Microtunneling through a Very Soft Soil Condition-The Contractor's Perspective of a Case History 10 Years Later

摘要

Waikiki Public Bath Force Main Replacement Project consists of 975m of 400mm force main that carries the wastewater from the Public Bath Pump Station to a gravity sewer on the Ala Wai Blvd. This force main transports wastewater underneath Kalakaua Ave upon which some of the most expensive real state in the world are located. This area is the heart of famous Waikiki beach and it is full of beautiful beaches, resorts, five star hotels, shopping, etc. The soil strata in this area can be briefly described as 2.50 to 3.00m deep of beach sand on top of half a meter to one meter of very hard coral ledge on top of very soft layer of silty sand. The N value in this silty sand layer ranges from zero to 8. The original design of the force main was in the beach sand layer, but an old buried wooden bridge was discovered during the preparation for construction that mandated lowering the elevation to be through and underneath the ledge layer.The specified construction method for this project was microtunneling of 650 mm Permalok steel pipe with a 400mm PVC carrier pipe inside the steel pipe. The annular space between the two pipes was grouted with low density grout. Many problems were encountered during the construction phase of this project. This paper is a retrospective review of the project from the contractor's point of view. It covers the design and construction aspects of this project in addition to the encountered problems and the lessons learned from that project.