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Mining Subsidence • • <br />Surface <br />o : o ~ ' <br />QUATERNARY _ ~ 3i Pleistocene: Sand, Clay; <br />_ - _ ~ unconsolidated. <br />TERTIARY _ 9Om. Eocene: Clay; unconsolidated. <br />_ _ ~ <br />- - <br />N <br />N <br />0-12Om. Muschelkalk: Marl. <br />N /~/ <br />TRIASSIC <br />q 18Om. Upper Bunter Claystone; coarse <br />~ „Red Beds" <br />A <br />5O m. ROT SA LT <br />1 <br />:A A <br />~: A Lower Bunter <br />Figure 2. Geological profile of the Akzo concession "Twenthe Rijn", east Netherlands. <br />I, have produced 1,385,000 tons of salt. This is about the <br />same tonnage of salt from an equal number of wells in <br />Boekelo, but in Hengelo the production rate is higher and <br />[he cavities are interconnected as can be seen in Figure 4. In <br />this figure the cavity diameter is also reproduced on the <br />basis of calculations after production figures, while assum- <br />ing the shape o(an inverted tone. Then in our second half- <br />yearly periodical measurements in 1963, a subsidence of 30 <br />centimeters or 12 inches was found in comparison with the <br />first measurement in 1963. In Figure 5 the results are given <br />465 <br />for the first four years. The bench marks 43 to 56 pass <br />through the center of the subsidence bowl at centers of about <br />20 meters. The vertical movements are eiven iri millime- <br />ters. In 1966 the subsidence measured in [he cener was <br />about 6.1" or 1,625 mm. The horizontal displacement of the <br />bench marks or the strain is given in millimeters per ten <br />meters of length. The measurements showed similarities <br />with subsidence figures for the Dutch coalfields under static <br />conditions of the winning front, as can be seen in this figure <br />by the stable zero strain point. <br />