The Anacostia river tunnel is one of a series of new CSO tunnels designed to capture sewage overflows that currently feed into Washington D.C.'s rivers and divert them to the blue plains advanced wastewater treatment plant. The TBM assembly and main work shaft for the tunnel consists of twin 21.3-m (70-ft) diameter shafts connected by a 30.5-m (100-ft) long tunnel mined by the sequential excavation method (SEM). During the hand mining of the inter-shaft connector tunnel, a ground loss event occurred which resulted in approximately 1900 m~3 (2,500 yd~3 ) of material flowing into the tunnel and formation of a sinkhole which encompassed the area between the two shafts. The immediate response included flooding the shaft and tunnel, backfilling of the sinkhole, and an array of exploratory holes to map the limits of disturbance. Jet grouting was selected as the ground improvement method to restore ground conditions in the crown of the SEM tunnel so that mining could resume. Several other ground improvement methods were required to support the jet grouting work, including non-cementitious grout for void filling, compaction grouting to create an interim bulkhead, and jet grouting to create a contiguous improved ground structure and provide a stable face for the subsequent recommencement of tunneling. After six months of remedial work, the connector tunnel was completed and the TBM was launched. This paper describes the design steps and implementation of this complex, challenging and time-sensitive remediation effort.
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