Experimental estimation of the fallout size and reinforcement design of a tunnel under excavation

A number of cylindrical fallouts were observed in a tunnel under construction and temporary grouting as well as immediate backfilling was performed in order to control cavity expansion. The exact dimension of the largest fallout, i.e., height, cannot be measured due to limited access to the site and, therefore, experimental as well as empirical approaches were employed to estimate the fallout size. Additional boreholes were drilled immediately after the installation of permanent reinforcement and the accuracy of the proposed Tunnel Electromagnetic Prospecting System (TEPS), a non-destructive experimental testing method used to estimate the fallout volume, is verified with borehole core data. It is found that the empirical approach based on a bulking factor is easy to apply if a reference value is available. It is also proven that the TEPS method, which is fairly simple and inexpensive during the test process, gives satisfactory results when compared with borehole data. Finally, an effective reinforcement design is considered and the safety of the reinforced fallout area is evaluated through numerical analysis. Future work will be concentrated on the refinement of the measured data from TEPS and the reinforcement effect of the auxiliary techniques utilized in the fallout area.