geosynthetics - north dakota local technical assistance ...• all geosynthetics provided...
TRANSCRIPT
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Geosynthetics Subgrade Stabilization and Base Reinforcement
Eli Cuelho, P.E. Western Transportation Institute
Montana State University
October 24, 2012
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• Geotextile
• Geogrid
• Geocomposite
• Geonet
• Geomembrane
Geosynthetic Types
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• ASTM D4439: “A permeable geosynthetic
comprised solely of textiles.”
• Woven geotextile • monofilament
• multifilament
• slit film tape
• Non-woven geotextile • needle punched
• heat bonded
Geotextiles
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Geotextile: Woven Monofilament
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Geotextile: Woven Multifilament
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Geotextile: Woven Slit Film Tape
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Geotextile: Nonwoven Needle Punched
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Geotextile: Nonwoven Heat Bonded
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• ASTM D4439: “A geosynthetic formed by a
regular network of integrally connected
elements with apertures greater than ¼ in. to
allow interlocking with surrounding soil, rock,
earth, and other materials to function primarily
as reinforcement.”
• Categories based on junction type:
– Extruded geogrid
– Bonded geogrid
– Woven geogrid
Geogrid
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Geogrid: Biaxial Extruded or
Integrally-Formed
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Geogrid: Laser Welded
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Geogrid: Woven
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• ASTM D4439: “A product composed of
two or more materials, at least one of
which is a geosynthetic.”
• Common combinations:
– Geotextile and geonet
– Geotextile and geogrid
– Geotextile and drainage pipes
– Geonet and erosion mat
Geocomposite
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Geocomposite: Geotextile/Geonet
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Geocomposite: Geotextile/Pipe
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• ASTM D4439: “A geosynthetic consisting of
integrally connected parallel sets of ribs
overlying similar sets at various angles for
planar drainage of liquids and gases.”
Geonet
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1) Stabilization / Reinforcement
2) Separation
Wheel load support Base aggregate
Subgrade
Confinement
Geosynthetic
Tension
Geosynthetic Functions in Pavements
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3) Drainage
4) Filtration
Geosynthetic Functions in Pavements
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Placement and maintenance of aggregate that serves as a stable layer for support of the remaining pavement structure
What is Stabilization?
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Instabilities During Construction
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Instabilities During Operating Life
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Stabilization
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Stabilization: Separation Function
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– Lateral Restraint
– Bearing Capacity
Increase
– Membrane Tension
Support
Stabilization: Reinforcement Function
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Full-Scale Field Study of Geosynthetics
Used as Subgrade Stabilization
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Background
• Problem
• Lack of universally accepted design that uses
generic geosynthetic properties
• Understanding of which properties are most
relevant
• Objective – assess performance and
survivability of various geosynthetics when
used as subgrade stabilization
• Weak subgrade
• Constructed uniformly
• Controlled traffic
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Eli Cuelho – Research Engineer & Program Manager
(406) 994-7886
www.transcendlab.org
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Test Section Layout
Control 1 WeG-1 WeG-2 IFG-3 IFG-5 WeG-6 WoG-7 WoG-8 WoT-9 Control 2
20 m
20 m
15 m 15 m 15 m 15 m
15 m 15 m 15 m 15 m
CoG-4
20 m
15 m
Direction of trafficking
NWoT-10 4 m
Not to scale
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Geosynthetics
WeG-1 WeG-2 IFG-3 CoG-4 IFG-5
WeG-6 WoG-7 NWoT-10 WoT-9 WoG-8
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Constructing Trench
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Construction of Artificial Subgrade
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Tilling
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Moisture Control
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Compaction
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Pre and Post Trafficking Subgrade Strength
Co
mp
os
ite
CB
R
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Test Section
Contr
ol 1
WeG
-1
WeG
-2
IFG
-3
CoG
-4
IFG
-5
WeG
-6
WoG
-7W
oG
-8
WoT-9
NW
oT-1
0
Contr
ol 2
Pre-Trafficking Composite
Post-Trafficking Composite
Targeted
Range
Post Trafficking
Average
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Installation of Geosynthetics
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Base Course Aggregate
• Well-graded gravel
• 20 cm thick based on FHWA design
• Control sections ~100 mm of rut at 45 truck passes
• Geosynthetic sections ~100 mm rut at 455 truck passes
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Grading the Base Course
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Compacting the
Base Course
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Ready for Trafficking
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Trafficking
• Total weight = 46 kips (20,860 kg)
• Speed = 10 mph (15 kph)
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Final Layout
50 mm
25 mm
200 mmBase course
Artificial
Subgrade1 m
4 m
Original taxiway
50 mm
25 mm
200 mmBase course
Artificial
Subgrade1 m
4 m
50 mm
25 mm
200 mmBase course
Artificial
Subgrade1 m
4 m
50 mm
25 mm
200 mmBase course
Artificial
Subgrade1 m
4 m
Original taxiway
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Pass 1
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Pass 2
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Pass 3
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Pass 5
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Pass 20
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Pass 25
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Pass 40
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Filling in Ruts
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Rut Measurements
• Differences in elevation as rut accumulates
• Two outermost wheel ruts in each test section
• Relate traffic passes to specific rut levels
• 1 truck pass = 2.2 traffic passes
Apparent rut
Original
road surface
Elevation rut
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Mean Rut Depth (mm)
0 20 40 60 80 100 120
Na
dd
0
20
40
60
80
100
WeG-1
WeG-2
IFG-3
CoG-4
IFG-5
WeG-6
WoG-7
WoG-8
WoT-9
NWoT-10
Mean Rut Depth vs. Nadd
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Forensic
Investigations
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Extracting Geosynthetics
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Post Trafficking Measurements
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Conclusions
• All geosynthetics provided improvement when
compared to controls
• Welded, woven and stronger integrally formed
grids performed best
• Two textiles and weaker integrally formed grid
provided significantly less benefit
• Current design methods underpredicted base
layer thickness for this situation
• Tensile strength in cross-machine direction
plays a significant role in rut suppression
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Phase II Subgrade Stabilization Study
Objective: match geosynthetic material
properties to field performance
• Pooled-fund study (9 states, MT is lead)
• 17 full-scale test sections
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Phase II Test Section Layout
North
50 ft
50 ft
... Control
1 11 Tensar
BXType2
50 ft
50 ft 50 ft
Tensar
BXType2 15 ft Control
2
50 ft
2 3 4
50 ft 50 ft
50 ft
1
50 ft
Not to scale
Thickest
base
(24”)
Thicker
base
(16”)
Regular
base
(12”)
Control
3
50 ft
Regular
base
(~12”)
Tensar
BXType2
Regular
subgrade
CBR=1.7
Weaker
subgrade
CBR=1.4
Stronger
subgrade
CBR=2.0
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Base Reinforcement
• Improve long-term load bearing capacity
• Improve structural support
• Geosynthetics incorporated into design of
road structure
• Improve roadway
longevity
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Application
• Tend to be lower volume roads • AC thickness 2 to 4 inches
• Base thickness 8 to 16 inches
• CBR < 8
• Pavement surface distresses • Rutting
• Fatigue cracking
• Reinforcement placed at bottom of base layer
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Structural Contribution Based on
Empirical Methods
• Traffic Benefit Ratio (TBR) • Comparison of equivalent pavement systems
• Ratio of load applications in reinforced sections over
load applications in unreinforced sections
• Base Course Reduction Factor (BCR) • Comparison of equivalent traffic capacity
• Percent reduction in base thickness
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TBR
0
0.2
0.4
0.6
0.8
1
0 20,000 40,000 60,000 80,000
Traffic Passes
Ru
t D
ep
th (
inch
)
TBR= 75,000/12,500 = 6
12,500 Passes 75,500 Passes
TBR= 4
Unreinforced Reinforced
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BCR
BCR = (D2-U - D2-R)/D2-U
with identical life
GEOSYNTHETIC BASE
AC
D2-U D2-R
SUBGRADE
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Benefit Results
• Requires comparative studies
• Typical TBRs from test sections • Geogrids: 1.5 to 70
• Geotextiles: 1.5 to 10
• BCR • 22% to 50%
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Mechanistic-Empirical Design
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Geosynthetic Modeling
• Finite element model by Perkins et al. (2004) • Based on 2-D axisymmetric FEM contained in NCHRP
Project 1-37A
• Includes geosynthetic reinforcement
• Geosynthetic material models need constitutive
properties pertinent to pavement design • Elastic modulus in principal strength directions (tension tests)
• Soil-geosynthetic interaction (pullout tests)
• In-plane Poisson’s ratio (biaxial test)
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Cyclic Tension Tests
• Low-strain cyclic modulus (ASTM D7556)
0
2
4
6
8
10
12
14
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035
Strain (m/m)
Load (
kN
/m)
Monotonic Test
Cyclic Test
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Cyclic Pullout Tests
• Resilient interface shear modulus (ASTM
D7499)
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Biaxial Tension
• Poisson’s ratio
XMD
MD
P
e
MD
XMD
nXMD-MD
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Practical Use of This Information
• Areas of weak subgrade material • Need for stable platform to build road
• Maintain separation between layers
• Areas where gravel sources are limited or
costly
• Low-volume roads experiencing increased
truck traffic
• FHWA NHI Manual: Geosynthetic Design
& Construction Guidelines (2008)
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Thank you!
Presented by: Eli Cuelho, P.E. – Western Transportation Institute
[email protected] | (406) 994-7886
WesternTransportationInstitute.org