Breakthrough of the Tunnel with the Biggest Overburden in Croatia

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1 289 Breakthrough of the Tunnel with the Biggest Overburden in Croatia Redovnikovi, L., Ališi, I., and Džapo, M. University of Zagreb, Faculty of Geodesy, Fra Andrija Ka i a Mioši a 26, 1 Zagreb, Croatia, Web site: lredovnikovic@geof.hr, ialisic@geof.hr, mdzapo@geof.hr Abstract Breakthrough of a very long tunnel is a challenge for people involved in its construction. Croatia is a country characterized by karst mountain range, in an almost unbroken line, extending from Kvarner region to the south. This mountain range is an obstacle for transport connection between north and south. All road tunnels in Croatia, which are longer than 4 m, are passing beneath this range. This paper aims to present the experience acquired during the breakthrough of the tunnel St. Ilija through mountain Biokovo. There is a problem of developing external geodetic network in the rugged terrain and the risk of possible side effects of refraction due to very high overburden. Therefore in this paper a method of measuring and calculating the work on controlling the breakthrough of the tunnel shall be presented. Key words: tunnel, breakthrough, overburden, limestone 1 INTRODUCTION Probably no geographical fact has so much influenced the historical development of Croatia as the mountain range extending in almost continuous sequence from the southern Croatian Kvarner to the Croatian south (Nadilo 21). For hinterland residents mountain ranges represented an insurmountable transport barrier preventing their access to the sea and separating them from the encounter with other civilizations, but at the same time defending them from dangers that have always came from the sea. It kept them in isolation and poverty. Impoverished and war-devastated country could not invest in building roads that would connect them to others. Therefore, all transport links between the mainland and coastal Croatia were built by the foreign countries, primarily motivated by military reasons. After the collapse of Yugoslavia, Croatia became an independent country and started the construction of modern motorways towards the sea. Thus millennia-old separation of land and coastal was finally overcome. Symbols of those changes are the tunnels, which broke the coastal mountains, especially Tuhobi above the Rijeka and Sv. Rok near Zadar. After the enormous effort and sacrifice the mythical Velebit was broken through and the next to go had to be the high and precipitous mountain Biokovo. TS 7 Underground Structures INGEO th International Conference on Engineering Surveying Brijuni, Croatia, September 22-24, 211

2 29 INGEO 211 Biokovo was been a major obstacle to traffic between the coast and coastal areas. Ideas of a tunnel through the mountain slowly began to revive with the approach of the highway A1 Split and announcements that it will extend to Dubrovnik. In 1997 the preliminary design of tunnel St. Ilija was drafted. First mining was carried out on the north side on March 25th 28, while on the southern side, work on the breakthrough began in early. Due to the disagreement with the users of the quarry it was necessary to drill a lateral tunnel whose length was 58 m (Figure 1). Figure 1 Sketch of the southern entrance to the tunnel St. Ilija With its length of 425 m, St. Ilija is currently 4 th longest road tunnel in Croatia and with its overburden of 133 m, it is classified as one of the tunnel with the largest limestone overburden in Europe. Since in Croatia there was a lack of experience in breakthrough of tunnels with such high overburden, it was decided that the breakthrough of the tunnel should be approach with great caution. 2 GEOTHERMAL GRADIENT With the breakthrough of tunnel with very high overburden the earth's geothermal gradient often leads to the appearance of high-temperature of walls in the tunnel due to the increased influence of lateral refraction on the measurement of horizontal directions. In order to better prepare ourselves for a breakthrough of the tunnel St. Ilija we studied the paper from the Energy Institute Hrvoje Požar, from which it is evident that on the northern territory of Croatia we can expect very high geothermal gradients, while on the southern part of Croatia we can expect geothermal gradients lower than average (Figure 2). Figure 2 Average geothermal gradient of the Croatian territory (URL 1)

3 Redovnikovi, L. et al.: Breakthrough of the Tunnel with the Biggest 291 According to this study, we can expect in the Dinarides the average geothermal gradient of 18 C/km. But this gives no assurance that area of the tunnel St. Ilija will show a low geothermal gradient. Since there are many deep caves in Croatia, which are located in similar geological terrain and reach depths greater than 13 m, we inquired about the temperature conditions prevailing in the deep pits. From caver Darko Baksi from PDS ''Velebit'' we received the information about the temperatures measured in deep pits on the north Velebit (Fig. 3). Figure 3 Deep pits in the area of north Velebit For instance, in ''Lukina jama'' (the deepest pit in Croatia) temperature of only 5 C was measured at the bottom of the pit (Figure 4). 25 Air temperature in ''Lukina jama'' 2 Air temperature [ C] Depth [m] Figure 4 Measuring air temperature in Lukina jama. In ''Lukina jama'' two temperature gradients were measured, from the entrance on a depth of m, there was a negative temperature gradient, while in the deeper parts (from -165 m to m) positive temperature gradient were measured which was C/km (Figure 5).

4 292 INGEO 211 ''Lukina jama'' air temperature up to 165 m 13 Air temperature [ C] y = 18.94e.331x R 2 =.9325 y =.557x R 2 =.8266 mjerenje linijska regresija eksponencijalna regresija 5 5 Depth [m] ''Lukina jama'' air temperature from 165 to 1368 m Air temperature [ C] y =.35x.839 R 2 =.9744 mjerenje linijska regresija Depth [m] Figure 5 Decreasing temperature at the entry and growth due to the geothermal gradient In the cave system ''Velebita'' two temperature gradients were also measured. Negative (up to -1 m) dt / dh = C/km and positive (from -1 m to -1 m) dt / dh = C/km. Similar results were obtained in other deep pits of north Velebit (URL 2). Such results suggested that in case of breakthrough of the tunnel St. Ilija we might expect a smaller temperature gradient than the average geothermal gradient of the Earth's crust, which, as stated, is about 3 C/km. If this is the case there would be no great risk of side effects from the refraction on the measurements of horizontal directions. Since there was no previous experience with the breakthrough of the tunnel with such a high overburden, every possible precaution while measuring the tunnel St. Ilija was taken in order to avoid adverse impacts of lateral refraction. Last control measurement was carried out during the Christmas and New Year holidays when all work in the tunnel was suspended. Since the tunnel ventilation was not working there was an equalization of air temperature with the temperature of the rocks. By measuring the temperature in the tunnel, the results obtained on the south side of the tunnel were 14 C, and on the north side of the tunnel 8 C (Redovnikovi 211)

5 Redovnikovi, L. et al.: Breakthrough of the Tunnel with the Biggest CONTROL MEASUREMENTS FOR TUNNEL ST. ILIJA Portal micro-networks were controlled by GPS static method. The homogeneity of the micro-network was tested by developing a precise traverse over Biokovo. Due to very inaccessible terrain this job required a good physical preparation of the surveyors. Portal micro-networks were also controlled with terrestrial measurements. All terrestrial measurements were carried out in 3 cycles and with a precise total station Trimble S6, which had an angle accuracy of 1''. On the northern side the geodetic control points was stabilized with reinforced concrete pillars, while on the south side the points were stabilized in the vault of the tunnel. Since during the measuring process there was no traffic in tunnel, the standing points could be placed away from the tunnel wall. Thus the result of these actions was that the influence of lateral refraction on measurements of horizontal directions in the tunnel was insignificant. At the last control test, which was performed only 1 m prior the breakthrough, the following standard deviations were obtained for the points that were located on the foreheads of the tunnel (Table 1): Table 1 Standard deviation of the points before the breakthrough T n s y [mm] s x [mm] s z [mm] Point 317 was located 22 m from the northern portal, while the point 52 was situated 19 m from the south portal (Džapo 21). Breakthrough of the tunnel was successfully performed on 21 st January 29 (URL 3), to the satisfaction of all involved in its construction. 4 CONCLUSION The results of measurements of temperature in the tunnel St. Ilija were similar to the results of temperature measurements in the deep pits of north Velebit, suggesting that the Croatian karsts regions geothermal gradient is not high, as is the case for example in the Alps, where temperatures of tunnels rocks are known to exceed over 5 C. In Croatian karsts areas these temperatures are significantly lower. Stabilization of the points on the tunnel s sealing proved to be more efficient method that allows surveyors greater flexibility in their work. Geodetic measurements in the tunnel are much easier and more accurate if they are performed when there is no traffic in the tunnel. REFERENCES DŽAPO, M. - REDOVNIKOVI, L. - ALIŠI, I. (21): Control of the general control network of the tunnel St. Ilija Biokovo, Zagreb. NADILO, B. (29) Tunnel St. Ilija through Biokovo, Gra evinar, Croatia, ,

6 294 INGEO 211 REDOVNIKOVI, L. (211): Specificity of geodetic surveying in tunnel construction with special reference to lateral refraction influence, Doctoral thesis, University in Zagreb, Faculty of Geodesy, Zagreb. URL 1: Geothermal Energy ( ). URL 2: 9. Croatian speleologists meeting ''Kamanje 28'' Fskup.speleo-karlovac.hr%2Fimages%2FSkup%252Kamanje%25228%252- %252knjiga%252sazetaka.pdf&rct=j&q=speleo%2velebita%2gradijent&ei=y8e- TbSgG8nOsgbBxZzvBQ&usg=AFQjCNFOMUpiYUzcskfyBgUwhZg5pbP2vg&cad=rja ( ). URL 3: Dubrova ki vjesnik, ( ).