Microtunneling in the Pacific Ocean

The design and construction of a jacking shaft, ground improvement, and microtunneling in the ocean presented unique challenges due to the general lack of precedents. The need to construct a new 3.9-km long ocean outfall beneath coral reefs and environmentally sensitive ocean floor was successfully resolved by specifying over water shaft construction and underwater microtunneling along 800 m of the new outfall alignment. Geotechnical site characterization included over water drilling and sampling, and marine geophysical surveys. The selected microtunneling alignment avoided thick deposits (over 50 m) of very loose, liquefiable sands. Two approximately 400-m long microtunnel drives, one jacking shaft, and two under water retrieval pits were specified. The microtunneling jacking shaft, related jet grouting, and underwater retrieval pits were constructed in open ocean waters over 6 m deep, with the microtunneling shaft bottom at 12 m below sea level. The geotechnical and trenchless engineering design approach, overwater shaft construction and performance characteristics, and microtunneling performance in very loose sands to hard coral limestone will be discussed.