Tunnel Construction with Pressed-In Sheet Pile Walls Next to High-Rise Residential Buildings

Light rail transit (LRT) systems have been built in cities all over the world in order to cope with increasing population and heavy car traffic in their downtown cores and surrounding areas. Light rail tracks are often sharing the roadways with automobile traffic while some of them are on the dedicated trackage for safer and faster transit. For example, the twin cities of Minneapolis and St. Paul in the state of Minnesota in the U.S. have their Blue and Green LRT lines in operation with the latter line being extended by 23km to a southwestern suburb of Minneapolis. The mostly dedicated extension alignment includes an 875m long cut-and-cover tunnel because the alignment there is just next to a high-rise condominium complex, a cluster of townhouse buildings, and a parallel running railroad track. Sheet pile walls were selected as part of the earth retaining structures needed for this section of the extension project. The distance between the buildings and the sheet pile alignment is a mere 3m at their closest point. In order to minimize negative impacts to these existing structures from sheet pile installation, press-in piling was utilized. The soil conditions were alternating sand and silt layers with the SPT value varying between 23 and above 50, which necessitated the use of auger attachments. It took two press-in piling machines 14 months to install approximately 2,500 pairs of Z-shaped sheet piles with their lengths between 7.6m and 19.8m. After the sheet pile wall installation, excavation between the walls, installation of internal bracing, and construction of a concrete tunnel box followed.