Lake Mead: intake tunnel No. 3 pre-excavation grouting challenges using a high pressure slurry TBM.

The construction is described of the third intake tunnel at Lake Mead, Nevada, USA. The reservoir was formed by the Hoover Dam, which extends across the Colorado River between Nevada and Arizona. A hybrid shielded tunnel boring machine was employed with the capability of excavating in either closed or open mode depending on the hydrogeological conditions of the rock mass. The machine is designed to withstand a maximum hydraulic head pressure of 17 bar and is equipped with drill rigs to perform geological investigations or pre-excavation ground treatment. After about 280 m of excavation along the first curve of the alignment the tunnel boring machine penetration values became lower and lower and the machine stopped advancing. The decision was made to lower the face pressure to zero bar to inspect the cutterhead. It was important to estimate in advance the quantity of water inflow, and a procedure was developed in which the tunnel boring machine was used as a large-scale constant-head permeameter. Three permeability tests were conducted but all were aborted at 10 bar, and it was decided to drill and grout the ground ahead of the tunnel boring machine to reduce the permeability, increase the overall stability and allow personnel to enter the cutterhead. An inspection of the cutterhead was found to be possible using a camera installed on a steel pipe and indicated insignificant wear of the cutter. Subsequent camera inspections showed wear of the cutter and a series of pre-excavation grouting campaigns were carried out to allow for maintenance.
The construction is described of the third intake tunnel at Lake Mead, Nevada, USA. The reservoir was formed by the Hoover Dam, which extends across the Colorado River between Nevada and Arizona. A hybrid shielded tunnel boring machine was employed with the capability of excavating in either closed or open mode depending on the hydrogeological conditions of the rock mass. The machine is designed to withstand a maximum hydraulic head pressure of 17 bar and is equipped with drill rigs to perform geological investigations or pre-excavation ground treatment. After about 280 m of excavation along the first curve of the alignment the tunnel boring machine penetration values became lower and lower and the machine stopped advancing. The decision was made to lower the face pressure to zero bar to inspect the cutterhead. It was important to estimate in advance the quantity of water inflow, and a procedure was developed in which the tunnel boring machine was used as a large-scale constant-head permeameter. Three permeability tests were conducted but all were aborted at 10 bar, and it was decided to drill and grout the ground ahead of the tunnel boring machine to reduce the permeability, increase the overall stability and allow personnel to enter the cutterhead. An inspection of the cutterhead was found to be possible using a camera installed on a steel pipe and indicated insignificant wear of the cutter. Subsequent camera inspections showed wear of the cutter and a series of pre-excavation grouting campaigns were carried out to allow for maintenance.