Integration of Operations and Underground Construction: Sound ...

� The shaft will then be partially backfilled in lifts corresponding to the pile removal sequence above, with
CDF material to allow for the NB tunnel TBM to mine into the shaft.
Figure 5 shows an isometric view of the shaft.
Figure 5. View of shaft showing existing and additional support elements
Access Drift and Pile Removal Chamber
In order to avoid surface disruption to Pine Street a 3 × 3 m (10 × 10 ft) access has been designed to be driven from
the shaft above tunnel axis level within the CDF material between the NB and SB tunnels. This access drift will take
advantage of the existing PSST soldier piles on the east side for support. The drift excavation and chamber top
bench will be supported by partial steel sets, with lagging or shotcrete to ensure ground stability, placed in line with
the existing piles. At this stage it will be possible for the upper part of the SB tunnel break-out panel to be exposed
and removal of the concrete will begin. Subsequent benches will be excavated from the top down, exposing the
entire break-out panel for removal and the complete length of piles and laggings to be removed from the tunnel
envelope.
Beginning from the bottom bench, laggings, piles, and 0.9 to 1.2 m (3 to 4 ft) of existing tieback will be removed
after stabilizing the soldier piles. The lower portions of the pile removal chamber are expected to be in the overconsolidated
Qpgm and Qpgl materials, which are stiff to very stiff clays. The upper portion of the chamber will be
within the zone of ground treated soils, which should not become unstable during the short period that they are left
unsupported. Figure 6 illustrates a section through the fully developed access drift and pile removal chamber which
is larger than required to accommodate the tunnel envelope (SB tunnel profile shown) because of the presence of
tieback anchor points that connect with tiebacks intersecting the tunnel horizon, as well as to allow waterproofing,
mechanical, electrical, and systems connections to the existing PSST structure. To facilitate TBM excavation, the
tiebacks will be disconnected from their associated piles within the pile removal chamber.
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