Geogrid Reinforced Railway Embankment on Piles - Performance Monitoring 1994 - 1998
In 1994 a stretch of about 2 km total length of the German Railways -link Berlin-Magdeburg in areas with soft subsoil was rebuilt as geogrid-reinforced, relatively flat embankment on slender piles. It was the first application of such a system in a railroad in Germany. Due to the lack of experience in dimensioning and construction a monitoring programme has been performed from spring 1994 until autumn 1998. Subsoil conditions, system geometry and materials, dimen-sioning concepts and instrumentation are described. Important results of the system's behaviour dur-ing four years under traffic (160 km/h) are reported and analysed. It is probably the most detailed and long lasting monitoring programme for such structures. Based on the results the stretch was cer-tified for unlimited traffic by the German supervising authorities.
The measurements conducted over a period of four years under train traffic have revealed that the load-bearing system comprising a high-strength geogrid reinforced embankment on piles is capable of satisfying requirements. The properties of the poorly load-bearing soil influ-ence the deformation imposed on all bearing elements. The more unfavourable the soil con-ditions, the greater is the settlement of the over-all system and also the stress and forces in the soil reinforced with high-strength geogrids. De-formation of the reinforced embankment has increased with time, but the rate of deformation has fallen steadily off, so that the system has nearly reached a final state of equilibrium in the course of just a few years.
It is recognisable from the strain measure-ments in the geogrids that the reinforcement is subject to high stress and is therefore decisive in terms of the safety and serviceability of the load-bearing system. No difference has been observed between the tensile forces of the three reinforcement layers (top, middle and bottom). It can be concluded, that a membrane-bearing mechanism takes effect even in the three-level-reinforced system contrary to the plate-bearing and similar mechanisms which were alternative-ly assumed some years ago (see chapter 3).
The stress imposed on the geogrid is dis-tinctly greater under the track than outside of the track region. From the associated variability of the tensile forces in the reinforcement, it can be concluded that the interaction forces between geosynthetic reinforcement and soil and the compatibility of deformations are an important factor.
The measurements would lead to the conclu-sion that the system described has been suffi-ciently well dimensioned (to some extent on the safe side), including the geogrid-reinforcement. The "German" dimensioning concept described later with some modifications by Kempfert et al. (1997) seems reliable. The dynamic effects and the cumulation of remaining deformation from the first traffic loads would need to be ana-lysed in more detail.
The strains measured (and prognosticated by extrapolation) in the geogrids remain below the allowed level of 3 % established (conservative-ly) at the design stage. The stability of the rail road foundation is well guaranteed over time. The system is perfectly fit for use with little requirement for maintenance. The deformation recorded to date has not given rise to problems with the rail road. Personnel operating over the line in question have confirmed that trains run better on the sections comprising geogrid rein-forced embankment on piles than on those which underwent a conventional soil replace-ment. On the basis of the experience, observations and measurements the stretch with geo-grid-reinforcement on piles was officially certified for unlimited time by German supervising au-thorities in autumn 1998.
In the meantime further high-strength geo-grid-reinforced systems on piles are under traf-fic inclusive the German bullet trains (ICE) (Alexiew D. et al. 1999).