Control of Seepage Pressures beneath Cut-and-Cover Excavation in Varved Glacial Silt: Fid-Bau Portal

Control of Seepage Pressures beneath Cut-and-Cover Excavation in Varved Glacial Silt

Ho, Chu E.

This paper presents the design and implementation of groundwater cutoff and dewatering scheme for cut-and-cover construction of an underground subway station in Manhattan, New York. The excavation was approximately 60 ft wide and 1,800 ft long with depths ranging from 60 to 70 ft below street level. The invert of the station rested partially on rock at one end but mostly on weak glacial deposits for the remaining portion of the station and a transition tunnel. Groundwater cutoff was achieved using secant pile walls and diaphragm walls. A dewatering system consisting of a combination of shallow sumps and pressure relief ejector wells was adopted. Hydraulic properties of the soils were evaluated from particle size distribution, in situ falling head tests, and a full-scale pumping test. Field pumping volumes were recorded using flowmeters at discharge points to ensure pumping capacity was adequate at all times. Piezometric head within the excavation was observed using selected ejector wells as open standpipe piezometers. Hydraulic pressure build-up beneath the invert slabs were closely monitored as the pressure system was progressively deactivated to ensure the partially completed underground structure was stable against uplift water pressures. The varved character of the glacial deposits resulted in varying residual pressures acting at the invert base after the pressure relief system was de-activated.