Collaborative Research: Multi-scale Modeling and Measurement of Clay Aggregate Behavior

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The goal of this research is to advance the understanding of the mechanical behavior of clays as engineering materials. Clays comprise very fine ( 2 um), electrically charged, chemically active particles whose behavior and assembly are strongly affected by physico-chemical forces operating on individual particles at the smaller (i.e., molecular and interfacial) scales. Clay minerals of different varieties occur widely in natural geological environments and are generally found in the form of aggregates or clusters whose interactions control the macroscopic engineering properties of clay-rich soils used in the design and construction of infrastructure. While micromechanics-based experimental and computational research has advanced significantly the understanding of the behavior of sands (particle sizes greater than 75um), the importance of the particulate nature of clays has not been extensively investigated. This collaborative project aims to develop and validate a new multiscale framework for understanding the macroscopic (i.e., continuum scale) mechanical properties of clays by studying the microscale aggregation of elementary clay particles and the interactions between the resultant clay aggregates. The project will provide the fundamental understanding needed to develop the next generation of constitutive models for mechanical properties of clays that can be used for subsurface engineering, ultimately reducing risks and costs associated with the design of foundations, underground construction and use of geological energy resources (e.g., clay shales, etc.). The research will impact directly upon the education of future geotechnical engineering and geomechanics students through the creation of online modules related to the small-scale measurement and multiscale modeling of clay behavior. The research work comprises a closely-integrated program of multiscale experimentation, atomistic and coarse-grained multiscale simulations for one common clay mineral, illite. The research involves the following main tasks: 1) modeling and experimental validation of individual clay aggregate behavior under a range of porewater chemistry conditions; 2) development of new modeling techniques and experimental methods for investigating aggregate-aggregate interactions and fabric (i.e., quantitative measurement of particle orientation and of orientation distribution function); and 3) validation of multiscale model predictions through comparison with macroscopic measurements of clay properties. Experimental measurements will use existing laboratory facilities at the University of Massachusetts Amherst (UMass) as well as the analytical facilities at the Argonne National Laboratory, while numerical simulations at the Massachusetts Institute of Technology (MIT) will take advantages of NSF eXtreme Science and Engineering Discovery Environment (XSEDE) national supercomputing resources. By developing a framework for understanding particulate-based clay micromechanics, the research aims to provide an innovative multiscale perspective for explaining the underlying basis of continuum-based clay properties such as cohesion and creep, which have to date only been observed phenomenologically via macroscopic approaches. The long-term goal is to generalize the methodology to enable bottom-up prediction of mechanical properties for complex natural soils that comprise mixtures of different clay minerals with silt- and sand-sized particles as well as varying porewater chemistry.