Insitu Determination of Residual Soil Shear Strength Parameters Using the Standard Penetration Test with Torque
Residual soils are found throughout the world and often behave differently than transported soils. Residual soils are identified as materials developed from the insitu weathering of rock that has remained in the location of its origin as opposed to transported soils which are developed though processes that include erosion, transport, and deposition. Geotechnical engineers rely on insitu and laboratory soil investigation techniques to determine soil properties, even though there is little within the current standard procedures to quantitatively distinguish between residual and transported soils. Due to the increasing volume of infrastructure development in areas where residual soils are present, there exists a need to better understand and quantify the properties and behaviors of residual soils. The objective of this dissertation was to develop direct and indirect relationships between the insitu standard penetration test with torque (SPT-T) and the laboratory shear strength of residual soils. Three research sites, located in Charlotte, North Carolina near the geographic center of the Southern Piedmont region, were characterized using the SPT-T, the consolidated drained triaxial test, the dilatometer test, soil classification tests, and an interface shear test. The results suggest that a possible relationship exists between the SPT-T maximum torque and the cohesion. The investigation also demonstrated that the maximum torque can provide an assessment of triaxial shear strength, although the relationship was largely influenced by the vertical effective stress. Various relationships within the data set were explored and many produced low correlations, with the majority of the R2 values below 0.4. The most promising relationship for a geotechnical engineer was maximum torque multiplied by the insitu water content versus triaxial shear strength which suggests that the insitu water content at the time of the SPT-T has an influence on the measured maximum torque. The most promising geologic correlations were chroma versus traixial shear strength and minor mineral percentage versus triaxial shear strength. These two trends suggest that geologic parameters can provide an assessment of triaxial shear strength. Another important result was that the undisturbed residual soils investigated did show inherent cohesion, which is generally neglected in design. Additionally, an exploratory investigation of remolded residual soils suggests that the cohesion decreases and effective angle of internal friction increases due to remolding. Also, both the SPT-T and dilatometer tests provided un-conservative friction values for undisturbed residual soils. This dissertation illustrates the importance of quantitatively distinguishing between residual and transported soils. The main drawback of the study was the limited number of tests/test sites available for the research. The results clearly show the feasibility of the methods and justify further research. Ultimately, the implementation of the quick and simple torque test to a site investigation can provide valuable data for geotechnical design.