The Rosanna and Macleod branch sewers are located within the North Yarra and Rosanna SCAP, which is approximately 10 km northeast of the central business district of Melbourne. It includes a land area of approximately 13,500 hectares and includes the suburbs of Heidelberg, Ivanhoe East, Rosanna, Viewbank and Macleod.

“A hydraulic analysis using a calibrated computerised model of the North Yarra and Rosanna SCAP indicated that the majority of the Rosanna and Macleod Branch Sewers were unable to contain sewerage during extreme wet weather conditions,” Yarra Valley Water Project Manager Louie Grivas said.

These storms, with a one in five year average recurrence interval, resulted in spills from two emergency relief structures and several manholes in and around customer properties.

“The main objectives of the project were to increase the capacity of the sewer using sustainability principles and eliminate sewerage spills to the environment, and to ensure the immediate sewerage needs of the community are met while also accommodating for future growth, at least community cost.”

The benefits of reducing the volume of sewerage spills include improvements to public health, reduced community inconvenience; improved amenity associated with the discharge of diluted sewerage onto customer properties, and contamination of surface and creek waters and associated riparian vegetation.

Assessing the options

“There were two major options assessment studies conducted during the planning phase to ascertain the preferred construction option and alignment for the upgrade,” Mr Grivas said.

Several complex issues needed to be resolved as the current alignment ran underneath Rosanna Road (part of Melbourne’s highway system that carries up to 50,000 vehicles per day) and approvals for closing the road during construction would have been met with resistance from the state road authority, VicRoads. Additionally, choosing a different alignment would require sewers and access shafts to be built 20 m deep.

Some of the options considered included a larger replacement sewer to be constructed via open cut and boring techniques; upsizing the existing sewer by using a hydraulic percussion tool to expand the existing sewer line in which a new upsized pipe is pulled through; building several pump stations on a different alignment to replacement the existing sewer; and, the construction of a wet weather relieving sewer.

Yarra Valley Water chose to build the relieving sewer, retaining the existing sewer and constructing the new sewer along a different alignment by sinking shafts and boring new lines up to 20 m deep in built up residential streets. This option was similar to installing a replacement sewer, however no existing connections needed to be transferred.

Local liaison

Communication to residents was a critical component to this project since there were some significant disruptions to vehicle traffic during work hours. Yarra Valley Water said that its communication strategy consisted of speaking directly to residents prior to commencing construction, as well as a letter box drop with printed project information within the local area. An information session was held and working hours of between 7:30 am to 4:30 pm were agreed upon with residents. Portable traffic lights were erected near a local primary school for safety and after hours access was ensured through the construction zone. For more information, see Anna Grutzner article on page 22.

Boring the sewer

Construction on the relieving sewer began in June 2007 and was completed in June this year.

Mr Grivas said “The success of this project hinged on the requirement to place a laser boring machine - requiring a working footprint of 6 m by 2.5 m - 20 m deep in unpredictable ground conditions.”

However, the requirement by the residents and Council was to limit the road excavation to 3 m by 3 m. Contractor MFJ Constructions and subcontractor Pezzimenti Laserbore came up with a working solution by excavating a 2.4 m diameter shaft using a hydraulic auger. These shafts were then sleeved with 2.1 m cylindrical steel sections that also acted as the shaft supports. A further 6 m of the shaft was hand dug and ring and timber supports were set in place.

“At the bottom of the shaft two small tunnels approximately 2 m long were excavated in opposite directions, which gave enough working room for the laser bore to be set up and operate,” Mr Grivas said.

In the larger shafts, a mini excavator fitted with a remote hydraulic power pack was used as the ground became too hard to manually dig. The hydraulic power pack was situated above ground and well away from the shaft, which provided the hydraulic fluid to operate the excavator. The use of a remote power source to power the mini excavator eliminated the heat and fumes generated by a normal diesel operated mini excavator within a confined space.

The boring machine was placed into the base of the shaft and commenced the boring of 900 mm diameter tunnel. Once boring was completed 700 mm ID VC pipes were pipe jacked through the tunnel.

Problematic ground conditions

Whilst boring through mudstone, 20 m into the 95 m bore between access chamber 3 and 4, clay was encountered. This meant that the cutter head had to be changed regularly to suit the conditions.

This constant change destabilised the clay seam bringing about mini cave-ins, which were enough to impact the laser used to guide the head. The loss of laser signal decreased the accuracy of the bore, with a <50 mm deviation over entire bore length.

To rectify the problem a carrier pipe was used as a sleeve and progressively pipe jacked while the line was bored. The line was bored successfully taking an additional 47 days on top of the estimated 18 day construction program.

Excavation within built up residential streets

Due to the nature of the residential area, large earthmoving equipment could not be used. Additionally, one of Yarra Valley Water’s requirements was to minimise the footprint of the excavation and to maintain public access through the road after hours.

The solution was to use an auger to excavate a 2.5 m wide by 14 m deep shaft with a total surface area of 3 m x 3 m for each shaft. The remaining 6 m shaft was hand dug using a combination of hand held electric jackhammers for the initial 4 m and a mini excavator for the last 2 m, retrofitted with an electric power pack.

The electric mini excavator eliminated the need for CSE requirements as no toxic diesel fumes or heat were released into the shaft. Spoil removal was through a 450 mm diameter bucket, which was lowered and raised by a mini crane from the surface.

Existing curved sewer line under Rosanna Road

A curved sewer was found to be near the preferred alignment, with detailed analysis suggesting that Yarra Valley Water would have about 750 mm clearance.

When boring commenced, the cutter head stopped as it encountered steel. The existing curved sewer was found to have been hand tunnelled with steel supports inserted during construction. The solution was to hand dig another shaft with a depth of 8 m in close proximity to the obstruction and hand dig a 15 m tunnel to the obstruction. Once there, the old steel supports were removed, the cutter head re-attached and boring of the line continued.

Future project knowledge

Mr Grivas said that a number of lessons were learned while completing the relieving sewer project. These included the realisation that sound understanding and good occupational health and safety management is critical on projects such as these; and, all issues can be overcome with a ‘can do’ attitude and a collaborative team between the client, contractor and subcontractor.

In addition, Yarra Valley Water demonstrated that gravity sewers can be constructed up to 25 m deep with similar techniques in built up areas and by undertaking this option, the company achieved the project objectives at least financial and community cost, minimal disruption and low to insignificant environmental impact.

“The key learning we discovered is that old engineering techniques coupled with new technology can be applied to make a project successful,” Mr Grivas said.