Ballast preservation system still providing protection against ballast fouling - Geosynthetics Magazine

Features | October 10, 2024 | By: Keith C. Brooks, P.E.

In February 2023, Geosynthetics magazine published a feature article titled “Innovative solution for ballast fouling.” The feature focused on the phenomenon of ballast fouling where mud from saturated soil conditions migrates up into the sub-ballast and ballast material in rail applications. The ballast fouling issue is prevalent in coastal areas where high groundwater is often present and rail lines are necessary for operations. The feature article in the February 2023 issue focused on understanding the ballast fouling problem, and highlighted a case study where ballast fouling was requiring the Port of Texas City to full-depth reconstruct a section of rail every 24 to 36 months.

Remy Steffer, director of engineering at the Port of Texas City, Texas City Terminal Railway, and Houston Belt and Terminal Railway, decided to try a new technology from Industrial Fabrics Inc. called BaseLok® BallastGuard™, which is specifically designed to address ballast fouling. When the article for the February 2023 issue was drafted, the section had been in place approximately 18 months and was performing very well. The purpose of this article is to examine how engineers approach a rail design with BaseLok BallastGuard, to discuss how the product is installed and to revisit the current state of the Port of Texas City section.

Geosynthetics have been used in rail applications for many years. BaseLok BallastGuard is a composite that consists of a high-strength biaxial geogrid and a heavy-weight non-woven geotextile fabric. For decades, the AREMA Manual for Railway Engineering has included guidelines for both geotextile fabrics and geogrids. In the 2023 edition, Geotextile and Geocomposite Guidelines for Railroad Separation/Stabilization Applications is in Chapter 1, Section 10.1; Geogrid Guidelines for Ballast and Sub-Ballast Reinforcement is in Chapter 1, Section 10.6. It should be noted that both the geogrid and geotextile components of the BaseLok BallastGuard significantly exceed the guidelines found in these sections.

Traditionally, non-woven geotextiles are utilized for separation, while geogrids are utilized for ballast or sub-ballast reinforcement, as noted by the titles of the referenced sections above. As such, when considering the design of the railway section, the geogrid is what governs the design thickness, while the geotextile provides separation, filtration and durability.

In the early 1900s, Professor A.N. Talbot developed an empirical formula that looks at the pressure below the tie compared to the allowable bearing capacity of the subgrade, then calculates the amount of combined sub-ballast and ballast below the tie needed to support the wheel loads from the train. The 2024 AREMA Manual for Railway Engineering, Chapter 1, Section 2.11 Sub-ballast Recommended Practices includes subsection 2.11.2.3 Depth of Ballast and Sub-ballast, which provides guidance on utilizing the Talbot equation, shown below.

h = thickness of combined ballast and sub-ballast (in)

Pa = applied pressure at the bottom of the cross tie (psi)

Pc = allowable bearing capacity at the top of subgrade (psi)

The first step is understanding Pa and Pc. Pa is a function of both train factors and superstructure factors. The train factors are used to calculate an influence factor (IF), and the superstructure factors are used to determine the distribution factor (DF). The train and superstructure factors are shown below.

The Influence Factor is calculated with the following equation:

V = maximum speed (mph)

D = train wheel diameter (in)

The distribution factor (DF, vertical axis) is the percentage of the axle load carried by a single tie. The distribution factor is determined from the following figure once the center-to-center tie spacing and tie type are known.

Once the IF and DF are determined, Pa can be calculated with the following equation:

w = wheel load (lbs)

A = cross-tie area at the base of the tie (in2)

The next step is determining Pc. As previously mentioned, Pc is the allowable bearing capacity of the subgrade. In an unreinforced state, this is sometimes provided in the geotechnical engineering report; however, more often than not, a subgrade strength value is provided. This subgrade strength value can be utilized to calculate the allowable bearing capacity using guidelines from the U.S. Army Corps of Engineers Engineering Technical Letter (ETL) 1110-1-189 dated February 14, 2003. Section 2.2.4 states that the subgrade bearing capacity can be determined by multiplying the subgrade shear strength (C) by the bearing capacity factor (Nc). Undrained shear strength is most commonly used, so we will refer to the shear strength as Cu.

Section 2.2.3 discusses appropriate Nc values and suggests a value of 2.8 for unreinforced conditions and 5.8 for geogrid reinforced conditions. The section states that these values were determined using empirical data from full-scale ERDC test sections. This research was completed well before the invention of BaseLok BallastGuard; however, the BaseLok BL6 geogrid used as the geogrid component in BallastGuard significantly exceeds the minimum values published in Table 3 of ETL 1110-1-189. Because of the greater strength, Industrial Fabrics Inc. often recommends additional Nc enhancements when utilizing its stiffer and strong BaseLok geogrids such as BL6 and BL7. The equation for calculating Pc is shown below.

The factor of safety is typically a value between 1.5 and 3.0 and should be provided by the engineer of record.

Determine the required combined ballast and sub-ballast thickness for a storage yard with following design parameters:

Loading = E80 Cooper (40,000 lbs (1814.3 kilograms) wheel load)

Wood ties (soft) = 8.5 feet (2.5 meters) x 8 inches (20.3 centimeters) x 9 inches (22.8 centimeters) on 24-inch (60.9 centimeters) centers

Wheel diameter = 36 inches (91.4 centimeters)

Maximum velocity = 20 mph

Design factor of safety = 2.0

Undrained shear strength = 5 psi (70 pound-force/square foot)

Step 1: Calculate IF

Step 2: Determine DF

DF = 29% or 0.29

Step 3: Calculate Pa

Step 4: Calculate Pc unreinforced

Step 5: Calculate Pc reinforced

Step 6: Utilize Talbot equation to determine combined ballast and sub-ballast thickness unreinforced

Step 7: Utilize Talbot equation to determine combined ballast and sub-ballast thickness reinforced

The addition of the geogrid allows for a significant reduction in the total thickness of combined ballast and sub-ballast. Most railways have a minimum ballast requirement of between 8 (20.3 centimeters) and 12 inches (30.4 centimeters) below the tie. To determine the amount of each component, use the minimum ballast depth requirement provided by the railway owner or engineer of record and combine it with the amount of sub-ballast needed to reach the calculated required total thickness.

The BaseLok BallastGuard product is heavy. Each 100-foot-long (30.4 meters) roll weighs more than 350 pounds (158.7 kg). As such, loaders with forks are typically utilized to position the BaseLok BallastGuard prior to rolling out the product. A protective wrap helps keep the product dry prior to installation. This protective wrap is removed prior to installation. Once positioned, the product is manually rolled out. (See Figures 4-6)

When necessary, the BaseLok BallastGuard should be overlapped 1 foot (0.3 meters) to 3 feet (0.9 meters), both side to side and end to end to prevent separation during sub-ballast installation. Subgrade strength and site conditions are taken into consideration when determining the appropriate overlap.

Once the BaseLok BallastGuard has been correctly deployed, the next step is the sub-ballast installation. Industrial Fabrics typically recommends that sub-ballast be placed directly on the BaseLok BallastGuard; however, a larger aperture geogrid is available and can be utilized in the manufacturing of the BaseLok BallastGuard in the event ballast material needs to be placed directly on the BaseLok BallastGuard. Additionally, a minimum of 6 inches of ballast or sub-ballast should be placed as fill material on the BaseLok BallastGuard.

As mentioned at the beginning of this article, the first installation of BaseLok BallastGuard occurred at the Port of Texas City in May 2021 under the direction of Remy Steffer, director of engineering. As is typical when a product is tested, one of the most challenging locations was chosen. Located along a seawall, this particular section of rail needed to be completely reconstructed every 24 to 36 months.

In the February 2023 issue of Geosynthetics magazine, this project was the subject of a feature article discussing the issues with ballast fouling and how the Port of Texas City was addressing the issue. At the time of the writing of that article, the section had been in place for 18 months and appeared to have no fouling issues whatsoever.

Given the proximity of the Port of Texas City project to Industrial Fabrics’ Houston manufacturing facility, monitoring the performance of the BaseLok BallastGuard has been convenient. The pictures shown were taken July 1, 2024—37 months post-installation. The BaseLok BallastGuard section continues to perform as designed, and no evidence of ballast fouling has been observed in the past three years.

The Port of Texas City and Texas City Rail Terminal continue to be pleased with how well this section has held up, and they have begun to use the BaseLok BallastGuard at other locations (R. Steffer, personal communication, July 1, 2024). Several Class 1 railroads have also begun to utilize the BaseLok BallastGuard to mitigate issues with ballast fouling with test sections currently being evaluated.

Ballast fouling continues to be an issue for rail yards and main lines throughout the country. Advancements are being made in measuring the level of ballast fouling through innovations with ground-penetrating radar, which is helping companies identify problem areas before they become extreme. The ground-penetrating radar data is being correlated to a fouling index to quantify the level of ballast fouling present. This data allows companies to prioritize repairs and maximize their maintenance efforts and dollars. As companies begin to repair identified areas of concern, BaseLok BallastGuard is available to prolong the lives of these repairs, maximizing construction dollars and allowing for a safer, more structurally competent rail system.

AREMA. (2024). Manual for Railway Engineering.

U.S. Army Corps of Engineers. (2003). Use of Geogrids in Pavement Construction, ETL1110-1-189.

Keith C. Brooks, P.E. is the director of engineering at Industrial Fabrics, Inc. in Houston, Texas. He has over two decades of experience designing with geosynthetics.

All photos courtesy of Keith Brooks.

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