Using a Clegg Impact Hammer to Measure Snow Strength

The development of strong and durable snow roads is critical to operations in and around McMurdo Station, Antarctica. As part of improving and assuring the health of the snow road transportation system, and for comparing different construction and maintenance techniques, the strength of the snow road is monitored. Historically, snow strength is measured using a Rammsonde snow penetrometer, which measures the resistance to penetration. While the entire snow profile is important, it is primarily the top layer (approximately 30 cm) that changes from day to day with different maintenance practices and road use. The Rammsonde provides a good measure of the strength profile, but does not characterize the near-surface well. The Clegg Impact Hammer is an instrument used to assess the bearing capacity of soils, as measured from the surface, and therefore could be useful for snow road assessment in this way. The Clegg Impact Hammer provides a strength index in response to a mass free-falling from a specific height and displaying the deceleration upon impact. During the austral summer of 2009/2010, Clegg Impact Hammers of three different masses were used on the snow roads at McMurdo Station. Tests were performed on a uniform snow surface and on the operational snow road for comparison. Because snow is more compressible than soils, several methods to analyze the Clegg data were explored. The equations used to relate the Clegg measurement to a California Bearing Ratio is also discussed.