University Link Project Cross Passage 17:Collaborative Approach Is the Key to Success

摘要

Transit tunnels require cross passages between adjacent main tunnels to provide a safe meansrnof egress or a safe haven in case of emergency. Cross passages are generally excavated under free air usingrnmechanical (sometimes manual) means and present one of the major technical challenges of transit tunnelrnprojects.rnCross Passage 17 is one of 16 cross passages constructed under Contract U220 of Central Puget SoundrnRegional Transit Authority’s University Link Project in Seattle, WA. The cross passage was moved 35 feetrnsouth from its design location after review of data developed during TBM inspection stops indicated groundrnconditions in the originally planned location were poorly suited for excavation. Pre-excavation probe holesrndrilled at the revised location showed no signs for concern, allowing excavation to proceed using the SequentialrnExcavation Method beginning with excavation of the top heading. After completion of the top heading, groundrnwater seepage into the excavation steadily increased reaching a maximum flow rate of more than 200 gallonsrnper minute, along with about 20 to 40 cubic yards of sand adding to the difficulties.rnThis paper presents the cooperative process undertaken by the owner, designer, construction managerrnand contractor which resulted in development of the path forward to successfully complete the cross passagernexcavation, install the final lining and ensure long term stability of the area. Issues and challenges that werernencountered during development and execution of the cross passage work are identified along with steps takenrnto mitigate them.