Modeling the performance of an air convection embankment (ACE) with thermal berm over ice-rich permafrost, Lost Chicken Creek, Alaska

Abstract

Construction of roadway embankments over permafrost often results in settlement due to thawing of ice-rich foundation soils. The air convection embankment (ACE) is a relatively new design developed to reduce thaw settlement. In 2012 an ACE and an adjacent thermal berm were constructed as part of a realignment of the Taylor Highway near Lost Chicken Creek, Alaska. The Taylor Highway, a minor roadway with a gravel surface, is closed and not maintained during the winter months. To evaluate the thermal performance of the newly-constructed ACE and thermal berm, we performed field work and laboratory testing to determine foundation soil and embankment properties, measured temperatures at the base of the embankment, and developed two-dimensional finite element models to estimate long-term stability. Measurements indicated temperatures beneath the ACE were significantly colder than beneath the thermal berm. In the modeling, we simulated plowed (PC) and snow-covered (SC) conditions. Both models indicated that the ACE experienced density-driven air convection during the winter months. The PC scenario produced significantly colder temperatures within the ACE and underlying foundation soils. The SC model results more closely matched measured temperatures beneath the ACE, which is reasonable considering the lack of maintenance on the Taylor Highway during winter. The modeling indicates that ACE performance is improved through plowing of the surface, and that an ACE is more effective in maintaining frozen conditions in the foundation soils than the thermal berm; in fact, the modeling and measured temperatures indicate that the thermal berm actually raises the temperature in the foundation soils. Based on the results, we expect that thaw settlement will occur beneath the thermal berm until thermal equilibrium is reached, whereas the majority of the ACE will remain stable.