The key to a successful trenchless project can usually be attributed to an array of different information, issues and undertakings. These include correct design, product, knowledge of the ground conditions, understanding the limitations of technology, comprehensive tender documentation, monitoring of the project and of course the selected contractor.

Every trenchless project must be designed and undertaken as ‘site specific’. Due to the specialised nature of the trenchless industry, it can be difficult for the client to find relevant information which allows them to correctly assess, design and undertake a trenchless project.

Recently Eire Contractors completed a challenging but successful trenchless project by involving a specialist trenchless consultant, Trenchless Advisor, to assist them with a wide range of issues including achievability in available ground conditions, additional geotechnical information requirements, design of the crossing and selecting the most suitable trenchless method.

Eire Contractors also engaged Trenchless Advisor to manage the trenchless contractor and their work site along with their work practices plus overall reporting of the trenchless project during its construction, in Redhead, south of Newcastle in New South Wales.

The project

Hunter Water Corporation requested the upgrading of the existing Redhead 1 wastewater pumping station (WWPS) including the upgrade of pumps and switchboards, demolition of the above ground portion of the existing Redhead 1 WWPS and conversion of wet well/ dry well clover leaf station to a wet well submersible station, construction of an emergency storage structure. The project included approximately 30 metres of DN2700 pipeline and the construction of approximately 1.7 kilometres of new DN300 rising main that included approximately 180 metres of trenchless crossing under Jewells Swamp.

Hunter Water noted within its tender that Jewells Swamp is identified as a coastal wetland under the provisions of State Environmental Planning Policy No. 14 (SEPP14). The organisation specified that the crossing of the wetland by the sewer rising main shall be achieved by Trenchless Technology. No excavation or construction activities shall be undertaken inside the designated wetland, or within two metres of its boundary. All practicable means must be used to prevent release of water, sediment, drilling fluid, or any other material into the wetland.

In late 2008, Eire Contractors was awarded the challenging project from Hunter Water Corporation for construction of the Redhead 1 WWPS. The original details of the trenchless portion made available to Eire had limited geotechnical information surrounding the wetlands including unknown sub-ground conditions. An indicative trenchless design was provided, which involved a DN300 PVC Class 12 pipe to be installed using Trenchless Technology on a grade of -0.5 per cent over its total distance. This approach required both an entry and exit pit at a depth of approximately 5 metres to achieve the desired depth of 3 to 3.5 metres under the swamp.

Initial geotechnical information did show variable ground conditions up to 15 metres deep. Even the geotechnical contractors were finding it difficult to get suitable soil samples due to the poor ground conditions in the boreholes.

Due to the difficulty of the project, current design and its potential environmental problems which could occur from drilling under the swamp, Eire Contractors contacted Sydney based Trenchless Advisor to help assess the current design, achievability and undertaking of the project. Trenchless Advisor undertook an initial site visit which included a meeting with Hunter Water to discuss all issues. This was followed by an in depth site appraisal including current design achievability. Trenchless Advisor had been contacted previously by a local horizontal directional drilling (HDD) contractor working with Eire Contractors to assess the possibility of undertaking the project, therefore some prior knowledge was already at hand.

After the initial site visit, design review and appraisal report, Trenchless Advisor recommended that the installation method be undertaken using HDD due to its surface launch (shallow pits) and steering capability. As the pipe was a rising main and not required to be on grade, the bore path would not be required to be at its original depth and could now be designed to be constructed in more suitable ground conditions using achievable bend radius. The only other concerns that would to be considered would be the critical buckling pressure of the product and the current pumps achievability if the pipe was installed deeper than currently designed. It was then recommended that a DN355PE100PN16 polyethylene pipe be used so as to keep in line with Hunter Water flow requirements and achievable buckling pressures.

Due to the length of the bore and current geotechnical information available, there was a probability that differing ground conditions could be encountered as construction progressed. If the bore was to be undertaken using HDD, then a more comprehensive bore plan would be required that would take into account ground conditions, entry and exit angles, achievable bend radius, achievable bore depths, amount of drilling fluid to be produced during the construction period (for drill slurry containment size facility) and machine set up requirements. A more comprehensive geotechnical investigation would also be required, along with a drilling additive program to help stabilise the borehole and minimise any potential frac-outs that could occur (a frac-out is caused when excessive drilling pressure results in drilling mud propagating toward the surface).

Environmental considerations One issue that was still a major concern for both Hunter Water and Eire Contractors was the protection of the environment. Due to the SEPP14 category, minimising the potential for a frac-out occurring was of utmost importance to Hunter Water. Trenchless Advisor approached this problem with a request for additional geotechnical boreholes to better understand the geology of the area. Unfortunately all four geological boreholes showed different ground conditions. These varied from gravel, fine silty sands, clays, shale and rock. One borehole showed no results for the first eight metres due to the water-charged ground and silty sands. These troubling conditions increased the potential of a frac-out occurring during the trenchless undertaking.

To counter the poor ground conditions, Trenchless Advisor compiled a comprehensive drill additive program including drill additive contingencies, proper work practices required by the chosen HDD contractor (reamer selection, pullback and rotation speeds) and safe boring depths to minimise any potential frac-out occurring.

After several additional reports, detailed methodology, environmental and safety requirements and a redesign to suit HDD, Eire Contractors was given the go-ahead to undertake the project using HDD. A suitable HDD contractor was then sourced with experience with variable ground conditions together with a suitable HDD machine and equipment. Trenchless Advisor was also approached by Eire Contractors to monitor the project and keep a record of day to day undertakings.

Once Eire Contractors had located a suitable HDD contractor, a construction time frame of two weeks was given and methodology put into place along with a Safe Work Method Statement (SWMS), Environment Management Plan (EMP) and contingency plans including a drilling additive program. Before any work began, a detailed meeting took place between Hunter Water, the HDD contractor, Eire Contractors and Trenchless Advisor to ensure all issues were understood and met.

Within the first few days the pilot bore was successfully drilled to the exit side with no environmental issues arising. A pump was then placed in the exit pit with a return line laid across the swamp into the main entry pit to allow the drill slurry to be collected and removed via a vacuum truck. A 320 mm outer diameter backreamer was then pulled through the borehole whilst trailing rods, in case of any ground collapse. Once complete, this process was then followed by a 500 mm outer diameter backreamer. During the reaming, process the drill additives had to be constantly modified and monitored to suit the changing ground conditions.

Over the course of the project, heavy rainfall caused minimum disruption to the project. At one stage the HDD machine broke a drive chain, which took five days to replace. During this period of time the drill rods were intermittently rotated and the borehole flooded with drilling additive to prevent any borehole collapse or rods getting stuck. The return line also became blocked towards the end of the final reamer with a new return line being sourced immediately. The product pipe was fused together and tested successfully during this period.

Completion

After 21 days of construction including machine and return line problems, the product pipe was successfully installed with minimum pressure required from the machine and no frac-outs occurring.

Eire Contractors engaged Trenchless Advisor after being recommended by the Department of Commerce as an expert in the field.

Managing Director of Eire Contractors Tom Kelliher said “We engaged them right from the start and their advice, knowledge and dedication proved the secret to a successful crossing.”

At a recent project review meeting Eire Contractors was highly commended by Hunter Water for successfully completing a challenging bore, Eire gave full thanks for this to the Trenchless Advisor team.

Directional Drilling is an area where Eire Contractors, as a contractor, has to rely heavily on advice from drilling specialists. “It was a great pleasure working with the team from Trenchless Advisor who, at all times, provided a professional approach and dedication to their work,” said Mr Kelliher.