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• III IIIIIIIIIIIII III _ 999 DC>G. Na~le~• ~~JeC.Co~rs ~'XLt-f.~(-l" i~~~: ~-~~-5~ Mining Subsidence in the East Netherlands Th. Hans Wassmann Ak;o. Zour Chemir ~\rdrrlund. Pnsrbw ?5 7550 GC Hniyrln. The Nrdirrlruul.r ABSTRACT A method for forecasting the vertical and horizontal movements of points at the surface owing to mining subsidence has been developed for the special case of the Hengelo field. All possible data wire collected and studied of a first case of caving-in of cavities created by the solution mining of salt. A theory was set up to explain two striking differences with other subsiding areas, i.e. the duration in time a(the subsidence and the small area inFluenced at the surface in relation to the depth of the cavity mined out. Special attention is paid to the behavior of the Triassic claystone formation ("Red Beds") above the salt layer. In 1973 this theur}' was used to predict the rata of subsidence of a second case for the forthcoming 10 ytan- period. This second area of subsidence was situated just under a ne++' brine purifica- tion and vacuum plant. For each part of the installation within this area the predicted movement was calculated in time. With these figures it u~as possible to arrangr a long-term planning for prevemive main[enance and by doing so to keep the com- plete plant operating. INTRODUCTION In 1918, KNZ started solution minim: of salt in the Boekelo area, East Netherlands (Fig. I)~in 1933. KNZ moved to the nearby Hengelo area because of beuer trans- port facilities available after the Twente-Rijn canal had been due during the time of the economic crisis. The total production of salt in Boekelo amounted to about 1,4.15,000 metric tons from ei;=ht singlz production wells. The production in Boekelo \vas ceased in 195?. In our 1977 leveling no subsidence whatsoever was measured. In Boekelo and Hengelo the geological profile down to the base of the salt is about [he same and given in Figure 2. The Quaternary and Tertiary formations consist of an un- consolidated very plastic clay with only some sand in the upper thirty meters. The Triassic begins directly under this clay at a depth of about 120 meters with the Muschelkalk mars. Oniy in the northern part of the Hengelo concession is this Muschelkalk missing. Below the Muschelkalk we find the Upper Bunter claystone or "Red Beds" with the Riitsalt Formation at their base with an average thickness of 50 meters. The Red Bed claystones are dry, tight and im- permeable. This formation is very consistent throughout the whole concession with a thickness of 180 meters. Figure 3 gives an idea of the depth, strike and dip of the top of the salt. The strike is nonh+vest-southeast with a dip of abau six decrees towards the southwest. In 1963 the lint subsidence was meusured at area I (Fig. 3), just within the Muschelkalk border. Ten czars later ;t second case of subsidence occurred in area 2 just a fe\+ hundred meters ro the north and jtnt outside the area of the \luschelkalk deposit. Unfortunately this second subsi- dence was situated under the ne+v brine purification and vacuum pl.mt. The ten years' lapse between these two cases provided an opportunin• to thoroughly study the behavior of area I. Armed with this knowledge, it was possible to predict the second subsidence and to prevent severe damage to the salt production plain. SUBSIDENCE CASE 1 Well number 18 was drilled in area I in 19-09 and (ol- IOKYd in 1951 by well number ?-1 at a distance of only ?5 meters (Fig. 4). Both wells Wert taken out of production in 1958, 5 years before any subsidence was measured. It is difficult to trace accurately the production of salt from each individual well because of the numerous connections exist- ing between the cavities of neighbouring wells. In total [he wells 16 to 2?, plus well 23, all more or Tess situated in area 463