Park Road Over/Under Tunnels: Problems and Solutions

摘要

The Park Road Tunnels is a 3220-foot connector between two stations in an over/under configuration down a narrow public right-of-way. An over/under configuration was chosen to avoid the necessity of taking a church and 15 residences. Other design problems included known hydrocarbon contamination and squeezing a double deck fan shaft and emergency exit into a small footprint. This project is one of five that comprise the Mid-City E Route segment of the Washington Metropolitan Area Transit Authority's (WMATA) effort to fast track construction of the remaining 13.5 miles of the systemGeology for this project is soft ground with the upper tunnel predominantly in sands and with the lower tunnel in sands, clays, and decomposed rock. Ground modification was designed and split between an observational dewatering system and chemical grout injections. Also, an open face digger shield was specified for both tunnels.As construction progressed, it became apparent that the designed observational dewatering system had failed. Tunneling commenced on the lower tunnel as specified and ran into a flowing ground condition almost immediately. Although the contract provided for partnering, the contractor and WMATA had developed a deep sense of mistrust in each other. The problem was referred to a Disputes Review Board who found a offering site condition existed but that neither WMATA ncr the section designer should be held at fault. An independent consultant, retained by WMATA also found that neither WMATA nor the section designer could have reasonablely anticipated the ground behavior during the design process.WMATA, working with consultants and the contractor, agreed on a stepped methodology to overcome the differing site condition; first by a hybrid system of chemical grout injections and additional dewatering wells; and second by abandoning most of the dewatering effort and concentrating on chemicalgrout injections. Tunneling is complete and final liner concreting is underway. The cost in time is approximately 14 months and the cost in dollars did not quite double the bid price of the contract. From WMATA's point of view, the most important part is that the problem was solved and the segment will be complete on time and within budget.