Bald Hill Reserve Upgrade, Stanwell Tops – Stage 2



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Introduction

The Bald Hill Reserve upgrade is a $7.4 million improvement project of the existing Bald Hill viewing area and surrounding reserve. The project is a major investment in the local area and seeks to renew the iconic tourist and aerosport site in Stanwell Tops. It is to be completed in three stages and will include rebuilding the lower car park and installing site services, a new plaza and visitor viewing platform, footpaths, landscaping and street furniture. The project is being funded through the New South Wales Government’s Restart NSW Illawarra Infrastructure Fund and the Wollongong City Council. The full scope of the works is expected to be completed in mid-2016.

Project Scope

The contract works required the design and construction of a sewer pipeline to the new site amenities on Bald Hill. This included the installation of a horizontal directional drilling (HDD) bore from the base of the Bald Hill reserve starting in Chellow Den Avenue, through the hillside itself and exiting at the Stanwell Tops lookout point.

UEA was contracted to complete the following scope:

  • Design of HDD bore profile to accommodate 121 metres of vertical elevation difference
  • Drilling fluid design of drilling product to be used during the HDD works
  • Supply and installation of 345 metres of DN75 PN25 HDPE
  • Dewatering management plan
  • Pressure testing of installed main

Techniques Used

  • Utilisation of D100 x 120 Vermeer HDD Rig
  • Use of specialist support equipment including ultra high pressure mud pump for significant elevation bore construction
  • Engineered drilling rig thrust block and procedures to overcome unique and significant thrust pressures associated with bore design
  • Steering Engineer employed to track bore path using ParaTrack 2 wireline tracking

The Works

UEA completed the highest elevation HDD known to be completed in NSW, successfully reaching 121 metres of vertical elevation over 345 metres of horizontal distance, and with the deepest point reaching 45 metres of vertical depth. Through the skilled and professional operation of the HDD crew and with the assistance of a Steering Engineer, UEA was able to deliver the trenchless works to specification, on time and on budget to the client’s exact requirements.

Design

The design of a steep elevation bore needed to take into consideration a large range of constraints including the hydraulic design of pipeline, transverse distance, terrain, ground conditions, geotechnical data, groundwater and drilling products that would allow drilled cuttings to be removed from the bore.

The entry point located on Chellow Den Avenue is also the location of the defunct Stanwell Tops heritage listed railway line. The bore design needed to ensure that all drilling operations would not affect this asset and meet the strict and onerous Review of Environmental Factors. The culturally significant location of the railway line and its surrounding dictated the angle and location of entry, as well as the allowable placement of HDD support equipment.

UEA, in conjunction with our drilling product specialist Mi Swaco, designed a bore mud plan specifically for this project. A critical factor in this process was to ensure drilled cuttings remained suspended long enough to be transported out of the bore hole.

Site Constraints

UEA had to be inventive and resourceful whilst completing the works due to the challenging job and limited site access. Major challenges were presented throughout the project, including a detailed and lengthy design input to ensure that works could be completed within the historically significant Stanwell railway corridor.

The entry point was located on a narrow resident access road which was surrounded by environmentally sensitive fauna and land categorised as culturally significant, and the entry point was adjacent to the heritage listed Stanwell Tops disused railway tracks.

The equipment selected to complete the job, including the Vermeer D100x120 drilling rig and DFE 450 drilling fluid recycling system, had a large footprint and posed a significant challenge when mobilising to the entry point. Good pre-planning and numerous site visits, combined with community, client and council consultation, ensured all equipment was mobilised without damage to surrounding heritage or environment and minimal disturbance to residents. Mobilisation plans were created and approved to confirm the sequence and timing of deliveries to maintain existing access and activities.

Tracking the Bore

The bore was designed to transverse through Bald Hill, entering at Chellow Den Avenue, travel through the Bald Hill headland reserve, and exit at the top of the Stanwell Tops iconic lookout – 121 metres higher than the entry point. The Trenchless team needed to track the bore 345 meters through thick, steep bushland as well as reaching 121 metres of vertical elevation and a depth of 45 meters below surface level. Due to these requirements, UEA could not use conventional tracking methods alone, and employed Highside Steering Services, steering engineer specialists to utilise a combination of walk over and ParaTrack 2 wireline tracking techniques. The combination of these techniques allowed for the most cost effective, efficient and accurate means to track the bore.

Pipe Supply

Due to the length of the bore, site constraints at exit, and the need to maintain an internal bead free pipeline, a string out and weld method could not be completed for pipe assembly. As an alternative to traditional methods UEA sourced a specially manufactured 400 metre coil of PN25 75mm HDPE pipe from Perth with the aid of KenKar plastics, specifically for this project. This enabled one continuous pipe pull, more manageable site access and generally a lower risk for the project itself.