DESIGN ISSUES FOR CONSTRUCTION OF A REROUTED MIS THROUGH BOULDERY, GASOLINE CONTAMINATED GROUND

Abandonment of old, deteriorating metropolitan interceptor sewer (MIS) segments and improvement of flow between a large combined sewer and the MIS system required construction of a new, 400 m (1300 feet) long, 2.2 m (84-inch) diameter sewer through treacherous ground conditions beneath a city street. Limited alignment alternatives and existing sewer grades required that the new sewer be constructed at depths ranging from 15 to 21 m (50 to 70 feet) through glacial soils containing dense bouldery sand and gravel (outwash). Located along the alignment was a former service station that had leaking underground storage tanks (LUST). Free-product gasoline was present approximately 5 m (16 feet) above the tunnel zone and was in the process of being remediated. In order to minimize risk of toxic and explosive gasoline compounds from migrating into the tunnel and from spreading beyond the pre-construction contamination plume, the specifications prohibited dewatering. The dewatering restriction made boulder removal more complicated and required the use of special tunnel boring machine requirements for isolating groundwater and for boulder splitting and removal.