Measurement, interpretation and recommended use of laboratory strength properties of fibrous peat

Abstract

Peat deposits are complex natural formations encountered in many geographical areas. Peat itself is considered as a challenging geomaterial, with many aspects of its behaviour seemingly remaining enigmas. This state-of-the-art review paper provides an extensive summary of current knowledge on the strength and shear behaviour of fibrous peats. Critical assessments and interpretations of the undrained strength, effective-stress strength and at-rest earth pressure coefficient parameter values determined for fibrous peat materials using standard and advanced laboratory apparatus are presented, focusing particularly on the consolidated undrained triaxial compression and direct simple shear approaches. Based on documented case histories of embankments, dikes and natural peat slopes, guidance is given for operational shear strength assessments in peat, aimed at bearing capacity and slope stability calculations, considering both the normalised undrained strength ratio and limit equilibrium effective-stress strength approaches. In particular, the botanical origin, structural anisotropy, humification level, likely initial overconsolidated state, tensile resistance associated with the peat fibres and possible scale effect related to the test-specimen size are discussed in the context of the strength mobilised for different testing apparatus. The test methodologies and interpretations of experimental results presented for fibrous peat in this paper should be transferable to other fibrous soil and soil-like materials. Notation

Experimental determination of the shear strength of peat from standard undrained triaxial tests: correcting for the effects of end restraint

“…Landva & La Rochelle, 1983;Stark & Vettel, 1992;Farrell & Hebib, 1998;Ogino et al, 2002;Komatsu et al, 2011). In the recent practice, direct simple shear is widely preferred, assuming that more uniform stress and strain states are guaranteed compared to the direct and ring shear, that the effects of fibres stretch are prevented and, especially, that the test better replicates the deformation mode characterising failure in the field (Farrell et al, 1999;Boylan & Long, 2008;Zwanenburg et al, 2012;Den Haan & Grognet, 2014;O'Kelly, 2017). In reality, all these assumptions are debatable.…”

“…Different alternative operational criteria have been proposed in the literature to reduce these exceptionally high shear strength parameters, by limiting the reference deviatoric stress at failure. Some criteria are based on different extrapolation of the experimental data in the stressstrain or in the stress path planes (O'Kelly, 2017). Kanmuri et al (1998), and more recently Hendry et al (2012), proposed to associate failure to the start of the ultimate linear strain hardening response in undrained triaxial compression tests.…”

Landslide investigations during pandemic restrictions: initial assessment of recent peat landslides in Ireland

“…Quantitative data from blanket bogs are now providing a basis for investigating the geotechnical properties and behaviour of peat in new and more detailed ways to improve our quantitative assessment of in situ peat strength (e.g. O’Kelly 2017 ) and understanding of mechanisms of failure. Table 1 presents data obtained from lower catotelm peat, i.e.…”

MPeat2D – A fully coupled mechanical-ecohydrological model of peatland development in two dimensions

“…The analysis of surface patterning is typically developed based on ecohydrological feedback, which encompasses the interactions between water table position, vegetation communities, nutrient availability, and peat hydraulic properties (Eppinga et al, 2009;Eppinga et al, 2008;Morris et al, 2013;Béguin et al, 2019). However, as a porous medium with relatively low shear and tensile strength (Long, 2005;Boylan et al, 2008;O'kelly, 2017;Dykes, 2008), mechanical instability also determines the process of surface patterning on the peatland. The simulation from Briggs et al (2007) indicates that the peatland surface might experience wrinkles due to the changing pattern between tensile and compressive stresses.…”