Geotechnical Engineering Aspects of Deep Microtunneled Pipeline Crossings of Honolulu Harbor in Very Soft Sediments

This paper summarizes the design phase geotechnical evaluations performed to develop the bidding and construction requirements pertaining to microtunneling and deep shafts for a recently completed harbor entrance channel pipeline crossing in Honolulu. The dual 7,500 linear feet (LF) 63-inch I.D. sewer force mains were installed inside 81-inch diameter jacked steel casings by microtunneling method. To cross under the only entrance to Honolulu Harbor, 1,580 LF of the sewer and jacked steel casing was installed at -85 feet below mean sea level (msl) through very soft to very loose sediments and coralline formations, and the microtunneling machine egressed and ingressed through 90-foot deep jacking and receiving shafts sunk through very loose soils to very hard basalt using the vertical shaft sinking method. From the deep shafts the pipeline profiles required microtunneling to be performed on slopes of 4.28% (1,450 LF) and 11.86% (590 LF) to shallower, approximately 25 feet deep shafts. This paper summarized the geotechnical evaluations performed to guide the harbor crossing pipelines design, bidding and construction.