Foundation measurement, design for special trackwork: One often-overlooked key to extended service life for special trackwork lies beneath the surface, out of sight, but not out of mind - NRCNews - Transportation Technology Center Inc - Brief Article

Railway Track and Structures, May, 2001 by David D. Davis, Satya P. Singh, Don Guillen

Considerable research and development has been accomplished by Transportation Technology Center, Inc., to improve the performance of special trackwork for heavy axle loads and other high-dynamic load service.

Under the Association of American Railroads' strategic research program, TTCI has investigated foundation-improvement methods for both conventional track and special trackwork.

TTCI's theoretical work suggests that foundations can be designed to minimize the effects of high-dynamic loading seen at switches and frogs. Foundation stiffness is important for controlling track settlement and alignment, and foundation damping is important for minimizing vertical dynamic loads. Performance measurements of three subgrade-strengthening methods have shown that they are effective at increasing the stiffness of the track. The effect on damping characteristics is material specific, with the concrete slab decreasing damping, the cellular confinement system increasing damping and the hot-mix asphalt having no effect.

Design of foundations for special trackwork adds many complexities not found in conventional trackwork. These include:

High dynamic loading. Running surface discontinuities, such as flange-way gaps and switch-entry angles, can generate dynamic loads that are more than twice the static wheel load. The load environment differs from the surrounding track.

Different load application rates from having more than one traffic level in the same structure. Turnouts and crossing diamond frogs have three traffic levels: each incoming track and the combined track or frogs.

Significant changes in track stiffness. Due to functional requirements, switches and frogs are often stiffer and occasionally less stiff than the surrounding track, both laterally and vertically. These stiffness changes often occur over a few inches, creating running surface discontinuities and impact locations in the track under load.

Lack of access to the ballast for effective maintenance. Special trackwork often has big superstructure elements and obstructions, such as plate-work, timbers and switch motion devices, which reduce access to the ballast. These large elements are also difficult to effectively tamp with conventional equipment.

Dating back to the original construction of the railroads, special trackwork often has the same foundation as surrounding track. However, the loading conditions can be quite different. Since frogs generate dynamic loads from the flangeways and vertical stiffness changes, the dynamic load environment at a turnout or crossing diamond can be significantly more severe than in surrounding track. Dynamic loads that are two or more times the static wheel load are not uncommon in special trackwork. Thus, frogs and switches tend to become low spots in track.

Further complications arise in trying to maintain track surface and alignment at special trackwork. Large timbers, control rods and plate work inhibit effective surfacing with mechanized methods. Conventional tamping can be largely ineffective at correcting surface defects at large frogs and crossing diamonds.

Most machines have difficulty in manipulating such large and atypically-placed track components. Tamping, along with he high dynamic loads at the frogs, further breaks down the ballast and, over time, fouls the drainage at the frog or crossing diamond. The resulting surface increases dynamic loading and accelerates deterioration.

There are two philosophies for designing special trackwork: (1) Make the frog or switch as strong and rigid as possible, i.e., "bigger is better," and (2) design to minimize dynamics loading, i.e., "better mousetrap." Both philosophies are successful up to a point and the best approach for frogs uses some from both.

However, as wheel loads increase and available track time decreases, the latter approach becomes more attractive. The additional benefit of this approach is that the lower loading also lessens damage to the train.

The best ways to eliminate the discontinuities in special trackwork are presently being implemented or explored. These include 1ow-entry-angle switches with and without spiral geometry and various methods to eliminate unsupported flange-way gaps.

Innovative designs that can drastically reduce the dynamic loading include movable wing or" spring" and movable point or "swing-nose" frogs for turnouts. Flange-bearing frogs appear to be an attractive, viable solution for high-angle crossings.

These are all relatively high-cost solutions best suited to locations with significant amounts of traffic. Many lower-cost solutions are also available that are economic to wider range of applications.

One solution to reduce vertical dynamic loading is to have a better longitudinal running surface design at the frog. The ramped-corner crossing diamond and renewed interest in turnout frog point slope design are examples.

Another method TTCI has evaluated is to optimize the damping characteristics of the frog to minimize vertical dynamic loads. TTCI has modeled the behavior of loaded freight cars going over conventional high-angle frogs to determine the effects of track stiffness and damping on vertical loads.