The Effect of Drainage on the Functionality of Railway Track Sub-ballast

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The significance of railway track drainage has typically been emphasised in various guidelines, but little research data exists on its actual effects. Based on the literature, water content clearly has an impact on the loading resistance of earth structures, but the occurrence of detrimental displacements and geometry deterioration depend on the actual loads. In particular, there was a lack of information on the degree to which geometry problems could be reduced by increasing the drainage depth along railway tracks.

Several shortcomings were found in the implementation of railway track drainage
in Finland. For the most part, these problems are relatively simple issues, such as silted ditches, blocked culverts or subsurface drains. The original level of drainage has also been low in many places. Site drainage poses a more challenging problem, as conducting water away from the track area has been difficult in places due to reasons such as low height differences. Drainage maintenance design has largely been based on visual inspection, and maintenance contracts that call for maintenance ‘as necessary’ proved to be difficult to assess. This may have caused the functionality of drainage to have deteriorated over an extended period of time. Therefore, it is obvious that methods producing numerical data on the state of drainage should be adopted in order to optimise maintenance.

Three drainage monitoring stations were built along the Rantarata track in Southern Finland in places where the plan was to improve drainage. Material samples were collected from the sub-ballast layer at these monitoring stations and examined with several methods at the laboratory. The static triaxial tests found a clear increase in shear strength in all of the materials examined when the water content dropped below 7%. The effect of apparent cohesion was clear, and, at best, maximum shear stresses more than twice as high were achieved in a drier state. In the cyclic tests the samples with the highest water content proved to be the weakest. When the materials’ behaviour is examined in the predicted operating stress range, the Km44 and Km98 materials can withstand loading with relatively minor deformations, while the uniformly grained Km137 material, which has the smallest mean grain size, proved to be weak at high saturation degrees. However, the Km137 material also appears to withstand the impact of the axle loads of passenger traffic, even when saturated.

Based on field measurements, the water content of the railway track sub-ballast layer is largely determined according to the level of the (ground)water table at the site. The impact of rain on water content was lower than expected, and rain was unable to increase the degree of saturation beyond 60–70% due to the effect of absorption; instead, the impact of rain was primarily evident in the rise in the
(ground)water table. Water content varied by site, as expected. The drainage
improvement carried out at site Km44, which previously had poor drainage, resulted in a major improvement. The high saturation degrees corresponding to seasonal variation practically disappeared entirely, and the water content decreased significantly at the sub-ballast layer.

Based on the monitoring results, no connection could be established between the vertical displacements along the railway tracks and the water content of the subballast layer. The same observation was also made when examining supplementary
data from frost monitoring stations. However, this observation can be explained
with the laboratory tests conducted, as the load increase caused by passenger traffic in particular at the depth of the insulation layer is so small that weaker materials can also withstand the load as long as the intermediate layer is not completely saturated. The axle loads of freight traffic cause slightly more deformations, but heavy axle load(250 kN) trains usually operate only on mainly overhauled main track lines. The vertical displacements occurring at site Km44 could not be changed with the help of the drainage improvement carried out at the site. The most common causes of these displacements appear to be subgrade settlement, discontinuity points along the track or thaw softening after frost heave. Therefore, it is likely that in many places the formation of geometry irregularities cannot be prevented by improving drainage alone.

Based on the results, recommendations were issued for planning railway track
drainage. In principle, drainage should always be kept operative, but draining all of the structural layers, particularly in rail sections with a low traffic volume, may be cost-ineffective. The key content of the new guidelines is that the ballast layer and intermediate layer must be drained under all circumstances. With busy or heavy traffic, the drainage level must go even deeper than this. Based on this study, the axle loads currently in use in Finland do not necessarily require the drainage of sub-ballast to be extended to the design depth of frost.
ISBN (elektroninen)978-952-03-3301-0
TilaJulkaistu - 2024
OKM-julkaisutyyppiG5 Artikkeliväitöskirja


NimiTampere University Dissertations - Tampereen yliopiston väitöskirjat
ISSN (painettu)2489-9860
ISSN (elektroninen)2490-0028


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