Evolution of Adfreeze Strength of Pile Foundations in Warming Permafrost

Abdulghader Abdulrahman

In the proceedings of: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: COLD REGIONS ENGINEERING

ABSTRACT: An experimental investigation was carried out to evaluate the thermal exposure effect on the load carrying capacity of steel piles embedded in ice-poor frozen soils using steel-soil interface tests. The interface testing was conducted in a walk-in cold room to enable testing at various temperatures below the freezing point. A series of stress-displacement curves were established at different temperatures and under various normal stresses. The results showed a significant reduction in adfreeze strength of the pile-soil interface as the exposure surface temperature increased. The interface strength decreased approximately 30% when the exposure temperature increased from -1.5 to zero degrees Celsius. Such condition can probably be witnessed in warm permafrost that may experience temperature ranges from -3 to 0. The shear stress strain behaviour was shown to be brittle and followed by significant loss of bearing capacity.

RÉSUMÉ: Une étude expérimentale a été menée pour évaluer l'effet de l'exposition thermique sur la capacité de charge des pieux d'acier incorporés dans les sols gelés mais pauvres en glace au moyen dd'interface acier-sol. L d'interface a été réalisé dans une chambre froide de plain-pied pour permettre des essais à différentes températures en dessous du point de congélation. Une série de courbes de contrainte versus déplacement ont été établies à différentes températures et sous différentes contraintes normales. Les résultats ont montré une réduction significative de la force « adfreeze » de l'interface pieu-sol quand la température de surface d'exposition augmente. La force d'interface a diminué d'environ 30% lorsque la température d'exposition a augmenté de -1,5 à zéro degrés Celsius. Cette condition peut probablement être observée dans le pergélisol chaud qui peut éprouver des plages de température de -3 à 0 . Le comportement de la contrainte de cisaillement versus déplacement et suivie par une perte importante de la capacité portante du pieu. 1. INTRODUCTION Foundation design in frozen ground must satisfy both thermal and serviceability considerations (Weaver and Morgenstern, 1981). Several researchers (e.g., Maksimov 1967; Nixon 1978; Linell and Lobacz 1980) considered the thermal aspect in design of piles in frozen soils using well established analytical solutions. On the other hand, the serviceability and settlement was thought to lack clear guidelines. Therefore, efforts have been dedicated mostly to evaluate the bearing capacity and settlement behavior for piles in frozen grounds with scant attention to the thermal role. The performance of piles in frozen ground is a function of soil type, temperature, loading rate, and pile type. In the past, pile design was primarily based on adfreeze bond strengths at the pile-soil interface where the pile failure is defined by the rupture of the adfreeze bond and excessive settlement at the pile tip. Therefore, sufficient adfreeze bond is required to sustain both the structural load and any downdrag or frost heave loads subjected to the pile following to frost thawing or soil freezing respectively. The US Army/Air Force (1967) suggested that the end bearing capacity should be neglected from design consideration for piles with base diameter smaller than 150 mm attributing that greater movement is required for mobilizing the end bearing compared to the much smaller movement for adfreeze rupture. In general, the design criteria for piles in frozen ground can be expressed using Coulomb failure criterion similar to unfrozen ground as follow: [1] Where, and are the interface shear strength parameters and is the normal stress acting on the pile-soil interface. Weaver and Morgenstern, (1981) correlated the adfreeze shear strength to the long-term shear strength of the frozen soil using an the pile surface. They suggested that for a very rough pile surface, the adfreeze strength could reach the shear strength of the frozen soil. In this correlation they neglected contribution of the friction at the interface of the piles attributing that to the small confining pressure ( ) acting on the pile shaft in frozen grounds. Therefore, the long-term adfreeze strength of the piles in frozen ground was expressed in the following form: Where is the long-term adfreeze strength, is the long-that describes the surface roughness of the pile and with a value of 0.6 for steel, 0.7 for concrete and 1 for corrugated steel pipe piles. Ladanyi and Theriault (1990) reported that the long-term capacity of the pile shaft in frozen ground does not solely depends on the long-term adfreeze, but also on the residual friction angle at the interface, and respectively on the total lateral ground stress. The lateral stress acting on the pile

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