Evaluation of peat strength for stability assessments

In this paper guidance is given for the assessment of peat strength for stability assessments based on laboratory undrained simple shear tests. When considering the stability of peat, these tests will yield a conservative estimation of the in situ strength of the peat mass. The study was motivated by recent interest in renewable energy developments in upland peat areas. The results of more than 111 simple shear tests from 16 sites in Ireland, Scotland and the Netherlands were studied. It was found that the strength of peat is strongly influenced by its stress history, and also varies as a function of the water content and degree of decomposition (fibre content). The normally consolidated normalised strength ratio (⁠$su/σ′v$⁠) from simple shear tests of peat was found to be approximately 0·4, which is towards the lower bound of previously published data for peat. Comparisons of strengths derived from simple shear and field vane tests showed that the ratio of the strength derived from the two tests was influenced by the degree of decomposition, and that previously published correction factors for field vane strengths are inappropriate. Guidance is given for engineers working on future schemes on upland peat areas.