At shallow depth, in situ stress can affect significantly the behavior of the rock mass around excavations, but stress measurements are usually subjected to high levels of uncertainty. This is the case at the Odenplan railway station in Stockholm where unexpected large values of displacements were recorded during the excavation. A sensitivity analysis of in situ stress in this area is thus required. As the rock mass is highly discontinuous, discrete modeling using 3DEC software is performed with three different in situ stress cases estimated from measurements. The results show that joint slip and rock mass dilatancy explain the large displacements and heaving of the ground and that a continuum approach is not reliable for this study. A comparison between the displacements from numerical modeling and those measured in situ provides indication on the range of in situ stress to be used in the future for this area.

1 Introduction

Underground constructions in urban areas constitute an important part of the infrastructure needed for society. Being at shallow depth, their potential impact on the environment and surrounding structures can be significant. Particularly in Scandinavia, where horizontal stresses are high, performing excavations at shallow depth in brittle hard rock, combined with irregular surface topology and open fractures, can lead to large deformations and upheaving of the ground. The estimation of in situ stress is required in order to study the behavior of the rock mass around such excavations and limit its potential deformation. However, data concerning in situ stress field is usually associated with high uncertainties particularly at shallow depth where it is affected by the topography of the rock surface, near-surface weathering, exfoliation joints… Additionally, stress measurements are performed at specific points and are usually subjected to high scatter. It is therefore important to complement these measurements with back analysis at a larger scale (Kaiser et al. 2000), in order to gain insight into the in situ stress field in a specific area.

This study deals with a preliminary back analysis of in situ stress in the Stockholm area using displacement measurements performed during the excavation of Odenplan station, which is part of the Citybanan, a railway tunnel under construction by Trafikverket. Around this station, in situ overcoring stress measurements have been performed (Perman & Sjöberg 2007) and present a considerable scatter. It is also important to note that the rock mass surrounding Odenplan station is highly discontinuous. Available data from mapping shows that open fractures are unusually persistent especially those with a dip angle from 50 to 70 degrees. During the excavation of the railway tunnel, large displacements were recorded in the area where the tunnel crosses under the surface subway track. Measurements in extensometers installed between the surface and the tunnel (up to 10 mm of extension) suggest heaving of the surface rock at some locations. Such amplitudes of displacements were not accounted for during the design. Thus, a precise analysis is required to explain the phenomena observed. The access at the Odenplan station to a wide set of data (rock mass and joint properties, joint geometry, deformation and stress measurements) makes it a unique opportunity to perform a sensitivity analysis of in situ stress for the first time in this area.

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