Northern Line tunnel reconstruction at Old Street

Abstract

For many years, London Underground's Northern Line tunnels just south of Old Street station had suffered from attack by sulphuric acid. These tunnels were constructed between 1899 and 1901, and were enlarged between 1922 and 1924. Acid began to seep into the tunnels in 1945 and cracks appeared in the tunnel linings in 1960. Over the years, to ensure safety, London Underground Limited (LUL) monitored the increasing levels of distortion and cracking of the tunnel linings and installed temporary strengthening until the need for innovative long-term solutions became evident. This paper describes various investigations into the problem, the formation of a solution and the consequent construction works, which had to be carried out with minimal disruption to services within the complex operating environment of the two Northern Line branches. The solution was to replace the grey cast-iron linings with larger diameter acid resistant linings made from cast duplex stainless steel. The works were carried out at night using a special shield, through which the trains passed during the day. The project took six years from initial concept to completion, including more than four years of research and design, a precasting contract for the linings and finally nine months of installation works. In total, the project cost approximately £15 million.