Special design methods for construction of tunnel portals in areas with extremely steep morphology

Abstract

The highway ''Athens - Corinth - Patra'', which is an Olympic Games 2004 project, connects Athens to Peloponese. It is part of PATHE (Patra-Athens-Thessaloniki-Evzoni) highway project that links Peloponese to Northern Greece. In the section of the highway called ''Kakia Skala'', located approximately 50 km outside Athens, the highway tunnel AS1 is under construction. Tunnel AS1 comprises a double-bore tunnel, whose portals are located almost aside to the existing national highway. The length of the right bore (AS1d) is 845 m, of the left bore (AS1a) it is 280 m and the area of the cross-sections of the tunnel is up to 240 m2 (approximate height 14 m and width 19 m). The rock mass consists of slightly to intensively cataclastic limestones, conglomerates and tectonic breccia. The inclination of the open cuts in the tunnel portals is 80-90 degrees relative to the horizontal. Due to the existing steep morphology of the ground surface, the presence of an active seismic extended fault zone and the current environmental restrictions, the east portal (entrance) of the right bore of the tunnel AS1 and the west portal (exit) of the left bore of the same tunnel required the application of special design methods and construction techniques, which are presented in this paper. Considering the significance of ensuring slope stability in the area of the tunnel portals and the degree of uncertainty concerning the geological and geotechnical conditions of the area as well as the environmental restrictions, it was decided to limit the excavations for the construction of the tunnel portals through the application of innovative design methods, which involve the construction of two reinforced concrete structures (RCSP and RCS) and the application of the required support measures. The main difference between the two portals was that, in the east portal the tunnel cross section was about 30% inside the existing slope (RCSP), while on the west portal the tunnel cross section was about 70% inside the existing slope (RCS). The design and construction method applied for the modulation of the two portals is almost similar. However, in the former case a significant amount of concrete was replaced by a composite construction of polysterine boxes, which resulted in higher rate of excavation progress and lower cost. Numerical analyses were carried out in order to design and dimension the RCSP and RCS, as well as the support measures, which mainly included concrete pile walls, micropiles, pre-stressed anchors, fiberglass bolts and cantilever beams. Furthermore, the aim of the analyses was to ensure the stability of the reinforced concrete structures and the efficiency of the support measures. An extensive program of geotechnical measurements was undertaken during and after the construction of the two tunnel portals in order to assess the behavior of the open cuts, the function of the retaining system, the support measures and the reinforced concrete structures. The tunnel portals were successfully constructed and the tunnel AS1 was effectively excavated. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.