Integration of Operations and Underground Construction: Sound ...

Figure 6: Transverse section showing access drift and SB pile removal chamber
Tieback Removal through the TBM
During construction of the PSST headwall structure, the temporary excavation support soldier pile wall running
northwest was supported by a tieback anchoring system. The tieback system was arranged in five rows at intervals
of 3.4 to 3.7 m (10 to 12 ft), which intersect the proposed SB tunnel envelope as shown in Figure 6. The tiebacks
consist of steel cables anchored over a minimum 4.6 m (15 ft) length at the cable terminus, and intersect the SB
tunnels to varying degrees. The TBM is likely to encounter tiebacks over a 13.7 m (45 ft) long interval, starting
approximately 16.8 m (55 ft) before the PSST headwall. In accordance with the specification an earth pressure
balance (EPB) TBM will excavate in closed mode (pressurized face) up to this position and then convert to open
mode (non-pressurized face) while excavating under the cover of the jet-grouted tieback zone. Following each of the
7 to 8 ring excavation sequences required to mine through the tieback zone, interventions are to be carried out as
necessary to cut the cables engaged by the cutterhead or exposed in the face. This is anticipated to ensure that at no
time there will be more than 1.5 m (5 ft) of cable exposed which could become entangled in the TBM cutterhead.
Stability of the crown during these interventions will be provided by the ground treatment zone. Figure 7 shows a
perspective view of the intersection of tiebacks with the SB tunnel envelope.
Figure 7: Perspective of tiebacks intersecting the SB tunnel envelope (ground treatment not shown for clarity)
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