From the magazine, Microtunnelling, Tunnelling

Boring the urban environment

The project is part of Jemena Asset Management’s ongoing upgrade of the high pressure natural gas system in Sydney and involves the construction of approximately 2,500 metres of 200 mm diameter secondary pressure steel main to provide capacity for growth and security of supply to Sydney’s eastern suburbs.

The X42 steel pipeline has a wall thickness of 6.4 mm and operates at a 1,050 kPa maximum allowable operating pressure.

Connection points for the pipeline include a 200 mm diameter connection on the southern side of Oxford Street and a 150 mm diameter connection at Marathon Road, Darling Point, between Marathon Lane and Adelaide Street.

The project differed from others in terms of the location of the works and the ground conditions in which the pipe was being installed. The pipeline route went from Centennial Park to Darling Point near Rushcutters Bay and was predominantly located within the road carriageway in residential and commercial areas. This route location – coupled with varying ground conditions of sandstone, ironstone rock and sand – posed challenges for the project.

Road crossings

The project involved two major road crossings – across Oxford Street and New South Head Road.

After investigation, it was decided that case boring would be the best method for the Oxford Street crossing and before the commencement of works, a detailed services search and location exercise was carried out at night to locate and identify all existing services in the path.

A bore hole was constructed over three days in Victoria Avenue for the approximately 55 metres crossing through solid sandstone rock.

The crossing at New South Head Road was required to be open cut due to the restricted space for a bore hole and the amount of existing services located within the area. These works were carried out overnight and required a total of four vertical alignment changes on the pipe to avoid the existing services.

Codmah purchased its first Vermeer trencher, the T800B, sixteen years ago, and now owns and operates the T800B rock trencher, a T755 rock trencher and three 24×40 horizontal directional drilling rigs.

Mr Saunders says “We have an ongoing relationship with Vermeer, with both sales and service, which we believe has benefited both parties.”

Route selection in an urban area

Codmah says that selecting a pipeline route with the least impact on community and traffic was challenging. However, the main challenge lay in the ongoing design of the pipeline to suit the quantity of existing services along the route.

“Existing utilities are always a challenge within built up areas and this was certainly no exception,” says Mr Saunders.

“High and medium pressure natural gas pipelines, water mains and services, sewer mains and connections, stormwater, communications and electricity – both above and below ground – were all present along the route.”

Mr Saunders explains that the number of crossings required was double to triple that of other projects in built up areas due to the average length of house frontage to the street being 4-5 metres wide. This meant that there were two to three connections every 5 metres.

Ground conditions

The ground conditions on the project were challenging, as rock encountered was mainly sandstone and ironstone, requiring large machinery in small residential streets to trench, break, rip and cut the rock for removal.

“We encountered a surprise on Mona Street when we encountered fine sand under the road pavement and down to the trench base This caused the trench to be continually widening out as the sand walls collapsed and threatened to undermine the roadway,” says Mr Saunders.

“Sheets of ply were used to drive into the trench sides and help minimise the subsidence to allow works to continue.”

A job well done

The Paddington pipeline project was varied and required different methods of pipeline installation to successfully construct in the urban environment.

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