Field investigation on long-term deformation characteristics of long-and-narrow basements in soft clays

Two long-and-narrow station basements, constructed using zoned and staged ramp excavation sequences, are extensively monitored to study the long-term ground and structure deformations. During the main excavation stages (i.e., soil removal), the lateral wall movements increase rapidly with the excavation depth. However, the rate of increase in the lateral wall movements decreases dramatically during casting of concrete slabs. Meanwhile, the post-excavation induced ground surface settlements increase by up to 29.7%. The long-term wall deflections and soil settlements are significant, but these long-term deformations are suppressed after casting the top concrete slabs. Although the bottom-up construction technique is used for the basement excavation, the measured ground surface settlements are substantially smaller than those associated with the top-down construction technique. This is primarily attributed to the adoption of zoned and staged ramp excavation sequences, which effectively control ground settlements. Unlike wall deflections and ground surface settlements, a decrease is observed in the column and wall heaves during post-excavation. The measured heaves of the interior columns and retaining walls are 0.08%–0.15% H (excavation depth) and 0.04%–0.11% H, respectively. A series of deformation envelopes are proposed to estimate ground and structural deformations due to the long-and-narrow basement excavations.