Integration of Operations and Underground Construction: Sound ...

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Operational considerations and Neighborhood stakeholder concerns at the PSST Early in the design of the connection design it was made clear to us that disruption of the Sound Transit and King County Metro operations within the PSST had to be held to a minimum. After some reflection on all of the construction activity that could not be avoided within the PSST and the risks this posed to ongoing transit operations the concept of “demising walls” was developed. The “demising walls” are fixed bulkheads fitted out with roller and personnel access doors constructed between 15 and 20 m (50 and 65 feet) from the PSST headwall in order to create a construction exclusion work zone. These bulkheads have been designed to prevent the communication of dust and noise from the zone, control personnel access into the active transit operations area, and maintain the integrity of the existing FLS (Fire Life Safety) ventilation. Installation of the bulkheads could not avoid impacts to the PSST. Relocation of the light rail “bumper posts” reduces the available storage length for light rail vehicles by approximately 60 feet and restricts Sound Transit to two-car travel. However, the advantages of the bulkheads outweighs this temporary inconvenience to operations and Sound Transit will not need a three-car service until the completion of University Link in 2016. Once the bulkheads have been installed they will remain in place until all systems and other finishing works have been carried so that the seamless integration of the U-Link with the PSST can be completed. To address the concerns that site neighbors and other stakeholders would have of further construction being carried out that would disrupt traffic on Pine St. the design team came up with a feasible approach that would ensure that for the most part construction activities would take place within the site boundaries, only stepping outside into the side walk areas for very specific operations such as the angled jet-grouting below Pine St as shown in Figure 3. Figure 3. Jet grouting below Pine St. 4
Figure 4. Plan view showing the adopted final design alternative connection (sections arrows provide some idea of the detailed engineering required to make the design work) Temporary Shaft Support Construction of the temporary access/retrieval shaft for the NB TBM tunnel has been designed to proceed according to the following steps: � A roughly rectangular shaft will be constructed so that the PSST headwall and additional temporary soldier piles will support the shaft excavation from elevation 52 m (170 ft) to the base of the PSST structure. The layout of the piles avoids the electrical duct bank and the overhang of the existing vent structure. � The 22 m (70 ft) shaft will rely on the temporary soldier piles, wales at 2.4 to 3.7 m (8 to 12 ft) level intervals, and timber lagging are anticipated, similar to the successful model used for temporary excavation support of the PSST. � Temporary soldier piles will be installed in order to safely excavate the shaft to the level of the access drift and provide access for removal of the existing soldier piles within the shaft. � Upon completion of the works for the temporary pile removal chamber, the shaft will be excavated to a point where the NB tunnel headwall break-out panel can be removed. � Existing soldier piles that were used as temporary support for the PSST and are within the Temporary Access Shaft will be removed. � Once this work has been completed, all the soldier piles within the tunnel envelope will be cut or extracted after bracing the existing piles in lifts, with removal carried up to 0.6 m (2 ft) above the crown. 5
Page 1 and 2: Integration of Operations and Under
Page 3: � removing the heavily reinforced
Page 7 and 8: Figure 6: Transverse section showin
Page 9: adjacent Pine Street; eliminates SE

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