Special segments design for cross passages and shaft passages
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Published:2021
Xavier Torelló, MSc Civil Engineering, Dominik Hoerrle, MSc Civil Engineering, Helena Castellvi, MSc Civil Engineering, George Doulkas, MSc, Civil Engineering, Tomasz Rogala, MSc, MICE IEng, 2021. "Special segments design for cross passages and shaft passages", High Speed Two (HS2): Infrastructure Design and Construction (Volume 1)
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Abstract
Operational considerations for the High Speed Two (HS2) TBM twin tunnels require cross passage connections and ventilation openings that are critical for not only the safe operation but also for the construction process. Conventionally, temporary support is used to ensure the stability of these openings in the tunnel lining. The use of special segments reduces the negative impact on the construction programme and TBM logistics.
This paper focuses on the design of Northolt Tunnel West (NTW) which is the 8.8m internal diameter twin bore tunnel located between the 15,600 and 23,400 kilometre section of HS2 Phase One.
The special segments are geometrically equal to the standard segments but are more heavily reinforced and contain embedded structural frames and tension rods to transfer the load to the adjacent rings and to avoid stepping between segments.
In the NTW, the design of the special segments has been achieved with the following procedure:
Ground load derivation: Accurate estimation of the loads acting on the segments around the opening using Plaxis® 3D, considering the ground properties, the construction stages and the most unfavourable dewatering hypothesis.
Member forces and stepping between segments calculation: Accurate structural analysis carried out using SAP2000® considering the derived ground loads, non-linear links between rings acting under compression only and a remaining residual propulsion force.
Study of the construction compatibility with TBM advance, with low interference in the TBM advance ratio and feasibility to be executed from the TBM.
Design based on accurate tolerances of the embedded steel frames inside the special segments compatible with the construction sequence.
This design procedure allows, although with a higher design effort, a more effective construction process with no additional temporary structures required around the openings to prop the tunnel lining and little interference in the TBM advancement ratio.
