ABSTRACT: The San Marco bell tower dates back to 12th Century. On July 14th, 1902 the tower bricks masonry collapsed and 14,000 tons of debris dropped on the San Marco square causing the breakdown of the NE corner of the Marciana library and of the Sansovino’s loggetta. The reconstruction started in 1903 and was completed in 1908. The designers decided to enlarge the stone masonry foundation from the original area of 222 m2 to 407 m2. The new stone masonry was locked together to the old one. Since early ‘50’s, some cracks were discovered in the foundation block, indicating the detachment of the new masonry from the pre-existing one. The progressive widening of the cracks opening as been monitored since their early discovery and recent data triggered the decision of the Ministry of Infrastructures and Trasportation, through the Concessionary Consorzio Venezia Nuova, to reinforce the foundation. The intervention consists of inserting at two levels along the perimeter of the plinth prestressed titanium rebars, to increase the overall flexural stiffness of the foundation and stop further cracks opening. The intervention required excavations below the ground water level in soft/loose lagoon deposits within an extremely important monumental area and in presence of buried archeological remains, many of which were unknown. In such very sensitive environment, the works required severe precautionary measures to prevent even little settlements of the tower and surrounding monuments during excavation, dewatering and retrieval of the buried archeological remains. To achieve this goal a large number of deep in-place mixed columns was foreseen, to create both watertight retaining walls and bottom plugs of the pits to be excavated. The paper will briefly illustrate the main features of the strengthening project and present a detailed description of geotechnical works that made the implementation of the project feasible.