GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 2.3 437 A seismic event can result in damages to the tank only if it is full of fluid. Since the structure is a buffer storage tank, it is almost empty most of the time, except in the case of extraordinary maintenance on the pipelines. The probability of an earthquake occurrence at the same time of rare and short maintenance operations is very low. Therefore, the scenario to be taken into consideration is the one in which the first shock causes the rupture of the pipeline, the tank is filled in order to restore the pipeline and then subject to a second quake. The first step of the study was the seismic site characterization, in order to define the soil condition for the design spectra determination. Passive seismic methods (Refraction Microtremors, Re.Mi.; Multichannel Analysis of Surface Waves, MASW) have been used to define the shear wave velocity profile. To calculate the base shear stress, the overturning moment and the height of the sloshing wave, the simplified procedure proposed by Malhotra et al. (2000), integrated in Eurocode8, has been used. The method involves spectral acceleration with damping ratio of 2% and 0.5%, therefore the spectra have been calculated from a set of strong motion data, since the standard formulations were not valid for damping ratio lower than 5%. The amplification related to site effects was calculated through the 1D modeling by the software Pshake (Sanò and Pugliese, 1991), assuming as input those spectra, the stratigraphy and the Vs profile obtained from the geophysical analysis. Finally, the maximum vertical displacement of the fluid surface due to sloshing has been computed, and the safety condition verified. The moment and shear stress resistance calculation, instead, is still work in progress. Geological characterization. The SIOT tank is located in the Friuli Venezia Giulia region, in the municipality of Paluzza (Udine, north-eastern Italy). The industrial plant is located in the flood plain of the Bût River at the average altitude of about 550 m. The River Basin Authority classifies the area as P1 class, with a moderate hydraulic hazard (Piano Stralcio per l’Assetto Idrogeologico, http://www.adbve.it/ ). The area is characterized by Holocene stabilized alluvial deposits, made of coarse gravels, sands and fluvioglacial silt, surrounded by Permo-Trias carbonate rocks of the so-called Paleocarnic Chain (Venturini, 2009). The tectonic framework is complex and partially buried under Quaternary coverage. Normal faults and thrusts are evidenced in geological surface mapping, and the main one is the Comeglians-Paularo thrust (Servizio Geologico per l’Italia, Dipartimento per i Servizi Tecnici Regionali, 2002). The knowledge of the soils and underground structure of the valley is very poor, and no stratigraphic investigations are available. An initial three-dimensional model of the valley was developed overlapping the orthophotos of the regional numerical technical map and the digital elevation model (RAFVG, 2014).The valley bedrock depth in correspondence to the industrial site is estimated at about 86 m, and this value was compared with those obtained with geophysical investigations. Geophysical surveys. In order to better characterize the seismic properties of the shallow soils (i.e. unconsolidated sediments) at the test area, S-wave array measurements have been carried out by analyzing and inverting the dispersion curves of Rayleigh wave’s fundamental mode. Seismic signals were induced by active and passive (ambient noise) sources. A multichannel survey was performed using a 24 channel digital system SoilSpy Rosina (Micromed). Multiple weight drops were used to record redundant seismic data. The layout of the seismic profile was oriented along the longitudinal axis of the valley, in order to meet the physical conditions of 1-D medium and remove the apparent surface wave arrivals. With this test it was possible to define the “limits” of the velocity model (Fig. 1). It is achievable to read the chart from 50 Hz, with a correspondent speed of approximately 260 m/s, and therefore a 2.08 m depth. The dispersion curve starts to be not interpretable at about 10 Hz at a speed of 470 m/s. To define the profile, the dispersion curve in the fundamental mode was connected to the fitting of H/V at the site, obtaining a shear wave value for the upper 30 m of about 438 m/s, that set the site in B class, according Eurocode 8 classification. Ambient seismic noise has been recorded all around the area where the tank is located to establish stationary site conditions. Data were acquired also on the top of the tank structure