Discrimination of the flow law for subglacial sediment using in situ measurements and an interpretation model

Kavanaugh, Jeffrey L.; Clarke, Garry K. C.

doi:10.1029/2005jf000346

Cited by 56 publications

(82 citation statements)

References 98 publications

(138 reference statements)

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“…Christoffersen and Tulaczyk, 2003). As a result, ice-infiltrated till, rather than relatively clean ice, may commonly be in contact with the surfaces of soft beds, contrary to existing treatments of slip at the bed surface (Brown and others, 1987;Alley, 1989;Iverson, 1999;Kavanaugh and Clarke, 2006). Consistent with expectation, the glacier slipped across the bed surface at near zero pore-water pressure; deformation of the prism was activated by pumping water to it, which increased pore pressure and weakened the till.…”

Section: Discussionsupporting

confidence: 65%

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Soft-bed experiments beneath Engabreen, Norway:regelation infiltration, basal slip and bed deformation

Iverson¹,

Hooyer

Fischer

et al. 2007

J. Glaciol.

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ABSTRACT. To avoid some of the limitations of studying soft-bed processes through boreholes, a prism of simulated till (1.8 m Â Â 1.6 m Â 0.45 m) with extensive instrumentation was constructed in a trough blasted in the rock bed of Engabreen, a temperate glacier in Norway. Tunnels there provide access to the bed beneath 213 m of ice. Pore-water pressure was regulated in the prism by pumping water to it. During experiments lasting 7-12 days, the glacier regelated downward into the prism to depths of 50-80 mm, accreting ice-infiltrated till at rates predicted by theory. During periods of sustained high porewater pressure (70-100% of overburden), ice commonly slipped over the prism, due to a water layer at the prism surface. Deformation of the prism was activated when this layer thinned to a sub-millimeter thickness. Shear strain in the till was pervasive and decreased with depth. A model of slip by ploughing of ice-infiltrated till across the prism surface accounts for the slip that occurred when effective pressure was sufficiently low or high. Slip at low effective pressures resulted from water-layer thickening that increased non-linearly with decreasing effective pressure. If sufficiently widespread, such slip over soft glacier beds, which involves no viscous deformation resistance, may instigate abrupt increases in glacier velocity.

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Section: Discussionsupporting

confidence: 65%

“…Alley, 1989;Kavanaugh and Clarke, 2006). Thus, increases in the rate of basal motion will not be accompanied by increases in slip resistance.…”

Section: Discussionmentioning

confidence: 99%

Soft-bed experiments beneath Engabreen, Norway:regelation infiltration, basal slip and bed deformation

Iverson¹,

Hooyer

Fischer

et al. 2007

J. Glaciol.

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“…Laboratory studies have, however, strongly favored the notion of till having a plastic, Mohr-Coulomb rheology, with a very small rate dependence in the case of critical-state deformation (Kamb, 1991;Iverson et al, 1998;Tulaczyk et al, 2000a;Rathbun et al, 2008). This Mohr-Coulomb rheology is also supported by field investigations (Truffer et al, 2000;Kavanaugh and Clarke, 2006). However, a rate-weakening rheology is expected in the case where obstacles plough through a soft and deformable bed (Iverson et al, 1994;Iverson, 1999; Thomason and Iverson, 2008).…”

mentioning

confidence: 78%

“…Assuming a Mohr-Coulomb constitutive relation of the basal till strength, an increase in pore-water pressure weakens the bed by reducing the effective stress, and this may facilitate basal movement if the driving shear stresses become sufficient to overcome the sediment yield strength (Kamb, 1991;Iverson et al, 1998;Tulaczyk et al, 2000a;Fischer and Clarke, 2001; Kavanaugh and Clarke, 2006).…”

mentioning

confidence: 99%

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

et al. 2015

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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.

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“…It has also been pointed out that since till is a granular medium its rheological behaviour should fall within the range commonly exhibited by such materials (Iverson et al, 1998;Tulaczyk et al, 2000a,b;Truffer et al, 2000;Kamb, 2001;Kavanaugh and Clarke, 2006). Therefore, it should show Coulomb-plastic rheology (termed "treiboplastic" by Kamb, 2001), with a yield stress which is controlled by intergranular friction and which depends only slightly, if at all, on the strain rate (Kamb, 1991;Iverson et al, 1998).…”

Section: Modelling the Advance Of A Glaciermentioning

confidence: 99%

A numerical study of glacier advance over deforming till

Vieli

Gudmundsson

2010

The Cryosphere

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Abstract. The advance of a glacier over a deforming sediment layer is analysed numerically. We treat this problem as a contact problem involving two slowly-deforming viscous bodies. The surface evolution of the two bodies, and of the contact interface between them, is followed through time. Using various different non-linear till rheologies, we show how the mode of advance depends on the relative effective viscosities of ice and till. Three modes of advances are observed: (1) overriding, where the glacier advances through ice deformation only and without deforming the sediment; (2) plug-flow, where the sediment is strongly deformed, the ice moves forward as a block and a bulge is built in front of the glacier; and (3) mixed-flow, where the glacier advances through both ice and sediment deformation. For the cases of both overriding and mixed-flow, an inverse depth-age relationship within the ice is obtained. A series of model experiments show the contrast in effective viscosity between ice and till to be the single most important model parameter defining the mode of advance and the resulting thickness distribution of the till. Our model experiments indicate that the thickness of the deforming till layer is greatest close to the glacier front. Measurements of till thickness taken in such locations may not be representative of deforming till thickness elsewhere. Given sufficiently large contrast in effective viscosity between ice and till, a sediment bulge is formed in front of the glacier. During glacier advance, the bulge quickly reaches a steady state form strongly resembling single-crested push moraines. Inspection of particle paths within the sediment bulge, shows that particles within the till travel at a different speed from the bulge itself, and the push moraine to advance as a form-conserving non-linear wave.

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Discrimination of the flow law for subglacial sediment using in situ measurements and an interpretation model

Cited by 56 publications

References 98 publications

Soft-bed experiments beneath Engabreen, Norway:regelation infiltration, basal slip and bed deformation

Soft-bed experiments beneath Engabreen, Norway:regelation infiltration, basal slip and bed deformation

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

A numerical study of glacier advance over deforming till

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