The construction lot U2/2, an extension segment of the underground line U2, is part of the third extension phase of the Viennese underground system.Two single-track tunnels are located within tertiary sediments below a densely populated urban area. In the forefront of tunnelling, the groundwater level had to be lowered by the means of wells over the whole construction length. Various equipment concepts for tunnelling were developed and a cost and performance analysis was performed. Due to gaps in the well-system and the complex geology the subsoil could not be drained completely. As a consequence jet grouting had to be used. In a new cost and performance analysis the various concepts were re-evaluated and assessed.