The Microstructure of Compacted Moist Sand and Its Effect on Stress Transmission

An experimental investigation was conducted to provide a better understanding of the development of microstructure in unsaturated sand and its effect on dynamic behavior. Previous research has shown that compacting moist sands with varying amounts of water prior to dynamic loading can increase the stress transmission ratio by as much as a factor of two. The compaction method and amount of moisture have also been shown to influence static and dynamic behavior of sands In this study, sand specimens were dynamically compacted to a constant dry density at various saturations, using a standard Proctor hammer and subjected to high strain rate testing using the Split-Hopkinson Pressure Bar device. The influence of soil microstructure on compassion energy, stress transmission, -and compressional wave speed for Ottawa 20-30 sand was evaluated. Experimental results suggest that the dynamic behavior of the sand depends on soil microstructure, which is a function of the initial moisture content during compaction. An attempt was made to relate differences in dynamic soil behavior to to variations in soil microstructure formed during compaction. A two-dimensional grain orientation analysis was performed on epoxied specimens of compacted sand. Angular frequency histograms and orientation statistics were generated. Vector magnitude was analyzed as a function of saturation and an inverse relationship was found between vector magnitude and stress transmission ratio as a function of saturation. Results from the microstructural analysis suggest that microstructure may be a significant factor affecting the dynamic behavior of unsaturated soils. However, a clear and concise explanation of the microstructural development process is not now available.