67797 SpectraBrochure OP-Cindy6.0 9/24/08 2:30 PM Page 2 · For over two decades, Tensar® Biaxial (BX) Geogrids have offered owners, engineers and contractors signiﬁcant value

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67797_SpectraBrochure_OP-Cindy6.0 9/24/08 2:30 PM Page 2

With the Spectra® Roadway Improvement System from

Tensar International Corporation, road design has never

been easier. Tensar® Biaxial (BX) Geogrids provide a

simple, reliable and cost-effective solution in this patented

soil reinforcement application.

The Spectra System can improve both paved and unpaved

roads by:

• Reducing required materials

• Simplifying construction

• Increasing pavement life

• Reducing future road maintenance

Even in less than ideal construction situations (i.e., weak

subgrades, heavy loads, thick fill layers, high aggregate

costs, contaminated subgrades, shallow utilities, etc.)

a predictable, engineered solution is possible with the

Spectra System. This solution relies on Tensar BX Geogrids

to provide patented soil reinforcement for unbound

aggregate layers in two distinct but related applications:

• Base Course Reinforcement – Enhances

performance or reduces the thickness of a permanent

road when constructed on a relatively firm foundation.

• Subgrade Improvement – Provides a stable

foundation layer for a permanent or temporary

road/working surface when weak subgrade

conditions (CBR < 3) are encountered.

The benefits of using the Spectra System for roadway

applications can include:

• Less aggregate

• Increased pavement life

• Less undercut

• More consistent compaction and

controlled settlement

The Spectra System has been used successfully for a

variety of projects such as: highways and secondary roads;

unpaved haul roads and working surfaces; parking areas

for commercial and industrial facilities; airport runways

and taxiways; freight truck distribution centers and

terminals; port, rail, industrial and intermodal facilities.

The Spectra® System provides a

predictable, cost-effective solution

for soil reinforcement.

Changing the Way Roads are Designed >

Tensar® Geogrids

The Spectra System owes its strength anddurability to Biaxial (BX) Geogrids, Tensar’spatented geosynthetic reinforcement grids.These geogrids stand the test of time,performing better than other commerciallyavailable geosynthetics due to their stiffinterlocking capability. For more information,visit www.tensar-international.com.

BX Geogrid awaiting installation for a project in Texas.


For aggregate reinforcement to work, it is necessary to

transfer the loads from the unbound aggregate into the

stiff geogrid. With Tensar Biaxial Geogrids, this is done

through mechanical interlock, a process where the

granular particles partially penetrate the apertures

of the geogrid and lock into place as the material

is placed and compacted (Figure 1). This process is

also often referred to as “mechanical confinement.”

Mechanical interlock can be illustrated using a billiard

ball rack (Figure 2). The rack confines the balls with

its stiffness and strength at the corners of the triangle

(i.e., the junctions). To confine the balls effectively, the

rack must have thick, squared edges. The unique process

used to manufacture Tensar BX Geogrids results in a grid

structure with full strength junctions and stiff ribs. This

presents a square, thick leading edge to the aggregate,

promoting effective mechanical interlock.

The billiard ball rack also demonstrates that effective

confinement at the aggregate section produces an

“improved zone” extending well beyond the aggregate –

geogrid interface. Roadway performance is significantly

improved as the aggregate is locked in place, yielding

a stiffer composite of pavement components.

The strength and stiffness of BX Geogrids at low strain is

critical to their performance in roadway applications. After

mechanical interlock is established during the placement

and compaction of aggregate on top of the geogrid, the

reinforcement benefits are generated at the onset of

traffic loading. The bottom line: Tensar BX Geogrids greatly

enhance the stiffness of unbound granular layers.

This results in lower cost, longer lasting and more

reliable pavements.

2

Mechanics of Aggregate Reinforcement Using BX Geogrids >

Demonstration ofmechanical interlockusing Tensar BX Geogrids

The unique structure allows the grid to get

a good “grip” on the aggregate particles and results in

effective mechanical interlock.

The unique shape of the geogrid ribs confines

aggregate particles due to its high stiffness and the strength

at the corners (junctions), just like a rack confines billiard balls.

Figure 1 Figure 2


For over two decades, Tensar® Biaxial (BX) Geogrids have

offered owners, engineers and contractors significant value

for building over soft and competent soils for a variety of

paved and unpaved surfaces. Our BX Geogrids are single

layer, integrally formed products manufactured from high

grades of polypropylene resin that exhibit high strength

at low strain and are chemically inert in practically all soil

conditions and environments. Tensar BX Geogrids exhibit

significant resistance to installation damage due to a

unique manufacturing process whereby an extruded sheet

is punched and drawn into a highly oriented grid structure.

Performance Offers Proof of Value

The performance and value of Tensar BX Geogrids have

been demonstrated over the years through full-scale

independent research and thousands of installations

worldwide. Independent agencies agree that in-ground

performance ultimately determines how geogrid products

are evaluated, approved for use and utilized for projects.

Tensar BX Geogrids have proven their value through

dozens of high profile research projects that have helped

to define the relevant civil engineering applications

whereby BX Geogrids can offer financial benefits:

• Subgrade improvement over soft soils

(both unpaved and paved surfaces)

• Base reinforcement of aggregate base course

(paved surfaces)

• Foundation improvement for shallow foundations

• Embankment stability over compressible soils

The U.S. Army Corps of Engineers concluded in 19921

through full-scale research that no single index

property, like tensile strength, is a primary indicator

of performance. Rather, the sum of these unique

characteristics predict the ability of any geogrid to

1) confine aggregate particles 2) distribute live load

(i.e., the “snowshoe effect”) and 3) span weak soils

through the tensile membrane effect.

The unique properties of

Tensar ® BX Geogrid provide

the Engineered Advantage.™

Tensar Biaxial Geogrid for Paved and Unpaved Surfaces >

PROPERTY SPECIFICATIONS FOR TENSAR® BIAXIAL GEOGRIDSIndex Properties

GEOGRID PROPERTIES TEST METHOD BX1100 BX1200 BX1300

Geometry MD CMD MD CMD MD CMD

Aperture Size I.D. Callipered 1.0 in. (25 mm) 1.3 in. (33 mm) 1.0 in. (25 mm) 1.3 in. (33 mm) 1.8 in. (46 mm) 2.5 in. (64 mm)

Open Area Measured 70% 70% 70% 70% 75% 75%

Minimum Rib Thickness Callipered 0.03 in. (0.76 mm) 0.03 in. (0.76 mm) 0.05 in. (1.27 mm) 0.05 in. (1.27 mm) 0.05 in. (1.27 mm) 0.05 in. (1.27 mm)

Junction Thickness Callipered 0.114 in. (2.9 mm) 0.114 in. (2.9 mm) 0.157 in. (4.0 mm) 0.157 in. (4.0 mm) 0.174 in. (4.4 mm) 0.174 in. (4.4 mm)

GEOGRID PROPERTIES TEST METHOD BX1100 BX1200 BX1300

Load Capacity/Structural Integrity MD CMD MD CMD MD CMD

Tensile Strength @ 2% Strain ASTM D 6637-01 280 lb/ft (4.1 kN/m) 450 lb/ft (6.6 kN/m) 410 lb/ft (6.0 kN/m) 620 lb/ft (9.0 kN/m) 380 lb/ft (5.5 kN/m) 650 lb/ft (9.5 kN/m)

Tensile Strength @ 5% Strain ASTM D 6637-01 580 lb/ft (8.5 kN/m) 920 lb/ft (13.4 kN/m) 810 lb/ft (11.8 kN/m) 1,340 lb/ft (19.6 kN/m) 720 lb/ft (10.5 kN/m) 1,200 lb/ft (17.5 kN/m)

Junction Strength GRI-GG2-01 98 lb (0.44 kN) 111 lb (0.49 kN) 151 lb (0.67 kN) 167 lb (0.74 kN) 156 lb (0.69 kN) 276 lb (1.23 kN)

Junction Effi ciency Calculated 93% 93% 93% 93% 93% 93%

Flexural Rigidity ASTM D 5732-95 250,000 mg-cm 250,000 mg-cm 750,000 mg-cm 750,000 mg-cm 450,000 mg-cm 450,000 mg-cm

Aperture Stability Modulus COE-Kinney, 2001 3.2 kg-cm/deg 3.2 kg-cm/deg 6.5 kg-cm/deg 6.5 kg-cm/deg 5.8 kg-cm/deg 5.8 kg-cm/deg

Tensar International Corporation (Tensar) reserves the right to change its product specifi cations at any time. It is the responsibility of the specifi er and purchaser to ensure that product specifi cations used for design and procurement purposes are current and consistent with the products used in each instance. Tensar warrants that at the time of delivery the geogrid furnished hereunder shall meet its published specifi cations as of the date of manufacture of the product. NO OTHER WARRANTY, EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY OF FITNESS FOR A PARTICULAR PURPOSE, IF PROVIDED AND ANY AND ALL SUCH OTHER WARRANTIES ARE SPECIFICALLY EXCLUDED. The sole remedy to the purchaser or user of our products for breech of the above-mentioned warranty is the replacement of the geogrid material. Notifi cation of any such breech shall be made within three (3) months of product delivery and prior to installation. The applicable product specifi cation supersedes all prior specifi cations for the product. Unless indicated otherwise, values shown are minimum average roll values determined in accordance with ASTM D4759.

1Department of the Army, U.S. Army Corps of Engineers, “Geogrid Reinforced Base

Courses for Flexible Pavements for Light Aircraft: Test Section Construction, Behavior

Under Traffic, Laboratory Tests, and Design Criteria,” DOT/FAA/RD-92/25, p.26.


4

Structural Geogrid Properties – The Significance Explained >

The Flexural Rigidity Test demonstrates that Tensar

BX Geogrids are stiff and do not sag like geotextiles.

The aperture stability modulus testing apparatus

for biaxial geogrids.

Aperture Size: The inside dimension of the geogrid

openings measured with calipers. The openings must be

large enough to permit “strike through” of aggregate fill,

but small enough to ensure thorough interlocking with

aggregate particles.

Open Area: The area of openings as a percentage of the

total (horizontal) area [Percent open area is measured by

photocopying a representative specimen of geogrid laid

flat, and then adding the weights of (cut-out) pieces of

paper representing apertures and dividing this sum by

the total weight of paper]. A relatively high percentage of

the horizontal coverage area must be open to facilitate

interlocking and stress transfer between geogrid ribs

and aggregate particles.

Rib & Junction Thickness: The vertical, top-to-bottom,

dimension of the geogrid. Geogrid must be sufficiently

thick to secure aggregate particles (which strike through

the geogrid plane) in place, and thus restrict their lateral

movement. Geogrid thickness also relates to robustness;

the geogrid must be sufficiently robust to withstand heavy

earthwork construction-induced stresses to prevent

installation damage.

Secant Aperture Stability Modulus (a.k.a. TorsionalStiffness): The geogrid’s resistance to deformation under

torque – specifically a given torque (moment) of 20 cm-kg.

The test quantifies the geogrid’s capacity to maintain its

aperture configuration. Since each aperture is created by a

junction of ribs at each corner, the procedure essentially

amounts to holding a junction node and then twisting it

in its horizontal plane, as one can demonstrate with thumb

and forefingers. Torsional stiffness provides the single-best

correlation with Traffic Improvement Factor (a.k.a. Traffic

Benefit Ratio) from full-scale independent tests.

Flexural Rigidity (a.k.a. Flexural Stiffness): The geogrid’s

resistance to sag under its own weight. The test quantifies

the geogrid’s capacity to maintain its “diving-board” shape.

In the test procedure, a geogrid specimen is pushed off

the edge of a platform as a cantilever. Flexural rigidity is the

only direct measure of the so-called “snowshoe effect” of

load distribution.

Junction Strength/Efficiency: The geogrid’s capacity at

the intersections of longitudinal and transverse ribs. The

test procedure is analogous to holding one’s outstretched

arms while pulling the legs. Geogrid junctions must

efficiently transfer load from rib to rib in any direction within

the horizontal plane to be effective in load distribution.


Flexible pavement systems often fail prematurely due to

progressive lateral displacement and a weakening of the

base course. This results in rutting and eventual cracking

of the pavement surface. The Spectra System provides

confinement of the aggregate particles in the base course,

thereby maintaining structural capacity and improving the

performance of the pavement system.

Base reinforcement applications are traditionally used

with permanent roads paved with asphalt cement concrete.

In contrast with the primary application of geogrids –

improvement of weak subgrades – base reinforcement

is characterized by relatively firm subgrades with a

CBR > 3. If the subgrade is weak, an additional layer

of aggregate and BX Geogrid may be used to strengthen

it before placing the base reinforcement layers.

In base reinforcement applications, the critical failure

mechanism is lateral spreading of the base course

away from the wheel path. The inclusion of BX Geogrids

provides lateral confinement of the base to enhance

pavement performance with an increase in pavement

life, a decrease in required pavement thickness or

a combination of the two (Figure 3).

Reduce Component Thickness

BX Geogrids can reduce the base thickness required to

support a specified amount of heavy traffic by as much

as 25% to 50%.

In a trial conducted by the Wyoming Department of

Transportation, two test sections of a state highway

were monitored for performance. The control section

was designed to carry 310 ESAL’s per day for a 20-year

design life. The adjacent test section’s base thickness

was reduced by one-third, and Tensar BX1100 Geogrid

was installed at its base-subgrade interface.

Using a falling weight deflectometer (FWD) to measure the

rut depth at the surface and the surface deflection, the

relative performance of the two test sections was found to

be virtually identical after a three-year period (Figure 4).

Increase Pavement Life

Numerous trials have been conducted to assess the

benefits of using BX Geogrids in base reinforcement

applications. In these trials, the performance of an

unreinforced control section was compared after

Base Reinforcement >

The Spectra System

extends the life

of a roadway by

providing confinement

and maintaining

structural capacity.

Figure 3

AsphaltAggregate

Base Course

SubgradeUnreinforced –Aggregate moveslaterally under traffic loading.

Reinforced – Aggregateis confined in the zoneimmediately above the BX Geogrid andtherefore lateralmovement is reduced.

Asphalt Concrete

Granular Fill or Subbase

Tensar BX

Geogrid

Aggregate BaseAggregate Base

The Spectra System is used to improve the subgrade, to reinforce the base course, or do both. Shown here are typical placement elevations for Tensar BX Geogridsbeneath paved roadways.

Subgrade


Traffic Benefit Ratio (TBR)

trafficking to the performance of a section that included

a geosynthetic.

When addressing extension of pavement life, it is

necessary to consider the traffic benefit ratio (TBR).

TBR is defined as the ratio of cycles-to-failure in a geogrid-

reinforced section compared with an unreinforced section

of the same thickness. As indicated in independent,

full-scale testing conducted by a number of research

entities, the TBR varies significantly with different geogrids.

Once the TBR has been determined for a specific

geogrid, it can be multiplied by the design ESAL’s for

an unreinforced pavement section to determine the

performance of that section when a geogrid is included.

For relatively thin aggregate base sections, the geogrid

is normally placed at the bottom of the layer; for thicker

bases (10 inches or greater), it is placed toward the center

of the layer.

Retention of Stiffness

The biaxial geogrid within a reinforced pavement is able

to maintain the stiffness characteristics of the aggregate

base throughout the life of the pavement structure. This

enables the base layer to be modeled with mechanistic

design methods to ensure the service life of the structure.

With full-scale research and empirical data collected from

projects, stiffness retention has been demonstrated to

be a vital feature of base reinforcement applications.

The confinement mechanism unique to biaxial geogrid

reinforcement captures the residual stress of the aggregate,

resulting in a base layer modulus that is maintained

throughout the life of the pavement structure. This results

in longer lasting pavements and lower maintenance costs.

Proven Through Research and in the Field

The Spectra Roadway Improvement System has been

studied for base reinforcement applications for over

20 years. It can also handle the toughest test of all for base

reinforcement – real world performance. Tensar BX Geogrids

have been used by the FHWA, state DOTs, local county and

municipality agencies, and private owners and developers,

consistently demonstrating their economic and structural

value. Without question, the Spectra System has proven

its reliability in a variety of conditions time and time again.

6

0.79 in. friction course

4 in. hot mix

granular base

Subgrade(CBR = 4)

11 in.

17 in.

ControlSection

TensarBX1100

Wyoming Department of Transportation Designed for 310 ESAL’sper day for 20 years

0.20

0.15

0.10

0.05

0

With BX1100

Control

Measured performanceafter 3 years in service

0.116 in.

0.013 in.

Rut Depth

Measurement method

Defl

ecti

on/R

utin

.

FWD Deflection

0.012 in.

0.123 in.

Number of passesin reinforced section

Number of passesin control section

DeformationControl

200 600

Number ofpasses

Reinforced1.0 in.

0.50 in.

600= = 3

200TBR for 1 in. =deformation

Comparison of unreinforced and reinforced pavements in Wyoming.Figure 4


Empirical data reveals three distinct features that

Tensar® BX Geogrids provide in base reinforcement

of flexible pavements:

• Reduction in pavement components

• Extension of service life

• Retention of stiffness over time

University of Illinois at Urbana-Champaign

A full-scale accelerated pavement test was conducted at

the University of Illinois at Urbana-Champaign, under the

direction of Dr. Imad L. Al-Qadi and Dr. Erol Tutumluer, to

evaluate the performance of Tensar Geogrids in pavements.

The study’s main objective was to develop a mechanistic

analysis model for inclusion of geogrids in flexible

pavements by testing full-scale pavement sections and

measuring the pavement’s response to loading using

pavement instrumentation.

Nine instrumented, full-scale flexible pavement test

sections were designed and built to evaluate and quantify

the effectiveness of geogrid reinforcement. The sections

were divided into three categories based on the total

thickness of the pavement system structure.

The UIUC research revealed that the presence of geogrid

in the reinforced sections had a pronounced impact on

the response and performance of the base course in

comparison with the unreinforced section. The incremental

improvement demonstrated by the geogrid-reinforced

sections validated the assumption that the degree of

enhancement offered by base reinforcement varies with:

• Aggregate thickness

• Asphalt thickness

• Subgrade support

• Aggregate quality

• Geogrid type and placement

• Moisture, traffic and other factors

The research resulted in the development of a mechanistic

model through which the incremental benefit associated

with Tensar Geogrid reinforcement can now be reliably

predicted. These findings aid in validating historical and

empirical full-scale research, which continues to set the

foundation for base reinforcement applications throughout

the world.

Base Reinforcement (continued) >

The University of Illinoisused the AcceleratedTransportation LoadingSystem (ATLAS) to testthe effectiveness ofTensar® BX Geogrid. The responses of thepavement sectionswere then measured.

The post-trafficked trench

section (on the right)

reveals a dramatic

reduction in asphalt and

aggregate deformation

due to the inclusion

of Tensar BX Geogrid

when compared to the

unreinforced section

pictured on the left.

Fatigue CrackingPavement

Deformation

Reinforced with

BX Geogrid


8

Distance from load centerline (mm)

-400 -200 0 200 400

Dyn

amic

sur

face

de

form

atio

n (m

m) 3

2

1

0

ControlWoven GeotextileTensar BX

Cycle number0 50000 100000 200000

Dyn

amic

ver

tica

l str

ess

(kPa

)

80

60

40

20

0

ControlTensar BX

Figure 5 Figure 6

0.9

Grid M Grid T Control Grid F

14 in. Aggregate Base

Grid C BX1100 BX1200

0.9 1.0 1.1

1.6

2.7

4.7

Subgrade (CBR = 3)

Traf

fic B

enefi

t Rat

io

Results from a U.S. Army Corps of Engineers Trial

5

4

3

2

1

0

2 in. Asphalt Concrete

Pavement Section

U.S. Army Corps of Engineers

In 1990 and 1991, the U.S. Army Corps of Engineers,

on behalf of the Federal Aviation Administration (FAA),

investigated geogrid-reinforced base courses for flexible

pavements used by light aircraft (prior research used tank

and truck loadings). This investigation involved full-scale

field-testing of reinforced pavement sections using a

30,000-pound single-tire load. The objective of the

research was to determine the traffic benefit ratio (TBR)

for flexible pavements for light aircraft.

Surface deformation measurements were periodically

taken after a specified number of load cycles for each test

section. The test sections consisted of two inches of

asphalt, 14 inches of base aggregate and a subgrade CBR

of 3%. The TBR at one-inch deformation for Tensar BX1100

Geogrid and Tensar BX1200 Geogrid was 2.7 and 4.7,

respectively. The TBR for the other commercially available

geogrids tested varied from 0.9 to 1.6. These results

confirmed that the inclusion of Tensar BX Geogrids

significantly enhanced the performance of flexible

pavements which led the FAA to draft a performance-

based specification that captured the necessary physical

properties for geogrid to enhance performance.

Montana State University – Bozeman

From 1996–1999, Montana Department of Transportation

researchers were tasked with providing experimental data

that could be used to further identify the mechanisms

of geosynthetic reinforcement that lead to enhanced

pavement performance. A small-scale stationary plate

was used to distribute stress to an asphalt-aggregate-

subgrade system; Tensar BX Geogrid, geotextile and a

control section were all tested to monitor deformation

at the surface during and after loading.

This study clearly demonstrated Tensar BX Geogrid’s

reinforcement and confinement mechanism associated

with base reinforcement applications. The stiffness of the

aggregate layer was enhanced as a result of the geogrid

placement at the base-subgrade interface; dynamic surface

deformations were reduced by a factor of nearly 3.0

(Figure 5). In addition, the stiffness retention mechanism

was demonstrated through stress measurements taken

within the subgrade, indicating that Tensar BX Geogrid

greatly reduced the load influence over repeated load

cycles in comparison with the control section (Figure 6).


Expedient – But Efficient?

Phased construction has become a common practice in

recent years, particularly in residential developments, in

order to gain site access quickly. With phased construction,

contractors build a roadway, first placing a layer of

aggregate followed by an asphalt base. Once overall site

construction is complete, the remaining asphalt surface

course is applied.

The process may be expedient, but it’s not always

structurally efficient; phased construction can result in an

80% reduction in trafficking capacity of the finished road,

leading ultimately to premature road failure. Since a

roadway is subjected to its most punishing traffic during

site construction, surface cracking can begin to appear

within a year of its completion, most commonly in the form

of “alligator cracking.” Once cracking begins, deterioration

accelerates quickly. The problem is literally below the

surface, deep within the pavement structure, and standard

surface rehabilitation measures will not resolve it.

BX Geogrids Increase Capacity, Improve Long-Term Performance

These three pavement section diagrams, reflecting current

AASHTO methods for the design of flexible pavements,

illustrate the benefits of using Tensar BX1200 Geogrid

during phased construction. An unreinforced roadway

(Figure 8) can lose 80% of its trafficking capacity as a

result of phased construction. But a roadway reinforced

with BX1200 Geogrid (Figure 9) during phased

construction can outperform an unreinforced roadway with

its final asphalt surface layer applied (Figure 7). Clearly,

reinforcement with Tensar BX1200 Geogrid is the cost-

efficient alternative, providing exceptional performance

both during phased construction and throughout the

service life of the pavement.

Phased Construction of Asphalt Pavements >

Tensar BX Geogrids add performance

and cost efficiencies to today’s

phased construction practices.

Unreinforced pavement during phased

construction (10,000 ESAL’s)

Pavement reinforced with BX1200

Geogrid during phased construction (60,000 ESAL’s)

Traffic capacities calculated with SpectraPave3™

Software using current AASHTO design guidelines.

Figure 8 Figure 9Unreinforced finished pavement

(55,000 ESAL’s)

Figure 7


Creating a Uniform Subgrade Elevation

Uneven Elevations Compromise PerformancePavement sections at large retail store developments

are typically designed to accommodate both heavy-duty

(delivery trucks) and light-duty (shoppers) trafficking

conditions. The need to install thicker layers of asphalt or

unreinforced aggregate in heavy-duty sections can result in

aggregate-subgrade interfaces at lower depths (Figure 10)

relative to the light-duty section.

This may cause a number of problems. Of greatest

significance, water entering the pavement can accumulate

in these recesses. As a result, subgrade strength will

diminish as the base becomes saturated, leaving the

pavement weaker in areas where the greatest strength

is required.

BX Geogrids: the Uniform SolutionBy reducing required aggregate and asphalt thicknesses,

Tensar BX Geogrids can be used to create a uniform

subgrade elevation for both heavy-duty and light-duty

pavement sections (Figure 11), resulting in a number

of engineering and economic benefits:

• Settlement of water into deeper recesses of heavy-

duty sections is eliminated, maintaining pavement

strength and reducing the possibility of freeze-thaw.

• Light-duty sections better withstand unexpected

loads from delivery trucks and other heavy traffic.

• Construction is simplified with reduced stake-out

procedures, soil excavation and disposal.

• Fewer materials and quicker construction save

overall project time and expense.

By creating a uniform subgrade elevation, Tensar

BX Geogrids significantly improve the performance

of pavement sections throughout larger retail

store developments.

Large Retail Store Developments >

10

Tensar BX Geogrids create

uniform surfaces for light-duty and

heavy-duty pavements common

to retail developments.

1.5 in. ACC Surface 1.5 in. ACC Surface1.5 in. ACC Surface

3 in. ACC Base 3 in. ACC Base4 in. ACC Base

9 in. Aggregate etagerggA .ni 9etagerggA .ni 9

Subgrade SubgradeSubgrade

Standard Duty Heavy Duty Standard Duty

142,000 ESAL’s 321,000 ESAL’s 142,000 ESAL’s

1.5 in. ACC Surface 1.5 in. ACC Surface1.5 in. ACC Surface

3 in. ACC Base 3 in. ACC Base3 in. ACC Base

6 in. Aggregate etagerggA .ni 6etagerggA .ni 6

Subgrade SubgradeSubgrade

BX1200 Geogrid dirgoeG 0011XBdirgoeG 0011XB

Standard Duty Heavy Duty Standard Duty

165,000 ESAL’s 330,000 ESAL’s 165,000 ESAL’s

Pavement section reinforced with BX Geogrids

Traffic capacities calculated using current AASHTO design

guidelines; for details, consult current (1993) Design Guide

and Interim Standard PP 46-01.

Figure 11Unreinforced pavement section.Figure 10


Weak foundation soils (subgrades) are a common problem

in road construction. This could be a short-term issue

if constructing a temporary access road, or a long-term

problem if a permanent road is built over a soft subgrade.

In either case, large deformation of the subgrade will lead

to a rapid deterioration of the paved or unpaved surface.

The interlocking action of aggregate and Tensar BX Geogrids

results in a stiffened granular platform. This platform

enhances load distribution, much like a snowshoe

distributes a person’s weight evenly over soft snow. This

“snowshoe effect” generated from BX Geogrids reduces

the stress applied to the subgrade (Figure 12).

Improve Site Access

BX Geogrids provide access to a site even in the most

severe soft soil conditions. The geogrid is simply rolled

out with a layer of granular fill over the top resulting in a

free-draining working platform or a reliable access road.

Reduce Aggregate

In test after test, Tensar BX Geogrids have shown their value.

They enable a thinner layer of fill to be used with the same

capacity and serviceability as a thicker unreinforced layer.

Generally, the fill reduction is between 40% and 60%

(Figure 13). The Giroud-Han Design Method discussed on

pages 13 and 15 outlines the approach to determine the

required granular fill thickness.

Reduce Undercut and Backfill

Removing contaminated or poor quality soil is expensive –

especially when disposal to a licensed landfill is necessary.

An additional benefit of the reduced aggregate thickness

described above is that the amount of undercut is also

reduced. This results in savings of both time and money.

Subgrade Improvement >

By creating a stiffened platform

and evenly distributing load,

BX Geogrids reduce the stress

applied to the subgrade.

By using BX Geogrids the unbound aggregate can be

reduced by about 40% to 60%.

Unreinforced

Vertical pressurewithout Geogrid

Vertical pressurewith Geogrid

Reinforced

Soft Subgrade

Tensar GeogridSub-base

Granular Fill

Figure 13Snowshoe Effect – Tensar BX Geogrids distribute heavy

loads over soft soils just like a snowshoe supports the weight of a man

over soft snow.

Figure 12


A Cost-Effective Alternative to Chemical Stabilization

When chemical stabilization (lime, cement, etc.) is being

considered for a particular project, BX Geogrids provide a

cost-effective alternative with several advantages such as:

• Simple design procedures

• Easy installation (no skilled labor required)

• Better drainage

• No environmental concerns

• Immediate results (no curing period)

Reduced Maintenance

Tensar BX Geogrids are reliable and consistent. With

a reduced load spread evenly over highly variable

subgrades, compaction tends to be more uniform than

when no reinforcement is used. This leads to decreased

maintenance and repair costs both during construction

and over the life of the roadway.

Simple Installation

Installation of BX Geogrids is quick and simple. They are

supplied in lightweight rolls that are easy to handle and

cut in the field. They easily adapt to curves in the roadway,

utility projections and other unforeseen site obstructions.

“Stiffer is Better”

Tensar Biaxial Geogrids are stiff – they have high strength

in two directions. In fact, in Report DOT/FAA/RD–92-25,

sponsored by the USDOT/FAA, researchers at the U.S.

Army Corps of Engineers Waterways Experiment Station

(WES) concluded that, in evaluating geogrids “Stiffer

is better.” It’s this rigidity that outperforms other

commercially available geogrids. Over weak subgrades,

Tensar BX Geogrids distribute loads over a larger area,

reducing the pressure on the subgrade.

To keep weak soils from becoming a problem, choose

Tensar BX Geogrids for subgrade improvement

applications. BX Geogrids distribute loads more efficiently,

increase the effective bearing capacity of the subgrade,

reduce rutting and provide a better alternative to costly,

conventional stabilization methods.

12


Subgrade Improvement Design Using The Giroud-Han Design Method

The most significant advancement in geosynthetic-

reinforced road design in the last 20 years was published

by Drs. J.P. Giroud and Jie Han in the August 2004 edition of

the ASCE Journal of Geotechnical and Geoenvironmental

Engineering. Their approach combines bearing capacity

theory, empirical data from full-scale test sections and

monitored in-service roads.

The major advantages of using the Giroud-Han method are:

• Consideration of the effects of variation in base

course strength.

• Consideration of the number and size of load cycles

(axle passes) and the desired roadway performance.

• Consideration of how the load distribution angle

within the base course changes with time.

• Recognition that geotextiles and geogrids perform

differently in roads.

• Recognition that not all geogrids perform the same,

and that geogrid index properties can be used to

predict how a particular material will perform.

• Calibration and validation of the theoretical results

with laboratory and full-scale test data.

This new method facilitates the most accurate, reliable

and cost effective determination of the required aggregate

thickness for unpaved roads or working platforms when

designing with geogrid, geotextiles or aggregate only.

It has revolutionized how unpaved roads are designed

in subgrade improvement applications.

Subgrade Improvement Design Using theU.S. Army Corps of Engineers’ Method

In February 2003, the U.S. Army Corps of Engineers

published ETL 1110-1-189 titled, Use of Geogrids in

Pavement Construction.Their approach, based on the

methodology originally developed by the U.S. Forest

Service, distinguishes the performance of geotextiles

and geogrids as reinforcement components in subgrade

improvement applications.

The required aggregate thickness values determined using

the Corps’ design method are in general agreement with

those determined using the Giroud-Han method. However,

the latter provides more flexibility and therefore allows the

designer to consider a much wider range of design parameters.

Design Methods>

Quantifying the features

of Tensar BX Geogrids for

paved and unpaved surfaces.

The Giroud-Han Design Methodology, a

significant advancement in unsurfaced

roadway design, was published in the

ASCE Journal of Geotechnical andGeoenvironmental Engineering.


Base Reinforcement Design Using AASHTO Guidelines

Guidance for the design of flexible pavements is provided

in the current AASHTO (1993) Flexible Pavement Design

Guide. The AASHTO approach uses the thickness, layer

coefficient and drainage factor for each pavement

component to obtain an overall structural number. This

is then considered along with other parameters such

as defining the subgrade strength and level of reliability

required to obtain an allowable trafficking level (ESAL’s)

for a particular pavement section.

Geogrids can be included in a flexible pavement design by

referring to the method prescribed in AASHTO’s Provisional

Standard PP 46-01, “Recommended Practice for

Geosynthetic Reinforcement of the Aggregate Base Course

of Flexible Pavement Structures.” The allowable traffic load

determined for the unreinforced pavement is multiplied by

an appropriate traffic benefit ratio (TBR). The TBR value

used for a particular geogrid must be developed through

evaluation of the product in full-scale test sections.

The magnitude of the TBR determined in such tests

varies depending on several factors such as type of

geogrid and the overall geometry of the pavement

section being considered.

This empirical approach is the standard of practice

for base reinforcement design. However, new, more

accurate mechanistic-empirical design methods

incorporating BX Geogrids are currently being developed.

See page 7 for results from a study conducted by the

University of Illinois at Urbana-Champaign that validated

the new mechanistic-empirical model for designing

flexible pavements with Tensar® BX Geogrids.

Base Reinforcement Design Using the Kentucky Transportation Cabinet’sMethodology

In an adaptation of AASHTO’s methodology, the Kentucky

Transportation Cabinet’s (KYTC) Design Memorandum

Ref. 17-04, published in December 2004, describes an

approved method for inclusion of geogrids in base

reinforcement applications.

Using this approach, the target structural number for

a given number of ESAL’s is reduced in pavement sections

that contain geogrid reinforcement. The effect of the

geogrid’s inclusion is evident – the thickness of the

pavement section can be reduced with no performance

loss (Figure 14).

14

Control SectionTarget SN = 5.72

6 in.

23 in.

ACC

ABC

ACC

ABC

ACC

ABC

6 in.

18 in.

6 in.

14.5 in.

BX1100Target SN = 5.02

BX1200Target SN = 4.50

Equivalent Designs

for 5 million ESAL’s,

Subgrade CBR = 6

based on the

Kentucky

Transportation

Cabinet’s Design

Methodology.

With a reduced target

structural number, the

pavement section can

be reduced with no

performance loss.

Figure 14

ACC = Asphalt Cement Concrete

ABC = Aggregate Base Course


Cost Benefits of Tensar Biaxial Geogrids >

Tensar BX Geogrids prove

their value time and time again.

0.868 + (0.661 – 1.006J2)

1 + 0.204

logN PRr2

h = – 1

– 1 1–0.9e Nc Fc CBRsg

r

1.5rh

sfs

3.48CBRbc0.3

CBRsg

r

h

2

The cost savings offered by Tensar Biaxial Geogrids used

in conjunction with unpaved surfaces over soft soils are

realized through the Giroud-Han design method entitled,

“Design Method for Geogrid-Reinforced Unpaved Roads.”

Many owners, engineers and contractors recognize the

value of BX Geogrids through the aggregate reduction

calculated using this state-of-the-art design equation.

A review of the design equation and an example of this

method and the value offered by BX Geogrids are

discussed below.

Giroud-Han Method Equation

This method predicts the required fill thickness for an

unpaved surface based on live loading, subgrade support

and a prescribed level of serviceability. The equation for

the determination of unreinforced and geogrid reinforced

fills over soft soils per the Giroud-Han Method is shown:

Where:h = base course thickness (in./mm)

P = wheel load = axle load/2

J = aperture stability modulus of the geosynthetic

(m-N/degree)

Junreinforced = 0 JBX1100 = 3.2

Jgeotextile = 0 JBX1200 = 6.5

r = radius of the equivalent tire contact

area (meters)

CBRbc= aggregate CBR

CBRsg = subgrade CBR

N = number of axle passes

s = maximum rut depth (mm)

fs = rut depth factor = 75 (when s quoted in mm)

fc = factor relating CBR of subgrade to

equivalent cu value

Nc = bearing capacity factor

Nc–unreinforced = 3.14 Nc–geotextile = 5.14

Nc–geogrid = 5.71

cu = undrained cohesion of subgrade soil (kPa)

The equation is solved iteratively until the value base course thickness,

h, is determined.


Example Problem

A haul road is needed to accommodate 400 passes of a

3-axle dump truck, with axles carrying 20,000 lbs apiece

and tires inflated to 100 psi. The subgrade soil, fill

material, and serviceability of the road (rut depth) are

characterized below:

• Subgrade Soil = Medium/Stiff Consistency

• Measured Subgrade CBR = 1.6

• Aggregate Fill CBR = 80

• Allowable Rut Depth = 1.5 inches

• Aggregate Cost (installed) = $20.00/ton

Determine cost savings per unit area (SY) with a Tensar®

Biaxial Geogrid and a woven geotextile.

16

$150,000

$100,000

$50,000

$0

1.000 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00

Cost Savings vs. Subgrade CBR

Subgrade CBR

$ S

avin

gs/

Runnin

g M

ile

Tensar BX1200 Tensar BX1100 Woven Geotextile

Typical cost savings per running mile of a 20 ft wide permanent haul road. Aggregate Cost = $20.00/ton in-place.

Example of cost savings realized by a BX Geogrid reinforced haul

road compared to an unreinforced and geotextile section.

Figure 15

Solution:For 1,200 axles passes (400 x 3 = 1,200),

thicknesses yielded from the Giroud-Han Method,

solving iteratively for “h” are:

• Unreinforced: 24 inches

• With Tensar BX1200: 11 inches

• With Tensar BX1100: 15 inches

• With Woven Geotextile: 18 inches

For the given example, the savings offered by

Tensar BX1200, BX1100 and a woven geotextile are:

• With Tensar BX1200: 24 - 11 = 13 inches

• With Tensar BX1100: 24 - 15 = 9 inches

• With Woven Geotextile: 24 - 18 = 6 inches

Based upon an in-place aggregate cost of $20.00/ton, the

corresponding unit price per SY-inch of stone is $1.00. That is,

for every inch of fill reduced by the geosynthetic relative to the

unreinforced design, the end user will realize a $1.00/SY savings.

Therefore, the cost savings realized for each product is:

• Tensar BX1200 = $13.00/SY minus in-place cost for the geogrid

• Tensar BX1100 = $9.00/SY minus in-place cost for the geogrid

• Woven Geotextile = $6.00/SY minus in-place cost for the geotextile

See Figure 15 for an example of how Tensar BX Geogrids offer the

highest cost savings.

For pricing information on Tensar Biaxial Geogrids or to find a local

stocking distributor, please call 1-800-TENSAR-1.

Assumed Aggregate Unit Weight:

AGGREGATE IN-PLACE COST CONVERSION CHART

$/TON $/CY $/SY-in. $/TON $/CY $/SY-in.

$ 5.00 $ 8.98 $ 0.25 $ 22.00 $ 39.50 $ 1.10

$ 6.00 $ 10.77 $ 0.30 $ 23.00 $ 41.29 $ 1.15

$ 7.00 $ 12.57 $ 0.35 $ 24.00 $ 43.09 $ 1.20

$ 8.00 $ 14.36 $ 0.40 $ 25.00 $ 44.88 $ 1.25

$ 9.00 $ 16.16 $ 0.45 $ 26.00 $ 46.88 $ 1.30

$ 10.00 $ 17.95 $ 0.50 $ 27.00 $ 48.47 $ 1.35

$ 11.00 $ 19.75 $ 0.55 $ 28.00 $ 50.27 $ 1.40

$ 12.00 $ 21.54 $ 0.60 $ 29.00 $ 52.06 $ 1.45

$ 13.00 $ 23.34 $ 0.65 $ 30.00 $ 53.86 $ 1.50

$ 14.00 $ 25.13 $ 0.70 $ 31.00 $ 55.66 $ 1.55

$ 15.00 $ 26.93 $ 0.75 $ 32.00 $ 57.45 $ 1.60

$ 16.00 $ 28.73 $ 0.80 $ 33.00 $ 59.25 $ 1.65

$ 17.00 $ 30.52 $ 0.85 $ 34.00 $ 61.04 $ 1.70

$ 18.00 $ 32.32 $ 0.90 $ 35.00 $ 62.84 $ 1.75

$ 19.00 $ 34.11 $ 0.95 $ 36.00 $ 64.63 $ 1.80

$ 20.00 $ 35.91 $ 1.00 $ 37.00 $ 66.43 $ 1.85

$ 21.00 $ 37.70 $ 1.05 $ 38.00 $ 68.22 $ 1.90


Natural Filtration of Confined Aggregate

Separation is a term used to describe the ability of a

geosynthetic to restrain fine soil particles from migrating

into a courser overlying fill due to the presence of excess

moisture and pressure. This “contamination” phenomenon

is a common issue when building over soft soils. Common

sense might suggest that geogrids, because of their

apertures, cannot provide effective separation when

placed between aggregate fill and a soft, fine subgrade.

However, this is not the case. Testing and experience

have shown that stiff BX Geogrids do facilitate separation.

When properly graded aggregate fill is used, the aggregate

performs as a “natural filter.”

Implications of Routinely UsingGeotextiles for Separation

When geotextiles are used to construct roads over soft

soils, there is often no consideration of the consequences

of this action – the “blinding out” or clogging that may

occur as fine particles migrate into the geotextile layer.

Tensar BX Geogrids do not clog, but allow moisture

to move through their apertures, thus expediting the

consolidation of the underlying weak subgrade material.

Evidence That Separation Can Indeed Be Achieved Using Geogrid

Identical pavement sections using two different

BX Geogrids and a geotextile were constructed in a study

undertaken for the FHWA in the Department of Civil

Engineering at Montana State University; full results of

the study can be found in the report ref. FHWA/MT-99-

001/8138. The subgrade consisted of high plasticity clay

mixed and placed to attain a CBR value of 1.5. The study

concluded that, “For all test sections, mixing of the

subgrade and base course aggregate was not observed.”

A similar study was undertaken on behalf of the U.S.

Department of Transportation and the Federal Aviation

Administration and performed at the U.S. Army Corps of

Engineers Waterways Experiment Station Geotechnical

Laboratory (DOT/FAA/RD-92/25). Similar findings were

reported, “The BX Geogrid confines the subgrade material

below the base, preventing or limiting the amount of

subgrade rutting upheaval from penetrating into or

through the base material. Without geogrid confinement,

rutting upheaval can penetrate through the base layer.”

At a major truck parking lot, the opportunity arose to

excavate and inspect the condition of an unbound roadway

Separation >

Tensar BX Geogrid reinforced

aggregate provides effective

separation by using the natural

filtration properties of the

granular fill.

After 11 years of traffic in a truck parking lot, the boundary between the base and subgrade

remains distinct. Stiff Tensar BX Geogrid was the only material used to separate these materials.

Figure 16


following 11 years of heavy traffic. The original subgrade

consisted of a soft organic soil; a BX Geogrid had been

placed at the interface between the weak subgrade and

overlying aggregate. It can be seen in Figure 16 that the

interface is distinct, and there is no evidence of upward

pumping of the subgrade or downward migration of the

aggregate. It can also be seen that the geogrid itself

is flat and level.

Natural Separation Mechanism

In order to understand the mechanism that causes

separation, we can consider how a sieve functions. A - in.

sieve does not allow soil particles smaller than - in. to

pass through unless the sieve itself is vibrated. BX Geogrids

interlock with aggregate fill and inhibit movement at the

base-subgrade interface, preventing the movement of soil

particles, which causes the subgrade to “pump” up into the

aggregate. Without lateral or vertical movement, well-graded

aggregate base material can form a natural filter and

prevent pumping from occurring, keeping the base aggregate

uncontaminated (Figures 17 and 18). In addition, the pressure

reduction, which results from the snowshoe effect, helps to

reduce pore water pressures in the subgrade. This results in

a reduced driving force to maintain the movement of fines

up into the base material.

18

LEGEND:

In-place soil

D85 soilentrapped in filter

Soil migrated intofilter, held by D85size soil particles

Spherical particle “B” willjust pass through porespace between the threespheres. “A” = 6- timesthe diameter of “B.”

A

B

Particle Sizes Fundamentalto Piping Ratio Concept

D15 (filter)

D85 (subgrade)

Water Movement

Subgrade Nominal boundarybefore movement

Preventing Pumping with Natural Filters

Figure 17

Figure 18

The storage yard for

Tensar’s manufacturing

plant in Morrow, GA,

rests on “Georgia Red

Clay” topped by a

Tensar® BX Geogrid and

crushed aggregate.


It is possible to calculate whether a filter is required when

considering a BX Geogrid for the reinforcement of an

unbound aggregate layer. The potential for “pumping”

to occur between two materials can be determined by

calculating the Piping Ratio (as defined below). In order

to prevent “pumping,” it is necessary that the Piping Ratio

is less than 5.

Piping Ratio =

Where • D15 Filter = the grain size of the smallest

15% of the base aggregate or granular fill

• D85 Subgrade = the grain size of the largest

85% of the subgrade soil

Low plasticity subgrades consisting of silt or very fine sand

are inherently more mobile in moist conditions than clay

subgrades. When these soils are present, it is necessary

to undertake an additional filter check:

Average Size Ratio =

Where • D50f = the average grain size

of the base aggregate

• D50s = the average grain size

of the subgrade soil

Mechanical Separation Case Study –Tensar’s Manufacturing Plant, Morrow, Georgia

The storage yard for Tensar’s manufacturing plant in

Morrow, Georgia, rests on “Georgia Red Clay” topped by

Tensar BX Geogrid and a standard, well-graded DOT-type

aggregate base. The hard standing area supports a high

volume of heavy lifting equipment that generates

significant subsurface pressures. Examination of several

test pits excavated at the site revealed that the aggregate

fill, after several years’ service, is continuing to function

as a “natural” filter and that the geogrid reinforcement

maintains a clear separation between the aggregate and

the underlying subsoil (Figures 20 and 21). An examination

of the grading curve data shown below for the aggregate

and underlying “Georgia Red Clay” illustrates why this

is the case (Figure 19).

D15 Filter

D85 Subgrade

D50 Filter

D50 Subgrade< 25

“Georgia Red Clay,” like

most naturally occurring subgrades,

is not 100% clay-sized. In many

instances, like this one, there are as

many sand-sized particles as clay-sized

particles. As such, a DOT-grade base

is a very effective filter, as evidenced

by the piping ratio calculation.

Subgrade PI > 7; there is therefore,

no need to check the Average Size

Ratio (D50f/D50s).

Piping Ratio= D15f/D85s= 0.22/0.62= 0.4 < 5 OK*

3 in.

85%

15%

% F

iner

by

Wei

ght

100

80

60

40

20

0

Gravel

Subgrade (Georgia Red Clay)Pl = ~20 – 25

D15 = 0.22mmD85 = 0.62mm

Aggregate Fill(Graded Aggregate Base)

#4 #200 Silt & Clay

Filter Analysis – Manufacturing Plant Site

Sand

100 10

Grain Size (mm)

1 0.1 0.01 0.001

Figure 19

*

Separation (continued) >

< 5


20

A storage yard is created by applying a single lift

of graded aggregate base (GAB) atop Tensar BX Geogrid laid

over “Georgia Red Clay.”

After several years, an excavation through the BX Geogrid

interface shows that underlying red clay particles do not exist within

the overlying aggregate base.

Figure 20 Figure 21


There is often confusion regarding the performance

of geogrids versus geotextiles in paved and unpaved

roadway applications. Geotextiles can provide separation

and filtration functions but are typically not as effective

as Tensar BX Geogrids when used for reinforcement

purposes alone.

In soil reinforcement applications, the strength and stiffness

of a geosynthetic is only significant if it can be transferred

efficiently into the surrounding soil. With Tensar BX Geogrids,

this is achieved by “mechanical interlock.” For geotextiles,

the load from the soil cannot be transferred in the same

manner; instead, the geosynthetic acts as a “tensioned

membrane” (Figure 22).

Some of the disadvantages of geotextiles in reinforced

roadway applications include:

• Their extreme flexibility can cause “kinks” even

after installation. As a result, the benefits of the

“membrane effect” are only realized after significant

deformation of the roadway, which is required

to build up tension within the geotextile. This

can cause premature distress to the structure

and require early maintenance.

• Longer embedments of geotextile can be required

to provide anchorage beyond the loaded area

of a roadway.

• Fixed wheel paths are required in order to ensure

the long-term performance of geotextile-reinforced

roadways. As geotextiles are generally not

pretensioned in the field during construction, rutting

of the subgrade needs to take place before heavy

traffic loads can be carried. It is essential that this

rutting is maintained and that additional ruts do

not form in adjacent areas along the subgrade-base

course interface.

• The formation of ruts in the subgrade, causes water

to accumulate. This results in the deterioration of

the roadway being accelerated.

As a result of the above disadvantages, the effective

use of geotextiles as roadway reinforcement is typically

restricted to narrow, unsurfaced haul roads.

Reinforcement Comparison with Geotextiles >

Geotextiles and geogrids perform differently.

When used for reinforcement,

Tensar BX Geogrids outperform

geotextiles. Tensar GeogridReinforcement Geotextile

Confinement versus membrane effect.

TensarGeogrid Geotextile

Confinement effect Tensioned membrane effect

Figure 22


Geogrid Testimony

In the early 1990s, the U.S. Army Corps of Engineers

conducted a full-scale pavement trial comparing the

performance of four geotextiles and a Tensar BX Geogrid to

an equivalent pavement section containing no geosynthetic

reinforcement. One of the geotextile sections performed

approximately the same as the control section while

the other three performed significantly worse

(Figures 23 and 24).

It is interesting to note that in the Corps’ test, the section

containing the strongest geotextile performed the worst

of all the test sections. It is believed that at the relatively

low strains encountered in roadway applications, there

is no reinforcing effect of the geotextile, and its smooth

surfaces may promote sliding of the aggregate. In this

particular example, it is likely that the strongest geotextile

promoted the most slippage of the aggregate.

The Tensar BX Geogrid test section performed well

compared to the control section in the Corps’ trial. Based

on this work and a review of more than 100 technical

papers and publications, the Corps concluded, “If

geotextiles are included in the structure, no structural

support should be attributed to the geotextiles.”

22

130 lbs

Control G2 G4 G5 G6 Tensar BXGeogrid

247 lbs

472 lbs 1,000 lbs

BX gridG6G5G4G2Control

Number of Passes

Pass

esfo

r 2

in.R

utRu

tdep

th (i

n.)

576 lbs/ft at5% strain

1000 2000 3000 4000 5000 60000

5t Military truck

G2, G4, G5 and G6geotextiles, value isgrab strength

6000

5000

4000

3000

2000

1000

0

0

1

2

3

4

ControlTensar BX GeogridG5

G2G4G6

Comparison of geotextiles and geogrids in

USACE trafficking trial.

Trial Section

This access road near Mobile, Alabama, was a contractor’s

nightmare. The unpaved road, built with fabrics, rapidly failed

as trucks were delivering fill to the site.

This is the same site with the same soil conditions. However, there’s

an important difference – this time the road was built with Tensar BX

Geogrids and it held up under repeated truck traffic.

Figure 23

Figure 24


Chemical stabilization using lime is a traditional solution

when soft or expansive clay subgrade soils are encountered.

While this method of subgrade stabilization has been used

successfully in the past, there are a number of important

factors to consider when selecting lime.

Performance Considerations

Versatility: Geogrids are effective for all types of soil.

Lime works best in heavy clays that are free of soluble

sulfates, which can create damaging heave after a

pavement is constructed. The chemical reaction, known

as sulfate-induced heave, can significantly affect the

smoothness of a finished pavement surface. Repair of

the pavement can be time consuming and costly.

Durability: In areas where frequent wet-dry or freeze-thaw

cycles are common, the long-term durability of lime

stabilized subgrades can be significantly affected by the

solubility of lime. Geogrids are unaffected by freeze-thaw

cycling and bridge seasonally soft soils by stiffening the

base. Lime modified soils can lose strength over time,

particularly in climates with frequent wet-dry cycles. These

actions lead to cracking and in some cases, premature

failure of the pavement.

Drainage: The drainage characteristics of a pavement

are key to its long-term performance. Lime stabilization

does nothing to improve the drainage capacity of a

road founded on weak, impermeable soil. In fact, the

technique can make a poor situation even worse.

A geogrid-reinforced granular base or sub-base layer

promotes “positive drainage” of the road, leading

to enhanced long-term performance.

Strength: Geogrids increase the effective bearing capacity

of subgrade soils. Lime improves strength only within the

treated depth, and its strengthening effects can deteriorate

with time based upon a variety of factors. Due to the

confinement of the overlying aggregate fill, geogrid

reinforcement immobilizes the potential horizontal and

vertical movement of aggregate due to applied construction

and in-service pavement loading. This increase in the

aggregate base stiffness improves the bearing capacity of

the subgrade without having to chemically or mechanically

modify the subgrade soil itself.

Comparison with Lime Stabilization >

With Tensar BX Geogrids,

there’s no need for specialized

labor and equipment.


Uniform Installation: Geogrids provide uniform

strength and performance in areas where they are

installed. Lime-treated areas may be highly variable

with respect to the rate of application, depth of treatment,

uniformity of mixture into the soil, and soil type

consistency. Lime installation is highly dependent

on the contractor’s expertise.

Design Considerations

Simplicity: In order to have full confidence in a soft

ground solution incorporating lime stabilization, the

designer requires complete knowledge of the soil type,

strength and plasticity for all subsoil materials likely to

be encountered. In practice, this data is rarely available

for the entire area being treated; and when it is, there

is no accurate account of material variation across the

job site. A geogrid solution is engineered through

traditional bearing capacity analysis and requires an

indication of subgrade strength alone. This information

is generally freely available to the designer, and variation

in subgrade strength across the site can be easily and

inexpensively identified.

Efficiency: Tensar BX Geogrids are manufactured under

controlled conditions and provide a uniform product with

well-known design characteristics that are not subject to

field variability. Lime must be mixed on site under much

less controlled conditions; in practice additional lime is

typically specified to account for field variability, adding

unnecessary cost to the project.

Safety/Environmental: Geogrids do not require special

environmental controls. Lime stabilization requires

personal safety standards and environmental controls.

With lime, leaching is often a problem in the presence of

moisture flow and therefore this technique is inappropriate

in environmentally sensitive areas such as wetlands. The

high alkalinity of lime-treated soils can affect vegetation

adjacent to the area being treated. Precautions should

be taken prior to installation where vegetation must

be kept intact.

24

Soils containing soluble sulfates are not good candidates for lime

treatment. Note: The calcium in lime reacts to form ettrignite and the

resulting reaction can cause significant damage to pavement structures.

Even when using proper specialized equipment and experienced

contractors, construction with lime is highly dependent upon favorable

weather conditions.


Effective in all types of soils and

unaffected by weather conditions,

BX Geogrids can be installed quickly

and easily while keeping the project

on schedule.

Installation Considerations

General Access: Lime equipment may be difficult or even

impossible to operate over very soft soils. However, the

“snowshoe” effect provided by BX Geogrids allows

conventional construction equipment to operate over

these soils. Field research performed by the U.S. Army

Corps of Engineers revealed that Tensar BX Geogrids

enable equipment to be mobilized over extremely weak

subgrades (CBR = 0.4).

Uniformity: Geogrids are uniform products that install

easily and predictably. Lime must be skillfully mixed,

placed and compacted for optimum performance.

Non-uniformity of subgrade soils chemically modified

or stabilized with lime will affect the consistency of the

subgrade support.

Stability: Geogrids are suitable for installation in the

coldest and most inclement weather. Lime requires

dry conditions with little or no wind and temperatures

in excess of 40°F. It is also a requirement that the

moisture content of the subgrade soil be maintained

close to optimum.

Speed: Locally available geogrids install quickly over

problem soils. Lime stabilization requires mixing, curing

(mellowing) and compaction, which can take several

days even in optimum weather conditions. Geogrids

offer construction expediency over lime treatment,

because they provide mechanical rather than

chemical solution.

Uncomplicated: Tensar BX Geogrids do not require

a significant investment in equipment and labor to ensure

a quality installation, as is the case with lime stabilization.

Conventional labor and equipment may be utilized to

install the lightweight rolls of geogrid and aggregate.

Comparison with Lime Stabilization (continued) >

The dangerous environmental effects of lime to the air are often obvious.Lime stabilization requires mixing, curing and compaction, which can take

several days even in optimum weather conditions.


26


SpectraPave3™ Software

Tensar International Corporation breaks new ground

with the release of our industry-leading SpectraPave3™

Software. This new software allows the user to accurately

predict the performance of geogrid reinforced and

unreinforced conditions of both paved and unpaved

surfaces. The software offers three distinct analyses

modules (Figure 25) and cost analyses tools to evaluate

design options for paved roads, unpaved roads and

working surfaces which include:

• Subgrade Improvement

• Base Course Reinforcement – Standard Method

• Base Course Reinforcement – Advanced Method

Subgrade ImprovementDeveloped in accordance with the latest Giroud-Han

design methodology, this module allows the designer

to consider the cost benefits of using BX Geogrids

in unreinforced aggregate sections. Similar sections

containing geotextiles or other geogrid materials

can also be analyzed. The program output includes

a breakdown of aggregate savings, undercut savings

and overall project savings.

Base Reinforcement

This module was developed to allow the designer to

consider the benefits of using BX Geogrids in paved

roadway applications generally subjected to heavy

traffic over long periods of time. The calculations

undertaken are in full accordance with AASHTO’s

(1993) Flexible Pavement Design Guide and

Provisional Standard PP 46-01 (Figure 26).

Cost Analysis Tools

What’s more economical: a road reinforced with

BX Geogrids or a conventional section? The cost analysis

tools provide total in-place costs (and savings) for each

design option in dollars per unit area. This flexibility also

enables you to quickly predict performance and value

Design Tools >

Users have the option to navigate to three unique analysis

modules as well as a project information folder from the home page

of SpectraPave3 Software.

Both base reinforcement modules allow designers to

analyze an unreinforced and geogrid reinforced condition to realize

component reduction and/or extended pavement service life in a

flexible pavement section.

Figure 25 Figure 26

By using SpectraPave3™ Software

or the Subgrade Improvement Slide

Rule, cost savings can be calculated

quickly and easily.


28

engineer comparisons for a range of design scenarios

for the life of the pavement through the life-cycle cost

analysis tool found in the Advanced Method.

SpectraPave3 Software is available in CD-ROM format

supporting Windows 95, 98, 2000, XP and NT operating

systems. It may also be downloaded free of charge from

www.tensar-international.com. For more information or to

request your free copy, call 800-TENSAR-1 or contact your

local Tensar BX Geogrid distributor.

Subgrade Improvement Slide Rule

This handy, pocket-sized tool uses the same

state-of-the-art Giroud-Han Design Methodology

used in SpectraPave3 Software and also contains

a cost calculator to determine the financial advantages

of using BX Geogrids to reduce the required aggregate

thickness. The slide rule is primarily intended for use on

earthwork sites where quick decisions need to be made

concerning the use of geogrid reinforcement in subgrade

improvement applications. If the user knows the typical

prices for the supply of aggregate and the excavation and

disposal of any undercut material, a few simple steps can

be taken to reveal the economic advantages of using the

Spectra System (Figure 27).

The slide rule output is based on the SpectraPave3 Software Subgrade

Improvement module, which uses the state-of-the-art Giroud-Han Design Methodology.

The slide rule brings this advanced technology right to where it is needed most – the

project site.

Figure 27

The slide rule is available in three versions: English units,

metric units and translated into Spanish with metric units.

This easy-to-use tool will eliminate the uncomfortable

guesswork often associated with soft soils, and allows

the user to recommend economical solutions quickly

and with confidence. To order a Subgrade Improvement

Slide Rule, call 800-TENSAR-1 or contact your local

Tensar BX Geogrid distributor.


Installati0n >

Recommended Overlaps

SOIL DESCRIPTION CBR OVERLAP

Firm ≥ 4 1 ft

Soft Ground 2-4 1-2 ft

Very Soft Ground 0.5-2 2-3 ft

Extremely Soft Ground ≤ 0.5 3 ft

Figure 28

Site Preparation

Tensar BX Geogrids are quick and easy to install. A smooth

graded surface is ideal for placement of Tensar BX

Geogrids. Tree stumps, boulders and other protruding

objects should be removed or cut at ground level.

Conventional practices may be used (clearing and

grabbing, topsoil removal and compaction) to properly

prepare a subgrade prior to placement of the geogrid.

Unique measures should be considered for extremely

soft subgrades (CBR < 1.0) to minimize disturbance

of the site soils.

Placement of Geogrids

After preparing the site, the geogrids are rolled out over

the subgrade. A 1- to 3-ft overlap (shingle style, in the

direction of fill advancement) is sufficient to ensure

stability across the installation. The actual overlap length

and any anchoring pattern required is dependent on the

subgrade strength. The weaker the subgrade, the greater

the overlap length required (Figure 28).

Placement of Fill

The required fill thickness is dependent upon the subgrade

strength, quality of fill used and the expected amount of

traffic. This can be quantified by using the SpectraPave3

Software or the Subgrade Improvement Slide Rule. In

order to minimize potential damage to the geogrid during

compaction, the initial lift should be at least 6 in., but under

no circumstances less than 4 in. The initial lift thickness

used should be based on the subgrade strength and the

loadings imposed by the placement and compaction

machinery. In very soft soil conditions, it is prudent to dump

fill on stable ground and then push it out over the geogrid.

The dozer blade should be “feathered”upward and raised

as each lift is pushed out at the leading edge of fill

advancement. After the geogrid and overlying fill have been

placed, normal compaction methods may be used.

Utility Concerns

Utility trenches often need to be excavated after geogrids

and fill material have been placed. This raises the concern

that the geogrid’s ability to provide reinforcement in these

areas will be compromised. The solution is a simple repair

process that overlaps the geogrids as the service trench

is backfilled. Detailed installation requirements for

Tensar BX Geogrids are provided in the Spectra System

Installation Guide.

Tensar BX Geogrids can be used for projects involving utility trenches.


The Solution That Works Every Time >

The Spectra System Advantage

For more than 20 years, industry professionals have

been using structural geogrids from Tensar International

Corporation to reinforce unstable subgrades. With clear

advantages in performance, design and installation,

mechanical stabilization with Tensar BX Geogrids

provides a preferred technology for building economical,

long-lasting structures over challenging soils.

Our entire worldwide distribution team is dedicated to

providing the highest-quality products, service and

support. With a technically trained sales staff and an

in-house engineering department, Tensar International

Corporation keeps its systems at the forefront of today’s

design technology and market trends.

For more information on the Spectra Roadway

Improvement System please call 800-TENSAR-1, visit

www.tensar-international.com or e-mail [email protected].

We are happy to supply you with additional BX Geogrid

information, complete installation guidelines, system

specifications, design details, conceptual designs,

preliminary cost estimates, case studies, software

and much more.

For information on how Tensar BX Geogrids can be

used in rail applications, see our Spectra Rail System

overview brochure.

30

REFERENCES:

1 Webster, S.L. 1992. Geogrid Reinforced Base Course for Flexible Pavementsfor Light Aircraft: Test Section Construction, Laboratory Tests and DesignCriteria. U.S. Army Corps of Engineers Report No. DOT/FAA/RD-92-25. UnitedStates Army Corps of Engineers, Washington, D.C.

2 Huntington, George & Ksaibati, Khaled. 1999. Evaluation of Geogrid-Reinforced Granular Base, Proceedings: Recent Advances in theCharacterization of Geo-Materials, (June 13–17), University of Illinois –Champagne.

3 Al-Qadi, Tutumluer, Kwon, Dessouky. 2007. Accelerated Full-Scale Testing of Geogrid-Reinforced Flexible Pavements, Proceedings: TransportationResearch Board 86th Annual Meeting, (January 21–25), Washington, D.C.

4 Perkins, S.W. 1999. Geosynthetic Reinforcement of Flexible Pavements: Laboratory Based Pavement Test Sections. Final Report, FHWA/MT-99-001/8138. United States Department of Transportation, Federal HighwayAdministration, Washington, D.C.

5 Giroud, J.P. & Han, Jie. 2004. Design Method for Geogrid-Reinforced Unpaved Roads. I. Development of Design Method, American Society of Civil Engineers

Journal of Geotechnical and Geoenvironmental Engineering, (August); Volume 130, Number 8.

6 Department of the Army, U.S. Army Corps of Engineers. 2003. Use of Geogrids in Pavement Construction, Engineering Technical Letter 1110-1-189.

7 AASHTO. 1993. AASHTO Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, D.C.

8 AASHTO. 2003. Recommended Practice for Geosynthetic Reinforcement ofthe Aggregate Base Course of Flexible Pavement Structures. AASHTOPublication PP46-01. American Association of State Highway andTransportation Officials, Washington, D.C.

9 Kentucky Transportation Cabinet. 2004. Design Procedures for Roadbed Stabilization with Geogrids. Design Memorandum No. 17-04.

10 Cedergen, Harry R. 1989. See page, Drainage, and Flow Nets, Third Edition. John Wiley & Sons, New York, pgs. 153-156.

11 White, D.W. 1990. Literature Review of Geotextiles to Improve Pavements forGeneral Aviation Airports. U.S. Army Corps of Engineers Report No.DOT/FAA/RD-90/26. United States Army Corps of Engineers, Washington, D.C.


©2008, Tensar International Corporation. Certain products and/or applications described or illustrated herein are protectedunder one or more U.S. patents. Other U.S. patents are pending, and certain foreign patents and patent applications mayalso exist. Trademark rights also apply as indicated herein. Final determination of the suitability of any information ormaterial for the use contemplated, and its manner of use, is the sole responsibility of the user. Printed in the U.S.A.

Distributed by:

SPECTRA_BRO_7.08

Tensar International Corporation

5883 Glenridge Drive, Suite 200

Atlanta, Georgia 30328

800-TENSAR-1

www.tensar-international.com


Documents

67797 SpectraBrochure OP-Cindy6.0 9/24/08 2:30 PM Page 2 · For over two decades, Tensar® Biaxial (BX) Geogrids have offered owners, engineers and contractors signiﬁcant value