Abstract
Total plastic deformation in tunnels passing through weak and schistose rock mass consists of both time-independent and time-dependent deformations. The extent of this total deformation is heavily influenced by the rock mass deformability properties and in situ stress condition prevailing in the area. If in situ stress is not isotropic, the deformation magnitude is not only different along the longitudinal alignment but also along the periphery of the tunnel wall. This manuscript first evaluates the long-term plastic deformation records of three tunnel projects from the Nepal Himalaya and identifies interlink between the time-independent and time-dependent deformations using the convergence law proposed by Sulem et al. (Int J Rock Mech Min Sci Geomech 24(3):145–154, 1987a, Int J Rock Mech Min Sci Geomech 24(3):155–164, 1987b). Secondly, the manuscript attempts to establish a correlation between plastic deformations (tunnel strain) and rock mass deformable properties, support pressure and in situ stress conditions. Finally, patterns of time-independent and time-dependent plastic deformations are also evaluated and discussed. The long-term plastic deformation records of 24 tunnel sections representing four different rock types of three different headrace tunnel cases from Nepal Himalaya are extensively used in this endeavor. The authors believe that the proposed findings will be a step further in analysis of plastic deformations in tunnels passing through weak and schistose rock mass and along the anisotropic stress conditions.
Highlights
Stability of tunnels passing through weak and schistose rock mass is influenced by two important considerations, which are assessment of extent of tunnel deformation and requirement of support pressure to contain the deformation
A comprehensive assessment of the recorded plastic deformation of the selected headrace tunnel sections helped to understand on how stress anisotropy plays an important role in the extent of rock mass deformation in the tunnel periphery
It is highlighted that the time-independent deformation is mostly dominating and often the most crucial part of the plastic deformation, which takes place immediately after the tunnel excavation and until the tunnel face effect is ceased
Summary
Stability of tunnels passing through weak and schistose rock mass is influenced by two important considerations, which are assessment of extent of tunnel deformation and requirement of support pressure (stiffness) to contain the deformation. Wang et al (2014) had proposed an analytical formulation for the stress and displacement fields in the host rock and in the liners by accounting sequential excavation for lined circular tunnels excavated in viscoelastic rock and supported by any number of elastic liners installed at various stages Their approach is relevant and applicable to a tunnel of large cross-sectional area where sequential excavation method is applied. Relation between time-independent (instantaneous) and total tunnel deformation (tunnel strain), and rock mass deformability properties, in situ stress and support pressure is established and discussed To achieve this goal, the measured deformation records of 24 tunnel sections passing through graphitic, siliceous and micaceous phyllite as well as in schist and schistose mica gneiss of three different headrace tunnels of the hydropower project located in the Nepal Himalaya are exploited. The comprehensive tunnel monitoring records, rock mass quality records, laboratory testing results, topographic and geological information of the project areas are used as background material for the assessment
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