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).
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.
Specifically, in the track section between the cities
of Sentjur and Celie, bioriented geogrids in PP TENAX LBO
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.
The embankment was completed in winter of 1996 and the railway line
is fully operative.
The geogrids installed were monitored by means
of extensometers to verify the local shear stresses acting on the
The low mobilised geogrid strains show that the geogrid
is fully performing within the reinforced section as a rigid load
transfer platform; thus minimizing the shear displacement and consequent
loss of bearing capacity and settlements.
Compaction of the foundation base
Installation of the first course of 0.30 cm
Installation of the geotextile and geogrid
Typical cross section