Subgrade Improvement

7/29/2019 Subgrade Improvement

1/12

Subgrade Improvement

for Paved and Unpaved Surfaces

Using Geogrids By Stephen Archer, P.E.

October 2008


2/12Enter #xxx at gostructural.com/infodirect


3/12Professional Development Advertising Section CONTECH Construction Products Inc. PDH 3

The use of geogrid reinforce-

ment is a common practice

for engineers, owners, andcontractors for building

structures over soft soil conditions.

First introduced in the United States

in the early 1980s, the unique char-

acteristics and mechanisms specific

to geogrids offer significant benefits

compared with the following conven-

tional construction practices:

excavation and replacement with

select fill material,

thick structural (pavement) sections

to account for weak subgrade soil

conditions,

chemical stabilization or modifica-

tion with calcium-based materials

(i.e., cement, lime, fly ash), and

stabilization with woven or non-

woven geotextiles.

A geogrid is defined as a geosyn-

thetic material consisting of

connected parallel sets of tensile

ribs with apertures of sufficient size

to allow strike-through of surround-ing soil, stone, or other geotechnical

material (Koerner, 1998; see Figure 1

on page PDH 4). Commercial geogrid

products marketed and sold today

include extruded punched-and-

drawn geogrids, woven and coated

geogrids, welded geogrids, and

geogrid composites. Structural biax-

ial geogrids can be used to reinforce

earth fill over soft ground and provide

a stable subgrade under flexible and

rigid pavements, unpaved roads,

railroad track beds, industrial yards,

equipment work platforms, parking

areas, and building foundations.

Many small and full-scale stud-

ies have been performed to better

understand how geosynthetics inter-

act with fill materials to contrast their

performance with unreinforced condi-

tions in a variety of civil engineering

applications. This historical empirical

data is the basis for development of a

number of design methods to quan-tify the fill thickness required over a

geosynthetic reinforcement element

to achieve a minimum level of service-

ability. As the use of geosynthetics in

soft soil conditions has evolved during

the last three decades, so has the

number of design methodologies and

criteria by which geosynthetics are

evaluated. This article addresses the

following two current design meth-

ods commonly used by engineers

within the United States and abroad:

The Giroud-Han Design Method

(2004) and The U.S. Army Corps of

Engineers Design Method (2003).

Geogrid reinforcementmechanisms

A subgrade soil beneath a paved or

unpaved surface can fail under load

in two ways: localized shear failure

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Learning Objectives Understand the primary mechanisms by which geogrids

reinforce granular fill over soft soils and how these

features translate to quantifiable fill savings relative to

conventional practice.

Develop an understanding of two design methods

endorsed by the American Society of Civil Engineers

and the U.S. Army Corps of Engineers for designinggeogrid-reinforced unpaved surfaces over soft soils.

Translate the granular fill savings to cost benefits that

can be realized relative to a geotextile solution and an

unreinforced solution.

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CONTECH Construction Products Inc.

Tensar International Corporation

Professional Development Series

Subgrade Improvement for Paved and Unpaved Surfaces Using Geogrids

By Stephen Archer, P.E.


4/124 PDH Professional Development Advertising Section CONTECH Construction Products Inc.


and deeper-seated

bearing capacity fail-

ure. Localized shear

failure, or base punch-

ing, typically occurs in the form of

severe deformation or rutting in soft

saturated subgrades when loadingexceeds the subgrade shear strength.

The subgrade beneath an unrein-

forced fill will fail in localized shear

failure at about half of the stress level

than the ultimate bearing capacity

of the subgrade. Premature failure of

a paved or unpaved surface due to

weak subgrades leads to costly full-

depth repairs that can be avoided

with good engineering judgment

at the time the section is designed.

Geogrids offer protection over weak

foundation soils because of the ability

of the material to act as a snowshoe

over soft, rut-prone conditions.

Geogrid reinforcement of granu-

lar fills over soft ground can prevent

localized shear failure of the subgrade

and therefore significantly increase

the effective bearing capacity of

the subgrade. In addition, geogrids

reinforce the granular fill through

confinement of the particles, stiffen-

ing the base layer for improved load

distribution.The net effect of these mechanisms

is a reduction in the fill thickness

required to provide stable foundation

support for a paving operation or for

the immediate trafficking of unpaved

structures such as haul roads or work-

ing platforms (see Figure 2).

In 2003, the U.S. Army Corps

of Engineers (Corps) identified and

defined the primary applications

for biaxial geogrid reinforcement

for paved and unpaved structures:

mechanical subgrade stabilization and

base reinforcement. In an engineer-

ing technical letter (ETL), the Corps

referenced three primary mechanisms

as being relevant to the interaction of

geogrid reinforcement and pavement

materials: lateral restraint, improved

bearing capacity, and tensioned

membrane effect (Perkins and Ismeik,

1997a; see Figure 3). The following

is summarized from the Corps ETL

1110-1-189 (page 3).

Lateral restraint Considered

the primary reinforcement mecha-

nism by the Corps document, lateral

restraint describes the ability of the

aperture geometry of a grid to confine

aggregate particles within the plane

of the material. This feature yields

a stiffening effect to the reinforced

granular material, both above and

below the geogrid (in the case of the

material being installed at the mid-

point of a granular fill), that results

in an increase in modulus of the rein-

forced layer.

Improved bearing capacity Typically associated with geogrid use

over soft subgrades, improved bear-

ing capacity describes a change in

the potential failure mechanism of

the subgrade from a localized shear

generally characterized as a deep

rutting failure to a general bear-

Figure 1: Biaxial geogrid and aggregate Figure 2: Granular fill thickness reduction achieved through a biaxial geogrid layer

Figure 3: Geogrid reinforcement mecha-

nisms of lateral restraint (top), improved

bearing capacity (center), and tensioned

membrane effect (bottom).



ing capacity failure. The result is an

improved effective bearing capacity of

the subgrade resulting from pressure

dissipation at the geogrid-subgrade

interface.

Tensioned membrane effect

Initial research suggested thatthe tensioned membrane effect was

the primary mechanism of geogrid

over soft ground. Subsequent stud-

ies have proven that geogrid offers

discernable structural enhancement

without significant rutting of the

subgrade layer. This is a key distinc-

tion of geogrids when compared with

geotextiles as it relates strain accumu-

lation within each layer of a paved or

unpaved structure. As punched and

drawn geogrids are manufactured by

pre-straining the polymer, yielding

an effective stress transfer of vertical

and horizontal stress, both woven and

non-woven geotextiles require the

strain be induced after the product is

installed, leading to rut accumulation

in the aggregate layer and subgrade

layer. The result is a structure that

may require frequent rehabilitation or

premature replacement, depending

on serviceability requirements and life

cycle cost valuation of the structure.

These mechanisms, unique togeogrid reinforcement, collectively

contribute to the interaction of gran-

ular fill with the open structure of

the geosynthetic. Research and thou-

sands of full-scale applications during

the last 30 years have yielded two

reliable design methods that now

give guidance for the use of geogrids,

as well as geotextiles, for constructing

unpaved surfaces over soft soils.

Giroud-Han Method (2004)Recognizing a need to advance

geosynthetic design for unpaved

surfaces, J.P. Giroud, Ph.D., and

Jie Han, Ph.D., published a design

method in the August 2004 edition of

the American Society of Civil Engineers

(ASCE) Journal of Geotechnical and

Geoenvironmental Engineering. Their

approach combines bearing capac-

ity theory with empirical data from

full-scale test sections and monitored

unpaved roads. Some distinctions

of the Giroud-Han method relative

to conventional geosynthetic road

design practice include the following:

consideration of the effects of varia-

tion in base course strength,

consideration of the number andsize of load cycles (axle passes) and

the desired roadway performance,

consideration of how the load distri-

bution 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

calibration and validation of the

theoretical results with laboratory

and full-scale test data.

The method accounts for, in addi-

tion to the factors considered by the

Giroud and Noiray (1981) methods

developed for the then U.S. Forest

Service, the strength/modulus of the

base material, the variations of the

stress distribution angles through the

base course, and the aperture stability

modulus strength property of the

geogrid. The theoretical model that

was initially developed was calibratedusing data from large-scale, cyclic plate

load tests directed by Mohammed

Gabr, Ph.D., at North Carolina State

University. These tests

were run for both rein-

forced and unreinforced

conditions with 6- and

10-inch-thick base courses placed on

a soft subgrade. Two reinforcement

geogrids were used for the testing Tensar BX1100 and BX1200.

The tests yielded data for pressures

on the subgrade and deformations

at the surface as functions of the

number of load cycles for the various

combinations of reinforcement and

base thickness. The pressure data was

used to estimate the load distribu-

tion angle and to quantify the effects

of base reinforcement and thickness

on both the initial angle and on the

changes in the angle with continued

applications of load. Layer elastic

theory was used to assess the effect

of the base course modulus on the

stress distribution angle. The newly

available test data made it possible

to develop this more comprehensive

and statistically accurate unpaved

road design method.

Giroud and Han (2004a) summa-

rized the significance of this cali-

bration effort: The design method

presented in this paper and the

companion paper is theoreticallybased and experimentally calibrated.

Therefore, it more accurately predicts

performance for both geogrid- and

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Figure 4: Aggregate thickness values using the Giroud-Han method for 2-inch rut depth,

80-psi tire pressure, 20-kip axle, and 1,000-axle-pass loading criteria




geotextile-reinforced

unpaved roads and for

unreinforced, unpaved

roads than do earlier meth-

ods developed by Giroud and Noiray

(1981) and Giroud et al. (1985). As

such, the method presented hereinsupersedes these previous methods.

In consideration of these principles

and the conventional practice of

load distribution theory, the follow-

ing equation was derived to predict

the required thickness of fill (h) to

provide the prescribed serviceability

for the given loading conditions and

soil subgrade support. In using this

equation, the designer is required to

solve iteratively for fill thickness (h):

where:

h = required base course thickness

(m)

J= geogrid aperture stability modu-

lus (m-N/degree)N= number of axle passes

P= wheel load (kN)

r= radius of the equivalent tire

contact area (m)

CBRsg= California bearing ratio (CBR)

of the subgrade soil

CBRbc = CBR of the base course

s = allowable rut depth (mm)

fs = factor equal to 75 mm

fc = factor equal to 30 kPa

Nc = bearing capacity factor, in which

Nc = 3.14 and J = 0 for unrein-forced base course;Nc = 5.14 and

J = 0 for geotextile-reinforced

base course; Nc = 5.71 and J =

0.32 m-N/degree for Tensar

BX1100-reinforced base course;

and Nc = 5.71 and J = 0.65

m-N/degree for Tensar BX1200-

reinforced base course.

The Giroud-Han Method is unique

in its approach of combining standard

bearing capacity theory and observed

practical performance. As such, it

presents the design engineer with the

most reliable method currently avail-

able for the design of unpaved roads.

The method was developed, cali-

brated, and validated with data fromfull-scale, field and laboratory tests

considering different geogrids. The

formulation can be refined further

to consider new geogrid products

and new research data as it becomes

available.

The Giroud-Han Method can be

expected, and it can be shown, to

give the most accurate predictions of

field performance for similar loading

conditions, base and subgrade prop-

erties, and for the specific geogrids

used in the various test programs.

Figure 4 demonstrates a specific

example comparing the output of a

design for an unreinforced section andsections reinforced with a geogrid and

a geotextile. Once output has been

calculated for the available options,

a cost-benefit analysis can then be

undertaken given the in-place costs

for both the aggregate fill and the

geosynthetic(s).

U.S. Army Corps of EngineersMethod (2003)

In February 2003, the Corps

published a design method consider-ing the use of geogrids and geotex-

tiles for paved and unpaved roads.

Its approach for unpaved surfaces,

based on the methodology originally

developed by the U.S. Forest Service,

distinguishes the performance of

geotextiles and geogrids as rein-

forcement components in subgrade

improvement applications.

The design charts developed by

the Corps are based on empirical data

obtained from full-scale test sections

undertaken at the Corps Research and

Development Center in Vicksburg,

Miss. This data was combined with the

old bearing-capacity design methodol-

ogy developed by Steward, et al., for

the U.S. Forest Service (1977). Basedupon the Corps independent, full-

scale testing (Webster, 1992), a mate-

rial specification was developed for

geotextile and geogrid products. The

geogrid specification recommended in

this document is shown in Table 1 on

page PDH 7.

Geogrid-reinforced aggregate

surface design using the Corps

method requires the design engi-

neer to select an appropriate Bearing

Capacity Factor, Nc

, for the geosyn-

thetic type being considered. The

Corps recommended the following

Ncvalues:

Nc = 2.8 without a geosynthetic,

Nc = 3.6 with a geotextile for conser-

vative designs, and

Nc = 5.8 with a geogrid.

The next step in determining an

appropriate granular fill thickness

is to determine the subgrade shear

strength, C (psi). This may be deter-

mined through conventional sheartesting in situ (shear vane, torvane,

pocket penetrometer, et cetera), or by

laboratory tests on extruded, undis-

turbed samples. The shear strength

of the soil can also be correlated from

alternative tests (field CBR, dynamic

cone penetrometer, et cetera). The

relationship recommended by the

Corps between the cone index, CBR,

and shear strength is presented in

Figure 5 on page PDH 7.

The subgrade bearing capacityused to calculate the required aggre-

gate thickness is determined in accor-

dance with Equation 2:

Subgrade Bearing Capacity =

C* Nc (psi) (Equation 2)

Once the subgrade bearing capac-

ity has been determined, the designer

can reference one of the three relevant

design charts (single wheel, dual wheel,

(Equation 1)



and tandem gear wheel weight) in theETL document to calculate the required

aggregate thickness. An example chart

for a single wheel load condition is

presented in Figure 6 on page PDH 8.

Resulting thickness savings with

the geosynthetic relative to the unre-inforced sections are substantial. A

minimum aggregate thickness of 6

inches is recommended by the Corps

for aggregate-surfaced pavements. To

facilitate a comparison of the design

methods described in

this article, an analy-

sis has been performed

using the same design

criteria used in the earlier example

describing the Giroud-Han method.

The results for the Corps method arepresented in Figure 7 on page PDH 8.

Design method comparisonA comparison of the Giroud-Han

and the Corps design input and output

reveal both similarities and differences

between the two methods (see Table

2 on page PDH 8). A similar sensi-

tivity study of the Giroud-Han and

the Corps methods was performed to

compare the predicted fill thickness

outputs for given conditions (Tingle

and Jersey, 2007). Figures 4 and 7

plot sample output such that each

method for the two types of geosyn-

thetic (geogrid and geotextile) can

be compared. A direct comparison of

required aggregate fill thickness using

each method reveals the following:

Generally, the Giroud-Han method

yields thicker aggregate required for

both the unreinforced and geotex-

tile-reinforced relative to the Corps

method.

Except for extremely soft subgradeconditions (CBR 0.5), the

Giroud-Han method yields thinner

aggregate required for the geogrid-

reinforced relative to the Corps

method.

Figure 5: Relationship between cone index, CBR, and shear strength, C (TM 5-518-8)

Table 1: Minimum biaxial geogrid specification requirements per the Corps method (2003)



The minimum thick-

ness allowed for traffick-

ing allowed by the Corps

method is 6 inches, while

the Giroud-Han method allows for a

minimum of 4 inches of granular fill.

Both methods suggest that geogridreinforcement requires less aggre-

gate fill when compared with

a geotextile for the same level of

serviceability and design criteria

Of the two methods reviewed, only

the Giroud-Han method addresses

the difference in index properties of

geogrids. Engineers, owners, and

contractors routinely compare the index

properties of commercially available

geosynthetics to determine the proper

selection of a product for a given appli-

cation. However, research has shown

that index properties alone do not

correlate to in-ground performance.

Accordingly, designers are encouraged

to seek manufacturer-specific, full-scale

empirical evidence that proves that the

performance predicted by each of the

methods reviewed in this article indeed

correlate to the geosynthetic manufac-

turer brand in question.

Cost-benefit analysisEssential to any design analy-

sis is the need for the in-place cost

of an alternative solution relative to

conventional practice. The primary

benefit that owners, engineers, and

contractors seek in using geosynthet-

ics is the potential for front-end cost

savings associated with raw material

Figure 7: Aggregate thickness values using the U.S. Army Corps of Engineers method for

2-inch rut depth, 80-psi tire pressure, 10,000-pound wheel load, and 1,000-axle-pass

loading criteria

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Figure 6: Aggregate-surfaced pavement design curves for single-wheel loads


8 PDH Professional Development Advertising Section CONTECH Construction Products Inc.

Table 2: Comparison of required aggregate thickness: Giroud-Han (G-H) and U.S. Army Corps of Engineers (USACOE) methods for

2-inch rut depth, 80-psi tire pressure, 20-kip axle, and 1,000-axle-pass loading criteria



use. In the case of aggregate-surfaced

roads, the raw material in question

is the aggregate itself. To realize the

value of the geosynthetic, a designer

is encouraged to explore the in-place

cost of both the geosynthetic and the

aggregate fill required to provide thedesigned service life of the structure

in question.

This relatively simple analysis can

be performed through weighted

average price data that is available

from most state departments of

transportation and other public enti-

ties that publish this information on

a monthly, quarterly, or annual basis.

The steps involved for such an analy-

sis include the following:

1) Determine the in-place cost of

aggregate per square yard-inch of

depth (see Figure 8).

2) Determine the in-place cost of the

geosynthetic of choice (geogrid or

geotextile).

3) Determine the required aggregate

fill thickness for an unreinforced

case for the given loading and

serviceability using either method

reviewed above.

4) Determine the required aggre-

gate fill thickness for a reinforced

case for the same loading andserviceability using geogrid and/

or geotextile.

5) Subtract the required reinforced

thickness from the required unre-

inforced thickness to determine

aggregate fill thickness savings for

each reinforced section.

6) Calculate the cost savings by multi-

plying the aggregate fill thickness

savings in inches (obtained in step

5) by the in-place aggregate cost

per square yard-inch of depth, andthen subtract the in-place cost of

the geosynthetic per square yard.

The output from the Giroud-Han

method shown in Figure 4 demon-

strates the potential cost savings

that can be realized using a layer of

geogrid reinforcement over soft soil.

For example, if a haul road is to be

constructed over a subgrade CBR

equal to 1.5 for a 2-inch rut depth,

20-kip axle load, and 80-psi tire infla-

tion, the required aggregate thickness

necessary is represented below:

unreinforced = 20 inches,

geotextile-reinforced = 14 inches,

and

geogrid-reinforced = 7 inches

If aggregate costs $20/ton in-place,

the savings realized for the geosyn-

thetic solutions equate to:

Geotextile-reinforced = (20

inches 14 inches) = 6 inches x ($1/

square yard-inch) = $6/square yard

minus geotextile cost

Geogrid-reinforced = (20 inches

7 inches) = 13 inches x ($1/square

yard-inch) = $13/square yard minus

geogrid cost

SummaryGiven present day challenges asso-

ciated with increasing raw material

pricing and dwin-

dling project fund-

ing, geosynthetics offer

owners and engineers a

proven, cost-effective alternative to

conventional building practice for

constructing unpaved haul roads andworking surfaces over soft subgrade

soil conditions. Significant initial cost

and construction time savings can

be realized through the inclusion

of a geogrid layer. Much empirical

evidence, along with full-scale and

small-scale research, has yielded reli-

able design methods for quantifying

the benefits of geosynthetics relative

to expensive alternates such as under-

cut-and-replace and chemical stabili-

zation or modification.

Current methods developed by

Giroud-Han and the U.S. Army Corps

of Engineers offer guidance in deter-

mining both the proper selection of

the geosynthetic type and the neces-

sary granular fill thickness to provide

Figure 8: Aggregate in-place cost conversion chart (unit weight = 133 pounds/cubic foot)




U.S. Army Corps of Engineers, 2003, Use of Geogrids in Pavement Construction, ETL

1110-1-189.

Giroud, J.P., and L. Noiray, 1981, Geotextiles-Reinforced Unpaved Road Design,

Journal of Geotechnical Engineering, Vol. 107, No. 9, pages 1233-1253, ASCE.

Giroud, J.P., and Han, J., 2004a, Design Method for Geosynthetic-Reinforced

Unpaved Roads: Part I Development of Design Method, Journal of Geotechnical

and Geoenvironmental Engineering, in press, ASCE.

Koerner, Robert M., 1998, Designing With Geosynthetics, Fourth Edition, Prentice

Hall, Upper Saddle River, N.J.

Perkins, S. W., and Ismeik, M., 1997a, A Synthesis and Evaluation of Geosynthetic

Reinforced Base Layers in Flexible Pavements: Part I, Geosynthetics International, Vol.

4, No. 6, pages 605-621.

Tingle, Jeb S., and Jersey, Sarah R., 2007, Empirical Design Methods for Geosynthetic-

Reinforced Low-Volume Roads, Transportation Research Record: Journal of the

Transportation Research Board, No. 1989, Vol. 2, Washington D.C., pages 91-101.

Webster, S.L., 1992, Geogrid Reinforced Base Course for Flexible Pavements for

Light Aircraft: Test Section Construction, Laboratory Tests and Design Criteria, U.S.

Army Corps of Engineers Report No. DOT/FAA/RD-92-25

References

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CE News Professional Development Series Reporting Form

Article Title: Subgrade Improvement for Paved and Unpaved Surfaces Using GeogridsPublication Date: October 2008 Valid for credit until: October 2010

Sponsors: CONTECH Construction Products Inc., Tensar International Corporation

Instructions: Select one answer for each quiz question and clearly circle the appropriate letter. Provide all of the requested contact infor-mation. Fax this Reporting Form to 513-645-7993. (You do not need to send the Quiz; only this Reporting Form is necessary to be

submitted.)

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optimal performance.

Given these methods,

the cost benefits of each

method and geosynthetic

solution may be realized.

Participants are encouraged to

download the U.S. Army Corps ofEngineers method reviewed within this

article, available online at www.usace.

army.mil/publications/eng-tech-ltrs/

etl1110-1-189/entire.pdf.



Professional Development Series Quiz

1. Of the reinforcement mechanisms described in the Corps'

ETL, which is considered to be the primary reinforcement

mechanism in unpaved structures?

a) Tensile membrane effect b) Lateral restraint

c) Improved bearing capacity d) Separation

2. Which of the following are true statements?

a) The Giroud-Han method considers how the load distribution

angle within the base course changes with time.

b) The Giroud-Han method considers the number and size of load

cycles (axle passes) and the desired roadway performance.

c) The Giroud-Han method recognizes that not all geogrids perform

the same.

d) The Giroud-Han method recognizes that geotextiles and

geogrids perform differently in roads.

e) All of the above statements are true.

3. According to the Giroud-Han method, what is the key index

property used to model the strength characteristic of the

geogrid element within the design equation?

a) Tensile strength at 5 percent strain

b) Tensile strength at ultimate strain

c) Junction strength

d) Aperture stability modulus

4. The Giroud-Han method is considered to have superseded

which of the following methods for designing unpaved

roads over soft ground conditions?

a) Giroud and Noiray (1981)

b) U.S. Army Corps of Engineers (2003)

c) Giroud et al. (1985)

d) Both a and c

e) All of the above

5. What are the key parameters needed to run a cost-benefit

analysis comparing an unreinforced design section with a

section reinforced with geogrid?

a) The in-place cost of the geogrid

b) The in-place unit cost of the granular fill ($/ton)

c) The unit weight of the granular fill

d) The thickness savings yielded for a geogrid-reinforced design section

e) All of the above

6. Which of the following statements is false?

a) Both the Giroud-Han method and the U.S. Army Corps of

Engineers method recognize that geogrids and geotextiles

perform differently in unpaved roads.

b) Both design methods demonstrate that aggregate fill thickness

required for geogrid is less than for geotextiles.

c) Both methods account for subgrade strength, wheel load, tirepressure, and the geogrid aperture stability modulus.

d) The Giroud-Han method was calibrated and validated around

full-scale testing.

7. Dynamic cone penetration tests reveala soft subgrade soil strength, CBR = 1.5.

Using the U.S. Army Corps of Engineersmethod, determine the required aggregatethickness (round up to the nearest inch) of anunreinforced, geotextile-reinforced, and geogrid-reinforced unpaved road section for a single-wheel loadweighing 10,000 pounds.

a) 15 inches (unreinforced); 13 inches (geotextile); and 9 inches(geogrid)

b) 18 inches (unreinforced); 16 inches (geotextile); and 12 inches(geogrid)

c) 12 inches (unreinforced); 10 inches (geotextile) and 6 inches(geogrid)

d) 16 inches (unreinforced), 10 inches (geotextile) and 9 inches(geogrid)

8. An analysis using the U.S. Army Corps of Engineers methodreveals a design for an unpaved road requires 14, 12, and8 inches of aggregate for unreinforced, geotextile, andgeogrid reinforced roads, respectively. What would be the

potential cost savings (per square yard) for a geogrid sectionrelative to an unreinforced design, given an in-place cost foraggregate fill of $22/ton?

a) $5.50/square yard minus the in-place cost of the geogrid

b) $6.60/square yard minus the in-place cost of the geogrid

c) $7.70/square yard minus the in-place cost of the geogrid

d) $8.80/square yard minus the in-place cost of the geogrid

9. Dynamic cone penetration tests reveal a soft subgrade

soil strength, CBR = 1.0. Using the Giroud-Han method,determine the required aggregate thickness (round up tothe nearest inch) of an unreinforced, geotextile-reinforced,

and geogrid-reinforced unpaved road section for an axleload weighing 20,000 pounds, 80-psi tire pressure, 2-inchlimiting rut depth, and 1,000 axle passes.

a) 21 inches (unreinforced); 14 inches (geotextile); and 10 inches(geogrid)

b) 14 inches (unreinforced); 20 inches (geotextile); and 7 inches(geogrid)

c) 23 inches (unreinforced); 17 inches (geotextile); and 9 inches(geogrid)

d) 26 inches (unreinforced); 19 inches (geotextile); and 11 inches(geogrid)

10. An analysis using the Giroud-Han method reveals a

design for an unpaved road requires 23, 14, and 9 inchesof aggregate for unreinforced, geotextile, and geogridreinforced, respectively. Assuming an installed costfor geotextile of $1.25/square yard, what would be thepotential cost savings (per square yard) for a geogridsection relative to a geotextile design given an in-place costfor aggregate fill of $23.50/ton?

a) $7.12/square yard minus the in-place cost of geogrid

b) $9.33/square yard minus the in-place cost of geogrid

c) $4.63/square yard minus the in-place cost of geogrid

d) $5.88/square yard minus the in-place cost of geogrid


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Subgrade Improvement

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