PSI - Issue 29

Michele Paradiso et al. / Procedia Structural Integrity 29 (2020) 87–94 Michele Paradiso, Sara Garuglieri and Viola Ferrarini / Structural Integrity Procedia 00 (2019) 000 – 000

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In a genera lizedway it canbe sa id that the roofs and floors represent themost deteriorated and advancedelements of degradationwithin theentire convent. Theoriginalwalls made upof sackwalls in stoneandearthandothers entirely in tapial are very deteriorated a lso due to the lost of the original plasters which exposed the structures to widespread erosion, biological a ttacks and the presents of large infestedvegetation. The set of arches inside thefirst cloister is the most unsafe structural part of the whole complex, presentinga wide range of cracks spread in correspondence of the kidneys and in keys, whose incidencegets bigger and bigger approaching the south-east corner of the cloister (Figure 2). In this point the arches resting on the four-lobed corner pillar are affected by the grea test number of cracks and therefore fully proppedup; thewa lking surface is a t a lower height than the adjacent columnsof 12cm, equal to about 4% of the arch light. The arch structures in this area are affectedby secondand third typecollapse mechanisms. The cracks show a state of serious pre-collapse where the flexural collapse mechanism, caused by external actions of a sta tic type, is accentuated by the vertical subsistence mentionedaboveby thewedge mechanism. 1.1. Analysis of materials and construction techniques Various construction techniques were used for the construction of the Convent in rela tion to the temporal phases of execution as well as the economic availability of the nuns: inconsistent masonry, sack wall in caliza jaimanita stone (loca l limestonewith tiny natural fossils) andearth, tapial wa lls and walls executed with the entremado technique. The first cloister, belonging to thefirst constructionphaseandconsistingof more complexandvaluable wall systems, is completely surrounded by a portico with round arches on the ground floor and polycentric on the first floor on Tuscancolumns in caliza stone. Thecolumns havea diameter of 60cm, with theexceptionof thosewith a four-lobed angle, and theyare arrangeda t an irregular center distance(2,7 m on average). All the horizontal structures aremade up of beams (of theSpanish type) andplanks, with double slope wooden roof of alfarje coveredwith creole tiles. As previously mentioned, the worryingpre-collapse phenomena persistingon the south-east corner of the cloister are mostly a ttributable to seabed subsidence; as a consequence, a specific survey in the foundation state was made. The excavationwas carried out both at the investigatedcorner andonother positions, in order toestablish thegeometric na tureof the foundationand theprominent share, looking for thepossible causes of thefailure; thef oundational surface is a lmost insufficient and superficial (50- 20 cm deep) consisting of “ argamasa ”, an incongruous conglomerate of aggrega tes, stones andearthas a binder. During this survey, some samples of the excavatedearthand the tapial walls were taken and analyzed by the Italian Na tional Research Council CNR researcher Fabio Fratini by X-ray diffractometry (XRD) for determining the main minera logical composition, the amount of calciumcarbonate content andwet sieving for a granulometric analysis rate. The analyzes made on the soils have highlighted the presence of smectite, a mineral whose crystal la ttice changes volume as humidity changes. The ma terial used in the f oundation was compatible with the composition of the surrounding soil; sensitivity to changes in humidityand its lowcohesivepower lead us to assume that this material is not suitable to be used for foundations and that, therefore, the la tter are inadequate for their required performance. Like the foundation, the soil used for the construction of the tapial wa lls was found to be mainly made up of sand (87%) of predominantly calcitic composition: a very thin soil with low cohesive capacity and therefore not suitable for use in this sense. 1.2. Insights related to thesouth-east corner of the cloister On the basis of the informationprovidedby the surveyandby the analyses, the followinghypothesis was made about the structural status of conservation of the south-east corner: the hinge articulation probably depended on static and perhaps dynamic actions (excessive loads, changes intended use, earthquakes and hurricanes) which have made the arc structure lose its degree of hyperstaticity. This a lready precarious situa tion was superimposed by vertical subsidence, whose consequences were overcoming the frictional resistance between ashlar and ashlar and the consequent sliding downward of some portions of the arch. In general, the cause of these injuries can be mainly a ttributed to the subsidence of the soil as well as to the current lack or instability of the horizontal elements, to the dila tion caused by the infiltra tion of wa ter into these wooden elements (which, by expanding, have damaged the reta iningwa lls), and to the lack ofmaintenance. Afurtherminor cause consists a lsoof the overloadof theseelements duringa ll the transformations of use undergone by the structure. It was initia lly assumed that the seabed subsidence was due to the presence of landfill, an hypothesis la ter denied by laboratory analysis that did not identifydifferences