Evaluation of Uncertainty for Soil Water Characteristic Curve Measurements and Implications for Predicting the Hydro-mechanical Behavior of Unsaturated Soils
AuthorFissel, Lauren Elizabeth
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In engineering design and classical soil mechanics, the assumption of saturated soil is the most conservative, weakest state that can be assumed. Classical soil mechanics involves the use of saturated soil parameters, including effective stress, effective cohesion, and effective friction angle. The use of unsaturated soils in design would be less conservative, but could lead to savings. Unsaturated soil mechanics also involves additional stress state variables net normal stress and matric suction. Because of the involvement of matric suction, determination of the soil water characteristic curve (SWCC) must occur simultaneously with strength testing of unsaturated soils. The calculation of unsaturated soil strength in design should have reliable reproducibility because unsaturated soil is a less conservative state. Variability in predictions of unsaturated shear strength may result do to variability in SWCC parameters. This possibility was investigated in this study, with emphasis on SWCC parameter variability. Variability of inter-specimen SWCC measurements was quantified for six soil samples. The hanging column method, an evaporation method with the UMS HYPROP apparatus, and the dew point potentiometer method provided measurement of discrete SWCC data. The van Genuchten SWCC function was parameterized using regression on data from at least 10 HYPROP trials per soil, individual measurements from the hanging column, and the same two dew point potentiometer data per soil. The HYPROP apparatus measures suction with upper and lower tensiometers within the soil specimens. Variability in water content was + 0.13 to + 0.16 at any given suction, indicating random variability in SWCC prediction despite identical specimen preparation. Variability in replicate parameter estimates for a single soil may be greater than 14% of median estimates. This result may impact testing programs that commonly rely upon a single SWCC measurement and subsequent parameterization. Additionally, this variability could affect strength testing reliant upon replicate specimen preparation. Unsaturated shear strength was predicted with a model that relies on the SWCC parameters estimated in this study. Based on this model, SWCC parameter variability resulted in minimal unsaturated strength prediction variability at suctions less than 1 kPa and increasing variability beyond this point. However, the same model failed to predict unsaturated shear strength results of this study obtained from unsaturated direct shear testing.