Drill Blast Excavation to Within 1m Existing Steel Lined Penstock Tunnel

Abstract

Phase 1 of the Tie-In Works of a tunnel twinning project in northern British Columbia required drill and blast excavation of access tunnels leading to interconnection tie-in chambers that were excavated in hard rock to within 3m of the existing pressurized steel lined penstock.  The eventual geometrically complex removal of the pillar of rock and backfill to allow the connection of steel wye's at 96-inch diameter by a combination of drill and blast and mechanical excavation was subsequently undertaken during a limited shutdown period when the penstocks were depressurized.

This paper focuses on the methodology used to determine and review drill and blast constraints employed to protect the existing penstocks from blast induced deformation and the measures taken to maintain the stability of the temporary rock pillar for a challenging excavation geometry.  Blast monitoring and geotechnical instrumentation results collected during construction are reviewed against the original design and the conceivable benefits of onsite construction monitoring that led to the successful completion in 2013 of the tie-in excavation to the existing penstock tunnels are discussed.