Compacted Clay Barriers for Waste Containment: Keys to ...

Compacted Clay Barriers for Waste
Containment: Keys to Successful
Implementation and Field Performance
Craig H. Benson, PhD, PE, DGE
Wisconsin Distinguished Professor of Geological Engineering
University of Wisconsin-Madison
Madison, Wisconsin USA 53706
chbenson@wisc.edu
©
University of Wisconsin-Madison 2010

Objectives
Leachate Collection Layer
Clay Liner (> 600 mm)
(1) Construct a clay barrier with low saturated
hydraulic conductivity (< 10 -7 cm/s)
(2) Protect the clay barrier from damage that may
increase hydraulic conductivity

Three Principles for
Success
• Selecting soil with suitable properties
• Processing and compaction conditions
that yield low hydraulic conductivity (K)
• Protection of the constructed barrier

Criteria for Suitable Soil
• Composition
– particle size characteristics
– clay mineralogy/Atterberg limits
• Availability
– volume
– distance
– cost
• Heterogeneity
– uniform?
– pick & choose?

Source Control
Clay liner quality begins with controlling the quality of the source
material. Some borrow pits are very uniform, and require little
inspection. This borrow pit near Detroit, MI, was extremely
uniform.

Total distance ~ 100 m
Clay
Clayey
Silt
Silty
Sand
This borrow pit near Racine, WI was highly heterogeneous, and
required constant inspection.

Percent Finer (%)
100
80
60
40
Particle Size Distribution
4.8
mm
75
mm
2
mm
Definitions:
Gravel: > No. 4 (4.8 mm)
Sand: < No. 4
> No. 200 (75 mm)
Fines: < No. 200 (75 mm)
Clay: < 5 mm or 2 mm
2 mm clay definition most
common
20
For this soil:
0
100
Gravel
10
Sand Fines Clay
1 0.1 0.01 0.001
Particle Size (mm)
0.0001
Gravel = 11%
Sand = 55%
Fines = 34%
Clay = 20%

Gravel Content
Lab data
suggest that
gravel
content can
be as much
as 50%.
Data from Shakoor and Cook
Practical
upper bound
20-25%.

Gravel Content
Lab data
suggest that
gravel
content can
be as much
as 60%.
Data from Shelly and Daniel
Practical
upper bound
20-25%.

Fines (P200)
Fines consist
of silt and clay.
Soil with 50%
fines can be
50% silt or
50% clay
(probably in
between).
Data from Benson et al. (1994)
Practical lower
bound 30%.

2 micron Clay Content
Better
correspondence
between K and
clay content.
Data from Benson et al. (1994)
Practical lower
bound 15%.

Atterberg Limits and Hydraulic Conductivity
Data from Benson et al. (1994) Data from Benson et al. (1994)
Greater clay fraction and/or more active clay
mineral yields lower K.
Reasonable lower bounds: LL > 20 and PI > 10.

Recommended Minimum Properties
for Liner Soils
Gravel Content < 20%
Fines Content > 30%
Clay Content (2 mm) > 15%
Liquid Limit > 20
Plasticity Index > 10
Soils meeting these criteria will classify as CL,
CH, SC, SC-CL, or SC-CH. Not all are ‘clays.’
For more detail: Benson, C., Zhai, H., and Wang, X. (1994), Estimating the Hydraulic Conductivity of Compacted Clay Liners, J. of Geotech. Eng., ASCE, 120(2), 366-387.

Hydraulic Conductivity (cm /s)
Previous recommendations do NOT apply to soil-bentonite
mixtures. Custom-design for base soil.
10 -4
Soil-Bentonite Mixtures
6.2% = Compaction
W ater Content
Dry of Optimum
W et of Optimum
Add various bentonite
contents to soil and
compact specimens
for hydraulic
conductivity testing
10 -5
10 -6
10 -3 0 1 2 3 4 5 6
17.3% 5.4%
Determine bentonite
content to achieve
target hydraulic
conductivity.
10 -7
10 -8
13.9%
Bentonite Content (%)
13.1%
14.7%
Add 1-2% extra for
additional safety
margin.

Conditioning and Placement
• Compaction control – water content and
dry unit weight
• Water content tempering
• Compaction methods & control

Compaction Curve
Modified
Proctor
Zero Air
Voids
ASTM
Standards:
Dry Unit Weight
Max. Dry Unit
Weight, g dm
Standard
Proctor
Lab Data
Line of
Optimums
D 698 –
standard
Proctor
D 1557 –
modified
Proctor
Optimum Water
Content, w opt
Compaction Water Content

Hydraulic Conductivity Curve
Hydraulic Conductivity
Optimum Water
Content, w opt
Lab Data
Compaction Water Content
Hydraulic
conductivity
greatly affected by
water content.
Higher K occurs for
compaction dry of
optimum water
content.
Low K occurs for
compaction wet of
optimum water
content.

Compaction
Conditions
Hydraulic conductivity affected
by water content and
compactive effort.
Key to low K is compaction
wet of optimum water content
for any compactive effort.
Optimum water content varies
with compaction energy; field
energy ambiguous.
Compact wet of line of
optimums.
Data from Mitchell et al. (1965)
line of
optimums

Effect of Water Content and Compactive Effort on
Remolding of Clods and Hydraulic Conductivity of Clay
A
Highly Plastic Beaumont Clay
w o = 17% for Std. Proctor
w o = 10% for Mod. Proctor
Photograph: specimen
compacted with standard
Proctor energy (ASTM D 698)
at w = 12% (5% dry of
optimum)
Large interclod pores are
readily visible. K ~ 10 -4 cm/s

Specimen compacted with
standard Proctor energy
(ASTM D 698) at w = 16% (1%
dry of optimum)
Large interclod pores are still
visible, but clay is visible
softer and more remolding is
apparent
K ~ 10 -5 cm/s
B

Specimen compacted with
standard Proctor energy
(ASTM D 698) at w = 20% (3%
wet of optimum)
Only micro-scale pores exist.
Clods fully remolded and
interclod voids are
eliminated.
K ~ 10 -9 cm/s
C

Influence of Compactive Effort
Standard Proctor
w = 16%, w opt = 17%
K ~ 10 -5 cm/s
Modified Proctor
w = 16%, w opt = 10%
K ~ 10 -9 cm/s
B
D

Dry Unit Weight
Ways to Cross the Line of Optimums
D
Zero Air
Voids
MP
A
SP
B
C
Line of
Optimums
Compaction Water Content
For more detail: Benson, C. and Daniel, D. (1990), Influence of Clods on the Hydraulic Conductivity of Compacted Clay, J. of Geotech. Eng., ASCE, 116(8), 1231-1248..

Compaction Specifications
Traditional Modern

4-Step Procedure: Daniel & Benson (1990)
For more information: Daniel, D. and Benson, C. (1990), Water Content-Density Criteria for Compacted Soil Liners, J. of Geotech. Eng., ASCE, 116(12), 1811-1830.

Example: upper bound normally placed on
water content to ensure adequate shear
strength and trafficability.
Leroueil et al. (J. of Geotech. Engr., 1992)
Undrained
shear
strength
decreases
as water
content
increases.

Clod Size
Larger
(< 19 mm)
clods
Smaller (< 4.8
mm) clods
Sensitivity to compaction water content is reduced if clod size is
reduced. Smaller clods more readily hydrated and remolded.
Benson, C. and Daniel, D. (1990), Influence of Clods on the Hydraulic Conductivity of Compacted Clay, J. of Geotech. Eng., ASCE, 116(8), 1231-1248.

Clods dumped from scraper or dump truck can be very
large, and should be reduced in size (< 50 mm).

Agricultural disk may not be
effective with stiffer clays.

Hydraulically
actuated disk is
more effective.

Road Reclaimer – good for dry
clays & claystones

Carbide tipped teeth rapidly crush clods

Water Content Adjustment
Modest adjustments
(

Hydraulic Conductivity (cm/s)
Swell (mm)
Tempering Time
Hydraulic Conductivity
Swell
15
10
Sufficient
“tempering”
(hydration) time
needed to
penetrate and
soften clods.
10 -6 0
10 -7
10 -8
0 10 20 30 40 50
Hydration Time (hours)
5
24 hr tempering
good rule of
thumb.
Add water, blend,
stockpile/temper,
blend again
Benson, C., Gunter, J., Boutwell, G., Trautwein, S., and Berzanskis, P. (1997), Comparison of Four Methods to Assess Hydraulic Conductivity, J. of Geotech. and Geoenvironmental Eng., ASCE, 123(10), 929-937.

Clay with water content that is too low for compaction.
Note remnant clods and interclod pores. Compactor
footprints are not noticeable.

Clay at
appropriate
water content
for compaction.
Note remolding
and penetration
of compactor
feet.

Compacting clay for landfill cell. Wet and soft clay makes for
clay liners with very low hydraulic conductivity.

Compactors
Data from Mitchell et al. (1965)
Smooth Drum
Conventional “Sheepsfoot”
Partially-penetrating
pad foot
Fully-penetrating
tamping foot
Kneading action yields lower K…more
effective in remolding clods.

Compactor Type – UW Database
Kneading
Static
In terms of
‘average’
conditions, UW
field data in good
agreement with
data from
Mitchell et al.
(1965).

Compactor Weight
1 ton ~ 10 kN
UW database
suggest that
heavier
compactors yield
lower K,
although not
particularly
important for
weight > 20 tons.

Tamping foot compactor (CAT 815B)

Tamping Feet

Tow-behind tamping foot compactor

Traditional Sheepsfoot

Protection of the Barrier
Frost Damage:
Freezing causes:
- formation of ice lenses:
cracking
- formation of desiccation cracks
as water moves to freezing front
- cracking that causes increases
in hydraulic conductivity
Protect the clay barrier
with insulation
(synthetic or burial).

Cracks

Hydraulic Conductivity (cm/sec)
How much does the hydraulic conductivity
increase?
10 -5 0 2 4 6 8 10
At least 10 X
10 -6
Typically 100 to
1000 X
Occasionally
10,000 X
10 -7
10 -8
10 -9
Blue - Laboratory ASTM D 6035
Red - Field
Number of Freeze-Thaw Cycles
Benson, C., Abichou, T., Olson, M., and Bosscher, P. (1995), Winter Effects on the Hydraulic Conductivity of Compacted Clay, J. of Geotech. Eng., ASCE,
121(1), 69-79.

Hydraulic Conductivity (cm/s)
Desiccation Damage
Drying of compacted clay barriers causes desiccation
cracks to form, increasing the hydraulic conductivity.
10 -4 10 15 20 25 30 35
w opt
Standard Proctor
10 -5
10 -6
10 -7
10 -8
# drying cycles
N = 0
N = 1
N = 2
N = 3
N = 4
N = 5
10 -9
Compaction Water Content (%)
Large-scale cracks may form, as in this clay barrier in southern
Wisconsin five years after construction.
Albrecht, B. and Benson, C. (2001), Effect of Desiccation on Compacted Natural Clays, J. of Geotech. and Geoenvironmental Eng., 127(1), 67-76.

Is the increase permanent?
Damage due to
frost or
desiccation
permanent.
Does not
“heal,” but
stress will close
cracks.
Othman, M. and Benson, C. (1994), Effect of Freeze-Thaw on the Hydraulic Conductivity and Morphology of Compacted Clay, Canadian Geotech. J., 30(2), 236-246.

Three Principles for
Success
• Selecting soil with suitable properties
• Processing and compaction conditions
that yield low hydraulic conductivity (K)
• Protection of the constructed barrier

EXTRAS

Summary Remarks
• Identified index properties for suitable clay
liner soil.
• Suitable soils may not classify as ‘clay.’
• Hydraulic conductivity greatly affected by
compaction condition (water content and dry
unit weight).
• Compact wet of the line of optimums to
achieve low hydraulic conductivity.

Consider Soil from PSD Slide
Gravel Content < 20% 11% OK
Fines Content > 30% 34% OK
Clay Content (2 mm) > 15% 20% OK
Liquid Limit > 20 28 OK
Plasticity Index > 10 14 OK
USCS Classification = SC
K = 2.9x10 -8 to 1.1x10 -7 cm/s

Mineralogy – Atterberg Limits
LL = liquid limit
PL = plastic limit
PI = LL – PL = plasticity index
C = clay
M = silt
O = organic
L = low plasticity
H = high plasticity

Effect of Clay Content on Atterberg Limits
LL and PI - affected by clay type and clay content
Soil from PSD slide had 20% clay, LL = 28, and PI = 14

Subgrade Preparation
Subgrade should be firm and moist. Dry subgrades will desiccate clay liners from
below and wet/soft subgrades will impede compaction.
Compaction near optimum water content and dry unit weight is appropriate (w opt
± 2%, >0.95g dm ).
Provide an outlet for runoff or a sump to facilitate construction after rainfall.