ABSTRACT
As part of the ongoing “New Water Supply Oslo” project, the clean water tunnel construction includes excavation with a double shielded TBM through the Ekeberg Fault, a prominent regional fault in Oslo. This area presents a complex environment for tunnelling projects. The experience with TBM-excavation through this fault highlights the convergence of multiple geological challenges, making it a notably difficult area for such undertakings. The combination of heavily fractured rock caused by the fault, substantial water inflow risks, minimal rock overburden of only 5 to 10 meters, and over 70 meters of sensitive clay created a uniquely challenging environment. These factors required careful handling to prevent settlement issues that could affect the surface structures above. The area above the tunnel includes Norway’s busiest road, a Alnabru Railway Terminal, and a shopping centre, all highly sensitive to ground movements caused by tunnelling activities. Another significant challenge was the presence of acid-forming alum shale in and after the fault. This required special attention and adjustments in the TBM-excavation process to manage its acid-generating potential, adhering to stringent environmental standards. This uncertainty necessitated continuous monitoring to ensure the TBM could navigate through the fault zone effectively.
To address these challenges, a TBM with an innovative design was employed, equipped with an advanced drilling shield. This shield offers additional space for high-tech drilling jumbos, facilitating efficient probe drilling. Extensive pre-excavation investigations and a comprehensive pre-grouting program through the zone, along with continuous monitoring of pore pressure, were essential to ensure project success while safeguarding the overlying infrastructure and maintaining environmental compliance.
This article summarizes the lessons learned from the TBM-excavation through the Ekeberg Regional Fault, focusing on the innovative design and operational strategies employed to navigate the complex geological conditions. The experiences gained will provide valuable insights for future tunnelling projects facing similar challenges.
