ABSTRACT
The NSS is a 4620-m long tunnel under construction in Rio de Janeiro state, Brazil. The excavation works started in November 2014 and, due to administrative issues, were interrupted in July 2016, when 3200 m had already been excavated in rock mass predominantly Classes I and II, with cover up to 260 m.
In November 2017, 14 months after the interruption of the excavation, a sinkhole occurred on the surface, 70 m exactly above the tunnel gallery. At first, it was believed that the tunnel had collapsed and caused the event at the surface. However, the investigation proved otherwise.
The investigations began immediately, to find technical elements necessary to support the studies in order to determine the eventual relation between the tunnel and the sinkhole. At the site, SPT and rotary borehole surveys were performed, as well as GPR investigation, among many others. At the office, the geological mapping and worksite staff follow-up reports issued by the constructor were analyzed.
This paper aims at showing the process and results of field surveys, the technical analysis of the data produced in the site and the analysis made to determine the cause of the sinkhole.
The conclusion was that the sinkhole is just a doline, formed by infiltration of water into sub soil and thus not related to the tunnel. It also concludes that tunnel did not cause the sinkhole but, instead, a collapse in the tunnel crown could have been caused by the sinkhole.
