The assessment of the seismic fragility of water networks is the key to characterizing their risk exposure to strong earthquakes. In the HORIZON2020 IMPROVER project, we focused our attention on the water network of Barreiro, a municipality close to Lisbon, Portugal. To determine vulnerability to ground shaking, the seismic fragility of each key component of the network needed to be determined. To exemplify our approach, we concentrate here on assessing the fragility of a semi-buried RC square water tank; previous studies have overwhelmingly focused on the seismic fragility of cylindrical (and mainly steel) water tanks. Relevant fragility parameters are inevitably uncertain, and our study made use of a sample of experts’ judgements, pooled using the Cooke’s Classical Model. This structured approach derives performance weights for experts’ abilities to quantify uncertainty in a statistically accurate sense and informatively, and then applies these weights to their uncertainty judgements on modelling parameters of concern. The unique empirical control in this procedure ensures an objective, rational consensus is obtained on the uncertainties to ascribe to different contributory factors. The main challenge in our study is to construct a suitable discrete damage scale, and to explore the possibility the tank might suffer liquefaction damage, given its proximity to the River Tagus and groundwater conditions at site. Provisional findings from the exercise are presented here and compared to existing fragility curves constructed for cylindrical RC tanks. The square tank was found to be more vulnerable than a cylindrical one.