Ice-marginal thrusting of drift and bedrock: thermal regime, subglacial aquifers, and glacial surges

1990 ◽  
Vol 27 (6) ◽  
pp. 849-862 ◽  
Author(s):  
H. D. Mooers
Keyword(s):  
Pore Water ◽  
Thermal Regime ◽  
Pore Water Pressure ◽  
System Development ◽  
Water Pressure ◽  
Laurentide Ice Sheet ◽  
Shear Stresses ◽  
Thrust System ◽  
Basal Shear ◽  
Subglacial Meltwater

Glacial thrust systems composed of blocks of drift and bedrock, associated with hummocky stagnation moraine along the margin of the Rainy lobe of the Laurentide Ice Sheet in Minnesota, are used in conjunction with paleoecological studies to constrain a numerical model of the ice-marginal thermal regime. Subglacial meltwater production in the thawed-bed zone was at least two orders of magnitude greater than the amount that could refreeze to the base of the glacier near the margin. The excess water recharged a thick subglacial aquifer, and thrust-system development was enhanced by the presence of a frozen toe and high pore-water pressures beneath the outer 2 km the glacier. The pore-water pressure required for thrusting is calculated from overburden pressures and basal shear stresses determined by numerical modeling. The heat generated by flow of water through the subglacial aquifer substantially affects the ice-marginal thermal regime, making a steady-state frozen toe 1.0–2.0 km in width unstable. Thrusting apparently occurred during multiple oscillations, or surges, when the ice was advancing over permafrost.

Earthquake-induced deformation analyses of the Upper San Fernando Dam under the 1971 San Fernando Earthquake

2001 ◽  
Vol 38 (1) ◽  
pp. 1-15 ◽  
Author(s):  
Guoxi Wu
Keyword(s):  
Finite Element ◽  
Pore Water ◽  
Effective Stress ◽  
Pore Water Pressure ◽  
Ground Deformation ◽  
Water Pressure ◽  
Soil Liquefaction ◽  
Shear Stresses ◽  
Cyclic Shear ◽  
San Fernando

A nonlinear effective stress finite element approach for dynamic analysis of soil structure is described in the paper. Major features of this approach include the use of a third parameter in the two-parameter hyperbolic stress-strain model, a modified expression for unloading–reloading modulus in the Martin–Finn–Seed pore-water pressure model, and an additional pore-water pressure model based on cyclic shear stress. The additional pore-water pressure model uses the equivalent number of uniform cyclic shear stresses for the assessment of pore-water pressure. Dynamic analyses were then conducted to simulate the seismically induced soil liquefaction and ground deformation of the Upper San Fernando Dam under the 1971 San Fernando Earthquake. The analyses were conducted using the finite element computer program VERSAT. The computed zones of liquefaction and deformation are compared with the measured response and with results obtained by others.Key words: effective stress method, finite element analysis, Upper San Fernando Dam, earthquake deformation, VERSAT.

scholarly journals FUNCTION OF INTERCEPTION, EVAPOTRANSPIRATION AND ROOT REINFORCEMENT OF PLANT ON SLOPE STABILIZATION

2020 ◽  
Vol 15 (1) ◽  
pp. 19-26
Author(s):  
Euthalia Hanggari Sittadewi
Keyword(s):  
Slope Stability ◽  
Pore Water ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Tree Canopy ◽  
Shear Stresses ◽  
Slope Stabilization ◽  
Root Reinforcement ◽  
Soil Shear Strength ◽  
Positive Effect

The ability of plants to carry out the functions of interception, evapotranspiration and root reinforcement provides an effective and contributes to an increase in slope stability. Canopy has a role in the process of interception related to the reduction of amount the infiltrated water and the rapid fulfilment of soil moisture. Through the evapotranspiration mechanism, plants can reduce pore water pressure in the soil so that the trigger force for landslides can be reduced and the soil will be more stable. The roots mechanically strengthen the soil, through the transfer of shear stresses in the soil into tensile resistance in the roots. Roots also bind soil particles and increase surface roughness, thereby reducing the process of soil displacement or erosion. There is a positive relationship between the density of the tree canopy with the value of rainfall interception, evapotranspiration with a decrease in pore water pressure in the soil and the ability of root anchoring and binding with an increase in soil shear strength, indicating that the function of interception, evapotranspiration and strengthening of plant roots have a positive effect on increasing slope stability. Plants selection that considers the level of interception, the rate of evapotranspiration and root reinforcement by adjusting environmental and slopes conditions will determine the success of slope stabilization efforts by vegetative methods.Keywords : interception, evapotranspiration, root reinforcement, slope stabilization.

scholarly journals Formation of thrust-block moraines at the margins of dry-based glaciers, south Victoria Land, Antarctica

1996 ◽  
Vol 22 ◽  
pp. 68-74 ◽  
Author(s):  
Sean J. Fitzsimons
Keyword(s):  
Pore Water ◽  
Pore Water Pressure ◽  
Freezing Point ◽  
Water Pressure ◽  
Lacustrine Sediment ◽  
Dry Valleys ◽  
Bed Deformation ◽  
Sedimentary Structures ◽  
Victoria Land ◽  
Subglacial Meltwater

Several dry-based alpine glaciers in the Dry Valleys of south Victoria Land, Antarctica, have prominent end moraines. Examination of their morphology, structure and sedimentology shows they consist of blocks of sand, gravel and organic silt within which sedimentary structures unrelated to entrainment and transportation by ice are well preserved. The nature and preservation of sedimentary structures, together with the presence of algae mats in the sediment, suggest formation by proglacial entrainment, transportation and deposition of frozen blocks of lacustrine sediment. Previous explanations of the formation of thrust-block moraines, including those that stress the importance of elevated pore-water pressure and Weertman’s ice-debris accretion hypothesis, depend on the presence of subglacial meltwater or the 0° C isotherm being situated close to the glacier bed. These models appear inappropriate for cold, dry-based glaciers because their basal temperatures are well below freezing point and they rest on deep permafrost. Three alternative models for the formation of thrust-block moraines at the margins of dry-based glaciers are examined in this paper: block entrainment of sediment associated with frozen-bed deformation; entrainment by overriding and accretion of marginal-ice and debris aprons; and transient wet-based conditions associated with glaciers flowing into ice-marginal lakes.

scholarly journals Formation of thrust-block moraines at the margins of dry-based glaciers, south Victoria Land, Antarctica

1996 ◽  
Vol 22 ◽  
pp. 68-74 ◽  
Author(s):  
Sean J. Fitzsimons
Keyword(s):  
Pore Water ◽  
Pore Water Pressure ◽  
Freezing Point ◽  
Water Pressure ◽  
Lacustrine Sediment ◽  
Dry Valleys ◽  
Bed Deformation ◽  
Sedimentary Structures ◽  
Victoria Land ◽  
Subglacial Meltwater

Several dry-based alpine glaciers in the Dry Valleys of south Victoria Land, Antarctica, have prominent end moraines. Examination of their morphology, structure and sedimentology shows they consist of blocks of sand, gravel and organic silt within which sedimentary structures unrelated to entrainment and transportation by ice are well preserved. The nature and preservation of sedimentary structures, together with the presence of algae mats in the sediment, suggest formation by proglacial entrainment, transportation and deposition of frozen blocks of lacustrine sediment. Previous explanations of the formation of thrust-block moraines, including those that stress the importance of elevated pore-water pressure and Weertman’s ice-debris accretion hypothesis, depend on the presence of subglacial meltwater or the 0° C isotherm being situated close to the glacier bed. These models appear inappropriate for cold, dry-based glaciers because their basal temperatures are well below freezing point and they rest on deep permafrost. Three alternative models for the formation of thrust-block moraines at the margins of dry-based glaciers are examined in this paper: block entrainment of sediment associated with frozen-bed deformation; entrainment by overriding and accretion of marginal-ice and debris aprons; and transient wet-based conditions associated with glaciers flowing into ice-marginal lakes.

scholarly journals Surface profiles of the laurentide ice sheet in its marginal areas

1974 ◽  
Vol 13 (67) ◽  
pp. 37-43 ◽  
Author(s):  
W. H. Mathews
Keyword(s):  
Late Pleistocene ◽  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Active Movement ◽  
Shear Stresses ◽  
Marginal Areas ◽  
Image Position ◽  
Basal Shear

Surface slopes of ice lobes can be estimated from the gradients of their margins as shown by ice limits, by contemporaneous recessional moraines, or by lateral melt-water channels, with allowance being made for the dip of an ice lobe laterally, as well as forward, toward its extremities. Profiles can be fitted approximately to a parabola with the equation in which h is the height above and x the distance up-stream from the terminus, in the same units, and A is a coefficient which varies from glacier to glacier. The coefficient A has a value of 4.7 m1 for both the Antarctic ice sheet inland from Mirny and the west central Greenland ice sheet. Several examples of late Pleistocene ice lobes within mountainous terrain of North America and New Zealand have values of A ranging from 2.9 ml to about 4.1 m1. For several ice lobes in the south-western part of the late Pleistocene Laurentide ice sheet, however, values are from about 0.3 to 1.0 m1, corresponding to basal shear stress of from about 0.07 to 0.22 bar. A major problem exists in accounting for the active movement of ice here under such low surface gradients and basal shear stresses. Evidence of basal slip, aided by high subglacial water pressure, should be looked for in the field. Alternatively, other possibilities for the explanation of such low surface gradients should be sought.

scholarly journals A model of glacial tectonics, applied to the ice­pushed ridges in the Central Netherlands

1985 ◽  
Vol 34 ◽  
pp. 55-74 ◽  
Author(s):  
Dick F. M. van der Wateren
Keyword(s):  
Shear Stress ◽  
The Netherlands ◽  
Stress Field ◽  
Sliding Friction ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Large Degree ◽  
Shear Stresses ◽  
Thrust Sheets ◽  
Basal Shear

During a detailed structural geological and geomorphological survey of ice-pushed ridges around the Gel­derse Vallei (centre of the Netherlands) several questions arose about the origin of these landforms. The Gelderse Vallei is a Saalian glacial basin filled with younger sediments, 40 km long and up to 20 km wide, running NNW-SSE. Thrust sheets which build up the ice-pushed ridges on either side of the valley were transported away from the centre of the basin. They are up to 25 m thick and are pushed to a level about 100 m above the decollement. The thrust sheets, mainly consisting of coarse sand layers, moved as rigid masses, while only the finegrained basal layers deformed by heterogeneous simple shear. Pore water pressure plays an important role by greatly reducing the sliding friction in the decollement layer. Once a glacial thrust sheet is formed, the weight of the upward moving frontal part (the toe) offers a resistance to movement, which can not be overcome by the basal shear stress of the glacier alone. Basal shear stresses in most glaciers are in the order of 0.1 MPa (1 bar) and appear to be incapable of lifting a toe of the size found in most ice-pushed ridges in the Netherlands. The article presents a model of a system ice lobe - substratum, which is to a large degree controlled by gravitational forces. The dilemma may be solved by the concept of the gradient stress field under the marginal area of an ice lobe. This concept was first formulated by Rotnicki (1976). Because ice thickness decreases towards the margin, the substratum is subjected to a decreasing load in the same direction. If the increments of stress difference under a slab of ice in the marginal zone are summed, a gradient stress is arrived at which is di­rected towards the ice edge. An estimate of the forces involved in the static equilibrium around the glacier margin indicates that the contribution by the gradient stress field is sufficient to move the toe. In the model proposed in the article, the energy to move and imbicate glacial thrust sheets around the margins of an ice lobe, is supplied by the continuous flow of ice into the ice lobe. The dimensions of ice­pushed ridges are thus to a high degree functions of the thickness of the ice lobe or icecap. The Gelderse Vallei ice lobe will have been at least 250 m thick to be able to form the ridges. The basal shear stress contributes less to the tectonic transport, the larger the thrust sheets and the high­er the ice-pushed ridges.

scholarly journals Surface profiles of the laurentide ice sheet in its marginal areas

1974 ◽  
Vol 13 (67) ◽  
pp. 37-43 ◽  
Author(s):  
W. H. Mathews
Keyword(s):  
Late Pleistocene ◽  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Active Movement ◽  
Shear Stresses ◽  
Marginal Areas ◽  
Image Position ◽  
Basal Shear

Surface slopes of ice lobes can be estimated from the gradients of their margins as shown by ice limits, by contemporaneous recessional moraines, or by lateral melt-water channels, with allowance being made for the dip of an ice lobe laterally, as well as forward, toward its extremities. Profiles can be fitted approximately to a parabola with the equationin whichhis the height above andxthe distance up-stream from the terminus, in the same units, andAis a coefficient which varies from glacier to glacier. The coefficientAhas a value of 4.7 m1for both the Antarctic ice sheet inland from Mirny and the west central Greenland ice sheet. Several examples of late Pleistocene ice lobes within mountainous terrain of North America and New Zealand have values ofAranging from 2.9 mlto about 4.1 m1. For several ice lobes in the south-western part of the late Pleistocene Laurentide ice sheet, however, values are from about 0.3 to 1.0 m1, corresponding to basal shear stress of from about 0.07 to 0.22 bar. A major problem exists in accounting for the active movement of ice here under such low surface gradients and basal shear stresses. Evidence of basal slip, aided by high subglacial water pressure, should be looked for in the field. Alternatively, other possibilities for the explanation of such low surface gradients should be sought.

Drained deformation and failure due to cyclic pore pressures in soft natural clay at low stresses

1987 ◽  
Vol 24 (2) ◽  
pp. 208-215 ◽  
Author(s):  
K. D. Eigenbrod ◽  
J.-P. Burak ◽  
J. Graham
Keyword(s):  
Pore Water ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Shear Stresses ◽  
Alternative Mechanism ◽  
Natural Clay ◽  
Deformation And Failure ◽  
First Approximation ◽  
Pore Water Pressures ◽  
Normal And Shear Stresses

Slow, recurring downslope movements in northern climates are frequently referred to as "creep movements," and are usually related to outwards freezing followed by vertical thawing movements. An alternative mechanism is examined in the reported test data.Undisturbed block samples of proglacial clay from a slope near yellowknife, N.W.T., have been tested by cyclically varying the pore-water pressure in triaxial specimens by an amount Δu, and measuring the resulting strains per cycle. The specimens were initially anisotropically consolidated with normal and shear stresses corresponding to those in the moving mantle. Drainage was permitted throughout the testing. This procedure represents changes that can occur in a natural slope from (a) seasonal groundwater level changes and (b) elevated pore-water pressures that accompany thawing. After 60–100 cycles, the pore-water pressure was systematically increased to the value Δuf at which the samples failed. This occurred on a steep, low-stress envelope, approximately c′ = 4 KPa, [Formula: see text]. The envelope is probably controlled by the nuggety macrostructure of the clay and appears to be slightly to the left of the [Formula: see text] locus.The strains per cycle were approximately linear in the range 30–100 cycles. As a first approximation they have been modelled as varying linearly with the ratio Δu/Δuf almost up to failure at Δu/Δuf = 1.0. Key words: downslope creep, solifluction, slope stability, clay, pore-water pressure, cyclic loading, low-stress failure.

Pore-water pressure response during undrained isotropic load changes in layered soils

1999 ◽  
Vol 36 (3) ◽  
pp. 544-555
Author(s):  
K D Eigenbrod ◽  
W H Wurmnest
Keyword(s):  
Pore Pressure ◽  
Pore Water ◽  
Pore Water Pressure ◽  
Water Pressure ◽  
Pressure Coefficient ◽  
Shear Stresses ◽  
Pressure Equalization ◽  
Pore Water Pressures ◽  
Internal Pore ◽  
Varved Clay

Low pore-water pressure responses observed during undrained isotropic loading of thinly interbedded varved clay specimens were related to internal pore-water pressure equalization and internal shearing between soft clay seams and stiff silt layers of the varved clay. Both processes were analyzed in two separate models: a finite element analysis of the layered soil specimen with different elastic properties for each layer showed that shear stresses can develop along the layer interfaces during undrained isotropic loading. However, because the shear stresses are small and restricted to a narrow zone close to the surface of the cylindrical specimen, it appeared that the effect of shearing on the overall pore-water responses is negligible. The analysis of the pore-water pressures during undrained, isotropic loading demonstrated that hydraulic gradients between the two layers will develop. As a result, pore water will drain from the clay into the silt, leading to consolidation of the clay and swelling of the silt seams. The stabilized pore-water pressures should be the same as the pore-water pressures measured for the overall specimen, if the effect of internal shearing is negligible. Comparison of the computed with the measured overall pore-water pressure responses during testing for Skempton's pore-pressure coefficient B indicated reasonable agreement.Key words: Skempton's pore-pressure coefficient B, pore-water pressure response, varved clays, internal shearing, internal pore-water pressure equalization.

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