problem
The railroad line Trieste Vienna, is crossing the Slovenian territory on very poor foundation soils (soil foundation modulus always lower than 20 MPa and locally equal to 5-10 MPa).
On such conditions of excessive settlements, a drastic reduction of the trains were forced to travel at a low speed of 40-50 km/h) until major reconstruction was completed. Since it was necessary to substitute only one track at a time, increasing the traffic on the adjoining track, the possibilities to remove poor superficial soil (clayey silt and clay) were very little (limited to the first 0.60 m).
In order to assure the minimum soil elastic modulus required by the Slovenian Railway, it was necessary to stabilize the foundation layers (50 MPa) by using TENAX geogrids.

solution: Šentjur-Celje
Specifically, in the track section between the cities of Sentjur and Celie, bioriented geogrids in PP TENAX LBO 401 SAMP were used to stabilize the railways foundation.
TENAX LBO 401 SAMP geogrids are manufactured from polypropylene (PP) by a patent extrusion process and successively oriented in both MD and TD directions thus creating biaxial geogrids.
After the removal of the ballast and partial foundation soil, the foundation was compacted by means of a compactor roller (photo n. 1); then a geotextile, a TENAX LBO 401 SAMP geogrid (photo n. 2) and 60 cm layer of granular material were placed (laid and compacted in two different lifts) (photo n. 3) and successively covered with 30 cm of ballast. A nonwoven geotextile below the bioriented geogrid in PP to has been installed at the base of the ballast.
The geotextile provides the filtering effects between the foundation soil and the ballast, while the geogrid prevents the geotextile from excessive deformation and stiffening the subbase (reinforcement and separation functions).
In this way the so called risk of "pumping" of fine particles of the foundation inside the granular material of the ballast, due to the compression and decompressing cycles caused by the rapid passage of trains, was avoided.
The grain size variation, due to the increasing portion of the fine fraction, generates a progressive water retention within the ballast, with consequent greater settlements. Besides, the separation layer at the toe of the ballast allows a filtering action between the ballast and the foundation soil, avoiding the capillary raise from foundation.