Tunnel Surveying Works: A Technical Challenge of 10 km-long tunnel under the Arabian Gulf-Qatar

The Musaimeer Pump Station & Outfall Tunnel project (MPSO) was constructed as one of the longest Outfall tunnels in the World with a total length of 10.189m and at the same time being the longest undersea tunnel in the Middle East. MPSO will manage ground- and storm-water from the Southern Doha District and dispose of this water via a diffuser bed connected to the outfall tunnel. Difficulties faced during the project execution were firstly, the tunnel network itself. It was necessary to maintain precision and reliability of the 10km network, in a tunnel of smaller than 5m in diameter, starting from a 26m diameter drop shaft. Secondly, the breakthrough was into a 3m diameter vertically positioned riser shaft encased in a 5m x 5m concrete block which was positioned and constructed offshore by utilizing different network of 10km Global Positioning System Real-Time Kinematic (GPS RTK) readings. The key problem was confirming that both survey networks used were homogenous as the marine tolerances in comparison to the tunnel tolerances were very different. Thirdly, the actual Tunnel Boring Machine (TBM) connection was under the sea. The TBM excavated through a concrete filled shaft 0.6m beneath the riser shaft invert, then excavated 35m to create working space for civil works. The tunnel's crown was then cut using a 3m diameter opening to connect into the bottom of the riser shaft. This study serves to describe the Survey technical issues faced in the construction and coordination of underground tunneling and interfacing with offshore works.