AN EVALUATION OF POTENTIAL CLOGGING OF GEONETS AND GEOCOMPOSITES DUE TO SUSPENDED SOIL PARTICLES

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1 AN EVALUATION OF POTENTIAL CLOGGING OF GEONETS AND GEOCOMPOSITES DUE TO SUSPENDED SOIL PARTICLES Dhani B. Narejo GSE Lining Technology, Inc., Houston, TX USA ABSTRACT A series of tests were performed to evaluate the flow behavior of soil-water suspensions through synthetic drainage media. The objective of the test program was to determine whether soil particles can get trapped within the drainage medium - leading to clogging - or continue to remain in suspension. A non-plastic soil passing #200 ASTM sieve (finer than mm) was mixed with water to prepare mixtures of 0.01 to 1% (100 to 10,000 mg/liter) concentration. This mixture was then used to perform transmissivity tests in equipment that was designed and custom built for these tests. Tests were performed at 0.04, 0.1 and 0.3 gradients. Each test was run on the order of 200 to 300 hours to reach a stable value of transmissivity. The negative effect of particulates on transmissivity, i.e., the clogging of the drainage medium, increased as both the gradient and concentration were increased. The drainage medium remained clean, with no effect on transmissivity, for lower gradients throughout the tested range of concentrations. The results indicate that both the type of flow regime as well as the concentration of suspended solids is important. Most commercial drainage products can handle the range of responses observed, as long as there is not an extremely high concentration of solids. BACKGROUND Water flowing through drainage media is almost always laden with suspended solids. In some cases these can be inert soil particles, such as in potable and storm water. In other cases, such as landfill leachate and municipal waste water, suspended particles can be almost completely biochemical in nature. In general, the size of suspended solids in liquids is in the range of 0.1 to 500 microns (Henry and Heinke, 1989). In terms of soil, this represents fine sand, silt and clay size particles. Suspended particles carried by a flowing liquid moving through a drainage medium may either stay in suspension or get trapped. In the latter case, there could be a significant negative influence on flow capacity of the drainage medium. Finding at least a qualitative answer to this problem is the objective of this paper. Current design methodology for applications utilizing drainage geonets and geocomposites requires performing a 100-hour transmissivity test under site-specific gradient and boundary conditions according to ASTM test method D4716. The resulting

2 transmissivity value, referred-to as θ 100, is then modified as per GRI method GC8 according to the following equation: θ allow = RF cr θ 100 xrf cc xrf bc Where, θ allow = allowable transmissivity, θ 100 = 100-hour transmissivity, RF cr = reduction factor for creep of geonet core, RF cc = reduction factor for chemical clogging, and RF bc = reduction factor for biological clogging. Notice that the above equation does not explicitly address any reduction in transmissivity due to particulate clogging. This is probably because a paucity of practical information exists on the topic of particulate clogging of drainage media. As a result there is no reason to add another reduction factor in the equation to address particulate clogging. Moreover, one can argue that for applications involving landfill leachate, such as primary and secondary leachate collection and removal systems, particulates are a complex mixture of biological organisms, chemical species and inert soil particles. For such an environment, it probably makes more sense to include the effect of particulate clogging in the reduction factors for biological clogging and chemical clogging already present in the above equation. The intent of this paper is to develop a fundamental understanding of the flow of suspended particles through synthetic drainage media. The current test program is not comprehensive enough to lead to a reduction factor that could be included in the denominator of the above equation. However, this experimental work identifies important variables that should be further evaluated if a comprehensive response on this topic is to be developed. MATERIALS AND VARIABLES Suspended solids concentration in flowing liquids varies significantly depending on source, environment and end-use. Typical values found in literature are in the range of a few mg/l for potable water to 1000 mg/l for some landfill leachates. Suspended solids in storm water flow are typically less than 200 mg/l (Henry and Heinke, 1989). Considering the wide range of concentrations expected in practice, it was decided to perform tests with concentrations in the range of 0.01 % (100 mg/l) to 1% (10,000 mg/l). The former value represents relatively clean water, or leachate passing through a filter prior to entering the drainage medium. The latter value represents extremely turbid water or an un-filtered leachate with high concentration of biochemical solids. The author expects concentrations of to 0.1% to be encountered in most drainage applications.

3 A non-plastic soil passing ASTM sieve size #200 (finer than mm) was used in all the tests. This is slightly on the higher side of 0.1 to 500 micron suspended particle size range typically encountered in flowing liquids (Henry and Heinke, 1989). Filters are used in most, but not all, applications of synthetic drainage media. This size approximately represents what author expects to pass through a filter and enter a drainage medium. Three different gradients were tested: 0.04, 0.1 and 0.3. This represents approximately two orders of magnitude variation in flow velocity (assuming velocity being proportional to gradient). The lower value of gradient is probably close to a laminar flow state; however, the author did not verify this claim. The higher value of gradient is certainly within the turbulent flow regime. Tests were performed with only one type of geonet and one type of geocomposite. Properties of these products are presented in Table 1. The geonet product is a standard biplanar geonet with a nominal thickness of 6.3 mm (250 mils). The drainage geocomposite is the same geonet heat-bonded on both sides with a nominal 200 grams/square meter (6 oz. / square yard) nonwoven needlepunched geotextile. Table 1 Properties of Geonet and Geocomposite Evaluated. Geonet Properties Structure Biplanar Thickness, mm (mils) 6.3 (250) Density, grams/cm Porosity 1 85% Approximate Opening Size, mm (inch) 2 10 (0.4) Tensile Strength (Machine Direction), N/mm (lbs/inch) 9.6 (55) Carbon Black Content (%) 2 to 3 Geocomposite Properties Structure Same geonet with 200 g/m 2 geotextile bonded on the each side µ 1) Porosity is calculated from equation n = 1, where µ = mass per unit area (g/cm 2 ), ρ = density ρ t (g/cm 3 ) and t = thickness (cm). 2) This is taken as rib to rib distance.

4 TEST EQUIPMENT AND PROCEDURE A fairly clear understanding of the equipment and procedure can be gained by carefully studying Figures 1 and 2. Clearly, the equipment consists of a central housing section for placing the test specimen and two gradient tanks on the either side. There is a lid that is placed over the specimen and sealed so that liquid does not escape up at the top of the specimen. A minimal seating load of approximately 4 kpa is placed over the lid to keep the specimen in place and avoid any gaps along the boundaries. The gradient tanks are intended to maintain gradient and to keep particles in suspension. However, primary mixing and stirring is performed in another tank (referred to as recipe tank) which can be seen in the foreground in Figure 2. A soil-water mixture of one of the three target concentrations of 0.01% (100 mg/l), 0.1% (1,000 mg/l) or 1% (10,000 mg/l) is prepared by adding a measured quantity of the soil into a known volume of water. The temperature of the mixture is continuously recorded so that proper adjustments can be made to measured transmissivity. Mixture is kept in suspension through stirrers placed in recipe tank as well as in gradient tanks. A submersible pump at the bottom of the recipe tank pumps mixture to the upstream tank. The mixture is continuously re-circulated through the specimen. The required gradient is maintained across the specimen by opening or closing the ports on the upstream and downstream gradient tanks. Some tests required the use of a chiller to keep the water temperature within reasonable limits. This is necessary as the water temperature tends to rise as the mixture is continuously circulated. However, the chiller does not maintain a particular temperature and it is necessary to record the temperature and make adjustments to transmissivity. A particular test starts with clean tap water. The resulting transmissivity value provides a baseline measurement against which transmissivity with a mixture can be compared. Once the baseline transmissivity with tap water is obtained, the same test is continued with a mixture. Intermittently, transmissivity readings are taken and compared with preceding values of transmissivity. Also measured is the soil remaining in suspension by using Buchner Funnel and filter. Any soil that is trapped within the specimen is not refurbished. The test is continued as long as necessary to reach a state where there is no further decrease in transmissivity with time. This state also corresponds with a stable concentration of soil within the recipe tank. ANALYSIS AND DISCUSSION Typical output of each test is demonstrated in Figure 3. The normalized value of transmissivity plotted in the figure is calculated by dividing the transmissivity value at a

5 Top Plate Downstream Gradient Tank Sample Placement Area 90 cm Upstream Gradient Tank Recipe Tank 30 cm 30 cm 180 cm 30 cm Note: Hoses and Stirrers Not Shown; Refer to the Photo in Figure 2 Submersible Pump Figure 1 Sketch of the Equipment Used in the Study. Figure 2 Photograph of the Test Equipment Used in the Study.

6 Normalized Transmissivity Transmissivity Suspended Soilds Normalized Suspended Soilds Time (Hours) Figure 3 Normalized Transmissivity and Suspended Solids with Time for Geonet at 0.3 Gradient and 1% Concentration. particular time by the baseline transmissivity value with tape water at the beginning of the test. The suspended solids curve represents the normalized value of suspended solids remaining in suspension at any particular moment. The normalized value of concentration is calculated by dividing the present concentration within the recipe tank by the initial concentration. For example, if the present concentration is 0.005% and the original concentration was 0.01% then the normalized value of concentration is 0.5. The curves in Figure 3 show that the normalized transmissivity and concentration values at the end of the test are approximately 0.75 and 0.70 respectively. The normalized value of transmissivity resulting from these plots is of further interest, as discussed in the next section. So far, twelve tests of the nature indicated in Figure 3 have been performed. Each test represents many days of equipment preparation, sealing and then actually performing the test. The end-of-the test normalized transmissivity for each test is provided in Table 2. The values of normalized transmissivity in Table 2 represent condition at which there was no further significant drop in transmissivity with time. This determination was made visually by plotting normalized transmissivity and normalized suspended solids against time. The test results summarized in Table 2 are plotted in Figure 4.

7 Table 2 Summary of Test Results Test No. Initial Concentration (%) Gradient Normalized Transmissivity Drainage Medium Geonet Geonet Geonet Geonet Geonet Geonet Geonet Geonet Geocomposite Geonet Geonet Geocomposite It can be seen from Table 2 and Figure 4 that the tendency to clog, i.e., for soil to get trapped within the geonet or geocomposite, increases as gradient is increased. In fact, at lower gradients of 0.04 and 0.1 there is almost no clogging of the drainage medium. This is counter-intuitive as one would expect the higher velocity, and the resulting kinetic energy of flow, at higher gradients to keep the particles moving and prevent accumulation within the drainage path. However, flow is very likely turbulent, or at least more so, at 0.3 gradient compared to 0.04 gradient. The probability of soil particles striking each other or with the strands of the drainage medium is higher during turbulent flow. This probably explains the difference in tendency of the particles to get trapped. At the 0.1 and 0.3 gradients, and especially at the latter, the trend is towards increased clogging as concentration is increased. This increase occurs over two orders of magnitude increase in the concentration of solids. There is a decrease in transmissivity of around 30% at 0.3 gradient and 1% concentration. However, 1% concentration is probably too extreme for most applications of synthetic drainage media. If one assumes 0.1% concentration as being representative of field conditions then the maximum decrease is 15%, that is, there is a reduction in transmissivity by a factor of The two tests performed on the geocomposite did not show a significant difference in response from the geonet. It is likely that within the parameters studied, there is no influence of the type of geonet or drainage geocomposite. The main difference between various types of geonets and geocomposites, as it relates to this test program, is the difference in opening size. It is possible that the test will not show the

8 1 0.9 Normalized Transmissivity Gradient = 0.04 Gradient = 0.1 Gradient = 0.3 Note: 1% = 10,000 ppm or 10 grams/liter Initial Concentration of Suspended Soil in Water (%) Figure 4 Relationship Between Initial Concentration of Suspended Soil in Water and Normalized Transmissivity. clogging to be a function of the drainage medium within the range of products currently available in the market. This must be confirmed with further tests. Two tests performed to-date are certainly not enough to reach this conclusion. CONCLUSIONS A test program was developed to study the transport of suspended particles through synthetic drainage media. This paper presented interim results of this test program. Intensity of clogging was seen to increase with increasing gradient and concentration of suspended solids. The data so far indicates that there may be a reduction in transmissivity on the order of 1.1 due to particulates settling within the drainage medium. Under extremely high concentrations, there may be correspondingly higher clogging if the flow is turbulent. Further testing must be performed to develop more confidence in the data and study other important variables such as the type of drainage medium and the size of suspended particles.

9 ACKNOWLEDGEMENTS Jarrett Nelson of TRI/Environmental, TX, performed all testing presented in this paper. The author sincerely appreciates many days of hard work and persistence on his part that made this work possible. REFERENCES Henry J.G. and Heinke, W.H. (1989), Environmental Science and Engineering. Prentice Hall, Englewood Cliffs, NJ, USA ASTM Test Method D4716 (2001), Test Method for Determining the (In-plane) Flow Rate per Unit Width and Hydraulic Transmissivity of a Geosynthetic Using a Constant Head, American Society of Testing and Materials, West Conshohocken, PA GRI Test Method GC8 (2001), Standard Guide for Determination of the Allowable Flow Rate of a Drainage Geocomposite, Geosynthetic Institute, Folsom, PA