Experimental behaviour study of peat at small stress level
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Date
2024
Authors
Wang, Di
Journal Title
Journal ISSN
Volume Title
Publisher
University College Cork
Published Version
Abstract
Peatland is widely distributed in Ireland, Scotland, Northern Eurasia, and North America, as well as other regions across the world. Compared to fine-grained mineral soils, peat usually contains a much higher content of organic matter and water, which in turn results in lower shear strength, high compressibility, and other unfavourable engineering characteristics.
To investigate the shear behaviours of peat, a series of standard classification tests is first conducted to determine the physical properties of peat, followed by 28 unconsolidated undrained triaxial tests. In addition, 14 consolidated undrained triaxial compression tests on samples from 1.65 m depth subjected to relatively small stress levels from 10.4 kPa to 40.5 kPa are conducted in this study.
The results show that the undrained deviatoric shear resistance of peat grows continuously with increasing shear strain, but peat has no obvious peak strength even at a very large strain (i.e. 25%). To compare the stiffnesses of peat specimens, the secant shear modulus G is normalised with the shear modulus G_(0.1%) at a relatively small strain of 0.1%. A hyperbolic function is then adopted to fit the relationship between the normalised stiffness and the normalised shear strain. Over 78% of the data falls within a ±30 % margin within 5% shear strain, while the predicted stiffness outside the margin is usually underestimated.
For peaty soil, the membrane correction effect on peat shear resistance is strain dependent and becomes significant above 10% shear strain. A critical state line for peat is determined based on the maximum curvature approach. Of the data recorded for peat, 78% falls within the range of 30–60°. This increases to 90.4% when excluding points lower than 10 kPa. Previous test data for very low stress levels (less than 10 kPa) might not be sufficiently reliable due to the limitations of conventional triaxial testing apparatuses, specimen preparation, and other experimental constraints. In addition, organic content plays an important role in the peat shear behaviour. In general, when the organic content exceeds 75%, the deviator stress mimics that of organic soils. Otherwise, the peat behaves more like a mineral soil. In peat samples with organic content greater than 75%, the direct shear box test gives higher estimates of shear strength than the triaxial shear test. However, these values may not be entirely accurate as the mechanism of direct shear acts only at the centre of the specimen. Triaxial testing allows for the development of shear failure throughout the entire specimen, providing a more representative assessment of shear behaviour.
The findings from this study provide additional insight into the mechanical behaviour of peat, particularly at small stress levels. The proposed stiffness degradation curve and critical state line based upon the maximum curvature approach are of value for infrastructure construction and assessments on/across peatland (e.g., wind farms, floating roads) in the light of growing energy and transportation demands.
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Keywords
Peat , Shear behaviours , Triaxial tests
Citation
Wang, D. 2024. Experimental behaviour study of peat at small stress level. PhD Thesis, University College Cork.