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Preliminary conclusions from this research are:1. The landslides were caused by high intensity rainfall resulting in high water pressures within the hillslopes.2. The nature of slope failure was controlled by position on any given hillslope and the presence of subsurfacewater drainage and an extensive iron pan in the subsoil.3. Drainage ditches do not appear to have been a significant contributing factor in failure.4. Approximately 177,000 m 3 of peat and soil were removed by the landslides. Some of this material was lefton the lower mountain slopes but much of it entered streams, rivers and the sea.5. These events are consistent with observations of peat landslides across Ireland and elsewhere in the world.Further laboratory analysis of the slope materials, detailed computer modelling of stability conditions of theslopes, and quantification of the sediment run-outs using GIS techniques, are continuing to resolve theseissues. This research has also highlighted a number of other key factors that require future research. Theseinclude the precise hydrological and geotechnical role of the subsurface iron pan in the soil profile, and thenature and distribution of subsurface pipes and other hydrological and structural discontinuities in the stabilityof intact mountain slopes.A geophysical investigation of a large scale peat slide on Dooncarton MountainShane Murphy (Project assisted by GSI)A geophysical and engineering investigation was carried out on a discrete peat slide on Dooncarton Mountain,Co. Mayo, where 41 separate peat slides occurred on the 19 th September 2003.Geophysical fieldwork was carried out between the 19 th to 28 th June, and the 4 th to 6 th July 2004 using a Sensorsand Software pulseEKKO 100 ground penetrating radar (GPR) unit and an ABEM Mark 6 seismogram. Thecollected GPR and seismic data was processed using ReflexW3.5 software developed by Sandmeier ScientificSoftware.In the field, the refraction survey proved to be inappropriate for investigating the thickness of peat and was notsubsequently processed. S-surveys however, provided Poisson’s ratios of 0.442 and 0.431 with Young’s modulusof 8.65±0.05MPa and 39.25±0.25MPa for the respective peat and weathered layers were calculated.All processed GPR profiles displayed a strong, continuous reflector that corresponded to the peat-weatheredrock layer boundary. A possible discontinuous iron pan reflector was located just below the top of the weatheredlayer. Naturally occurring pipes and sub terrain cracks were imaged in the peat using 100 and 200MHz antennae,although truthing or prior knowledge is generally required for interpretation of these features.Laboratory tests performed on peat cores taken in the field showed the peat to have a density of 0.92 ± 0.02 g/cm 3 while a rough index test proved that the peat had a low permeability. A simple two layer model with the peatsitting on top of the bedrock was back analysed using Janbu’s Simplified Method for three cross sectionsprovided by the GPR survey. By constraining the back analysis results with index shear strength results thecohesion of the peat was determined to be 8kPa and the internal angle of friction to range between 30 o and 40 o .The GPR, as a tool of investigation, proved successful in determining the failure plane in the peat where theseismic surveys were ineffective in determining the cause of the failure. The GPR and engineering analysiscombined to prove that water flowed through the cracks of the impermeable peat and caused the peat tobecome buoyant and susceptible to failure resulting in the peat slides.Fig. 8.3 A GPR profile along a survey line above the scarthat is located to the south of the survey. Yellowrepresents the acrotelm-catrotelm boundary, green thestart of the weathered layer, and blue a possible hard panin the weathered layer. Shane Murphy82