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

Abstract

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.

Modelling free gas overpressure in peat layers

“…7(b) for the case of K w /K = 667. The variation of the pore liquid pressure is dampened by the presence of gas, which increases the compressibility of the pore fluid ( [10]). The results in Fig.…”

“…Despite the magnitude of the excess pore pressure being limited to few kPa, its effect on the response of earth embankments on peats should not be disregarded, as shown by Acharya et al [9] and Jommi et al [10] in dedicated experimental studies. A fully coupled hydromechanical numerical approach is proposed here to quantify the effects of gas on the pore liquid phase response.…”

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

“…Loss of ignition tests gave an average organic content of 85,7 % (D2974-14, 2014; Skempton & Petley, 1970). Full details on the experimental procedures for the triaxial tests and the drying tests are reported by Jommi et al (2019) andTrivellato (2014), respectively.…”

“…Recent experimental studies are providing evidence of the consequences of increasing duration of drought periods on the hydro-mechanical properties of peats (Price, 2003;Peng et al, 2007;Gebhardt et al, 2010;Acharia et al, 2016;Jommi et al, 2019). However, the development of comprehensive geotechnical models reliable enough to be included in assessment tools still lags behind.…”

“…The model did not include gas exsolution-dissolution and the gas bubbles were assumed to remain confined within the pore fluid without interaction with the soil skeleton. However, recent experimental results by Acharya et al (2016) and Jommi et al (2019) suggest that the amount of gas exsolution and expansion is ruled by the mutual interaction between the gas phase and the solid skeleton.…”