USE OF TRENCH DRAINS TO IMPROVE CHRONIC EMBANKMENT STABILITY PROBLEMS. and SHANNON & WILSON, INC. ABSTRACT

Transcription

1 USE OF TRENCH DRAINS TO IMPROVE CHRONIC EMBANKMENT STABILITY PROBLEMS by Steven R. McMullen, P.E. Principal Engineer and Dexter N. McCulloch, C.E.G. Senior Vice President SHANNON & WILSON, INC. ABSTRACT Chronic instability of existing railroad track substructures and embankments cause track deflections that require frequent maintenance, slow orders, and overall inefficiency of railroad operations. Substructure instability can be caused weak subgrade soils, poorquality ballast, or inadequate thickness of the ballast and subballast layers. Typical problems that develop are subgrade shear failure, ballast pocket formation, and fouled ballast. As these problems develop, drainage becomes inhibited and the presence of water accelerates the problem. Slope instability problems that affect rail operations include embankment fill slope failures and landslides in natural slopes. Water, or water pressure, is often the primary cause of these types of instability. Trench drains are simple ballast-filled excavations that provide an effective means of draining water from the track substructure, embankment slopes and landslide areas. Removing water from these environments can lead to an immediate improvement in stability. Trench drains are extremely adaptable to various site and subsurface conditions. They provide several advantages over other stabilization methods currently in use. They are simple to understand, economical to construct, and have proven effective on thousands of sites around the country. Page 1 of 1

2 USE OF TRENCH DRAINS TO IMPROVE CHRONIC EMBANKMENT STABILITY PROBLEMS INTRODUCTION Poor drainage can be cited as a contributing factor in nearly all railroad embankment and track stability problems. Several methods are in current use to improve the stability of existing embankments. Perhaps the least sophisticated, yet in our experience, the most effective of these methods is the trench drain. In addition to their high rate of success in improving embankment stability problems, trench drains offer distinct advantages over other methods. This paper provides a description of the applicability, advantages, design and construction of the trench drains. COMMON TYPES OF INSTABILITY Track alignment and grade deflections can be caused by many different failure mechanisms. Some of the more common types of problems and their effects on the track structure and embankment are described as follows: Fouled Ballast Fine-grained clay and silt particles in the ballast voids can be derived from several sources including ballast breakdown, infiltration from surface sources, underlying granular layers or the subgrade soil, and tie wear. The presence of fines in the ballast reduces its permeability and the ballast layer can become saturated. Under train loadings, excess fluid pressures develop in the saturated ballast layer. This pressure is dissipated by mud pumping up to the surface. The contaminated ballast cannot resist the forces applied to the ties and track geometry problems result. In addition to pumping mud problems, fouled ballast can also contribute to subgrade failures. Dry, clean ballast will distribute the tie loads so that the stresses on the subgrade Page 2 of 2

3 are greatly reduced. Fouling and saturation cause the ballast layer to lose its load spreading ability and high stresses are transmitted to the subgrade soil through water pressure (Li, 1998). Subgrade Attrition Pumping mud can also occur from subgrade attrition. Attrition can occur when the subgrade consists of a soft rock or a hard clay layer that is in direct contact with the ballast and water is present above the contact between the two materials. During loading, the ballast is pushed into the hard layer causing local subgrade failure (Selig, 1994). A mud slurry is produced from failure of the fine-grained subgrade. The mud migrates upward during subsequent loading cycles fouling the ballast and eventually pumping at the ground surface. Progressive Shear Failure This failure mode occurs in fine-grained clay and silt subgrades. Initiation of progressive shear failure is often the result of inadequate ballast thickness (Li and Selig, 1998). The failure develops at the subgrade surface as the soil is sheared and remolded due to repeated overstressing. The soil moves outward and upward and cross-level develops in the track. This failure mode is usually apparent from soil heave along the track shoulders. The rotational movement of the subgrade and addition of ballast during resurfacing create a depression known as a ballast pocket below the track. Water becomes trapped in the ballast pocket causing a further strength reduction in the subgrade soil, and the situation worsens. Excessive Plastic Deformation Ballast pockets also develop from permanent cumulative strain of soft, fine-grained subgrade soil as it consolidates or compacts when subjected to repeated loading (Chrismer, 1998). Vertical displacements occur as the soil compresses and addition of Page 3 of 3

4 ballast is required to maintain track grade. Water becomes trapped in the ballast pocket reducing the shear strength of the surrounding soil. Embankment Slope Failures and Landslides Slope failures occur by processes that increase shear stresses in the slope or that decrease the shear strength of the soil mass. An example of an activity that increases shear stresses in a slope is excavation at the toe. However, the most common cause of slope instability is an increase in soil pore-water pressures caused by heavy rainfall. As the slope soils become saturated, the pore-water pressures increase causing a decrease in the soil shear strength. Movement of the soil mass is resisted by the shear strength along potential failure surfaces. If the shear strength is reduced sufficiently, the shear stresses will exceed the shear strength and sliding will occur. Cracks can develop and fill with water. Water in the cracks exerts hydrostatic pressure on the slide mass causing additional sliding. STABILIZATION METHODS Fouled ballast problems can be treated by removing the degraded ballast and replacing it with a more durable material. Placing a protective blanket of subballast between the hard subgrade layer and the ballast can often control subgrade attrition. Several methods are in use for treating progressive shear failure and ballast pocket problems. These methods include, but are not limited to, increasing the ballast thickness, placing an asphalt layer between the ballast and subgrade, and installing geosynthetics or cellular confinement layers over the subgrade. Another method involves pressure injection of various lime, fly ash, and cement mixes to fill ballast pocket voids and/or chemically stabilize the subgrade soils. Each of the above methods may be appropriate under certain circumstances; however, each has its limitations. Undercutting to remove fouled ballast is expensive, costing as much as $58,000 per mile (Wood, 1998). Increasing the ballast thickness to treat Page 4 of 4

5 progressive shear and plastic deformation problems may be limited by clearance constraints. In addition, the ballast pockets should still be drained or the increased ballast thickness alone may not solve the problem. Installing asphalt layers or geosynthetic products requires removal of the entire track structure. Slurry injection methods can be used without removing the track; however, their effectiveness is highly susceptible to mix design, subgrade soil properties, injection patterns and methods. Furthermore, proper implementation of these methods requires a thorough understanding of the subsurface conditions. Subsurface explorations and laboratory testing should be performed and proper design procedures used when available. Slope stabilization methods commonly used by the railroads include removal and replacement of the slide mass, toe buttresses, various retaining wall schemes, pile driving, excavation to unload the head of the slide, and subsurface drainage techniques. The appropriate stabilization method will depend on site constraints and subsurface conditions. In many cases, several methods may be appropriate for a particular site and the method selected is based solely on cost. TRENCH DRAINS Description Trench drains consist simply of an excavated trench backfilled with ballast. While water may not be the cause of all of the problems previously described, their occurrence generally leads to the introduction of water, which accelerates the problem. Removal of the water from the ballast, ballast pockets, and subgrade soils will do nothing but improve stability. Trench drains function by providing a high permeability path for water to escape from the embankment. Page 5 of 5

6 Ballast is usually selected as the backfill material because the railroads can acquired it conveniently and at a relatively low cost compared to other drainage materials. One criticism of trench drains is that the ballast does not satisfy standard filter criteria when in direct contact with fine-grained subgrade soils, and therefore they may be susceptible to soil piping. For piping to occur, a large pressure gradient must exist. Extremely large pressure gradients exist between the toe of a dam and the reservoir it retains. In railroad embankment situations, pressure gradients are usually very low and generally become negligible as the trench drain serves its intended purpose. In addition, the permeability of the backfill material must be high enough to rapidly drain open cracks and permeable ballast pockets. To our knowledge, no piping failures have occurred on any of the projects that we have installed trench drains. Applicability Trench drains are applicable to all of the stability problems previously mentioned. In embankments where fouled ballast and pumping mud problems occur, closely spaced trench drains oriented perpendicular to, and extending under, the track provide a dramatic increase in the overall permeability of the track substructure and help to slow the rate of ballast degradation. In progressive shear failure applications, trench drains oriented perpendicular to the track extend into the ballast pocket. The depth of the drain is controlled by the depth of the failure surface or by the embankment height. When possible, trench drains are installed so that they extend deeper than the failure surface. In this way, the drains not only provide ballast pocket drainage, but have the added benefit of increasing shearing resistance along the failure surface since the ballast backfill has a higher shear strength than the subgrade soil. Shear keys consisting of wide trench drains oriented parallel to the track are sometimes combined with perpendicular drains to provide increased shear resistance along the entire length of the heaved shoulder. Page 6 of 6

7 In embankment slope failures and landslides, trench drains can be very effective in reducing water pressures in cracks and along failure surfaces. Trench drains are appropriate for rotational and translational landslides that are generally less than 30 feet deep, where the critical groundwater level is not deeper than 15 feet, and where slope angles are flatter than 1.5H:1V (horizontal:vertical). Trench drains are commonly installed in landslides in conjunction with other stabilization techniques. For example, following mass excavation of material in the head of a slide, trench drains can be installed around its perimeter. The excavation reduces driving forces on the slide mass and enables the trench drains to extend to deeper elevations. Spoils from the trench excavations are often used to construct a buttress at the toe of the slide which increases the forces that resist sliding. Slope stability analyses should be performed to evaluate the effectiveness of trench drains for landslides. Advantages Trench drains have several advantages over other stabilization methods. First, each trench excavation serves as a subsurface exploration. The open trench provides a full view of the subsurface conditions. Before backfilling with ballast, the trench is logged as if it were a test pit excavation. A sketch is made of the trench with descriptions of the subsurface materials, dimensions, location and description of the failure surface, seepage, etc. Samples of the materials can be obtained if desired. By visually monitoring conditions in each trench, the failure mechanism can be evaluated and drain design parameters, such as spacing and depth, can be modified as needed to obtain the maximum improvement in stability. A second advantage of trench drains is that they can be installed while the track is in service. Drains that extend under the track in low embankments can be installed quickly. Track and Time permits or Form B Track Bulletins usually provide adequate protection. Longer work windows are sometimes needed for deeper drains in landslides. Only in rare cases does a track need to be removed from service in order to construct the drains. Page 7 of 7

8 A third advantage of trench drains is that construction requires no special equipment. Conventional earthwork equipment such as readily available excavators and dozers are used and can be operated by railroad or contractor personnel. The equipment is adequate for grading the ground surface and therefore, surface drainage improvements are routinely performed as part of the trench drain work. Finally, the simplicity of trench drains has to be considered among their advantages. Pipes and geosynthetic materials are used in some circumstances, but in most applications, ballast is the only material needed for a project. Planning and Design This section describes the typical steps involved in developing and constructing an embankment stabilization project. 1. The railroad determines the need to correct unstable sites based on slow orders, frequency of maintenance and severity of the problems. 2. The site or sites are visited with railroad personnel who have detailed knowledge of the unstable locations. 3. Data for each problem area are recorded, including milepost location, length of instability, track deflections, and embankment dimensions. Some stability problems and landslides may require more detailed site reconnaissance and subsurface explorations before remedial measures can be designed. If it is determined that trench drains are feasible, an initial estimate of the number of drains and ballast quantity required is recorded. 4. A report is prepared presenting the data collected in the field, recommended course of action, (which may not always include trench drains), and an estimated cost. 5. The railroad approves funding to perform the remedial work. 6. Ballast is dumped at appropriate locations. 7. Construction equipment is mobilized. Page 8 of 8

9 8. The work is performed with the assistance of an on-site geotechnical engineer or geologist. 9. A report summarizing the construction work is prepared. Trench Drain Installation Material Delivery The most common method of delivering ballast to project sites is by air-dump cars. These cars typically have capacities of 35 to 50 cubic yards. Georgetown Conveyor Trains are more flexible at placing ballast in desired locations and are ideal for sites with limited dumping space. Hopper ballast cars can carry up to 80 cubic yards of ballast. Doors on the bottom of the car can be opened to discharge ballast on either side of the track or in the center. On a few projects, trenches were backfilled by discharging ballast directly from the hopper cars that had been pulled over the open excavations. Finally, material can be delivered by truck if the site is accessible. Installation Equipment Equipment needs will vary depending on specific site conditions. A rubber-tired backhoe is often adequate for low embankments. For most embankment and landslide projects, a hydraulic excavator is required. Installation of drains in steep slopes or limited access areas may require a long-reach hydraulic excavator. Small tilt-blade dozers are generally used to backfill the drains, grade the excavation spoils and improve surface drainage. A dozer with a slopeboard attachment is useful for grading slopes. Front-end loaders are necessary when the ballast needs to be hauled from the stockpile area to the drain locations. Page 9 of 9

10 Economics of Trench Drains The application of trench drains offers economical solutions to the stability problems discussed above. Trench drains can be constructed under the track structure with minimal impacts to train traffic. Engineered trench drain solutions are typically 5 to 10 times less costly than structural or removal and replacement methods for solving embankment and slope failures. This cost difference does not include the cost benefit of keeping the track in service during the remediation work. The cost of installing trench drains in chronic unstable areas can frequently be less than the annual maintenance costs required to keep the track in service. When operational cost impacts of slow orders and reduced efficiency is considered, the return on investments in trench drain solutions is even quicker. CONCLUSIONS Trench drains provide an excellent means of improving drainage from chronically unstable embankments. They are extremely adaptable to various site and subsurface conditions. Trench drains provide several advantages over other stabilization methods. They are simple to understand, economical to construct, and have proven effective on thousands of sites around the country. Page 10 of 10

11 REFERENCES 1. Chrismer, S., Design Considerations for Track Substructure, Roadbed Stabilization and Ballast Symposium, AREMA, July 28-29, 1998, pg Li, D., How to Design Ballast Depths for New Track Construction, Roadbed Stabilization and Ballast Symposium, AREMA, July 28-29, 1998, pp Li, D., and Selig, E.T., Method for Railroad Track Foundation Design I: Development, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 124, No. 4, pg Selig, E.T., Track Geotechnology and Substructure Management, Thomas Telford Services Ltd., 1994, pp Wood, J.G., Drainage Related to Subgrade and Ballast, Roadbed Stabilization and Ballast Symposium, AREMA, July 28-29, 1998, pg. 4. Page 11 of 11