Tuzla City (BiH): an example of geohazard induced by salt extraction
Creators: name: Stecchi, Francesco <1977>
Publisher: name: Alma Mater Studiorum - Università di Bologna
Year: 2008
Subjects: GEO/02 Geologia stratigrafica e sedimentologica

description: Salt deposits characterize the subsurface of Tuzla (BiH) and made it famous since the ancient
times. Archeological discoveries demonstrate the presence of a Neolithic pile-dwelling settlement
related to the existence of saltwater springs that contributed to make the most of the area a swampy
ground.
Since the Roman times, the town is reported as “the City of Salt deposits and Springs”; "tuz"
is the Turkish word for salt, as the Ottomans renamed the settlement in the 15th century following
their conquest of the medieval Bosnia (Donia and Fine, 1994). Natural brine springs were located
everywhere and salt has been evaporated by means of hot charcoals since pre-Roman times.
The ancient use of salt was just a small exploitation compared to the massive salt production
carried out during the 20th century by means of classical mine methodologies and especially wild
brine pumping.
In the past salt extraction was practised tapping natural brine springs, while the modern
technique consists in about 100 boreholes with pumps tapped to the natural underground brine runs,
at an average depth of 400-500 m. The mining operation changed the hydrogeological conditions
enabling the downward flow of fresh water causing additional salt dissolution.
This process induced severe ground subsidence during the last 60 years reaching up to 10
meters of sinking in the most affected area. Stress and strain of the overlying rocks induced the
formation of numerous fractures over a conspicuous area (3 Km2). Consequently serious damages
occurred to buildings and infrastructures such as water supply system, sewage networks and power
lines.
Downtown urban life was compromised by the destruction of more than 2000 buildings that
collapsed or needed to be demolished causing the resettlement of about 15000 inhabitants (Tatić,
1979).
Recently salt extraction activities have been strongly reduced, but the underground water
system is returning to his natural conditions, threatening the flooding of the most collapsed area.
During the last 60 years local government developed a monitoring system of the phenomenon,
collecting several data about geodetic measurements, amount of brine pumped, piezometry,
lithostratigraphy, extension of the salt body and geotechnical parameters. A database was created
within a scientific cooperation between the municipality of Tuzla and the city of Rotterdam (D.O.O.
Mining Institute Tuzla, 2000).
The scientific investigation presented in this dissertation has been financially supported by a
cooperation project between the Municipality of Tuzla, The University of Bologna (CIRSA) and the
Province of Ravenna. The University of Tuzla (RGGF) gave an important scientific support in
particular about the geological and hydrogeological features.
Subsidence damage resulting from evaporite dissolution generates substantial losses
throughout the world, but the causes are only well understood in a few areas (Gutierrez et al., 2008).
The subject of this study is the collapsing phenomenon occurring in Tuzla area with the aim
to identify and quantify the several factors involved in the system and their correlations.
Tuzla subsidence phenomenon can be defined as geohazard, which represents the
consequence of an adverse combination of geological processes and ground conditions precipitated
by human activity with the potential to cause harm (Rosenbaum and Culshaw, 2003).
Where an hazard induces a risk to a vulnerable element, a risk management process is
required.
The single factors involved in the subsidence of Tuzla can be considered as hazards. The final
objective of this dissertation represents a preliminary risk assessment procedure and guidelines,
developed in order to quantify the buildings vulnerability in relation to the overall geohazard that
affect the town.
The historical available database, never fully processed, have been analyzed by means of
geographic information systems and mathematical interpolators (PART I).
Modern geomatic applications have been implemented to deeply investigate the most relevant
hazards (PART II). In order to monitor and quantify the actual subsidence rates, geodetic GPS
technologies have been implemented and 4 survey campaigns have been carried out once a year.
Subsidence related fractures system has been identified by means of field surveys and
mathematical interpretations of the sinking surface, called curvature analysis. The comparison of
mapped and predicted fractures leaded to a better comprehension of the problem. Results confirmed
the reliability of fractures identification using curvature analysis applied to sinking data instead of
topographic or seismic data.
Urban changes evolution has been reconstructed analyzing topographic maps and satellite
imageries, identifying the most damaged areas. This part of the investigation was very important for
the quantification of buildings vulnerability.