Drill & Blast

Legacy Way: Solving a materials handling challenge

In the early 2000s, the TransApex project underground tunnel proposed by then aspiring lord mayor (and now Queensland Premier) Campbell Newman was born out of the need for improved traffic infrastructure in Brisbane and the conceptual designs of forward thinking engineers and planners.

The third project in an integrated series of five planned underground tunnel and surface road systems, the Legacy Way Project, following the Clem 7 and Airport Link tunnel projects, commenced planning in 2010 and the tunnel is expected to be opened in 2015.

{{image3-a:r-w:300}}At the early stages of planning, it was indicated by Mr Newman that road truck transport for the disposal of tunnel boring machine (TBM) spoil was not an option and that the spoil would be placed in Mount Coot-tha Quarry. This option would eliminate the requirement for 96,000 truck trips to transport the material to landfill sites, a major benefit to the local community and to the environment.
The Mount Coot-tha hornfels quarry is owned and operated by Brisbane City Council (BCC). It has been operational for more than 80 years and is the closest operating quarry to the Brisbane central business district (CBD), being located less than 4km away. The majority of the quarried product is used in BCC asphalt plants for council road construction and maintenance.

The quarry boundary, flanked by vegetation buffers to the Botanical Gardens, state forest and the access road to Mount Coot-tha Lookout, has a footprint of less than 20ha, with about 5ha of extraction pit area. Current production rates are limited to 410,000 tonnes per annum with a projected life span to 2033.

To locate the TBM spoil in the quarry, Mount Coot-tha Quarry manager Robert Bell commissioned Ausrocks Consulting to complete a life of quarry plan for Mount Coot-tha, in conjunction with a resource drilling program. Under the direction of the BCC?s Major Infrastructure Projects Organisation (MIPO), this plan was now to be extensively modified by Ausrocks to take into account the placement of 2.7 metric tonnes or one million bench cubic metres (Mbcm) of TBM spoil over a period of 12 months as the two parallel TBMs, Annabell (TBM 1) and Joyce (TBM 2), burrowed their 12.4m diameter tunnels 4.2km from the Western Freeway at the Mount Coot-tha Botanical Gardens to the Inner City Bypass at Herston.

There were a number of options to get the TBM spoil from the Legacy Way portal to the quarry, including an overland conveyor system through the Botanical Gardens and a conveyor tunnel directly beneath the Botanical Gardens to enter the quarry at 30mRL, directly below the access ramp.

In 2011, the winning tenders for Legacy Way were the Transcity team, consisting of Australian civil contractor BMD, Italian construction company Ghella and Spanish tunneller Acciona. Transcity opted to develop, by drill and blast methods, a 530m long, nominally 4m x 4m conveyor tunnel to transport the TBM spoil from the Legacy Way western portal, under the Botanical Gardens to a fixed radial stacker located at the 30mRL portal for distribution via excavator and truck to the southern end of the quarry, commencing at 0mRL and filling up to above 70mRL.

From August 2011 onwards, Bell had to manage a complex construction and material placement program within the quarry, as well as the normal quarry operations. There were a number of issues that had to be handled by Bell and the Legacy Way team:

  • Developing the 530m conveyor tunnel from the quarry and the construction of the radial stacker in the quarry with associated logistics, traffic and health and safety issues.
  • Constructing the radial stacker and tunnel conveying system to link into the conveying systems in the two Legacy Way tunnels. The TBMs operated at 20 hours per day and the performance statistics are as follows:
  • Maximum advance in a 24 hour period: 24 rings or 49.58m advance.
  • Average daily advance from start up to 14 June 2013: 29.41m per day combined performance.
  • Best seven day performance: 253.8m (82.000t per week, equivalent to 4Mt per annum).
  • Best combined performance (when two TBMs operating): 81.7m per day.
  • Developing a new water storage sump of 21 megalitre capacity in the floor of the quarry prior to TBM spoil placement.
  • Organising a reversal of the conveying system after the TBM breakthroughs are complete to allow backfilling of the tunnel inverts with selected screened TBM spoil.
  • Scheduling quarry blasts so there was no interference or fly rock damage to the Legacy Way infrastructure in the quarry.
  • Ensuring minimum disturbance to the local residents in terms of noise, vibration and dust emissions.

A comparison of the specifications of the three TransApex tunnels constructed to date that relate to materials handling is shown in Table 1. Innovative use of conveyors to transport TBM spoil off-site at the Airport Link project reduced road truck spoil movements by 80,000.

{{image5-a:r-w:200}}{{image4-a:r-w:200}}TBM SPOIL PLACEMENT
Placement of the TBM spoil from the Legacy Way tunnel commenced in the fourth quarter of 2012. Two TBM tunnels have been driven in parallel, producing 2.7Mt of spoil. The material cut by the TBMs consists mainly of Bunya phyllite rock units (phyllites, hornfels and greenstones) but towards the eastern portal the harder Neranleigh Fernvale rock units (argillites and arenites) are intersected. The current location of the spoil placement at the southwestern end of the quarry has allowed both quarrying and spoil placement operations at 15m high benches with 15m wide berms and roadways to be carried out concurrently, using an excavator and rigid trucks (Komatsu HD465).

The spoil is being placed from 0mRL to 70mRL based on the expected compaction factor giving a compacted density of about 2.1t/m3 with a California bearing ratio generally in excess of 100. In the first stage of spoil placement, it was required that both spoil placement operations and quarry trucks share a common access ramp to the current base of the quarry at 15mRL. As part of this stage a ramp from 30mRL was built and compacted to haul road specifications to serve as the main access to 30mRL and 15mRL for quarry trucks. The current access from the existing main haul route to 45mRL at the southwest of the pit will be maintained as the main quarry access but another connecting ramp from 45mRL to 30mRL will be built to serve as the main access to 30mRL.

{{image6-a:l-w:200}}The ramp is running in an east to west direction of the quarry adjacent to the new sump. The current access from the existing main haul route to 45mRL at the southwest of the pit will be maintained. Access to the 60mRL and 70mRL on the spoil placement area is a ramp from 45mRL along the face of the spoil profile to serve as major access to the top of the profile for maintenance at the end of the operation. Figures 1 and 2 show the Mt Coot-tha Quarry in May 2011 prior to relocation of the sump and the quarry in May 2013, and in May 2013 towards the end of the TBM spoil placement, scheduled for completion in June 2013. Figure 3 shows a southwestern view into the TBM spoil pile and shows the ramp constructed in the spoil from 15mRL to 70mRL.

The success to date of the integrated spoil placement of TBM spoil from the Legacy Way project into Mount Coot-tha Quarry has been based on the fact that the materials handling process of the tunnel and the quarry has progressed without major delays for the duration of the TBM project. This is in spite of the TBMs finishing the excavation process in 10 months rather than the original schedule of 18 months. The added benefit was the removal of 96,000 truck trips for disposal of the TBM spoil elsewhere.

{{image7-a:r-w:200}}Ausrocks has estimated that the conveyor waste disposal process at Mt Coot-tha has resulted in a considerable saving over truck transport, such that using a 3km long conveyor tunnel would be cost-effective over truck transport.
This means that the TBM portal can be up to 3km from the quarry or even further if surface conveyors can be used. Another suggestion from Transcity is that the portal for the proposed Brisbane Cross River Rail tunnel be located near the Brisbane River at Yeerongpilly and that the tunnel spoil be used to raise the level of potential flood areas, eg spoil from twin tunnels at 8m diameter could raise a 50ha flood plain by 1m. Future planning for tunnels should consider the location of adjacent quarries or suitable areas that can accept the TBM spoil as fill or, if suitable, for processing into construction materials.

The integration of the Legacy Way spoil placement at Mount Coot-tha quarry is truly a legacy that may become a benchmark for future projects associated with infrastructure development and the preservation of reusable materials. In many countries, including Australia, the recycling of construction/quarrying materials has proven to be practical and economical, particularly for roadbase blending and in paving construction works. ?

Adebayo Bayooke and Alan Robertson are the respective senior mining engineer senior and director of Ausrocks Pty Ltd. Robert Bell is the quarry manager of Mount Coot-tha Quarry.

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