31 March 2022
The mechanisms of the Derrybrien peatslide
On 16 October 2003 a large peatslide occurred at the site of an under-construction windfarm in Derrybrien in the Republic of Ireland. This was one of the a number of such landslides that have occurred in Ireland in recent years, but it has been particularly high profile for the fact that the European Court of Justice made a ruling against the Irish government on the basis that an environmental impact assessment was not properly produced ahead of the works. Construction at the site was temporarily suspended, but the works were completed in 2006. The Derrybrien Wind Farm remains in operation today.
Aidan Forde kindly highlighted on Twitter that the Remedial Environmental Impact Assessment was published in August 2020 (better late than never I guess), which provides some detail of the mechanisms of the 2003 landslide. The report contains this description of the landslide:
The peat slide source area contained approximately 450,000 m^3 of peat at the time of the slide and it is estimated that approximately 250,000 m^3 of this material flowed from the source area into the downslope stream where it was routed down the mountain slopes to an elevation of approximately 200 mAOD where it spread out on a relatively flat area of agricultural land and to approximately the Black Road Bridge. Following some heavy rainfall, the debris material was forced further downslope along the Owendalluleagh River channel before reaching a phase of dispersion in the river water approximately 6.9 km downstream of the source area.
At the time of the failure, peat was being excavated from the site of the turbine and the arisings were being placed or “sidecast” onto the intact peat slopes on the downslope side of the turbine and floating road. Other drainage works were being carried out along the floating road within the slide area approximately 300 m downslope from T68. Consultants investigating the slide concluded that the construction activity, and particularly the placing of arisings onto the intact peat slopes, most likely triggered the slide.
The report contains the following schematic to illustrate the process:
So, it seems that the mechanism was that excavated material was emplaced on the peat adjacent to the hole. Peat is weak and has a very high water content. The emplaced material drove a localised shear failure, allowing a small landslide to develop. This then induced an additional load downslope, triggering the development of a much larger mass movement.
This appears to me to be a viable mechanism, but it also implies no retrogression occurred. This is intriguing as that was certainly not the case for the Meenbog peat landslide in November 2020. I am slightly fascinated by the hint of a failure on the upslope side of the excavation in the schematic diagram. I wonder why this was included.
It would be fascinating to see the geotechnical reports after the 2003 landslide. Does anyone know if these are available?