CE 527 Solid Waste Management
Clay Liners and Membrane Liners
Dr. S. K. Ong
Clay Liners
 To achieve a hydraulic conductivity of 1 x 10-7 cm/s
Rule of Thumb
- at least _____ fines (silt and clay, < 0.002 mm), but > _____ would be more appropriate
- % gravel should be less than ____, although some studies have shown that up to 50% have not
significant impact on the hydraulic conductivity (very much dependent on the composition of the
remaining fraction)
- Plasticity index (PI) should be greater than 10, ideal PI between ______, between 30 to 40 – soil
will be sticky and difficult to work
[The definition of clay varies depending on the standard used, see Figure 15 – 1].
ASTM - sand sized particles passing through No. 4 sieve but not able to pass a No. 200 sieve
(0.075 mm to 4.74 mm)
- particles passing through a No. 200 sieve are treated as fines (silt and clay)
- clay is distinguished from silt entirely based on the plasticity criteria
- plasticity of a soil refers to the capability of a material to behave as a plastic, moldable
material
- four states: liquid, plastic, semisolid and solid
- plasticity characteristics of a soil are quantified by three parameters
LL – Liquid limit
PL – Plastic limit
PI – Plastic index
LL – the water content corresponding to the arbitrary limit between the liquid and plastic
states of consistency of a soil
PL – water content corresponding to the arbitrary limit between the plastic and semisolid
states of consistency of a soil
- dividing line between liquid and plastic is the LL and plastic and semisolid is PL
LL of a soil – water content at which the groove is closed ½ in. when the soil sample is
jarred in the standard manner by exactly 25 drops (or blows) from a height of 1 cm in a
standardized liquid limit device
PL of a soil - water content at which the soil begins to crumble when rolled into small
threads (1/8 in. in diameter without the threads breaking into pieces).
Materials used in Natural Soil Liners
- many different types of clay exist
- generally three main groups are used for clay liners
nonswelling clay such as kaolinite, illite (see Table)
- generally good for compacted clay liners of depth of 0.5 m or greater
swelling clays such as montmorillonite (smectite clay), bentonite, a naturally occurring material
with montmorillonite as a common mineral present
(note the expansion index of sodium montmorillonite about 2.5 as compared to that of
calcium montmorillonite)
preferably use calcium rather than sodium smectite group
(minerals is made of:
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kaolinite – octahedral and tetrahedral layer
smectite – tetra – octa – tetra – Exchange cation – tetra – octa – tetra )
Table
Clay Group
Cation Exchange
Capacity (meq/100 g)
Spec. Surface
Area (m2/g)
Expansion
Index
Kaolinite
Sodium
Calcium
Illite
Sodium
Calcium
Montmorillonite
(smectite)
Sodium
Calcium
Other factors
Molding Water Content
 The degree of saturation of soil liner material at the time of compaction is perhaps the single most
important variable that controls the engineering properties of the compacted material.
 The change in hydraulic conductivity with molding content is shown below:
Where d is the dry unit weight = /(1+ )
 = total unit weight or wet density
 = water content of each compacted specimen
 Soil with less than the optimum water content has high hydraulic conductivity and soils with greater than
optimum water content tend to have low hydraulic conductivity and low strength.
Compaction Energy
 increase compaction energy, hydraulic conductivity 
 compaction is dependent on the weight of the roller, number of passes, thickness of the soil lift to be
compacted
Desiccation
 desiccation of soil liners occurs whenever the soil liner dries during or after construction
 cause shrinkage and cracking , resulting in preferential pathways
 overburden stress on liner components may assist in preventing a change in the hydraulic conductivity
(for bottom liners and not true for final cover).
Waste- Soil Liner Interactions
Effects of Inorganic Compounds such as Cations
 Leachate has a variety of high valency cations present that can be exchanged with the cations present in
the clay
 Clay are collodial particles with a negative surface charge. Positively charged cations and water are
attracted to the surface of the clay forming a zone of water and ions surrounding the clay particles
 Due to the strong electrostatic forces, a layer of cations and water will be immobile relative to the clay
particle. This layer of immobile water and cations is referred to as the diffuse double layer (DDL).
 Fluid flowing through the clay layer will be affected by the immobile layer. On a microscale level, the
hydraulic conductivity of the soil will be controlled by the thickness of the DDL. When the DDL shrinks.
More area will be opened for flow - resulting in higher hydraulic conductivity. The reverse is true.
 The thickness of the DDl can be estimated using the Gouy-Chapman equation where
where
 - dielectric constant (dimensionless)
n - electrolyte concentration (mol/cm3)
z - cation valence
Assume a compacted soil liner of sodium bentonite is used. Polyvalent cations, e.g., Ca 2+, Fe2+, Fe3+ in
the leachate if there is a leak will displace the sodium, resulting in a decrease in the thickness of the DDL
and there fore increasing the hydraulic conductivity. Note that the presence of the polyvalent cations will
also cause flocculation and aggregation of the clay.
 Note that we do not have long-term data on the effects of leachate on the clay liners. The limited
evidence available shows that the long-term effect of exposure to leachate may not be as severe as
originally thought. One reason is that the clay liner is under several thousand tons of waste and any change
will be corrected for by the weight of the trash.
 Precipitation and the development of active biomass near the upper surface of the soil liner produce a
clogging action that reduces the effective hydraulic conductivity of the barrier.
 A study on a clay liner that was exposed to leachate for 15 years showed that the upper 15 cm contained
high Fe, Zn, Cu and Pb. In addition, the first 20 cm was black and oily, suggesting microbial activity.
Beyond 15 cm, the concentrations of polyvalent cations were at background levels. Diffusion transport of
Cl and Na reached 1.5 m into the clay in the 15 years.
 Please note that if you have cracks in the soil liner, the leachate will move very rapidly out of the clay
liner system.
Neutral Organic Compounds
 The hydraulic conductivity of clay can change in the presence of organic compounds
 Water has a dielectric constant, , of approximately 80.
 Based on the Guoy-Chapman equation, the DDL will shrink in the presence of organic solvents,
effectively opening up flow paths (see Figure 2-26).
 Pure organic liquids of several types can substantially increase the permeability. Note that dilute aqueous
solutions of organic liquids do not appear to affect the permeability of clay liners. A rule of thumb is that
concentrations up to 0.1% by weight can be tolerated.
Flexible Membrane Liners (FML) or Geosynthetics
 synthetic materials, mostly plastics, used in place of or to enhance the function of natural soil materials.
Function of geomembranes are generally used for isolation of liquids or as a vapor barrier. EPA's de
minimis leakage requirement is 1 gallon/acre/day for geomembranes.
 Other types of similar materials include
geotextile - for reinforcement. Separation, filtration and drainage
geonets - for drainage
geogrids - for slope stability
geomats - for prevention of erosion of exposed slopes such as landfill caps
 Major types of polymers, two major classes thermoplastic - can be repeatedly reworked to the desired shape by heating and cooling
thermoset. - can be processed once only
 Two most commonly used polymers - polyvinyl chloride and polyethylene
Major types of Geomembranes in Current Use
Thermoplastic Polymers
Thermoset Polymers
Combinations
Polyvinyl chloride (PVC)
Butyl or isopreneIsobutylene (IIR)
Epichlorohydin rubber
PVC - nitrile rubber
Ethylene propylene
Diene monomer (EPDM)
Polychloroprene (neoprene)
PVC-ethyl vinyl acetate
Polyethylene (VLDPE,
LLDPE, MDPE, HDPE)
Chlorinated polyethylene
PE-EPDM
chlorosulfonated
Polyethylene (CSPE or hypalon)
 Composition of geomembranes consists of four major components
________
- the polymer
__________
- e.g., epoxides, phosphates, polyesters
-Impact flexibility to the compound, although some plasticizers are subjected to
microbial attack
- improve handling by modifying physical and mechanical properties
_________
- e.g., mineral particles, metallic oxides, fibers and reclaimed polymers
- increase the stiffness of the geomembrane without altering the permeability
_________
- general stabilization purpose, prevent ultraviolet light degradation
_________
- e.g., antioxidant - to reduce aging from ozone
Typical Formulation of Geomembranes
Type
Resin
Plasticizer
Carbon Black
and/or Filler
Additives
PVC
CSPE
EIA
VLDPE
HDPE
Leakage Rates
 There is no such thing as an impermeable geomembrane
 They have very low permeability but still allow some leakage
 Moisture moves through by diffusion. Vapor transmission test conducted using ASTM E96. Sample is
placed on top of a small aluminum cup containing a small amount of water. Cup placed in a controlled
humidity and temperature chamber of 20% RH. Humidity in cup is 100%. Moisture diffuses through the
membrane and rate of diffusion estimated based on weight loss.
Vapor Transmission
Material
Thickness
(mil)
PVC
30
Water Vapor
Transmission
(g/m2day)*
1.9
Thickness
(mil)
10
20
Methane
Transmission
(mL/m2/day.atm)
4.4
3.3
CPE
CSPE
HDPE
40
0.4
40
0.4
34
1.6
30
0.02
98
0.06
34
1.4
LLDPE
18
2.3
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* in terms of gal/acre day multiply by 1.07
Pinholes
 Leakage through geomembrane can also occur due to pinholes and large holes
 leakage through geomembrane is drastically reduced when placed on a low permeability soil such as clay
 such composite liner is only effective if the geomembrane and clay are in close contact over the entire
surface
Typical Rates of Leakage through Primary Liner - (liters/heactare/day)
Hydraulic Head - 3 cm
Hydraulic Head - 30 cm
Geomembrane/clay
Geomembrane/silt
Geomembrane/sand
Geomembrane/gravel
Note - 5 holes/ha, each hole 2 mm diameter
Attachment of Geomembranes
Geomembranes are seamed using
Thermal seaming - heat only at > 260o C,
- include hot air bonding or hot wedge or knife bonding
- good for thermoplastics
Chemical seaming
- use solvents to dissolve the FML or by vulcanizing the surface
- long term integrity suspect when it comes into contact with the leachate
Degradation Concerns
Ultraviolet Degradation
- short wavelengths of sunlight (315 to 380 nm) enter into the polymer system and cause chain
scission and bond breaking
- carbon black is used as a blocking agent to retard UV degradation
- manufactured with 2 - 3 % carbon
- FML should be buried within 6 to 8 weeks of laying out
Chemical Degradation
- various chemicals can be attack the membrane
- to evaluate capability use EPA 9090 method to assess chemical resistance (immerse FML in
leachate for at least 120 days at 23 and 55o C. Perform physical and analytical tests on unexposed
and exposed FML to establish baseline data on changes on samples exposed to the leachate. Tests
include puncture, tensile strength, etc.)
See attached compatibility chart.
Swelling Degradation
- all polymers swell when exposed to liquid including water
- in general HDPE swells the least, PVC the most
- secondary effects that cause degradation
Extraction Degradation
- components of geomembrane formulation are extracted causing the properties of the remaining
materials to be compromised, e.g., breaking the bond of plasticizers, resulting in the extraction of
the plasticiizer with time.
Delamination Degradation
- if the geomembrane is fabricated in layers, there is a possibility that the liquid entering between
layers may cause delamination and failure
Thermal Expansion and Contraction
- FML may be stretched tight in locations which may lead to failure of seams.
Oxidation Degradation
- oxidation of polymers caused by gases or liquids interfacing with the geomembranes, e.g., free
radicals such as oxygen atom may diffuse and react with polymers
- antioxidant added to scavenge oxygen and free radicals
Biological Degradation
- unlikely for resin portion to degrade
- microorganisms can interact with plasticizers or fillers
Comparison of 3 FML for Landfill Applications
Property
CSPE
Heat Resistance
Excellent
Microbial resistance
Good
Chemical resistance
Very good
(poor to aromatic
hydrocarbons)
Ultraviolet Resistance
Excellent
Puncture Resistance
Fair to good
Field seams
Best made on
Warm day
Ease of placement
Good
PVC
Poor above 140o C
-very good
HDPE
Excellent
Good
excellent
Poor
Good
Excellent
If thick, good
If thin, fair
Fusion or
extrusion welding used
in field, no glues needed
Fair
Good, but tendency
To lose plasticizer and
Shrink - must be
Installed loosely
Cost
High
Low
Moderate
Tensile Strength
No information
High
Excellent
Ozone resistance
good
Poor
No information
Cold Weather Difficulties Good resistance to
Stiff and brittle
-Cracking
in cold weather
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Cost (1994 prices)
20 mil PVC = $_____/m2
100 mil HDPE = $_____/m2
or ______ cent per mil per square foot.