The excavation of urban tunnels in ground with low cohesion can produce over-excavations leading to frontal instability phenomena (the most typical, produced in front of and above the shield). But, in the event of there existing layers of sands without any fines above the crown and then resistant ground on top of that, over-excavations, cavities in the sand and surface cave-ins take place up to a distance of a diameter and a half towards the rear (dorsal instability). This paper describes the phenomenon and presents theoretical studies of it, along with solutions for avoiding the problem.