Very often, the speed of support installation affects the progress of shielded soft ground tunneling operations.  Consequently, the tunneling industry has seen a continual improvement in the development of segmental lining systems from brick to cast iron and steel to the precast concrete systems that are in common use today.  These developments, combined with advances in tunnel boring machines (TBM) such as the development of Earth Pressure Balance (EPB) and slurry tunnel boring machines, have made the safe construction of tunnels in soft ground beneath the water table both technically and economically feasible.

In response to the high capital costs for such tunnel boring machines and ever increasing labor costs, the trend for improvements in segment quality and tunneling productivity continues. Advancements have been made in materials (gasket, concrete, reinforcing, and inserts), geometric design (longer rings, taper systems, segment shape), erection sequence, construction tolerances, analytical capability (geomechanical software) and overall segment quality.
 
Traylor Shea Ghazi Precast (TSGP) set up a dedicated manufacturing plant in Littlerock, CA, to produce nearly 40km (25 miles) of precast concrete segmental tunnel liners in the L.A. area including the ECIS (18,126m or 59,470´) and NEIS (8,421m or 27,630´) tunnel projects.



For more information on precast tunnel segment production, view this article: 
http://www.hatchmott.com/documents/adobe/Segment%20Design_RETC%202003-from%20CD_Kramer%20et%20al-L.pdf