Due to modern earthworks technologies, it is possible to more intensively compact the load-bearing subsoil material used in the construction of railway substructure. At the same time concepts are being developed to minimize the costly incorporation of base layers by preparing the formation and subgrade protection layers with binders.
These measures usually result in the earthworks having better characteristics with respect to their load-bearing capacity and functionality. However, the improved compaction of the soils and the load-bearing subsoil materials as well as the treatment of the soils with binders are also associated with lower elastic deflection of the rail and the carrying system when the train passes by. As less elastic deflection results in increased stress, stiffening of the subgrade and protection layers leads to increased stress and damage of the railway ballast bed and the railroad substructure. A solution to avoid excessive damage to the railway ballast bed is the installation of a multi-functional geocomposite beneath the ballast layer.
1. Problem definition
With the further development of the technologies in earthwork construction it becomes possible to compact the in-situ soils and used base course materials in railway substructures more intensely. At the same time concepts are developed to minimize the expensive installation of base courses by preparing the in-situ soils by using binding agents.
These measures generally lead to better properties of the earthwork constructions as to their bearing capacity and their usability. The better compaction of the soils and base course materials as well as especially the soil treatment with binding agents are however associated with lower elastic deflection of the track and the bearing system during the train crossings. However, the higher the stiffness of the substructure, the more concentrated is the load transfer and less sleepers are involved in this load transfer. The result is an increase of the surface pressure under a sleeper and thus the ballast load which will lead to a damage of the ballast and the railway substructure. An approach to avoid the overstressing of the ballast bed is the use of additional elastic elements.
NAUE GmbH & Co. KG in cooperation with the Dresden University of Applied Sciences and the GEPRO Ingenieurgesellschaft Dresden developed a new product by using nonwovens. This product installed under the ballast bed takes over the following functions within the system of the railway track:
- ensure the elasticity of the railway track,
- ensure the mechanical filter stability of the track ballast towards different soils,
- protect the base course surface from overloads and damages caused by the track ballast,
- drain the rainwater away from above the railway substructure,
- ensure a sufficient interlocking of the ballast stones with the geocomposite,
- reduction of the structure-borne noise by elastic decoupling.
Another aspect of railway traffic, which becomes increasingly important – especially in areas with a high density of population – is the noise caused by the railway traffic as well as the resulting vibrations. Also in this case the installation of elastic elements in the track can lead to a reduction of the structure-borne noise by decoupling. By the installation of the elastic geocomposite under the track ballast the complete superstructure is elastically decoupled from the substructure. Thus the transmission of vibrations into the substructure is considerably reduced.
2. Product description
The new geocomposite Secutex® EDF 5000 consists of two layers of mechanically bonded geosynthetics (polypropylene) (Figure 1). A combination of nonwoven and woven is used as carrier layer, a nonwoven is used as cover layer. During the manufacturing process of the geocomposite a rubber granulate is placed between these geosynthetics and finally these two layers are needle-punched over their entire length. Thus a permanent, shear strength transmitting compound over all layers of the geocomposite is achieved. At the same time the mechanical properties of the geocomposite can be defined by this manufacturing process.
Figure 1
3. Laboratory test results
In the laboratory for transportation at the Dresden University of Applied Sciences comprehensive tests concerning the mechanic and hydraulic properties as well as their changes as a result of constant loads as well as tests concerning the durability are carried out. The results of these tests are published in [1], [2] and [3].
These tests carried out under laboratory conditions show that the new type of geocomposite Secutex® EDF 5000 on the one hand in case of stiff subsoil brings an additional elasticity into the system of the railway track and at the same time shows a protection, separation filtration and drainage efficiency towards the formation. As a result the load conditions for the superstructure of railway tracks can be designed more convenient and the influences in the superstructure and substructure can be reduced.
4. Field test
4.1. Characteristic of the test section
The test field is located in the north of Berlin in the city of Hohen Neuendorf within the line 6087 on the railway bridge Puschkinallee at km 14,821. In this section a track rehabilitation of track 1 was planned including the installation of a sub-ballast layer (Figure 2).
Figure 2 View of the railway bridge which serves as test field
A special deformation resistant installation of the backfill area using cement stabilized or compacted soil wedge of high quality according to the examples given in figure 1 and 2 of the Ril 836.0504 [4] has not been carried out. The 20 cm thick sub-ballast layer was placed up to the point to the bridge abutment. The installation of the geocomposite Secutex® EDF 5000 was carried out with a length of approx. 6 m and approx. 9 m in front and behind the bridge. In this test field vibration acceleration measurements were carried out.
4.2. Evaluation of the vibration measurements
Altogether 11 rail crossings of mainly fully loaded freight trains with on average 25 waggons each and a velocity of 70 – 90 km/h could be recorded. In each relevant measurement cross-section the vibration accelerations have been measured. On the basis of this data the vibration velocities have been determined by single time integration and the vibration distances (elastic deflection) have been determined by double time integration.
For the evaluation, especially the measurement cross-sections in the embankment area (MQ) 1 without geocomposite and (MQ) 3 with geocomposite have been considered. Because their superstructure properties are comparable as well as because of similar soil dynamic properties of the substructure and the foundation these two measurement cross-sections were very suitable for the evaluation of the effectiveness of the geocomposite Secutex® EDF 5000
4.3. Determination of the insertion loss
The only difference between measurement cross-section (MQ) 1 and 3 is the existence of the geosynthetic composite Secutex® EDF 5000 in the measurement cross-section (MQ) 3. All other factors like the dynamic properties of superstructure, sub-base and subgrade as well as the dynamic stress caused by train crossings are however almost equal. As a consequence the level of insertion loss of the geocomposite Secutex® EDF 5000 can be determined according to [5] resulting from the difference of the two maximum “Third-Rapid Levels” LF, max (fTn). The level of insertion loss which is determined is shown in Figure 3.
Figure 3 Insertion loss – Comparison between MQ 1 and MQ 3 in the level of the sub-ballast layer
Despite the low stiffness of the sub-ballast/subgrade which is typical for existing lines, an improvement of up to 7 dB, on average of approx. 3 dB can be observed. In the case of a stiffer subgrade (e. g. in case of a binder-stabilized embankment) this value would correspondingly be higher. In case of a rigid subgrade (e. g. in case of application inside tunnel) values of up to approx. 20 dB can be achieved according to [6]. The (minimum) deterioration in the area between 12.5 and 16 s-1 is typical for sub-ballast mats and is a result of the resonance exaggeration in the area of the natural frequency of the geocomposite Secutex® EDF 5000.
Summary
The first experiences gained in laboratory and field tests showed, that by using the geocomposite Secutex® EDF 5000
- the filtration and separation stability between the track ballast and the substructure/foundation is guaranteed,
- an interlocking effect is achieved by pushing the track ballast into the geocomposite,
- the elasticity of the superstructure is increased,
- the dynamic loads are reduced because of its damping properties,
- an equalization of the load conditions is achieved and
- an extension of the working life of the ballast superstructure can be achieved.
As a consequence the new geocomposite Secutex® EDF 5000 offers – in the case of inhomogeneous, but above all in the case of load conditions with low deformations – a protection of the superstructure / the substructure / the foundation. This leads to an improved track position and to an increase of working life.
The following preferred fields of application must be considered:
- transitional area / locally restricted points of failure,
- soil treatment with binding agents,
- high-quality protection layers and
- rock as well as
- tunnels and bridges.
References
Großmann, S.; Lieberenz, K.; Ehrenberg, H. (2011): Entwicklung eines Geoverbundstoffes zur Verbesserung der Auflagerbedingungen von Gleisschotter bei Eisenbahnstrecken, 7. Geokunststoff-Kolloquium, Dresden, 20.-21.01.2011.
Weisemann, U.; Großmann, S. (2011): Untersuchungsbericht zum Geoverbundstoff Secutex® EDF 5000, Hochschule für Technik und Wirtschaft Dresden Fakultät Bauingenieurwesen / Architektur Verkehrsbau / Eisenbahnbau, Dresden, 08.07.2011.
Weisemann, U.; Großmann, S.; Ehrenberg, H.; Ewert, W.F. (2012): Development of a Geocomposite to Improve the Bed Support Conditions of Railway Track Gravel for Railroad Tracks, 5. European Geosynthetics Congress, Valencia, 16.-19.09.2012.
Deutsche Bahn AG: Richtlinie 836 – Erdbauwerke und sonstige geotechnische Bauwerke planen, bauen und instand halten. – Fassung 20.12.1999a mit 2. Aktualisierung, Frankfurt am Main, 01.02.2013.
Wettschureck, R. G.: Körperschall an Schienenverkehrswegen.-Gastbeitrag im Rahmen der Vorlesung Technische Akustik II, TU Berlin, Sommersemester 2010.
Wettschureck, R. G.; Kurze, U. J.: Einfügungsdämmmaß von Unterschottermatten.-Seiten 177 – 182, Acustica 58, 1985.
Prof. Dr. Ing. Ulrike Weisemann
Hochschule für Technik und Wirtschaft Dresden
E-mail: weisemann@htw-dresden.de
Dipl. Ing. Markus Hempel
NAUE GmbH & Co. KG
E-mail: mhempel@naue.com
