This issue of the "Technical Bulletin" summarizes an EPA Report entitled "Quantification of Leak Rates Through Holes in Landfill liners", published in August, 1992. The report is written by K. W Brown and J. C. Thomas of the Texas Agricultural Experiment Station-College Station Texas; and R. I. Lytton, P. Jayawickrama and S. C Bahrt of the Texas Engineering Experiment Station - College Station, Texas. A complete copy of the report can be obtained by contacting the Hazardous Waste Engineering Research Laboratory, Office of Research and Development, US Environmental Protection Agency, Cincinnati Ohio 45268. Reference report #600/2-87/1062 when inquiring.
A study was undertaken to evaluate the rate at which liquids leak though flaws in the Flexible Membrane Liner (FML) component of composite FML soil liners. The variables studied were: flaw size and shape, FML type and thickness, the influence of a geotextile between the compacted soil and the FML, the conductivity of the soil subbase, the liquid head and the liquid characteristics. Testing was done in 60 cm diameter permeameters. Soils were compacted in the permeameter and overlain with the FMLs to be tested with either round holes, slits or seam flaws. A 15 cm layer of gravel was placed over the FML to provide ballast, and a head chamber was used to apply as much as 100 cm of head on the FML. Tests were conducted with a gravel subbase to determine the influence of the flaw alone on the flow rate followed by soil subbases having nominal conductivities of I x I0-4 and I x I0.6 cms-l. A calculational procedure was developed and was modified to allow calculation of leak rates underfield conditions.
Kays (1977) categorized FML failures into those resulting from physical, chemical or biological failures. Physical failure mechanisms include punctures that may result from traffic pressure even when the liners are covered (Gunkel, 1981). These may include tears, creep, freeze-thaw cracking, wet-dry cracking, differential settling, thermal stress, differential hydrostatic pressure, abrasion; and seam failures (Haxo, 1982). Chemical failure is cased by deterioration that results from ultraviolet light, ozone, hydrolysis ionic species attack, extraction, ion species incompatibility and solvent attack which may dissolve either the plastic or the plasticizer. Biological degradation includes microbial attack, attack by burrowing animals or damage resulting from animals trying to escape from inside the facility.
At the present time, there is a dearth of knowledge on leakage rates through flaws in FMLs. It is, therefore, important that these leakage rates be quantified and the principles governing leakage rates be understood so that predictions of leakage rates can be made. Leaks in new installations may result from improper seaming, punctures from overlying or underlying sharp objects, such as stones, dropped or carelessly handled objects such as buckets or hand tools and traffic such as heavy construction equipment. The leakage rate
may be affected by the following parameters:
- Type of FML.
- FML thickness.
- Size and shape of flaw.
- Characteristics of the subbase material.
- Presence or absence of geotextile between subbase and FML.
- Head of liquid above the flaw.
- Characteristics of the liquid to be retained.
The thrust of this research was to evaluate the effect of each of the above factors on leakage rate through flaws in FMLs. Physical measurements of flow rates were made and used to develop a calculational procedure for predicting leakage from any given set of input parameters.
Geosynthetic Materials Used:
- PVC: 20 mil and 30 mil
- HDPE: 30 mil, 80 mil, I00 mil
- EDPM: 30 mil
- CSPE (Hypalon): 36 mil, 45 mil
- Geotextile materials: Hoescht 1127 and Foss 90.
Leakage Test Parameters:
Flaws consisted of:
- .08 cm round holes
- .16 cm round holes
- .64 cm round holes
- .27 cm round holes
- 5 cm long vertical slits
- 15 cm long vertical slits
Subbase Materials Used:
- Gravel with nominal conductivity of 10 – 1 cms –1
- Sandy loam with conductivity of 1 x 10 –4
- Clay/Soil mixture with conductivity of 1 x 10 -6
Liquids Used in Test:
- Tap water
- Synthetic landfill leachate
Pressure Application in Test Chambers:
- .01 Mpa approximating a liquid depth of 10 meters
- Overburden pressure applied to FML - 156 Kpa.
The PVC and CSPE liners had significantly lower discharge rates than the EDPM at both heads and was significantly lower than the HDPE at the 50 cm head. Thus, the PVC and CSPE materials seem to offer more resistance to flow through a hole. And, thus have greater head loss lower discharge rates when the flow beneath the liners is obstructed.
In regards to flow rates with and without geotextile underlayments, four of the six sets of data did not differ significantly. The two data sets which did differ were of different hole sizes suggesting the absence of pattern concerning the impact of geotextiles on flow rates when gravel underlies a defective FML.
At a hydraulic head of 100 cm of water, there was no significant change in flow rate due to liner thickness. At a 50 cm head, there were no significant differences in the PVC and CSPE liners.
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