Planning and Implementing of Effective Enabling Works for a Successful TBM Tunnelling Project

Abstract

The outfall tunnel, which forms part of the Musaimer Pump Station and Outfall project (MPSO), is a 10.2 km long subsea tunnel that extends from the south coast of Doha, starting onshore, and continues under the Arabian Gulf to end in a connection with a diffuser field via riser shaft. The excavation of the outfall tunnel using a Tunnel Boring Machine (TBM) with a 4.42 m diameter and 180 m long backup, starts from a 40 m deep shaft and has an inclination of 0.05% upward to the riser shaft. The tunnel is constructed by precast concrete elements of 3.7 m internal diameter passing through different geological formations typical in the State of Qatar. Although planning a TBM tunnel involves a significant amount of engineering knowledge and experience, what is equally challenging is the enabling works which are required to be designed and constructed before the TBM even arrives at the site. On this project, these works which were substantial within themselves commenced 16 months before the TBM arrived on site. This paper will discuss the planning and execution of these outstanding underground enabling works, the logistic shaft required to facilitate the assembly and launch of the TBM at the initial stage of tunnelling and also how to arrange the sequence and space in terms of TBM logistics supply requirements to allow simultaneous construction of key permanent structures, namely the drop shaft structure which resulted in 6 months saving on the overall completion on this section of the works.