The Queen 's Gate Tunnel ( Welsh : Twnnel Porth y Frenhines ) , also known as the Butetown Tunnel , is underneath southern Butetown and follow a line underneath the Wales Millennium Centre . The 715 m ( 2 @,@ 346 ft ) twin tunnel was constructed using the cut and cover method of construction , which involve digging a trench for the tunnel and then roofing it over . The tunnel was constructed using reinforced concrete and a central wall separate the two sets of traffic .