Characterization of unbound and stabilized granular materials using field strains in low volume roads

Low volume roads (LVR) account for approximately 70% of the world’s roads, equivalent to 26 million kilometers. In LVR, unbound and stabilized granular layers give structural strength to the road. Since LVR present lower traffic volumes and loads compared to high traffic roads, lower standards are demanded, resulting in limited studies for understanding their performance. The main objective of this research is the in-situ characterization of unbound and stabilized granular materials used in LVR layers by means of field strains. The selected strain coil system for the measurement of field strains, installed in different road sections located in different areas of Chile, gave reliable results with repetitive measurements and low electric noise. The collected elastic strains indicated a significant degree of anisotropy or dilation of the unbound granular materials in the horizontal plane of the road. The in-situ and stress-dependent resilient modulus and Poisson’s ratio of the road materials were estimated from field strains. Resilient modulus obtained from laboratory tests was approximately three times less than field modulus. Laboratory tests are normally accurate and reliable; however, these results suggest that an effort should be made in the laboratory to better replicate real material conditions found in LVR. The distress of the road materials was estimated using a model that relates the granular material vertical elastic strain and the number of load repetitions with the surface plastic deformation or rutting. The model predicted that, if laboratory strains instead of field strains are used for the rutting calculation, the material performance is overestimated, which could lead to the overdesign of LVR. Overall, the study concludes that an in-situ strain coil system installed in unbound granular and stabilized layers in LVR gives realistic and reliable measurements that could be used for the characterization of unbound and stabilized granular materials. These measurements will help to improve the structural design of this type of roads and other type of pavement structures in the long-term, although further research will be needed to collect more data from the field.