Completed works (looking north).
Summary
ROD was engaged by Donegal County Council to provide construction and handover stage consulting engineering services for this road development scheme located just north of Glenties, at the north-west edge of the Blue Stack Mountains. The appointment was made under the Transport Infrastructure Ireland (TII) Technical Consultancy Services Framework Lot 1C.
The scheme comprises:
- 4.1km of Type 3 single carriageway
- 2.5m wide cycletrack
- 0.6km of pavement overlay
- realignment of five local side roads and direct accesses
- a new arch bridge structure
- reinforced soil wing walls
- part demolition of Sruhangarve Bridge
Construction began in early 2021, with Wills Bros Ltd. as the main contractor, and was largely completed by November 2022.
Highlights
The project was shortlisted for International Project of the Year at the Ground Engineering Awards 2023.
Innovative geotechnical construction solutions, including the staged construction of large embankments on very soft peat soils, were implemented on the project to mitigate the risk to sensitive environmental receptors.
In demonstrating how peat excavations can be avoided, this project signals the potential for developing sustainable geotechnical engineering solutions that reduce greenhouse gas emissions from excavated peat (depending on import material costs and transport needs).
Approach
The open and collaborative approach adopted by the office, site and construction staff resulted in the successful mitigation of environmental restrictions, construction of 6m+ high embankments on extremely low strength peat and the safe delivery of the scheme.
Responsibilities
In addition to fulfilling our responsibilities as Employer’s Representative, our team oversaw:
- construction of the temporary floating road over peat
- supplemental geotechnical in-situ testing and geophysical surveying
- implementation of an inspection and monitoring regime in advance of the construction of 2.5km of surcharged embankment over soft ground
- targeted changes in the proposed arrangements for monitoring ground movements during intermediate and final stage holds, which informed the release of suitable areas to the contractor without unnecessary delays
- excavation and replacement of unacceptable material where necessary
- installation of pre-cast concrete culverts
Solution
The project team undertook supplemental investigations, conducted risk analysis based on geophysical profiling, and carried out piezometric cone penetration testing and specialist laboratory scheduling on undisturbed samples obtained from locations of specified instrumentation.
Discussions with the original designer, client and contractor produced several improvements, including changes in geosynthetic materials specifications and detailing.
Supplemental site and laboratory testing informed stability analyses and allowed us to improve the detailing and filling rate criteria used to mitigate the reduced peat strength observed on-site.
While significant embankment settlements of up to 1.5m high were observed during construction, overall stability was controlled by the observation of lateral movements and pore water pressures.
A temporary works crossover was constructed to minimise risk to users of the temporary road while excavation of the cutting section, through the adjacent peat and in areas outside the catchment, was ongoing.
Challenges
The scheme involved construction close to and within the West of Adara/Maas Road Special Area of Conservation (SAC) and the Owenea catchment, where freshwater pearl mussels have been recorded. To ensure compliance with planning conditions, our ecological Clerk of Works (eCOW) visited the site and carried out audits of the contractor’s environmental mitigation measures.
The risk of potential impacts to the highly sensitive aquaculture was a key consideration and necessitated the avoidance of peat excavations.
Detailed ground investigation information was limited at several key locations on site.
Services provided:
- Construction and handover stage consulting engineering services
- Employer’s Representative and site supervision
- Contributions to related TII pilot trials and university research currently being undertaken at University of Galway
