tests can be conducted in less time with the constantvolume system because the flows can be read precisely using capillary tubes and inflow is forced to equal outflow. In general, the constant-volume tests were conducted about 40 times quicker than the open system tests and yielded comparable, but slightly lower hydraulic conductivities. The higher effective stress in the constant volume tests is believed to be responsible for the slightly lower hydraulic conductivities obtained from constant volume testing. Tests conducted with a falling-head constant-volume (FHCV) system exhibited decreasing hydraulic conductivity during the initial 60 min, which is probably caused by rapidly changing pore water pressures. After the initial 60-min period, the hydraulic conductivities were essentially steady. This difficulty is minimized using the constant-head constant-volume (CHCV) procedure. In addition, the effective stress remains constant during constant-head testing, whereas it continually changes during falling-head testing. Acknowledgments Stephen Trautwein of Trautwein Soil Testing Equipment, Inc. supplied the FHCV permometers used in this study. Richard Ladd provided the derivation of the equation for calculating hydraulic conductivity in the FHCV test. The authors appreciate their assistance. Kenneth Criley of Vector Engineering is thanked for supplying his data from OS and FHCV systems. References Bjerrum, L. and Huder, J., 1957, Measurement of the Permeability of Compacted Clays, Proceedings of the Fourth International Conference on Soil Mechanics and Foundation Engineering, Butterworths, London, Vol. 1, pp Daniel, D., Bowders, J., and Gilbert, R., 1997, Laboratory Hydraulic Conductivity Testing in Flexible-Wall Permeameters, Testing and Acceptance Criteria for Geosynthetic Clay Liners, ASTM STP 1308, L. Well, Ed., pp GRI (1993), Standard Test Method for Permeability of Geosynthetic Clay Liners, GRI Test Method GCL-2, Geosynthetic Research Institute, Drexel University, Philadelphia, PA. Trautwein Soil Testing, 1995, Permometer P Instruction Manual, Trautwein Soil Testing Equipment, Inc., Houston, TX. WCC, 1991, Test Procedure for Permeability Testing with Backpressure Using the Constant Volume-Falling Head Apparatus, Woodward-Clyde Consultants Clifton Laboratory, Test Procedure No. CL-T30.0.