66kV Marshland ZS to Belfast ZS

Isaac Construction were engaged to complete civil works for installation of 3 x 66KV cables between Prestons Substation to Belfast Substation which included trenching, construction of reinforced concrete base, shoring the sides of the trenches with timber, liaising with the electrical contractor for cable lay, backfill with thermal concrete, and reinstatement of trenches.

Project overview

Location Prestons Road to Blakes Road, Belfast, Christchurch

Scope $3.4M

Date of completion 2021

Isaac Construction were engaged to complete civil works for installation of 3 x 66KV cables between Prestons Substation to Belfast Substation which included trenching, construction of reinforced concrete base, shoring the sides of the trenches with timber, liaising with the electrical contractor for cable lay, backfill with thermal concrete, and reinstatement of trenches.

9825aa

Purpose

Isaac Construction were engaged to complete civil works for installation of 3 x 66KV cables between Prestons Substation to Belfast Substation which included trenching, construction of reinforced concrete base, shoring the sides of the trenches with timber, liaising with the electrical contractor for cable lay, backfill with thermal concrete, and reinstatement of trenches.

Overview

This involved over 4200m of trenching with 460m of this required to be ducted. This work trench alignment also went through the adjacent Lower Styx Bridge upgrade.

There were three separate contracts for this work; this included a civil contract (Isaac Construction), electrical contract (design and supply of cable, laying of cable) and the substation construction contract. Communication between all three contractors was very important as each contract had works that were interlinked and had their own milestones to meet.

The team was required to meet strict time constraints as each cable pull required the client to organise specialised sub-contractors to mobilise to site. Once the cable was installed in the trench, the team was then required to backfill the trench with special “thermal concrete” as quickly as possible to eliminate the need for security to watch over the cable.

The trench also needed to meet strict hygiene and alignment requirements, as any damage to a cable or not meeting the design alignment would have resulted in extremely costly remedial work. The concrete base needed to be completely clean and free of any debris (leaves or aggregates) as it was critical to not damage the armour of the cable or to alter the thermal properties of the concrete. The trench depth was also critical as there was tight cover and clearance tolerances. It was critical to have a collaborative approach with both the client and cable lay contractor to ensure all clearance from existing services was met.

Challenges

The trench was required to be fully open from joint bay to joint bay (approx. 700m) to allow the cables to be pulled. This meant having large sections of trench open at a time which needed to be delineated and made safe for our staff, the cable lay contractor and general public.

With the trench being on the lower side of the road this meant that in large rain events the shoring had to be reinforced and extra bracing from the inside was required to prevent collapse. During these events the de-watering also required close monitoring. The resource consent included a tight tolerance of 50ppm of total suspended solids - if the number was greater than this we would have been required to turn dewatering off and have our trenches flooded.

Fulton Hogan were working on a separate drainage contract on Blakes Road, so it was critical that we worked with them to ensure that our newly laid concrete and cable alignment did not interfere with their drainage alignment. To achieve this, we needed to lower the vertical alignment which meant trench shields and heavy dewatering was necessary as the trench was over 2m deep through an underground stream.

Dewatering took place at three sections of this job and was handled in three different ways as explained below.

Marshlands Road

For this part there was not a great deal of ground water approximately 50-100mm of groundwater in the bottom of the trench at the start of the day, and in order to keep costs down for the client water was pumped into a water cart and removed from the trench at the start of each day. In order to mitigate this cost we changed our methodology to dig shorter sections and have the trench dug, shored and foundation layer of concrete poured by the end of each day.

Belfast Road

For this section we encountered a significant amount of ground water at around 0.7m bgl, and this required spears to be installed to a depth of 2.1m. Prime Pump, a subcontractor who utilised a new well pointing methodology, was engaged for this work.

Prime Pump’s well pointing was believed to be one the first of its kind in New Zealand. Two Netherlands designed and manufactured BBA PT150 piston pumps were deployed side by side on a 200m line, rather than the conventional configuration of one vacuum pump per 100m section of line. Prime Pump Account Manager, Hayden Powell, says the configuration saves space and time: “You don’t have to break the line and it’s a lot easier for the contractor”. Another critical factor to the well pointing’s success was the use of a filter agent to surround the spears themselves - this avoided issues with liquefaction and provided an extremely clean out flow.

Blakes Road

As mentioned above we were required to trench under the proposed SW alignment to a depth of 2m where we encountered a running underground stream which required dewatering. We considered utilising the same dewatering with spears as we had used effectively on Belfast Road, however as the location was at the entrance of the new Belfast substation we talked to the civil contractor and they had indicated that from their experience the ground water was localised in seams that ran through the site. For this reason we chose to incorporate a 6 inch pump with a large single spear to reduce costs to the client. This proved to be effective and once we passed the outflow through a settling tank we were pumping crystal clear water.

Locating Ducts and extending at CNC

Connectics had previously installed 180mm ducts on the cable route at four locations, Prestons / Marshlands intersection, Kapotone creek, Guthries Road and under the new CNC motorway. Due to no as-built data these proved difficult to locate, however the team was always up for a challenge. One of the greatest challenges was on the “island” between Guthries Road and the new CNC. Due to the ground conditions the overpass had been specifically engineered and we were not allowed to dig within 3m off the toe of this embankment. The team identified that the ducts were within this zone which required consultation with the Engineers from the CNC project, it was agreed that we could dig up the ends of these ducts but we needed to extend them to be 3m off of the toe. When we exposed these ducts, we also found them to be 200mm ducts not the 180mm specified so we needed to add spigout reducers at short notice to ensure the cable lay stayed on programme. One of these reducers was required to be machined down to the right size.

Prestons Road

The trench needed to be excavated between the existing 11KV and 66KV cables on Prestons Road, and this meant that we were required to do extensive potholing (every 7.5m) on this 312m section. This was the most time critical stage of the project and this extensive potholing could not be completed any earlier than once we had closed the west bound lane on Prestons Road. Traffic Management started early to ensure the lane was closed prior to 6:00 AM on Monday morning before traffic was due to peak. We then proceeded to cut and remove the asphalt on this area as quickly as possible - four hydro excavation trucks were used so that we could complete as much potholing as possible in the shortest amount of time. The planning for this was critical as every minute counted. Once we had located the parallel 66KV and 11KV cables we placed maker pegs on top of the duct/concrete and then backfilled these holes. This meant that the crews and client knew exactly where the services were while making the site safe for us to accelerate the program. We were able to get our digging crews started at both ends of the trench on the first day which then accelerated our program. This also meant that most of this critical potholing was completed on the first day and the team could concentrate on open trenching.

The proximity to the BP petrol station also meant that within 50m of this area we were required to take PID (Photo-Ionisation Detector) readings at three heights every five metres. This required training by Tonkin and Taylor and the use of a specialised detector.

Testimonials

Michelle Flanagan – Orion Ltd

I am pleased to be sending a summary of the feedback we received following the reopening of Prestons Road after the 66kV cable installation last week.

On Friday we sent an email update to our stakeholders – primarily businesses and residents in the area who were affected by the works. The responses received have been overwhelmingly positive, with all expressing their thanks for the speedy completion of the works. This is a great result, and huge thanks to the Isaac and Connectics crews who worked hard to get this work done ahead of programme.

To say thank you, the community wants to have a little morning tea for the crews on site. I will put some thought into how and when we can make this happen, and will be in touch.

Facebook Posts

“I’m not sure who exactly to express my gratitude to, but I just wanted to say Thanks! It’s a massive relief that it’s been so quick! It really impacted our family for school and work, so a sincere thank you to all involved. The detour was well managed too.”

“I just wanted to take a minute to say thank you to you and everybody that has worked so hard to get this stage of the project completed so quickly! I really appreciate the extra mile you went to in order to minimise disruption to the local residents”