This project involved the structural strengthening and stabilisation of a 25‐metre stretch of bastions underlying Council Square and part of the Xara Palace Hotel. This was a pilot project in view of the longer tract of bastions also showing signs of displacement. In particular, this was happening further to the south underlying Vilhena Palace. The displacement was characterised by a downward movement of the walls made evident by the cracks in the walls themselves and the buildings above. Before undertaking any interventions our office monitored the cracks over a 12‐month period and established that the cracks were live with significant movements occurring over the twelve months.
The contractors for this project were Ballut Blocks. TBA Periti were engaged as their restoration consultants and structural engineers.
Such movements had been noted for a long time with the first report of damage dating back to 1658, and some collapses occurring in 1956. Various studies of the problem have been carried out over the years. The most detailed investigation and appraisal of the situation was completed in 2003 after a long period of monitoring by the Università di Bologna.
After studying these reports and assessing the movement that occurred over the twelve months, our office formulated the hypothesis that the settlement was the result of differential movement of the underlying rock strata. The bastion was built on the edge of the Upper Coralline outcrop, lying on the Blue Clay layer which forms the typical slopes beneath the rock plateau. The varying moisture content in the clay led to successive softening, hardening, and fissuring cycles corresponding to seasonal rhythms. The settlement mechanism was not limited to a simple downward movement but was accompanied by an outward one.
