Expedient Frost Layer Measurements

Abstract

During the winter of 2004, several agencies interested in quick and easy methods for measuring the competency of frozen ground gathered for a field test in northern Minnesota. The objective of the experiment was to test various tools for quickly measuring frost depth and strength in natural areas ranging from forests to peatlands. Six tools were evaluated for effectiveness and ease of operation. The goal was to find the best tool for determining frost depth and strength. Foresters, loggers, oilfield personnel, land managers, and military personnel need better guidance to maximize operability and minimize site damage from timber harvesting activities, oilfield operations, and military training. The participants in the study included personnel from the University of Minnesota, Department of Forest Resources; the Minnesota Department of Natural Resources, Division of Forestry; the Alaska Department of Natural Resources, Division of Land; and the Cold Regions Research and Engineering Laboratory. The tools evaluated were: 1) Alaska Slide Hammer, 2) Hilti Drill, 3) Dor-Cone (drop cone penetrometer), 4) Dynamic Cone Penetrometer (DCP), 5) Russian Cone Penetrometer, and 6) Generic (Orange) Slide Hammer. The frost depth or strength was measured using each tool, efficiency was noted, and correlations between penetration and resisting force for frost strength estimates were evaluated. The fastest method to measure depth was to drill through the frost layer using the Hilti drill; however this did not result in any strength parameters. Each of the slide hammers and penetrometers yielded a strength index profile, but the speed of the measurement varied, depending on the nature of the frost (soil type and wetness) and the geometry and weight of the tool. In some cases, the measurement took much too long to be feasible, based on the slow rate of penetration and number of blows. For shallow frost depths, which would be most critical for limiting terrain disturbance, the small diameter penetrometers (Russian Cone Penetrometer, Generic Slide Hammer, and Alaskan Slide Hammer) give the most information in the least amount of time. These instruments could be calibrated against standard Civil Engineering tools for more meaningful strength values. Comments on design and comparison of the strength indices between instruments are also discussed.