Session K34: Soft Earth Geophysics

Name: Douglas J. JerolmackTitle: The Fragile Earth

It is all too clear that Earth is fragile, as human disturbances disrupt the natural cycles that sustain a habitable planet. Earth is also fragile matter: apparently solid soil can suddenly lose rigidity when stressed by rainfall or earthquake shaking; and earth materials exhibit creep, aging and hysteresis. In this talk I consider some existential challenges of living sustainably on a fragile (matter) planet, and advocate for the need of Soft Matter Physics to address these challenges.

To first order, we may consider Soft Earth materials -- and their associated geophysical flows -- to be composed of three ingredients: frictional particles (sand/silt), cohesive particles (clay, fine silt, organic material), and water. In this talk I demonstrate how the origins of complex behaviors of Earth materials arise from these ingredients, with surprises for both geologists and physicists. Experiments show that creep hardens hillslope soils and river beds, that landscapes `breathe' in response to thermal forcing, and that the structure and dynamics of granular creep are similar to glass. Current granular frameworks, such as jamming, cannot account for these behaviors. I then examine the surprising explanatory power that idealized models of yielding in amorphous solids have for predicting failure and flow behavior of complex Soft Earth materials. Finally, I show how nature assembles robust, hierarchical aggregates from evaporation of polydisperse suspensions, and speculate on how we may harness this process to mitigate soil erosion and create sustainable geomaterials for construction. When possible, I illustrate how concepts and experimental techniques can be taken out of the lab and into the field.   

3D Discrete Element Model and Continuum Theory for Quasi-static Granular Flow of Ice Mélange

Ice mélange is a granular pack of sea ice and icebergs that is tightly packed in tidewater glacier fjords and can suppress iceberg calving by providing resisting stresses called buttressing. Without measurements of buttressing forces provided by ice mélange, it remains challenging to predict glacial calving events and thus the mass loss of ice sheets.

To quantify the buttressing force on the glacier terminus, previous work has developed 2D discrete element models (DEM) for ice mélange. However, recent observations and experiments suggest that the mélange thickness affects its buttressing force. Here we develop a 3D DEM that simulates a moving terminus pushing against a collection of cubic icebergs that is confined within a channel. To study the effect of fjord friction on mélange behaviors, we adopt straight and rugged channel configurations. In a straight channel, the mélange moves like a plug flow with a constant buttressing force. In a rugged channel, the mélange forms shear bands and undergoes stick-slip cycles, which is evidenced by slight fluctuations in buttressing force during the steady state. We developed a 3D continuum theory to describe ice mélange in the quasi-static regime. The resulting analytical model reveals that the buttressing force depends on the square of the mélange thickness, exhibiting an excellent agreement with DEM simulations. Validated by remote observations, we conclude that the mélange thickness at the terminus must be larger than 145 meters to provide effective buttressing.   

Rheological fingerprints of soft earth suspensions

Soft earth suspensions, such as debris flows, are fast-flowing slurries of soil matter, that cause huge loss of human life and infrastructure. These dense suspensions-based geophysical flows are becoming more dangerous and increasing in frequency, exacerbated by the climate change effects. Today, a dichotomy exists, between granular pore pressure and complex fluid rheology, in explaining the origins of flow behavior of these dense soil slurries, prompting a fundamental question: Is there a generalized way to capture their `failure' and `flow' mechanics to propose better hazard prediction models? Here, we experimentally show that a simple model complex fluid mixture -- comprising kaolin clay and silica sand -- captures the non-inertial flow behavior of debris mixture, observed in both laboratory experiments and natural flows. Surprisingly, we find that the physics of debris flow mechanics are a strong function of the `clay ratio', which is the volumetric ratio of clay to total solids, controlling the cohesive-to-frictional interactions in the suspension mixture. The shear thinning exponent associated with yielding in these sand-clay suspension mixtures decreases with increasing sand concentration. Our observations suggest that the attenuating shear-thinning exponent results from soft soil material microstructural annealing changing the failure mode from plastic dissipation-like gradual yielding in pure clay suspensions to a Mohr-Coloumb mode granular failure in sand-rich suspensions. These insights contribute to a deeper understanding of the flow mechanics of debris mixtures, potentially leading to improved mitigation strategies and risk assessment measures.   

Impact of Salinity on Rheology Properties and Erosion Threshold of Sand-Clay Mixture

Abstract

 

 

Mud, consisting of clay and sand, is ubiquitous on each surface and is critical in land surface processes, such as coastal erosion. However, a fundamental understanding of clay's erosional and rheological properties under varying salinity is currently lacking. Here we investigate the influence of salinity on the rheology properties and erosion threshold of the mixture through systematical laboratory experiments.  First, we developed homogeneous sand-clay mixtures with varying sea salt concentrations to simulate mud in environments with different salinity.  The mixtures were made using a custom-built experimental setup involving a vacuum pump, desiccator, CO2 tank, and DI water. Rheological characteristics of the samples were measured using a Discovery Hybrid Rheometer to estimate flow curves and obtain the storage-loss modulus distribution. We also quantified the yield stress of the samples as the limit of the linear viscoelastic region.

Additionally, we measured the critical shear stress to erode the mud in a flume, using a digital camera and an Acoustic Doppler Velocimeter that measures 3D water velocity fluctuations. Our results show that the stirring or the mixing procedure, which influences the homogeneity of clay-sand mixtures, can impact their yield stress by about a factor of 6. In addition, we show that salinity significantly impacts the viscosity and yield stress of the clay-sand mixture. Our results provide critical insights into the erosion of salinity-responsive sand-clay mixtures, commonly found in wetlands, coastal areas, and estuaries. The dependence of erosion threshold and rheological properties on salinity we identified will facilitate future designs of erosion control structures and prediction of sediment transport in these environments.    

Force signatures of creep in a photoelastic granular medium

Creep is the subsurface, slow movement of constituents in a granular packing due to applied stress and the disordered nature of its grain-scale interactions, as in the example of slowly evolving sloped hillsides. We explore creep through experiments of quasi two-dimensional piles of disks that are made from a birefringent material, which allows us to use image acquisition to observe both grain movements and grain-scale force networks. Controlled disturbances to the pile are used to instigate creep events. We investigate the evolution of the force network and particle rearrangements to illuminate signatures of these events. We find that shifts in force chains provide a precursor to larger, avalanche-scale disruptions that can predict where an avalanche will occur. In addition, changes in force chain structure manifested at greater depth than any noticeable particle shifts, suggesting that there is a distinct "flowing" layer that transitions to creep behavior deeper in the pile.   

Findings and lessons learned while translating statistical physics to Soft Earth materials

A hallmark of many processes within Earth's subsurface is the transitions in states that natural granular materials undergo, from structurally arrested to creeping to flowing. These transitions in material states can produce geohazards (e.g., landslides, ground-rupturing earthquakes, and liquefaction) that cause loss of life and destroy infrastructure. Using continuum models primarily, the geoscience community has yet to identify reliable ways to forecast if, when, and how structurally arrested buried sediments will creep, flow, and or suddenly fail. Granular physics research indicates that the relationships between shear stress and the spatial distribution and magnitude of mesoscale features such as interparticle friction, pore pressure, force chain distribution, and coordination number exert control on the rigidity and flow of lab-reconstituted and simulated granular materials (e.g., a column of 2-D disks or glass spheres). Thus, a natural next step for the geoscience community is to assess whether such tools, developed for more idealized systems, are useful for studying natural granular materials, which tend to be more disordered. This talk discusses findings and lessons from viewing natural earth materials through a granular lens. Specifically, we report on the use of a series of field and lab experiments to assess the degree to which (1) energy serviced during deposition influences the configurational geometry, strains, and rigidity of natural earth materials, (2) whether the frictional jamming framework for forecasting transitions between solid-like and fluid-like states provides insights into the development and erasure of deformation memory and granular flow within shallow fault zones, (3) whether various forms of entropies provide useful ways to characterize the geometric complexities of natural granular materials, and (4) whether the density of excited vibrational modes, which has been shown to provide a potential precursor signal to unjamming, may be extracted from seismograms collected within natural sediments. The main lessons are that there are significant benefits to viewing the Earth through a granular lens and to working interdisciplinarily to improve existing granular physics frameworks so they capture the hysteresis and disorder within natural granular materials.    

Earthquake Nucleation and the Initiation of Frictional Ruptures

Recent experiments have demonstrated that rapid rupture fronts, that are actually shear cracks, mediate the transition to frictional motion. Moreover, once these dynamic rupture fronts ("laboratory earthquakes") are created, their singular form, dynamics and arrest are well-described by fracture mechanics. Ruptures, however, need to be created within initially rough frictional interfaces, before they are able to propagate. This formation process, occurring bellow the critical ('Griffith') length for crack instability, is not described by our current understanding of fracture mechanics.
By conducting controlled nucleation experiments, in which the interface is continuously imaged, we have been able to gain a detailed description of the nucleation process. We find that the expansion of the nucleation patch is qualitatively different than the propagation of the fully formed rupture front. It occurs at extremely slow and constant velocities, and it is 2D in nature. Some of the features of this expansion, such as self-similar evolution and stress-dependent timescales, are general. However, the details of this process are governed by the local conditions at the nucleation region. Due to the slow rates of expansion, local variations in the surface toughness (the 'fracture energy') can influence characteristics such as the exact nucleation point, the shape of the patch, and the stress threshold that is needed for nucleation to occur.
As nucleation is not described by the usual frameworks that are used to explain rupture propagation, understanding the driving mechanism of it is of fundamental importance to questions ranging from earthquake nucleation and prediction to processes governing material failure. We propose a theoretical model for this mechanism, and show that it captures the unique characteristics of the nucleation process as well as the transition to dynamic rupture propagation that is described by standard fracture mechanics.   

Earthquakes at the lab scale and their memory effect

We study an experimental reproduction of a seismic fault by submitting a granular sample of glass beads to a biaxial test and looking at the stationary phase following the deformation's localisation into shear bands. A technique based on interferometry allows us to measure the local deformation occurring in the sample. We observe fluctuation of the plasticity inside the shear bands, similar to the way earthquakes occur along  seismic fault lines. In both cases, the total displacement is the accumulation of the individual slip of each event. The spatio-temporal dynamics of these lab-scale earthquakes follow the same statistical properties as real ones. Indeed, the amplitude distribution of the events follows a power law with the same exponent as the Gutenberg-Richter law[1]. We also observe that lab-earthquakes tend to cluster together in time and space forming aftershock sequences which follow Omori's law.

In my talk, I will present recent experimental results where we investigated the effect of the strain rate we impose on our sample on the memory effect we observe[2]. We showed that the aftershock sequences are driven by deformation and not time. I will also discuss a cellular automaton model reproducing this behaviour.   

Local failure of gradually tilted cohesive granular beds

In the presence of inter-particle cohesion, gradually inclining a rectangular bed of particles can lead to localized mobilization of grains on the downstream edge. Experiments of such failure are observed to display a loop-like structure on the top surface from where material is mobilized. By controlling the amplitude of cohesion, the bed geometry, and the grain size, we show that such features are characteristic of local cohesive failures at free edges or cliffs. Specifically, a gradient of cohesion in depth seems to be necessary to observe this phenomenon. In this study, experiments with controlled cohesive grains are used to investigate the observed shape of failure, using a 3D topographic reconstruction.   

Failing Just Right: Unraveling the Seabed Sediment Collapse Phenomenon

The dynamics of seabed granular avalanches underwater is a consequence of granular mixing with ambient fluid. Generally, underwater avalanches are characterized by the transition of granular packing from a solid to liquid phase, by a phenomenon called “slumping”, similar to a melting phase transition. However, under specific conditions, the granular packing can instead transition from a solid to a gas phase, in a process called “breaching.” This granular sublimation phenomenon produces turbidity currents, similar to those observed on the ocean floor. Previous studies have demonstrated the importance of packing fraction and pore pressure on dictating the failure mode of underwater avalanches. However, the effect of particle size on the nature of the failure transition remains elusive.  Here, we perform controlled experiments that delineate the transition from slumping to breaching in a two-parameter phase space: grain size and packing fraction. In our experiments, we use silica sand, classified by size, in a custom-built chamber with two-fold capabilities: an in-built fluidization chamber that adjusts the initial packing fraction of the granular material, and a pneumatic valve that mimics a controlled “dam break” releasing the material from the fluidized pressure chamber to ambient water. As grain size decreases and packing fraction increases, we observe a transition from slumping to breaching. We attribute the physical mechanism as a competition between particle and pore pressures; we predict that breaching occurs where pore pressure is greater than particle pressure. Our work provides a general framework to predict the failure of seabed sediments, which can aid in protecting underwater infrastructure.   

Statistical laws of stick-slip friction at mesoscale

Friction between two rough solid surfaces often involves local stick-slip events with different slip lengths. Here, we report a systematic study of stick-slip friction over a mesoscale contact area using a hanging-beam lateral atomic-force-microscope, which is capable of resolving frictional force fluctuations generated by individual slip events and measuring their statistical properties at the single-slip resolution. The measured probability density functions (PDFs) of the slip length dx, the maximal force Fc needed to trigger the local slips, and the local force gradient k of the asperity-induced pinning force field provide a comprehensive statistical description of stick-slip friction that is often associated with the avalanche dynamics at a critical state. In particular, the measured PDF of dx obeys a power law distribution and the power-law exponent is explained by a new theoretical model for the under-damped spring-block motion under a Brownian-correlated pinning force field. This model provides a long-sought physical mechanism for the avalanche dynamics in stick-slip friction at mesoscale.