Thermal induced pore pressure within peat

M.P. Acharya, M.T. Hendry, C.D. Martin, T. Edwards

Dans les comptes rendus d’articles de la conférence: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Laboratory and Field Testing

ABSTRACT: This paper presents the results of a study investigating the long-term behaviour of peat under new construction. Instrumentation to measure temperature, pore water pressure, and vertical displacements was installed at different depths within the peat below a recently constructed embankment. During this investigation, the pore pressures increased significantly during the warmer months and were accompanied by a rapid decrease in pressure. From the available literature, a hypothesis was developed wherein this pressure was generated due to the development and expansion of gases within the peat. Further investigation was conducted in the laboratory to confirm this hypothesis. Peat was placed within a triaxial cell at constant confining pressure, and the temperatures were incrementally increased while the resulting change in pressure and volume of gas was monitored. The results of this investigation indicate that the increase in temperature and the expansion of gases cause the measured pore pressure response.

RÉSUMÉ: Cet article présente les résultats d'une étude portant sur le comportement à long terme de la tourbe dans les nouvelles constructions. Instrumentation pour mesurer la température, la pression interstitielle de l'eau, et les déplacements verticaux a été installé à différentes profondeurs dans la tourbe en dessous d'un remblai de construction récente. Au cours de cette enquête, les pressions interstitielles ont considérablement augmenté au cours des mois les plus chauds et ont été accompagnés par une diminution rapide de la pression. D'après la littérature disponible, une hypothèse a été élaborée, dans laquelle cette pression a été généré en raison du développement et de l'expansion des gaz à l'intérieur de la tourbe. Une enquête plus approfondie a été menée dans le laboratoire pour confirmer cette hypothèse. La tourbe a été placé dans une cellule triaxiale à pression de confinement constante, et les températures ont augmenté de manière incrémentielle pendant le changement résultant de la pression et le volume de gaz a été surveillée. Les résultats de cette étude indiquent que l'augmentation de la température et de l'expansion des gaz provoquent la réponse de la pression de pore mesurée. 1. INTRODUCTION The Canadian railway network operates through a wide variety of geographical and climatic conditions. Long stretches of this railway network, especially in the northern regions of the country, have been constructed over thick peat deposits. In Alberta, the railway line between Edmonton and Fort McMurray, known as the Lac-la-Biche subdivision (LLBS), is estimated to have more than 120 km of embankment constructed over peat formations. These embankments are often subject to ongoing settlement and stability issues. Canada National Railway (CN) has been upgrading this railway line to transport freight north to Fort McMurray and oil products south. These upgrades are being done to allow the railway line to handle heavier axle loads and increasing amounts of traffic. Thus, it has become necessary to study the long-term response and stability of these peat subgrades. As part of this study, a railway embankment on the LLBS was instrumented to monitor the long-term behaviour of the peat foundations at milepost (MP) 263.9. During the monitoring of the site, an anomalous pore pressure response was measured that could not be readily explained. The measured pore pressures at peat strata showed a strong correlation to seasonal fluctuations in temperature, with significant increases in pore pressure (>10 kPa) during the summer months. The pore pressure variations were confirmed to not be a result of fluctuations in ground water level. This paper presents the current research being undertaken at the University of Alberta, which includes the instrumentation of a second site, at MP 263.5 on the LLBS, to corroborate the results measured at the first site; and followed up with laboratory testing. To date, this research has demonstrated that the pore pressure behaviour is a result of the formation of gas bubbles due to the natural decomposition of peat and the thermal expansion of gas bubbles. 2. BACKGROUND Peat is generally a normally consolidated organic material consisting largely of organic residues that has accumulated due to slow decomposition (Stanek 1980; Landva 2007). Peat characteristically has a very high water content and high degree of compressibility and its physical and mechanical properties are highly variable (Landva and Rochele 1983). Peat exists between two extremes: a fibrous peat comparable to felt and an amorphous-granular peat (Landva 2007). The former is more common in Canada. The structure of peat consists of a large network of well-connected voids. These voids

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