In the Hengelo area, East Netherlands, solution mining of rock salt is carried out by Akzo-Nobel from a 50 m thick salt deposit of Triassic age, located at about 350 m depth. Until recently surface subsidence was mainly ascribed to upward migration (stoping) above overmined caverns. However, in the oldest part of the brine field the extraction ratio is so high, that also deformation of the remaining salt pillars has to be taken into account. The effect of pillar creep on surface subsidence was investigated by the construction of a series of subsidence rate maps, per year from 1947 to 2003, and the analysis of the relationship of the pattern of these subsidence rates and the most likely cavern and pillar outlines.

This study not only confirmed the existence of several subsidence areas as a result of stoping, but also revealed evidence of a pillar deformation-induced subsidence area. The stress state in this pillar proved to be such that significant dilatancy must be considered. It was concluded that in the dilatant pillar solution-precipitation creep can contribute to the total deformation at the same amount as dislocation processes. It is recommended to incorporate solution-precipitation processes in future geomechanical models of pillar creep, even under non-dilatant conditions.