The dimensions, loads and foundation of the new tanks are identical to the old tanks which were erected 40 years ago and demolished a few years ago. Despite the fact that the new tanks are identical to the old tanks, the stability calculation for the foundation of the new storage tanks did not fulfill the requirements of the present Eurocode 7-1. To ensure sufficient stability, additional measures like soil improvement are necessary. This is due to the fact that safety levels have much increased in the last 40 years. In order to optimize the design, it was decided to recalculate the stability using the code for existing structures "NEN 8707", which is currently being developed. According to NEN 8707 it is allowed to use a lower value for the reliability index for existing structures compared to new structures, because of proven strength. Thus, lower partial load and material factors can be applied. These calculations with the adjusted partial factors resulted in a sufficient overall safety factor for the ultimate limit state for the hydro-test phase and the operational phase. Thus, using NEN8707, the tanks can be built on a crushed rock ring foundation without an expensive soil improvement.