Geosynthetics Subgrade Stabilization Stabilization and Base Reinforcement
Eli Cuelho, P.E. Western Transportation Institute Montana State University October 24, 2012
Geosynthetic Types •
Geotextile
•
Geogrid
•
Geocomposite
•
Geonet
•
Geomembrane
Geotextiles D4439: “ A • ASTM D4439: A permeable permeable geosynthetic geosynthetic comprised solely of textiles.” textiles. ” •
•
Woven geotextile •
monofilament
•
multifilament
•
slit film tape
Non-woven geotextile •
needle punched
•
heat bonded
Geotextile: Woven Monofilament
Geotextile: Woven Multifilament
Geotextile: Woven Slit Film Tape
Geotextile: Nonwoven Needle Punched
Geotextile: Nonwoven Heat Bonded
Geogrid • 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: Biaxial Extruded or Integrally-Formed
Geogrid: Laser Welded
Geogrid: Woven
Geocomposite • 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: Geotextile/Geonet
Geocomposite: Geotextile/Pipe
Geonet • 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.”
Geosynthetic Functions in Pavements 1) Stabilization / Reinforcement Base aggregate
Wheel load support
Subgrade Confinement
2) Separation
Geosynthetic Tension
Geosynthetic Functions in Pavements 3) Drainage
4) Filtration
What is Stabilization? Placement and maintenance of aggregate that serves as a stable layer for support of the remaining pavement structure
Instabilities During Construction
Instabilities During Operating Life
Stabilization
Stabilization: Separation Function
Stabilization: Reinforcement Function – Lateral Restraint
– Bearing Capacity Increase
– Membrane Tension Support
Full-Scale Field Study of Geosynthetics Used as Subgrade Stabilization
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
Eli Cuelho – Research Engineer & Program Manager (406) 994-7886
[email protected]
www.transcendlab.org
Test Section Layout
Direction of trafficking 15 m
20 m
4m
Control 1
WeG-1
20 m
WeG-2
15 m
IFG-3
15 m
CoG-4
15 m
IFG-5
15 m
15 m
15 m
WeG-6 WoG-7 WoG-8 WoT-9 NWoT-10 Control 2
15 m
15 m
20 m
Not to scale
Geosynthetics
WeG-1
WeG-2
IFG-3
CoG-4
WeG-6
WoG-7
WoG-8
WoT-9
IFG-5
NWoT-10
Constructing Trench
Construction of Artificial Subgrade
Tilling
Moisture Control
Compaction
Pre and Post Trafficking Subgrade Strength 3.0 Pre-Trafficking Composite Post-Trafficking Composite
R
2.5
2.0
B s
i
et
Targeted Range
C 1.5
o
Post Trafficking Average p C
o
m 1.0
0.5
0.0
2 7 - 1 - T 9 - G 3 - G - 6 - 4 5 - G 8 l 1 l 2 1 0 G o o G G G G T e e o W o o e I F F t r o I o t r n n W C W W W W o W o C N C Test Section
Installation of Geosynthetics
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
Grading the Base Course
Compacting the Base Course
Ready for Trafficking
Trafficking
• Total weight = 46 kips (20,860 kg) • Speed = 10 mph (15 kph)
Final Layout
200 mm
Base course 50 mm
Original taxiway
25 mm Artificial Subgrade 1m
4m
Pass 1
Pass 2
Pass 3
Pass 5
Pass 20
Pass 25
Pass 40
Filling in Ruts
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
Original road surface Apparent rut
Elevation rut
Mean Rut Depth vs. N add 100 WeG-1 WeG-2 IFG-3 CoG-4 IFG-5 WeG-6 WoG-7 WoG-8 WoT-9 NWoT-10
80
60 d d a
N
40
20
0 0
20
40
60
80
Mean Rut Depth (mm)
100
120
Forensic Investigations
Extracting Geosynthetics
Post Trafficking Measurements
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
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
Phase II Test Section Layout North Not to scale 50 ft
15 ft
50 ft
Control Control Control 1 2 3
50 ft
50 ft
1
50 ft
2
3
50 ft
Thickest Thicker Regular base base base (24”) (16”) (~12”)
4
...
50 ft
50 ft
50 ft
11
Tensar Tensar Tensar BXType2BXType2BXType2
50 ft
50 ft
Regular base (12”)
Regular subgrade CBR=1.7
Weaker Stronger subgrade subgrade CBR=1.4 CBR=2.0
Base Reinforcement • Improve long-term load bearing capacity • Improve structural support • Geosynthetics incorporated into design of road structure • Improve roadway longevity
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
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
TBR Unreinforced TBR= 75,000/12,500 = 6
1
Reinforced
) h 0.8 c n i (
0.6
h t p e 0.4 D t u 0.2 R
TBR= 4
12,500 Passes
0 0
20,000
40,000
75,500 Passes 60,000
Traffic Passes
80,000
BCR BCR = (D2-U - D2-R)/D2-U with identical life
AC BASE
D2-R
D2-U
SUBGRADE
GEOSYNTHETIC
Benefit Results • Requires comparative studies • Typical TBRs from test sections • Geogrids: 1.5 to 70 • Geotextiles: 1.5 to 10
• BCR • 22% to 50%
Mechanistic-Empirical Design
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)
Cyclic Tension Tests • Low-strain cyclic modulus (ASTM D7556) Monotonic Test
14 12 ) m / N k ( d a o L
10 8 6
Cyclic Test
4 2 0 0
0.005
0.01
0.015
0.02
Strain (m/m)
0.025
0.03
0.035
Cyclic Pullout Tests • Resilient interface shear modulus (ASTM D7499)
Biaxial Tension • Poisson’s ratio P
XMD MD
e
MD
XMD
n
XMD-MD
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)
