Author 1: Joseph A. Sopko, Ph.D., P.E. Author 2: Behzad Khorshidi, Ph.D Author 3: Adam Curry, P.E ABSTRACT: The 15 km long Southeast Collector (SeC) Gravity Trunk Sewer will augment the existing York Durham Sewage System (YDSS) accommodating the anticipated population growth in the York Region in Great Toronto Area, Canada. The project includes the construction of three shafts for diversion facilities, 13 permanent access and maintenance shafts and two drop structures. The sewer tunnel will be excavated by four Earth Pressure Balance Machines (EPBM). In advance of the Tunnel Boring Machine (TBM) drives to utilize launch, inspection and retrieval of the TBMs, 16 shafts and three strutted excavations will be constructed along the sewer alignment. The overburden soils consist of preconsolidated glacial till and glaciofluvial gravel, sand, silt and clay deposits overlaying Ordovician-age bedrock. The shafts are excavated in overburden soil material to a maximum depth of 51 m below the surface. The bedrock formation, consisting of shale, is utilized by several deeper shafts for groundwater cutoff. The diameters of the shoring system range from 4.6 m to 15.6 m. Active dewatering is generally not acceptable for the project to avoid influencing the groundwater table. For the temporary support of the shaft excavations, secant pile walls, diaphragm walls, shotcrete lining, ground freezing and combinations are used. Unexpected difficulties during one shaft excavation required changing the approach completely and converting to ground freezing. The previously installed shotcrete lining, was not designed for the forces associated with the freezing process. Frost pressure on the shoring of the shafts and existing sewer and manhole, as well as mitigation methods are addressed. The behavior of the frozen ground was evaluated using both thermal and structural modeling programs. In-situ stress and deformation was measured to confirm the results of the modeling program and to gain insight to the interaction of the frozen earth and the temporary shaft lining for use on subsequent shafts on the project.