[EN] This Final Degree Project presents an analysis of a water tank in the municipality of Sagunto, which was taken out of service after being filled for the first time after excessive movement in the retaining walls. This water tank is located just 500 meters west of the town of Sagunto, where formerly there was a quarry. The water storage tank that forms the subject of this study is divided into two sections 250 m long by 100 m in length. The retaining walls are 6 m high with a concrete grid slab above them. The water tank investigation and diagnosis has consisted mainly of: 1. Observed damage 2. Geotechnical survey 3. Assessment of the stability of the water tank 4. Structural calculation of the water tank After several field visits to assess the damage ¿in situ¿ a diagnosis was made which permitted the causes of the failure to be stablished. One wall was seen to have moved a considerable distance on the top of the wall, due to the overturning of the wall. This movement produces the collapse of part of the cover slab that impacted the top of the wall, which rests in elastomeric bearings. In consequence, the movement also affected the expansion joints, where appeared considerable gaps in the cover slab and retaining walls. In order to examine the state of the ground below the tank, an extensive geotechnical survey was performed. It consisted in an 11 boreholes carried out by the SPT method in the large failure area. Furthermore, three trial pits were taken through a backhoe in the large of the wall with the most damage in order to know if the drainage and waterproofing on the foundations is correct. The survey reflects that the ground below one of the retaining walls was filled with insufficiently compacted material. It was also observed from the geotechnical characterization that in the zone affected by wall movement PVC sheeting had been detected. The presence of PVC sheeting between the foundations and the soil would reduce the safety factor for sliding. Respect the stability of the water tank, the safety factor of the walls for sliding and overturning has been calculated. The safety factor for overturning is correct, but the safety factor for sliding is not properly safeguarded according to the norm. Therefore, a remedial action should be stablished. A numerical analysis was also performed with the geotechnical finite elements software PLAXIS. The structural calculation is based on the structural elements verification, such as columns, foundations, retaining walls and the concrete grid slab. It was verified that the elements reach the EH-91 rules, which is the current regulation when the water tank was built and designed.