Effect of yielding supports on the behavior of deep tunnels
As part of the extension works for the Underground Research Laboratory (URL) at Bure, different support profiles are used during the excavation of the GVA2 gallery, oriented along the minor horizontal stress. Classical lining segments are installed in zones 1 and 2. The annular gap is filled by a classical mortar in zone 1 and by a compressible mortar in zone 2. In zone 3, lining segments made compressible by a softer layer on the extrados are used.
Itasca has performed a two-dimensional numerical analysis with FLAC to verify the tunnel stability and the effect of support profiles on the tunnel behavior.
In this study, the Callovo-Oxfordian argillite surrounding the tunnel is modeled by a viscoplastic model which generalizes two models: one for the short-term response in which damage is approached by plasticity theory using the Hoek-Brown criterion; the other for the long-term behavior according to a modified Lemaître’smodel.
The conventional materials (concrete segment, classic mortar) are simulated using a Mohr – Coulomb model while the compressible materials (compressible concrete segment, compressible mortar) are represented using a Double – Yield model which includes, in addition to shear and tensile failure envelopes, a volumetric yield surface (or “cap”). The model parameters are calibrated from the results of laboratory tests.
Due to the high deformability of the compressible materials, the long-term tunnel convergences are significantly higher in zones 2 (with compressible mortar) and 3 (with compressible concrete segments) than in zone 1 where the traditional support has been installed (Fig.1).
Moreover, the compressible materials reduce and homogenize – even in the markedly anisotropic stress field created by the EDZ around the excavation - the normal force in the concrete segments (Fig.2).