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 m 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.

Construction on the pipeline commenced in October 2008 and the project was commissioned in early April this year, in line with the expected completion date.

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.

T755 to the rescue

For the Paddington project, Codmah chose to use Vermeer’s T755 trencher.

Codmah Construction Manager Andy Saunders says “The T755 was our original method for carrying out the excavation in the rock areas, but after a detailed services search was carried out, it was deemed only efficient to use the trencher in areas where the quantity and frequency of services was above one service per ten metres of trench.

“In these particular areas, the trencher doubled the production rate and reduced the trench widths and therefore the overall quantities of spoil removal, imported backfill material and restorations.”

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 24x40 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 m wide. This meant that there were two to three connections every 5 m.

Storage

Mr Saunders says that a site compound storage area for the construction of the pipeline was not able to be sourced.

“All materials for the project had to be taken directly to the site from the supplier, all excavated material was required to be removed to licensed facilities straight from the excavation, and all equipment was required to be kept onsite at all times.”

Mr Saunders says that in order to overcome this challenge, a 10 m long x 3 m wide x 2.5 m deep cased bore hole was utilised to temporarily hold excess materials that had be excavated.

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.”

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 hold was constructed over three days in Victoria Avenue for the approximately 55 m 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.

A job well done

The Paddington pipeline project was varied and required different methods of pipeline installation.

In the areas where trenching was required, Mr Saunders says that utilising the Vermeer rock trencher had several advantages over conventional rock excavation methods. These included:

• Speed and efficiency of trench excavation;
• Lack of vibration caused by hammering;
• Lack of noise caused by hammering;
• Minimising trench width resulting in less permanent restorations of sealed surfaces; and,
• Direct loading of trucks with excavated material via a machine mounted conveyor.

Mr Saunders says that Codmah has successfully used both the T755 and T800B \trenchers on many urban projects such as the Sydney Primary Loop, Launceston Gas Feeder Main, Cremorne to Balgowlah Pipeline, Sydney West Reinforcement Project Stage 1 and 2, as well as a number of projects in the ACT.