Content uploaded by Agnieszka Maj
Author content
All content in this area was uploaded by Agnieszka Maj on May 18, 2017
Content may be subject to copyright.
175
http://dx.doi.org/10.7494/geol.2013.39.3.175
1 The Strata Mechanics Research Institute of the Polish Academy of Sciences;
ul. Reymonta 27, 30-059 Krakow, Poland;
e-mail: kortas@img-pan.krakow.pl, maj@img-pan.krakow.pl
2 Inowrocław Salt Mines „Solino” SA;
ul. św. Ducha 26a, 88-100 Inowrocław, Poland;
e-mail: jacek.drogowski@solino.pl
: Occurrence of land surface subsidence is a result of rock salt extraction. The process is obse-
rved by geodetic measurements. On the Palędzie I salt mining field, such measurements are conducted
every five years. The most recent series of measurements was carried out in 2009. The analysis of land
levelling results indicated that a twin-centre subsidence is still being formed above the salt mining
area. Its maximum load on the SW side of the salt dome exceeded −100 mm in 1986–2009. The second
subsidence centre behind the NE edge pillar is 50% smaller than the first one. Along with the extraction
moving up to shallower areas of the salt bed, the rate of land surface subsidence is increasing, with
the decreasing perimeter of the depression. The indicators that describe the land surface subsidence
– vertical displacement of benchmarks and the caverns volume – are presented on the function of time
and a parameter, determining the distance from the measurement point to the exploitation field edges.
: salt domes, land subsidence, salt caverns, twin-centre trough
The recognition of the influence of the salt bed exploitation on land surface on the basis
of the coal and ore mining operations is not adequate for the description of the influence
of mining operations in salt deposits, in solution mining in particular. The diversity of geo-
logical and mining conditions existing in the salt dome consists in definitely rheological
Geology, Geophysics & Environment 2013 Vol. 39 No. 3 175–187
G. Kortas, A. Maj & J. Drogowski 176
properties of the salt medium, specific form of solution mining operations, where the range
of opening is several times smaller than the height and direction of the movement of the min-
ing operating front from the bottom to the ceiling.
The application of the physical models of the elastic-viscous medium to describe
the rock salt mass (Beiley 1929, Norton 1929, Ślizowski 2006) opened the way to the gen-
eral determination of the influence of soil mining on the rock mass and land surface (Kortas
2008, 2009). Consequently, a separate type of subsidence distribution was identified, includ-
ing a previously unknown type of the influence of mining operations resulting in the produc-
tion of a twin-centre subsidence trough. An example of that phenomenon has been observed
in the distribution of the exploitation effects above the Mogilno salt dome.
The purpose of this study is to present the current state of the surface subsidence rec-
ognition above the solution mining facility in Mogilno, taking into account the observations
conducted in 2009, the distribution of the subsidence with the exploitation frontage move-
ment, and the development of long- and short-term influences. On that basis, we identified
the trends of the changing rates of subsidence and the subsidence distribution, appearing
in recent years.
The Mogilno salt dome (Fig. 1) has the shape of a flat ellipsis in a horizontal cross-sec-
tion, with the salt mirror situated at ca. 250 m under the ground level. At ca. 600 m, the length
of the dome is 5.5 km, and the width is ranging from 0.8 km to 1.5 km. The NE dome wall
is sloping at the angle of ca. 80°, while the opposite SW one at 90° and more, creating over-
hangs. On the SE side of the salt dome, the salt ceiling is situated at the depth of 550–650 m.
The internal dome structure has been recognized primarily owing to the drilling op-
erations along the exploitation cavern axes. In recent years, research works were conducted
on the use of borehole geo-radars to determine the internal deposit structures in the Góra
and Mogilno salt mines (Tadych et al. 2010). The main mass of the salt dome is made
of the coarse-crystal older salt. It is contaminated by anhydrite which occurs either in dis-
persed or laminar forms. The transition strata are made of fine-crystal salts, with silt-an-
hydrite flasers and the traces of potassium-magnesium salts. The NaCl content determined
for the particular cyclothems is ranging in the deposit from 90.0% to 98.5%. As a result
of halokinetic tectonic stresses, particular strata were subjected to strong, folded deforma-
tions, pressing, and wearing. Locally thick layers appear. The recorded wall angles range
from 70° to 90°, with a frequent reversal of layer sequence.
The salt rocks of the Mogilno salt dome are characterized by diverse geo-mechanical
properties, deformations, and the strength of both long-term and temporary nature (Grzy-
bowski et al. 2008). Flexibility to creep, increasing with depth, is a feature of that medium.
The salt’s flexibility to creep and dissolution results from the sequence of strata, mineral com-
position diversity, grain and crystal sizes, and natural fractioning in particular formations.
Land subsidence caused by solution mining of the Mogilno salt dome 177
Geological cross-section of the Mogilno salt dome (Szybist in: Kortas red. 2008): 1 – salt
series, 2 – gypsum cap + anhydrite coat, 3 – Mesozoic, 4 – brown coal, 5 – Tertiary and Quaternary
In the SE section, the deposit has been mined by underground leaching since 1986,
and a cavern gas storage facility has been situated in the NE section. The solution mining
method was started at the depth of 1,400 m and continued up to the ceiling shelf at 400 m.
The leaching openings were set within a triangle grid of 100 m sides. The cavern diameters
depend on cavern depths and range from 45 m in the lower zone to 55 m in the upper one
on the average. On the area of the potassium-magnesium salt occurrence, the openings are
locally larger. Proper shelves for conducting mining operations were left in the beds which
are not useful for extraction.
Owing to the mining method, the changeability of the susceptibility to the dissolution is
especially arduous. K-Mg salts and coarse-crystal halite are quicker to dissolve than the me-
dium- and fine-crystal rock salts. K-Mg salts are still dissolving even in the fully NaCl
saturated brine. They create the so-called heavy potassium-magnesium alkalis. Vertical bed-
ding is favourable for alkali dropping and light brine rise. Consequently, constantly active
G. Kortas, A. Maj & J. Drogowski 178
leaching places occur, with the development of unintended connections between caverns.
Those are the reasons of uneven, non-cylindrical cavern shaping.
The cavern geometry is identified on the basis of the sonar measurements. Owing to
the inaccessibility of certain leached out areas for taking measurements, within the so-called
acoustic grey zones, the cavern volume is determined mainly on the basis of the extraction
output data. Despite the breaching pillar space, considerable resistant virgin masses are pre-
served in the rock mass between the caverns. The mining ratio, or the proportion between
the cavern volume and space determined by the mining field boundaries (excluding the edge
pillars and the mine’s ceiling shelf), is estimated at 0.23 in the Mogilno Salt Mine. That
value is similar to the indicators obtained in underground room-and-pillar salt mining con-
ducted in the Kłodawa Salt Mine, as well as in the Solno and Wapno salt mines exploited
in the past. With a similar use of mining ratio, the geo-mechanical conditions are more ben-
eficial in the salt mines using the solution method than in the case of the underground dry
extraction. That is caused of body rock support by brine under its hydrostatic pressure, as
well as the leaching, non explosive mining method effects.
Convergence, uplift of cavern bottoms, and land surface displacements are measurable
effects of geo-mechanical actions that affect the rock mass during salt mining. Since the cav-
erns are not separated, the convergence of cavern environment deformations has not been
studied yet. However, the measurements of displacemens within the mining area have been
regularly taken. Six observations by the precise levelling method, class II, were conducted
between the start of the field mining operations in 1986 and the last series of measurements
taken in 2009, with the average error of ±0.7 mm/km. The number of benchmarks observed
since 1986 was reduced, although new ones were set up as well. It is estimated that the net-
work observation capability indicator reached 0.63, in the scale from 0 to 1.0, based on
the criteria determined in the literature (Maj 2004).
In the case of the observation of slowly increasing displacements, the time gap between
observations is selected in respect of the accuracy of measurements. The subsidence deter-
mination error in geodetic measurements increases with the length of levelling sequences
and the sum of levelling deviation squares determined for the grid mesh, although the reli-
ability of subsidence measurements also depends significantly on the stability of link points.
When analyzing the influence of the Wieliczka Salt Mine on the ground surface, Szewczyk
(2008) appraised the average subsidence height error at ±5 mm. The author stated, however,
that, omitting the errors between link points and stable points, the accuracy of height deter-
mination by the precise levelling method, after a measurement result adjustment, can reach
±0.5 mm on the average (oral communication). However, taking into account the diversity
of link-point selection in the levelling grid applied in Mogilno and the uncertain stability
of link points, we can assume that the upper error limit of subsidence height determination
does not exceed ±2 mm.
Land subsidence caused by solution mining of the Mogilno salt dome 179
The distribution of subsidence on the Palędzie I mining field in 1986–2009 is shown
in Figure 2. Subsidence areas do not exceed –105 mm. The subsidence of −148 mm appeared
above the southern salt dome wall, but it was omitted in the presentation of the subsidence
distribution in this study, owing to the incidental increase of subsidence recently. The maxi-
mum average the subsidence rate was −4.8 mm/year.
Land subsidence in 1986–2009 (milimiters), with the locations of mining openings
The distribution of the subsidence in 2004–2009, presented on Figure 3, shows that
it was mainly the area adjacent to the SW salt dome wall which subsided in that period.
The maximum subsidence reached −51 mm, and the average rate of maximum subsidence
amounted to −10 mm/year in that period.
G. Kortas, A. Maj & J. Drogowski 180
Increase of the land subsidence trough in 2004–2009 (milimiters)
The Horizontal displacements were dominating the component movements in the cav-
ern environment. The Land surface subsidence is the result of such a rock mass movement
dissipation. Owing to the shape of the vertical cross-section of the mine structure and salt
dome, the range of the effects moving in the eastern direction is the smallest, and we can
estimate it at 400 m. In the western direction, however, the zero isoline of the subsidence ap-
pears outside the edge pillar, at a distance not exceeding 1,000 m, within the influence zone
of the underground cavern gas storage facility. The uplift effect measured on about a dozen
of the benchmarks located on the eastern section of the subsidence trough (Figs 2, 3) can
indicate that the link points of the levelling measurements were not stable or, which is also
probable, the analysts observed the salt dome uplift activity caused by the mining operations.
However, the stability of the observation grid link points requires clarification first of all.
Land subsidence caused by solution mining of the Mogilno salt dome 181
The variations of the land subsidence distribution in 1986–2009 are illustrated by
a set of profiles specified for the direction which is perpendicular to the longitudinal axis
of the salt dome (Fig. 4).
SW-NE profiles of land subsidence in 2009
The analysis of the subsidence distribution above the solution mining area in Mogilno
indicates the following:
- the distribution of subsidence does not create a regular trough, but rather a twin-centre
trough is shaped along the axis which is perpendicular to the longer side of the mining field;
- the maximum subsidence (max S and max N on Figure 4) occurs on the SW side
of the salt dome, 400 m from the field centre, and that distance decreased in recent years;
- the second centre, with smaller subsidence, is located in the same distance from the field
centre but in the opposite direction (NE); that distance increased in recent years;
- the range of effects on the SW side is larger than in the NE side, and it reaches the dis-
tance double of the mining depth.
However, the present geophysical recognition of the salt dome boundaries is not accu-
rate enough. Generally, the edge pillars are wider on the NE side than on the SW side (Fig. 4),
therefore, the consequences of possible leaching out of the deposit at the SW border can be
reflected in the form of the land subsidence.
Model studies explain that the twin-centricity of the subsidence trough is the result
of the slim vertical mining structure and the more conspicuous it is the higher is the techno-
logic pressure. The technologic pressure applied in mining is useful for overcoming the ex-
tracted brine’s flow resistance. The shift of the mining frontage up to the ceiling section
of the deposit allows for the reduction of the pressure applied.
0 500 1000 1500 2000 2500 3000 3500 4000 4500
-11 0
-10 0
-9 0
-8 0
-7 0
-6 0
-5 0
-4 0
-3 0
-2 0
-1 0
0
1 0
ma x S M-1 3 ma x N
100 0
800
600
400
200
0
Profiles in particular periods:
1986 - 1993
1986 - 1994
1986 - 1999
1986 - 2004
1986 - 2009
S u b s i d e n c e [ m m ]
S W
N E
D is ta n c e [m ]
S a ltd o m d e p th [ m ]
Mo g i ln o
S a l tD o m e
G. Kortas, A. Maj & J. Drogowski 182
Consequently:
- the rock mass uplift gets smaller in the central part of the subsidence trough,
- the pressure in the cavern decreases, and the same with the hydraulic support on the body
rock in the lower extracted section of the deposit,
- convergence increases in the lower section of the deposit,
- land surface subsidence height increases, especially close to the boreholes field.
The model studies conducted in 2011 (Kortas et al. 2011) indicated that expanding defor-
mations are present in the close vicinity of the caverns. In the case of excessive process pres-
sures, those deformations could cause local fracture in the rock mass. The difference between
the volumetric convergence of the caverns and the subsidence trough volume corresponds
to the increase of the rock mass volume in the zone affected by the mining operations. Thus,
the smaller the trough volume the larger volumetric deformations can be expected. They reach
maximum values if no land surface subsidence occurs together with cavern convergence.
The trends of rock mass movement and land trough shaping can be determined on the ba-
sis of benchmark lowering observations in time. The benchmark lowering above the solution
workings during the 1986–2009 period is presented on Figure 5.
Benchmark lowering in the central part of the subsidence trough
198 5 1 990 1 99 5 20 00 20 0 5 201 0 201 5
-1 00
-9 0
-8 0
-7 0
-6 0
-5 0
-4 0
-3 0
-2 0
-1 0
0
78
1102
73
1312 2903
5001
3705
3701
3704 1310
77
3603
3404
1005
3101
3902
4001
13021304
73...
77...
78...
1005
1102
1302
1304
1310
1312
2903
3101
3404
3603
3701 3704
3705
3902
4001
5001
56000 56200 56400 56600 56800 57000 57 200 57400 57600 57800
94200
94400
94600
94800
95000
95200
95400
95600
95800
(max N)
(max S)
T i m e [ y e a r ]
S u b s i d e n c e [m m ]
Land subsidence caused by solution mining of the Mogilno salt dome 183
Observations revealed a non-linear increase of subsidence. On that background,
short temporary abrupt subsidence rate changes are observed as well. Those were rather
caused by the consequences of the methods of linking the grid and its equalization than
any geological or mining aspects, which was better visible on the areas subjected to big-
ger subsidence. The dependence of the subsidence on time is the increasing function, with
the trend which is similar to large and small subsidence heights. Therefore, the trend is
characterizing the behaviour of the rock mass and land surface areas affected by the min-
ing operations.
The relationship between land subsidence and time can be expressed by the follow-
ing exponential function:
w(t)= w0 [exp(CDt) – 1] (1)
where Dt (year) is the time increase since the start of the mining operations in 1986,
and the C (1/year) parameter increase with the distance between the benchmarks
and the mining field boundaries. The parameter w0 = −20 mm can be associated with
the average extraction intensity. Figure 6 shows the graphs of function (1) for several
values of parameter C.
Benchmark lowering rate trend
1985 199 0 1995 2000 200 5 2010 2015
-1 40
-1 20
-1 00
-8 0
-6 0
-4 0
-2 0
0
T i m e [y e a r ]
C = 0 , 0 1 4
C = 0 , 0 3 4
C = 0 , 0 7 4
S u b s i d en c e [m m ]
G. Kortas, A. Maj & J. Drogowski 184
The proportion of the trough volume VN(t) to the workings’ volume VK(t) is the coef-
ficient of the workings’ influence on the affected land surface.
The trough volume in the function of time VN(t) was estimated on the basis of the sub-
sidence observations. That is approximated by the following function, similar to (1):
f=VN(t)=0.085 mln m3 exp CΔt
( )
−1
[ ]
(2)
where VN(1) = 5,100 m3 is the increase of the trough volume subsidence in the first year
of the mining operation, and the parameter is C = 0.06 1/year.
Extraction, caverns and trough volumes in the time function
The cavern volume VK(t) was determined by assuming the calculation coefficient
of j =1.68 Mg/m3, expressing the proportion of the workings’ volume to the extracted min-
eral volume. Generally, three mining periods were distinguished: 1986–1993, 1993–2004,
and 2004–2009 (Fig. 7). The estimated proportion of the trough volume to the cavern vol-
umes reached the following values in the respective periods: 11‰, 17‰, and 20‰. The sub-
sidence trough volume increase rate was still small: VN/VK = 0.321 Mm3 / 14.4 Mm3 = 22.3‰.
The increasing caverns’ volume was the main cause of the land subsidence rate but their
0
1
2
3
4
5
6
7
8
9
10
11
12
1984 1 986 1988 199 0 1992 1 994 1996 1998 20 00 2002 2 004 2006 200 8 2010 2 012 2014
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0.0 0
0.0 2
0.0 4
0.0 6
0.0 8
0.1 0
0.1 2
0.1 4
0.1 6
0.1 8
0.2 0
0.2 2
0.2 4
0.2 6
0.2 8
0.3 0
0.3 2
0.3 4
0.3 6
0.3 8
0.4 0
C a v e r n v o lu m e [m i o . m 3 ].
Total extraction [mio. Mg]
CavernvolumeV
k
Annual extraction
T r ou g h v o lu m e [m io . m
3
]
TroughvolumeV
n
P e r io d 1
19 8 6 1 9 93
Vn /Vk= 1 1 ‰ P e r io d 2
19932004
Vn/Vk = 2 7 ‰
P e r io d 3
20042009
Vn/Vk = 2 0 ‰
T i m e [y e a r]
Land subsidence caused by solution mining of the Mogilno salt dome 185
distribution in the last period was the result of the extraction of the upper part of the salt
dome. The trend stability indicates that in the future we can expect further subsidence rate
increases, bigger on the mining field area and surrounding lands, smaller on farther outside
these areas.
The stability of the land surface subsidence trend, specified by equation (2), would lead
to a considerable depression on the land surface in a longer period. The increase of subsid-
ence rates should slow down after discontinuation of deposit exploitation.
1. The distribution of subsidence in 1986–2009 formed a twin-centre trough, with the max-
imum subsidence of −105 mm on the SW side of the mining field, which was explained
mainly by the vertical slenderness of the mining field. The range of the effects identified
on the SW side was larger than on the NE side, and it corresponded to the double maxi-
mum depth of the caverns.
2. The increase of the subsidence in 2004–2009 indicated a further increase of the subsid-
ence rate, which was primarily caused by the increase of the caverns’ volume.
3. The proportion between the subsidence trough volume and the cavern volume, describ-
ing the influence of the exploitation on the land surface, is presently reaching ca. 22‰.
Small land subsidence values cause, in view of the extensive trough, that the related
deformations are not harmful for ground surface.
4. The evaluation of the rock mass requires previous cavern convergence measurements
which will be possible after mining operations in the caverns terminate.
Bailey R.W., 1929. Creep of steel under simple and compound stresses, and the use of high
initial temperature in steam power plants. The transactions of the Tokyo Sectional Meet-
ing, World Power Conference, Tokyo, October 29–November 7, 1929; vol. 3: Power for
use in transportation, better efficiency in power production, 1089–1121.
Grzybowski Ł., Wilkosz P. & Flisiak D., 2008. Mechanical properties of rock salt from
Mogilno salt dome. Gospodarka Surowcami Mineralnymi [Przegląd Solny], 24, 3/2,
141–157.
Kortas G. (red.), 2008. Ruch górotworu w rejonie wysadów solnych. Wydawnictwo Instytutu
Gospodarki Surowcami Mineralnymi i Energią PAN, Kraków.
Kortas G., 2009. Singularities of the rock mass movement during the minning of salt domes.
Conference Papers, Solution Mining Research Institute – Spring 2009, Kraków, 14 mar-
ca 2009.
G. Kortas, A. Maj & J. Drogowski 186
Kortas G., Maj A., Flisiak D., Kortas Ł., Jagiełło W. & Kot P., 2011. Określenie optymalnych
lokalizacji oraz geometrii komór eksploatacyjnych w południowo-wschodniej części
złoża „Mogilno I”. GeoConsulting Kraków, IKS Solino SA [unpublished].
Maj A., 2004. Obserwacja osiadań powierzchni nad wyrobiskami w wysadzie solnym
na przykładzie kopalni Kłodawa. WUG: Bezpieczeństwo Pracy i Ochrona Środowiska
w Górnictwie, 9, 25–28.
Norton F.H., 1929. The Creep of Steel at High Temperatures. McGraw-Hill, New York.
Szewczyk J., 2008. Kopalnia Soli „Wieliczka” – 80 lat obserwacji deformacji górniczych.
Gospodarka Surowcami Mineralnymi [Przegląd Solny], 24, 3/2, 251–272.
Ślizowski J., 2006. Geomechaniczne podstawy projektowania komór magazynowych gazu
ziemnego w złożach soli kamiennej. Studia, Rozprawy, Monografie – Polska Akademia
Nauk. Instytut Gospodarki Surowcami Mineralnymi i Energią, 137, Wyd. IGSMiE PAN,
Kraków.
Tadych J., Drogowski J., Grzybowski Ł., Kleczar M., Enghardt J. & Bornemann O., 2010. Op-
tymalizacja procesu eksploatacji soli kamiennej w oparciu o geologiczną interpretację
pomiarów georadarem złóż soli kamiennej „Góra” i „Mogilno I”. XV Międzynarodowe
Sympozjum Solne Quo Vadis Sal, Świeradów-Zdrój 21–22 października 2010 r., Polskie
Stowarzyszenie Górnictwa Solnego, 62–64.
Wpływy eksploatacji soli na powierzchnię terenu określane są na podstawie okreso-
wych pomiarów niwelacyjnych. Analiza wyników tych pomiarów pozwala na kontrolowanie
i prognozowanie ruchu górotworu.
Wysad solny Mogilno ma kształt wydłużonej elipsy ze zwierciadłem solnym około
250 m p.p.t. (Fig. 1). Na głębokości około 600 m długość wysadu wynosi 5,5 km, a szerokość
waha się od 0,8 km do 1,5 km. Ściana NE wysadu jest nachylona pod kątem ok. 80°, nato-
miast przeciwległa SW 90° i więcej, tworząc przewieszenia. W SE części wysadu strop soli
obniża się do 550–650 m.
Otworowa eksploatacja złoża w 1986 r. rozpoczęła się od głębokości 1400 m do półki
stropowej na głębokości 400 m. Otwory ługownicze założono w siatce trójkątów o bokach
100 m. Średnice kawern zależą od głębokości, wynosząc średnio od 45 w dolnej strefie
do 55 m w górnej. W obszarach występowania soli potasowo-magnezowych są większe.
W złożu nieprzydatnym do prowadzenia eksploatacji pozostawiono między komorami od-
powiednie półki.
Pomiary przemieszczeń w strefie oddziaływania kopalni prowadzone są od 1986 r.
metodą niwelacji precyzyjnej ze średnim błędem ±0,7 mm/km, ostatni wykonano w 2009 r.
Obniżenia terenu w latach 1986–2009 nie przekraczają −105 mm (Fig. 2). Stwierdzany
w pomiarach efekt podnoszenia się terenu po stronie NE niecki osiadań wskazuje, że punkty
dowiązania pomiarów niwelacyjnych nie są stałe. W latach 2004–2009 obniżał się głównie
rejon przyległy do SW ściany wysadu (Fig. 3).
Land subsidence caused by solution mining of the Mogilno salt dome 187
Osiadania terenu wykształcają się w formie dwucentrycznej niecki z maksimami po SW
i NE stronie wysadu (Fig. 4). Przyczyną osobliwego rozkładu osiadań jest smukłość for-
my kopalni i reżim stosowanych ciśnień technologicznych. Wyniki obserwacji ujawniają
zróżnicowany zasięg wpływów eksploatacji, mniejszy w kierunku NE niż w kierunku SW,
co spowodowane jest budową złoża. Zasięg ten po stronie SW wysadu osiąga dwukrotną
głębokość spągu komór.
Obniżenia terenu zależą od wydobycia, a prędkość ich rośnie w czasie (Fig. 5, 6).
W ciągu 25 lat określa je funkcja wykładnicza z parametrem C: w(t) = w0 [exp(CΔt) − 1], gdzie
C jest proporcjonalne do odległości reperów od pola górniczego. Objętość niecki aproksy-
muje także funkcja wykładnicza: VN(t) = 0,085 mln m3 [exp(CΔt) − 1], gdzie C = 0,06 1/rok.
Wskaźnikiem oddziaływania wyrobisk na powierzchnię terenu jest stosunek objętości
niecki osiadań VN do objętości wyrobisk VK, proporcjonalnej do wydobycia. W latach
1986–1993, 1993–2004, 2004–2009 wskaźnik f(t) osiągnął wartości 11‰, 17‰ i 20‰
(Fig. 7).
Przyczyną wzrostu prędkości obniżeń terenu jest przede wszystkim rosnąca objętość
komór. Przechodzenie strefy ługowania od dołu ku górze powoduje spadek stosowanych
ciśnień technologicznych, odpowiedni do zmniejszającej się jej głębokości eksploatacji.
Skutkiem tego jest zmniejszenia się podparcia hydraulicznego ścian komór. Z analizy wy-
nika, że w następnych latach można się podziewać dalszego zwiększenia prędkości obniżeń
w rejonie nadległym i przyległym do pola górniczego przy utrzymywaniu się prawie stałej
prędkości obniżeń w znacznym oddaleniu od wyrobisk.
