In 2001 the New South Wales Roads and Traffic Authority (RTA) tendered the financing, construction and 33 year operation of the US$530M Cross City Tunnel (CCT). The project consists of 7km of driven and cut and cover tunnels designed to remove more than 90,000 vehicles/day from Sydney’s congested city centre, providing travel reductions of up to 20 minutes by bypassing 18 sets of traffic lights.
The Cross City Motorway (CCM) consortium was declared the successful bidder (T&TI April 2002, p9) with the agreement finally signed in December 2002. The Baulderstone Hornibrook Bilfinger Berger JV (BHBB JV) is the contractor, with CW-DC providing principal civil and structural design for the driven tunnels. Downer Engineering (formerly ABB) is acting as subcontractor for the design and construction of M&E services, while Chubb Fire Safety (formerly FFE) is carrying out fire fighting hydraulic design and construction. Hyder Consulting is the independent verifier. Hassell has provided architectural design services.
Project works
The main project works involve the construction of 7km of road tunnels (Figures 1 and 2), comprising twin dual-carriageway mainline tunnels, four single-carriageway connection tunnels, seven portals, three Y junctions (where road carriageways merge and diverge) and seven reinforced concrete bridge decks (for CCT tunnel crossovers, or provision for future Metrowest rail tunnels). Other underground works include a 2km long ventilation tunnel and three ventilation stations, as well as four substations and water treatment chambers.
Emergency egress facilities include refuge doors every 120m with safe waiting spaces for mobility-impaired persons, 18 cross passages (up to 150m long), and nine stair shafts (including five to the surface).
The surface works include road re-configuration with modified intersections, new transitways and cycle lanes, a 40m wide Landbridge over existing traffic, footbridges, a 60m high ventilation stack, and a Tunnel Control Centre. Urban streetscape improvements are also being undertaken over several kilometres of Sydney’s business district.
The majority of the driven tunnel works are being excavated by seven roadheaders. Where the tunnel approaches the surface, a reinforced concrete structure is being built by cut and cover.
The driven tunnels are predominantly excavated in sandstone of varying quality, with support types ranging from random rock bolting to canopy tubes, spiling bars and steel sets. A thin layer of steel fibre reinforced shotcrete also supplements rockbolt support. Generally the driven tunnel has a flat roof profile due to the horizontally bedded nature of the rock, but where tunnelling is in poor quality rock and/or in close proximity to existing infrastructure or tunnels, an arched profile with heavy support is used to limit settlement and maintain roof stability.
Cut and cover roof structures are constructed of either cast in-situ or precast concrete elements, as dictated by traffic staging and utility requirements. Depending on the ground conditions, the excavated walls below the roof are bored pile and panel, secant or contiguous concrete piled walls, or exposed rock. Floors are structural propping slabs or continuously reinforced pavements.
Alignment
The alignment is often adjacent to the basements of major buildings, the Eastern Distributor road tunnel, existing and future rail tunnels, existing viaducts, and utility services. The absolute requirement to remain within the land available to the project was an additional constraint.
The original alignment proposed by the RTA crossed over the Eastern Distributor road tunnel and exited into the western end of the Kings Cross Tunnel.
The current alignment is a successful alternative developed by CW-DC and BHBB during the tender design phase, that now sees the CCT pass under the Eastern Distributor road tunnel and exit east of the Kings Cross Tunnel.
Despite being 30m deeper and 300m longer, the alternative was successful as it eliminated the ‘challenge’ of mixing several hundred metres of cut and cover construction with the heavy traffic along William Street. Also, by incorporating larger radii and tunnel widening at the roadway curves, the line of sight and stopping sight distances were improved permitting the design speed to generally increase from 70kmph to 80kmph, increasing the tunnel’s traffic capacity.
Driven tunnel design inputs
Inputs for the driven tunnel design can be broadly divided into the following categories:
Ground support design for the driven tunnels is sensitive to the assumptions made regarding the expected ground conditions. Ground conditions in the Sydney Basin consist of sedimentary sandstones and shales. Local engineering practice divides these rock types into five Classes, with Class I representing the best ground conditions and Class V representing extremely weathered, extremely low strength rock. Six types of ground support have been designed, primarily based on these rock classes, support stiffness and its ability to minimise ground movement. These support types have been developed for each of the generic tunnel cross-sections, such as the single and double lane carriageways, with and without longitudinal passages for emergency egress.
Design philosophy
The design philosophy for rock Classes I to III was based on the Voussoir Beam Algorithm as developed by Diedrichs(1). The algorithm utilises the horizontal laminations that are a characteristic of the higher classes of the sedimentary rock types in the Sydney Basin. A flat roof tunnel profile is used in these ground conditions. The algorithm was used to determine the required rock bolt length and spacing for ground stability. A surcharge has been included to account for ground water pressure. Steel fibre reinforced shotcrete was subsequently designed to enhance the ground support, taking into account several modes of failure such as adhesion, direct shear, flexural failure, and punching shear failure between the rock bolts.
For rock Classes IV and V, a typical rock load was derived for each rock class using empirical methods and then applied in the design of a composite support system of rock bolts and steel fibre reinforced shotcrete. These support types utilise an arched roof tunnel profile. Where the rock load exceeds rock bolt and shotcrete capacity, steel sets are provided to assist in supporting this load. A Beam Spring model was used to size the steel sets and the shotcrete. Where low cover, poor ground and adjacent infrastructure demand support that minimises ground movement, pre-reinforcement in the form of canopy tubes and spiling bars are included in the generic support types.
Customised support design is required at the intersection of roads, chambers, passages (where the span reaches up to 18m), at portals and where the rock pillars between the tunnels attract additional loads. Rock pillar analysis required firstly an assessment of the need to replace the rock with mass concrete, and secondly an evaluation of the confining pressure required to maintain stability of the pillar.
The extent of support types was estimated based upon the geological interpretative model, surcharge from adjacent buildings loading on to the tunnel, settlement estimates and estimated building impacts. The level of building impact, such as cracking, is influenced by the building type, its proximity to the alignment, the size of the tunnel’s cross-section and the anticipated ground conditions. A stiffer support was nominated where excessive ground movement would result in damage to nearby and adjacent buildings. Over 200 buildings, many taller than 20 storeys, were assessed by inspection, survey and examination of structural drawings.
Ground support hardware and components
Nine different rock bolt types were specified, five of which are for permanent support with a 100-year design life. Durability for steel rock bolts is achieved through a double corrosion protection system of a grouted annulus and plastic sheathing. The designed shotcrete thicknesses take into account the exposure classification of the tunnel and provide additional thickness for durability. This shotcrete also covers the components of the rock bolts that protrude from the excavation face thus ensuring their durability.
Fire protection
The default fire rating required for most tunnel structures is four hours to the ISO time temperature curve. A 2hr rating for hydrocarbon fires is also required where tunnel collapse could cause damage to buildings and infrastructure resulting in injury to persons, and for separation of tunnels.
CW-DC resolved this requirement by undertaking a qualitative risk assessment of each nearby building and tunnel so as to identify potentially critical locations. Then, in the case of driven tunnels, critical sections were confirmed using a detailed assessment based on the Rock Tunnel Quality Q Index. As a result, passive protection for the 2hr hydrocarbon fire is to be applied in several sections of the driven tunnel.
Drainage
Plastic strip drains in the tunnel roof and walls, channel groundwater inflows to the tunnel drainage system that is pumped from the tunnel via a water treatment plant. The strip drains are fastened to the excavated rock face and then covered in shotcrete or are fastened to the shotcrete lining, depending on when the inflow occurs.
Construction progress
A total of seven Mitsui, Eickhoff, Alpine and Boart Longyear roadheaders are being used for driven tunnel excavation, while rock saws and breakers are used for detailed excavation. Most tunnels are being excavated using two headings. While ground support is installed in accordance with the construction sequences specified in the CW-DC design, BHBB have instigated some modifications including the use of temporary ground support at the face of the single lane tunnels, as permanent rock bolting cannot be safely carried out from the roadheader. The temporary rock bolt pattern is the same as the required permanent rock bolt pattern. Permanent rock bolts are then installed behind the roadheader with shotcrete following shortly afterward.
Due to space limitations in the city, construction access to tunnels is via deep shafts and ramp lowerings excavated at points between tunnel portals. The main access point on William Street has been covered by an acoustic shed for noise and dust containment. Spoil is hauled by truck from this site using the existing Eastern Distributor Tunnel network.
At the end of 2003, 30% of the total driven tunnel excavation and permanent ground support works had been completed, including:
In addition to these permanent works, two temporary access shafts have also been completed and fitting-out has commenced with laying of the structural pavement in the Riley Street and Harbour Street trough structures.
Excavation of the single lane tunnels is being undertaken at a rate of 6m per 24hr day, and is expected to be completed in January 2005. The contract date for completion of the entire road tunnel has been set for October 2005.
Building movements, surface settlements, tunnel convergence and groundwater levels are monitored on a daily basis near current construction activities and on a weekly basis at less critical locations. To date negligible movements have been observed. Excavation has not yet reached areas particularly sensitive to ground movement such as Town Hall Station and a number of multi-storey buildings. These areas present some of the greatest challenges on the project and the performance of the support systems at these locations offer some of the best opportunities for CW-DC to verify and improve the designs.
Related Files
Map of the Cross City Tunnel in Sydney, Australia
