Instability of blanket bog slopes on Cuilcagh Mountain, N.W. Ireland

There are many accounts of slope failures on blanket bogs, but their nature and controls are poorly understood. This study investigates the mechanisms of blanket bog failure on Cuilcagh Mountain, north-west Ireland, and identifies the critical factors affecting the stability of peatland hillslopes. This is achieved by means of extensive field investigations involving hydrological monitoring, soil sampling and comprehensive laboratory analyses to determine the physical, hydrological and geotechnical properties of the blanket peat. The results from these
investigations form the basis of hillslope hydrology and slope stability modelling using finite-element modelling programmes (commercial SEEP/W and SLOPE/W software). A total of 47 failures scars were identified on Cuilcagh Mountain involving an estimated 300,000 m3 of peat. Detailed field investigations revealed two main types of peatland slope failure: shallow translational peat slides associated with the failure of clay underlying the blanket peat, and bog flows in which failure occurs as a slurrytype
plastic flow with the failure zone located within the peat. Peat slides were more prevalent on the steeper slopes (7.0-17.00) of Cuilcagh, whereas bog flows were found exclusively on low gradient slopes (1.5-7.50) with deep accumulations of peat (typically up to 2.5 m). Previously it had been suggested that bog failures were confined to steep slopes or peripheral areas of blanket bogs. However, on Cuilcagh Mountain they appear to be
an integral part of the natural evolution of the main peatland, with the presence of many failure scars at different stages ofre-vegetation and recovery. Conventional methods of slope stability analysis (Factor of Safety using limit equilibrium methods) were found to be adequate for use on peat slide failures, but were not as suitable for analysing the slopes prone to bog flows.
Sensitivity analyses indicate that cohesion of the failure material (Le. the catotelm peat for bog flows and the underlying clay for peat slides) is the most critical factor contributing to slope failure. A reduction in cohesion is thought to be related to decomposition andlor progressive failure of the peat, or weathering and creep of the underlying clay. Increased overburden pressure from continuing peat accumulation is also an important factor in reducing the overall stability of a peatland slope. The initiation of bog flows and peat slides can occur from the progressive failure of the material in question, but there is more evidence to suggest that both types of failure are more frequently initiated as a result of a specific trigger event usually associated with high intensity rainfall.