PERMFILE51395

• Permit No: M1999002
• IBM Index Class Name: Permit File
• Doc Date: 6/18/1999
• Doc Name: OBJECTORS EXHIBIT
• Media Type: D
• Archive: No

470 Fitfh International Symposium on Salt-Northern Ohio Geological Society
moistened h}' water or brine so that the claystone loses its
structural strencth. As has been noticed before in other
abandoned but not plugged wells, the roof of the cavities
will collapse in beds of about 10 to 30 meters thickness over
the years.
In 1963 the caving could finally reach the unconsolidated
Tertiary clay'. From that moment on a pronounced subsi-
dznce was measured on the surface. After a czrtain amount
of time thz subsidence is no loner directly related to the
caving of the roof formations. The compaction o(the rubble
in the disturbed la}cn will ht resporoible for a vzq' slow
dzcreasinc but lone-lasine subsidencz at the surfacz.
Nccerihelzss the small aria intluenczd :u the ~ur(ace with a
diameter of ooh '_70 mcier> could not he explained in rzla-
tiun to thz depth ul the original Gavin' ut 3.10 mztzrs lancle
u( drain).
SI:BSIDENCE CASE 2
In 1973 subsidznce was meawred in a szcond case dur-
ine the periodic leveling. Thz center of this new arza i'as
situated near the old well 1. After a yzar of thoroueh
measurine this nei~ situation could he jud,_ed and the
similarities and dilizrences be szcn bztween this case and
case I. Thz Geological prolilz of tell 4 is about the samz as
that of svclls I8 and 21. Only the IS meters of P7uschelkalk
are ahSen[ here.
\Vell 1 ias taken ow of production in 1919. By that time
this cavity' had connected to several other cavities and seas
situated in the middle of e T-crossins (Fig. 1). From the
salt production (icures and thz history of repair jobs on this
i~ell, it was known that the dissolving height in the salt la}'er
had been lost very fast. Because of this fact the shape of the
cavity i~ill look like a small inverted cont. The connections
to the neighbourin, wells were made just under the roof of
the rock salt. This means that the cavity of well 4 had less
space (or sioraee of broken rock material. It was no sur-
prise, thereforz, that a much slower rate of subsidence was
measured at the sur(acz in this urea.
Figure 7 shows the diflzrznce in the subsidence rate of
two similar bench marks in both areas. By similar bench
marks are meant bench marks at the same distance from the
cemer of the subsidence bowl. As can be seen in this (ieure,
the subsidence rate o(area I is much higher in comparison
to that of aria 2 for the first three years. After three years
the rate in area 1 has decreased to a more or Icss constant
f_ure of about RO mm a year. In area 2 the rate of subsi-
dence has stabilized for the time beine at a figure of about
60 mm a year after the same period o(three years. In area 2
thz same comparably' small area of influencz was found
also.
From laboratory experimznls on cores, a maximum ad-
cisablz diamrter of about 80 meters i~as calculated for the
cax'ities in the Hengelo field. Hosvzver, in these calcula-
Lions chances in the mechanical properties usvim_ to waning
of the roof formations, and especially the Red Beds, were
not considered. Now it~is known that water or brine will
penetrate into the normally tight and impermeable clay-
stones owing to capillary forces. But it is though[ to be
possible only in an upward direction where the internal
stresses are morn or less released oi•ine to undzrcutting by
the open cavity. The walls of the cavity' are still under the
influence of the overburden przssure and not accessible for
pznetrating fluids. This process will creep slowly but stead-
il} upwards depznding on thz height of the capillary action,
and bed after bed of the roof formations will collapse into
the cavity. There is some proof from loe~in~ in old, but still
accessihlz tells, that this process actually takz> place.
Eventually' this Iznds to thz followin_ picturz of the un-
Jerground developments IFi_. 8). The moistening of the
roof rock ahovz thz existim~ eosin' will crzate a chimney
iillzd iiih broken rock matzrial. Dzpznding on the storage
capacity' of the cavity, this chimney either rill reach the un-
consolidated formations or not. If thz unconsolidated clay is
rzached, a normal subsidence boi'I will develop i•ith a slope
equal to the angle of draw oF15 degrees (or this kind of rock.
This explains the relatively' small area of influence in relation
ro [he depth of the original cavity. The plane of break or frac-
ture has been found ut a much steeper an~lz. This plane
intzrsecis the surfacz at a lint connective the points of the
lamest extension. This anclz is about 80 degrees in the
Hengelo field. If one reckons with a 159c saelline of the
claystone h}' moistening, with a caviq' shape of an inverted
cone and the calculated voltimz of 360,000 cubic meters for
tell 1, this cavity' can just store the wholz Triassic c}'lindri-
cal column. So in this case, the slow rate of subsidence
measured is caused by the compaction of the rubble pile
above the cavity. The base of the unconsolidated Tertiary
clay is found at a depth of 135 meters. Using the angle of
draw of 15 degrees (more or less [he same angle as has been
found in the Dutch coalfields), the radius for the area inFlu-
enced can be calculated to be about I35 meters. This figure
is in extremely good correspondence i•ith the figure actually
found in both cases.
Considering this theory and thz differences between the
nvo cases, an anempt was made to forecast the vertical and
horizontal movements at the surface in area 2 (or the sub-
sequent tzn years' period. This task was of utmost impor-
tance in keeping the salt factory running. The purification
plant, quite an intricate plant, is buih on an overflow princi-
plz and flow directions would be reversed due to the subsi-
dence. Numerous pipelines would be subjected to horizon-
tal forces so that measures had to be taken to avoid rupture
of these pipelines.
Table I shows some o! the predictions madz in 1971. In
thz first column the measured subsidence o(bench mark I8'
in area 1 is shorn, while in thz next column it is compared
i ith thz predicted subsidence of bench mark H in area ?,
both for thz first ten years' period. Finally, the last column
rives the actual measured subsidence of bench mark H until