PSI - Issue 64

Nicoletta Bianchini et al. / Procedia Structural Integrity 64 (2024) 352–359 Bianchini N., Sabra Z, Green K.. Wrigth, R. Structural Integrity Procedia 00 (2024) 000 – 000

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always be achieved using traditional surveying techniques or photogrammetry so when surveys are required for terrain covered in vegetation this system is ideal. Indeed, LiDAR can penetrate dense tree canopy, enabling the capture of bare earth terrain models beneath the vegetation. Nevertheless, Aerial LiDAR Survey global accuracies of less than 5cm with point densities up to 700 points per sqm is assured, assuming all excess vegetation have been removed. The general arrangement profile was compared with that of the photos and found to be accurate with a deviation value expected given the method of measuring the mid-rise and quarter-rise points. Subsequently, all the materials involved were analysed with a level of detail which depends on accessibility. In particular, eight structural cores were extracted from the intrados in numerous locations (Fig. 3). The main aims of these investigations were to identify the inner geometrical section of the arches, the quality of the mortar, units’ composition, and the presence of backfill. From the samples obtained, it was observed that the predominant material was strong dark brown sandstone, developing for a length of 700-780 mm and weak light grey mortar, determined by visual inspections only. More developed laboratory analysis, e.g. digital in-line holographic microscopy based on BS4551:2005+A2:2013 or scanning electron microscopy (SEM) and petrographic examination would have been ideal to better characterize the mortar composition, however a higher quantity of material would have been needed, which would have fallen outside the Client’s project brief . a b

Fig. 3. Coring samples (a) location; (b) SC02 sample.

Ground-penetrating radar (GPR) tests was performed to characterize the internal morphology of construction elements on homogeneity, voids and irregularities which resulted to be quite homogeneous and very compacted. The trial pits were also drilled to investigate the fill depth and the presence of any backing which was not detected. The main finding of this nondestructive tests was the actual thickness of the barrel was thicker (from 250 mm to 720 mm) than what observed before by only qualitative estimation and this concurred to avoid underestimation of capacity of the bridge. 2.2. Flooding events Principal and General inspections revealed the effects of frequent flooding that reached the key of the arch barrel, often accompanied by significant debris accumulation obstructing the bridge span, as illustrated in (by Fig. 4). Thus, to assess the impact of these historical floods on the structure's health, 25 years of water level data from national archives were analysed (Fig. 5). a b

Fig. 4. (a) 1987 Diving inspection; (b) 2016 Principal inspection.