Numerical assessment of tunnel collapse: a case study of a tunnel at the East–West Algerian highway

Construction of long tunnels in difficult sites is associated with various risks. Such risks are usually resulted on one hand from the lack of geological and geotechnical studies of the surrounding ground, and on the other hand from the inappropriate support system. On January 1, 2014, the left tube of T1 tunnel located in Constantine (Algeria) collapsed during construction. The collapse incident caused enormous damage and significant economic losses. T1 tunnel is mainly excavated in a formation which consists principally of a poor behaviour altered argillite. Preliminary investigations have shown that the collapse for the most damaged part is due to stress concentration around the tunnel and the low mechanical strength of the preliminary support, which could not support the development of ground pressures. In this paper, two-dimensional numerical modelling using a finite difference calculation code was performed. In order to identify the causes of the collapse incident, the attention is therefore focused on the loads generated within the lining system. The final results are in good agreement with measurements, and also clearly highlight the inefficiency of the adopted support system. In order to resume and get back on the project, necessary appropriate remedies with considerable resources have been implemented and the primary lining rigidity has been improved.