The South East Busway & Transit project involves the construction of a two-lane dedicated busway and transit lanes linking the south eastern part of Brisbane with the central business district and in addition it will serve the Woolloongabba Stadium where a number of games will be played as part of the Olympic soccer competition.

Part of the project involves the construction of nearly 600m of driven and cut-and-cover tunnel and approaches, through a heavily populated area of the city. It consists of a cut/cover section that crosses three sets of railway tracks, the underpinning of a three-storey concrete and masonry building, a 410m driven tunnel, which runs under the building and crosses Vulture Street (a busy arterial road) and a further section of cut and cover tunnel adjacent to Mater Hospital, a large public/private facility.

The construction value of this section is A$40M (US$27M). Peabody Mning Services is contractor for the driven section.

Ground conditions

The tunnel generally encounters rock for most of its length. The approaches decline through clay and highly weathered rock. Most of the cut-and-cover and all of the driven tunnel passes through slightly to moderately weathered metamorphic rock. A fault was known to exist locally along the tunnel alignment but it is not seismically active. On investigation it was found to consist of a sheared zone and did not greatly hinder tunnelling activities. It was generally anticipated that most tunnelling materials would be excavatable with hydraulic hammers and road headers. This was subsequently found to be correct.

Although the tunnel alignment passes below the level of the Brisbane River, which is located at about 200m distance, ground conditions were anticipated to be such that ground water inflows would be minimal. This was also found to be the case.

Cut and cover tunnels

The cut-and-cover tunnels under the rail tracks and roadway are constructed with pile walls and a precast concrete deck unit roof. Each roof has a composite slab and a waterproof membrane with a protection layer above. Minimal leakage has occurred to date. The tunnel under the rail tracks was constructed with limited closure of the electrified rail lines above. These rail lines are part of a busy urban commuter network and could not be closed for significant periods.

Driven tunnel

The 410m of driven tunnel (figure 1) accommodates a two-lane bi-directional bus and emergency vehicle carriageway in shallow cover. It travels beneath the suburban railway lines and platforms of Vulture Street railway station, an existing three-storey building and beneath extensive services existing in Vulture Street including telecommunications and gas.

The driven tunnel has a typical excavation width of 12.6m, and has been designed to allow for a completed width (including shoulders) of 10.2m, rigid concrete barriers, services including ventilation, lighting, traffic control (intelligent transport systems), fire services, and pavement and tunnel drainage.

Key features include a partial waterproof membrane installed along the entire length of the tunnel to limit water seepage, three-fan enlargement sections (figure 2) for the provision of ventilation services, an emergency egress passage and escape shaft, and a Y-junction for the future provision of a light rail link.

Support was provided by a two-stage system. The initial support (primary lining) typically consisted of 3.5m long chemical rockbolts installed through steel ribs encased in 200mm thick shotcrete. Steel fibre-reinforced shotcrete, typically 100mm thick without steel ribs, was used where ground conditions were favourable. An 80MPa steel fibre-reinforced shotcrete designed to achieve a strength of 4.8MPa at 3mm deflection was specially developed for the project.

The final support (secondary lining) was constructed after the waterproof membrane had been applied, and was provided by a 300mm thick cast in-situ unreinforced concrete lining. The secondary lining was installed in sections using an 11m collapsible steel travelling form along the inside of the tunnel working on a 24-hour production cycle.

Excavation of the tunnel proceeded from two portals using a heading and bench construction sequence. A Paurat E242 roadheader was used at the western (Vulture) portal, and a Mitsui S-200 roadheader at the eastern (Mater) portal.

The proximity of commercial and residential property meant that stability and minimisation of ground settlement were critical factors at all stages of excavation and construction of the driven tunnel. As this section passes through low-strength rock and has very little soil/rock cover between the tunnel roof and the ground surface (typically 2-3m), design and construction methods had to be adopted to ensure the settlement of the railway and building were minimised.

The support system was designed to maximise the strength and stiffness of the combined rock and support, and to allow tunnelling at such low cover. Modelling predicted up to 40mm settlement – in the event only 5mm was recorded.

Two small tunnels were excavated using a Mitsui

S-65 roadheader at the outer edges of the main driven tunnel zone, and concrete drift beams were poured into these tunnels to provide an adequate bearing foundation for the primary support arch.

Following the excavation of each small section of the tunnel, canopy tubes were installed around the crown to provide longitudinal support in advance of the heading, thus reducing ground surface movement and settlement. Glass fibre face nails were also used to prevent movement of the ground at the tunnel face. Excavation through the canopy tube sections was limited to typically 0.75m-1m. Additional support was provided by steel ribs encased in shotcrete. Heintzmann Expanding steel sets were used to fit the varying tunnel cross-section throughout the splayed tube sections.

Provision had to be made to allow for the possible future conversion of the busway to a co-located light rail facility or a full light rail facility without buses. The plan produced by the Brisbane Light Rail Project indicated that the light rail alignment would potentially come down Grey Street in a tunnel and link into the Vulture Street tunnel. This meant that a Y-junction had to be accommodated to connect the future light rail tunnel into the Vulture Street tunnel, thereby creating an underground intersection beneath the Vulture Street/Grey Street intersection.

With a maximum span of approximately 24m, and approximately 7m of cover, the Y-junction presented a major design challenge. This was met with innovative support methods and a detailed construction sequence. Because of construction constraints, and the fact that the excavation was advancing away from the Y, a shaft was constructed from the surface at the nose of the Y-junction. This was so that a concrete filler wall could be constructed to accommodate the loads transferred by the driven tunnel linings on either side of the Y, as the strength of the rock pillar was inadequate to handle the induced rock stresses.

The Y-junction roof support was provided by approximately 130 cablebolts installed from the surface above, which were plated as they were exposed during excavation. These were successfully installed without halting the surface traffic flow and avoiding the extensive services located in the area. Rockbolts were installed from within the tunnel, between the rows of cablebolts and a final lining was provided which consisted of lattice girders encased in high-strength steel fibre-reinforced shotcrete.

The construction sequence for the Y-junction was carefully worked out and excavation advances were limited because of the large span and small amount of cover. Excavation was performed for half a heading on either side of the Y-junction which left a central pillar for stability and support. Once the rock support was installed and built up on both sides, the central pillar was progressively excavated in stages, and the middle section of the support arch completed. Excavation of the central pillar was limited to typically 1m advances, before support had to be installed. Convergence of the tunnel and surface settlement were closely monitored during construction.

The final lining was constructed using shotcrete rather than concrete, and did not include a waterproof membrane, because of the complex geometry of the Y-junction. The shotcrete was sprayed in panels of 4m maximum width. Construction joints between panels were installed with an injection hose system, which can be injected with a hydrophilic grout to prevent leaks, and then flushed clean for future injections should conditions require.

Tunnel drainage systems accommodated a water quality treatment device so that water could be treated to remove any oils and sediment particles before discharge to the storm-water system. This was consistent with the water quality objectives of the project. It also ensured that the invert level of each portal was above any potential flood level of the Brisbane River, so that flooding in the river would not result in flooding of the tunnel.

The design engineer for the driven tunnel excavation and support design was Connell Wagner. The checking engineer was the specialist tunnelling design firm of EJ Nye & Associates, as subconsultant to Sinclair Knight Merz, which was the main/civil structural engineer.

Tunnel approaches

Close bored pile walls with infill shotcrete panels were used for most of the open approaches. These piles are 1200mm diameter and cantilever up to 8m in locations where ground anchors could not be placed under adjoining properties. Where pre-tensioned ground anchors could be used, these were of the permanent type with a design life of 100 years. The pile walls are concealed by precast concrete panel walls with an exposed aggregate finish.

Tunnel fit-out

The tunnel, and all of the busway, has a concrete pavement for long maintenance-free life. A thin stone mastic asphalt overlay is provided for ride quality. The concrete pavement is jointed at 6-8m centres, and varies between 225mm and 270mm in thickness. The pavement was planned and designed for the possible future overlay of a light rail track slab. The pavement profile and surface contouring of the final pavement profile had to be carefully considered because of the stringent requirements for light rail cant and horizontal/vertical geometry.

Extruded concrete barriers support a lightweight wall lining in the tunnel, and precast concrete wall panels on the approaches. The lightweight wall lining consists of steel framing supporting a fibrous cement sheet material. The wall lining is light reflective and forms part of the tunnel lighting system. The fibrous cement sheeting has a baked paint finish. Durability concerns have led to the use of paint coatings over the galvanised finish of exposed steelwork. ISO exposure category 3 to 4 is considered appropriate given the proximity of diesel fumes from the buses, and possible moisture from condensation and any leakage of groundwater.

The tunnel ventilation system utilises a series of jet fans. These were manufactured in Sweden. Three banks – each consisting of three fans – are used, housed in tunnel roof enlargements. The fans generally direct air away from the Mater Hospital portal. Discharge of tunnel air is at the portals. Vent stack chimneys are not required, and each fan is reversible.

Graded lighting is provided as is normally the case for road tunnels. Lighting design is to Australian Standards and PIARC recommendations.

An array of sensors (CO, NO2, visibility and air

velocity) are connected to a PLC that controls the operation of the fan. An intelligent transport system for the busway has overriding control of the tunnel systems. Sprinklers are not provided in the tunnel. At the mid-length of the tunnel, a pressurised stair and lift shaft provide safe egress.

Fire safety

The design provides for safe egress, in the event of an incident in the tunnel, by means of an egress shaft. A stair and lift brings passengers up 5m from the lowest point of the tunnel to surface level. The stair/lift shaft is positively pressurised to prevent smoke entry. The performance provisions of the Building Code of Australia were used in the design of this egress shaft, although building codes do not specifically cover these kinds of structures.

Should an incident occur in the tunnel, then the following provisions have been made:

– Jet fans operate to evacuate smoke. The jet fans can operate for two hours in a 250o C air stream

– Electrical cabling is fire-rated Radox type

– Emergency exit lighting leading to the tunnel portals and the egress shaft is provided

Linear heat detector wires send a signal to the fire indicator panel located outside the tunnel.

Those parts of the roof supporting load by flexure (bending) have a sprayed on vermiculite insulation layer giving a two-hour rating for a hydrocarbon fire.

The tunnel is on programme for opening prior to the Olympic games in Sydney in September, 2000.

Related Files
Figure 1: Typical section of driven tunnel
Figure 2: Enlarged section