Consistent deterioration of the Lenihan Dam’s steel pipe outlet structure required construction to replace it with a new outlet pipe at Lexington Reservoir, Santa Clara Valley Water District’s second largest reservoir. Since tunneling occurred directly adjacent to and under the active reservoir, extensive probe hole drilling, spiling, and cut-off grouting was required. Drill Tech excavated through a Franciscan Mélange material ranging from extremely hard rock to crushed clay. A heavy invert slab was required to control the squeezing ground conditions in the clay, and extensive convergence monitoring was performed.

Drill Tech excavated the tunnel and performed initial lining that consisted of W6x25 steel sets spaced on variable centers and variable thickness steel fiber reinforced shotcrete lagging over timber lagging and crown bars. After driving the tunnel, a final cast in place concrete lining was installed and a 54″ diameter outlet pipe was installed in the tunnel. The tunnel was connected to the surface of the lake by a 16′ inside diameter, 50′ deep shaft, that was lined with secant piles and constructed less than 10 feet from the lake’s edge.

There were several logistically challenging aspects of this project that included small work areas, difficult environmental protection regulations, and the potential for high penalty liquidated damages in the event of late pipe service connection. Additionally, crew members had to access the site through public walking and biking trails that remained open during construction. Drill Tech finished this project ahead of schedule and performed the necessary planning and coordination among contractors, stakeholders, and the public for maximum efficiency of space.