Physical and geotechnical influences on peat instability

There is an urgent need to develop robust tools and policies for stability and hazard risk assessments in order to manage upland peat landslides in locations such as the British Isles where they have frequently occurred and caused harm to the environment. One of the particular difficulties is that reliable values of peat strength are difficult to obtain. The objectives of this research were to establish the nature of any relationships between the strength characteristics and the botanical, physical and chemical properties of the peat, and to determine whether palaeobotanical analyses of samples of the basal peat can provide a reliable indication of potential instability in upland blanket bogs. The research was carried out at the Straduff Townland (Co. Sligo), Slieve Anierin (Co. Leitrim) and Slieve Rushen (Co. Cavan) landslides, all located in northwest Ireland, from the margins of which monolith and core peat samples were collected. Standard and validated paleobotanical, chemical and geotechnical protocols, modified or refined where necessary to suit the nature of the peat, were used in the study. The triaxial, direct shear and tensile strength tests were conducted using experimental very low stress conditions in order to fully replicate in-situ conditions. The reliability of the measured strength parameters was examined by performing deterministic and probabilistic stability analyses of the failed slopes using industry-standard ÔÇ×limit equilibriumÔǃ software (SLOPE/W). The nature, extent and spatial distribution of the hydrocarbons unexpectedly found in the basal peats during the fieldwork were also investigated. This research found that blanket peat dominated by monocotyledons (with mainly E. vaginatum) is likely to be susceptible to failure because its ÔÇ×effective structural propertiesÔǃ, specifically the high degree of humification and low fibre content of its basal peat, cause it to have very low strength and also therefore a very low bearing capacity. Furthermore, monocotyledons or its remains in peat have morphological, chemical, biological features that can promote bogflow-type failure. These may include for example (i) their parallel and elongated leaf veinations that promote flow, (ii) the genesis of hydrocarbons such as bitumen from their lignified tissues and (ii) being host to a hydrocarbon-producing aphid Colopha compressa. Laboratory measurements of undrained strength of the weak basal peats were consistently < 3 kPa, and deterministic stability analyses revealed that the value of the tensile strength can be used as an indicator of the undrained shear strength. A new classification (i.e. "the modified fibre content scheme") and a modified procedure for assessing upland peat failure for construction projects has been proposed based on peat fibre and humification characteristics and their apparent influences on peat strength. Deposits of hydrocarbons such as bitumens within the basal peat constitute a previously unrecognised factor that probably contributed to the occurrence of the studied landslides due to their hydrophobic properties.