Integration of Operations and Underground Construction: Sound ...

� removing the heavily reinforced concrete tunnel “eyes” without undue disturbance to the ongoing
transit operations
ALTERNATIVE DESIGN
A few alternative approaches were originally considered, in addition to the PE design. One of the initial ideas was to
determine whether any of the TBM breakthrough preparation work could be carried out under the PSST contract,
which was still underway at the beginning of the University Link design period. This preparation work would have
consisted of the removal of the partially exposed soldier piles and part of the anchors that intersect the proposed
tunnel alignment from the surface prior to the restoration of Pine Street, as well as removal of the break-out panels
in the PSST headwall. However, it was quickly ascertained that this idea would be difficult to implement given the
necessity for a very late change to the scope and schedule of the PSST contract, which was near completion at that
time.
The alternative discussed in this paper was developed as part of the Capitol Hill Tunnel contract (U230), and
eliminated the need for any further construction work within Pine Street, minimizing disruption to residences and
businesses in the area. This alternative was used in the final design, and involved the following activities:
� Ground treatment to facilitate tieback removal through the TBM cutterhead for the SB tunnel, and
stabilizing TBM break-ins for both tunnels.
� Installation of “demising wall” bulkheads within the PSST to facilitate removal of the NB and SB breakout
panels and installation of utility connections and light rail within an agreed length of the PSST
� Temporary access/retrieval shaft construction for the NB tunnel only, taking advantage of the PSST
headwall and the existing Controlled Density Fill (CDF) backfill on two of the four shaft sides.
� Access drift from the temporary shaft to a temporary chamber, constructed within the safety of the CDF
located between the existing soldier pile wall and the PSST headwall.
� Removal of soldier piles from the temporary chamber and replacement with CDF backfill.
� In-tunnel disassembly of the SB tunnel TBM.
� Removal of soldier piles from NB retrieval shaft.
Figure 3 shows a general layout of the alternative used in the final design.
Ground Treatment
Due to the presence of both recent alluvium deposits and landslide deposits below the groundwater and overlying
the over-consolidated glacial soils, a limited ground treatment zone was determined to be required for both tunnels.
The ground treatment zones, as shown in Figure 3, vary for each tunnel. For the NB tunnel, the zone is large enough
to provide a stable face to allow for bottom removal of the east soldier piles, which are used for support of the
retrieval shaft and later be removed from the path of the TBM. For the SB tunnel, the zone also needed to provide a
stable face for the east soldier pile removal, but also provide stability for the tunnel heading to allow removal of
tiebacks from within the face of the TBM, to be carried out under atmospheric pressure. The SB tunnel geometry
was also dictated by an existing sewer that needs to stay in operation throughout the tunnel construction phase.
Ground treatment to stabilize the tunnel crown and improve the soil standup time was designed as jet grouting
because of the high silt content of the in situ soils, and used to create a consolidated block of material in the zone of
landslide debris between the alluvium and over-consolidated glacial soils. This work has been planned to be carried
out from the Sound Transit staging site shown in Figure 1 next to Pine Street and extend at an angle below the street
to prevent further surface disruption and minimize any potential traffic impacts.
3