Subgrade soil is the foundation soil that supports the different pavement layers and the dynamic load of traffic. American Association of State Highway and Transportation Officials guide for the design of pavement structures recommends the use of soil resilient modulus to represent the dynamic strength of subgrade soil. Soil resilient modulus can be obtained from repeated load tests. The instrumentation and technical experience required for this test are not available in many soil testing laboratories. For certain types of soil, resilient modulus is sensitive to the variation of moisture content. Pavement design guide requires the use of an effective value for subgrade resilient modulus that reflects the seasonal variation of soil moisture content. The objective of this research is to evaluate the sensitivity of resilient modulus of typical Manitoba soils to the variation of moisture content. A repeated load triaxial test was conducted on three types of soils: sandy silt, sandy clay, and high plastic clay. Soil samples were prepared at four levels of moisture contents that cover the dry and wet sides of standard proctor compaction curve (below and above the optimum moisture content). Soil samples were subjected to loading combinations with different dynamic loads and confining pressures. Results showed that cohesive soils are more sensitive to moisture variation than cohesionless soils. The results of these tests will be used to develop effective values for subgrade resilient modulus which will be incorporated in the structural design of new pavements. The results of these tests will also be used as reference values for assessment of basic techniques to improve resilient modulus and reduce its sensitivity to moisture content variation.
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