Naslov
Experiance with tunnelling in carst region

Autori
Vrkljan, Ivan ; Ožanić, Nevenka ; Ženko, Tomislav

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Advances in Hydro Science and Engineering : Book of Abstracts / Piasecki, Michael - Philadelphia (PA) : Drexel University, 2006, 363-364

Skup
Advances in Hydro Science and Engineering

Mjesto i datum
Philadelphia (PA), Sjedinjene Američke Države, 11.09.2006. - 13.09.2006

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
tunnel excavation; karst; underground waters; water protection areas

Sažetak
In Croatia a karst area are widely distributed. Their total area is about 25.790 km2 that represents approximately 45% of the total territory. These areas are primarily located along the coast and in near vicinity. Therefore numerous researchers have studied and described karst phenomena in this particular area. Several internationally adopted expressions originated from Croatian language (dolina, polje, vrulja etc.). The national Institute for Karst Research located in Gospić was founded. Until now about 9 000 caves and cavities of diferent shapes and dimensions were investigated. On two main roadways connecting the capital of Croatia, Zagreb and the Adriatic coast, during the last 10 years a total number of 27 tunnels with cumulative length of over 53 km were constructed. Namely these are highways Zagreb-Rijeka and Zagreb-Split-Dubrovnik. If we add to these also the tunnels in the city of Rijeka and the “ Učka” tunnel that connects Istria and Kvarner regions we are reaching the length of over 60 km road tunnels. They are all constructed in karst areas and some of them are located near water spring of drinking water, or near national parks and parks of nature. That emphasizes the necessity of the protection of surface and underground waterways. During their construction numerous caverns of different dimensions and shapes were discovered. About 730 caverns, e.g. speleological formations without natural entrances, were investigated. The highest density is recorded at the location of twice tube tunnel Brinje (2x540 m) where 73 caverns were discovered. The principal objective of the paper is to inform professional community about our experience while placing special emphasis on difficulties that may be expected during realization of underground works in karst terrain. Consequently, two aspects of building in karst surrounding are emphasized: a) the necessity for preservation and protection of great amount of underground water during highways construction and exploitation ; b) the risks that accompany project making and building in karst areas from the perspective of choosing the adequate technology for excavation and stabilization of underground environment. In karst areas, most of the precipitation water flow is happening along the underground water systems, which in favorable hydro geological condition can give considerable quantities of drinking water. In comparison with other porous media karst aquifers are unpredictable and, on the other hand very vulnerable during the episodes of pollution. Hydro geological characteristics of karst aquifers cause the fast underground water circulation and, consequently, shorter water holding up in the underground and low self purification capability. One of the basic measures for the protection of karst water springs and aquifers is the determination of sanitary protection zones and performing of protective measures on them. Regulations regarding the defining of water protection zones should ensure the greater homogeneity of criteria for the protection of present water resources in karst from both, regular activities and accidental water pollutions on catchment areas of karst water springs. The basic sources of pollution are precipitation waters that wash out the pollutants from the road surfaces and could infiltrate the surrounding terrain and water streams. The pollutants are of different origin such as: losses from the motive power and lubrication systems tire residues, products of wearing out of the surface layers and the emission of combustion products. The most polluted waters are those during the first 30 minutes of precipitation, especially after a longer dry period. The combustion products (gasses and aerosols) are deposited on the surrounding ground and plants. It is estimated that approximately 50% of lead emitted with the exhausted gasses is precipitated in the diameter of 30 m. Other heavy metals under certain conditions could be very mobile and represent a great threat as pollutants of underground waters because they are very difficult to control. Accidents on roads and in tunnels could cause spilling of noxious and dangerous materials from vehicles. In water protection zones on vulnerable karst ground such accidents could cause irreparable damages on potable water springs with long term consequences. It has been proved that polluters once reached underground have very complex mechanism of outflow. They could then reach underground areas located on very distant locations outside the water protection zone defined according to the criterion of time of water withholding in the underground. For further avoidance of all of the mentioned consequences during the planning and projecting of such roads and tunnels, it is necessary to think about such solutions that could prevent or minimalize accident consequences to the acceptable level. It could also be suggested that the placement of most risky elements of these constructions should avoid the most susceptible areas. All those measures should be based on positive legislation for the protection of water quality of drinking and surface water, ensuring the safe drainage of wastewater and maintaining the water regime. The overburden thickness registered on above mentioned tunnels attained up to 600 m. The excavation of twin tube tunnels has revealed the extent of unpredictability that may be expected when forecasting geological features in karst terrain. Thus engineering geological properties in individual tubes of a twin tube tunnel may differ considerably regardless of the small axis-to-axis distance between the tubes (25 m). The actual geological situation was determined through detailed engineering-geological investigations performed during tunnel excavation. The analysis of accuracy obtained in forecasting engineering-geological conditions based on preliminary investigations (geophysical survey, exploratory drilling, geological mapping, analysis of aerial photographs, etc.) is presented in the paper. Difficulties encountered during rock mass classification in karst areas are also described. Numerous karst phenomena are often at the origin of cave-ins, some of which even affect the ground surface if the overburden thickness is small. These occurrences were classified into several categories. Appropriate repair measures were defined for each category.

Izvorni jezik
Engleski

Znanstvena područja
Građevinarstvo

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