Acquiring approval for trenchless works

A recent project undertaken in Sydney’s west required a new underground high voltage 11 kV feeder to run from a new substation, through a residential area and under an existing rail corridor before connecting to an existing high voltage overhead mains network. The project will redistribute heavy load demands currently faced in the area on all existing 11 kV feeders, and improve overall network reliability to all customers.

Due to the complexity of the project and the approval requirements, including the safety aspects, the client decided to get some expert advice from Sydney-based consultancy firm Trenchless Advisor. Onsite discussions were held and several key requirements were decided upon. These included a detailed trenchless feasibility and concept design report; advice on methodology; technical drawings and other required documentation; and assistance in the submission to the rail authority for approval of the design prior to engaging a contractor to undertake the works.

This information was needed to outline the most effective and appropriate trenchless method for this part of the project. Trenchless Advisor was also tasked with sourcing, pricing and managing the appropriate sub-contractors to undertake geotechnical investigations, surveys and existing utility location investigation, all of which were required for the gathering of vital information needed for the feasibility, concept design and rail authority submission.

Before any works were undertaken, a timeline for seeking rail approval and even an approximate start date for construction was determined between the client and Trenchless Advisor. As with most projects, there was very little time available to acquire the correct approval from the rail authority before the project was to be tendered and construction was to begin.

Only seven months were allocated for Trenchless Advisor to acquire “÷approval in principle’ for the project. This timeline had to include completion of all sub-contractors’ works and their reports had to be supplied to Trenchless Advisor. The feasibility and concept design, technical drawings and track-monitoring-plan then had to be compiled, allowing sufficient time within the timeline for the appropriate engineers and personnel from the rail authority to review the information and provide Approval in Principle. This would allow the client to move forward with the tender documents and selection of a contractor to undertake the works.

The proposed crossing was to include one underbore of 450 mm to install four 140 mm diameter conduits and two 63 mm conduits. One conduit will house an 11 kV underground cable and the others will remain spares for possible future works and in case of emergency repair. The feasibility and concept design report took into account geotechnical investigation results, existing utilities, risks and impacts, construction method and technical specifications along with the current rail authority, client requirements and Australian Standards.

Like any project, a few obstacles were encountered along the way. These included the creek that weaved its way through the rail corridor, and the several large and small drains guiding the water flow from the surrounding areas. The drains in particular were a point of concern, as they limited the exit location for the proposed bore and dictated the depth at which the bore was to be situated.

To begin the required works, Trenchless Advisor organised for the survey and geotechnical investigation to be conducted, including a borehole on either side of the rail corridor to give an indication of the subsurface conditions. The information gathered during the survey and geotechnical investigation was vital for accurately assessing the chosen method and depths required for the underbore. It also helped predict a possible settlement within the rail corridor, and allowed a track-monitoring plan to be followed during construction. A detailed existing utility search was also undertaken, allowing a true mapping of all assets in the area.

Once all the necessary information was collected, a preliminary investigation began, including compiling an electronic bore plan to ensure the bore’s achievability. The information used to compile the electronic bore plan consisted of a wide range of data including expected ground conditions, existing utility location, preferred entry and exit sites as specified by the client, and specifications and requirements put forward by the rail authority. Other information also important to the bore plan design included appropriate Australian Standards, HDD machine requirements, bend radius, bore depths and product specifications, to ensure a safe and practical design was achieved.

The results of the preliminary investigation showed that HDD was the most effective and appropriate method for the project. The subsequent investigation and profile indicated a borehole that could be constructed within the silty clay ground conditions from relatively shallow entry and exit pits with minimal chance of settlement. It was also considered the safest method available when crossing the rail corridor, and would cause less impact to the surrounding area and flora. The predicted ground settlement, based on the ground conditions, depth below rail and method of installation, were shown as being limited to approximately 1 mm.

HDD was chosen to install the conduits due to the distance of the underbore, the existing ground conditions, the depth required under rail, and the client’s requirements. An in-depth assessment showed that the underbores could be achieved with minimal risks. HDD has the ability to both start and finish at shallow depths whilst maximising the achievable depth under rail, limiting the risks during construction.

In a review of the proposed conduit requirements, the data shows that due to the depth of the underbore, the method of installation, and the expected loads, the PN10 140 and 63 mm diameter PE conduit was safe for use. A safety factor was built into the design specifications to ensure a minimum of 100 years life is achieved with the product. The product was also preliminarily assessed against the effects of electrolysis and vibration with the expectation that any possible affects would be low.

All information compiled by Trenchless Advisor was then collated and sent to the client for submission to the rail authority. Contact was kept between all parties during the submission to ensure any issues, if they arose, would be quickly dealt within. In early June, five months into the works, an approval in principle from the rail authority was received by the client. This fell well within the allotted time schedule and provided the client additional time to compile the required tender documentation and source a contractor.

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