A very dangerous situation of the West wall of the left transept of the Cathedral of Lucca was discovered by chance in 1994. The masonry structure is a typical three-layered one: the two external faces enclose an internal less regular brickwork, the so-called "muratura a sacco". During the eight centuries of life it underwent numerous interventions and repairs. Unfortunately in one occasion gypsum mortars were used. Due to water infiltration, in several areas the mortar was completely ruined. The damage was discovered by chance. Seismic tomographies were the first quantitative diagnostic tool to show the very critical situation of the structure. Very low velocity zones were very evident: they correspond to very low dynamic elastic moduli, less than 1000 daN/cm2. The tomographies put in evidence many structural details. The heavy loads due to the 30 m high structure of the transept act only on the external layers of the wall structure. The presence of an important portal makes the situation worst. Our description of the situation has been well confirmed by a series of other very accurate tests and historical investigations that were performed in the following years. A series of seismic tomographies were later performed across other parts of the wall of the church. Luckily the rest of the building did not show other problems. Since that period a number of different diagnostic tools were used and tested on this structure. The structures of this building in fact are a very interesting test-site, as there are very different situations in the different sides of the church and because of the very detailed information that were obtained by different methods. A number of microseismic and electromagnetic experiments are shown.


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