A bit of a local one here
The Birkenhead (Queensway) Mersey tunnel appears to be semi-circular, but is actually a complete circle. The space below the road deck is known as "Central Avenue" and was originally intended for use as a tramway, but these plans were eventually scrapped. At one point, the space was used to run gas pipes from one side of the river to the other, but now it is just used for maintenance and drainage.
The road deck in the tunnel has over 10,000 documented cracks in it and has already been strengthened three times. It can't be strengthened again without completely re-designing the tunnel's ventilation system.
And at £1.30 a go is still a rip off