South East Queensland is Australia’s fastest-growing region, with an estimated 1000 new residents arriving every week to enjoy the benefits of the ‘sunshine state’. The Queensland government has invested heavily in infrastructure to meet the demands placed on the region’s water, housing and transport. While large-scale tunnel projects seem all the fashion around town, there is a ‘quiet achiever’, which deserves its own share of the limelight – the Boggo Road busway tunnel. Dedicated busways are a very important component of Brisbane’s public transport initiatives.

The Queensland government-funded Boggo Road Busway Alliance (BRBA) is a partnership between Queensland Transport, Thiess Pty Ltd and Sinclair Knight Merz. The alliance is responsible for the delivery of 1.7km of dedicated busway linking the M1 South East Freeway to the University of Queensland via the existing Eleanor Schonell Bridge which opened in December 2006. The busway takes its name from the iconic heritage-listed jail under which a different sort of tunnel has been excavated.

The busway’s termination at the state’s largest university is warranted. The University of Queensland is the third largest traffic generator in Brisbane after the central business district and the airport.

The busway will open in mid 2009.

Tunnelling

Stage Two of BRBA is a 640m long tunnel, of which 430m is true shallow-cover driven tunnel. The excavated tunnel cross section is a typical busway horseshoe shape, 14m wide and 7m high, with an average excavated cross-sectional area of 105m2. Total spoil to be removed is approximately 50,000bcm over a 57 week period from August 2007 to early October 2008.

The driven tunnel is being excavated with a refurbished ATM105 roadheader. This roadheader had a previous life on the Eastlink road tunnels in Melbourne. Advance is achieved with a 5.5m high top heading followed by a 1.2m high benching activity.

Geological conditions are extremely variable within the driven tunnel section of the works, although an added benefit is the lack of groundwater ingress to the works. The geological profile is generally divided into three distinct sections.

  • Initial 150m in weathered Brisbane Tuff material – typically in UCS range 20-120MPa
  • 150m in the Tingalpa formation (siltstones and sandstones) – typically in UCS range 10-40MPa
  • 130m back in the stronger Brisbane Tuff

Within each of these formations are several small faults, conglomerates, breccias and claystones.

Three distinct temporary ground support types are used during the roadheader top heading excavation.

  • The 117m of tunnel beneath heritage-listed Boggo Road Jail utilises 12m long horizontal fibreglass face nails, 12m long canopy tubes and lattice girders at 1000mm centres encased with 300mm of plain shotcrete. Each canopy tube array consists of 27 No. tubes at 500mm centres. Face nail numbers varied typically from 20-30 each array. Canopy tubes were supplied by Alwag, a subsidiary of DSI. The average advance rate for this ground support type was in the order of 7 lineal metres/week for an 11 shift week
  • Lattice girders at 1200mm or 1500 centres encased with 300mm or 250mm thick plain shotcrete respectively for average advance rate 10-11m/week. In what we believe to be a first in Australia, self-drilling friction stabiliser rockbolts supplied by DSI have been used to pin the lattice girders to the crown in these areas where there are no canopy tubes, and this has allowed for all persons in the work area to remain separate to the support installation and not work close to unsupported ground
  • 3600mm long resin encapsulated rockbolts on 1200mm or 1500mm grid laterally and longitudinally for average advance rates 16-20m/week. All resin and bolts are installed remotely with a Tamrock Minimatic two boom jumbo drill.

The geological model

Rather than go into detail about what can sometimes be mundane descriptions of “how we sprayed shotcrete” or “how we installed rockbolts”, the project has some interesting innovations and has incorporated some “lessons learnt” from recent Australian tunnelling projects and put them into practice quite effectively. Discussion around the geological model is the first item of note.

The tunnel, except for the old jail, has the unique advantage of not passing under any other buildings or significant structures along its 430m alignment. This attribute enabled the tunnelling team in the early conceptual stages, to drill 10 boreholes on or very close to the tunnel centreline, some within the jail perimeter. This provided in-situ core information at an average of less than 40m centres. From this information a very detailed and accurate geological model was developed. This in turn allowed the tunnel designers to accurately forecast ground support design.

Design and construction interfaces

The project has several success stories, but the one that stands out is the excellent relationship between the design team and the construction team. The unique alliance framework, now popular in Australia for the delivery of major infrastructure projects, facilitates the two disciplines to work closely together and two distinct advantages became apparent on this project:

During the project estimate and approval stage, the geological model was available and tunnel designers and constructors were able to analyse the information and make critical decisions on the ground support types and methods of excavation.

At the same time, the construction team was also able to work closely with the tunnel estimators, in the same office environment, to take the design and the work method statements and price the job accordingly.

Two fundamental decisions emerged from this process. One was the decision to install lighter-weight lattice girders underneath the canopy tube arrays through the jail area instead of traditional steel sets and use 300mm plain shotcrete as the primary stiff support. Secondly, the idea to use resin encapsulated rockbolts in lieu of shell anchored or grouted bolts because of the variability of the ground in the tunnel crown.

The Permit to Excavate process

The driven tunnel section of the Boggo Road busway is also unique for another reason – the shallow cover. The minimum ground cover beneath old Boggo Road Jail is 6m, the maximum cover along the alignment is 15.5m and the overall average cover to the crown of the excavation is less than 11m. Not much, considering the tunnel excavation is over 14m wide.

Being aware of this fact, the alliance tunnel team modified a process used recently on several Australian tunnelling projects, called the “Permit to Excavate”, and used it successfully to assist safe and efficient excavation. The core of its success lies in the fact that this process requires a member of the geological team, the design team and the construction team to sit together every day and analyse face mapping, the actual geological conditions versus predicted, general progress, quality, settlement results, tunnel convergence and water inflows, so that the conditions of advancement are agreed and signed off upon. This information is then passed to the tunnel supervision and excavation continues for another 24 hours under the new permit.

Boggo Road Jail and settlement

The initial 117m of the driven tunnel beneath the 107 year old Boggo Road Jail was the most challenging for a number of reasons and a steep learning curve through this section was experienced. But this was anticipated and the program adjusted accordingly. Drilling 12m long, 55mm diameter face nail holes and the same length 140mm diameter canopy tube holes in variable ground, with a new team and new equipment, at the start of a project, was always expected to present difficulties. It should be noted that the major obstacles were not related to ground conditions or design; they were linked to work crews developing safe, repetitive methods over time and modifications required to the jumbo drill to overlap with these methods as they developed. Despite a slower than expected start up, the canopy tube section was completed on budget and time. Rates of 10 No. 12m long tubes drilled and installed per 10 hour working shift were achieved towards the end of this operation. Canopy tubes were installed with a Tamrock Minimatic two boom jumbo drill. Each tube consists of 4 No. 3m long elements. The initial 3m element has a sacrificial drill bit and excellent alignment accuracy is achieved if the first section is drilled on line and grade. This accuracy allows neat excavation underneath the tubes, easy installation of lattice girders and reduces shotcrete wastage.

Each canopy tube array had a 9m effective length due to the 3m overlap between arrays. The use of lattice girders in lieu of traditional steel sets proved a good decision because although extra time is taken up spraying the thicker shotcrete, time and cost savings are achieved via the quick erection of the light weight girders and the fact that once the shotcrete is sprayed, no further secondary spraying at a later date is required. This surface is then ready for waterproofing. To our knowledge this is the longest section of canopy tube support in Australia and the first time that the lattice girder and shotcrete support under canopy tubes has been used in Australia.

Forecast settlement under the jail was 10mm and this was not exceeded. Surface settlement was some 7-10mm with no damage to the heritage structures recorded.

Plant and equipment

Boggo Road Busway Alliance was fortunate in starting tunnelling in August 2007 as the the Mitcham to Frankston tunnelling project in Melbourne, constructed by Thiess Holland JV, was disposing of equipment. Excellent quality equipment was sourced including an ATM105 roadheader, Meyco Suprema robotic shotcrete rig and Caterpillar 25 tonne moxy trucks. The quality of this equipment and of the refurbishment works, especially to the roadheader by Sandvik Mining & Construction at Tomago near Newcastle, in New South Wales, have been key to the success of the excavation.

Another successful innovation in the area of plant has been the installation of two concrete drop pipes from the surface linking the underground works. Two off-road concrete agitators have been purchased for the tunnel fleet, which allows concrete to be efficiently transferred to the underground works at any time of night or day in any weather and means subcontractor trucks with all their associated issues are not required to enter the tunnel.

People, production and safety

Demand for quality people in the Australian construction sector is at an all-time high. At the start of the project the recruitment of the tunnel workforce focussed on drawing on the workforce finishing in Melbourne and the relationships with the supervision also sourced from that project. Recruitment also targeted the mining sector and drawing on the existing unskilled surface workforce on other parts of BRBA. This three pronged attack proved very successful in building a quality excavation team of 31 people.

Tunnel engineering resources are even more difficult to find. Whilst key leadership roles in management and supervision roles were in place early, BRBA spent considerable time in attempting to recruit other site and project engineer roles with tunnelling experience. This proved unachievable and a different strategy was adopted. Quality engineers with no tunnel experience were identified as a solution to the issue. The Australian experience is that generally engineers specialise in areas of expertise and this is split between underground or surface environments. The key is to get them exposed to the underground environment and then provide strong leadership to bring them up to speed. One particular point of note is that the tunnel site engineers have spent dedicated periods working on shift with the tunnel crews, similar to the practice used in the Australian underground hard rock mining industry.

The quality of the people on the project is directly related to the safety performance. To date, the tunnelling works have exceeded 365 days Lost Time Injury Free.

BRBA has adopted a great program around work and lifestyle. Production for the week commences at 8pm on a Sunday night and finishes at 6am on a Saturday morning; a total of 11 No. 10 hour production shifts. Saturdays are dedicated maintenance and housekeeping. Production windows from 4pm to 8pm daily are covered by an overlapping afternoon shift which allows production to continue and negates the need for 12 hour shifts. All staff work a 5 day week on average and this is managed rigorously to achieve a work lifestyle balance and a safe tunnelling operation.


The heritage listed Boggo Jail 3D of the system of advance Alwag canopy tube installation using the Tamrock Boomer Measuring possible settlement at the Jail The Meyco Suprema preparing to spray a lattice girder WHAT ELSE BESIDES EXCAVATION?

Crack control requirements stipulated by the owner, the Queensland Government, require that the final concrete lining be reinforced, not a common requirement in Australian road tunnels. The following are all subsequent post excavation activities:
• Installation of in-floor drainage, working floor blinding and 200mm high concrete kicker curb pour – all prior to arch formwork for the concrete lining. The key decision was made to install drainage early to take pressure off subsequent civil, and M&E works
• Installation of a geofabric and PE waterproofing membrane from a custom gantry
• Steel fibre reinforced shotcrete final lining to 4 No. 20m long raised fan niches equally spaced along the tunnel alignment
• Final arch lining reinforcement installation – a unique method proposed whereby self supporting lattice girders are installed at 1800mm centres prior to 12mm and 16mm straight bar installation. This method negates the requirement to penetrate the waterproofing membrane to hold the reinforcement in place
• Form and pour 300mm (minimum) thick reinforced concrete lining. Arch formwork is currently being constructed in Malaysia by CAD2000 and due on site late September 2008. Formwork has been designed at a 9m length to match the sawtooth sections in the canopy tube excavation where the thickest concrete sections exceed 1000mm and to keep pour volumes around 100 cubes so that formwork can be turned around in a 24 hour period to achieve 5 pours per week
• Mechanical and electrical fitout by United Group working as a sub-Alliance including a 6mm/minute deluge system for fire control