Laser scanning covers the points (Featured in Tunnels and Tunnelling)

The Thames Water Ring Main (TWRM) was one of the engineering  achievements of the 1980s. Some 80km of concrete pipeline, mostly 2.54m in diameter, surrounds the capital and acts simultaneously as a reservoir and as a distribution system, supplying around 3.5 million Londoners. At the time of its construction, the TWRM was the longest tunnel in the UK – longer by 30km than the Channel Tunnel, which had opened less than six months earlier.
It was excavated by TBM, mainly through the easily-tunnelled and forgiving London clay – though at one point under Tooting Bec Common it unexpectedly passed through a bed of the Thanet Formation, which led to flooding and the temporary abandonment of a TBM.
The initial ring main was constructed by Thames Water between 1988 and 1993. Extensions and branches have been constructed, and further extensions are planned.