Collapse of the walls of Volterra: resistance crisis of the stone material?
The walls of Volterra, a work of military engineering dating back to the Etruscan era modified during the Middle Ages, were have already been affected by a similar collapse occurred ten years ago, during intense rains. Since then, the attention and investments of the Municipality and the Ministry have grown to map the situation of the entire city wall, carry out the most urgent restoration interventions and proceed with periodic checks. From the news in the press in recent days, it seems that the wall which collapsed on Sunday did not never showed any alarm signals, such as cracks or the presence of humidity.
The geologists interviewed directly during the inspection after the collapse, as deduced from the press reports, exclude that cause is due this time to rain infiltrations into the ground, having found the stones dry and not soaked in water.
One of the hypotheses, which will have to be explored in further investigations, could lead the cause back to resistance crisis of historical stone material. At the moment we have no further elements to confirm this hypothesis, but it still constitutes an element of reflection if, in addition to the pressures of the ground and external causes due to strong meteorological events, the residual resistance of the ancient walls represents a crucial parameter for their safety, also in light of the fragile collapse that we witnessed, which in fact gave no warning signal to the technicians in charge of periodic visual monitoring.
The decay of the ancient city walls
We must consider the ancient wall walls as structures that have suffered several failures over the centuries, in addition to the physiological deterioration of the materials over time.
«It is necessary to consider a historic building like the result of a real-life experiment that lasted years or centuries. To interpret the results of an experiment it is necessary to know what the building was like at the time of construction, which trauma has undergone over time, what changes and, finally, as it appears at the end of the test» (Decree of the Extraordinary Commissioner of the Government for the purposes of reconstruction in the territories affected by the seismic events that occurred starting from 24 August 2016, n. 456 of 13 October 2022 and the related MIC circular 14/11/2022, n. 48 /2022 – Operational indications for the restoration and reconstruction of buildings of cultural interest integrated by specific indications for buildings of worship, with attention to the seismic safety of buildings of cultural interest).
In particular, the decay of the mechanical performance of the mortars of bedding between the brick or stone elements plays a fundamental role in the deformability and resistance of the wall section. Ancient masonry buildings often have widespread lesions due to various causes, such as high self-weights, the effects of mortar creep and the interaction between creep and fatigue.
These phenomena are linked to original functionalityat construction techniques used and to theevolution of loads over time.
The importance of diagnosis and monitoring
The safety assessment of historic wall structures can also make use of non-invasive or minimally destructive diagnostic techniques to estimate their mechanical characteristics. Furthermore, in cases of greater complexity and relevance, it is advisable extend the use of monitoring techniquesfor an evaluation of stability over time through the interpretation of scientific data by specialized technical personnel, in analogy to what is already taking place on the infrastructural network after the tragic collapse of the Morandi bridge.
One of the monitoring techniques that could have picked up warning signals inside a wall facing is, for example, that of acoustic emissions. The technique is based on the spontaneous emission of pressure waves emitted by a material under stress as a result of irreversible phenomena such as damage, microcracks, degradation, which can also be caused by interaction with external stresses such as wind, earthquake or traffic vibrations, or by movements of the structure due to settlements. The acoustic emission signals are read by piezoelectric sensors, with frequencies between 50kHz and 1 MHz, whose data interpretation allows locate microcracks even inside the structure, when not visible externally, and to evaluate the trend in the severity of the damage over time.
The same technique is currently used in monitoring the stability of Asinelli and Garisenda towers in Bologna (Fig. 1). On the latter, in particular, the interpretation of the results deriving from instrumental monitoring with different techniques, conducted for some years, has made it possible to intercept an alarm phase for the safety of the structure, and to implement a series of urgent interventions provisional, still underway, waiting to develop a definitive reinforcement project in light of the vulnerabilities revealed by the monitoring plan itself.
The understanding ofactual state of health of our historical-architectural heritage is therefore an essential condition for evaluating its safety. The knowledge and monitoring phase should be part of the scheduled maintenance plans of many other monumental complexes, to prevent further events of sudden structural collapse.
