The Potential Use of the Light Drop-Weight Deflectometer to Control Subgrade Compaction on Forest Roads

Abstract

Improving subgrade strength can reduce a road's construction and maintenance costs. Additionally, it may lessen some of the environmental impacts from forest roads. However, most forest roads receive little effort to establish or control compaction because of the perceived increase in construction costs and potential delays in the flow of wood from the area. A light drop-weight deflectometer was tested on two forest roads in western Oregon to determine if this tool could aid with improving forest road construction by improving the quality of the road's subgrade. One road was built on a fine sand-silt soil and the other was constructed on a silt-clay soil. The goal was to correlate the easily collected subgrade values produced from the light drop-weight deflectometer and wet unit weight to the soaked and nonsoaked California bearing ratio (CBR) values for the subgrade, which are common design variables used to determine the minimum thickness of the aggregate surface. The light drop-weight deflectometer showed mixed results with regards to its ability to predict the values from the more expensive CBR laboratory test. The results from site 1, sand-silt soils, showed the potential of the light drop-weight deflectometer to aid in construction control as the values from light drop-weight deflectometer, when combined with the wet unit weight, explained a statistically significant amount of the variability in the soaked and nonsoaked laboratory CBR values. These variables explain approximately 60% of the variation in CBR values. The data from site 2, the silt-clay soils, were not able to significantly predict the subgrade strength variables, and the road will need to continue to use the traditional methods for establishing and controlling compaction.