Trackbed Isolation

GERB Spring Systems for Floating Trackbeds

Range of Applications

GERB Floating Slab Track Systems are the most effective and reliable solution for protection against mechanical vibrations and ground-borne noise generated by passing trains. They have been successfully installed in tunnels, on grade as well as on bridges and viaducts.

GERB Floating Slab Track Systems are available for nearly all types of rail traffic, including

  • LRT

  • MRT

  • Subway

  • Passenger and freight trains

  • High-speed trains

Description of the GERB Floating Slab Track System (FSTS)

In a floating slab track system, the rails are fixed to a concrete slab or ballasted trough, either directly by means of a fastening system or indirectly on rail ties. Dynamic forces are generated by passing trains at excitation frequencies above the natural frequency of the GERB FSTS. The slab or trough is isolated from the tunnel invert or other substructure by individual spring elements containing highly elastic helical steel springs. The springs form an elastic interface with the mass of the slab, effectively isolating the dynamic forces. Only the dead load of the train and a very small percentage of the dynamic forces are transferred through the springs into the substructure.


Helical Steel Springs

Helical steel springs are ideal for vibration isolation. Advantages include

  • High load-carrying capacity

  • High elasticity, providing a low system natural frequency

  • Specified stiffness in the 3 main directions

  • Linear load deflection curve

  • No difference in static and dynamic stiffness

  • No change in properties over time

  • Nearly unlimited lifetime


Isolation Efficiency

The efficiency of a floating trackbed system depends mainly on the vertical natural frequency of the system, with and without live load, damping and on the bending natural frequencies of the floating slabs. GERB spring systems are economically designed to natural frequencies as low as 4 to 8 Hz. They can provide very high attenuation values, up to 20 to 25 dB, even at low excitation frequencies. System damping can be added to the springs as required which also improves the mitigation effectiveness at higher frequency levels.

 

Types of GERB Spring Elements

GERB offers three different, standard types of elements for trackbed isolation:

  • Prestressable GP-Elements

  • Non-Prestressable K-Elements

  • Jack-Up GSI-Elements

For more information select the system of your interest.

 

Design Criteria

proper selection of elements for any axle load, including the 25-ton axle in the UIC 71 load configuration.

When selecting a spring, the relationship of the dynamic load to the static load is an important issue. For a given static spring deflection, the permissible dynamic deflection is limited, as shown below.

 

Fig. Example for the permissible compression of a spring

Fig. Example for the permissible compression of a spring


The permissible limits depend on the properties of a particular spring, based on the conditions set forth in DIN 2089, part 1.

The loaded parts of a spring element, including the steel spring, are fatigue tested under high loads to more than 2 mio. load cycles. Further tests performed by institutions in Japan and China have proven that the physical properties of the system are not expected to change over time.

 

Fig. Shaker on a floating test slab

Fig. Shaker on a floating test slab


The elements are typically arranged in pairs per track in the longitudinal direction. Spring type and spacing of spring elements depend on the following parameters:

  • the mass of the track slab

  • the stiffness of the track slab

  • the design axle load and velocity of the trains

  • the radius of the curve

  • the slope of the curve

  • the support areas in the substructure

GERB recommends the design of longer slabs, with a length between 20 and 50 m. Studies have shown that the low bending natural frequencies of these longer slabs have a positive influence on vibration and structure-borne noise control.

Longer slabs also keep the number of costly joints to a minimum. The gap between adjacent slabs should allow for thermal expansion of the concrete. Loads are transmitted from one slab to another by the rails, or by shear-dowels connecting the slab ends.

At the transition from the floating slab system to the conventional trackbed, the number of springs is increased to achieve a smooth and gradual adjustment of stiffness.

The system lay-out can either be done by GERB or other specialized consultants.

References for GERB floating trackbed systems, from Light Rapid Transit to High-Speed Train lines, are available.