Eastern Distributor to ease Sydney city congestion Feb 1999

Almost everything about the Eastern Distributor highway tunnel through central Sydney is remarkable. Its size, its shape, its interior configuration, and its lining and support are all aspects which set this tunnel apart from any built in the past. With a cross section of 156m², the 13.5m wide x 11.5m high tunnel will accommodate a double-deck six-lane highway. At various points this profile increases to 22m wide and 255m² to accommodate ramps feeding traffic in and out of the tunnel to major surface streets. The tunnel profile in Sydney's Hawkesbury Sandstone is rectangular with a flat roof and straight sides and its permanent lining and support will be rockbolts and shotcrete with no in-situ concrete (Fig 1). Perhaps excavation of the tunnel by roadheaders in the sandstone is predictable, in line with a tradition set by Australian engineers to push design and technology frontiers to maximise the engineering potential of the favourable sandstone medium.

North portal works at Cathedral Street showing side wall drifts either side of the 3.5m wide central pillar which is removed as final permanent support is installed

The 1.7km length of tunnel is part of a new 6km section of freeway to provide a link between Sydney's famous bridge and Immersed tube tunnel harbour crossings, its central business district (CBD), and the city's international airport and south-eastern suburbs (Fig 2). The project has been envisaged since the early 1950s and more particularly since cross harbour capacity increased with the opening of the immersed tube tunnel in 1992. The Distributor will alleviate chronic congestion at the southern end of the harbour crossings and will provide a highspeed free-flowing link to the airport, cutting some 10 minutes off the average 30 minute journey for the 10km trip from the CBD. As another of several projects designed to be completed as part of Sydney's preparations to host the 2000 Olympic Games, the project was prioritised by the New South Wales State Government and was set on a fast track, design-and-construct contract programme in 1997.

Funding and bids

The project is based on a 48-year BOOT concession awarded by the State Government's Roads and Traffic Authority (RTA) in August 1997 to the Airport Motorway Ltd company (AML). AML is the concession vehicle established by the Leighton Group (one of Australia's largest construction groups) and Macquarie Bank. AML has a full capitalisation of about $Aust700 million and has a fixed-price, lumpsum design-and-construct contract with Leighton Contractors Pty Ltd valued at about Aust$600 million. Leighton Group holds a financial stake in AML of less than 20% and Macquarie Bank, holder of the balance, has managed syndication of the project's debt. There is no government funding of any kind offered to the BOOT concession and no government guarantees to underwrite the risks associated with the fixed-price, lumpsum construction contract, the financial borrowing arrangements, or the toll income forecasts. When it opens, the toll of Aust$3/car and Aust$6/truck will be collected one-way from northbound traffic exiting the motorway at three toll plazas, one at William Street offramp, and two at the north end of the project as it joins the existing Cahill Expressway.

Having signed the contract in August 1997 Leighton mobilised on site in October to begin a tight 36-month construction programme to have the project finished and opened by August 2000. Prior to this, RTA had taken the project through its various environment approval stages. Approval from the State's Department of Urban Affairs and Planning was granted in 1997 but was subject to some 151 conditions. These mainly concerned protection of the urban environment. "With these conditions in mind, we worked with Maunsell (Australia), our appointed design engineer, and arrived at the approved project configuration," said Gordon Ralph, Project Manager for Leighton Contractors.

Fig 1. Design of the double-deck, six lane section with its square shape, flat roof and rockbolted and steel fibre reinforced shotcreted permanent support and final lining

As an extension of the Cahill Expressway, traffic passing behind the Opera House and the Royal Botanic Gardens will pass under a landbridge and into a cut-and-cover transition to the bored tunnel. At the far end, tunnel traffic transfers from the double deck configuration into a four-lane parkway along South Dowling Street through Moore Park and on to join the at-grade alignment of the existing Southern Cross Drive, the main dual carriageway south and to the airport. En-route there are connections to William Street, Moore Park Road and Anzac Parade, together with footbridges to Moore Park across the Parkway and major top-clown constructed tunnels under cross streets beyond the main tunnel.

"The 140m long landbridge over the Cahill Expressway at the harbour end," explained Ralph, "creates a continuous park environment from the New South Wales Art Gallery, through the Royal Botanic Gardens and down to the Opera House. The bored tunnel under Bourke and Flinders Streets contains traffic noise and pollution through the residential areas of Darlinghurst and Surry Hills. It also maintained the surface streets and prevented demolition of houses through this historic part of the city. Continuation of the tunnel along South Dowling Street through Moore Park was not necessary but setting the road 4-5m below street level in the Parkway further controls noise and removes sight of the freeway from the park landscape. A great deal of effort has gone into protection of the environment, and community liaison remains a project priority."

Fig 2. New highway designed to alleviate central Sydney congestion at the southern end of the bridge and tunnel harbour crossings

With Maunsell preparing final design documents for these works, Leighton, as main project contractor, decided to subcontract the tunnel excavation works and to complete a substantial part of the remaining construction work itself. After a competitive bid process, two subcontractors were appointed for the tunnel. The first, for all works progressing from the south end, was awarded in January 1998 to McConnell Dowell for a contract price of approximately AUD$20 million. The second, for all works progressing from the north end, was awarded in March 1998 to Concrete Constructions for approximately $28 million.

Both contractors are working towards a specified contract breakthrough middle point beneath Taylor Square. "Both contractors tendered for the two contracts and submitted a cost saving bid to complete both contracts as one," said Ralph, "but we decided to go with our original plan for two separate subcontracts working from both ends. The project has a very tight and uncompromising deadline; there is a large amount (about 350,000m³) of tunnel excavation to undertake; a single contract for all excavation would have been a huge call on the resources of any single company; and the natural competitive spirit generated by having two separate contractors working from opposite ends toward a common meeting point is healthy for the construction programme, for the management of each separate subcontract, and for the overall quality of the end product.

Both contractors have mobilised well, and we are very pleased with progress to date and the quality of work being achieved." Both contractors working on the tunnel are undertaking their own quality assurance procedures. Maunsell has a team of field surveillance engineers and it directs changes to tunnel support requirements as work proceeds. Leighton as principal contractor maintains all liaison with the client and audits all tunnel works.

Geology

The tunnel lies between 10 and 30m below ground surface and is excavated by roadheader through competent Hawkesbury Sandstone, a medium familiar to engineers and contractors involved in all types of tunnelling in Sydney. It already hosts several major underground structures including the Sydney Opera House car park, underground sewage treatment plant caverns and train stations, and more recently the Elgas LPG storage cavern.

Top heading span of 22m at the Williams Street junction

The sandstone is Permo-Triassic and has an average UCS of between 30 and 60MPa. It also has a much higher horizontal than vertical stress characteristic. This stress is believed to have been locked in by a massive overburden which has since eroded. The sedimentary rock is recognised for its dry, competent quality; its ability to host wide-span openings; its long self-supporting stand-up time; and its natural tendency to break back to a definite laminate layer when excavated, and assume a flat roof. In its design, Mausell, with design engineers from Leighton and specialist geotechnical subconsultants Pells, Sullivan, Meyninck (PSM), has made full use of these distinctive qualities to arrive at an innovative, technologically advanced excavation sequence and a cost-effective immediate and long-term support regime.

Phillip Pells, in particular, has worked extensively on the design of flat roofs and wider spans in Sydney's sandstone). At 22m wide maximum, the Eastern Distributor is the widest span so far to be attempted in Sydney. The underground pumping station at Bondi is 13m span, the widest Elgas cavern span is 14m and the Opera House car park span is 17m. "The linear arch theory (1942) is the model most applicable to Hawkesbury Sandstone," explained Andy Peck, the on-site engineer for Maunsell. "It provides a closed form solution to estimate the maximum unsupported span and the necessary minimum thickness of roof beam created by long vertical and inclined rockbolts to prevent the sandstone joints and bedding planes opening up." The UDEC computer modelling programme was used to define the appropriate design parameters.

View along the right hand side wall drift with the systematic rockbolting support keeping pace with tunnel advance

The decision to adopt a flat roof for the tunnel was taken early in the process. The second most interesting decision, to adopt rockbolts and shotcrete as the permanent lining and support, was the result of long discussions questioning the need for an internal in-situ concrete lining in such a dry, competent and stable tunnel when the primary support of rockbolts and shotcrete contribute such a high degree of the ultimate load carrying capability. "A great deal of evidence was brought forward to support the permanent rockbolt and shotcrete support argument," said Ralph, Tunnel Methods Engineer for the project, "and intense development and testing of the materials subsequently led to the adoption of the method."

Part of the design development included examination of a large number of bolts cored from existing installations. Those taken from Sydney-area sites after being 20 years in sandstone showed no significant deterioration, it was said, and bolts retrieved from the Snowy Mountains scheme after up to 40 years service in granite showed 'not much deterioration'. "The history of rockbolting is as yet limited," said Bendall. "Bolts may well last for 100 years or more. However, for our purposes, we are operating on a design life for the tunnel of 100 years; of 45 years for the intermediate precast concrete road deck; of 50 years for the rockbolts; and of 100 years for the shotcrete."

Long-term bolts

To maximise the design life of the rockbolts, those used in the permanent systematic support regime are epoxy coated, rather than galvanised, and are fully cement grouted. Galvanisation is considered good for 15 years, it was said, whereas epoxy coating is also considered good for 15 years but has a longer residual factor of safety, increasing the probable effectiveness of an epoxy coated bolt up to 50 years.

A great deal of additional development went into the installation of the rockbolts, paying particular attention to details such as the packaging, transportation, storage and handling of the bolts to avoid damaging the epoxy coatings, and into the design of holt plates and 'spider' bars which ensure a high interlocking key with the layers of the permanent shotcrete. Each bolt has spacers along the length to centre the bolt accurately in the bolt hole, and two polyurethane tubes - one running to the bottom of the bolt hole for grout injection, and the other the breather tube - are fitted to each bolt to permit high quality grouting. The bolt is fully cemented when grout flows from the breather tube.

Fig 3. Details of the permanent rockbolt and shotcrete lining and detail of the standard and high capacity bolts

Rockbolts used in the permanent support regime are 24mm diameter; are 3.5m to 9m long; and are divided into two principal categories, standard capacity and high capacity (Fig 3). Most permanent support bolts are also tensioned to specific load and within a minimum specified distance from the advancing face. The standard capacity bolts are expansion shell bolts of 3.6m to 5m long. These are tensioned initially to 120kN and retensioned after being fully grouted to 230kN. The high capacity bolts, with a shear nut/hanger arrangement, are between 7 and 9m long, and are tensioned initially to 200kN, and ultimately to 450kN.

Installation of the bolts is on a particular sequence and in accordance with the excavation programme. The tunnel is being excavated on a top heading and bench sequence with top headings of more than 13.5 m wide divided at all times into three phases; two side wall drifts; and a central pillar. The central pillar is a specified minimum of 3.5 m, giving the two side wall drifts a varying width of 5 m in the normal 13.5 m tunnel span to 9.25 min the maximum 22m span at the William Street junction.

Each subcontractor is able to arrange the sequence of excavation to suit his particular programme, although the faces of the side drifts are recommended to be staggered by a minimum 10m (to provide advanced notice of ground conditions) and the pillar cannot be removed unless and until the specified permanent support has been installed depending on the tunnel span (Fig 4). There is also a minimum advance of unsupported tunnel. In Class 2 sandstone (the average quality sandstone) this so-called 'minimum unsupported span' or MUS is 6m, while in Class 3 sandstone the MUS is limited to 4m.

Where necessary, the contractor must install immediate 'temporary' resin grouted and untensioned rockbolts to ensure safety and security of the workers in the MUS, but standard capacity bolts and high capacity bolts are installed on a specific systematic pattern according to rock conditions and tunnel span, and all bolts must be fully grouted and tensioned to ultimate levels within 3.5m of the central pillar excavation. In addition, the bolt holes for all permanent bolts installed on an angle are socketed to provide a flat surface on which to install the face plate and ensure uniform contact with the rock.

Fig 4. Plan and section of the systematic bolting pattern for spans of more than 13.5m

Steel-fibre shotcrete

Shotcrete used on the project is subject to similar rigorous testing, examination and formulation. Most shotcrete is specified wetmix steel-fibre reinforced, with a minimum of 45kg of fibre/m³ Additives such as superplactisisers and accelerators are strictly controlled and the minimum design strength criteria for the shotcrete mixes as developed by each of the two tunnel subcontractors is 40MPa.

The minimum thickness and application of each layer of shotcrete, to the optimum thickness of 175mm, is also strictly controlled. An initial layer of 50mm of fibre reinforced shotcrete can precede installation of the first row of rock bolts if preferred by the subcontractor, and this is followed by a minimum 85mm secondary layer of fibre shotcrete which must cover the face plate spider bars by a minimum 15mm all round. Finally, the tunnel is finished with a minimum 40mm thick layer of unreinforced shotcrete as the permanent and final lining of the underground facility.

It is on the rockbolting regime that the tunnel relies for its permanent stand up support, and the design and installation of the rockbolts are being crossed checked and monitored very closely. John Sharp and his company Geo Engineering is the design verifier engaged by Maunsell, and according to his recommendation more of the bolts in the permanent bolting pattern are tensioned.

On the practical side, subcontractor Concrete Constructions working on the north portal contract of the tunnel has had an alternative design for the 7-9m long high capacity bolts approved. Instead of an epoxy coated and coupled rebar bolt (the coupling needed for installation of 6-9m long bolts in a 6m high x 6m wide side wall drift), Concrete Constructions proposed a flexible, two strand bolt which is greased and sheathed (for sufficient protection) and sufficiently flexible to install in long lengths.

Ground movement monitoring is, of course, a high priority along the bored tunnel alignment, and a comprehensive array of instrumentation and surveying is monitored. This includes strict and regular convergence surveying within the tunnel and the reading of extensometers installed from the surface and from within the tunnel.

Full profile top heading of Anzac Parade ramp heading where support included lattice girders and progressive layers of steel fibre reinforced shotcrete

Progress to date

During a site visit to this impressive project in mid-1998, both tunnel subcontractors had made positive starts on the top heading phase from their respective ends of the project. McConnell Dowell, from the south portal, had mobilised two roadheaders: a 120tonne Paurat from the main South Dowling Street portal and a Mitsui S200 in the Anzac Parade access ramp to the Drivers Triangle junction. Shotcrete is applied by a Jacon shotcreting system with a telescopic nozzle boom, and bolt holes are drilled using a new Tamrock drillrig.

At the opposite end, Concrete Constructions is using three Voest Alpine AM 105 roadheaders, two in the main tunnel top heading from the Cathedral Street portal and the other advancing the William Street feeder tunnel to the main tunnel. Concrete Constructions is also using Jacon shotcreting machines, and bolt holes are drilled with a Tamrock DHA 600 track drill.

At the time of the site visit, both contractors were making positive progress. McConnell Dowell with its Paurat roadheader was advancing an average 6m/10h shift in the 6m x 6m sidewall drifts from the main South Dowling Street portal, while Concrete Constructions was making equally positive rates up to 30m/day total in the top headings of both working sites.

As competent as the Hawkesbury Sandstone is, however, it is interrupted by several significant geological faults and features. Early into its drive from Cathedral Street, Concrete Constructions had to negotiate the Woolloomooloo Fault Zone, a 150m thick zone of weak rock, and the Oxford Street Dyke was encountered on the William Street heading. Immediate temporary support of flash shotcrete, lattice girders (designed especially by Maunsell to suit the tunnel's rectangular profile), and the addition of spiling on occasions (or canopy tubes as it is known in Australia) were required in these zones, and progress was retarded accordingly.

Full profile top heading of Anzac Parade ramp heading where support included lattice girders and progressive layers of steel fibre reinforced shotcrete

At the south end, McConnell Dowell had to use similar immediate support elements to pass through the 6m wide Great Sydney Dyke, an intrusion of clay and weak material encountered shortly into the tunnel alignment from the South Dowling Street portal. The two subcontractors are responsible for the sourcing and purchase of all materials used on their individual contracts. Leighton, as the main contractor, will be responsible for construction of the intermediate precast concrete road deck and all other civil works in the tunnel, and for installing all its necessary electrical and mechanical equipment.

According to latest reports, tunnel excavation by both subcontractors has continued to progress positively. All top heading excavation, including the central pillar of the main tunnel alignment, holed through in early December 1998, and benching have started from both ends. Installation of the permanent support requirement has kept pace with excavation, and all excavation is scheduled to be complete by late March/early April 1999. There have been no major causes of concern during excavation to date, and instrumentation readings confirm that settlement of the surface above the widest top heading span of 22m, some 23m below the surface at the William Street junction, is well within expectations. Similar readings of very limited movement or settlement are now being recorded as the benching operations pass through the instrumentation stations.

Acknowledgements

This article is presented with the cooperation and assistance of Gordon Ralph, the project manager for Leighton Contractors; Annie Luczka, the project's Community Relations Manager; Maurice Bendall, the project's tunnel methods engineer at the time of writing; and after interviews with the managers of the two tunnelling subcontractors, David Logan for McConnell Dowell and Alistair Rowe for Concrete Constructions. Engineering drawings are reproduced with the kind permission of Maunsell, the principal design engineers, and by Leighton and AML.