Geodetic Basis of the Longest Tunnel in the Republic of Croatia

Abstract

The tunnel “Mala Kapela”, 5760 m long, is the longest tunnel in the Republic of Croatia. This paper gives the presentation and the analysis of the geodetic basis in the tunnel “Mala Kapela”. The tunnel geodetic basis consists of the overhead and underground part. The overhead geodetic basis consists of:1. two geodetic rectangles (one in the vicinity of the northern, and the other in the vicinity of the southern portal)2. a precise traverse connecting positionally these two micro-networks3. a precise levelling figure connecting vertically the micro-networks of the northern and southern portals.The underground geodetic basis consists of:1. two networks in the right tunnel tube in the form of a triangle chain (one of them at the northern, and the other at the southern tunnel portal)2. two traverses in the left tunnel tube (one of them at the northern, and the other at the southern part of the tunnel) connecting the underground networks in the right tunnel tube and the overhead geodetic basis at the northern, i.e. the southern portal of the tunnel3. two closed precise levelling figures (one at the northern and the other one at the southern part of the tunnel).This is the first time that an underground geodetic basis in the form of a triangle chain has been applied in the Republic of Croatia. It was chosen because it best meets the set criteria related to the accuracy of cutting the tunnel “Mala Kapela”. The error sources in direction measurement were processed with special attention given to the error of the initial grid bearing and direction orientation. The article gives an outline of formulas used to calculate corrections and reductions of the lengths measured with electro optical distance meter. At the end, there is an illustration of the accuracy of cutting the tunnel “Mala Kapela”. The transverse deviation for the cutting point in the right tunnel tube was sQ = 0.8 cm and the longitudinal deviation sL = 2.1 cm. The transverse deviation for the cutting point in the left tunnel tube was sQ = 1.0 cm and the longitudinal deviation sL = 2.2 cm. The vertical deviation for the cutting point in the right tunnel tube was sH = 1.9 cm, and for the cutting point in the left tunnel tube sH = 2.1 cm.