Sustained growth in urban development in southern Ontario has led to an increased activity in tunnelling in rocks. These sedimentary rocks are bedded and anisotropic in behaviour, In situ horizontal stresses considerably higher than the vertical overburden pressure exist in these rock formations. The combination of these two factors, together with the difficulties encountered in recent tunnelling projects, necessitates a reappraisal of current design concepts.Theoretical studies show that, because of these two factors, high stress concentration occurs at the crown and invert, causing failure in the weaker layers of the bedded rocks. In addition, the pattern and sense of movement are reversed when compared to 'normal' conditions of in situ stress state.Field observations made in three tunnels are described. The instability at the roof and floor and the observed pattern of radial displacements are consistent with the approach employed.Some requirements for temporary support under these conditions are outlined. Available field data on the timedependent deformation of these openings are presented and the consequent influence on the choice of permanent lining is discussed.