Microtunneling Challenges: Crossing Under Major Railroad and Highways in Very Soft Glacial Soils—The Evolution of a Ground Treatment Assessment Process

摘要

Bergen County Utilities Authority (BCUA), New Jersey has initiated an ambitious programto invest US$65 million to carryout a significant CSO improvement program to meet a State DEP consentorder by 2010. The New Overpeck Valley Parallel Relief Sewer Project is part of the overall scheme toprovide additional capacity and redundancy for sewage conveyance from the BCUA service areas to theBCUA Wastewater Treatment Plant (WTTP). A major portion of this CSO improvement program utilizedmicrotunneling techniques to complete 1.2 miles (1.8 km) of new sewer. A Herrenknecht AVND1800ABmicrotunnel boring machine (MTBM) was used to install 72 in (1800 mm) diameter reinforced concrete pipe(RCP). This machine was equipped with two articulation joints and was selected by the Contractor due to itsincreased ability to steer to maintain line and grade. The microtunnel alignment included drives through 2,788ft (850 m) of extremely soft glaciolacustrine varved clay, transitioning to Deltaic deposits of loose to mediumdense mixed soils containing silty sands, gravels, and boulders. Included within these soft ground drives werecrossings beneath the New Jersey Turn Pike and the 12 track CSX Railroad Intermodal Yard and a branchmainline. During construction, the Contractor proposed a significant jet grouting program within the sensitivevarved clays to increase the bearing capacity of the soft material required to support the weight of their MTBMand trailing equipment. A thorough bearing capacity analysis and advanced geotechnical investigation programwas developed and used to eliminate the need for extensive jet grouting in the soft soils along the entirealignment. Recommendations were also provided during construction to change the Contractor’s microtunnelcutting head to reduce the weight of the MTBM at the front of the machine and improve the overall factor ofsafety for bearing capacity for the proposed equipment. Challenging conditions were also encountered whenmicrotunneling under US Highway Route 46. These challenges included excavation within soft varved clayswith also the potential to encounter boulder(s) within rock fill. This paper discusses the extremely challengingground conditions encountered along the alignment and presents findings of the detailed ground treatmentassessment process used on this project which can be applied to other similar soft soil installations. This paperalso discusses as a case history the significant engineering challenges related to all drives that include groundcharacterization, key MTBM features and performance, and ground monitoring results.