DIRECTIONAL drilling beneath a watercourse has been carried out as part of a project to install a new pipeline for Scottish Water in North Ayrshire.
The publicly owned utility is upgrading the rising sewer main between its pumping station in Saltcoats and the treatment works in Stevenson. This work comes on the back of several pipe bursts and spot repairs in recent years, caused by the ageing infrastructure.
Civil engineering contractor George Leslie was tasked with supporting the delivery of the works on site.
A particularly problematic section of the pipeline was located underneath the Stevenson Burn watercourse, which runs alongside a nature reserve and caravan park.
Allan Ralston, project manager at George Leslie, explained, “To replace the old pipeline at this section, our proposed plan of works involved sinking two cofferdams, one either side of the watercourse and 70m apart (a drive pit and reception pit) and installing a large diameter steel by-pass pipework system. We have directionally drilled underneath the watercourse, using a 914mm diameter steel sleeve. Our teams will then install the new 710 PE pipe and tie-in to the existing network, before decommissioning the old pipeline.”
To facilitate the safe delivery of the project, an extensive series of temporary works was required, including a temporary bridge to provide access across the watercourse for plant, materials and equipment, as well as groundworks for the various cofferdams and excavations. Following recommendations by the drilling contractor, the drive pit had to be excavated to a depth of 10m – a minimum of 2m underneath the riverbed to avoid water ingress.
George Leslie turned to temporary works supplier Mabey Hire for support, having worked successfully with the firm on a similar project in nearby Stewarton, which also involved a directional drill and an 8m deep excavation.
In addition to the temporary access bridge, designed for vehicles up to 26t and formed out of Mabey Hire’s flat top beam bridge product, spanning 12m, the supplier also designed and supplied a range of its groundworks support equipment for the various cofferdams on site, using a combination of its Larsen piles, trench sheets and heavy bracing frames.
Given the close proximity of the site to a residential area, Mabey Hire supplied its vibration monitoring sensors for installation during the works.
With multiple excavations required, including the reception and drive pit for the drilling rig and the two tie-in points, the project required a carefully planned phased approach, while the site location also presented challenging ground conditions.
Jamie McLachlan, associate sales director – Scotland at Mabey Hire added, “As well as the proximity to the watercourse, we were presented with a high-water table, meaning the team had to consider the resulting groundwater levels in the cofferdams. To counteract this, George Leslie installed a de-watering system in the proposed excavation locations. It involved the installation of small-diameter wells, connected to a header pipe and positioned around the perimeter of each cofferdam, with a vacuum pump system then used to lower the groundwater level.
“We also worked with the pile supplier to provide a sealant pre-delivery, to alleviate groundwater ingress. Through a combination of early engagement and excellent collaboration with George Leslie and the wider team, we were able to ensure that this didn’t cause issues or delays to the project timeline.”
