The causes of excessive settlement above Trojane Tunnel and remedial measures

The 2,900 m long twin Trojane tunnels, located on the motorway section AC A10 Ljubljana-Celje, are currently under construction. Starting from the east portal the construction works advanced on both tunnels to the most demanding section, which was immediately beneath the Trojane village. The tunnels are of about 11m diameter, at a spacing of about 50 m between axis, and are constructed using principles of the New Austrian Tunnelling Method (NATM). Difficult ground conditions, low overburden and the presence of the urban development above the tunnels all congregated at this particular section. The geological sequence consisted of weak sandstone, siltstone, claystone and numerous fault gauges. Due to the vicinity of a strong fault zone these strata are strongly deformed and crushed, creating almost chaotic geological sequence. The tunnel construction was carried out well above the water table. To lower the settlements the reinforcement of the primary lining was gradually introduced. The settlements were kept under control using the feed-back mechanism that was established through the wide observational scheme. The majority of the buildings in the village suffered only minor disturbance, some of them were damaged but not structurally challenged. There was one building that was severely damaged due to an inadequate foundation, which was unfortunately unknown prior to the event. The building C28, a barn, was founded shallow in the cut, partly on the rock and partly on the embankment of the material deposited from the cut. As the tunnel approached the structure, the part of the foundation on the soft embankment gave away and the building suffered severe cracking in the middle. The immediate measures, shown schematically in the figure, consisted of the structure that combined a row of 400 mm, 25 m long micropiles and the anchor-beam. Every two meters the anchor beam was supported by the pair of geotechnical anchors with a nominal capacity of 400 kN. These measures were not intended to control the settlement of the ground per se but to prevent possible complications of the event that could have included the collapse of the embankment and the building. The immediate intervention measures were carried out and successfully removed the threat of the collapse of the building. (A) "Reprinted with permission from Elsevier". For the covering abstract see ITRD E124500.