Development of a Correlation Model between a 20kg Clegg Hammer and Field CBR for Measuring Subgrade Strength in Forest Roads in Western Oregon

Abstract

Much of the forest industry in the western United States does not require control of compaction on forest road subgrades despite the potential economic and environmental gains from improved subgrade construction. Development of tools such as the Clegg impact hammer allow for rapid testing of the subgrade during construction, and Clegg Impact Values (CIV) have been correlated with more robust soil test values such as the California Bearing Ratio (CBR) that are often used for road design. A new correlative model that relates the CBR to 20 kg CIV was developed entirely from field data, differing from previously developed models that used laboratoryprepared soils. Another advantage of this model is that it uses a 20kg Clegg impact hammer. The 20kg Clegg impact hammer has a larger testing area than the more common 4.5kg Clegg impact hammer; thus, there is less influence from small deviations in the soil matrix due to items in the fill such as rocks or organic matter. The correlation coefficient for this model is 0.71, which is lower than that for models previously developed from laboratory results. The use of a lower bound, onetailed, 90% confidence limit is presented as a useful design model to determine a conservative of soil strength that can be measured directly during construction.