Prestons Pressure Sewer

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Project Overview: New Horizontal Civil Infrastructure: Gravity Wastewater Installation, Parklands West, Christchurch. Project subtasks.

Fitting a steel 90° “elbow” bend in the main sewage line at over 4m depth in very challenging, saturated sandy ground, and within 2m of an existing operating wastewater pipeline. This ingenious back-to-front drilling method both keeps the entry pit dry, and yet also allows pulling both ends of the 350mm Wastewater line through the steel walls of the pit for subsequent welding to the right-angle bend—all without damaging the new pipe, or disturbing the existing infrastructure.


Client: City Care

Principal Stakeholder: Ngai Tahu Property

Duration: Sept–Dec 2014

 

3D Google map infographic of HDD drillshot, corner Rothesay Rd, Putake Dr, Parklands, Christchurch, New Zealand

 


Prestons Pressure Sewer

 

The soils of eastern Christchurch are predominantly silty sands and contain a very high water table – approx. 1 -2 metres from surface.  Truline Civil Ltd were engaged to install a 350mm pressure main for approximately 1 km in Rothersay Rd and Pukake Dr to pipe the sewer from the new Prestons Subdivision downstream to a point where it could be connected to the existing eastern feeder main to the Bromley treatment plant.

This project was necessitated by the urban extension and development of greenfield sites within Christchurch city, post-earthquake.  The development in question is the Prestons urban residential subdivision, both sides of Prestons Rd and the largest urban addition to the city to date.  Developer Ngai Tahu with partners CDL Land NZ, and Foodstuffs, have created an environment encompassing 2600 additional homes and 8000 new residents.  However the area was well under capacity for the conveyance of sewer to the city’s treatment facility at Bromley requiring this new section of pipeline.  The only realistic corridor was through the new subdivision at Tumara Park via Rothesay Rd and Putake Drive.  Tumara Park is a relatively recent high quality urban development and trenching through this pristine developed environment was not a realistic option.  Trenchless Directional Drilling was identified as a good option due to its low reinstatement impact.

The pipe size was 350mm dia laid to a depth of 3.5m to 5 m in free-flowing running saturated sands.

Initial location of existing services was undertaken using Ground Penetrating Radar and utility plans.  These services were physically located by vacuum excavation to determine exact depth and position, which also confirmed design parameters for lay route of new pipeline.

Drill entry and receiving pits all required wellpoint dewatering, excavation and shoring.  Pressure pipe “strings” were welded together from 18 m lengths giving section install lengths of 108 m. Pipeline was drilled and installed to a gradient of 1:500 requiring critical care in the drilling process. 

The corner of Rothersay and Putake required a 90 deg bend, 4m deep, immediately adjacent to the live 400mm Belfast sewer main.  This required sheetpiling the excavation, full wellpoint dewatering, and localised pumping of the excavation through settlement tanks.

 

Directional drilling was undertaken away from the sheetpiled excavation in both directions, with a penetration through the wall allowing entry of the drill head, and pullback of the pipeline.  Following pilot drilling to grade, an oversize backreamer is fitted – the reaming and pullback of the pipeline take place simultaneously.  The back reamer displaces the additional 225 mm diameter of the borehole as cuttings.  The ability to suspend the cuttings and flow them out the drill hole as a sediment laden slurry is important.  The Drillflex fluid system is designed to create a “tunnel wall” preventing the sand from continually collapsing the drill hole and allowing pullback installation of the pipeline.

Pipeline install was completed to a good standard, within specs, on time, and within budget.

Download the print ready pdf of the Parklands Case Study.

  

 

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TruLine Civil is determined to incrementally improve the service we deliver our clients, and the quality and range of civil engineering services that we offer to owners and stakeholders who expect excellence.

Excellence in Infrastructure.

TruLine Civil.

Interested in our services? You can contact us +64 (21) 228-5090 and ask for Geoff Powell or e-mail me at geoff@trulinecivil.com 

Christchurch Address 

Greywacke Road extension,
Harewood, 
Christchurch, 
New Zealand 


 

Credits: Photos, infographic and case study documentation by Shaun Waugh www.magentadotbrands.com


 

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Prestons Pressure Sewer

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