Frost Heaving of Bedrock in Permafrost Regions

Abstract

Frost heaving of bedrock in permafrost areas is a widespread process that produces a variety of landforms and exhibits movements of possible concern to engineering endeavors. While the study of ice growth in soils is an actively researched subdiscipline of soil mechanics, the same process in rock escaped notice until major engineering projects were considered in areas outside the dominantly soil covered parts of the North American arctic. Geology and hydrology exert a fundamental control on the style of the rock heave process. Displacement of single blocks, panels, and domes has taken place where saturated zones in the active layer become confined by the downward-advancing freezing front. The attempted expulsion of water by continued freezing supplies the heaving force, much in the manner of the closed-system pingo. This mechanism is favoured where the water table lies close to the surface in the massive granites, gneisses, and quartzites of the Canadian Shield. The mechanism requires that ice-filled cracks be able to seal in high pore water pressures. Field measurements show that water pressures up to 400 kPa are attained within the confined water bodies, a pressure considerably in excess of that needed to offset the overburden pressure. Where drier conditions prevail and rocks less resistant to weathering are present, the accumulation of weathering products in cracks can provide a site for heaving. In this case, the forces are produced by the growth of segregated ice in soil-filled cracks. This mechanism is common in sandstones and limestones and contributes to the mechanical breakdown of outcrops. Laboratory experiments confirm that horizontal movements can be produced by vertically-downward freezing in vertical soil-filled cracks. This mechanism is widespread because saturated conditions are not necessary. Resurveying of markers at several locations from Churchill, Manitoba to the central Arctic Islands reveals yearly movements of up to 5 cm, horizontally and vertically. Recorded movements are most common at sites subject to the ice segregation mechanism. Vertical movements tend to be cyclical due to settlement during thaw. Features formed by the water expulsion mechanism presumably reach an equilibrium height and show little further movement.