CONSIDERATIONS FOR VERY SHALLOW CONVENTIONAL (SEM) TUNNELING IN URBAN SETTINGS

摘要

Construction of shallow mined tunnels is frequently undertaken by use of conventionalrntunneling known as the Sequential Excavation Method (SEM) and sometimesrncalled NATM. In soft ground at shallow cover the method relies on the use of presupportrnmethods and ground improvement to satisfy permissible deformations of affectedrnfacilities. A recent project involved tunneling for twin single-track Metrorail guidewayrntunnels at Tysons Corner, Virginia at depths as shallow as 7–8 feet overburden overrnthe crown. The paper addresses the range of considerations, including ground conditions,rninsurer’s requirements, utilities, and existing structures, that prompted designrnand construction refinements. These led to a robust design and successful executionrnnowadays called for in urban environments.