TECHNICAL | MINED TUNNELING
UNDERPINNING OF A SUBWAY AIDED BY MINED GALLERIES
Mined galleries were excavated below groundwater level to help underpin an operational subway box structure during construction of the new Eglinton Crosstown LRT in Toronto. The novel solution is discussed by Dr Angelos Gakis and Thomas Schwind of Dr Sauer & Partners, in Salzburg, and Elmar Feil and Gerald Skalla of Dr G Sauer & Partners, Canada
INTRODUCTION
Cedarvale station was built as part of the new Eglinton Crosstown LRT in Toronto, at the intersection with the Yonge-University (Spadina) line under the existing Toronto Transit Commission (TTC) boxed tunnel of the Eglinton West Station. The new station, an open-cut excavation, required an underpinning of the operational TTC box to ensure an unhindered service. A novel solution was implemented to provide access
to install a series of ‘needle beams’, utilizing six small, mined galleries. The galleries were supported by a temporary shotcrete lining on their sides and a concrete slab at their invert. Rebar dowels were placed at the connection of the shotcrete lining to the overlying TTC box base slab. Additionally, temporary support was provided during the excavation of the galleries by a series of steel props, supported on precast concrete pads, mechanically connected to the invert slabs. All the excavations extended below the groundwater
level. Dewatering was undertaken prior to the excavation of the galleries to lower the groundwater level to below
their invert level and to control groundwater ingress and surface water flow during excavation. The TTC box and the new Cedarvale station as well
as the main underpinning elements are shown in Figure 1. The multidisciplinary design team on the works
consisted of Dr. G. Sauer & Partners (mined elements), Entuitive (structural), and Isherwood (support of excavation). The prime contractor of Cedarvale station was Crosslinx Transit Solutions Constructors (CTSC), a construction consortium of SNC Lavalin Inc., ACS- Dragados, Aecon Infrastructure, and EllisDon Civil. Sub-contractor for the mining works was HC Matcon Inc. (HCM).
EXISTING STATION The existing TTC box is formed of reinforced concrete units, 12.2m-long, 10m-wide and 5.7m-high. The top and bottom slabs and the external walls are approximately 600mm-thick and the inner wall is approximately 460mm-thick.
TTC box joint Unit 1604 Unit 1608
Existing TTC spadina box
Unit 1612 T/R Shotcrete Needle beams T/R
Lowered water table
Underpinning caissons
Underpinning girders
Above, figure 1: Existing TTC box, new station, and main underpinning elements 12 | Winter 2023
SOUTH SIDE
NORTH SIDE
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