Assessment of damage zone thickness and wall convergence for tunnels excavated in strain-softening rock masses

Abstract

There are two fundamental issues in all underground excavations, which are safety and economy. To ensure safety and expedite excavations, level of tunnel wall convergences and damage zone thickness should be predicted before the excavation starts, should be determined accurately, and monitored during the excavation by the tunnel designer. However, accurate prediction of these two parameters is difficult unless in-situ stress and deformation measurement tools are used. In this study, damage zone thickness and tunnel wall convergence relation was investigated for horseshoe-shaped highway tunnels by using empirical methods. For this aim two-dimensional plane strain finite element models were used. Totally 9 tunnel sections were selected for the empirical analysis from 5 different ongoing tunnel excavations, which are excavated in weak and fair quality rock masses and show strain softening post-failure behavior. With the help of data gathered through in-situ convergence measurements and predicted convergences and rock mass geotechnical properties an empirical analysis was conducted. Engineering characteristics of the tunnel routes were determined by means of geological mapping, drillings and laboratory studies. Tunnel wall convergences were measured by optical measurement devices in three-dimensional space. Then, numerical models have been created for each of the tunnel sections studied. For evaluation of damaged zone thickness; yielded elements, volumetric strain, and principal stress concentrations have been used. Damage zone thickness and convergences were estimated from the numerical models and the whole data were compared with the real convergence results. Findings have been compared with previous researchers’ convergence predictions and plastic zone calculation approaches. The results are in agreement both with the field measurements and previous empirical approaches. In this way, an empirical relation has been obtained for tunnel wall convergences and damage zone thicknesses. Besides, through an analysis of the relations of convergences at each query points in plane strain finite element model, a new empirical relation has also been put forth that gives “Convergence Constant” for modelled cross-section. This constant can be used as an auxiliary tool for prediction of next section convergences, on the condition that excavation has similar geological, geotechnical properties and topography.