The railway track was built about 150 years ago and is made of coarse-grained rock, with an average height of 13.0 m and a base width of about 30.0 m. Under the track, work was carried out to lay a gas pipeline, for which a tunnel with a diameter of 0.65 m was excavated through the base of the track.

During the work, settlement of the ballast layer was detected. The tunnel laying was suspended until the ballast layer was restored and safety at the work site was ensured. According to the geological survey report, the base was made of homogeneous stone material, most likely irregular stones 5–20 cm in size, without fine aggregate. This structure is characterised by a significant void content. According to data from similar work on the Zagreb–Rijeka line, the approximate porosity coefficient is 35% in the upper part and 30% in the lower part. The most likely cause of the settlement was the collapse of the pipeline channel due to vibrations associated with the excavation. After the protective pipe was removed, it was decided to cement the section affected by the settlement. The attempt failed because the voids between the stones were too large to hold the concrete. Meanwhile, the settlement of the embankment continued (about 5 cm per week), and to compensate for the deformations, the ballast layer was repeatedly topped up with new stones at short intervals.

Goals
Thanks to a precise analysis, the most affected areas at the tunnel excavation site were identified, where injection using URETEK® technology was planned. The main goal was to stabilise the current situation and prevent future deformations.

Technology applied:
For this facility, the patented URETEK DEEP INJECTIONS® technology – the deep injection method – was chosen. This technology made it possible to reduce the number and size of voids in the embankment by filling the space between ballast layer particles, significantly increasing strength. This was achieved by injecting a rapidly expanding polymer composition with high pressure upon expansion.

Thanks to precise studies, the areas most subject to stress were identified, and injection points were concentrated there. Injection tubes with a diameter of 12 mm were installed in specially drilled holes in the embankment. To accurately distribute the injections, tubes of different lengths were placed into each hole.

Result
Train traffic on this section was not closed during the repair and restoration work, but for safety reasons the train speed was reduced to 20 km/h. Overall, the entire operation took about 15 working days, including drilling and grouting operations.