Pore pressure response and ground displacements in Chicago clay during tunneling and over long term

In Evanston, Illinois, four test sections were installed in the path of a tunnel in a deep deposit of Chicago clay ranging in consistency from medium to hard. The tunnel was constructed 12 ft in diameter at 70-ft depth below ground surface, in 2000. The initial tunnel lining relied on ribs and timber lagging at 4-ft spacing and the final tunnel lining was made of cast-in-place reinforced concrete. At the location of test section 1, the ribs were installed at 2-ft spacing.; Two test sections were in the vicinity of rail lines where membranes were installed behind the ribs and lagging to limit drainage from the clay and resulting consolidation, while the other two were outside the membranes. In the four test sections, multi-port and pneumatic piezometers revealed low initial pore pressures in the clay due to drainage toward dolomitic limestone bedrock and into the deep TARP tunnel excavated in the rock about 19 years previously. The lowest pore pressures were in two test sections closest to the TARP tunnel.; Ground movements and pore pressures were measured ahead of the shield, during shield advance, and long after shield passage at the test sections (up to a period of about 540 days), using a series of geotechnical instruments installed at various distances from the tunnel. Tunnel advance induced immediate ground movements and pore pressure response typical of undrained clay behavior.; Ground deformations around the tunnel developed in two stages. The short term involved (a) partial dissipation of excess pore pressure induced by total stress changes due to tunneling, (b) immediate ground loss into tunnel, and (c) creation of disturbed zone around shield with pore pressure dissipation and consolidation. The long term was controlled mainly by (a) dissipation of excavation-related excess pore pressure and (b) pore pressure reduction due to drainage into tunnel.; The use of membrane influenced ground deformations around the tunnel mostly in the long term stage where it successfully (a) reduced dissipation rate of excavation-related excess pore pressure and thereby the rate of consolidation and (b) decreased both rate and magnitude of consolidation due to drainage into the tunnel.