Deep Excavation of the Gate of the Orient in Suzhou Stiff Clay: Composite Earth-Retaining Systems and Dewatering Plans

From 2005 to 2010, a large excavation (approximately 26,000 m2 in plane) was conducted in stiff clay deposits for the Gate of the Orient building in Suzhou, China. To reduce project cost and shorten construction duration, composite earth-retaining systems were designed for this excavation, i.e., removal of the uppermost 7.85 m of soil was conducted mostly by the sloped open-cut method, leaving one side supported by soil nailing wall (SNW); subsequent excavation to a depth of 21.5 m was retained by multipropped continuous bored pile (CBP) wall; final excavation of two inner pits to 30.0–30.7 m deep was supported by a SNW along with jet-grouting of basal soils. In spite of this, lateral wall displacements in this case were comparable to those of multipropped excavations in Suzhou. Because of a high phreatic level and confined artesian water, composite dewatering plans were adopted. Because the waterproof curtain did not extend deeply into the underlying confined aquifer, for the sake of saving cost, discharging artesian water inside the pit incurred a dramatic drawdown in the artesian level outside the pit, accompanied by significant ground settlements. It turned out that the methods in the literature for predicting excavation-induced ground settlements were not applicable to this case any more. For a large excavation not following zoned-construction procedure, corner-stiffening behavior was significant. In this case, deformations near the pit middle span were up to 2.5 times those near corners. Beyond recognition, excavation in stiff clay incurred noticeable basal rebound as well, which was up to one-fourth that in soft clays. Generally, excavations in stiff clay caused smaller lateral wall displacement than excavations in soft clays; their maximum lateral wall deflections mostly occurred above the excavation surface instead of equally above and below the excavation surface in soft clays.