A Study of the Strength and the Physical Properties of Glacier-ice Runways - Assesment of Troll Airfield, Antarctica
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Airfields located in Antarctica are unique because the runways are constructed by glacier ice. Ice as a construction material differs from conventional runway materials in sense of strength, hardness and temperature dependency, and new techniques and tools are therefore required for monitoring the runway strength. Troll Airfield is an example of such an airport. However, this airfield experiences an additional challenge related to particle accumulation on the runway surface. The minerals absorb heat from the sun due to the dark colour and thereby melt the surrounding ice. The holes are repaired by removing the particles and freezing new ice, a method called patching. A field investigation program was conducted at Troll Airfield where the objective was to determine the strength and the physical properties of the runway ice. The difference between the glacier ice and the patch ice was emphasized. For this purpose, an indentation test and a penetration test were conducted along the runway, and ice specimens were brought back to NTNU for determination of the physical properties. The indentation test was conducted by use of a recently developed borehole jack. This series involved a 30-mm diameter piston indenting a borehole wall at constant displacement rate of ~0.9 mm/s. The results indicated that the ice strength was mainly related to ice temperature, especially for temperatures above ~ -5 degree C. The patch ice was found to absorb more heat from sun radiation than glacier ice, and the patch ice strength was therefore in general weaker than the glacier ice. Furthermore, the failure behaviour of the stress-time curves were studied, and a thorough literature review is presented on this topic. The collected data showed that ductile failure behaviours dominated for temperatures above -8 degree C, and that brittle failure behaviours became relevant in the colder temperature range. This observation is supported in the literature, where a corresponding ductile-to-brittle transition is found for increasing indentation rates. In particular, a "sawtooth" loading pattern was studied and was related to a microstructurally modified "layer" in the ice adjacent to the piston. A Russian snow penetrometer test was recommended by Cold Regions Research and Engineering Laboratory (CRREL) as a monitoring procedure of the runway strength at Troll Airfield. However, this test proved to be unsuited for the ice pavement at Troll because the ice was too hard and dens, and sufficient penetration was thereby suppressed. Thin sections and CT scanning of the runway ice specimens have been performed in the cold laboratories at NTNU. The glacier ice structure is characterized as granular with moderate grain elongation in vertical direction. The average grain size is 4-5 mm in the horizontal plane, and the density is 845 kg/m^3. A small degree of anisotropy is observed. The patch ice has an isotropic, granular structure with grain sizes varying from 1-7 mm. The patch ice density is 898 kg/m^3. The CT scan of the ice specimens showed that the patch ice contains less air bubbles than the glacier ice.