ABSTRACT
In High Speed 2’s Northolt Tunnels West, innovative special segments were installed avoiding temporary supports around openings. This solution maximizes space proofing during the excavation while ensuring long term stability of the lining without permanent collars. The local ground conditions, consisting of clayey soil with thick sand channels and high-water pressures, required ground freezing to be adopted to provide a safe cross passage excavation. Therefore, the special segments had to cope with demanding freezing loads due to ground expansion, making this project a reference in the tunnel openings field. High-strength concrete along with pre-stressed connections for high shear and tensile forces transfer stiffen the ring joints reducing the risk of future stepping between rings and water leakage. The design required non-linear staged construction finite element analysis and extensive laboratory testing. This paper discusses the performance of the special segments under ground freezing load and compares laboratory, numerical and monitoring data.
