From the magazine, HDD

Knowledge equals success

The installation was required as the existing overhead cables were damaged by a passing yacht as it made its way up the Shoalhaven River system. The proposed crossing ran from the mainland, under the river and exited on an island.

This project was originally flagged by the client’s environmental department as high risk due to potential frac-outs occurring during the drilling process. The crossing required the installation of 2×125 mm internal diameter conduits. One conduit would house the initial installation of a 240 sq mm, three core AL XLPE, 11 kV underground cable and the other would remain a spare for possible future works and in case of emergency repair.

Sydney-based horizontal directional drilling (HDD) specialist, Trenchless Advisor, was contracted to provide a Trenchless Feasibility and Concept Design report for the proposed underbore section of the project. The feasibility and concept design report for the installation was to take into account geotechnical investigation results, survey, existing utilities, risks and impacts to the environment, achievable construction methods. The designs would also provide practical and achievable technical specifications to suit the client’s cabling requirements.

Before any feasibility or design could be undertaken, Trenchless Advisor sourced and managed a suitable geotechnical contractor (Geotechnique) to drill two geotechnical boreholes on either side of the river, which were needed to give an indication of the subsurface conditions that would be encountered during construction. The results of the survey and geotechnical investigation were studied and used to compile a detailed report to assess the risks, construction method, expected ground conditions, proposed bore depths and conduit suitability for the underbore.

Field work for the geotechnical investigation was carried out on 29 and 30 August 2012. This consisted of drilling two boreholes (BH1 and BH2) using a truck mounted drilling rig fully equipped for geotechnical investigation. Both boreholes were drilled to V-bit refusal in possible gravel bed at approximately 35 m from existing ground surface. Standard Penetration Tests (SPT) were conducted at regular depth intervals and disturbed soil samples were recovered for visual classification. Groundwater levels in the boreholes were also measured.

Trenchless Advisor proposed that the installation of the electrical conduits undertake a profile to maximise the depth below the channel to maintain sufficient overburden pressure reducing the risk of frac-out whilst not encountering the bedrock which may add additional time and costs to the project.

An existing utility search was conducted along the proposed route and surrounding area, indicating the presence of both electrical and communication cables. The existing electrical was currently overhead and no underground electrical cables were present. Telstra plans indicated a submarine cable crossing the waterway from the mainland to the island, although this was away from the proposed alignment. It was noted however, that there would be a land-based crossing of the existing Telstra cable on either side of the river.

During a site visit it was observed that the location indicated on the Dial Before You Dig (DBYD) plans appeared to be different to the location of a marker visible on-site. This was seen as a risk and noted in the report although, as significant depth of cover was included in the underbore design at these locations, minimum risk of a conflict during construction was expected. Before any ground engaging was undertaken, the chosen contractor was required to pothole the communication cable at these locations to confirm depth and alignment.

As Shoalhaven Water did not subscribe to DBYD, initial contact was made and a water and sewer plan was provided that showed an existing watermain in the area. The watermain was shown as being on the northern side of the road and on the mainland side only. This was not seen as a risk due to the bore being proposed on the southern-side of the road.

Once all the data was gathered and reviewed, the feasibility and concept design, including a computer generated bore profile and downhole calculations, was compiled using practical entry and exit angles, achievable bend radius and safe boring depths under the river. A detailed risk assessment was completed on the proposed bore’s route with a list of risks and contingency noted. The bore was to cross the river no shallower than 11 m under the riverbed with a total bore length of 277 m. The borehole size was calculated at 450 mm to easily accommodate the required conduits. The feasibility also stated that the minimum HDD machine required for this project was a 40,000 lbs rig in conjunction with a wireline system to accurately track the drillhead.

Once the feasibility and concept design was completed and reviewed by the client, the project was tendered for construction with the successful drilling company selected as Rightline Plumbing and Civil Excavations.

Works commence

Rightline commenced works on 22 May 2013 with the inclusion of specialist steering engineers Highside Drilling Services being engaged to ensure the pilot bore was accurate and as per the client’s requirements. The chosen HDD rig was the American Auger DD6 (60,000 lbs). The exit point for this bore was critical as it was located 2 m off the main road and if not precisely on target the risk of over shooting by a further 2 m would have seen it exit into private property. The pilot bore was completed after only three days. A wire line steering tool was used to ensure that the bend radius for the pipes remained within manufacturer tolerances and that the pipe was installed within the narrow pipeline easement. No additional issues were encountered during the pilot bore as Trenchless Advisors’ report was both relevant and concise. This enabled the correct Baroid drill additives to be programmed and on-hand as required. The mud program allowed for environmental contingencies due to the sensitive nature of the works crossing the river.

Reaming commenced on 27 May 2013 and it was decided due to the changing sandy gravel and clay bands that rods were to be trailed. At no stage during the reaming phases did Rightline encounter lost flow, with all cuttings returning to the exit pit consistently. Due to the limited access at the exit side, a punt was used to ferry both materials and plant to the other side. The local punt was identified in the Trenchless Advisor design as only capable of a 20 tonne max payload. This restriction inhibited filling the vacuum tankers transporting the cuttings back to the launch side to their normal operating weight and all scales on the vehicles were calibrated prior to commencement to ensure weight limits were not exceeded.

The conduit installation commenced early on the morning of 30 May 2013 and was completed by midday. A testament to the Rightline crew’s drilling practices and borehole conditioning was the minimum pull back pressure required during the entire installation process. The project was a complete success from start to finish and highlighted the effectiveness of undertaking an independent, detailed feasibility and concept design before going to tender. This allowed the client to understand all the risks and issues with its construction whilst still having the ability to change aspects of the design to better suit their requirements before any contractor was on-site.

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