LAKE PLEASANT PIPELINE TUNNEL

Phoenix, Arizona

The project is part of an eight mile long, 76-in. inside diameter water transmission pipeline from a new water treatment plant in northwestern metropolitan Phoenix . The tunnel was required to minimize or eliminate construction disturbance to an existing City of Phoenix Mountain Preserve Park with sensitive desert vegetation. The tunnel alignment is 1,700-ft long in variably weathered granitic bedrock and is designed to parallel the existing Carefree Highway and the northern boundary of the Mountain Preserve. The alignment passes below Skunk Creek and a small knoll comprising the northern portion of the Mountain Park . The west shaft is 60-ft deep constructed through dense alluvial soils and completely weathered bedrock and the east portal approximately 40-ft deep constructed through weathered rock. Tunnel excavation used drill and blast and roadheader methods, placement of temporary excavation support followed by pipe placement and annular backfill using cellular concrete. Subsurface explorations were performed using portable drill rigs, carried by hand, to each drill hole location to minimize disturbance to the Preserve. Tunnel design included provisions for noise mitigation and portal/shaft were located outside the Park boundary to minimize disturbance.

Lyman Henn provided oversight of subsurface investigations performed by a local geotechnical engineering firm, prepared full contract documents for design of the tunnel and provided designer coordination and field inspection services during tunnel construction. The tunnel design was done under a very aggressive schedule to accommodate the overall project schedule requirements for pipeline completion. Construction services included full-time resident inspection of tunnel excavation, support pipe installation and backfill grouting construction activities.