The presented analysis concerns deflections of the reinforced concrete slab in the fire-fighting water storage tank with volume of 950 m3. It was built on human-altered soil which led to deflection of the tank. When water was pumped out from the tank, rectification was performed. The tank and its slab foundation were non-uniformly elevated by means of hydraulic jacks. These jacks were installed under the slab, on foundation made of concrete block stacks, which were pressed into the ground. The computational analysis was conducted for displacements and deflections of the slab supported on the jacks. The number of jacks under the slab and stiffness of jack supports on the stacks were the variable parameters of the model. Stiffness of the jack supports was found to have non-significant impact on deflections of the foundation slab of the rectified tank. On the other hand, the number of jacks under the tank affected both deflections of the slab and displacements of the whole tank. The greatest deflection of the tank slab supported on three jacks was 15.233 mm, and the smallest one was 10.435 mm at 32 jacks.