Ground conditions delay Lake Mead9 December 2010
A second inflow of material at the tunnel face has further delayed the start of tunnelling on Intake No. 3 at Lake Mead, in Las Vegas, and may affect its alignment.
Contractor Vegas Tunnel Constructors (VTC), a JV of S.A. Healy and Impregilo and its client, the Southern Nevada Water Authority (SNWA) are working on a contingency plan to deal with geologic instabilities following a second inflow of loose material to the starter tunnel.
It is unknown when the 6.36m diameter Herrenknecht TBM delivered this May will begin boring the 4.83km tunnel. And SNWA confirmed completion has been moved back from 2013 to 2014.
Excavations on the starter tunnel stopped in July when VTC encountered a pocket of loose rock and soil behind the face of the starter tunnel. Following nearly four months of recovery work, the second inflow happened in late October.
VTC was able to install pre-fabricated temporary bulkhead to contain the material where they were working, about 40ft (12m) from the tunnel face, roughly the same place as the first incident.
“This second inflow demonstrated that the geologic feature, an ancient fault in the rock, is still unstable and will require greater care and effort to surpass than previously anticipated,” said Marcus Jensen, director of engineering, SNWA.
“Contingency planning will include the option of bypassing the fault by adjusting the tunnel alignment, if necessary,” he said.
In the meantime, additional precautions of increased grouting and setting up more drainage are being carried out.