Presentation Topics DESIGNING WITH GEOSYNTHETIC CLAY LINERS (GCLs)

DESIGNING WITH
GEOSYNTHETIC CLAY LINERS (GCLs)
Presentation Topics
• About Bentonite
• GCL Products and Properties
• Design Considerations
– Hydraulic Performance
– Chemical Compatibility
– GCL Shear Strength and
Slope Stability
Chris Athanassopoulos, P.E.
CETCO Construction Engineering Group
catha@cetco.com
Why is bentonite such a
good lining material?
What is Bentonite?
• Bentonite is an ore whose main
ingredient is the clay mineral
montmorillonite
• It was formed hundreds of millions of
years ago through the aqueous
deposition of volcanic ash
•
•
•
•
Myth #1:
Myth: “Powdered bentonite is
superior to granular bentonite”
Truth: There is no difference in
hydraulic performance between
powdered and granular. The
individual bentonite platelets
(approximately 1 nm thick and 0.2
to 2 microns in length) are the
same in both materials.
It swells when hydrated!
Bentonite swells up to
15 times its original
volume if unconfined.
Bentonite expands and
conforms to irregular
surfaces, penetrations,
and infiltrates cracks
and voids.
Swell pressure makes it
an active sealing agent.
DRY
WETTED
Powdered Bentonite
GCL Properties
• A manufactured hydraulic
barrier consisting of clay
bonded to a layer or
layers of geosynthetics
(ASTM D4439)
• Very low permeability
(< 5 x 10-9 cm/sec)
• Self-healing if punctured
• Self-seaming at overlaps
• Replaces compacted
clay liners (CCLs)
Bentomat DN
(Bentoliner BL200 and BL300)
– World’s most popular
GCL, established 1990.
– Ideal for most sloping
applications
– Certified permeability of
5 x 10-9 cm/sec.
– Used in landfill liners,
closures, tank farms,
wetlands, heap leach
pads
Key Design Elements
• Hydraulic Performance—will the GCL
provide the required level of
containment?
• Chemical Compatibility—what about
compatibility with contaminated liquids?
• Shear Strength and Slope Stability—will
the liner system remain stable on
sloping surfaces, berms, ramps, and
roadways?
– Reinforced GCL with
two high interface
friction nonwoven
geotextiles
– Needlepunched to
same specifications
as Bentomat ST
– High tensile strength
and elongation
Hydraulic Performance
• Governed by Darcy’s Law:
Q
Bentomat ST
(Bentoliner BL080 and BL100)
k uiu A
where:
Q = flow through liner
k = permeability
i = hydraulic gradient = (head + thickness) / thickness
A = lined area
• Note: Flux = q = Q / A = k i
The “Permeability Test”
• It’s really a flux test!
• k is calculated by
solving Darcy’s Law
• ASTM D 5084 is for
determining k value
of soil
• ASTM D 5887 is for
determining flux
(and now k) of
GCLs.
What Darcy Didn’t Tell You:
Composite Liner Performance
GCL Permeability varies with Effective Stress!
• A composite liner will leak only where there is a hole in
the geomembrane (diffusion is ignored).
• Flow from the hole depends on:
K (c m /s e c )
1.00E-07
1.00E-08
10x
–
–
–
–
Size of hole
Hydraulic head on liner system
Permeability of materials above and below the geomembrane
Contact between membrane and underlying soil
Landfill
Liners
Liners/Slopes
Landfill Bottom
Caps, Mine
Closures,
PondsLeach Pads
Heap
1.00E-09
1.00E-10
1
10
100
1,000
10,000
Effective Stress (kPa)
Petrov, R.J. et al (1997) “Selected Factors Influencing GCL Hydraulic Conductivity”, Journal of
Geotechnical and Geoenvironmental Engineering, ASCE
Giroud Leakage Equation
Q Cqo 1 0.1 ˜ h t s 0.95
a
0 .1
˜ h 0.9 ˜ k 0.74
where:
Q = flow through hole in geomembrane
Cqo = quality of intimate contact between membrane/soil
h = liquid head on liner system (< 3 m)
ts = thickness of soil beneath geomembrane
a = area of hole in geomembrane (circular, max diam 25 mm)
Ks = permeability of soil under geomembrane (constant)
Giroud Equation Results
• Shows that GCL-based composite liner
outperforms CCL-based composite liner
by a factor of 30
• These calculations include many
assumptions that require designer’s
best judgment—actual results will vary
Reference: Giroud, J.P., (1997) "Equations for Calculating the Rate of Liquid Migration Through Composite
Liners Due to Geomembrane Defects,“ Geosynthetics International, Volume 4, Nos. 3-4, pp. 335-348.
Liner Leakage Rates
• There is ACTUAL leakage data available!
• Major study conducted for USEPA
(Bonaparte, Daniel, and Koerner - 1999)
• Encompassed 91 Landfills and 287 cells
• Cells monitored for 10 years
• Double-lined
• Leakage through primary liner (either
GM/CCL or GM/GCL) was evaluated by
measuring flow into the underlying leachate
detection system
Field Performance Comparison
Composite Liner Leakage Rate
Performance (Bonaparte, et al. 2002)
400
Sand as leak
detection layer
300
Leakage,
lphd
GM
200
GM/CCL
100
GM/GCL
0
1
2
Life Cycle Stage
3
As a result of this data,
USEPA recognizes GCLs
as equivalent or superior
to CCLs as the clay
component of MSW
landfill liners.
GCL Chemical Compatibility
is a function of:
•
•
•
•
Chemical Compatibility
How will the GCL perform when
exposed to leachate and other
chemicals?
Diffuse Double Layer Theory
Type of chemicals and concentration
Prehydration
Exposure to wet/dry cycles
Confining pressure
Daniel, D. E. and Stark, T. D., GCLs for Containment of Wastes, Chemicals and Liquids,
Short Course, University of Illinois, 2000.
Contaminants of Concern
• Main contaminants of concern:
– Cations (Na+, Ca+2, Mg+2)
– Miscible (water-soluble) Organic Solvents
at high (> 50%) concentrations
Testing for Compatibility
• ASTM D6141, Standard Guide for
Screening the Clay Portion of a GCL for
Chemical Compatibility to Liquids.
• Uses free swell and fluid loss tests
• Results using test liquid are compared
to results using clean water
Fluid Loss Test
Swell Index
Air Pressure
FS • 24 mL/2g in
clean water
Filter Paper
Slurry
FL < 18 mL in
clean water
Alternate Test Method
Concentration Effects on Permeability
for a more conclusive assessment
• D6766, Standard Test
Method for Evaluation
of Hydraulic
Properties of GCLs
Permeated with
Potentially
Incompatible Fluids.
Mg = 240 ppm
– Modified version of
D5887 and D5084.
– Termination criteria:
OUTFLOW
Na = 2,300 ppm
Ca = 400 ppm
Jo et al. (2001)
• E.C.INFLOW§E.C.OUTFLOW
INFLOW
Landfill Bottom Liner Research
(TR-254)
Jo, H.Y. et al., Hydraulic Conductivity and Swelling of Nonprehydrated GCLs
Permeated with Single-Species Salt Solutions, Journal of Geotechnical and
Geoenvironmental Engineering, ASCE, July 2001.
Kolstad (2004) modified
• Kolstad (2004) tested long-term compatibility
(hydraulic conductivity) of numerous GCLs in
contact with various salt solutions.
• Found that a GCL’s long-term hydraulic
conductivity depends on 2 properties:
– Ionic Strength, P
– Ratio of Monovalent to Divalent Cations, RMD
RMD
>Na @ >K @
>Ca @ >Mg @
2
2
Most leachates causes very
little increase in k
Prehydration Benefits
Confining Pressure
Mine Waste
with
EC = 4,000 Pmho/cm
Prehydration improves k
Hydrating the GCL with fresh water prior to contact with
contaminants can have a significant positive affect on
permeability
Shackelford, C.D. et al., Evaluating the Hydraulic Conductivity of GCLs Permeated with
Non-standard Liquids, Geotextiles and Geomembranes, Elsevier Ltd., Vol. 18, 2000.
Confining stress improves k
Daniel (2000)
Wet-Dry Cycling
Hydraulic Conductivity (cm/s)
10
10
10
10
10
10
Compatibility
Recommendations
-5
DI
DI-CaCl 2
-6
• Once hydrated, bentonite is compatible with
most waste leachates
Tap-CaCl 2
CaCl2
-7
– Compatible with most inorganics except high Ca
and Mg (salts).
• Prehydration with clean water and increased
confining pressure improve compatibility
• Desiccation should be avoided
• Project-specific conditions will influence GCL
response to contaminants
• GCL screening and permeability testing
should be performed in cases of uncertainty
-8
-9
-10
0
1
2
3
4
5
6
7
8
9
Number of Wetting Cycles
GCL Shear Strength and
Slope Stability
GCL Shear Strength
• GCLs need cover for physical
protection, overlap performance, and
lowest permeability
• Soil or waste cover on sloping surfaces
creates shear stresses on liner system
• Shear strength of liner system must
exceed shear stresses in order to be
stable
Bentonite Shear Strength
Bentonite Shear Strength
High Normal Loads
Low Normal Loads
3000
Peak
Unreinforced Bentonite @ Low Normal Loads
400
Daniel 1992, slow shear
Large displacement
Fox et al, 1998 peak
2500
Hyperbolic fit for peak
Fox et al, 1998, post peak
Hyperbolic fit for post peak
300
Shear Stress (psf)
Daniel & Shan 1991, slow shear
post peak
Gilbert et al, 1996 post peak
Shear stress (psf)
Leisher, 1992 peak
Peak Hyperbolic Curve
10º
Post peak Hyperbolic Curve
200
2000
Mesri & Olson
1500
1000
2.4º
100
500
7.0º
18º
0
0
200
400
600
800
1000
1200
1400
1600
-
5,000
10,000
Shear Strength of
Unreinforced Hydrated Bentonite
• Approximately 10o at low normal loads
(less than 3,000 psf)
• Decreases to 4o at high normal loads
• Not acceptable for most sites with
appreciable slopes
15,000
20,000
25,000
30,000
Normal Stress (psf)
Normal Stress (psf)
What do engineers want?
• We don’t want to rely on unreinforced
hydrated bentonite.
• We want the GCL internal shear
strength to be greater than interface
strength
H
1
4
Subgrade
L
MSW
(JI)
2
Liner I( )
Needlepunch Reinforcement
3 to 5x
improvement
in internal
shear
strength
1
Benefits of High Needlepunch
Density
• Higher internal shear strength
– Higher factor of safety
– Allows steeper slopes or higher shear loads
– Not the “weakest link” in a multi-layer system
• More consistent mass/area
– Fewer thick and thin spots
– Uniform hydraulic performance
– Minimal edge loss during handling/installation
• Better resistance to unconfined hydration
– Resists swelling pressure of bentonite
GCL Peak Strength Comparison
Myth #2:
Myth: “Thermal Locking
is necessary to achieve
high internal shear
strength”
Truth: There are other
GCL manufacturing
methods which can
yield even higher shear
strength.
Needled fibers heat-bonded to
geotextile
Bentomat with needled fibers
Slope Stability Issues
Bentomat (Not thermal locked)
33.5 degrees
Bentomat
BentofixGCL
Thermal Locked
28.9 degrees
Veneer Stability
(McCartney, 2002)
Veneer Stability Issues
Resisting Forces
Driving Forces
- Interface friction
- Tension
- Toe buttressing
- Weight of cover
- Pore pressure
If Downslope
driving forces >
Resisting forces,
slide could occur
Tension
Weight of cover
Toe buttressing
Interface
friction
Global Stability
Why do veneer slides occur?
1. Pore pressures (no. 1 cause)
2. Inadequate shear strength for the slope
conditions:
-Long-term static conditions
-Dynamic conditions (during construction
and earthquakes)
Consult the literature for methods of stability assessment
Global Stability Issues
Global Stability Analysis
for bottom liner systems
• Depends on geometry of
fill
• Evaluated by limit
equilibrium methods or
finite element analysis
• Too complex to address
here!
• Best practice is to ensure
that a large-displacement
FS > 1 is maintained for
all interfaces
Stability of the entire mass of waste or earth must be assessed
Internal failure mode
4
H
Interface failure mode
MSW
(JI )
1
Subgrade
L
1
2
Liner ( I )
Myth #3:
Torn Edge of
Geomembrane
Myth: “Tensile strength and
elongation are important
GCL performance
properties”
Truth: In waste sliding
failures, GCLs and
geomembranes are “tissue
paper” and should not be
relied upon for tensile
reinforcement.
NonNon-woven
geotextile
Exposed
clay liner
Myth #4:
Peel and shear strength
300
Myth: “Peel strength is not
an important GCL property”
250
P eak S hear Strength (k P a)
Truth: Peel strength is a
measure of internal
reinforcement, and as such,
an indicator of GCL internal
shear strength. Since it is
difficult to control, some
manufacturers do not certify
peel strength.
Torn edge of geotextile
200
150
100
50
0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
Peel Strength (N/cm)
14.0
16.0
18.0
20.0
Slope Stability
General Rules
• Consult the Data! CETCO has a vast
database of internal and interface shear
test results
• In general, the GCL interface with a
geomembrane is the most critical
• Internal shear strength usually exceeds
peak design requirements
Slope Stability Recommendations
Slope Stability
General Rules (cont’d)
• All interfaces have unique
characteristics and the designer must
perform testing to ensure that sloped
systems will be stable
• Conduct material-specific tests across
full range of normal loads
• Work with a lab well-experienced in
shear testing
For more information
• Web site: www.cetco.com/LTE
• Attempt to position critical plane above primary
geomembrane
• Good practice to check FS > 1 everywhere
using Residual strength (seismic events,
construction activities, waste
placement/settlement can cause liner
deformation)
• Use Residual strengths of interfaces that have
the lowest peak strengths over each normal
load range
• Think about unexpected pore pressure
scenarios
– Register to access free technical references,
specifications, and installation guide
• E-mail: catha@cetco.com
les@volclay.com.au
phil@polyfabrics.com.au