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The Use of Geogrids
in Railroad ApplicationsBy Jim Penman, CGeol, FGS, and Daniel J. Priest, P.E.
OCTOBER 2009
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The inclusion of geogrids
within the unbound aggre-
gate layers of unpaved and
paved road structures has
become increasingly common since the
products were first introduced more
than 25 years ago. However, there is
less awareness within the engineering
community that the same technol-
ogy is equally applicable to roadbed
structures in rail applications. The use
of geogrid reinforcement in the sub-
ballast and ballast layers of roadbed
sections has gained widespread accep-
tance in many parts of the world. For
example, national rail authorities in
some European countries have gone
so far as to provide formal guidance on
the use of these materials in their own
design codes. More recently here in
the United States, formal guidance on
the use of geogrids in rail applications
has been provided by the American
Railway Engineering and Maintenance-
of-way Association (AREMA).
This article is intended to provide
the background information neces-
sary to design roadbed structures that
include geogrids within the sub-ballast
and/or ballast layer. Reference will be
made to the extensive research that
has been undertaken during the last
20 years to quantify the performance
benefits associated with the use of
geogrids in this application. An addi-
tional section will describe the means
by which geogrids can provide simi-
lar benefits in the heavily loaded road
structures surrounding the rail tracks at
intermodal facilities.
Geogrid reinforcementmechanisms
A geogrid is defined as a
geosynthetic 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 geotechni-
cal material (Koerner, 1998). When
unbound aggregate is placed on top
of the geogrid, the coarser particles
partially penetrate through the aper-
tures and lock into position (Figure
1). This effect, commonly referred to
as mechanical interlock, leads to lateral
The Use of Geogridsin Railroad ApplicationsBy Jim Penman, CGeol, FGS, and Daniel J. Priest, P.E.
Figure 1: Mechanical interlock of aggre-gate with a geogrid
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Learning ObjectivesAfter reading this article you should understand:
How including geogrids within ballast and/or sub-
ballast layers can enhance the performance of road-bed structures
The value provided by including geogrid reinforce-
ment in rail applications
The design methods available for geogrid reinforced
roadbed structures
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The Use of Geogrids in Railroad Applications
confinement of the
unbound aggregate
and a general stiffening
of the layer.
The confining effect of a geogrid
is especially important in a roadbed
reinforcement application, as lateralmovement of granular particles is a
major cause of sub-ballast and ballast
settlement. However, the stiffening
effect is also important, particularly
when construction takes place on a
less competent subgrade (stiff clay or
worse). Under these circumstances, the
stiffer aggregate distributes load more
efficiently onto the underlying soil,
thereby reducing both the dynamic
movement (vertical track deflection
during a single load cycle) and the
longer term settlement of the roadbed
due to subgrade consolidation.
Previous research andfield studies
The earliest research undertaken to
quantify the benefits of using geogrids
as reinforcement within a roadbed
structure was carried out at Queens
University in Kingston, Ontario, by
Bathurst and Raymond (1987). An arti-
ficial subgrade (consisting of a series of
rubber-bonded cork mats) was placedat the bottom of a rigid test box. The
remainder of the box was filled with
ballast. Cyclic loads generated by a
hydraulic actuator were applied to a
cross-tie placed on the surface of the
ballast (Figure 2).
The results of the testing (Figure 3)
show how for a maximum limit of 25
mm (1 inch) of permanent settlement,
the presence of a geogrid within the
roadbed extended its service life. An
almost five-fold increase is indicatedfor cases where both a reasonably
competent subgrade is used (CBR of
39) and where a soft subgrade condi-
tion exists (CBR of 1).
In a further study reported by
Matharu (1994), a set of full-scale tests
were undertaken at the British Rail
(now called Network Rail) test facil-
ity located in Derby, England (Figure
4). The main results from the study
(Figure 5) effectively show that when
Figure 2: Cyclic loads are applied to across-tie placed on the ballast surface atQueens University.
Figure 4: Network Rail test facility in Derby,England
Figure 3: Results of cyclic load tests at Queens University.
Figure 5: Full-scale tests show that the rate of settlement for a roadbed constructed ona soft subgrade but reinforced with a geogrid is similar to that of the same roadbedconstructed on bedrock.
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a roadbed reinforced with a geogrid is
constructed on top of a soft subgrade,
its rate of settlement is similar to that of
the same roadbed section constructed
on bedrock. In addition, the dynamic
vertical movement occurring in the
track as the wheel of the train passes isreduced by approximately 40 percent
when a geogrid is used to reinforce
the roadbed (Figure 6).
A rail corridor constructed between
Hochstadt and Probstzella in Germany
(Figure 7), presented an opportu-
nity to observe the benefits of using
geogrids in a full-scale field situation.
Plate bearing tests undertaken by the
German rail authority (Deutsche Bahn)
demonstrated that the stiffness of a
400-mm-thick (16-inch-thick) sub-
ballast was approximately the same as
a 600-mm-thick (24-inch-thick) unre-
inforced section. Also, the stiffness of
both 400-mm (16-inch) and 600-mm
(24-inch) sections doubled when a
geogrid was included (Figure 8).
On a similar project constructednear Cologne, Germany, inclusion of a
geogrid within a roadbed constructed
over a soft formation allowed the sub-
ballast to be reduced from 1,050 mm
(42 inches) to 700 mm (28 inches).
Despite the thickness reduction, the
target modulus of 120 MPa (17,400
psi) was maintained.
On a mainline project in Nagykanizsa,
Hungary, the decision was made to
include a geogrid within the ballast
layer during a rehabilitation operation.
Prior to replacement of
the existing roadbed,
the rail line required
monthly re-surfacing
maintenance. The dynamic deflection
of the rail track was measured using
a rail car, both prior to and follow-
ing the inclusion of the geogrid within
the roadbed section (Figure 9). As can
be seen, the inclusion of the geogrid
resulted in a dramatic reduction in the
dynamic deflection taking place during
trafficking. Service disruptions due to
the requirement for frequent mainte-
nance have since been eliminated.
Previous experienceThere are more than 100 projects
where geogrids have been used to rein-
force the sub-ballast or ballast layers
within a roadbed structure. All of the
U.S. Class 1 rail companies have used
geogrids within their roadbed struc-
tures at one time or another and several
light/passenger rail companies also have
experience in the use of these products.
Following are summaries about a select
number of these projects.
Utah Transit Authority (UTA)
Light Rail Project, Salt Lake
City (2005-2008) The UTAs
FrontRunner commuter rail line runs44 miles from the city of Ogden in
Weber County south to Salt Lake City.
The line is located in an existing right-
of-way that runs parallel with the
Wasatch Mountains. The area is part
of a natural drainage basin, character-
ized by poor quality soils and shallow
groundwater. The subgrade beneath
the roadbed typically consisted of low-
Figure 6: Geogrid reinforcement in the roadbed reduces dynamic vertical movement in thetrack by about 40 percent as the wheel of a train passes.
Figure 7: A rail corridor between Hochstadtand Probstzella, Germany, enabled obser-vation of the benefits of using geogrids ina full-scale field situation.
Figure 8: In German tests, the stiffness of a 400-mm-thick (16-inch-thick) sub-ballast wasapproximately the same as a 600-mm-thick (24-inch-thick) unreinforced section.
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The Use of Geogrids in Railroad Applications
to medium-strength
cohesive soils and
loose to dense sand.
A conventional roadbed
design, consisting of a thicker sub-
ballast layer, proved cost prohibitive
because of the extremely high costassociated with the sourcing of local
aggregate. Calculations determined
that the required sub-ballast thick-
ness could be conservatively reduced
from 12 inches to 8 inches by includ-
ing a layer of geogrid at the inter-
face between the sub-ballast and the
underlying subgrade. Reducing the
sub-ballast depth provided signifi-
cant cost savings and expedited the
construction process less aggregate
could be placed in less time. In addi-
tion, this approach eliminated contact
with the shallow groundwater and the
need to relocate the existing buried
utilities for 900 feet of track.
Dallas Area Rapid Transit
Authority (DART), NW1A Rail
Section, Dallas (2003-2008)
DART has regularly used geogrids
within its roadbed sections since
2003. Traditionally, prior to construct-
ing its standard roadbed section, the
underlying cohesive soils would first
be mixed with lime to a depth of 6inches. Although this provided good
short-term support for the roadbed,
this approach resulted in some logisti-
cal problems.
Installation of the lime stabilization
was not well suited for the project
because of the generation of extensive
dust clouds in the urban project loca-
tion. Additionally, the lime stabilization
installation process requires dry, mild
weather conditions. Finally, protrud-
ing utility pipes were present at thetime the roadbed was constructed
(Figure 10). Since large vehicles are
required to install the lime, this would
have made things very difficult for the
contractor and slowed the construc-
tion process down significantly.
To provide an alternate solution
to the lime stabilization, a structural
analysis was undertaken to demon-
strate that the 6 inches of lime treat-
ment could be avoided by including a
layer of geogrid at the bottom of the
standard roadbed section. Installation
adopting this mechanical stabiliza-
tion technique was both simple and
fast, requiring only minor slitting of
the geogrid in the areas where the
protruding utilities were located.
Kansas City Southern Rail
(KCSR) Company, Victoria to
Rosenberg Line, Texas (2008)
During 2008 and early 2009, KCSR
undertook reconstruction of a 91-mile
section of former track in south Texas.
Where weaker subgrades were encoun-
tered or the original sub-ballast had
been removed or washed out, normal
construction methods required place-
ment of as much as 12 inches of new
sub-ballast. Instead, a geogrid was
placed at the bottom of the new sub-
ballast to reduce the required thick-
ness to only 6 inches (Figure 11).
With no local suppliers, sub-ballast
aggregate was transported from
quarries in Hatton, Ark., and Mexico,
resulting in a particularly high cost
for the installed material . On this
project, reducing the required sub-
ballast thickness resulted in significant
construction cost savings. In total,
approximately 237,000 square yards
of geogrid was installed.
Figure 9: Dynamic deflection of rail track measured using a rail car, both prior to andfollowing the inclusion of geogrid within the roadbed section.
Figure 10: Utility pipes protruding from a Dallas Area Rapid Transit Authority roadbed areaccommodated by minor slitting of the geogrid.
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Geogridreinforcedroadbed structures
The U.K. Approach
to design In the United Kingdom,
the national rail authority, Network
Rail, has been using geogridsbeneath its main line tracks since
the early 1990s. In addition, it has
undertaken several in-house test-
ing programs, both in the laboratory
and along in-service tracks, to quantify
the performance benefits associated
with the use of geogrids in roadbed
structures.
Design protocols for roadbed
structures are prescribed in Network
Rail (2005). The general approach
adopted is to attain a target stiffness
for the roadbed beneath the track. As
shown in Table 1, this value varies and
depends on whether or not a geogrid
is included within the roadbed struc-
ture a minimum dynamic sleeper
support stiffness (K) of 60 kN/mm is
required for an unreinforced roadbed,
whereas only 30 kN/mm is required
for the same acceptable level of perfor-
mance when a geogrid is included
within the roadbed section.
For a given set of subgrade condi-
tions and the same type of aggregate,the stiffness of a roadbed section is
essentially a function of the thickness
of the ballast and sub-ballast layers. As
indicated in the design chart shown in
Figure 12, the use of a geogrid results
in a thickness reduction for the roadbed
of around 8 inches (200 mm) relative
to a conventional unreinforced section.
It should be appreciated that
although the Network Rail design proto-
cols refer to geogrids generically no
specific manufacturers products areruled in or out any geogrid product
proposed for use on its system requires
a current PADS Certificate. This docu-
ment is obtained from Network Rail and
is only issued for products where the
performance benefits prescribed above
have been demonstrated in full-scale
laboratory and field tests. It is not suffi-
cient simply to offer an equivalent prod-
uct based on material properties, even
if these properties exceed those for a
Figure 11: Geogrid placed at the bottom of a roadbed in Texas reduced the required thick-ness of sub-ballast by 50 percent.
TABLE 1: Target roadbed stiffness
Minimum Dynamic Sleeper Support
Track Condition Stiffness, K (kN/mm/sleeper end)
Absolute Value 30
Existing Main Lines
With Geogrid
Reinforcement 30
Without Geogrid
Reinforcement 60
New Track Up to 100 mph 60
Above 100 mph 100
Figure 12: Network Rail design chart for determining the required roadbed thickness.
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The Use of Geogrids in Railroad Applications
product with an exist-
ing PADS Certificate. To
date, only stiff geogrids
with integral junctions have
obtained PADS certification.
The U.S. approach to design
In the United States, most of the freightand passenger rail companies have
standard roadbed sections for their rail
systems. When particularly unfavorable
soil conditions are encountered, extra
subgrade improvement measures may
be taken including over-excavation
and replacement, chemical treatment,
placement of an asphaltic layer at the
bottom of the roadbed section, etc.
Geogrids are also commonly used in
these circumstances, though in many
cases, a detailed analysis and design is
not necessarily undertaken.
When a more rigorous roadbed
design is conducted, the approach
originally developed by Professor
Arthur Newell Talbot at the University
of Illinois is still the method of choice
for many rail engineers. Although it is
based on research undertaken toward
the early part of the last century, the
Talbot method is still listed in the
current AREMA (2009) design manual.
The basic Talbot equation is defined
in Equation1:
where,
h = required thickness of ballast plus
sub-ballast below the ties (inches);
pc= allowable stress at depth hunder
cross-tie centerline (pounds per square
inch (psi)); and
pa= imposed pressure at face of cross-
tie (psi).
The parameter pa is determined
using Equation 2.
where,
IF = impact factor (function of wheel
diameter and maximum train
speed);
DF= distribution factor; and
A= surface area of cross-tie face
(square inches).
Given that this methodology was
developed almost half a century
before the first geogrids were
invented, it is not surprising that thereis no prescribed means to include the
benefits provided by these materi-
als. However, as mentioned previ-
ously, it is widely accepted that one
of the principle functions of a geogrid
is to stiffen an aggregate layer and
thereby provide a more efficient
means by which to transfer load onto
the underlying subgrade. If then,
based on full-scale testing, it can be
determined what these load-spread
benefits are for a particular geogrid
product, the allowable stress param-
eter (pc) in Equation 1 can be adjusted
accordingly. A decrease in the pres-
sure imposed at the subgrade level
would clearly lead to a reduction in
the required roadbed thickness.
Although, up to this point, no formal
guidance has been provided by AREMA
on the use of geogrids in roadbed
structures, a new section describing the
benefits associated with the use of these
products has recently been approved for
publication and will appear in the 2010edition of the AREMA design manual.
This new section will also include
details on how to specify geogrids in
this particular application.
Ballast life extensionIn the United States, geogrids are
most commonly used within the sub-
ballast layer to reduce the required
thickness of the roadbed. In some
cases, a detailed design is under-
taken in accordance with the proce-dures outlined above. In other cases, a
geogrid is simply added at the bottom
of a standard roadbed section to effec-
tively beef it up. Either way, the
issue being addressed by the use of a
geogrid is one associated with poten-
tial instability or excessive settlement
of the subgrade.
In mainland Europe, inclusion of a
geogrid at the bottom of the ballast
layer is a more common practice. Under
these circumstances, the principle
benefit provided by the geogrid is one
of service life extension. Although this
approach results in an increase in the
initial cost of the roadbed section or the
rehabilitation costs for an existing road-
bed structure, the ballast life extensioncan easily outweigh this initial invest-
ment. Based on the laboratory and field
testing undertaken to date, it is likely
that a three- to five-fold increase in the
life of the ballast will be attained when a
geogrid is included within this layer.
Heavy-duty pavementstructures
The loading and unloading area
around the rail tracks at intermodal
facilities and in industrial yards can
provide significant challenges to pave-
ment designers. The applied loads
from container stacking equipment are
extremely high. Even when relatively
firm subsoil conditions exist, these
loads can lead to excessive movement
of the base course and cracking of
asphaltic layers. Geogrids can be used
in these structures to help reduce or
eliminate these problems.
When soft soil conditions are
encountered, an initial layer of aggre-
gate is frequently placed prior toconstruction of the main pavement
structure. When a geogrid is used
within this layer, the required thick-
ness can generally be reduced by 50
percent to 60 percent. Full details
of this application and the design
methodology associated with it are
provided by Archer (2008).
When good quality subsoil condi-
tions exist, a geogrid can also be
included to enhance the performance
of the main pavement structure.Up-front construction cost savings can
be attained by reducing the required
thickness of the unbound aggregate
and/or asphaltic layers in flexible pave-
ments. Recent research has shown
that some of the newer geogrids can
achieve a thickness reduction of as
much as 50 percent in the aggregate
layer or 30 percent in the asphalt layer.
The long-term stiffening of the aggre-
gate layer can also be used to provide
(Equation 2)
(Equation 1)
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component reductions in rigid pave-
ment structures.
Alternatively, when life cycle cost
savings are considered more impor-
tant, geogrids can be used to extend
the life of a pavement structure.
Numerous full-scale tests have beenundertaken to quantify the extension
of life provided by geogrids in flexible
pavement structures. Typically, a three-
to six-fold increase can be expected
for the better quality geogrid products
currently available. A detailed descrip-
tion of the topic of pavement life exten-
sion in geogrid reinforced pavement
structures is provided by Penman and
Cavanaugh (2006), and further details
on how to quantify the benefits of
using geogrids in pavement structuresare provided in AASHTO (2001).
Installation considerationsFor sub-ballast reinforcement appli-
cations, or where a geogrid is to be
included within a
pavement structure,
the method of installa-
tion is straightforward. If
possible, the existing surface is graded
and crowned to promote positive
drainage away from the main areaof trafficking. Any protruding objects
such as tree stumps are removed prior
to rolling out the geogrid. Overlaps of
1 foot to 3 feet are generally sufficient
to ensure that full geogrid coverage is
maintained during the design life of the
structure. Only when extremely soft
conditions are encountered is it neces-
sary to peg or tie geogrids together.
Once the geogrid has been rolled
out, the overlying aggregate can
be placed and compacted immedi-
ately. Conventional dump trucks and
compaction equipment can be used
provided the underlying subgrade
has sufficient strength to carry the
imposed loads.
For ballast reinforcement applica-
tions, special provision needs to be
made because of the clean, coarse
nature of the aggregate generally used
for this layer. Under normal circum-
stances, trafficking with heavy-axle
trucks on a limited thickness of aggre-
gate underlain by a geogrid is not aproblem. However, in the case of a
ballast stone, the lack of a finer frac-
tion can result in shoving and exces-
sive rutting at the surface even after a
fairly limited number of vehicle passes.
For a relatively thin ballast layer, the
geogrid can be exposed and pulled
up during the installation process.
Therefore, truck trafficking must be
limited when a geogrid is included
within the ballast layer.
Limiting the amount of truck traf-ficking can be achieved in the follow-
ing ways: Use an access road along the side of
the rail line so the trucks can drive
on and off the track area only where
they are dumping aggregate. If an existing track is located adja-
cent to the track being constructed,
side dump aggregate from trains. Once the geogrid is placed directly
on the sub-ballast, the track can be
Figure 13: Geogrid is rolled out by a track-mounted undercutting machine prior to newballast being dropped in place.
REFERENCES
AASHTO, 2001, Recommended Pract ice for Geosynthetic Reinforcement of the
Aggregate Base Course of Flexible Pavements, American Association of State Highwayand Transportation Officials Provisional Standard PP46-01.
Archer, S., 2008, Subgrade Improvement for Paved and Unpaved Surfaces UsingGeogrids, CE NewsProfessional Development Series, October 2008.
AREMA, 2009, Manual for Railway Engineering, American Railroad Engineering andMaintenance-of-way Association.
Bathurst, R.J and Raymond, G.P., 1987, Geogrid Reinforcement of Ballasted Track,Transportation Research Board 66th Annual Meeting
Koerner, R.M., 1998, Designing with Geosynthetics, fourth edition, Prentice Hall,
Upper Saddle River, NJ.
Matharu, M., 1994, Geogrids Cut Ballast Settlement Rate on Soft Substructures,Railway Gazette International, March 1994.
Network Rail, 2005, Formation Treatments, Business Process Document NR/SP/
TRK/9039.
Penman, J. and Cavanaugh, J., 2006, Extending Flexible Pavement Life UsingGeogrids, CE NewsProfessional Development Series, September 2006.
8/14/2019 Use of Geogrids in Railroad Applications
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The Use of Geogrids in Railroad Applications
constructed directly
on top. Ballast cars can
then be used to place
aggregate before lifting the
track and compacting with conven-
tional vibrating tines.
When ballast is being placedmechanically (for example, using
a track-mounted undercutting
machine) the geogrid can easily be
installed as part of this process. A
simple modification is undertaken
to the underside of the machine
(Figure 13) and the geogrid is rolled
out immediately prior to the new
ballast being dropped into place.
SummaryThe use of geogrids to enhance
the performance of roadbed sections
has become much more widespread,
particularly during the last 10 years.
Initial cost savings on the order of
$30,000 per linear mile of track can
generally be achieved by installing a
geogrid within the sub-ballast layer and
reducing the overall roadbed thickness.
Alternatively, by including a geogridwithin the ballast layer, the period
between maintenance events can be
extended by a factor of three to five.
For heavy-duty pavement structures
at intermodal facilities and in indus-
trial yards, geogrids can be used on
soft soils to reduce by 50 percent to
60 percent the amount of aggregate
required to provide a stable platform on
which to construct the main pavement
structure. In addition, geogrids can be
used within the pavement structure
to reduce the thickness of the aggre-
gate layer by as much as 50 percent
and the thickness of the asphalt by as
much as 30 percent without a reduc-
tion in structural capacity.
Jim Penman, CGeol, FGS, direc-
tor of business development for TensarInternational Corp., is a geotechnical
engineer with more than 15 years experi-
ence in geosynthetics. He currently serves
on AREMA Committee 1 (Roadway
and Ballast) and can be contacted at
Daniel J. Priest, P.E., product
manager Road Solutions for CONTECH
Construction Products Inc., holds a
MSCE from Northwestern University
and has more than 10 years of experi-
ence in the geosynthetics industry and
geotechnical engineering. He can becontacted at [email protected].
10 PDH Professional Development Advertising Section CONTECH Construction Products Inc.
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8/14/2019 Use of Geogrids in Railroad Applications
11/12
1. The primary reinforcing mechanism for geogrids in
the reinforcement of roadbed structures is:
a) Tensioned Membrane Effect
b) Improved Bearing Capacity
c) Lateral Restraint through Mechanical Interlock
d) Tensile Resistance at low strain
e) All of the above
2. The confining effect of a geogrid:
a) Reduces lateral movement of the granular particles
b) Stiffens the aggregate
c) Helps distribute the applied load more efficiently
d) Helps reduce the long-term settlement of the
roadbed structure
e) All of the above
3. Research conducted by Network Rail determined that
a geogrid reinforced roadbed has:
a) A 40-percent reduction in the dynamic vertical
movement
b) More settlement than an unreinforced section
c) An increase in the subgrade modulus
d) Less stiffness than a comparable unreinforced section
e) All of the above
4. A full-scale field simulation by Deutsche Bahn
concluded that:
a) The stiffness of a 16-inch reinforced section was
approximately equivalent to that of a 24-inch
unreinforced section
b) The stiffness of a 16-inch reinforced section was
double that of a 16-inch unreinforced sectionc) Geogrid inclusion had no effect on the stiffness of a
24-inch thickness of ballast stone
d) a and b
e) All of the above
5. Which of the following statements are true with
respect to the design method developed by Network
Rail?
a) The maximum thickness reduction provided by a
geogrid is 4 inches
b) A reduction in the required thickness of the roadbed
is based on the additional roadbed stiffness resultingfrom confinement of the aggregate by a geogrid
c) All geogrids essentially perform the same and
therefore any geogrid can be used with the Network
Rail design method
d) Geogrids can only be used to reduce the required
roadbed thickness when the subgrade is soft
e) All of the above
6. The Talbot equation in the AREMA Manual for Railway
Engineering:
a) Is based purely on mechanistic principles
b) Was developed based on research
undertaken at the University of Iowa
c) Helps determine the required thickness
of the roadbed
d) Predicts the vertical settlement occurring beneath the
track structure for a given amount of trafficking
e) None of the above
7. How can geogrids be accounted for in a design
incorporating the AREMA (Talbot equation) design
method?
a) The imposed pressure at the face of a cross-tie is
reduced by a factor of 4
b) The distribution factor component of the equation
used to determine the parameter pais doubled
c) The allowable stress component (pc) in the equation
is adjusted to account for the presence of the geogrid
d) The unreinforced roadbed thickness is simply reduced
by 20 percent
e) They cant be accounted for since geogrids were not
invented when the Talbot equation was developed.
8. Geogrid inclusion beneath the ballast stone can:
a) Mitigate potential instability
b) Prevent excessive settlement
c) Provide a three- to five-times increase in ballast
service life
d) a and c
e) a, b, and c
9. Inclusion of a geogrid within a heavy pavement
structure will:a) Provide no benefit to the structural capacity of the
pavement
b) Provide a maximum of double the service life of the
pavement
c) Allow for the reduction of asphalt thickness by as
much as 30 percent without a reduction in service life
d) Allow for the reduction of aggregate thickness by
a maximum of 30 percent without a reduction in
service life
e) None of the above
10. During installation of the sub-ballast and ballastwhen a geogrid is to be included:
a) Conventional equipment and compaction methods
can be utilized
b) Special equipment should be utilized to ensure the
geogrid is not damaged
c) Truck trafficking should be kept off of the section at
all times
d) Mechanical ties are always required to prevent two
adjacent lengths of geogrid pulling apart
e) None of the above
Professional Development Series Quiz
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12/12
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