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Nicolaj Krog Larsen

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

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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.

The rheology of a water-saturated granular bed may include both plastic and
rate-dependent dilatant hardening or weakening components, depending on the
rate of deformation, the material state, clay mineral content, and the
hydrological properties of the material. The influence of the fluid phase is
negligible when relatively permeable sediment is deformed. However, by
reducing the local permeability, fast deformation can cause variations in
the pore-fluid pressure. The pressure variations weaken or strengthen the
granular phase, and in turn influence the distribution of shear strain with
depth. In permeable sediments the strain distribution is governed by the
grain-size distribution and effective normal stress and is typically on the
order of tens of centimeters. Significant dilatant strengthening in
impermeable sediments causes deformation to focus at the hydrologically more
stable ice-bed interface, and results in a very shallow cm-to-mm
deformational depth. The amount of strengthening felt by the glacier
depends on the hydraulic conductivity at the ice-bed interface.
Grain-fluid feedbacks can cause complex material properties that vary over
time, and which may be of importance for glacier stick-slip behavior.
OriginalsprogEngelsk
TidsskriftThe Cryosphere
Vol/bind9
Nummer6
Sider (fra-til)2183– 2200
ISSN1994-0416
DOI
StatusUdgivet - 20 nov. 2015

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