A Mechanical Model for Permafrost Thaw Subsidence

A mechanical model describing permafrost subsidence has been developed and correlated with a full scale field test performed by the Atlantic Richfield Company and Exxon Company, USA at Prudhoe Bay. The permafrost mechanical response is modeled with a linear stress-strain relation that incorporates pore pressure reduction as the subsidence loading mechanism. The pore pressure reduction is due to the phase change of pore ice upon thaw and was verified by field test measurements. The mechanical response of the well casing is included in the model with no slip assumed between casing and permafrost. The thaw subsidence model explains several features observed in the field test. The pore pressure reduction mechanism produces an upward rebound of the permafrost base, resulting in compressive casing strains above the base and tensile strains below. The pore pressure loading also produces an inward lateral motion of the thawed-frozen interface. The inward motion, together with layers of different soil types, produce the alternating compressive and tensile strains measured in the field test. These alternating strains can be significant, depending on permafrost lithology.