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| Titel |
A new methodology to simulate subglacial deformation of water-saturated granular material |
| VerfasserIn |
A. Damsgaard, D. L. Egholm, J. A. Piotrowski, S. Tulaczyk, N. K. Larsen, C. F. Brædstrup |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1994-0416
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| Digitales Dokument |
URL |
| Erschienen |
In: The Cryosphere ; 9, no. 6 ; Nr. 9, no. 6 (2015-11-20), S.2183-2200 |
| Datensatznummer |
250116874
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| Publikation (Nr.) |
 copernicus.org/tc-9-2183-2015.pdf |
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| Zusammenfassung |
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
grain-fluid 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. |
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