Introduction

Tunnelling in urban areas has become increasingly challenging because of city development. Urban tunnelling usually implies that neither an optimal tunnel orientation nor an optimal tunnel depth can be achieved due to existing underground facilities or particular design needs (e.g., construction of a new station). A difficulty that often arises in urban areas is that the tunnel has to be excavated through mixed-face conditions with both soil and rock, which can have a major effect on the construction techniques and on the surroundings (see e.g., Clough & Leca 1993; U.S. Department of Transportation 2009; CEDD 2012).

This paper describes a case study of tunnelling in mixed ground conditions in the Stockholm area, and the post-construction numerical analyses. The objective of the study was to increase the understanding and knowledge of ground behaviour around tunnels in mixed ground conditions for future application and implementation. Moreover, an assessment of the performance of the reinforcement measures used in the project, such as compensation grouting, umbrella arch system, shotcrete lining and bolts, was carried out via numerical analysis.

Structural elements in FLAC3D model after the completion of the 30 meter tunnel excavation.

Keywords: calibration, chimney cave, collapse, FLAC3D, jet grouting, mixed-face conditions, pipe umbrella, reinforcement, rock, shotcrete, sinkhole, soil, support, tunnelling, urban

Lope Álvarez, D., J. Sjöberg, M. Eriksson, R. Bertilsson, and D. Mas Ivars (2016). "Tunnelling and reinforcement in heterogeneous ground – A case study," Ground Support 2016, Eighth International Symposium on Ground Support in Mining and Underground Construction, 12-14 September, Kulturens Hus, Lulea, Sweden, E. Nordlund, T.H. Jones, and A. Eitzenberger (eds.), Paper 193, 14 pages.