ase study of multi-parameter monitoring for better
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forecasting capabilities of geohazards in mines
NERIS performed the early warning of the massive collapse of a large underground solution min-
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ing cavity. Integrating real-time microseismic, geotechnical and geodetic monitoring including remote au-
tomatic, innovative data processing and interpretation, into one unique system allowed to follow precisely
the evolution of the geosystem, to early detect the occurrence of the event and get new insight in the failure
mechanism. Operational implementation of such advanced monitoring strategies enhances the prediction
and prevention of such large ground failures in mining sites.
ontext. Fast large caving phenomena, followed by
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underground or surface collapses, are well-known geohazards
encountered in numerous mining situations. In the Lorraine region,
France, SOLVAY operates solution mining at
200 meters depth
through fresh water injection using a channel and drilling development.
This mining method leads to the formation of large surface sinkholes
resulting from the controlled collapse of the underground cavities
formed by the critical voids created. This strategy aims to avoid any
poisoning permanent risk in the future in terms of land management.
Nevertheless, the first major failure of the heading cavity is not without
risks related to safety at work. Considering all unknowns related to this
issue, detecting reliably its onset, full occurrence, quiescence and then
new equilibrium offers a obvious benefit to the mine management.
olution. In cooperation with the mine staff, INERIS set
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up an integrated geodetic, geotechnical and passive
microseismic monitoring to better control the growth and eventually
3D elevation view of the cavity from sonar
survey. Upper flat limit marks the top of the
the expected collapse. The applied methods were tested regarding
salt layer
their suitability to yield information on induced displacement field, rock
strata behaviour and fracturing processes. In situ surface displacement field was sampled by an automatic high
resolution infrared tacheometer profile covering the entire lateral cavity extension and a GPS real time kinematic
station placed along the vertical axis passing by the top of the cavity
determined from a previous sonar survey. Ground displacement
measurements and 2D profiles allowed to detect successive different elastic
regimes characterising the overall response of the overburden to the
growing cavity and also to constrain numerical models.
ase study of multi-parameter monitoring for better
C
forecasting capabilities of geohazards in mines
oreover, a multipoint extensometer deeply
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anchored in the upper roof provided information
on the behaviour of the bedding planes. Data
collected explained clearly the dominant role of a 10
meters thick stiff, resistant dolomite blocking the
progressive failure of the overlying strata with full
microseismic quiescence during several months.
ventually failure of the limestone strata led to the
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caving dynamics of the
100 meters
superincumbent burden monitored through passive
microseismic that recorded several tens of thousands
events. After 36 hours of strong underground activity
with no visible sign on surface, the decreasing
Autonomous SYTGEO RTK-GPS station with high sampling
frequency and radio transmission offered a very high quality
seismic rate and increasing subsidence rate
time series of the 4 meters vertical subsidence recorded du-
anticipated the appearance of the 150 meters crater
ring the last two days with an accuracy close to 1 cm.
shape opening in a few hours.
icroseismic post-processing source analysis
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indicated the presence of dominant thrust
faulting along pre-existing structures striking NNE-
SSW which could be later confirmed from aerial
photos showing the evolved state of the surface
crater several months and years after collapse. The
role of the limestone strata and these geological
features revealed of fundamental importance to
explain respectively the challenge to predict the
triggering time and the runaway kinetics of the
complete collapse.
essons learned. Knowledge and
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experience gained from this project helped to
validate the potential of multi-parameter monitoring
method, to develop an innovative online microseismic
event location method adapted to dense seismic
swarms and to improve the calibration of hybrid
numerical models.
uch technology is nowadays a key component of
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the safety procedure in a solution mine in Italy,
where sinkholes have been successfully anticipated
many times, improving the safety at work while
optimizing salt production.
Hybrid
geomechanic mode-
www.ineris.fr
ling, constrained by geotechnical
data, shows concentration of ca-
ving below the stiff dolomite layer
(FLAC) and location of discrete
fractures involved in its failure
(PFC).