Verification of underground excavation stability during tunnel construction

Summary

Abstract

Design of tunnels in rock mass using the New Austrian Method (NATM), using the results of geotechnical field investigations and rock mass classifications (GSI, RQD, Q), is most often carried out in the early design stage. Verification of underground opening stability using observational methods is carried out by measuring the behaviour of the reinforced rock mass during the tunnel construction. This paper will present the verification of underground excavation stability in the Mali Prolog and Šubir tunnels on the Zagreb-Split-Ploče highway, near the town of Vrgorac, Croatia. During the tunnel excavation, field investigations and observations were carried out to verify the efficiency of the primary tunnel support and the length of the installed rockbolts. During the tunnel construction, investigation drilling at the tunnel face, geophysical seismic measurements, video-prospecting of investigation boreholes, geological mapping of the tunnel face, convergence measurements, and anchor pull-out tests were carried out. The investigations were carried out in the II, III, and IV categories of rock mass (according to the RMR classification), in which excavation by drilling and blasting was envisaged. Multi-layer reinforced shotcrete and rockbolts were used as the primary tunnel support. Based on measurements and observations, it was necessary to confirm the achieved stability of the underground excavation. If the behaviour, or rather the stability of the underground excavation, significantly deviated from the design criteria, the input parameters of the rock mass were corrected, and the analysis was repeated based on test results and back-analyses. Changes in the support system resulted in additional gradual strengthening or, in some cases, reduction of the support elements of the primary support system.