scholarly journals Ploughing of subglacial sediment

1994 ◽  
Vol 40 (134) ◽  
pp. 97-106 ◽  
Author(s):  
URS H. Fischer ◽  
Garry K. C. Clarke
Keyword(s):  
Shear Stress ◽  
Effective Viscosity ◽  
Basal Layer ◽  
Shear Resistance ◽  
Yukon Territory ◽  
Rheological Parameters ◽  
Ideal Plastic ◽  
New Instrument ◽  
Newtonian Viscous Fluid ◽  
Basal Shear

AbstractMechanical conditions at the base of Trapridge Glacier, Yukon Territory, were investigated using a “ploughmeter”. We describe the physical characteristics and the theory of this new instrument as well as its operation. Observational results reveal variations in ploughmeter response that might be attributed to spatial variability in subglacial processes or spatial variation in sediment granulometry. Quantitative analysis of the interaction of the ploughmeter with the basal layer yields estimates of rheological parameters. If the sediment is assumed to behave as a Newtonian viscous fluid, the estimated effective viscosity is 3.0 × 109−3.1 × 1010Pas; if it is assumed to behave as an ideal plastic solid, the estimated yield strength is 48–57 kPa. In both cases, the estimated shear resistance of subglacial material is comparable to but somewhat less than that required to balance fully the applied basal shear stress.

scholarly journals Ploughing of subglacial sediment

1994 ◽  
Vol 40 (134) ◽  
pp. 97-106 ◽  
Author(s):  
URS H. Fischer ◽  
Garry K. C. Clarke
Keyword(s):  
Shear Stress ◽  
Effective Viscosity ◽  
Basal Layer ◽  
Shear Resistance ◽  
Yukon Territory ◽  
Rheological Parameters ◽  
Ideal Plastic ◽  
New Instrument ◽  
Newtonian Viscous Fluid ◽  
Basal Shear

AbstractMechanical conditions at the base of Trapridge Glacier, Yukon Territory, were investigated using a “ploughmeter”. We describe the physical characteristics and the theory of this new instrument as well as its operation. Observational results reveal variations in ploughmeter response that might be attributed to spatial variability in subglacial processes or spatial variation in sediment granulometry. Quantitative analysis of the interaction of the ploughmeter with the basal layer yields estimates of rheological parameters. If the sediment is assumed to behave as a Newtonian viscous fluid, the estimated effective viscosity is 3.0 × 109 −3.1 × 1010Pas; if it is assumed to behave as an ideal plastic solid, the estimated yield strength is 48–57 kPa. In both cases, the estimated shear resistance of subglacial material is comparable to but somewhat less than that required to balance fully the applied basal shear stress.

scholarly journals Inversion of flow Measurements for Stress and Rheological Parameters in a Valley Glacier

1973 ◽  
Vol 12 (64) ◽  
pp. 19-44
Author(s):  
Charles F. Raymond
Keyword(s):  
Shear Stress ◽  
Power Law ◽  
Measurement Errors ◽  
Effective Viscosity ◽  
Three Dimensional ◽  
Rheological Parameters ◽  
Mean Stress ◽  
Flow Measurements ◽  
Anomalous Zone ◽  
Flow Law

AbstractMethods are developed for determining the distributions of stress and effective viscosity in a glacier, under the assumptions: the ice is quasi-viscous, the flow is time independent, and acceleration forces are negligible. Measurements of the three-dimensional distribution of velocity are needed for their application. The differential equations of mechanical equilibrium, expressed in terms of viscosity, strain-rate components, mean stress, and their gradients, are viewed as equations to be solved for viscosity and mean stress subject to boundary conditions at the free upper surface. For certain rectilinear flow patterns, unique distributions of stress and effective viscosity can always be derived. For more complicated flow this is not necessarily so. However, it is still possible to choose the best values of rheological parameters in any trial flow law based on the requirement that the residuals to the equations of equilibrium be minimized in a mean-square sense. The techniques are applied to measurements of internal deformation made in nine bore holes on the Athabasca Glacier. At the center line the magnitude of the surface-parallel shear stress increases with depth more slowly than would be expected from a standard shape factor correction or the theoretical distribution of Nye. Correspondingly the lateral distribution of lateral shear stress shows the opposite relationships. In the lower one- to two-thirds of the depth corresponding to a range in effective stress from about 0.5 to 1.2 bars, the gross rheology of the ice is not distinguishably different from the experimentally determined flow law of Glen (n = 4.2, T = 0.02° C) as generalized by Nye. The results do not support the conclusion that the effective viscosity is higher than would be expected from Glen’s experiments as indicated by the more limited measurements of Paterson and Savage. Power-law parameters derived for the different bore holes considered separately show a spread, which suggests some rheological inhomogeneity. However, no definite conclusions can be drawn, because of direct measurement errors at the bore holes and less definable uncertainty in the interpolated distribution of velocity between the holes. The upper one- to two-thirds of the glacier constitutes an anomalous zone in which there is either a strong effect from a complex distribution of stress arising from longitudinal stress gradients or more complicated rheology than in a homogeneous power-law material.

scholarly journals Inversion of flow Measurements for Stress and Rheological Parameters in a Valley Glacier

1973 ◽  
Vol 12 (64) ◽  
pp. 19-44 ◽  
Author(s):  
Charles F. Raymond
Keyword(s):  
Shear Stress ◽  
Power Law ◽  
Measurement Errors ◽  
Effective Viscosity ◽  
Three Dimensional ◽  
Rheological Parameters ◽  
Mean Stress ◽  
Flow Measurements ◽  
Anomalous Zone ◽  
Flow Law

AbstractMethods are developed for determining the distributions of stress and effective viscosity in a glacier, under the assumptions: the ice is quasi-viscous, the flow is time independent, and acceleration forces are negligible. Measurements of the three-dimensional distribution of velocity are needed for their application. The differential equations of mechanical equilibrium, expressed in terms of viscosity, strain-rate components, mean stress, and their gradients, are viewed as equations to be solved for viscosity and mean stress subject to boundary conditions at the free upper surface. For certain rectilinear flow patterns, unique distributions of stress and effective viscosity can always be derived. For more complicated flow this is not necessarily so. However, it is still possible to choose the best values of rheological parameters in any trial flow law based on the requirement that the residuals to the equations of equilibrium be minimized in a mean-square sense. The techniques are applied to measurements of internal deformation made in nine bore holes on the Athabasca Glacier. At the center line the magnitude of the surface-parallel shear stress increases with depth more slowly than would be expected from a standard shape factor correction or the theoretical distribution of Nye. Correspondingly the lateral distribution of lateral shear stress shows the opposite relationships. In the lower one- to two-thirds of the depth corresponding to a range in effective stress from about 0.5 to 1.2 bars, the gross rheology of the ice is not distinguishably different from the experimentally determined flow law of Glen (n= 4.2,T= 0.02° C) as generalized by Nye. The results do not support the conclusion that the effective viscosity is higher than would be expected from Glen’s experiments as indicated by the more limited measurements of Paterson and Savage. Power-law parameters derived for the different bore holes considered separately show a spread, which suggests some rheological inhomogeneity. However, no definite conclusions can be drawn, because of direct measurement errors at the bore holes and less definable uncertainty in the interpolated distribution of velocity between the holes. The upper one- to two-thirds of the glacier constitutes an anomalous zone in which there is either a strong effect from a complex distribution of stress arising from longitudinal stress gradients or more complicated rheology than in a homogeneous power-law material.

scholarly journals Influence of geometrical boundary conditions on the estimation of rheological parameters

2000 ◽  
Vol 30 ◽  
pp. 102-106 ◽  
Author(s):  
L. Testut ◽  
I. E. Tabacco ◽  
C. Bianchi ◽  
F. Rémy
Keyword(s):  
Shear Stress ◽  
Boundary Conditions ◽  
Flow Direction ◽  
Rheological Parameters ◽  
Longitudinal Stress ◽  
Ice Flow ◽  
Flow Modelling ◽  
Altimetric Data ◽  
Basal Shear ◽  
Geometrical Boundary

AbstractImproved knowledge of geometrical boundary conditions, such as bedrock geometry and surface topography, can contribute significantly to glaciological studies including ice-sheet-flow modelling. Precise thickness and altimetric data allow an estimation of ice-flow direction, the balance velocity and the basal shear stress. These parameters are calculated along a 1160 km profile in East Antarctica using a relationship between shear stress, basal temperature, the Glen flow exponent and a parameter related to strain rate. Strong variations of the flow-law parameters and basal conditions are found to play a major role in the ice-flow pattern. Sliding, anisotropy and longitudinal stress strongly perturb the validity of the law, but their signature can be identified.

scholarly journals Sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions: a model study

2002 ◽  
Vol 48 (163) ◽  
pp. 552-558 ◽  
Author(s):  
Marjorie Schmeltz ◽  
Eric Rignot ◽  
Todd K. Dupont ◽  
Douglas R. MacAyeal
Keyword(s):  
Shear Stress ◽  
Element Model ◽  
Shelf Area ◽  
West Antarctica ◽  
Ice Shelf ◽  
Ice Stream ◽  
Model Study ◽  
Grounding Line ◽  
Glacier Velocity ◽  
Basal Shear

AbstractWe use a finite-element model of coupled ice-stream/ice-shelf flow to study the sensitivity of Pine Island Glacier, West Antarctica, to changes in ice-shelf and basal conditions. By tuning a softening coefficient of the ice along the glacier margins, and a basal friction coefficient controlling the distribution of basal shear stress underneath the ice stream, we are able to match model velocity to that observed with interferometric synthetic aperture radar (InSAR). We use the model to investigate the effect of small perturbations on ice flow. We find that a 5.5–13% reduction in our initial ice-shelf area increases the glacier velocity by 3.5–10% at the grounding line. The removal of the entire ice shelf increases the grounding-line velocity by > 70%. The changes in velocity associated with ice-shelf reduction are felt several tens of km inland. Alternatively, a 5% reduction in basal shear stress increases the glacier velocity by 13% at the grounding line. By contrast, softening of the glacier side margins would have to be increased a lot more to produce a comparable change in ice velocity. Hence, both the ice-shelf buttressing and the basal shear stress contribute significant resistance to the flow of Pine Island Glacier.

scholarly journals Abrupt glacier motion and reorganization of basal shear stress following the establishment of a connected drainage system

2001 ◽  
Vol 47 (158) ◽  
pp. 472-480 ◽  
Author(s):  
Jeffrey L. Kavanaugh ◽  
Garry K. C. Clarke
Keyword(s):  
Pressure Sensors ◽  
Drainage System ◽  
Yukon Territory ◽  
Stable Configuration ◽  
Drainage Patterns ◽  
Pressure Pulses ◽  
Gradual Failure ◽  
Basal Shear ◽  
Pressure Changes ◽  
Glacier Motion

AbstractThree episodes of strong basal motion occurred at Trapridge Glacier, Yukon Territory, Canada, on 11 June 1995 following the establishment of a connected subglacial drainage system. Responses to these “spring events” are noted in the records for 42 instruments and were recorded throughout the ∼60 000 m2 study area. Strong basal motion during the events is indicated by ploughmeter, load-bolt and vertical-strain records, and abrupt pressure changes in several transducer records denote damage caused by extreme pressure pulses. These pressure pulses, generated by the abrupt basal motion, also resulted in the failure of seven pressure sensors. Records for pressure, turbidity and conductivity sensors indicate that basal drainage patterns did not change significantly during the events. Geophone records suggest that the episodes of basal motion were precipitated by the gradual failure of a “sticky spot” following hydraulic connection of part of the study area. This failure resulted in the transfer of basal stress to the unconnected region of the bed during the course of the events. No evidence for strong basal motion is seen in the instrument records for several weeks following the events, suggesting that the mechanical adjustments resulted in a stable configuration of basal stresses. This event illustrates how unstable situations can be quickly accommodated by mechanical adjustments at the glacier bed.

Shear Resistance Study of Hybrid I-Beams Fabricated by HPS 485W and Q345 Steels

2011 ◽  
Vol 255-260 ◽  
pp. 1311-1314
Author(s):  
Lan Duan ◽  
Li Zheng ◽  
Chun Sheng Wang ◽  
Jing Yu Hu
Keyword(s):  
Shear Stress ◽  
High Performance ◽  
Shear Failure ◽  
Ultimate Load ◽  
Shear Resistance ◽  
Conventional Steel ◽  
Buckling Resistance ◽  
Shear Buckling ◽  
Post Buckling ◽  
Elastic Shear

This paper evaluates the shear resistance of hybrid I-beams fabricated by high performance steel and conventional steel. A number of hybrid I-beams are modeled and analyzed to determine their shear failure mechanism characteristics, considering parameters of web slenderness (hw/tw), frame action from end-stiffeners, ratio of flange width to web depth (bf/hw) and panel numbers. The analyses conclude that, in shear resistance calculation, plate beam with inter and slender webs often fail in inelastic or elastic shear buckling while ultimate shear resistance of compact webs is given by the shear strength of the material. What’s more, more rigid stiffeners provide more fixity to flange plates and increase the post-buckling resistance of plate beam. For plate beam with several panels, the shear stress at the ultimate load is similar. Finally, the I-beams with larger flange width to web depth ratio would develop larger shear strengths and then shear deformation cause formation of plastic hinges.

scholarly journals Stability of Sheet Water Flow Under a Glacier

1983 ◽  
Vol 29 (103) ◽  
pp. 374-382 ◽  
Author(s):  
J. Weertman ◽  
G. E. Birchfield
Keyword(s):  
Shear Stress ◽  
Water Flow ◽  
Sheet Thickness ◽  
The Other ◽  
Flowing Water ◽  
Ice Melting ◽  
Geothermal Heat ◽  
Melt Water ◽  
Primary Origin ◽  
Basal Shear

AbstractWalder recently analyzed the initial instability of water flow in a sheet under a glacier that is produced by greater heat production in the flowing water and thus the larger rate of ice melting where a perturbation has increased the water sheet thickness. We have looked at the problem from the other presumed final state. We assume that instabilities have finally caused all the water to flow in channels (R-channels) at the bed. We investigated whether these channels can collect enough of the water that is produced by the geothermal heat and the heat sliding to remain in existence. When a basal shear stress is present, the distance out to which a channel can collect water is not that much greater than the channel radius itself. It is concluded that it is not likely for the channel to be able to collect appreciable amounts of water that is produced at the bed. Hence despite the indication that an instability initially might grow in a water sheet, this instability either cannot develop to the point when the water flow is primarily in channels or else the water flow alternates cyclically between a state primarily of sheet flow and a state of channel flow. It would appear that the channels that do exist under a glacier have their primary origin in the melt water from the upper surface that pours down moulins and thus is already channelized by the time it reaches the bed.

scholarly journals The influence of base oil type on the rheological properties of ecological lubricating greases

Nafta-Gaz ◽  
2021 ◽  
Vol 77 (2) ◽  
pp. 127-135
Author(s):  
Rafał Kozdrach ◽  
Keyword(s):  
Shear Stress ◽  
Shear Rate ◽  
Rheological Properties ◽  
Measurement Data ◽  
Measuring System ◽  
Rheological Parameters ◽  
Flow Curves ◽  
Lubricating Grease ◽  
Dispersion Phase ◽  
Base Oil

The article presents the results of research on the influence the type of base oil in lubricating compositions has on the rheological parameters of selected lubricants. Vegetable, mineral, and synthetic dispersion phases were used to produce lubricating greases. The modified amorphous silica was used as the dispersed phase. However, as a modifying additive was used a substance containing the antioxidants, corrosion inhibitors, and EP/AW additives. The experiments on rheological properties were carried out using a Physica MCR 101 rotational rheometer (manufactured by Anton Paar), equipped with a diffusion air bearing and connected to a pneumatic supply – an oil-free Jun-Air compressor and air drying block. The device is equipped with a Peltier system for temperature control in the range of –20°C to 200°C and an external thermostatic VISCOTHERM V2 system, working in the temperature range of –20°C to 200°C. The rheometer control and measurement data analysis were performed using Rheoplus software. The tests were carried out using a cone-plate measuring system with a shear rate range of 0.01–100 s-1 at 20°C for lubricating compositions prepared on various oil bases. To evaluate the value of rheological parameters, the results of tests of the dependence between shear stress and shear rate (flow curves) were used. For the theoretical determined on the flow curves, the following rheological models were used: Bingham, Herschel–Bulkley, Casson, and Tscheuschner. The values of the shear stress (yield point) in depending on the type of dispersion phase has changed. This proves that the use of a base oil with the appropriate functional properties does not weaken, but reinforces the spatial structure of a lubricating grease. It has an important meaning when selecting construction parameters when designing a central lubrication system with grease made from a vegetable oil base (Abyssinian oil). The rheological properties of the lubricating grease are influenced by the type of base oil and thickener, any additives in the grease, the production technology of the grease, and the conditions in which it is used. The tests revealed an important influence of the base oil on the rheological parameters that describe the behaviour of lubricating compositions subjected to stresses and strains in a lubricating system.

scholarly journals The role of bed separation and friction in sliding over an undeformable bed

1992 ◽  
Vol 38 (128) ◽  
pp. 77-92 ◽  
Author(s):  
Jürg Schweizer ◽  
Almut Iken
Keyword(s):  
Shear Stress ◽  
Functional Relationship ◽  
Water Pressure ◽  
Important Variable ◽  
Sliding Velocity ◽  
Sliding Motion ◽  
Basal Ice ◽  
Bed Separation ◽  
Basal Shear

AbstractThe classic sliding theories usually assume that the sliding motion occurs frictionlessly. However, basal ice is debris-laden and friction exists between the substratum and rock particles embedded in the basal ice. The influence of debris concentration on the sliding process is investigated. The actual conditions where certain types of friction apply are defined, the effect for the case of bed separation due to a subglacial water pressure is studied and consequences for the sliding law are formulated. The numerical modelling of the sliding of an ice mass over an undulating bed, including the effect of both the subglacial water pressure and the friction, is done by using the finite-clement method. Friction, seen as a reduction of the driving shear stress due to gravity, can be included in existing sliding laws which should contain the critical pressure as an important variable. An approximate functional relationship between the sliding velocity, the effective basal shear stress and the subglacial water pressure is given.

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