Discrete element modeling of subglacial sediment deformation

Damsgaard, Anders; Egholm, David Lundbek; Piotrowski, Jan A.; Tulaczyk, S. M.; Larsen, Nicolaj K.; Tylmann, Karol

doi:10.1002/2013jf002830

Cited by 36 publications

(67 citation statements)

References 121 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In this study, we explore the interaction between the fluid and granular phases in water-saturated consolidated particle assemblages undergoing slow shear deformation. A dry granular assemblage deforms rate-independently in a pseudostatic manner when deformational rates are sufficiently low (GDR-MiDi, 2004;Damsgaard et al, 2013). Critical-state deformation of water-saturated granular materials is rate independent during slow shear, but transient changes in pore pressure may cause initiation of liquefaction at higher shear velocities (Goren et al, 2011).…”

Section: The Fluid Modelmentioning

confidence: 99%

“…The elastic stiffness is k n in the normal direction to the contact and k t parallel (tangential) to the contact interface. The magnitude of the tangential force f t is limited by the Coulomb frictional coefficient µ (Cundall and Strack, 1979;Luding, 2008;Radjaï and Dubois, 2011;Damsgaard et al, 2013). The bulk Coulomb friction coefficient of a granular assemblage depends on the elastic properties, the inter-particle frictional coefficient, and the granular packing arrangement (e.g., Booth et al, 2014).…”

Section: The Granular Modelmentioning

confidence: 99%

“…The large size allows us to perform the temporal integration with larger time steps (Radjaï and Dubois, 2011;Damsgaard et al, 2013). The frictional force between two bodies is independent of their size (Amontons' second law) but is proportional to the normal force on the contact interface (Mitchell and Soga, 2005), as reflected in the contact law in the discrete element method (Eq.…”

Section: Computational Experimentsmentioning

confidence: 99%

“…Temporal upscaling does not influence the mechanical behavior of dry granular materials, as long as the velocity is below a certain limiting velocity (GDR-MiDi, 2004;Damsgaard et al, 2013;Gu et al, 2014). The shearing velocity used here (2.32×10 −2 m s −1 ), although roughly 3 orders of magnitude greater than the velocities observed in subglacial environments (e.g., 316 m a −1 = 10 −5 m s −1 ), guarantees quasi-static, rate-independent deformation in the granular phase, identical to the behavior at lower strain rates.…”

Section: Scaling Of the Shear Velocitymentioning

confidence: 99%

“…A shear-rate dependence in a grain-fluid mixture can only originate from the fluid phase, since dry granular materials deform rate-independently under pseudo-static shear deformation (GDR-MiDi, 2004;Damsgaard et al, 2013). Rate dependence emerges, however, as soon as the flow of viscous pore fluids starts to influence the solid phase.…”

Section: Strain-rate Dependencymentioning

confidence: 99%

See 4 more Smart Citations

A new methodology to simulate subglacial deformation of water-saturated granular material

et al. 2015

View full text Add to dashboard Cite

Abstract. The dynamics of glaciers are to a large degree governed by processes operating at the ice-bed interface, and one of the primary mechanisms of glacier flow over soft unconsolidated sediments is subglacial deformation. However, it has proven difficult to constrain the mechanical response of subglacial sediment to the shear stress of an overriding glacier. In this study, we present a new methodology designed to simulate subglacial deformation using a coupled numerical model for computational experiments on grainfluid mixtures. The granular phase is simulated on a per-grain basis by the discrete element method. The pore water is modeled as a compressible Newtonian fluid without inertia. The numerical approach allows close monitoring of the internal behavior under a range of conditions.Our computational experiments support the findings of previous studies where the rheology of a slowly deforming water-saturated granular bed in the steady state generally conforms to the rate-independent plastic rheology. Before this so-called critical state, deformation is in many cases accompanied by volumetric changes as grain rearrangement in active shear zones changes the local porosity. For previously consolidated beds porosity increases can cause local pore-pressure decline, dependent on till permeability and shear rate. We observe that the pore-water pressure reduction strengthens inter-granular contacts, which results in increased shear strength of the granular material. In contrast, weakening takes place when shear deformation causes consolidation of dilated sediments or during rapid fabric development. Both processes of strengthening and weakening depend inversely on the sediment permeability and are transient phenomena tied to the porosity changes during the early stages of shear.We find that the transient strengthening and weakening in turn influences the distribution of shear strain in the granular bed. Dilatant strengthening has the ability to distribute strain during early deformation to large depths, if sediment dilatancy causes the water pressure at the ice-bed interface to decline. Oppositely, if the ice-bed interface is hydrologically stable the strengthening process is minimal and instead causes shallow deformation. The depth of deformation in subglacial beds thus seems to be governed by not only local grain and pore-water feedbacks but also larger-scale hydrological properties at the ice base.

show abstract

Section: The Fluid Modelmentioning

confidence: 99%

Section: The Granular Modelmentioning

confidence: 99%

Section: Computational Experimentsmentioning

confidence: 99%

Section: Scaling Of the Shear Velocitymentioning

confidence: 99%

Section: Strain-rate Dependencymentioning

confidence: 99%

See 3 more Smart Citations

A new methodology to simulate subglacial deformation of water-saturated granular material

et al. 2015

View full text Add to dashboard Cite

show abstract

Ice flow dynamics forced by water pressure variations in subglacial granular beds

Damsgaard

Egholm

Beem

et al. 2016

Geophysical Research Letters

Self Cite

View full text Add to dashboard Cite

Glaciers and ice streams can move by deforming underlying water‐saturated sediments, and the nonlinear mechanics of these materials are often invoked as the main reason for initiation, persistence, and shutdown of fast‐flowing ice streams. Existing models have failed to fully explain the internal mechanical processes driving transitions from stability to slip. We performed computational experiments that show how rearrangements of load‐bearing force chains within the granular sediments drive the mechanical transitions. Cyclic variations in pore water pressure give rise to rate‐dependent creeping motion at stress levels below the point of failure, while disruption of the force chain network induces fast rate‐independent flow above it. This finding contrasts previous descriptions of subglacial sediment mechanics, which either assume rate dependence regardless of mechanical state or unconditional stability before the sediment yield point. Our new micromechanical computational approach is capable of reproducing important transitions between these two end‐member models and can explain multimodal velocity patterns observed in glaciers, landslides, and slow‐moving tremor zones.

show abstract

A Theoretical Model of Drumlin Formation Based on Observations at Múlajökull, Iceland

et al. 2017

View full text Add to dashboard Cite

The drumlin field at the surge‐type glacier, Múlajökull, provides an unusual opportunity to build a model of drumlin formation based on field observations in a modern drumlin‐forming environment. These observations indicate that surges deposit till layers that drape the glacier forefield, conform to drumlin surfaces, and are deposited in shear. Observations also indicate that erosion helps create drumlin relief, effective stresses in subglacial till are highest between drumlins, and during quiescent flow, crevasses on the glacier surface overlie drumlins while subglacial channels occupy intervening swales. In the model, we consider gentle undulations on the bed bounded by subglacial channels at low water pressure. During quiescent flow, slip of temperate ice across these undulations and basal water flow toward bounding channels create an effective stress distribution that maximizes till entrainment in ice on the heads and flanks of drumlins. Crevasses amplify this effect but are not necessary for it. During surges, effective stresses are uniformly low, and the bed shears pervasively. Vigorous basal melting during surges releases debris from ice and deposits it on the bed, with deposition augmented by transport in the deforming bed. As surge cycles progress, drumlins migrate downglacier and grow at increasing rates, due to positive feedbacks that depend on drumlin height. Drumlin growth can be accompanied by either net aggradation or erosion of the bed, and drumlin heights and stratigraphy generally correspond with observations. This model highlights that drumlin growth can reflect instabilities other than those of bed shear instability models, which require heuristic till transport assumptions.

show abstract

Discrete element modeling of subglacial sediment deformation

Cited by 36 publications

References 121 publications

A new methodology to simulate subglacial deformation of water-saturated granular material

A new methodology to simulate subglacial deformation of water-saturated granular material

Ice flow dynamics forced by water pressure variations in subglacial granular beds

A Theoretical Model of Drumlin Formation Based on Observations at Múlajökull, Iceland

Contact Info

Product

Resources

About