Geohazard mitigation with blasting to remove a feature above the Homer tunnel on SH94 in the magnificent Fiordland region
Geovert assessed the feature with NZTA engineer’s Opus, and determined the rock mass was failing in a combination sliding
rotational toppling mechanism with less than 10% of the toe area
providing the only point of attachment.
As a result of this inspection the road was closed to all traffic until the drill and blast removal of a 25m high 2000 tonne unstable rock mass was removed. This was located 900m vertical above the western portal of the Homer Tunnel at 1800m ASL and the contract period began in May which meant dealing with severe alpine weather conditions.
The rock mass was deemed highly unstable and a drill and blast programme was implemented under urgency to remove it. A series of extensometer points were installed to monitor for any movement during drilling and for post blast inspection. The blast design was engineered to maximise fragmentation and reduce damage
to existing highway furniture including the newly completed
temporary portal extension structure. Drilling was completed
for 23 holes, with 300kg of high velocity explosives used. Post blast
scaling and inspection was completed to allow the road to reopen. Several storm events resulted in 200km/h winds, minus 18degtemperatures, numerous snow events with over 2m falling duringthe contract period and over 500ml of rain.