Evidences of the Effects of Free Gas on the Hydro-mechanical Behaviour of Peat

Jommi, Cristina; Muraro, Stefano; Trivellato, E.; Zwanenburg, C.

doi:10.1007/978-3-319-52773-4_12

Cited by 4 publications

(5 citation statements)

References 11 publications

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“…The pore pressures within the peat subgrade were found to increase significantly (by >10 kPa) during warmer months and were strongly correlated with the seasonal temperature changes (Acharya et al, 2016). Gas exsolution during isotropic undrained unloading produced further significant reductions in the effective confining stress, with the mechanical response during the following undrained TC stage dominated as well by the ratio between the compressibility of the fluid and the compressibility of the soil skeleton (Jommi et al, 2017). Further, the pore pressures of peat specimens placed within a triaxial cell apparatus and subjected to a constant confinement pressure showed a direct correlation with incremental increases in the temperature of the specimens -for example, a 10°C temperature increase produced a 30 kPa increase in pore pressure for the peat subgrade material investigated by Acharya et al (2016).…”

Section: Geotechnical Research Volume 4 Issue Gr3mentioning

confidence: 95%

“…The effects of decomposition on some physical properties of the Ballydermot Raised Bog fibrous peat material were reported by O' Kelly and Pichan (2014). Investigations on the effects of the by-product gases on the hydromechanical behaviour of fibrous peat include field monitoring over a 3-year period of the temperature, p wp and vertical settlement at different depths within the peat subgrade beneath a railway embankment (Acharya et al, 2016) and TC testing of undisturbed Markermeer peat specimens in which gas had been exsolved by flushing the specimens with carbonated water followed by isotropic undrained unloading (Jommi et al, 2017). The pore pressures within the peat subgrade were found to increase significantly (by >10 kPa) during warmer months and were strongly correlated with the seasonal temperature changes (Acharya et al, 2016).…”

Section: Geotechnical Research Volume 4 Issue Gr3mentioning

confidence: 99%

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

O’Kelly

2017

Geotechnical Research

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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

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Section: Geotechnical Research Volume 4 Issue Gr3mentioning

confidence: 95%

Section: Geotechnical Research Volume 4 Issue Gr3mentioning

confidence: 99%

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

O’Kelly

2017

Geotechnical Research

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“…These were interpreted as the result of local expansion, which allowed preferential gas flow paths towards the external part of the samples (Jommi et al, 2017).…”

Section: Confining Effects On Gas Exsolution and Expansionmentioning

confidence: 99%

“…Attention was raised on the influence of biogenic gas on the geotechnical response of flood defences in the Netherlands during a thorough field study of dykes founded on peat, where gas release was regularly observed (Zwanenburg, 2013). As the lack of systematic studies on the mechanical response of gassy peats increases the knowledge uncertainties on the shortterm performance and the long-term durability of the embankments, an experimental investigation was initiated at TU Delft to start filling the knowledge gap (Jommi et al, 2017).…”

Section: Introductionmentioning

confidence: 99%

Experimental results on the influence of gas on the mechanical response of peats

et al. 2019

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Direct observation of gas in peat layers, generated by slow degradation in anoxic conditions, raised concern in the Netherlands about its potential impact on the geotechnical response of dykes founded on peat. To address this issue, an experimental investigation was initiated aimed at quantifying the main consequences of the presence of gas on the mechanical response of peats. The results of a series of triaxial tests on natural peat samples flushed with carbonated water are presented and discussed. Controlled amounts of gas were exsolved by undrained isotropic unloading, and the samples were sheared under undrained conditions. During gas exsolution, the samples suffered volumetric expansion, at a rate which is ruled by the relative compressibility of the fluid and the soil skeleton. The gas in the pore fluid dominates the stress-strain response upon undrained shearing, causing lower excess pore pressure compared to fully saturated samples. The experimental results suggest that local fabric changes occur during gas exsolution. However, for the amounts of gas investigated, these fabric changes seem to be almost reversible upon compression. Although the ultimate shear strength is hardly affected by gas, the reduction in the mobilised shear strength at given axial strain thresholds is dramatic, compared to fully saturated samples. The study suggests that the presence of gas must be cautiously accounted for at low stresses, when a reference stiffness is chosen for serviceability limit states, and when operative shear strength definitions, based on mobilised strength for given strain thresholds, are chosen in the assessment of geotechnical structures on peats.

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“…In the section of Geoengineering at TU Delft, a research effort has been undertaken in the last years to investigate in depth the role of gas formation and venting on the coupled hydro-mechanical response of organic layers in the subsoil of water defence embankments. Preliminary laboratory tests performed on peats to fill this gap showed the role of increasing gas content on their compressibility and on the mobilised shear strength at given strains [4,5]. The volumetric response of peats including gas was tentatively interpreted with a simple non-linear elastic model, which proved able to model the experimental results [6].…”

mentioning

confidence: 99%

Evidence of gas formation and venting in organic soils: experimental evidence and modelling approach

De Wolf,

Xu,

Jommi

et al. 2023

SEG

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Peatlands have been recognised to provide a natural carbon sink thanks to waterlogged conditions, which keep summertime temperatures relatively low, increase their water holding capacity, decrease the organic soil decomposition rate by creating anoxic conditions and eventually keeping high water table. However, unfavourable environmental conditions due to increasing temperatures and more frequent droughts will reduce water retention of peats and the summertime insulation, in turn increasing their temperature sensitivity and their decomposition rate [1]. As a result, peatlands may start inverting their positive cycle and emitting greenhouse gases, including CO2 and CH4 [2], which suggests better investigating how increasing climate stresses will affect the efficiency of peats in the greenhouse gases cycle and CO2 sequestration. Some evidence of gas production from increasing decomposition rate in the Netherlands is coming from continuous pore pressure measurements in saturated layers below the water table, which are monitored to assess the safety of the water defence and the transportation infrastructures. Increasing water pressure in closed piezometers compared to vented ones seem to suggest that gas is produced and capped in the ground, until the breakthrough pressure is reached and the gas vents from cracks opened in the soil matrix. Besides the environmental issues, increasing gas production from decomposition is becoming of concern for the stability of embankments made of organic soils, where the effective stress may be lowered to such an extent to endanger their stability. As a matter of fact, in the last ten years, gas overpressure has been claimed to be a triggering or a contributing factor in few small failures experienced by regional dykes in the Netherlands. In spite of the evidence [e.g. 3] and the risk increasing with heat waves and drought events, the role of gas on the coupled hydromechanical response of organic soils has been seldom investigated nor properly understood yet. In the section of Geoengineering at TU Delft, a research effort has been undertaken in the last years to investigate in depth the role of gas formation and venting on the coupled hydro-mechanical response of organic layers in the subsoil of water defence embankments. Preliminary laboratory tests performed on peats to fill this gap showed the role of increasing gas content on their compressibility and on the mobilised shear strength at given strains [4, 5]. The volumetric response of peats including gas was tentatively interpreted with a simple non-linear elastic model, which proved able to model the experimental results [6]. A similar model was used to numerically investigate the relevance of gas production and venting on the response of a regional dyke in the Netherlands, where gas bubbles from venting were observed after excavating - unloading - the toe of the dyke during a stress test. Fully coupled three-phases hydromechanical numerical analyses were performed with CODE_Bright [7] to include gas overpressure. A gas content of 6% in volume was artificially generated in the peat layer, capped by a clay layer on top, and let reaching an equilibrium distribution, which depends on the stress-strain response of the different layers and their volumetric compressibility (Figure 1(a)). Gas venting is triggered by simulating excavation at the toe of the dyke, which allows gas escaping after the capping clay is removed. The variation in the operative stress, on which stiffness and strength are assumed to depend [6], is shown in Figure 1(b) over gas generation and venting. In spite of the small amount of gas generated, the predicted overpressure is enough to bring the operative stress to zero in the upper meter of soil at the toe of the embankment due to the light weight of peat and cover soil, which temporarily reduces the factor of safety of the water defence against global stability. As soon as the gas overpressure is released, the operative stress increases above the effective stress which would characterise saturated conditions, bringing the system back to safer conditions. These preliminary analyses are supporting an undergoing experimental and numerical thorough effort to better quantify the dynamics of gas generation and venting in organic soils to reduce the hazard associated with increasing climatic stresses.

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Evidences of the Effects of Free Gas on the Hydro-mechanical Behaviour of Peat

Cited by 4 publications

References 11 publications

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

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

Experimental results on the influence of gas on the mechanical response of peats

Evidence of gas formation and venting in organic soils: experimental evidence and modelling approach

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