Our group uses a combination of laboratory and numerical experiments and theoretical approaches to better understand and more quantitatively predict the flow and deformation of Earth materials in initial loading and boundary conditions relevant to Earth’s surface and near-surface processes. A major focus of the group currently is on the behavior of granular geomaterials – and other soft geomaterials, complex fluids, and disordered solids that share some similarity in their structural properties and dynamical behavior with granular materials – in dry, fluid-driven, saturated, and unsaturated conditions. 

For numerical experiments, we use and collaborate to develop further, Discrete Element Method (DEM), DEM coupled to Computational Fluid Dynamics (CFD) method, DEM coupled to Finite Element Method (FEM) method, and Boundary Element Method (BEM) simulations. We also have an advanced rheometry laboratory dedicated to our research group, which is capable of performing steady-state and transient rheology experiments on sediments, soils, granular materials, and other complex fluids. 

We seek collaborations with field-going scientists and are excited to work across disciplinary boundaries with geophysicists, geologists, engineers, applied mathematicians, and computational scientists.

⇧ Figure 1: Examples of granular materials and geomaterial systems — in two modes of deformation — that we have studied in numerical and laboratory experiments. Panel (A) shows the powdered materials produced due to the wear of the two pieces of rock that are sheared across each other. This is an image from Professor Terry Tullis’s experimental rock friction laboratory in the Department of Earth, Environmental, and Planetary Sciences at Brown University. This powdered material is called the fault gouge in the earthquake and rock physics communities (Image is modified after Scruggs & Tullis, Tectonophysics, 1998). Panel (B) shows a snapshot of a sample of sheared granular material realized in a DEM numerical experiment (Ferdowsi & Rubin, Journal of Geophysical Research-Solid Earth, 2020). Image sequences (C)-(E) illustrate two styles of flow (slow creeping motion well below the surface and faster dense flow close to the surface) of granular materials in sandpile experiments performed by Komatsu et al., 2001 (Image is modified after Komatsu et al., Physical Review Letters, 2001).

We are also broadly interested in further understanding the physics and chemistry of the processes that shape and change the Earth’s surface in short (~human life) and long (geological) timescales. These include the physics of sediment transport, sediment erosion and deposition processes, and soil production and transport. These processes are also not very far from granular physics because most soils at some length scales are made of grains, and sediment transport is a hydrogranular process. Figure 2 above exemplifies some broad Earth and Planetary Sciences as well as Engineering areas where a more complete understanding of the behavior of granular and similar fragmented and soft geomaterials may lead to significant scienitific progress.

⇧ Figure 2: Granular physics in the Earth’s near-surface processes. Panels (A-D) show example geological hazards and processes where transient frictional response (rate- and state-dependent friction) of a localized shear zone plays a crucial role (in most cases, it fully controls) in the onset of sliding and the mode of instability. Panel (A) is an image of the 2005 La Conchita landslide in the town of La Conchita, California (image credit to the US Geological Survey, USGS). In this case, there is increasing evidence that the initial frictional instability that gets the landslide going takes place in the basal shear zone (which often is a rather narrow layer where deformation is localized, and it is filled with fragmented rocks and soils of all types; there is also an interplay between changes and effect of humidity and fluid transport with the “solid” materials phase in the shear zone) of the slide. Panel B shows the San Andreas Fault in San Luis Obispo County in California (image credit Tom Bean, CORBIS). Earthquakes are initiated by the frictional instability within the fault gouge, such as the one shown in Fig. 1A (albeit the fault gouge of Fig. 1A is formed in an experiment). Panel (C) shows the formation of subglacial debris (basal till) that has melted out from the dark striped basal ice layer (Photo credit M. J. Hambrey, 1987; https://www.swisseduc.ch). The transient friction of the basal till (powdered rocks and sediments) controls the speed and mode of sliding of the glaciers. Panel (D) is an aerial photograph of the Kilauea Volcano’s summit on June 12, 2018, after the onset of the caldera collapse (image credit: Kyle Anderson U.S. Geological Survey; see this Scientific American article for more information). A recent paper by Segall and Anderson provides strong evidence that the collapse event was controlled by the frictional instability of a shear zone in the caldera structure and that the frictional (“rate-and-state”) parameters of that shear zone are close to, or at least are meaningful, with regard to laboratory measurements (Segall and Anderson, PNAS, 2021).

Panel (E) is a photo of the Elwha river, a gravel-bedded river in the state of Washington (image credit to the National Oceanic and Atmospheric Administration (NOAA) Fisheries; please also check the link for more information about the influence of gravel sorting and armoring (a granular phenomenon where larger grains tend to localize around each other) of the riverbed structure on the river habitat. Panels (F) and (G) illustrate the development of convex, rounded hillslopes and the formation of ridge-and-valley topography in a soil-mantled terrain (made of hillslopes and valleys), which Dietrich and Perron argue in their 2006 Nature paper, can be the result of a particular (empirical) form of soil transport (constitutive) law (photo credit of panel F is to Daniel Hobley; photo credit of panel G is to J. Kirchner, modified from Dietrich & Perron, Nature 2006).