Natural Energy Laboratory of HI Authority
Keahole Point, Kona, HI
DEEP SEAWATER & SURFACE SEAWATER PIPELINE SHORE CROSSINGS
The Keahole Point pipeline shore crossing project involved open water construction that included driving two sets of parallel 67″ steel cased tunnels, from a 50 foot deep drive shaft located 15 feet from the Pacific Ocean. These tunnels were unique in that the shaft was located on the beach while two of the tunnels were at approximately 8% downward grade and the other two at a 1% upward grade. The shafts required extensive engineering to withstand both the jacking and uplift forces. A seven-foot thick concrete base was placed for the floor slab and an innovative sealant technique was implemented to minimize water seepage into the shaft.
Nova utilized a microtunneling system specifically designed for this contract. The cutterhead is hydraulically driven by a 170 HP motor yielding 195,000 ft-lbs of torque. Steel sheet piles were driven to allow deep excavations while retaining the soil excavation to a minimum at the microtunnel entry and exit pits. Two of the drives were approximately 420 feet inland, sloping up at a 1% grade, and two of the drives were 575 feet seaward sloping down at an 8 % grade, holing out 80 feet below sea level.
The geotechnical conditions encountered during construction were lava flow formations with a very dense upper layer rock cap over hard basaltic boulders and cobbles, clinker and basaltic sands, coral, coralline deposits including coral gravel and sands.