N56 Reinforced Soil Structures

The N56 is a national secondary road in Ireland that runs from Donegal town in the south to Letterkenny in the north via Ardara, Glenties, Dungloe, Falcarragh and Dunfanaghy.

GDG was requested by Maccaferri to provide geotechnical design services for a reinforced earth retaining structure along the N56 from Kilkenny to Letterilly. The purpose of this design was to provide an overview of the existing ground conditions on site, interpretation of ground investigation information and provide an overview of applicable design assumptions, methods, calculations, safety verifications along with serviceability in line with I.S. EN 1997–1:2005.

The N56 Kilkenny to Letterilly road scheme involves the realignment of approximately 4km of the existing road between Glenties and Dungloe.

The Scope of Work

Preparation of a Geotechnical Design Report and Design Certificates including:

  • Appraisal of ground profiles and soil parameters used in calculations
  • Internal slope stability designs for the MSEW and RSS structures with up to two cross sections analysed for each structure
  • External and global stability for each structure
  • Design Risk Assessments
  • Separate Design Certificate for each structure
  • Preparation of drawings to include for each structure

The principal parties involved in the project were:

  • The employer in the main contract was Donegal County Council.
  • The employer’s representative, PSDP and designer in the main contract was ARUP.
  • Fox Building & Engineering Ltd. was the principle contractor, and Maccaferri’s client.
  • Maccaferri was the materials supplier.
  • GDG was the sub-consultant and the designer of various geotechnical elements between chainages Ch.17+900 and Ch.18+400 of the road scheme.

The N56 Kilkenny to Letterilly reinforced soil slope project by GDG delivers excellent sustainability benefits, including using 40% less embodied carbon than a concrete specimen.

The design of a reinforced slope, by means of optimised slope geometry, is an exemplary case of minimising the environmental impact of new infrastructure.

Further challenges successfully addressed by the project team included steep slopes above and below the existing N56, along with constraints on the road’s horizontal alignment.


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