Web-based interactive landfill design software On-line interactive landfill design

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1 Web-based interactive landfill design software On-line interactive landfill design calculators assist in proper design and regulatory compliance. Landfill designers rely upon a variety of calculations during the design process in order to demonstrate regulatory compliance and ensure proper design. Creating a simplified yet accurate means of completing these calculations is the goal of Among the calculators available are USEPA HELP Model analyses, landfill surface water drainage, landfill gas venting design, landfill slope stability, and others. Many of these calculators demonstrate the vital role that geosynthetic materials play in landfill design, from both cost and function perspectives. Geosynthetic-based designs are also available for such applications as Geotextile Filter Design, Leakage through Composite Barriers, Geomembrane Anchorage, and others. Selective design topics are presented here with descriptions of associated Web-based interactive design calculators. All of the design calculators are based on state-of-thepractice principles, in an open-box format Figure 1: List of design calculators. (i.e., design equations are given and relevant references provided). This format gives the user and reviewer confidence in the accuracy and appropriateness of each calculator. The authors welcome readers to use the Discussion Board on the Web site for any questions, comments or suggestions for other calculators. Figure 2: Single slope design calculators. Landfill lateral drainage system calculators Lateral drainage systems within modern landfills must perform one or more of the following functions: Limit the head acting on the liner systems At a minimum this is required for demonstration of regulatory compliance, which requires the maximum head on the primary liner in a municipal solid waste (MSW) landfill to be maintained at less than 30 cm (e.g., 40 CFR (2) for MSW landfills and 40 CFR for hazardous waste landfills). Detect leakage through the overlying barrier system Double liner systems collect leakage from the upper liner to monitor the performance of the upper liner and to limit the head acting on the lower liner. Ensure stability of slopes In particular, this refers to drainage of surface infiltration water that percolates down through the soil veneer above a barrier layer in either the liner or final cover systems. Two Web-based calculators were developed to determine the minimum transmissivity sufficient for ensuring unconfined flow condition, e.g., all flow is within the thickness of the geocomposite. The seepage forces in the cover soil will be, therefore, close to zero, and the head acting on an underlying liner is minimal. The first design approach is the Unit Gradient Method. This is recommended for designing landfill cover drainage systems in all but arid regions. This calculator finds the ultimate transmissivity when the impingement rate is considered to be equal to the saturated permeability of an overlying soil. The second design encompasses the McEnroe Equation and Giroud s Method. These methods require an estimation of the liquid impingement rate. The first method, based on the McEnroe Equation, requires the input of an impingement rate (q h ), drainage media permeability (k) and a liner slope (β). This information is used to find the liquid thickness on the liner. From the McEnroe Equation, the required permeability of a drainage media is calculated, and an iteration procedure is used to find the required permeability such that the liquid thickness is equal to the thickness of the liquid collection layer. This permeability multiplied by the thickness of the liquid collection layer results in the required transmissivity. The ultimate geocomposite transmissivity can then be calculated by incorporating the total serviceability factor (product of safety factor and reduction factors). Giroud s Method produces a very close solution when compared to McEnroe s equations. Giroud s method can calculate the required transmissivity directly. Giroud, J.P., Zornberg, J.G., and Zhao, A Hydraulic design of geosynthetic 30

2 By Aigen Zhao, Tim Bauters and Ghada Ellithy nos Giroud, J.P., Zhao, A., and Richardson, G.N Effect of thickness reduction on geosynthetic hydraulic transmissivity. Nos Giroud, J.P., Zhao, A., and Boneparte, R The myth of hydraulic transmissivity equivalency between geosynthetic nos McEnroe, B Maximum saturated depth over landfill liners. Journal of Environmental Engineering. Vol. 19, no. 2. Richardson, G.N., and Zhao, A Composite drains for side slopes in landfill final covers. Geotechnical Fabrics Report. Vol. 16, no Thiel, R.S., and Stewart, M.G Geosynthetic landfill cover design methodology and construction experience in the Pacific Northwest. Proceedings of Geosynthetics Conference Figure 3: Cover slope stability calculators. Landfill cover slope stability calculators A barrier layer within the final cover invites sliding failure of the cover due to a build up of pore water above the barrier, or landfill gas (LFG) pressures beneath the barrier. Both pore water and LFG pressures reduce the effective stress acting on veneer interfaces and thus reduce the sliding stability of the final cover. Special attention needs to be paid to areas with anticipated earthquake activity. Geosynthetic veneer reinforcement can be used to enhance the stability of cover slope. Four calculators were developed to assess landfill cover stability to account for seepage force, LFG pressure, seismic forces, and veneer reinforcement. These calculators are found under the cover soil slope stability design calculators of landfilldesign.com. Koerner, R.M. and Soong, T-Y Analysis and design of veneer cover soils. Proceedings of 6th International Conference on Geosynthetics. Vol Geosynthetic drain equivalency calculator This calculator determines the equivalency factor when a granular liquid collection layer is replaced with a geosynthetic liquid collection layer. It s often assumed that two liquid collection layers having the same hydraulic transmissivity are equivalent. In the United States, this approach is often mandated by regulations, as in the case of leachate collection layers and leakage detection and collection layers used in landfills. However, this is true only in the case of confined flow (i.e., if the liquid collection layer is completely filled with liquid). In reality, liquid collection layers should be designed for unconfined flow, as demonstrated by Giroud et al. (2000). To be equivalent under unconfined flow conditions, the minimum transmissivity of the geocomposite must be greater than the transmissivity of the natural layer multiplied by an equivalence factor, E. For natural drainage systems having maximum flow depths of 30 cm (1 ft.), E can be approximated as follows: E [ 1 + ( 0.88 L ) ( cos β tan β ) ] where L is the slope length and β is the slope inclination angle. Note that E increases with decreasing length of drainage and grade. Giroud, J.P., Zhao, A., and Boneparte, R The myth of hydraulic transmissivity equivalency between geosynthetic nos Figure 4: HELP model calculator. HELP Model calculator The US EPA s HELP (Hydraulic Evaluation of Landfill Performance) model (Schroeder et al. 1994) is a tool for analyzing water balance in landfill lining and capping systems. HELP Model version 3.07 can be downloaded free of charge from html (a link is provided at landfilldesign.com). However, a proper simulation of geocomposite lateral drainage layers in the HELP Model is not well established. A misinterpretation of the model s output results could lead to an unsafe design of the drainage systems in landfills. A discussion on how best to apply the HELP Model appears at help.htm. Ellithy, G., and Zhao, A Using HELP Model for designing geocomposite drainage systems in landfills. Proceedings of Geosynthetics Conference Giroud, J.P., Zornberg, J.G., and Zhao, A Hydraulic design of geosynthetic nos

3 Schroeder, P.R., Dozier, T.S., Zappi, P.A., McEnroe, B.M., Sjostrom, J.W., and Peton, R.L The Hydrologic Evaluation of Landfill Performance (HELP) Model: Engineering Documentation for Version 3. EPA/600/R-94/168b, U.S. Environmental Protection Agency, Risk Reduction Engineering Laboratory, Cincinnati, Ohio. Soong, T.Y., and Koerner, R.M The Design of Drainage Systems Over Geosynthetically Lined Slopes. Geosynthetic Research Institute Report #19. Geotextile filter calculator The function of a geotextile filter is to retain the soil while allowing the liquid to flow as freely as possible. In order to achieve this objective, a geotextile filter needs to meet the following criteria: (1) retention criterion the filter opening size must be sufficiently small to retain soil particles; (2) permeability criterion the filter must be sufficiently permeable to ensure that the liquid flow is as free as possible; and (3) porosity criterion the filter should maintain a high porosity so the probability for clogging is small. The latest filter criteria presented by Giroud (2000) are used in the calculation. It is recommended that AASHTO M minimum hydraulic requirements as shown in Table 1 also be considered in the selection of a geotextile filter. Retention criterion Giroud (2000) uses a linearization of the particle distribution curve that, when plotted with the classical log scale horizontal axis, is as close as possible to the actual particle distribution curve. A least-variance approach was used to determine the best linearization of the central portion. It Table 1: Geotextile criteria for subsurface drainage (after AASHTO M288-96). Filter Criteria Maximum Permittivity, ASTM D-4491 Maximum AOS, ASTM D-4751 should be noted that there is greater uncertainty on the two extremities (d 0 and d 100 ) of the actual particle size distribution. This justifies the use of the linear particle size distribution curve. The result obtained using Giroud s retention criterion is not affected by the truncation of the particle size distribution curve. Permeability criteria k f k s * I s against excessive pore water pressure. k f k s against excessive reduction of flow rate. In these expressions, k f is the hydraulic conductivity of the geotextile filter, k s is the hydraulic conductivity of the soil, and I s is the hydraulic gradient in the soil. Porosity criteria N GTX > 0.3 where N GTX is the porosity of geotextile filter. Percent Soil Passing No. 200 (0.075 mm) Sieve < > sec 0.2 sec 0.1 sec mm 0.25 mm 0.22 mm Giroud, J.P Filter criteria. Jubilee Volume 75th Anniversary of K. Terzaghi's Erdbaumechanik (Soil Mechanics). Technical University, Vienna, Austria. Vol 5/2000. Brandl, H., editor. Giroud, J.P Quantification of geosynthetics behavior. Special Lecture, Proceedings of the Fifth International Conference on Geotextiles, Geomembranes and Related Products, Singapore. Vol Giroud, J.P Review of geotextile filter criteria. Proceedings of the First Indian Geotextiles Conference, Bombay, India.1-6. Giroud, J.P Filter criteria for geotextiles. Proceedings of the Second International Conference of Geotextiles. Vol Richardson, G.N., Giroud, J.P., and Zhao, A Design Manual of Lateral Drainage Systems in Landfills. Landfill gas pressure relief layer calculator LFG pressure underneath a lined cover system can significantly reduce the effective normal stress on the liner, which can affect cover-soil stability. Landfill-cover slope failures, caused by an inadequate LFG venting layer, have been recorded over the past 32 Table 2: Retention criterion for the hyperstable case (C uc =3) expressed using d 85S. Soil Density Loose I D 35% Rc 86% O F (C' u ) 0.3 d' 85S O F (9/C' u 1.7 ) d' 85S Medium Dense Density Index (Relative Density) I D Relative Compaction(Rc) Linear coefficient of uniformity of the soil, C' u 1 C' u 3 C' u 3 35%<I D 65% 86%<Rc 92% O F 1.5(C' u ) 0.3 d' 85S O F (13.5/C' u 1.7 ) d' 85S Dense I D >65% Rc>92% O F 2(C' u ) 0.3 d' 85S O F (18/C' u 1.7 ) d' 85S

4 decade. According to the intrinsic permeability theory of gas transmission rates, the rate of LFG transmissivity is 10 times lower than the hydraulic transmissivity in any porous media. In the past, however, this relationship was believed to be inverse, i.e., the air transmissivity was believed to be 100 times greater than the hydraulic transmissivity. The resulting miscalculations significantly under-design the required transmissivity of the LFG venting layer. This calculator determines the required transmissivity for the LFG relief layer, the conversion from LFG transmissivity to hydraulic transmissivity (since LFG transmissivity is not routinely conducted), and also checks the Reynolds number to verify laminar flow. Web-based interactive landfill design software Figure 5: LFG relief layer calculator. Required LFG relief layer transmissivity The required gas transmissivity for a LFG relief layer is calculated based on LFG generation rate, maximum LFG pressure, and spacing between strip drains. Hydraulic transmissivity vs. LFG transmissivity The gas transmissivity can be converted to a hydraulic transmissivity for the same drainage medium. The intrinsic permeability variables for common liquids and gases are shown by Thiel (2000). Check for Reynolds Number The Reynolds number can be used to check if a flow is laminar or turbulent. Since laminar flow is the basis for the validity of Darcy s law, a calculator for Reynolds Number is presented. For sands, the flow is laminar if R e < 10; for pipes, the flow is laminar if R e < The critical R e for one geonet was reported to be about 500 (Richardson and Zhao, 2000). Giroud, J.P., and Bonaparte, R Waterproofing and drainage: Geomembrane and synthetic drainage layers. Plastics and Rubber Waterproofing in Civil Engineering, R.I.L.E.M., Symposium No. II, Liege, Belgium. Richardson, G.N., and Zhao, A Gas transmission in geocomposite systems. Geotechnical Fabrics Report. Vol. 18, no Thiel, R.S Design methodology for a gas pressure relief layer below a geomembrane landfill cover to improve slope stability. Geosynthetics International. Vol. 5, no Geomembrane puncture protection design A geomembrane is sometimes placed in direct contact with large-sized stones, on poorly prepared soil subgrade with stones protruding from the surface, or where stone materials are placed above the geomembrane as a drainage layer. Under these circumstances, a nonwoven Equation for determining P allow [ ] [ ] [ P allow = M 1 H 2 MF S * MF PD * MF A needlepunched geotextile is needed to provide puncture protection for the geomembrane. There are two design issues involved in selecting the geotextile: (1) required mass-per-unit area of a nonwoven geotextile for a given safety factor against geomembrane puncture, and (2) safety factor for a given nonwoven geotextile. Two design calculators were developed to address these issues. The method presented herein (Koerner, 1998) focuses on the protection of 1.5 mmthick HDPE geomembranes. The method uses the design-by-function approach: FS = P allow P actual where FS is the factor of safety against geomembrane puncture, P actual is the actual pressure due to the landfill contents or surface impoundment, and P allow is the allowable pressure using different types of geotextiles and site-specific conditions. P allow is determined by the equation for determining P allow (below), where M is the geotextile mass per unit area, H is the height of the protrusion above the subgrade. Additionally, MF S is the modification factor for protrusion shape, and MF A is the modification factor for arching in solids. 1 RF CR * RF CBD ] Also, RF CR is the reduction factor for long-term creep, and RF CBD is the reduction factor for long-term chemical/ biological degradation. Koerner, R.M., Wilson-Fahmy, R.F., and Narejo, D Puncture protection of geomembranes part III: Examples. Geosynthetics International. Vol. 3, no Koerner, R.M Designing with Geosynthetics. Prentice Hall Publishing Co., Englewood Cliffs, N.J. Narejo, D., Koerner, R.M., and Wilson-Fahmy, R.F Puncture protection of geomembranes part II: Experimental. Geosynthetics International. Vol. 3, no Wilson-Fahmy, R.F., Narejo, D., and Koerner, R.M Puncture protection of geomembranes part I: Theory. Geosynthetics International. Vol. 3, no

5 Web-based interactive landfill design software 34 Leakage rate through geomembrane liner This calculator computes the rate of leakage through defects in a geomembrane overlaying a very permeable medium. A geonet sandwiched between two geomembranes in a double liner system is one application of this calculator. The rate of leakage through a geomembrane liner due to geomembrane permeability is negligible compared to the rate of leakage through defects in the geomembrane. Hence, only leakage through defects will be considered. As proposed by Giroud (1984), Bernoulli s equation for free flow through an orifice can be used to evaluate the rate of leakage through a defect in a geomembrane underlain by a very permeable medium. This free flow condition occurs when the underlain porous medium has an average opening size that is greater than the diameter of the geomembrane defect. This free flow condition is valid if the hydraulic conductivity of the underlain media (e.g., gravel, geonet) in contact with the geomembrane is greater than 10-1 to 1 m/s if a = 0.1 cm 2 (10-5 m 2 ) and greater than 1 to 10 m/s if a = 1 cm 2 (10-4 m 2 ). A typical geonet/geocomposite has a hydraulic conductivity of 10-1 to 1 m/s, therefore, this leakage rate calculation is valid for geonet only when the defect size in the geomembrane is less than or equal to 0.1 cm 2. Bernoulli s Equation: Q A = 0.6. a. 2gh where Q = leakage rate (m 3 ), A = considered geomembrane surface area (m 2 ), and n = number of defects in the geomembrane area, a = area of a single defect (m 2 ), g = acceleration of gravity (m/s 2 ), and h = hydraulic head on top of the geomembrane (m). Note that Bernoulli s equation often overestimates the leakage rate, especially in landfills, where even absurd leakage rates are possible, e.g., the calculated rate through a defect in a geomembrane may be greater than the impingement rate above the geomembrane. Giroud et al. (1997) has extended this equation to include impeded flow. Design equations in this case are more complex and require iteration for calculating the leakage rate. Design charts are available in the abovereferenced paper. Figure 6: Calculator for leakage rate through composite liner. Giroud, J.P Impermeability: The myth and a rational approach. Proceedings of the International Conference on Geomembranes, Denver, Colo. Vol Giroud, J.P., and Bonaparte, R Leakage through liners constructed with geomembranes, part I: Geomembrane liners. Geotextiles and Geomembranes. Vol. 8, no Giroud, J.P., Tweneboah, K.B., and Soderman, K.L Evaluation of landfill liners. Fifth International Conference on Geotextiles, Geomembranes and Related Products, Singapore. Giroud, J.P., Khire, M.V., and Soderman, K.L Liquid migration through defects in a geomembrane overlain and underlain by permeable media. Geosynthetics International. Vol. 4, nos Leakage rate through a composite liner This calculator computes the rate of leakage through defects in a composite liner, i.e., geomembrane/compacted clay liner (CCL) or geomembrane/geosynthetic clay liner (GCL). The thickness of a CCL is between 0.3 to 1.5 m whereas the thickness of a hydrated GCL depends on the compressive stress applied during hydration. Typical values are between 5 and 10 mm; or on the order of 100 times less than the thickness of a CCL. Field evaluation, sponsored by USEPA, of leakage rate for double-lined landfills indicates that GM/GCL composite liners outperform GM/CCL liners (Othman et al. 1998). If there is a defect in the geomembrane, the liquid first passes through the defect, then it flows laterally some distance between the geomembrane and the low-permeability soil, and finally it infiltrates the low permeability soil. Flow between geomembrane and low-permeability soil is called interface flow, and is highly dependent upon the quality of contact between the two components (Bonaparte et al. 1989). Studies by Giroud and Bonaparte (1989) have shown that geomembrane liners installed with strict construction quality assurance could have one to two defects per acre (4000 m 2 ), with a typical defect diameter of 2 mm (i.e., a defect area of 3.14 * 10-6 m 2 ). A typical liner performance evaluation shows one defect per acre (4000 m 2 ), with a defect area of 0.1 cm 2 (equivalent to defect diameter of 3.5 mm). For a conservative design, a defect area of 1 cm 2 (equivalent defect diameter of 11 mm) can be considered (Giroud et al. 1994). Equation for leakage rate through a circular defect with diameter of d. Q = A n C qo [ (h/t s ) 0.95 ] d 0.2. h 0.9. k 0.74 s

6 The leakage rate through a circular defect with diameter of d is given by the equation for leakage rate through a circular defect with diameter d (bottom of previous page). In this equation, Q = Leakage rate through the considered geomembrane defect (m 3 /s), Cqo = Contact quality factor, t s = thickness of the low-permeability soil component of the composite liner (m), d = diameter of circular defect (m). Other geomembrane defect shapes are also considered in the calculator. Bonaparte, R., Giroud, J.P., and Gross, B.A Rates of leakage through land fill liners. Proceedings of Geosynthetics Conference Vol Giroud, J.P., and Bonaparte, R Leakage through liners constructed with geomembranes, part I. Geomembrane Liners, Geotextiles and Geomembranes. Vol. 8, no Giroud, J.P., Tweneboah, K.B., and Soderman, K.L Evaluation of landfill liners. Fifth International Conference on Geotextiles, Geomembranes and Related Products, Singapore. Giroud, J.P Equations for calculating the rate of liquid migration through composite liners due to geomembrane defects. Geosynthetics International. Vol. 4, nos Othman, M.A., Bonaparte, R., Gross, B.A., and Warren, D Evaluation of liquids management data for double-lined landfills. Draft Document Prepared for the U.S. Environmental Protection Agency, National Risk Management Laboratory, Cincinnati, Ohio. Acknowledgments The authors would like to thank Dr. Greg Richardson for his review and Karla Parker for editing this paper. Aigen Zhao, Ph.D., P.E., is executive vicepresident of Advanced Geotech Systems; Tim Bauters, Ph.D., is a design engineer for Advanced Geotech Systems; and Ghada Ellithy, Ph.D., P.E., is a project engineer for Advanced Geotech Systems. For a list of other, non-web-based geotechnical design programs, please see Software for geosynthetic engineering in the April 2001 issue of GFR (vol. 19, no.3), pages Ed. FREE information circle 3124 Leaks Can t Hide From LLSI Geomembrane Leak Location Services: Landfills with Soil Covers Impoundments and Tanks with Water Mine Leach Pads Bare Liners Leak Location Services, Inc. Tel: (210) Fax: (210) University Oak, San Antonio, TX llsi@texas.net FREE information circle 3047 Call us for a FREE Brochure! 35