A new technique to increasing safety of stone pavements through controlled evolution of unevenness

Stone element pavements constitute a typical feature for many artistically valuable urban environments in Italy and Europe. Unfortunately, their deterioration under traffic loads may generate discontinuities in road surfaces compromising ride safety especially for two-wheeled vehicles. Along tramways, due to pumping phenomena, such discontinuities may become remarkable, with local very severe elevation and rotation of the blocks. Besides a loss of safety for two-wheeled vehicles, other consequences are higher noise and derailment of trams. A complex multibody simulation allowed to set up a method able to quantify the danger level according to the actual road condition. Classical maintenance of this type of distresses along tramways was shown to be not effective at all, needing continuous repair with high costs and without being able to assure safety for users. In order to improve road safety a new rehabilitation technique of the pavement was developed. Such technique is based on the use of a polyurethane resin (set up in collaboration with chemical partners) able to seal the joints creating a structural continuity between the stone blocks. As consequence, the blocks are not allowed to locally displace as high stability is reached whereas, in case of permanent deformation of the pavement due to subgrade or foundation, the structural continuity supplied by the resin allows generation of only very long wavelength unevenness, less dangerous for riders respect to local discontinuities. In this way the evolution of unevenness is totally controlled. Since 1999 many laboratory and in situ tests (run in very important and trafficked Milan roads - still under control), enabled to set up and validate the right formulation for the resin and the whole technique. Such technique is now becoming a practice in Milan rehabilitation works of stone pavements. All the experience became also of interest for an international workgroup aimed at studying methods to increase safety of urban roads when tramways are embedded into the pavement. The present paper describes the research and the main results obtained with the aim to transfer acquired experience and knowledge to other Municipalities with face similar safety problems.