Gas exsolution and gas invasion in peat: towards a comprehensive modelling framework

Abstract

Increasing climatic stresses accelerate the degradation of highly organic soils, like peat, by increasing their drying rate above the water table and their decomposition rate under water. Recent experimental studies provide evidence of the consequences of these processes on the hydromechanical properties of peat. However, modelling the experimental evidence in a comprehensive framework remains challenging, especially in the case of anaerobic degradation, which is accompanied by gas generation, exsolution and expansion, into an initially saturated matrix of soil.In this research study, experimental results from undrained isotropic unloading on artificially gascharged peat samples are combined with data from drying tests on the same peat, in an attempt to develop a unified framework encompassing the two desaturation processes. As a first approximation, simple compression laws depending on the average stress acting on the soil skeleton are used to simulate the experimental results. The comparison between experimental data and model simulations suggests the possibility of modelling gas expansion similar to the gas invasion process occurring on drying. The modelling approach, stemming from unsaturated soil mechanics, is meant to offer a possible framework to include the hydro-mechanical consequences of the effects of degradation of peats in the engineering analysis.

Evaluation of the impact of freezing technique on pore-structure characteristics of highly decomposed peat using X-ray micro-computed tomography

“…However, both the short-and long-term modelling of peatland functioning, and in particular the impact of anthropogenic warming and direct human disturbance on atmospheric CO 2 , CH 4 and N 2 O, requires a detailed knowledge of the peat structure and of both water and gas flow with respect to the groundwater table level (e.g. Gharedaghloo et al, 2018;Zhao et al, 2020;Glaser et al, 2021;Müller and Fortunat, 2021;Swinnen et al, 2021;Wiedeveld et al, 2021). In order to achieve this, X-ray Computed Tomography, which is widely used in science as a non-invasive technique for the study of internal 2D and 3D structures, is a promising technique with which to analyse the structure of peats and their physical properties.…”

Effect of freezing on the microstructure of a highly decomposed peat material close to water saturation when used prior to X-ray micro computed tomography

“…However, the short-and long-term modelling of peatland functioning, and in particular the impact of anthropogenic warming and direct human disturbance on atmospheric CO 2 , CH 4 and N 2 O, requires detailed knowledge of the peat structure and of both water and gas flow with respect to the groundwater table level (e.g. Gharedaghloo et al, 2018;Zhao et al, 2020;Glaser et al, 2021;Muller & Fortunat, 2021;Swinnen et al, 2021;Wiedeveld et al, 2021). To achieve this, X-ray Computed Tomography, which is widely used in science as a non-invasive technique for the study of internal 2D and 3D structures, appears to be a promising technique to perform new analyses of the structure of peats and of their physical properties.…”

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

“…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].…”

“…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.…”