Problematika primárního ocelobetonového ostění podzemních děl

Abstract

The paper presents an alternative procedure that allows the specific characteristics of primary lining implemented using numerical modelling to be taken into account during its design. Primary lining, as it is structurally construed in accordance with the fundamental principles of the New Austrian Tunneling Method (NATM), is characterized by a time-dependent deformation property, which is determined by its design. The main design element is shotcrete, which, shortly after its application, provides structural interaction with the surrounding rock mass, which develops during the excavation until reaching the equilibrium stress-strain state of “rock mass – tunnel lining”. During this time, the structural interaction varies depending on the hardening of shotcrete, the rheological properties of rock mass, and other circumstances. The method to reinforce the primary lining is specific due to the distribution of steel reinforcement over the cross-sectional area. In addition to reinforcement meshes, which are evenly distributed in the cross-section of the lining, use is also made of reinforcing arched beams. These beams are fitted with an axial distance which is generally identical to the length of the burden, which consequently leads to uneven reinforcement of the cross-section. If it is necessary to take account of the reinforcing steel arches, such a structure then cannot be assessed as a reinforced concrete structure under the common Eurocode 2 procedure. As an alternative solution, use is made of the principles applied in the design of composite steel and concrete structures, or other alternative procedures. The procedure introduced in the paper provides a user-friendly alternative to take account of the above specific properties of primary lining. The solution uses the software Homogenization, developed by the Faculty of Civil Engineering, Department of Geotechnics and Underground Engineering. This software allows taking account of the heterogeneous structure of the lining by its replacement with a homogenous cross section. As a result, the parameters of homogeneous primary lining, which takes account of the presence of steel reinforcements and the time-dependent property of shotcrete, are included in numerical models.