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Luleå University of Technology

Technical Report / 2004:11

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TITLE
Stability of shallow seated constructions in hard rock: a pilot study

AUTHOR
Töyrä, Jimmy

DATE
2004-08-12

DEPARTMENT
Civil and Environmental Engineering / Rock Mechanics

SUMMARY
Due to lack of space on the ground in urban areas there is an increased need for underground constructions. These constructions are often situated at shallow depth. By a better understanding of the mechanical properties and the state of stress in the shallow parts of the bedrock it may possible to determine which factors that are of importance at a specific construction site.

The objective of this report is to identify and describe relevant factors that influence the behaviour and the stability of shallow seated tunnels. This pilot study consists of a literature review that contains studies of the mechanics and design of shallow seated tunnels and some case studies and introductory conceptual numerical analyses. These analyses include a sensitivity analysis to establish a reference model that will be used in further work as well as a preliminary study of a few chosen factors, namely the effect of the overburden and horizontal stress.

The first analysis showed that the tangential stress in the tunnel roof increased with decreased overburden, as long as the overburden is not damaged. However, the horizontal stress at the ground surface decreased with decreasing overburden. This is probably due to an increasing heave of the overburden with a decreasing thickness. This was shown by examining the vertical displacements of the tunnel roof.

The modelling of different stress situations showed that heaving of the overburden was strongly dependant on the horizontal stress. Furthermore, an increasing horizontal-to-vertical stress ratio induced tensile stresses at the ground surface, as well as heaving.

The preliminary studies showed that numerical analyses can be a powerful tool when studying the effect of different factors on the performance of shallow seated tunnels.

ISSN 1402-1536 / ISRN LTU-TR--04/11--SE / NR 2004:11

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