Whangarei Heads in situated in the Whangarei District along the eastern boundary of the main population area of Whangarei and is a coastal community. The proposed wastewater treatment system will cater for the existing 455 properties. Based on the potential occupation of dwellings, within five years the population could increase to something in the order of 1,500 properties.

The construction of the gravity reticulation, rising mains and pump stations associated with Stage Three of the Whangarei Heads Sewerage Scheme is the largest capital works contract ever let by the Whangarei District Council, with a value of $NZ14.5 million.

The works span a distance of approximately 14 km, and include the construction of the following key items of infrastructure:

* 15 pump stations and associated storage facilities, some of which are constructed up to 7 m below sea level. * 19,000 m of rising main with sizes ranging from 63 mm OD to 280 mm OD * 10,500 m of sewer gravity main (150 mm ID) * 450 house connections (100 mm ID)

The contract signed between United Civil Construction Limited (UCCL) and the Whangarei District Council (WDC), requires that this work is completed within a period of 18 months, with the further condition that approximately the first 60 per cent of the work value is completed in the first eight months. Given the relatively short timeline for a project of this size and considering the nature of the works and a local shortage of skilled staff, UCCL and Fulton Hogan Limited (FH), decided to pool their resources, with FH acting as a subcontractor to UCCL.

These tight timeframes, particularly for the first portion of the project, were a key consideration in electing to use considerably more directional drilling than had been allowed for in the design.

“Early on in the tendering process we identified the time constraints and the large liquidated damages associated with a delayed finish as a key risk,” said UCCL Managing Director Andrew Campbell.

“One way to mitigate this risk, yet still submit a realistic and competitive tender price, was to look at ways of doing things a little differently. Drilling a lot more of the pipe than what was shown by the design seemed an obvious way to achieve these objectives.

“While directional drilling may have seemed an obvious way of undertaking much of the work, choosing a subcontractor who could perform was a key consideration that was a little more difficult.

“Once we had made up our minds to drill as much as we could, we then had to work out who could do this for us. That wasn’t as easy, as there are several firms out there who are more than capable of doing the job. However, after looking at all the options and discussing with our major subcontractor we decided to go with Excel.”

Excel Managing Director Mark Millar and Operations Manager Ben Tombs had extensive experience in the industry, both in New Zealand and overseas (including Australia and Hong Kong) when they formed their company Excel Drilling Northland in 2002. After initially setting up their business with one drill, they soon bought another and have since built up a good track record with a large portfolio of key clients. One of these clients is UCCL, for whom Excel have worked in the past.

“While price is obviously a key consideration, we were also looking for a subcontractor who we knew could deliver the result we were after. Mark and Ben are two straight-up and capable guys who have performed well for us on past projects and they bring with them a high degree of professionalism and a can-do attitude,” said Mr Campbell.

The site

The works are located along the northern side of the Whangarei Harbour, extending from the outermost seaside community of Urquharts Bay at the harbour entrance, along a narrow strip of around 14 km in length, and connecting to the existing Whangarei sewerage reticulation system at Parua Bay.

Overlooking the harbour and the Bream Bay area are high and vertical rock faces, dominated by Mount Mania which provides an awesome vista and is a distinguishing feature of the area. While the landscape may be visually impressive, the subsurface conditions don’t make life that easy for the construction team.

The underlying strata vary from soft heavy clays and clayey silts to mudstone, andesite, ryolite and ignimbrite and even crystalline limestone in isolated areas.

“Variability of the ground conditions is a bit of a hassle on this job,” says Mark Millar of Excel. “You can be drilling through heavy clays, brown rock, cemented sandstone, volcanic rock, along with a few big boulders thrown in for good measure, all within the length of a 150 m string. It is taking a little bit longer to get some of this pipe in the ground than what we had hoped but we are getting there.”

In many areas the steep and craggy nature of the terrain means that retaining walls and intricate garden plantings feature extensively. Access to these areas with an excavator would be difficult and in many cases impossible without having a large construction footprint in the front garden of the properties of some residents.

“Many of the areas in which pipe is to be laid are on the side of steep hills, covered in bush and rock. Having to work in this sort of terrain and keep the peace was the other key factor in electing to go with as much drilling as we could,” said Mr Campbell. “Open trench construction in these areas would not only mean huge reinstatement costs, but a whole lot of inconvenience to the landowner.”

In fact, the benefits of drilling are obvious to many of the locals, with several landowners taking up the opportunity, at their own cost, and having the contractors extend the pipe further up into their property, beyond the nominal connection distance of 300 mm inside the property boundary provided for by the WDC. When the scheme is commissioned, connection of the landowner’s sewer pipework to the public system will simply be a matter of hooking up, avoiding the need to dig up lawns, gardens and BBQ areas.

Technology

The scope of the works, and the contractual requirements that the project be completed within a relatively short time frame, has seen Excel resource the project with two drills. A total of five staff supports the operation of both these machines.

The directional drilling machines used by Excel are a Vermeer 2440 and a Vermeer 3344, both of which were sourced second-hand from the United States. Each drill has its own eight wheeler transport truck to which is fitted a custom built 4,000 litre stainless steel mixing system, driven by a 4 inch diesel water pump. The drills use a variety of bits/heads, all of which are supplied by Melfred Borzall (US).

Locators used by Excel include an Eclipse Locator and a Mark IV Digitrack. A suction truck with a capacity of 2,000 litres is used to remove drilling fluid and slurry from the site of the works to an off-site disposal area.

Environmental impact

With the majority of the works being undertaken within 100 m of the ocean, and in many instances just above the high tide mark, the use of directional drilling and the resulting reduction in earthworks has significant environmental benefits.

Some pipelines, originally identified as being constructed by open excavation, run just above and parallel to the high tide mark and are in areas that are very difficult to access. Directionally drilling these pipelines has avoided the need to cut access tracks and cart spoil through environments of high ecological value.

In some instances however, access onto beach and into the coastal marine area with the drill remains necessary. Given the environmental importance of the foreshore and seabed, in each of these situations a specific Access Management Plan (AMP) is developed by UCCL in close consultation with Excel.

Detailed construction management plans for the drilling operation and associated checklists form part of these AMP’s. These plans are reviewed and approved by the supervising engineer and then the Northland Regional Council (NRC), the statutory body responsible for managing and monitoring activity within the marine area.

Consideration is given not only to how the work will be undertaken and the environmental impact minimised, but also contingency planning for spillages and mechanical failure. To date, these works have gone smoothly; with Excel taking hours to install pipelines which otherwise would have taken weeks to complete using open cut trenching.

Success of to date/outlook

The project is currently proceeding relatively smoothly. It is notable that the number of complaints from residents within the area of works is minimal, a fact that is largely attributable to the decision to use directional drilling wherever possible and avoid disturbance to private property. The contractor, subcontractors and engineer are working through issues in a constructive manner as they arise. As the project approaches the halfway stage, it is expected that the project will be completed on time and budget.