Analysis of the stabilising effect of ribs of reinforced sprayed concrete(RRS) in the Løren road tunnel

Abstract

The Løren tunnel is a road tunnel at Ring road 3 in Oslo, Norway. The tunnel has a length of 915 m in rock, has two tubes with three lanes and breakdown fields, and was first opened in 2013. For rock support in the case of weak rock masses, ribs of reinforced sprayed concrete (RRS) were used. The scope of this article is to present and analyse the results of a measurement programme carried out on three of these ribs. This is done by focusing on deformations in the rock and the support function of the ribs due to these deformations. The instrumented RRS had strain meters installed in the reinforcement and the concrete. From the surface above the RRS, multipoint borehole extensometers were placed to survey the soil and rock mass deformations caused by tunnel advancement. In addition, 2D and 3D rock stress measurements and rock property testing were conducted. The measurements and numerical modelling show that the deformations are too small to cause a considerable load on the installed support construction and that the 2D stress measurements seem to best fit the in-situ stress conditions. The rock mass quality in the area of this study is on the verge of where one usually starts using reinforced ribs. It is concluded that the RRS are not required because of deformations in the rock but, rather, because of the need to lock blocks, increase the friction in joints and prevent movement in larger filled joints. For this purpose, the RRS should probably be designed differently to get the most out of the materials used.