ABSTRACT:

This article deals with the numerical analysis of long-term behaviour of deep underground tunnel structures in claystone with "ductile lining" using PLAXIS 2D. The constructed gallery is a 2.6 m radius circular made of yielding sprayed concrete lining and is constructed at a depth of 500 m in a uniform claystone mass subjected to creep deformations. A visco-elastoplastic model has been used for describing the mechanical behaviour of the claystone. To prevent the development of excessive axial forces in the lining, yielding elements within the sprayed concrete lining have been used. They are modelled using a user-defined material model and special attention is dedicated in this article to the numerical formulation of such constitutive modelling. This article will finally present the short-term and long-term closure evolution results obtained in this context and highlight the good agreement between experimental measurements and numerical predictions, indicating the accuracy of the implemented creep law.

INTRODUCTION

Deep excavation in squeezing grounds for tunnel engineers normally implies large sometimes excessive tunnel convergences. The development of such deformations usually takes slowly lasting for one week, several months, or more than one year, after tunnel excavation is completed. Conventional rigid tunnel shotcrete linings, where rock deformations are strictly limited, are unable to work against great overburden pressure the excavation of which will almost systematically lead to considerable rock deformations and associated phenomena such as shotcrete falls or cracking, or even serious tunnel collapse.

To avoid shotcrete lining failure in deep excavation through squeezing grounds, the use of yielding elements in shotcrete lining, leading to the so-called "ductile lining", has gradually gained more attention. Yielding elements show a stronger deformability than shotcrete, providing shotcrete lining with high possible resistance and able to accommodate the controlled rock deformations through their compressible deformations.

Structural design of shotcrete lining with yielding elements requires the proper consideration of initial stresses in the rock mass as well as a correct evaluation of delayed deformation that will develop through time after the excavation process and support installation have taken place. This requires the use of appropriate constitutive models for the rock mass in which irreversible strains associated with creep effects as well as possible local shear failure can be considered. Proper consideration of soil-structure interaction is also essential for reliable assessment of structural force development over time. Finally, an accurate representation of the yielding elements and its effect on reducing the normal forces within the lining with acceptable safety margin must be achieved.

This content is only available via PDF.
You can access this article if you purchase or spend a download.