Publication: In-situ stress amplification due to geological factors in tunnels: The case of Pajares
tunnels, Spain
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Publication Date
2012
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Elsevier
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Abstract
Geological and geomechanical anisotropies can significantly increase the magnitude of in-situ stress in a rock
mass excavated for tunnelling purposes. Four tunnels for the new high-speed railway lines in Pajares in the
Cantabrian Mountains, N. Spain, were analysed and significant deformations was found to have occurred in
forty specific zones of these tunnels during excavation, requiring much more support than envisaged before
construction. Local factors influencing in-situ stress have been identified in these zones of the tunnels, related to
geological structures of high compressive tectonic stress regimes including thrust faults, folds with steep flank
dip, orwith geomechanical anisotropies in contact zones between rock formations of different strengths. Amethodological
procedurewas applied to four tunnels to assess the influence of the geological anisotropies on the insitu
stress magnitudes. This procedure is based on the analysis of tunnel deformations and the support pressure
needed to stabilise the excavations affected by geological anisotropies.
The increase of in-situ stress due to local factors is expressed by the Stress Amplification Factor (SAF) defined
by the ratio between the K(σH/σV)local value estimated in a particular rock mass tunnel zone and
the mean K(σH/σV)regional value representative of the whole rock mass tunnel. Kregional was estimated
from hydrofracture tests and from the TSI index. Klocal was calculated from the back analysis of the support
pressure required to stabilise the deformed tunnel zones. SAF values for the forty specific tunnel zones ranged
from 1.1 to 2.5. This significant increase in the magnitude of in-situ stress emphasises the influence of
geological and geomechanical anisotropies in tunnel stability and support design. The results provide a
quantitative approach for assessing structural stresses in rock masses for tunnelling excavations.