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Mining Subsidence <br />r1 <br />u <br />,~~ <br />asp <br />Akzo FACTORY HENGELO THE NETHERLANDS <br />~~~\ \ <br />' ' ~- ~ <br />o ~ PLANT ' 1Y •t <br />a ~ ° <br />\•v` Road `--0-----~3 ~ ~ o -- \ <br />15 <br />° <br />~~ <br />1- <br />1a ~ <br />° ~ r <br />~, (\ I <br />11 02 ~ ._ _. it 8 \, <br />0 <br />~ °10 <br />\ ° <br />9 _ <br />~ o <br />~~~~ <br />\... <br />~~ <br />t <br />\`, <br />0 50 100 150 zoc <br /> <br /> <br />CAVITY SHAPE <br />10m. <br />_z <br />17 ~, <br />0 <br />- , <br />., 1 , <br />z1 ~ , <br />°za <br />° o, , <br />°18 i <br />22 <br />~; 19 2~ [~ <br />t <br />~~ ~ ~~ <br />250 ~~~~ F <br />Figure 4. Reconstructed cavities based on salt production figures. Akzo Hengelo. Netherlands. <br />tend owing to a complete loss of circulation. As consequence <br />no cuttings reached the surface. Several cores were taken to <br />trace the guide horizons. In total 68.40 meters were cored <br />with a recovery of 65.390, whereas normally a recovery <br />between 959c and l00% is obtained in these formations. <br />The drilling was even successful in reaching the undis- <br />turbed and still undissolved base of the salt, as can be seen <br />in Figure 6. Between wells 18 and 24 some 8 meters of <br />rock salt were found. Figure 6 shows the reconstructed orig- <br />inal geological profile between wells IS and 24 before any <br />subsidence, in comparison to the profile actually found in <br />this exploration well 01. The most noteworthy guide hori- <br />zons have been marked in this figure. In this well a vertical <br />displacement of about 26 meters was found at a depth of <br />340 meters, whereas the subsidence at [he surface amounted <br />to only I.85 m at the same time. <br />35m. <br />_~ <br />The following conclusions could be made after comple- <br />tion of the exploration well 01: <br />I. The Quaternary and Tertiary clay formations are plastic <br />enough to avoid rupture or faulting. <br />2. The Triassic formations Muschelkalk and Upper Bunter <br />are completely broken. Open spalts and small cavities of <br />several meters could be traced during drilling and log• <br />Bing. <br />3. A cavity in the salt layer was not found because of the <br />collapsed roof. <br />Because the cavity collapsed several years after the <br />shut-down of the wells, the structural failure of the roof <br />rocks is due to more than the big span of this roof. There <br />must be another reason. The Bunter Red Beds can easily be <br />