Consolidated-Drained Triaxial Compression Testing of Peat

Abstract

Abstract:Recent peat soil problems, including failures of dykes, foundations and slopes in peat deposits, have focused greater attention on understanding the mechanical behavior of peat. Stability calculations routinely involve effective stress analysis, with pertinent strength and stiffness parameters often determined from standard triaxial testing, without special consideration given to internal tensile reinforcement provided by the fiber content and also the high compressibility of the peat material. This paper investigates consolidated-drained triaxial compression testing applied to peat soils. Significant differences in mini-structure and fiber content between test-specimens of undisturbed, reconstituted and blended peat materials were found not to cause significant differences in shear resistance under drained triaxial compression, with mobilized shear resistance increasing approximately linearly with increasing axial strain. Hence it was concluded that c and  deduced from drained triaxial -2-compression testing of peat are unlikely to be intrinsic material properties, and rather are largely a function of strain level, with higher values of  deduced for higher strain levels.End of primary consolidation should be deduced from pore-water pressure measurements rather than volume change response; although the repeatability of the triaxial consolidation tests was generally found to be poor on account of the natural variability of peat and small size of the test specimens.

Geotechnics of municipal sludges and residues for landfilling

“…For instance, in testing alum WTR 4, O'Kelly (2014a) observed that the bulk material could be easily remoulded by hand pressure at water contents significantly below the PL measured using the standard thread rolling method (BSI, 1990a), indicating that its value did not represent the lower limit of plasticity. Similar findings were reported for blended and sieved peat materials having LL = 757% and PL = 446% (O'Kelly and Zhang, 2013) and LL = 762% and PL = 446% (O'Kelly, 2015b). These findings cast doubt on the usefulness of measured and reported plastic limits in defining the lower water content limit for remoulding of these materials in practice.…”

“…Although usually quite sharp for a given material, undrained strength-liquidity index correlations can also vary widely between geomaterials since the strengths mobilised at the Casagrande LL and PL states are not unique (Haigh et al, 2013;Nagaraj et al, 2012;O'Kelly, 2013c). This is compounded by general difficulties in measuring and interpreting consistency limit values for organic soils (O'Kelly, 2013a(O'Kelly, , 2013b(O'Kelly, , 2014a(O'Kelly, , 2016bO'Kelly and Zhang, 2013). For instance, in testing alum WTR 4, O'Kelly (2014a) observed that the bulk material could be easily remoulded by hand pressure at water contents significantly below the PL measured using the standard thread rolling method (BSI, 1990a), indicating that its value did not represent the lower limit of plasticity.…”

Water Content Determinations for Peat and Other Organic Soils Using the Oven-Drying Method

“…According to the modified von Post peat classification system, [25] the Ballydermot peat deposit was classified as SCWPh-H 4-7 -B 3-4 -F 2 -R 2 -W 1 , the Clara peat material as SCN-H 4 -B 3 -F 3 (S)-R 1 (N)-W 1 (N), and the Derrybrien peat material as CErPh-H 3-4 -B 4 -F(Er) 2 -R(CPh) 3 -W 1. Full descriptions of these peat deposits and their geotechnical properties have been reported for the Ballydermot site by Pichan and O'Kelly [26,27] and O'Kelly and Pichan, [23] for the Clara site by O'Kelly and Zhang [28] and Zhang and O'Kelly, [29,30] and for the Derrybrien site by AGEC. [31] Refined Clara peat material (denoted by Cr) was also prepared for oven-drying tests by blending some of the remolded peat material using an electric handheld blender.…”

“…Note the PL condition could not be achieved for the Ballydermot and Clara peats, in that uniform soil threads could not be rolled out to 3-mm in diameter without crumbling/breaking on account of scale effects related to the fibrous particles. [27][28][29] Hence these materials were reported as non-plastic. In practice, however, the wet peats are readily remolded and therefore plastic.…”

Use of fall cones to determine Atterberg limits: a review

“…In the case of fibrous peat material, preloading (which gives the organic solids some stress history because of their compressible nature) produces lower LL values (O'Kelly, 2015). Greater mechanical breakdown of the peat solids during sample preparation produces lower LL, PL and I P values, especially for less humified material (O'Kelly, 2015), such that the measured plastic ranges are arbitrary and unlikely to sensibly correlate with mechanical behaviour (Hobbs, 1986;O'Kelly & Zhang, 2013;O'Kelly, 2015O'Kelly, , 2016a. Further, the pH of water affects the cation exchange capacity of finegrained soil, such that even usage of distilled water in changing the consistency of the soil material for laboratory testing can lead to different LL than what might happen for the field material (Torrance & Pirnat, 1984).…”