Bjørn Nilsen

Significance of Geological Parameters for Predicting Water Inflow in Hard Rock Tunnels

One of the most challenging aspects of tunnelling is prognostication of water inflows. More reliable prediction of groundwater inflow may give considerable economical saving for future tunnel projects and may also prevent damage of environment and installations on the surface. This paper is discussing the significance of eight hypotheses regarding geological parameters for predicting water inflow in tunnels. The respective hypotheses have been tested as part of a recent research project in Norway. Six Norwegian tunnels with different geological conditions were selected for the research; the Romeriksporten, Frøya, T-baneringen, Lunner, Skaugum, and Storsand tunnels. Based on detailed study of these tunnels, the hypotheses are tested by comparing water inflow with geological parameters and factors such as Q value, faulting, rock stress orientation, rock cover, thickness of permeable soil or depth of lake/sea above the tunnel, rock type, and width of weakness zones. It is found that four out of the eight tested hypotheses are supported, two have low to medium support and two are not supported. One unexpected result is that for the tunnels covered by this study, the water inflow was found to increase with rock cover.

Possible Concepts for Waterproofing of Norwegian TBM Railway Tunnels

The aim of this paper is to evaluate and compare the durability, life expectancy and maintenance needs of traditional Norwegian waterproofing concepts to the generally more rigid waterproofing concepts seen in other European countries. The focus will be on solutions for future Norwegian tunnel boring machine railway tunnels. Experiences from operation of newer and older tunnels with different waterproofing concepts have been gathered and analyzed. In the light of functional requirements for Norwegian rail tunnels, some preliminary conclusions about suitable concepts are drawn. Norwegian concepts such as polyethylene panels and lightweight concrete segments with membrane are ruled out. European concepts involving double shell draining systems (inner shell of cast concrete with membrane) and single shell undrained systems (waterproof concrete segments) are generally evaluated as favorable. Sprayable membranes and waterproof/insulating shotcrete are welcomed innovations, but more research is needed to verify their reliability and cost effectiveness compared to the typical European concepts. Increasing traffic and reliance on public transport systems in Norway result in high demand for durable and cost effective solutions.

Rock slope stability analysis according to Eurocode 7, discussion of some dilemmas with particular focus on limit equilibrium analysis

In Europe, rock slope stability analysis and slope design have to be based on Eurocode 7, which is today the basic standard for geotechnical, including rock engineering, design. According to Eurocode 7, several principles for stability analysis may be applied, including empirical, limit equilibrium, numerical, and probabilistic. Very often, particularly for excavated slopes, the limit equilibrium method is used for stability analysis and design. One particularly significant consequence of the introduction of Eurocode 7 is that for limit equilibrium analysis according to the Eurocode, the traditional principle of calculating a factor of safety cannot be used. Instead, the so-called partial factor principle is to be used. The intention of the Eurocodes seems to be that Reliability Based Design should be applied also in rock engineering. This is, however, not reflected by the guidelines and descriptions of the present (2004)-edition of Eurocode 7, and for this and several other reasons there is considerable confusion and uncertainty related to the use of Eurocode 7 for rock engineering analysis and design. This paper discusses this issue with particular focus on limit equilibrium analysis of rock slope stability. It is concluded that due to the uncertain and variable character of input parameters, the limit equilibrium approach has evident shortcomings for stability analysis of rock slopes, particularly when the partial factor principle is applied.