Post-work inspection for overbreak in tunnels: a case study application to the Alqueva hydroelectric plant

ABSTRACT This work seeks to contribute to the understanding of the intrinsic, geological-geotechnical and operational causes of overbreak in tunnels through the case study of the overbreak of two tunnels during the construction phase of the Alqueva hydroelectric plant in Portugal. The excavations were carried out in lithologies with good geotechnical characteristics, for this reason, overbreak was not expected. To determine if the overbreak was done due to operational negligence, mathematical models were developed to quantify (1) the overbreak by the peripheral divergence involving the specific load of the explosives used in the peripheral zone of the tunnel, (2) the geotechnical quality of the rock mass, and (3) the operational aspects. For the mathematical modelling, it was necessary to characterise the causes of excavation damage zone (EDZ), considering the participation of the developer and the contractor. To analyse the calculations and determine the acceptable limits, the Swiss standard SIA 198, which recommends a maximum tolerable overbreak, was used.