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.