Large-scale direct shear testing of compacted frozen soil under freezing and thawing conditions

Embankments in the Arctic are often constructed during winter conditions to preserve the underlying permafrost and minimize environmental impacts. However, there is a limited understanding as to how frozen soils, compacted during sub-freezing conditions behave, and how this impacts the overall performance of the embankment, especially during the first thawing season following winter construction. Fills are very difficult to compact at sub-zero temperatures with ground ice present in them. They are strong and stiff when frozen but they become softer and more compressible upon thawing. This reduction in shear strength due to thawing can lead to slope instability and embankment failure. This paper describes part of a larger study on the structural stability of embankments constructed in the winter along the new Inuvik-Tuktoyaktuk Highway in the Northwest Territories, Canada. A series of large-scale direct shear tests was conducted on laboratory-compacted frozen fill to determine its shear strength. Frozen soil chunks were compacted at −10 °C. Normal stresses of 25, 50, and 100 kPa were selected corresponding to the range of applied vertical stresses expected in the field. Horizontal and vertical displacements, applied normal stresses, and horizontal loads were recorded throughout testing. The tests were conducted in an environmental chamber under frozen, thawed, and cyclic freeze-thaw conditions. The frozen soil samples showed high shear strength when frozen, but upon thaw and following freeze-thaw conditions the shear strength was reduced by as much as 50%. The most critical condition, based on the tests conducted, occurs during the onset of the first thawing when the ice bonding in the soil matrix melts. Numerical models were developed using a finite difference program and calibrated with the results from the large-scale direct shear tests. It was demonstrated that restrained dilatancy affects the results of the simulations but the soil maintains the critical state friction angle for increased normal stresses.