Studia Geotechnica et Mechanica, Vol. 39, No. 2, 2017 DOI: 10.1515/sgem-2017-0013 GEOMECHANICAL NUMERICAL ANALYSIS AS A GUIDANCE FOR PRESERVATION WORKS OF THE “WIELICZKA” SALT MINE SITE KAJETAN D’OBYRN, ANTONI TAJDUŚ AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland, e-mail: [email protected], [email protected] Abstract: Salt was excavated at the “Wieliczka” Salt Mine for over 700 years. Underground mining operations terminated in 1996, by which time almost 2,400 chambers and 245 km of galleries had been created underground, situated on 9 levels and a few inter- levels. In 1978, the mine was included in the UNESCO World Heritage List, which stated that parts of the mine with historical value had to be preserved for future generations. In order to preserve the most valuable chambers and galleries, activities aimed at estab- lishing a protection pillar for excavations were conducted in the conservation area on Levels I–V. The need of large scope preserving works created the necessity to conduct a new and truly comprehensive geomechanical analysis. Such an analysis could only be done by means of advanced numerical modelling codes. Three-dimensional calculations were performed by means of FLAC 3D finite dif- ference code. Rock mass stability assessment in the vicinity of excavations was carried out on the basis of the distribution and range of the so called failure zones. This comprehensive geomechanical analysis allows for verification and give the directions for future preservation and closure works in the “Wieliczka” mine. Key words: geomechanical analysis, numerical modelling, physico-mechanical rock parameters, “Wieliczka” Salt Mine 1. INTRODUCTION associated costs. The scope and directions for further elimination works take into account the major dan- ger for the mine, i.e. the flooding hazard, especially The behaviour of historical and functional exca- at the northern boundary of the deposit into ac- vations, especially chambers in the “Wieliczka” Salt count. In light of findings from the first comprehen- Mine, is principally affected by the geological sive geomechanical analysis [1], [20], and analysis structure of the rock mass and hydrogeological and conducted by the “Wieliczka” Salt Mine and Krakow mining conditions. In rock masses such as rock salt, Saltworks Museum in Wieliczka [15], future actions the rheological and plasticity phenomena are acti- should primarily focus on preserving the most valu- vated as a result of rock mass pressure, which leads able parts, especially chamber excavations. In addi- to a constantly ongoing deformation process of the tion to geological and mining conditions, an impor- rock mass surrounding the excavations, and as a re- tant factor is provided through guidelines from the sult, their squeezing, loss of functionality and in Polish historical monument protection service, which many cases, their elimination. In 1978, after the mine recommends striving for maximum preservation of was included in the first UNESCO cultural and natu- the original substance and form of the excavations ral heritage list, comprehensive operations were recognized as historic in conducting mining preser- launched to create a strong support (in the form of vation works and define closure or excessive trans- a pillar) under the most historical, central part of the formation of the excavations as unacceptable and mine – Levels I to IV and partially Level V, through unjustified. eliminating (backfilling) excavations located at greater Work conducted on excavation elimination and depths (Levels VI–IX). These efforts have continued securing has so far resulted in the necessity to conduct to the present day. Despite these activities, preserv- a new and comprehensive geomechanical analysis, ing all the excavations, especially on Levels IV–V, which has been made possible by developments in has proved virtually impossible due to continuously numerical modelling capabilities [5]. deteriorating geomechanical and hydrogeological This analysis takes the target technical model of factors, technical condition of the excavations and the mine, the geological, hydrogeological and mining 26 K. D’OBYRN, A. TAJDUŚ conditions into account, to provide more precise knowl- clusters. Within the complex of Chodenice layers, the edge of the physical and mechanical parameters of the presence of sandy, conglomerate and clastic forma- Wieliczka rock mass, as well as technical condition of tions can also be observed. the excavations and the operations conducted so far. Within the deposit, two major parts are distin- This includes creating a protection pillar for excava- guished: the brecciated deposit and layered bed. In the tions in the historic area on Levels I–V. The minimum brecciated deposit, salt formations are strongly dislo- number of historic chamber excavations and excava- cated, while in the layered bed, despite tectonic tions performing technical functions has also been movements the continuity and succession of layers taken into account. This analysis therefore allows for have been generally preserved. The thickness of the verification and feasibility of the “Wieliczka” mine salt-bearing layer is approximately 350 m. target model to be achieved, in order to carry out fu- The layered bed is formed of three tectonic slices ture securing and elimination. thrust over one another. Within these thrust slices, the Stability of the rock mass in the “Wieliczka” Salt salt layers are subject to continuous deformation, i.e. Mine is determined by geological and mining condi- folding and local superimposition, leading to signifi- tions. In terms of mining factors, both historical condi- cant changes in their thickness. As a result of these tions resulting from mining activities (post-exploitation processes, the rigid gangue surrounding the deposit is voids) and the surface of the backfilled excavations highly fissured. The bed is formed of layers of the should be taken into account. oldest, layered green, shaft and spiza, salts. These salts vary in age, and were formed in a slightly differ- ent sedimentation conditions, and also contain differ- 2. GEOLOGICAL CONDITIONS ent impurities. The gangue in the deposit has formed as laminated mudstone transforming into claystone and sandstone. Rock salt deposit in the Wieliczka region is part The oldest salts form a complex of layers with of the Miocene salt-bearing formation that stretches a thickness of approximately 12 m. They are charac- in a narrow belt within the Carpathian Foredeep, from terized by varying grain size, variable amounts of Upper Silesia towards Kraków and Poland’s eastern terrigenous material and numerous veins of silty-clay border. The deposit extends in the east-west axis over and gypsum-anhydrite. The impurities in the salt make a length of about 10 km and its width in the area of up to 50% of its volume. The oldest salts display the the “Wieliczka” mine is of approx. 1.0 km. Within it, greatest variation of strength, particularly compres- three lithostratigraphic modules are distinguished: the sive strength. Skawina, the Wieliczka (the salt series) and the Cho- A higher stratigraphic module is formed of layered denice layers. The geological structure of the Wielic- green salts. These are distributed throughout the de- zka deposit has been the subject of many studies, both posit in the form of 3–5 layers with the thickness in the regional [16], and local context [13], and the between tens of centimetres and 2 meters, separated knowledge of its complexity, especially in tectonic by gangue. The size of the crystals in those is variable, terms, is regularly enriched [14], [23]. but large-crystal salt with crystals between a few to The Skawina layers are formed of green-grey marl a dozen centimetres in size prevails. with rich foraminifer fauna, sandstones and mud- Another stratigraphic module is formed by shaft stones. They overlie strongly dislocated formations of salt, characterized by the constancy of structure and is the older substrate, mostly Upper Jurassic limestone. present throughout the area. The fine- to medium- The thickness of these formations is highly variable crystal salt layer, with a thickness of approximately and ranges from approximately 50 m in the western 1.5 m, is often tectonically thickened due to the sus- part of the deposit to 350 m in the east. ceptibility of shaft salt to plastic deformation. This The Wieliczka deposit contains several layers layer is formed from crystals up to a few millime- of rock salt with variable thicknesses, structures and tres in size. NaCl content. Rock salt layers are separated by Above the shaft salt layer, spiza salts occur, ap- claystone, siltstone and sandstone formations as well proximately 30 m thick, with a variable granular as gypsum and anhydrite which represent the sul- structure, which has a content of terrigenous material. phate facies. The grain structure is predominant here, with a large The Chodenice layers occur in the form of dark- admixture of quartz and other terrigenous material grey claystone, shale and mudstone containing gyp- (up to 20%). The degree of contamination with sand sum as well as anhydrite, limestone and dolomite increases towards the roof of the salt spiza layer, which Geomechanical numerical analysis as a guidance for preservation works of the “Wieliczka” Salt Mine site 27 in places becomes sandy salt (so called “Makowica”) model constructed, the following rock layers were or unevenly overlying and distributed sandstone with distinguished: a halite binder in the roof part of the complex. In the Quaternary formations (clay, silty sediments with lower part of the layer, a pure variety of the spiza salt sand, gravel and flysch debris inserts); (the so called “eagle salt”) is usually found. The entire deposit lagging (the gypsum-clay cap); complex of spiza salt is interspersed with mudstone- the brecciated deposit (marly claystone, zubry, salt sandstone anhydrite inserts, with an average thickness blocks); of approximately 0.1 m. In the spiza salt layer, a cen- the layered bed: tral mudstone-sandstone-anhydrite insert with a thick- – spiza and sandy salt layer, ness of 2–5 meters occurs.