Taquari-Vassouras is an underground potash mine in northeastern of Brazil operated by Companhia Vale Do Rio Doce. Sylvinite is mined at depths ranging from 430 to 640 m where room and pillar panels are excavated using continuous mining equipment. The stratiform ore deposit consists of a lower and upper sylvinite layers separated by a barren halite interval. The underlying evaporites include a massive tachyhydrite layer separated from the lower sylvinite by a halite layer that in some areas is very thin or non-existent. Tachyhydrite is highly hygroscopic, weak, and creeps two orders of magnitude faster than the surrounding salts under similar conditions. One of the mine panels was successfully excavated in the area where tachyhydrite is in direct contact with sylvinite. An extensive array of convergence stations was installed in the panel and the panel was closely monitored. A series of room-and-pillar scale and panel-scale numerical analyses were carried out with the objective to establish the mechanism of tachyhydrite influence on the panel performance. It was established that despite operating difficulties related to floor heave, the presence of tachyhydrite has a positive influence on the stability of the roof.