The structural problems of the Mdina bastions, in particular in the area underlying Vilhena Palace, have long been recorded. In the past, various attempts to resolve this delicate situation were undertaken and, although several reports were drawn up, no interventions were carried out.
It is only recently that a pilot project was undertaken as part of a vast programme to consolidate and restore the historic ramparts of Mdina. This was based on concept designs drawn up by the University of Bologna in 2001, following a period of monitoring of ground conditions and movements in the area commissioned by the Restoration Unit of the Works Division.
The €661,000-funding of the pilot project was supported by grants from the European Economic Area and Norway, with 85 per cent of the project cost being funded through the EEA and Norwegian financial mechanisms and the remaining 15 per cent consisting of local funds.
A design and build tender for the consolidation of foundations and restoration of a 25-metre stretch of curtain wall behind Council Square, later extended by a further 20 per cent, was prepared by the Restoration Unit of the Works Division, then under the Ministry for Resources and Infrastructure, later the Ministry for Resources and Rural Affairs. This was eventually awarded to Ballut Blocks Services Ltd as main contractor having TBA Periti as consultants and Agius Stone Works as restoration sub-contractors.
The designs prepared by the contractor's consultants had to be modified following a unique archaeological find, the foundation of a semi-circular mediaeval tower.
Following a comprehensive investigation and documentation of the area, the new layout proposed three clusters 17 metres deep and 60 centimetres in diameter of reinforced concrete friction piles. The central cluster was to be housed within the perimeter of the semi-circular foundation, leaving the external face of the remains of the tower unaltered.
A total of 45 reinforced concrete friction piles were cast deep into the clay strata underlying the thin layer of rock that ends abruptly with the outer face of the curtain walls.
Before casting the pilecap and upstand beam, designed to take the loading of the permanent strand anchors, a process known as soil fracture treatment was applied. All encountered fissures and cavities were filled using low pressure grouting and concrete.
Permanent strand anchors inserted 27 metres deep into the ground were post tensioned, anchoring the entire reinforcing structure and underpinning the overlying historic structures. Stitching of the outer face of the wall by means of steel anchors varying from 12 metres to 17 metres in length was then carried out.
The pilot project was completed by the restoration intervention on the building fabric, carried out in conformity with the latest restoration conventions and techniques.