Rather complex subsurface and groundwater conditions have played an important role for the design and execution of the extensive reconstruction works on the existing underground station Rotterdam CS. The retaining wall is designed as a diaphragm wall for about 90% of the building pit circumference. The remaining 10% has been generated by means of ground freezing. This paper describes the results of 3D groundwater calculations using the MicroFem code; this model has been used for the design and during execution of the project to determine the effect of dewatering activities. Two project case histories are presented, illustrating that knowledge of the local groundwater regime provides key-information in case of unexpected events, like leakage through diaphragm walls and during the process of coming to the vital conclusion that 100% watertightness of the frozen soil structure has been achieved.