Project Details

Description

There are significant concerns regarding the structural integrity of the current A75 Ardachie culvert (structure 680 C25). The structure is past its serviceable life and remedial works are urgently required on the culvert to prolong the lifespan of the culvert, increase safety of road users, and repair scour to the culvert on both sides. Due to the confined nature of the culvert, it has not been possible to safely access inside the culvert to undertake temporary repairs. As a result, permanent repair works must be undertaken. Delays to these repairs could exacerbate the structural issues, leading to higher repair costs and increased safety risks. Scour has been occurring at the structure and, in the event that this is allowed to deteriorate further, it has the potential to cause significant damage and result in a catastrophic collapse of the structure.

Additionally, due to the narrow nature of the A75 over the structure and the small (non-standard) offset distances, the parapets of the structure are regularly damaged by passing vehicles. As a result, there is a need to setback the parapets to improve road safety and reduce maintenance costs. This requires increasing the length of the culvert to allow the verge to be widened and the installation of new parapets. These works will be informed by appropriate geotechnical investigations to ensure suitability of ground for foundations.

The construction activities include:

  • Temporary access to allow vehicle and pedestrian access to the culvert will be made for the duration of the works. This will comprise of hardcore materials such as crushed concrete, bricks, or stone.
  • Installation of a GRP Liner that will make the existing structure redundant. The GRP liners will be jacked through the culvert by a specialist sub-contractor, then a grout will be applied to any gaps between the new liner and existing masonry arch barrel culvert.
    • It may be possible to install the liner without restricting the flow of the watercourse; however, there remains a possibility that over-pumping may be required. Appropriate methods of construction will be determined by the sub-contractor.
    • Where it is not possible to install the structural liner under live flow conditions, a dry working area will be established. This will include the watercourse being temporarily dammed both upstream and downstream of the affected section. This will be achieved using sandbags or a similar approved method (e.g. inflatable dams or cofferdams), ensuring minimal disturbance to the riverbed and banks.
  • Installation of wing walls and a head wall at the culvert both upstream and downstream.
  • Installation of a compliant vehicle restraint system (VRS) which will require localised widening on the verge at the structure where they will be installed. Including GRP liner and wingwalls/apron, the culvert will be extended by approx. 4m on each side to accommodate the verge widening
  • To ensure embankment protection, a minimum of one rock roll will be installed on each side of the watercourse, both upstream and downstream of the culvert. Each rock roll has a diameter of 250mm, resulting in a total width of 500mm across both sides of the watercourse.

The plant and machinery required includes:

  • Hydraulic jack for pushing GRP liner through culvert.
  • Excavators to complete groundworks for verge widening.
  • A pump for grouting the liner.
  • Dumpers to be used for making the temporary access track.
  • Hand tools.
  • Welfare unit.
  • Water pumps (should over pumping be required).

Water-related construction is scheduled for July 2025, primarily during daylight hours, to align with seasonal restrictions linked to the fish spawning period. The remaining non-water-based activities will be undertaken later and are expected to take around 2 to 3 months to complete. Verge-widening work may be carried out overnight to help reduce traffic disruption.

It is likely full road closure will be required for the VRS installation and verge widening. When the A75 is closed, westbound traffic will be diverted by exiting the A75 by turning left onto the B735. Traffic will then continue along the B735 before turning right onto the B733 Main Street (Kirkcowan) and then left onto Station Road (B733). Traffic will carry on for 2.4km before turning left and rejoining the A75. Eastbound traffic follows the same route in reverse.

Please see PDF for Figure 1: Diversion route for closure of the A75, traffic diverted through Kirkcowan. This route adds 1.1km and approximately 2 minutes onto people’s journey for those travelling along the A75. For vehicles travelling from the point of the closure at each side of the structure, the detour will extend their journey by 6.5km, resulting in an additional 7 minutes of travel.

Location

The scheme is located on a remote section of the A75 in a rural area of Dumfries and Galloway (please see Figure 2: Scheme Location Plan). The scheme is located at the following National Grid References (NGRs):

  • Scheme Location: NX 32841 62901