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HDD - Akaroa Harbour Water Supply Reticulation Upgrade


New Horizontal Civil Infrastructure: Akaroa Water Supply Stage 3, Reticulation Upgrade.

Client: Hawkins Infrastructure

Principal Stakeholder: Christchurch City Council

Duration: March–August 2014

Project Engineer (Hawkins): Ben Whitham

Project Supervisor (Tru-Line): James Robinson

Consultant Engineer: David Carshalton of Beca (Beca Carter Hollings & Ferner)

HDD Operator (Tru-Line): Grant Senior

HDD Operator (Tru-Line): Liam Richardson

Akaroa Water Supply: System design abstract and project overview

A new water treatment plant at L'Aube Hill was completed in 2015, and this provides a water supply for Akaroa and Takamatua which meets the requirements of the Drinking Water Standards for New Zealand.

Prior to the upgrade water for the Akaroa water supply came from the catchment and wells in just one valley. The new scheme takes in water from three valley catchments that include four streams (Aylmers, Grehan, Balguerie and Takamatua) and two wells, one at Settlers Hill Road (138 metres deep) and one at Aylmers Valley (41 metres deep), which all needed to be integrated into a single water supply system.

The treatment process includes coagulation, flocculation, membrane filtration, chlorination and pH correction. The new treatment plant replaces three treatment plants, which were at L'Aube Hill, Aylmers Valley and Takamatua.

The installation required precise location of all existing services including; gravity sewers, gravity stormwaters, power cables and telecommumications cables. Installation involved a wide variety of terrain from flat urban streets of the town to steep inner harbour valley slopes, both urban and rural. Directional drilling and installation of the pipelines were undertaken to a good standard and without major incident. The project was completed on time and to budget to the satisfaction of the client stakeholder, Christchurch City Council.

For Hawkins Infrastructure Tru-line constructed new pipelines to connect Takamatua to the Akaroa system, to bring untreated water from Takamatua Stream to the treatment plant, and to pipe treated water back again to Takamatua. A new treated water reservoir was constructed t on Old Coach Road at the highest point of the pipeline to provide storage for Takamatua. There are two other main treated water reservoirs, one at the treatment plant site on L'Aube Hill and the other at the former treatment plant site at Aylmers Valley. There are also four smaller reservoirs. As well as storing treated water, these reservoirs supply pressure for the network and most properties in Akaroa are supplied by gravity from these reservoirs. In addition, there are four small pump stations to provide water for properties higher up the hill. The installation of the new system was completed by pressure testing and chlorination of the pipeline before bringing it into commission.

The Akaroa water supply scheme provides water to 1,095 properties in Akaroa and 132 properties in Takamatua.

Pipeline installation overview

The new system required the installation of approximately 5km of H.D.P.E. pressure main ranging in diameter from 100–280mm. 90% of the installation was undertaken by directional drilling. TruLine had two, sometimes three directional drills working on site.

Ground conditions, variable soil profiles

The ground conditions varied, generally the soils overlying the bedrock of the Akaroa harbour are composed of;

  • Weathered volcanic bedrock, typically <1m thick;
  • Volcanic Colluvium, typically <1m thick;
  • Loess, wind deposited sand and silt, typically <16m thick; 
  • Mixed Loess and Volcanic Colluvium, typically <20m thick;
  • Alluvium, in valley floors, the harbour head and in the Akaroa settlement near the shore

Loess is wind-blown silt, Colluvium is soil and rock material that has been shifted from its original location, usually by gradual down slope creep of materials, combined with discrete landslide type slope instability.

Alluvium comprises layers of sandy gravel (volcanic derived) and sandy silt (loess derived) deposited by fluid flow in the valley floors and harbour headwaters.

The upper soil profile in this Old Coach Road summit case study is typical of the consolidated reddish brown sticky soil, clay and rock material we experienced on the inner harbour slopes.

Old Coach Road summit drill shot overview

The Directional Drill operator maintains close communications with the crew of the Hydrovac truck down slope who are monitoring the backreamiing of the new pipe and any changes to ground conditions that might indicate the operator needs to make adjustments to the rate of pullback and the amount of drilling fluid being pumped to the backreamer.Pilot drill-shot diameter: 125mm

The gravity wastewater main pipe diameter: 180mm

Back reamer: reamed a 250mm hole on the way back

The backreamer displaces the additional 125mm diameter of the borehole as cuttings. The ability to suspend the cuttings, and the ability to flow cuttings out of the hole as a sediment laden slurry are important. The mixing ability of the back reamer is equally important. During the back-reaming process, the main objective is to mix the cuttings from the back reamer with the drilling fluid to create a slurry that can be displaced to the side of, or discharged out of, the bore hole to make room in the hole for the product being pulled back.


Trenchless Technology Method: HDD ‘Drillshot’ and Back Reaming

This document covers the penultimate Horizontal Directional Drilling (HDD) drillshot and back reaming of a 120m long drinking water supply pipe string, up slope to the summit of Old Coach Road, Akaroa. This 180mm diameter pipeline is designed to carry treated drinking water pumped up from a new drinking water treatment plant and pump booster station on the slopes of Old Coach Road to a new 500 cubic metre drinking water reservoir situated at the summit of Old Coach Rd on the ridge between Akaroa and Takamatua. 


The Akaroa Harbour basin has water shortages in many of its settlements during the summer months.

Potable water treatment and reticulation options

The existing treatment plant at Akaroa is old and in need of significant upgrade to ensure their water reliably meets the New Zealand Drinking Water Standards. With their expertise in Trenchless Technology, Tru-Line Civil has been contracted to reticulate the new drinking water system. A less intrusive, more cost-effective and rapid deployment of the reticulation systems is enabled by using HDD as the primary method for the installation of the new horizontal infrastructure system.

The reticulation model they are installing will deliver the most cost-effective way of improving consistency of supply throughout the summer and improving pressure and flow to end-users in Akaroa Harbour Basin.

Towing the welded pipestring from Akaroa township up Coachmans Road

HDD Methodology visual record

Slideshows 1–5: Complete Pilot Drillshot, exchange bit for macerator head, attach pipe string, commence back ream. 

For the purposes of a simple clear demonstration, the record of the morning’s work has been broken down into a series of chapters that describe the HDD process employed by the Tru-Line Civil team to pull back the 120m pipe-string uphill through clay soils with rocky volcanic incursions.

HDD01. Dig pressure relief well, verify borehole is to specification, ‘find’ the drill sonde. Click to view slideshow HDD01 >>

HDD02. HDD unit operations. Commencing completion of the drill-shot at the entry trench. Click to view slideshow HDD02 >>

HDD03. Completion the drill-shot in receiving trench. Exchange drill bit for the back reaming head. One function of the back reamer is to enlarge the 125mm bore hole to 250mm, this size is large enough to allow for the installation of the 180mm diameter pipeline. Visible to the right of the terminal trench is the new, aligned 125mm diameter pre-treatment water pipeline that Tru-Line has installed using HDD from it's source, the Takamatua Stream to the water treatment plant in Akaroa. Click to view slideshow HDD03 >>

HDD04. Attach 180m pipeline to back reamer head. Click to view slideshow HDD04 >>

HDD05. Commence back ream. The pipe string tail moving towards terminal trench. Click to view slideshow HDD05 >>

Slideshows 6–8: Hyro-vac truck crew ground pressure release tasks. Harbour rim HDD panoramas

Back-reamer passes through pressure release well.The drill-shot diameter is 125mm, the water main pipe diameter is 180mm, the back reamer reamed a 250mm hole on the way back. The back reamer displaces the additional 125mm diameter of the borehole as cuttings. The ability to suspend the cuttings, and the ability to flow cuttings out of the hole as a sediment laden slurry are important, and determined by the factors set out below. The mixing ability of the back reamer is equally important. During the back-reaming process, the main objective is to mix the cuttings from the back reamer with the drilling fluid to create a slurry that can be displaced to the side of, or discharged out of, the bore hole to allow room for the product being pulled back.

The frac-out releases depicted were caused by the pressurization of the borehole beyond the containment capability of the near surface geologic materials, and the high viscosity of the cuttings slurry (a mix of macerated clay soil and volcanic rock). The type and depth of these materials, as well as the back-ream head-unit water pressure, and the linear-metre rate of pullback are all key factors that determine the ability to prevent, mitigate and manage frac-outs. Drilling fluid is used to lubricate the back-reamer and render the near-surface materials into a sediment-laden slurry, or mud. The formulation of drilling fluid is tailored to the specific near surface geologic materials. 

The crew actively mitigate potential ecological impacts of sediment-laden water by installing pressure relief wells aligned with the borehole to minimize potential frac-outs. They also obtain geotechnical data in the location of the pipe alignment by making investigative boreholes and test pits that are hydro-vacced or excavated as required. The crew constantly monitor ground conditions aligned with the borehole and quickly respond to any inadvertent pressure releases that arise by hydro-vaccing pressure release bores offset from the borehole alignment. By these methods, and with close radio contact between the HDD operator, and the crew, Tru-Line succeed in minimizing ecological risk by effectively mitigating frac-outs and preventing sediment-laden fluids from entering watercourses

HDD06. Hydro-vac truck drainlayer crew ‘frac-out’ pressure release tasks. Back-reamer head passes through pressure release well. Click to view slideshow HDD06 >>

HDD07. HDD unit, entry trench and support truck portraits, Akaroa Harbour panoramas. Click to view slideshow HDD07 >>

HDD08. Back ream completion, removal of back reamer prior to relocating the HDD unit drill unit to its next drilling and back reaming site. Click to view slideshow HDD08 >>

Download the print version of the Case Study

Akaroa Harbour Water Supply Reticulation Upgrade.pdf

Contact us

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 (3) 768-6862 and ask for Geoff Powell or e-mail me at 

Christchurch Address 

Greywacke Road extension,
New Zealand 

Slideshow controls: [1] Double-click to "pause" any slide, [2] Double click to "play"/continue the show. [3] Jump to any slide in the show by clicking on the relevant thumbnail. [4] To escape at any time click "x" top right.

Credits: Photos and case study documentation by Shaun G. Waugh


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HDD - Akaroa Harbour Water Supply Reticulation Upgrade

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