Geosynthetic Stabilization for

Soft Subgrade

- Instrumentation and ME

Approach

Xiaochao Tang, Ph.D. Louisiana Transportation Research Center

Murad Abu-Farsakh, Ph.D., P.E. Louisiana Transportation Research Center

ACKNOWLEDGEMENTS

Financial Support from LA DOTD, Tensor, TenCate

Assistance from PRF

BACKGROUND Louisiana soil: soft subgrade soil leads to excessive deformation

Treatments for soft subgrade:

o Thicker base (constraints of resources, high cost)

o Cut and fill (considerable expense of excavation and transportation)

o Chemical stabilization (lime-reactivity and environment, cement-

shrinkage cracks)

An alternative mechanical treatment: geosynthetics placed at base-

subgrade interface

Asphalt

SUBGRADE

Base

Extruded geogrids: a polypropylene (PP) or polyethylene (PE) sheet,

punched and drawn (Biaxial &Triaxial)

Woven / welded geogrids: polypropylene (PP) or polyester (PET)

yarns, woven or welded

Geogrids

Geosynthetics Manufactured from polymeric material, used with geotech engineering

projects such as slopes, retaining walls, embankments, and pavements;

Includes geotextiles, geomembranes, geonets, geocells, and geogrids.

Woven Biaxial

Geotextiles Permeable textile structures: Woven & Nonwoven

Extruded Biaxial

Extruded Triaxial

Woven Geotextile

Asphalt

SUBGRADE

Base

SUBGRADE

Asphalt

Base

No Geosynthetics With Geosynthetics

Geosynthetic Functions in Pavements

1) Separation: minimize base degradation

2) Confinement effects (interface friction, interlocking):

minimize lateral spreading, increased modulus

3) Tensioned membrane effects:

reduce vertical stress on subgrade

(Perkins, 1999)

Evaluate and quantify effectiveness of geosynthetic

reinforcements for pavements built over natural soft

subgrade

Examine pre-rut effects on geosynthetics performance

Incorporate effects of geosynthetic reinforcement into

pavement MEPDG design

RESEARCH OBJECTIVES

EXPERIMENTAL PROGRAM

Cross Section

A total of 6 test sections

Two control sections: sand embankment and unreinforced

Triaxial geogrids: double-layer & at interface

High-strength geotextiles: 12-in & 18-in base at interface

Heavy clay (A-7-6), Mexico Limestone, Level 2 Superpave mixture

Geotextiles Geogrids Geogrids

Test Sections

Pavement Instrumentation

HMA

Aggregate Base

Soil Subgrade

3''

10''

RS580 iGeotextile

CL

5''

1.5''

Potentiometer

Earth Pressure Cell

Piezometer

TDR Strain Gage

Instrument positions in Section 6(longitudinal cross section, not to scale)

LVDT

2' 2' 2'2'4'2'2'

1'

8'8'

2'2'

1'

Thermocouple (Note: thermocouple will be installed at the edge between Sections 5 and 6)

North

Load-associated:

stress, strain, & deformation

Environment-associated:

water content & temperature

What to measure ?

Where to measure ?

Critical responses:

top of subgrade, base, bottom of

AC

Pressure Cell (Top of Subgrade)

Hydraulic type with semiconductor transducer

Measures total stress and dynamic stress

Earth Pressure Cell

Pavement Instrumentation (cont’d)

Piezometer

Piezometer (Top of Subgrade)

Measures static and dynamic pore water pressure

LVDT (Top of Subgrade)

Spring-loaded type with a contact disk of 2 in diameter

Measures both elastic and permanent overall subgrade deformations

Pavement Instrumentation (cont’d)

Customized potentiometer

Potentiometer (Mid-Base)

Measures strain over the length

Both elastic and permanent

Customized LVDT

Pavement Instrumentation (cont’d)

Foil Strain Gauge on Geosynthetics

On opposite sides of geosynthetics, along transverse direction

Measures permanent and dynamic strains

Pavement Instrumentation (cont’d)

TDR (Top of Subgrade & Mid-Base)

Moisture content

Thermocouple (Subgrade, Base, & Asphalt Layer)

Spatial and temporal variations of temperature

Pavement Instrumentation (cont’d)

Cable protection and organization

Sensor labeling

Site Characterization

Subgrade and Base

LWD, Geogauge, Nuclear Gauge, DCP, and FWD

Accelerated Loading Facility (ALF)

About 100-ft long and 55-ton

Unidirectional, dual wheel, half of a single axle

Adjustable axle load: 9,750 lb to 18,950 lb

Nominal speed: 10.5 mph,

40-ft wheel path, wander covering 30’’ transverse distance

Testing and Data Collection & Processing

Pre-rut on base & APT on AC

At select intervals: transverse rut

profile, instrumentation data (static &

dynamic)

Data processing: eliminating outliers,

data smoothing, identifying peaks

and valleys

Laser Profilometer

PRELIMINARY RESULTS

Mechanistic modeling: calibration and validation

Subgrade resilient deflection by LVDT

83

83.5

84

84.5

85

0 0.5 1 1.5 2

Def

lect

ion (m

m)

Time (s)

10

20

30

40

50

60

70

80

90

0 0.5 1 1.5 2

Ver

tica

l st

ress

(kP

a)

Time (s)

Vertical stresses atop subgrade measured by pressure cell

Response Measurement

Increase of dynamic vertical stress at the top of subgrade

Indication of decrease of base modulus – corresponds to LWD & Geogauge

As opposed to current MEPDG using constant modulus, possible to update

pavement layer properties along with traffic loading

Vertical stresses atop subgrade measured by pressure cell (section 6)

Response Measurement (cont’d)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

0 500 1000 1500 2000 2500

Ver

tica

l st

ress

on

top

of

sub

gra

de

(kP

a)

Applications of axle load

Empirical performance prediction model: permanent deformation

model for unbound pavement layers

Accumulated surface rutting Surface rut profile at different stages of trafficking

Performance Measurement

18

23

28

33

38

43

48

53

100 120 140 160 180 200

Rel

ativ

e el

evat

ion

(m

m)

Transverse distance (cm)

0205015041960080810081208

0

5

10

15

20

0 500 1000 1500 2000 2500

Acc

um

ula

ted p

erm

anen

t def

orm

atio

n (

mm

)Applications of axle load

avg._east wheel

avg._west wheel

After 1000 passes

Majority of rutting attributed to base

Accumulated subgrade permanent deformation

Performance Measurement (cont’d)

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0 500 1000 1500 2000 2500

Acc

um

ula

ted

su

bgra

de

per

man

ent

def

orm

atio

n (

mm

)

Applications of axle load

SUMMARY AND FUTURE WORK

Six test sections: geogrids and geotextiles, various structures

Pavement instrumentation

Interpretation and use of instrumentation data

Future work: continue APT testing, integrate instrumentation data in

design & analysis, incorporate effects of geosynthetic

reinforcements into MEPDG

Thank You!