Slope protection

Protection of rock slope cut 51 Lokve on the Zagreb-Rijeka railway line

The protection of rock slope cut 51 on the railway line M202 Zagreb GK – Rijeka (near the Lokve railway station) has just been completed. The works lasted from April to October 2018. The protection of the rock slope was carried out because of the rockfall danger on the track and other infrastructure elements. The company Geotech d.o.o. conducted the geotechnical supervision and survey supervision (Nekretnine d.o.o. Rijeka) and control tests (Geotest d.o.o. Zagreb). The investor is HŽ Infrastruktura d.o.o., the designer Geoekspert STM d.o.o. and the contractor Pružne građevine d.o.o.

Geotechnical exploration and investigation

Geotechnical exploration and investigation were conducted before the start of the project documentation (which consisted of detailed engineering geological mapping on a survey basis, measuring the discontinuous orientation, uniaxial compressive strength testing with a Schmidt hammer, the terrain classification and quantification of instability and processes on a slope). Also, samples of rock mass were taken and the uniaxial compressive strength in the laboratory was tested. These mentioned methods of exploration and investigation have enabled an insight into the physical and mechanical properties of the rock mass at the location on which the adequate protection of the slope was applied.

Designed measures of slope protection

The project documentation of slope-cut remediation was executed based on the geotechnical report.

The provided slope protection measures consisted of installing the supporting mesh (150 kN/m’ capacity) with anchors, supporting mesh (100 kN/m’ capacity) and the protective mesh.

Slope protection works

The survey stakeout was conducted before the slope protection works started and after the clearing of vegetation, clay, rocks and unstable blocks.

The designed slope protection needed to fulfil two components of general stability. The first component relates to satisfying of the global slope stability which is defined by the equilibrium equation of the mobilized slope strength and is described by the safety factor. The second component involves checking the local stability of slope segments (blocks) formed by the discontinuity orientation or special soil deposits.

Based on the conducted investigations, global and local analyses, it was concluded that there is a permanent possibility of local rockfall that endangers traffic safety. It was decided to provide slope protection by mesh installation to increase security against potential local instability. Two types of supporting mesh were installed along the section depending on the characteristics of the protected zone and are shown below.

Types of protection

Three types of rock slope – cut protection are foreseen:

• supporting mesh (150 kN/m’ capacity) with anchors
• supporting mesh (100 kN/m’ capacity)
• protective mesh

Mesh capacity 150 kN/m’

This type of protection was applied in places where the occurrence of the fall of larger rock blocks was expected to deteriorate the slope geometry and endanger traffic safety. The supporting mesh was set up in combination with the installation of the passive ground anchors diameter of 25mm, a length of 6.0m in increments of 3.0 m. On the supporting mesh and embedded anchors mounting plates were also set and tightened in order that in this way additionally preload the system over the entire surface of the slope.

Mesh capacity 100 kN/m’

Protecting mesh

For the stabilizing of local unstable blocks, a hexagonal double-twisted galvanized mesh reinforced with steel ropes is foreseen. At the bottom of the slope, the mesh is retained with 30x30x10 cm dimensions of concrete at intervals of 1.50 m.

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