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Design of a Steel Fibre Reinforced Concrete Segmental Tunnel Lining – City West Cable Tunnel Project, Sydney, Australia

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Author L Drowley and A Kuras
ID P200803061

Description

Segmental tunnel linings are becoming more common in Australia. One current
project employing a segmental lining is the City West Cable Tunnel (CWCT) in
Sydney. The project has been commissioned by Energy Australia and is currently
being constructed by Thiess Pty Ltd. The project includes 1.6 km of tunnel
boring machine (TBM) driven, segmentally lined tunnel. The majority of the CWCT
will be excavated through Hawkesbury Sandstone. The tunnel is required to carry
a 132 kV electrical circuit under Sydney’s central business district (CBD),
between Haymarket and Energy Australia’s City North Substation on Sussex
Street.


The segmental lining has been designed as a 3.2 m internal diameter, 200 mm
thick universal ring with six trapezoidal segments. The design and detailing of
this segmental lining presents some noteworthy challenges, including exposure to
saline groundwater, and the exclusive use of steel fibres for flexural and
tensile concrete reinforcement.


This paper
describes some of the key elements of the segmental lining design. These
include:
the case for choosing segmental linings over other forms of tunnel
construction;
the major design checks required of a segmental lining
including ground loading, hydrostatic loading, joint bearing, joint rotation,
ram loading and the early strength lifting case;
the benefits of using steel
fibres over conventional steel reinforcement;
the design approach used for
steel fibre reinforced concrete;
the application of 3D computer modelling in
the design;
the provision for components including connector dowels and
gaskets;
the design for passive fire resistance; and
the design for durability of steel fibre reinforced
concrete in a saline environment.


Segmental lining design requires the application of specialist design
approaches. Input is required by the construction team, as the methods of
demoulding, handling, transportation and installation impact on the lining
design.


As the segmental lining is functioning as immediate ground support and also a
permanent lining, high standards of manufacture and construction quality control
are required to ensure that the 100-year design life is met. The requirement for
a watertight tunnel means that detailing and installation of the EPDM gaskets is
of importance.