ABSTRACT
Oslo’s Fornebu Line metro project – part K2A consists of 2.5 km of tunnelling and four shafts excavated through Cambro-Silurian sedimentary rocks intruded by volcanic dikes, located in a densely populated urban environment. Challenges on the project involved the excavation of large underground caverns with complex geometry and overburdens as low as 3 m with numerous fault zones, including a soil section of the tunnel excavated by conventional methods.
A major challenge was the complex geometry of the shafts with limited space available at surface level. The excavation and ground support installation sequence was a key component of the success of the project. The excavation methods included conventional Drill & Blast, line drilling, pre-split, wire sawing and mechanical excavation.
Another challenge was the strict requirements for ground permeability and water ingress. This was overcome with a complex grouting and monitoring program. This reduced the permeability of the ground to such an extent that it eliminated the requirement for a concrete lining with a waterproof membrane in parts of the project most vulnerable to ground settlement.
The paper presents in detail how these challenges were solved, as well as covering sustainability criteria and how the ‘non-engineering aspects’ also made the project a success. These processes were driven by the Client, who was dedicated to assisting with ground investigations to obtain extra data whilst constantly striving for a safer, more efficient, and more sustainable project. Close collaboration between all parties was essential to ensure that these complex solutions were achieved.
