SGN Longman Junction Case Study – Groundwater Pumping Tests

A.k.a. “Alba’s Inverness Project”

Hi all! Better (a touch) late(r than anticipated) than never, as they say. Here’s your October/November edition of the newsletter, which focuses on Alba Dewatering’s groundwater pumping tests at Longman Junction. Enjoy!

Longman Landfill Site

Stadium Road

West Seafield



OS GRID REF NH67448 46849

Timeline: Invitation to Tender, April 2022

Site Meeting and discussions from Summer 2022 through to the end of the year

Alba Dewatering was awarded the contract earlier this year (2023) as Principal Contractor, with work starting in March and ending mid-April.

SGN identified a potential requirement for groundwater pumping tests situated at Longman Junction, Inverness, to determine viability for a new pipeline diversion needed due to proposed Transport Scotland-led highway improvement works. Invitation for tender was opened in April 2022, with the works being completed a year later. Alba Dewatering was brought on as Principal Contractor (CDM) to lead tests to determine the validity of the diversion through the prospective site (Longman landfill).

Tasks undertaken as CDM:

·       Co-Ordinator of testing

·       Surveying of site

·       Adherence to design

·       Record keeping, among other typical duties as outlined in regulatory requirements for principal contractors.

Project Background

The previous SGN high pressure gas pipeline was located close to and parallel to the M9/A9 Edinburgh-Stirling-Thurso Tunk Road.  The proposal was to construct the new pipeline diversion using open cut pipeline construction techniques with the pipe laid in a trench approximately 1.5m deep.  The pipeline diversion was connected to the existing pipeline using stopple techniques to prevent the pipeline from being shut down.  At either end of the diversion the new pipeline ties into the existing pipeline within two stopple pits, these pits were up to 3m in depth.  A deeper section of pipe trench was constructed and laid beneath a new culvert.   The pipeline diversion runs through Longman Landfill, which carried special considerations in terms of risks – both to on-site personnel and to the environment.

Risks and Environmental Considerations

The proposed pipeline diversion was located within the Longman Landfill with the pipe trench excavations and stopple pits being excavated in waste materials.  The Longman Landfill is and was closed at the time of works but was licensed to accept the following waste:

•                  Domestic waste.

•                  Industrial waste (chemical, medical and other); Animal waste,

•                  Asbestos.

•                  Very low levels radioactive waste (VLLRW) was recorded to have been tipped c.120m northeast of the investigation location.

There was also a very low potential for unexploded ordinance in the area, with 6 Alpha personnel commissioned to undertake under and over-ground CAT scans in the area north of Stadium Road to ensure site safety.

Moreover, ecological surveys from November 2021 concluded the presence of badger setts on the site, with exclusion zones/ buffers of 30m in place to reduce possible disruption there. Additionally, Japanese Knotweed was reported to be on site, with a high-voltage underground power cable adjacent to site works. All of these were considered in terms of risks and necessary mitigations were successfully implemented.

Daily groundwater level readings were required at the start and end of each day to fulfil Scottish Environment Protection Agency (SEPA) requirements, with hand dip measurement to be undertaken pre-, during, and after the test groundwater pumping operations. Water samples of flowing groundwater were collected from the v-notch weir tank at or approaching the end of each pumping test with six samples of groundwater required. These water samples were collected daily by a courier from the geotechnical laboratory on behalf of Alba Dewatering. In total, 12 boreholes were created, each to 12mbgi with a minimum 150mm diameter.

On-Site Insight

Courtesy of Mark Baldwin, Technical Director at Alba Dewatering

Were there any challenges that were not anticipated from the initial site report?

There was one: we didn’t anticipate the integrity of the capping on the landfill – normally there’s a layer of topsoil, but that wasn’t consistent with the ground being uneven in places. We had to navigate through trees and shrubbery to get our equipment through and in position. Additional care and considerations were required to conduct our activities without further disturbance to the site, and the client was understandably adamant that we shouldn’t expose any additional materials to compromise the site further.

That was something that was a bit of a surprise to us that wasn’t noted at the time of the initial site report or investigation, and only became evident when we arrived on site to start work.

What was the most important factor in finishing this project ahead of schedule?

We were focused on finishing to the schedule we were issued. As principal coordinator, it was important that we were efficiently coordinating between various parties, and to take point on handling the elements of the site we were tasked with. These elements included working alongside 6 Alpha personnel in terms of surveying for unexploded ordinance – which was a concern with the nature of the site– the principal designer, who was tasked with organising the pipeline diversion and civil works, and the specialist designer, who was handling the groundwater pumping test elements and hydrogeological surveys.

How did Alba Dewatering seek to differentiate its services during the tender phase of the project?

Good question, one thing was that the client required someone to take total ownership of the project as it was presented. The specialised nature of the project naturally catered to Alba Dewatering’s expertise in terms of managing a variety of elements that were hydrogeological, geological etc in nature. It couldn’t go to a ‘generic’ construction company without that history of experience.

We fit that bill, we could provide all of those services plus manpower and machinery, and all of the support equipment that was needed.

With regard to the contaminated nature of site, we also had access to the supplementary equipment that would not have normally been required for pump tests or hydrogeological surveys. There was special consideration given to creating vehicle access routes that didn’t result in a spread of contaminants across the site and up the access roads. We put a lot of measures in place to prevent that, as we’re experienced with working to such requirements.

Any other notes about this project specifically?

It was a really nice spot!

What was quite noteworthy was that we achieved a high degree of success in keeping coordination with all of the elements and teams on site. At the end of the day, it was well planned out, and well managed on site – a smooth project overall.

As I’ve touched on, the condition of the site was noteworthy in that we did not anticipate the level of surface disturbance that there was. We made it straightforward through organising ourselves, our manpower, and our equipment properly.

Testimonial from the Client:Very good experience on site with the Alba site supervisor leading a very smooth operation and completed the works ahead of schedule.”

Everything was restored to a good standard, and we are happy with the work you have carried out…. Thanks for making this part of the project run smoothly and we’ll hopefully work together again soon.”

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