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Use of Strips of High-Transmissivity Geocomposite in Leachate Collection Systems - Design Calculator
This calculator evaluates the equivalent system of parallel strips of high-transmissivity geocomposite associated with a layer of sand, to a leachate collection system entirely constructed with a layer of a regular geocomposite. The use of strips of high-transmissivity geocomposite in leachate collection systems has the advantages of reducing material and installation costs.
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(a) plan view with arrows showing the direction of leachate flow.
(b) cross-section perpendicular to flow direction, showing maximum leachate thickness.
Step One: The equivalency approach is valid only if a specified system is adequately designed. To verify the design of a specified system, the impingement rate, qh, must be known. qh can be determined by methods such as HELP model. The drainage factor of safety of a leachate collection system constructed with a continuous layer of regular geocomposite is calculated by:
If the design is sufficient, (FS >= 2.0) then the calculator will confirm that and continue on the equivalency demonstration. Otherwises, the specified system must be modified if continuous system is desired. The Lateral Drainage Design Calculator can be used to facilitate the design process. However, this calculator will still determine the spacing based on the impingment rate assuming FS = 2.0.
Step Two: The design of a system of parallel strips of high-transmissivity geocomposite must meet two criteria: (i) the geocomposite strips should have sufficient hydraulic transmissivity to convey all of the leachate collected over the entire surface area of the slope without pressure build-up in the geocomposite, and (ii) the thickness of leachate in the sand layer should be smaller than the thickness of the sand layer or smaller than a prescribed thickness (typically 0.3 m), whichever is smaller. When the transmissivity of the strip geocomposite is known, the design below seeks the spacing of the parallel strips that meet the following criteria:
Criterion 1: Maximum allowed spacing of parallel strips that limit the head with the geocomposite.
| Case A (FS >= 2.0) | Case B (FS < 2.0) |
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Criterion 2: Maximum allowed spacing of parallel strips that limit the head in the sand layer smaller than the prescribed thickness.
| Case A (FS >= 2.0) | Case B (FS < 2.0) |
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where:
| Slope Input | ||
| L | Slope length (m) | |
| β | Slope inclination angle (degree) | |
| Impingement Rate | qh | (m/s) |
| High-Transmissivity Strip Geocomposite input | ||
| B | Strip drain width (m) | |
| θstrip | Transmissivity of the strip geocomposite (m2/s) | |
| RFin-strip | Reduction Factor for Intrusion | |
| RFcr-strip | Reduction Factor for Creep | |
| RFcc-strip | Reduction Factor for Chemical Clogging | |
| RFbc-strip | Reduction Factor for Biological Clogging | |
| Continuous Geocomposite Input | ||
| θcont | Transmissivity of Continuous geocomposite (m2/s) | |
| RFin-cont | Reduction Factor for Intrusion | |
| RFcr-cont | Reduction Factor for Creep | |
| RFcc-cont | Reduction Factor for Chemical Clogging | |
| RFbc-cont | Reduction Factor for Biological Clogging | |
| Sand Input | ||
| k | Permeability of sand (m/s) | |
| RFcc-sand | Reduction Factor for Chemical Clogging | |
| RFbc-sand | Reduction Factor for Biological Clogging | |
| Tmax | Prescribed maximum liquid thickness (m) | |
[1] Intrusion reduction factor from 100 hour to design life. Giroud et. al (2000)
[2] Creep reduction factor from 100 hour to design life (for instance, 30 years). RFCR is determined from 10,000 hour compressive creep test, extrapolated to design life, GRI-GC8 (2001). RFCR is product and normal load specific.
[3] GRI-GC8
"GRI-GC8, Determination of the Allowable Flow Rate of a Drainage Geocomposite". Geosynthetics Research Institute, 2001.
"Designing with Geosynthetics". R.M. Koerner, Prentice Hall Publishing Co., Englewood Cliffs, NJ, 1998.
"Hydraulic Design of Geosynthetic and Granular Liquid Collection Layers". J. P. Giroud, J. G. Zornberg and A. Zhao, Geosynthetics International, Vol. 7, Nos 4-5.
"Lateral Drainage Design update - part 2". G. N. Richardson, J.P. Giroud and A. Zhao, Geotechnical Fabrics Report, March, 2002
"Maximum Saturated depth over Landfill Liners". B. McEnroe, Journal of Environmental Engineering (Vol. 19, No. 2, March/April, 1993).
"Use of Strips of High-Transmissivity Geocomposite in Leachate CollectionsSystems", Technical Note, Giroud, J.P. (2002)
Copyright 2001 Advanced Geotech Systems. All rights reserved.