Packer behavior under simple and coupled stresses

The packers are the contact elements normally used between tunnel segments with the purpose of distributing the loads in the joints without generating damaging stress concentrations. This material receives simultaneously normal and tangential stresses, being responsible for the behavior of the joint and for some of the anomalies observed during the construction of the tunnels. In spite of the importance it might have over the structural behavior of the lining, in the literature there are just a few experimental programs devoted to the characterization of the packer. The aim of this paper is to characterize the behavior of the packer under simple stress (normal) and under coupled stresses (normal and tangential), as well as to propose mathematical constitutive models to describe both behaviors properly. For this purpose, two new tests were designed to reproduce the conditions found in the tunnels and a thorough experimental program was performed with different types of packers. The results obtained for the test under coupled stress enables the identification of the possible failure mechanisms of the joints and confirm the hypothesis that there is another resistant mechanism that acts in parallel with the simple friction. The results obtained for the test under simple stress indicate that, contrarily to what is assumed in structural simulations, the packer present a nonlinear almost elastic mechanical behavior from the second load cycle onwards. In both cases, exponential models are proposed to represent the behavior of the packer. (C) 2011 Elsevier Ltd. All rights reserved.