The Use of Geogrids in Railroad Applications

By Jim Penman, CGeol, FGS, and Daniel J. Priest, P.E.
Expiration Date: October 2011

Overview

The inclusion of geogrids within the unbound aggregate 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 technology is equally applicable to roadbed structures in rail applications. The use of geogrid reinforcement in the subballast and ballast layers of roadbed sections has gained widespread acceptance 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 Maintenanceof- way Association (AREMA).

 

This article is intended to provide the background information necessary 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 additional section will describe the means by which geogrids can provide similar benefits in the heavily loaded road structures surrounding the rail tracks at intermodal facilities.

Geogrid reinforcement mechanisms

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 surrounding soil, stone, or other geotechnical material" (Koerner, 1998). When unbound aggregate is placed on top of the geogrid, the coarser particles partially penetrate through the apertures and lock into position (Figure 1). This effect, commonly referred to as mechanical interlock, leads to lateral 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 lateral movement 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 and field studies

The earliest research undertaken to quantify the benefits of using geogrids as reinforcement within a roadbed structure was carried out at Queen's University in Kingston, Ontario, by Bathurst and Raymond (1987). An artificial subgrade (consisting of a series of rubber-bonded cork mats) was placed at 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 indicated for cases where both a reasonably competent subgrade is used (CBR of 39) and where a soft subgrade condition 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 facility located in Derby, England (Figure 4).

The main results from the study (Figure 5) effectively show that when 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 is reduced 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 opportunity 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) subballast was approximately the same as a 600-mm-thick (24-inch-thick) unreinforced 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 constructed near Cologne, Germany, inclusion of a geogrid within a roadbed constructed over a soft formation allowed the subballast 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 following 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 maintenance have since been eliminated.

Previous experience

There are more than 100 projects where geogrids have been used to reinforce 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 structures 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 UTA's FrontRunner commuter rail line runs 44 miles from the city of Ogden in Weber County south to Salt Lake City. The line is located in an existing rightof- way that runs parallel with the Wasatch Mountains. The area is part of a natural drainage basin, characterized by poor quality soils and shallow groundwater. The subgrade beneath the roadbed typically consisted of low to medium-strength cohesive soils and loose to dense sand. A conventional roadbed design, consisting of a thicker subballast layer, proved cost prohibitive because of the extremely high cost associated with the sourcing of local aggregate. Calculations determined that the required sub-ballast thickness could be conservatively reduced from 12 inches to 8 inches by including a layer of geogrid at the interface between the sub-ballast and the underlying subgrade. Reducing the sub-ballast depth provided significant cost savings and expedited the construction process — less aggregate could be placed in less time. In addition, 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 constructing its standard roadbed section, the underlying cohesive soils would first be mixed with lime to a depth of 6 inches. Although this provided good short-term support for the roadbed, this approach resulted in some logistical 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 location. Additionally, the lime stabilization installation process requires dry, mild weather conditions. Finally, protruding utility pipes were present at the time 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 construction process down significantly.

To provide an alternate solution to the lime stabilization, a structural analysis was undertaken to demonstrate that the 6 inches of lime treatment could be avoided by including a layer of geogrid at the bottom of the standard roadbed section. Installation adopting this "mechanical stabilization" 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 encountered or the original sub-ballast had been removed or washed out, normal construction methods required placement of as much as 12 inches of new sub-ballast. Instead, a geogrid was placed at the bottom of the new subballast to reduce the required thickness 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 subballast thickness resulted in significant construction cost savings. In total, approximately 237,000 square yards of geogrid was installed.

Geogrid reinforced roadbed structures

The U.K. Approach to design — In the United Kingdom, the national rail authority, Network Rail, has been using geogrids beneath its main line tracks since the early 1990s. In addition, it has undertaken several in-house testing 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 structure — 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 performance when a geogrid is included within the roadbed section.

TABLE 1: Target roadbed stiffness

Track Condition		Minimum Dynamic Sleeper Support 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

For a given set of subgrade conditions 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 protocols refer to geogrids generically — no specific manufacturer's products are ruled in or out — any geogrid product proposed for use on its system requires a current PADS Certificate. This document 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 sufficient simply to offer an equivalent product based on material properties, even if these properties exceed those for a product with an existing 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 freight and 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:

Equation 1:

where:
h = required thickness of ballast plus sub-ballast below the ties (inches);
p_c = allowable stress at depth h under cross-tie centerline (pounds per square inch (psi)); and
p_a = imposed pressure at face of crosstie (psi).

The parameter pa is determined using Equation 2.

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 there is no prescribed means to include the benefits provided by these materials. However, as mentioned previously, 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 parameter (p_c) in Equation 1 can be adjusted accordingly. A decrease in the pressure 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 2010 edition of the AREMA design manual. This new section will also include details on how to specify geogrids in this particular application.

Ballast life extension

In the United States, geogrids are most commonly used within the subballast layer to reduce the required thickness of the roadbed. In some cases, a detailed design is undertaken in accordance with the procedures outlined above. In other cases, a geogrid is simply added at the bottom of a standard roadbed section to effectively "beef it up." Either way, the issue being addressed by the use of a geogrid is one associated with potential 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 roadbed structure, the ballast life extension can easily outweigh this initial investment. 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 pavement structures

The loading and unloading area around the rail tracks at intermodal facilities and in industrial yards can provide significant challenges to pavement 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 aggregate is frequently placed prior to construction of the main pavement structure. When a geogrid is used within this layer, the required thickness 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 conditions 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 pavements. 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 aggregate layer can also be used to provide component reductions in rigid pavement structures.

Alternatively, when life cycle cost savings are considered more important, geogrids can be used to extend the life of a pavement structure. Numerous full-scale tests have been undertaken 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 description of the topic of pavement life extension 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 structures are provided in AASHTO (2001).

Installation considerations

For sub-ballast reinforcement applications, or where a geogrid is to be included within a pavement structure, the method of installation is straightforward. If possible, the existing surface is graded and crowned to promote positive drainage away from the main area of 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 necessary to peg or tie geogrids together.

Once the geogrid has been rolled out, the overlying aggregate can be placed and compacted immediately. Conventional dump trucks and compaction equipment can be used provided the underlying subgrade has sufficient strength to carry the imposed loads.

For ballast reinforcement applications, special provision needs to be made because of the clean, coarse nature of the aggregate generally used for this layer. Under normal circumstances, trafficking with heavy-axle trucks on a limited thickness of aggregate underlain by a geogrid is not a problem. However, in the case of a ballast stone, the lack of a finer fraction can result in "shoving" and excessive 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 trafficking can be achieved in the following 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 adjacent to the track being constructed, side dump aggregate from trains.
• Once the geogrid is placed directly on the sub-ballast, the track can be constructed directly on top. Ballast cars can then be used to place aggregate before lifting the track and compacting with conventional vibrating tines.
• When ballast is being placed mechanically (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.

Summary

The 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 geogrid within 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 industrial 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 aggregate layer by as much as 50 percent and the thickness of the asphalt by as much as 30 percent without a reduction in structural capacity.

Quiz Questions

1. The primary reinforcing mechanism for geogrids in the reinforcement of roadbed structures is:
   1. Tensioned Membrane Effect
   2. Improved Bearing Capacity
   3. Lateral Restraint through Mechanical Interlock
   4. Tensile Resistance at low strain
   5. All of the above
2. The confining effect of a geogrid:
   1. Reduces lateral movement of the granular particles.
   2. Stiffens the aggregate.
   3. Helps distribute the applied load more efficiently
   4. Helps reduce the long-term settlement of the roadbed structure
   5. All of the above
3. Research conducted by Network Rail determined that a geogrid reinforced roadbed has:
   1. A 40-percent reduction in the dynamic vertical movement.
   2. More settlement than an unreinforced section
   3. An increase in the subgrade modulus
   4. Less stiffness than a comparable unreinforced section
   5. All of the above
4. A full-scale field simulation by Deutsche Bahn concluded that:
   1. The stiffness of a 16-inch reinforced section was approximately equivalent to that of a 24-inch unreinforced section
   2. The stiffness of a 16-inch reinforced section was double that of a 16-inch unreinforced section
   3. Geogrid inclusion had no effect on the stiffness of a 24-inch thickness of ballast stone
   4. a and b
   5. All of the above
5. Which of the following statements are true with respect to the design method developed by Network Rail?
   1. The maximum thickness reduction provided by a geogrid is 4 inches
   2. A reduction in the required thickness of the roadbed is based on the additional roadbed stiffness resulting from confinement of the aggregate by a geogrid
   3. All geogrids essentially perform the same and therefore any geogrid can be used with the Network Rail design method
   4. Geogrids can only be used to reduce the required roadbed thickness when the subgrade is soft
   5. All of the above
6. The Talbot equation in the AREMA Manual for Railway Engineering:
   1. Is based purely on mechanistic principles.
   2. Was developed based on research undertaken at the University of Iowa
   3. Helps determine the required thickness of the roadbed
   4. Predicts the vertical settlement occurring beneath the track structure for a given amount of trafficking
   5. none of the above
7. How can geogrids be accounted for in a design incorporating the AREMA (Talbot equation) design method?
   1. The imposed pressure at the face of a cross-tie is reduced by a factor of 4
   2. The distribution factor component of the equation used to determine the parameter p_a is doubled
   3. The allowable stress component(p_c) in the equation is adjusted to account for the presence of the geogrid
   4. The unreinforced roadbed thickness is simply reduced by 20 percent
   5. They can't be accounted for since geogrids were not invented when the Talbot equation was developed.
8. Geogrid inclusion beneath the ballast stone can:
   1. Mitigate potential instability
   2. Prevent excessive settlement
   3. Provide a three- to five-times increase in ballast service life
   4. Both a and c
   5. a, b, and c
9. Inclusion of a geogrid within a heavy pavement structure will:
   1. Provide no benefit to the structural capacity of the pavement
   2. Provide a maximum of double the service life of the pavement.
   3. Allow for the reduction of asphalt thickness by as much as 30 percent without a reduction in service life
   4. Allow for the reduction of aggregate thickness by a maximum of 30 percent without a reduction in service life
   5. None of the above
10. During installation of the sub-ballast and ballast when a geogrid is to be included:
    1. Conventional equipment and compaction methods can be utilized
    2. Special equipment should be utilized to ensure the geogrid is not damaged
    3. Truck trafficking should be kept off of the section at all times
    4. Mechanical ties are always required to prevent two adjacent lengths of geogrid pulling apart
    5. None of the above

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Jim Penman, CGeol, FGS, director of business development for Tensar International Corp., is a geotechnical engineer with more than 15 years experience in geosynthetics. He currently serves on AREMA Committee 1 (Roadway and Ballast)

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 experience in the geosynthetics industry and geotechnical engineering.

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REFERENCES

• AASHTO, 2001, Recommended Practice for Geosynthetic Reinforcement of the Aggregate Base Course of Flexible Pavements, American Association of State Highway and Transportation Officials Provisional Standard PP46-01.
• Archer, S., 2008, Subgrade Improvement for Paved and Unpaved Surfaces Using Geogrids, CE News Professional Development Series, October 2008.
• AREMA, 2009, Manual for Railway Engineering, American Railroad Engineering and Maintenance-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 Using Geogrids, CE News Professional Development Series, September 2006.