Experimental evidence of threshold-driven gas breakthrough in thawing permafrost

Introduction Gas breakthrough is an important but poorly constrained mechanism for abrupt methane release during permafrost warming. This study presents an experimental investigation of gas breakthrough in saturated permafrost analogue soils subjected to controlled freezing and progressive thawing. Methods A silty sand collected from Alaska’s North Slope was tested using an integrated program comprising unfrozen water content measurements, direct tensile tests, constant-volume gas breakthrough experiments, and stress-controlled breakthrough tests under confining pressures representative of shallow to deep permafrost. Results Unfrozen Water Content (UWC) measurements revealed a sharp, nonlinear reduction below 0 °C, with residual unfrozen water persisting at low temperatures and minor thermal hysteresis between freezing and thawing paths. Tensile strength increased nonlinearly with decreasing temperature, transitioning from ductile behavior near −2 °C to brittle failure at lower temperatures, while tensile resistance near −1 °C was negligible. Gas breakthrough occurred only when gas pressure exceeded thresholds governed by temperature-dependent unfrozen water content, tensile resistance, and confinement. Under high confinement, breakthrough pressures substantially exceeded unconfined tensile strength and occurred primarily during thawing near −1 °C. Discussion These results demonstrate that permafrost thawing significantly increases susceptibility to gas breakthrough and provide experimental constraints for assessing methane release from degrading permafrost.