The paper presents the results of centrifuge model tests and upper bound calculations on two-dimensional unlined tunnels in two different normally consolidated cohesive soils with the plasticity index of 10 and 25. The upper bound calculations were carried out by taking the variation of soil strength with depth and the strength anisotropy into account. It is found that the undrained stability of the tunnels increases with increasing tunnel cover-to-diameter ratio and that observed surface settlement profiles are different for the two soils. The critical tunnel pressures as well as the collapse mechanisms deduced from the bound calculation compare well with the observations. It is confirmed that the surface settlement profiles can be represented by the error functions fairly accurately.