A highway tunnel passing through the Alpine mountain series formed of flysch and ultrabasic rock, collapsed and caused a very significant delay in the progress of the work. The causes of the collapse are investigated, the repair measures are presented and conclusions are drawn
A Tunnel section collapse stopped the tunnel excavation works for almost 1.5 year. As a result of the failure the contractor was declared in default and effectively withdrawn from the site. Fortunately there were no fatalities. Nevertheless there was economic damage. This was due to:
The cost of repairing the tunel area affected.
The delay in providing services by a chain of projects with which the tunnel is linked, such as highways and ports.
The increase of the construction cost of the rest of the tunnel
Although for the particular project the last mentioned cost item may be the most important, its importance may vary from project to project. The unemployment of the people involved in the construction may constitute an additional social problem. The impact of such a collapse may sometimes be very serious to the economy of an economically small country. In order to avoid such an undesirable event, effort should be made to understand its mechanisms of failure. This case study illustrates the particular failure and its causes.
Section 2 describes the project and the geological formations through Which the tunnel is driven as well as the monitoring measures and their readings. Section 3 discusses the failure and the mechanisms causing it. The method used for the repair of it is given in section 4. Conclusions are drawn in section 5.
The 12 m wide excavated cross section of the 3500 m long highway tunnel is shown in Figure 2.1. The tunnel is at an altitude of 1100 m and it has a maximum overburden of 600 m. Local geology consists of ophiolite and sedimentary series. The former consists of basic and ultrabasic rocks such as peridotites, serpentinites and gabbros with limestone and chert inliers. The latter is a flysch formation consisting of intercalations of sandstone and mudstone. The two main rock series are separated along an over-thrust alpine nappe about 40 meter thick. The Tunnel is driven from both entrances and passes through the two rock series for about equal lengths. Of the ophiolites, the gabbros are competent rocks, whereas the serpentinites vary from competent (qu=60MPa) to very weak (qu=0.35MPa) according to weathering. The overthrust zone is completely degraded. The rock quality of the flysch is directly related to the proportion of sandstone and mudstone. The higher quality of the sandstones is found at the west entrance and these grade rapidly into siltstones and claystones (qu= 0.40MPa) going eastwards. The clay stone has medium plasticity and low swelling potential (4% volume expansion) due to a small presence of montmorillonite clay minerals. The bedding dip is shallow and parallel to the tunnel axis.
(Figure in full paper)