TBM Challenges on Musaimeer outfall tunnel

Abstract

Musaimeer outfall tunnel is the longest outfall tunnel in the Middle East. The tunnel was excavated by Earth Pressure Balance (EPB), Tunnel Boring Machine (TBM) and encountered three distinct rock masses namely Rus Formation, Midra-Shale, and Simsima Limestone along with water inflows at high pressure, complex mixed ground, and weaker ground strata prone to cavities with the presence of vertical and lateral fractures connected to the seabed. These conditions resulted in the TBM operating in very dry conditions where the addition of soil conditioners and water sprayed in the excavations chamber and cutter head were required to avoid clogging situations and create a pasty excavation material, which impacted TBM operation. The opposite scenario occurred when the TBM encountered very wet conditions while passing through weathered Simsima Limestone fully connected hydrostatically with the seabed. Such situations directly impacted the productions rates, quantities of consumables, extra cost on the tunnel enabling activities and finally, the need of both atmospheric and hyperbaric interventions into the cutter head up to 3.5 bar to replace cutting tools. Long tunnels with only one access shaft pose many programme problems for logistics, combine these with the requirement to drive directly out under the seabed for 10.2 km, and the programme issues become even more challenging. To meet and eventually improve on the planned completion date required a complex and extensive management of all tunnel logistics activities required to support consistent tunnelling for a two-year period. This technical paper will discuss the management issues and solutions implemented to meet the challenges of the outfall tunnel construction.