From the magazine

A host of technologies LinQing up Queensland

So far the Gold Coast light rail project, GoldLinQ, one of Australia’s biggest public transport projects and the state’s first light rail system, has applied a range of trenchless techniques.

These have spanned from horizontal directional drilling (HDD), thrust boring and microtunnelling – all required to upgrade and relocate underground track services for the project, which is anticipated to commence operation under “÷G:link’ in mid-2014.

A trenchless approach

Trenchless Technology has been imperative during the works in order to construct and relocate utilities, including watermains, sewers, electrical and telecommunication conduits.

The majority of utility relocations would not have been possible without the use of Trenchless Technology methods, making them vital for the successful delivery of the Gold Coast light rail project.

In total, trenchless techniques have been implemented across 2,385 m of the project.

The application of trenchless techniques meant disruption to busy intersections was significantly minimised, allowing additional works in the area to be carried out.

HDD was chosen a number of times during the project to assist when crossing waterways, while the use of microtunnelling proved to be the best option when the project was faced with obstacles such as reducing the need to remove a substantial amount of landfill during the construction process.

Design considerations

Design considerations took into account existing services surrounding the work area as well as how future works in the areas would be affected.

In some cases, the amount of existing service utilities required multiple relocations just to allow for several components of the construction process to be carried out.

Geotechnical testing was applied, and results indicated that in most areas the ground was expected to be composed of sand and gravel, additional geotechnical testing was also conducted in the middle of a waterway to ensure the ground composition was consistent.

Materials paving the way

There have been various materials used cross the vast GoldlinQ project; the longest thrust bore came in at a staggering 78 m in length to install a DN510 mild steel pipe, used in the middle of the Surfers Paradise precinct.

The longest HDD bore pipe used during the works was 310 m and was applied directly under the Nerang River.

The biggest bored pipe used was an 8 x PE 140 pipe that allowed for 95 m of electrical conduits to travel underneath a canal.

One of the microtunnel bores was 140 m in length; the longest microtunnel bore used within the project.

All the materials used via trenchless methods allowed for the works to be completed seamlessly and with minimal public disruption.

Trenchless by the sea

The sand in Surfers Paradise is fine, clean dune sand, which may sound like paradise but unfortunately poses a steering challenge for HDD drilling heads, causing friction when jacking casing pipes through the sand material.

The installation of the drilling entry and exit pits was often more difficult than the drilling operations due to the limited space caused by the surrounding urban environment and the existing utilities.

The constraints of working across diverse environments varied across the project due to the proximity of contrasting locations.

Construction workers could one day be working on the side of the highway and in the heart of Surfers Paradise the next.

The majority of the works were carried out in balmy Queensland summer months and the heavy rains during this season affected the de-watering process, with some spears often required to run 24 hours a day.

From the outset, the Gold Coast light rail project set high standards in regards to waste.

The bentonite mud was transported to an external location due to the material’s possible toxicity and there was constant testing on-site for acidic sulphate soils.

The project also implements strict sediment control measures.

Challenges when LinQing with HDD

During the project, GoldLinQ came across an obstacle when one HDD pilot and reams were taking longer than expected.

A second HDD rig was introduced to work from the opposing end, ensuring the progress wouldn’t be delayed (one push, one pull-back method).

The project’s design plan incorporated both vertical and horizontal curves that required, in some instances, a steering engineer to ensure that the pipe was not diving down or entering the grounds too shallow.

Another challenge for the project involved some difficult ground pressure that was encountered, requiring continual boring over a 24 hour period to break through.

HDD gets its fluids

The project applied the bentonite drilling fluid during the HDD, assisting with lubrication of the pilot head and reamers.

In addition, this drilling fluid allowed for the cooling of HDD equipment and prevented wearing down of the equipment.

The bentonite fluid also was pumped back and forth between retrieval and launch pits to prevent a trench collapse, which alongside any frac-outs, successfully did not occur during the works.

Business as usual for Gold Coast

The largest challenge for GoldLinQ during this project has been the logistics surrounding management of traffic, pedestrians and work noise.

Traffic control was put in place to allow for the recycle units, excavators and boring equipment to be delivered to the site.

GoldLinQ ensured that the excavation was managed appropriately to avoid significant disruption to the urban environment by allowing for the excavation of optimal sized trenches.

The typical size of the trench was between 5-7 m long x 3 m wide and up to 2 m deep, in addition, the shoring boxes were sunk into all launch pits.

A combination of steel (enveloper pipe), PVC and PE (product pipes) materials were used during the works.

The enveloper pipe was selected as both the jacking pipes and protection pipes underneath the track slab.

The steel pipes were made of mild steel (uncoated and unlined) and the annulus between the enveloper and carrier pipes were grouted to prevent voids in the sub grade.

With care taken to ensure sandy soils did not turn acidic, spoil was tested regularly to verify it had not turned into an acid sulphate soil.

If it was identified as acid sulphate the soil was treated and removed safely from site.

Traffic management was required in some cases during the works to allow enough space to string out the pipe.

During this project it became apparent that there was significant less disruption to the community in comparison to if there had been large, open trenches used.

Traffic and construction management was easier for the construction teams as opposed to the measures required when large open trenches were involved.

This allowed for cost efficiencies to be achieved across the project by GoldLinQ, especially when it came down to logistics.

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