Sunday 11th Apr, 2021

Conveyor solution for infrastructure spoil

Mobile Conveying Services was commissioned to remove spoil from the Northconnex tunnel and deliver it to a NSW quarry. ABHR learns how the company used an innovative approach to move the material.

Northconnex, a nine-kilometre tunnel linking the Hills M2 Motorway at West Pennant Hills (Sydney) to the M1 Pacific Motorway at Wahroonga, opened on 31 October 2020.

The Australian and NSW governments contributed equally to the project, which was delivered in partnership with private sector sponsors Transurban and the Westlink M7 Shareholders. The Lendlease Bouygues Joint Venture (LBJV) won the contract to design and build the tunnel.

Around 2.5 million tonnes of spoil was generated, of which around 40 per cent was used to backfill a disused quarry in Hornsby. This allows the Hornsby Shire to transform it into a 50-hectare recreational space, set to open in 2023.

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Delivery of spoil to the quarry commenced in early 2017 and the last truckload was tipped in late January 2019.

Mobile Conveying Services (MCS) first commenced discussions with LBJV on how conveyor systems could be used to deliver the spoil into the quarry in mid-2015 and it was started mobilising on site at the start of 2017.

Complete unload to place system

The system supplied was a mix of off the shelf conveyors and conveyors and related equipment either designed and built by MCS or substantially modified by them.

Key elements of the solution offered by MCS included:

  • tri-bay truck unloader with elevated control cab and interlocked light system for controlling truck movement and tipping
  • purpose-built downhill conveyor (its electric motor effectively became a generator), transferring spoil from the truck unloader to the pit
  • a telescopic radial stacker (58-metre reach), transferring spoil from the downhill conveyor into the pit
  • vibratory pan feeder for transferring material from the stockpile in the pit to the conveyors that distribute it around the pit
  • two telescopic radial stackers (46-metre reach) and two folding radial stackers (46-metre reach), all fitted with tracks for radial movement
  • an MCS crawler ‘tugger’ prime mover for moving the mobile conveyors around the site
  • a site maintenance and support facility with tools, parts storage and a 16-tonne telescopic handler for maintenance support
  • a rented house at Hornsby to accommodate site workers, who generally rotated on two-week rosters

While the energy generated by the downhill conveyor could have been used to supplement power on site, there was no suitable application and the energy was burned off by large resistors. In different circumstances, energy generated by a downhill conveyor could be harnessed for use on site. Graeme Cooney, MCS Director, said the size of resistor used at NorthConnex showed that this power, in a similar application, is significant.

Pioneering a mobile tri-bay truck unloader

MCS has been a long-term user of mobile truck unloaders in Australia, having bought both Ashross and Superior single bay unloaders (being significantly different in design, they have strengths in different applications and site lay-outs).

It has modified the Superior unloaders – initially developing hydraulic folding ramps so that the unloader does not require an assist machine to set up. It then converted a single bay unloader to a dual bay unloader to ensure continuity of flow for ship unloading.

While a dual bay truck unloader would have been theoretically capable of handling the required peak throughput of the NorthConnex project, there was potential for oversize material to be present in some loads, making it necessary to install grizzly screens in the tip bays to avoid damage to the belt. With some spoil having high moisture content, there was potential for the spoil to bridge over the grid and block the feed.

Due to these factors, MCS elected to develop a tri-bay unloader so that, in the event of a blockage, one bay could be closed while the blockage was cleared while the remaining bays stayed open. An excavator or loader was located at the unloader to assist with speedy clearing of blockages.

Other innovations in this machine were an enclosed operator control cabin in an elevated position and an interlocking light system to control truck movements and tipping as they passed through the unloader. Concrete barriers were used to delineate the bays and prevent trucks from damaging the equipment.

Given the need to maintain placement schedules to clear tunnel spoil, a dual bay truck unloader was parked on site to cover any extended downtime of the main unloader.

Options for pit transfers

MCS offered a gravity-fed stockpile reclaimer as an option for loading the transfer conveyor in the pit with spoil dropped from the surface. The contractor opted to use a vibratory pan feeder fed by an excavator to load spoil onto the conveyor.

Cooney says the workability of the stockpile reclaimer was proven subsequently on a WA mining project where it was part of a conveyor system supplied by MCS to replace short haul with a dump truck and excavator. The conveyor solution cut the per-tonne haulage cost to a third of its previous level.

The combination of a folding conveyor and a telescopic conveyor meant that the opposite side of the pit to where material was transferred from the surface could be reached.

Because of the problems with oversize material and high moisture material, trucks coming from sites with known problems were directed to a stockpile on the surface to minimise disruption to the truck unloader. The stockpiled material was transferred into the pit by truck.

MCS Director Graeme Cooney believes that, if these are known in advance, an alternative of dumping all material on the surface and running it through a sizer and then into the downhill conveyor should be considered as an alternative to the tri-bay truck unloader (with an over-belt magnet if steel contamination is also an issue).