Is Basal Reinforcement Essential for Long and Narrow Subway Excavation Bottoming Out in Shanghai Soft Clay?

Abstract

Because of its great length, one subway terminal excavation in Shanghai, China, was divided into two pits for sequential construction. Although it featured an adequate earth-supporting system and the field construction followed proper procedure, wall and ground displacements of the first excavated pit were up to 1.8–2.7 times local specifications. Then, basal strata of the second pit were jet grouted. Unfortunately, worse excavation performance was tracked. Technical review of design plans, construction logs, and field data, and postconstruction investigations disclosed that for a long and narrow deep excavation bottoming out in soft clays, reinforcing basal strata prior to excavation is essential and attention should be paid to the adverse effect arising from great wall exposure length, Le, during excavation. Although both basal treatment and Le are of decisive importance, lack of suitable basal treatment imposed more adverse effect on excavation performance when Le>4He, where He = final excavation depth.