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An Exploratory Experimental Evaluation of ε50 in c-ϕ soils
Civil and Environmental Engineering
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In this research, we explored the influence of several factors on the strain at 50% of maximum deviatoric principle stress, ε50, of c- ϕ (cohesive granular) soils. C- ϕ materials are described as Mixed Soils behaving like granular and cohesive soils with properties defined as containing 15%-35% of plastic clay/silt with PI>7. These soils are often encountered in the design of deep foundations and the common practice is to treat them either as cohesionless or cohesive soils (i.e. assuming one behavior or another). ε50 is one of the key parameters in the design of deep foundations under axial and lateral loading. Evans and Duncan (1980) provided correlations with Su (undrained shear strength) for cohesive soils and Norris (1977) suggested correlations with void ratio for various coefficients of uniformity (CU) for cohesionless soils. However, available literature does not provide any recommendations for c- ϕ materials. In this study, we investigated the effects of several parameters such as relative density, confining stress, PI, fines content, friction angle, cohesion, etc. on ε50 through a series of consolidated undrained triaxial tests on c-ϕ soil samples. Ultimately, this research attempted to establish correlations with key characteristics of c-ϕ soils. Furthermore, we collected and documented all available literature on recommended ε50 values for different soil types and summarized the recommendations in tables to be used in practice. In the end, we demonstrated the significance of ε50 selection through a field load test conducted on an axially loaded drilled shaft.