distributed shear of subglacial till due to coulomb slip in most models of the flow of glaciers on till beds it has been assumed that till behaves as a viscoplastic fluid despite contradictory evidence from laboratory studies in accord with this assumption displacement profiles measured in subglacial till have been fitted with viscoplastic models by estimating the stress distribution here we present a model that illustrates how observed displacement profiles can result from till deformation resisted solely by coulomb friction motion in the till bed is assumed to be driven by brief departures from static equilibrium caused by fluctuations in effective normal stress these fluctuations result from chains of particles that support intergranular forces that are higher than average and that form and fail at various depths in the bed during shearing newton s second law is used to calculate displacements along slip planes and the depth to which deformation extends in the bed consequent displacement profiles are convex upward similar to those measured by boulton and colleagues at breidamerkur jokull iceland the model results when considered together with the long term and widespread empirical support for coulomb models in soils engineering indicate that efforts to fit viscoplastic flow models to till displacement profiles may be misguided

shear mechanisms and viscoplastic effects during impulsive shearing in clay formations the co seismic reactivation of pre existing landslides involves dynamic shear actions on sliding surfaces characterised by attritive states impulsive shear motions were applied after the attainment of the fully softened and residual conditions by means of an appropriately modified ring shear apparatus the kinematics parameters that characterise shear motions are related to co seismic displacements of rigid landslide bodies as defined by newmark s approach the results of the tests performed on clay samples with different index properties highlight the influence of the initial spatial disposition of particles on the shear mechanisms in addition it seems that these mechanisms control both the inviscid and viscous components of the shear stress for high shear rates an increase of the viscous strength in the residual condition determines a greater viscoplastic strength than that in the fully softened condition on this basis different behaviours in seismically induced reactivat

distribution of terrigenous lipids in marine sediments off northeastern taiwan surface sediments on the continental margin off northeastern taiwan have been analyzed for terrigenous lipids including n alkanes n fatty alcohols and sterols marine input to the sediments is particularly low based on the average n c17 n c29 alkane and n c16 n c28 fatty alcohol ratios 0 15 0 13 and 0 13 0 06 respectively this may be due to the fact that marine lipids are more prone to degradation than terrestrial ones the study area has the highest plant wax n alkane contribution average carbon preference index 3 9 1 2 among the coastal marine areas surrounding taiwan lateral particle transport from the southern east china sea shelf and river runoff from the east taiwan coast are considered to be the major contributors the distributions of plant wax n alkane and n alkanol concentrations normalized to total organic carbon toc in the study area generally show maximum values on the upper slope of the southernmost okinawa trough but not for phytosterols linear regression of toc versus plant wax n alkane concentrations show a weak relationship r 0 64 p 0 001 and an even weaker relationship r 0 42 p 0 05 between toc and plant wax n fatty alcohol concentrations is found this could be attributed to several factors 1 a complex input not a point source of terrigenous organic matter to the study area 2 toc also including marine organic matter 3 temporal variations in river flow due to flooding and 4 different rates of degradation for toc and individual biomarkers however in spite of those factors toc and phytosterol concentrations are positively linearly correlated r 0 85 p 0 001 implying that the dilution of phytosterols in terrigenous organic carbon with marine organic carbon with or without the phytosterols follows a nearly constant ratio which is remarkable in addition the predominant source of diploptene in the sediments does not appear to be of higher plant origin

dynamic centrifuge testing of slickensided shear surfaces movement along preexisting slickensided rupture surfaces in overconsolidated clay and clay shale slopes can represent a critical sliding mechanism during earthquakes the seismic behavior of preexisting slickensided surfaces in overconsolidated clay was examined by performing dynamic centrifuge model tests of two slickensided sliding block models constructed using rancho solano lean clay dynamic shear displacements were concentrated along the preformed slickensided surfaces the peak shear resistances mobilized along the slickensided surfaces during dynamic loading were 90 120 higher than the drained residual strength measured prior to shaking to accurately predict the displacements of the sliding blocks using newmark s method it was necessary to use dynamic strengths that were 37 64 larger than the drained residual strength of the soil dynamic loading caused a positive pore pressure response in the soil surrounding the slickensided planes the postshaking shear strengths were 17 31 higher than those measured prior to shaking

shear resistance and continuity of subglacial till hydrology rules the field observations of g s boulton stimulated widespread interest in deformable beds shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure during unsteady deformation pseudo viscous shear resistance can be caused by dilation of consolidated tills and resultant pore pressure decline this effect is probably uncommon however because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective pressure transients stick slip motion at whillans ice stream antarctica indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip recurrence intervals for rapid slip episodes there 6 18 hours indicate that till strength variations if driven by changes in pore pressure either related or unrelated to basal freezing are focused in the uppermost several centimeters of the bed ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate weakening during rapid slip with pore pressure decay causing strengthening between slip episodes by promoting shallow sluggish subglacial water flow and low effective pressure soft beds may help sustain themselves by slowing their own transport soft bed shear resistance kinematics and continuity are problems rooted in subglacial hydrology

silicon dynamics within an intense open ocean diatom bloom in the pacific sector of the southern ocean abstract an intense diatom bloom developed within a strong meridional silicic acid gradient across the antarctic polar front at 61 s 170 w following stratification of the water column in late october early november 1997 the region of high diatom biomass and the silicic acid gradient propogated southward across the seasonal ice zone through time with the maximum diatom biomass tracking the center of the silicic acid gradient high diatom biomass and high rates of silica production persisted within the silicic acid gradient until the end of january 1998 ca 70 d driving the gradient over 500 km to the south of its original position at the polar front the bloom consumed 30 to 40 m si oh 4 in the euphotic zone between about 60 and 66 s leaving near surface concentrations our observations indicate that high diatom biomass and high silica production rates previously observed in the marginal seas around antarctica also occur in the deep ocean near the polar front the bloom we observed propagated across the latitudinal band overlying the sedimentary opal belt which encircles most of antarctica implying a role for such blooms in the formation of those sediments comparison of our surface silica production rates with new estimates of opal accumulation rates in the abyssal sediments of the southern ocean which have been corrected for sediment focusing indicate a burial efficiency of 4 6 for biogenic silica that efficiency is considerably lower than previous estimates for the southern ocean

a laboratory study of particle ploughing and pore pressure feedback a velocity weakening mechanism for soft glacier beds if basal water discharge and pressure are sufficiently high a soft bedded glacier will slip over its bed by ploughing the process in which particles that span the ice bed interface are dragged across the bed surface results of laboratory experiments indicate that resistance to ploughing can decrease with increasing ploughing velocity velocity weakening during ploughing at various velocities 15 400 m a 1 till was compacted in front of idealized particles causing pore pressures there that were orders of magnitude higher than the ambient value this excess pore pressure locally weakened the till in shear thereby decreasing ploughing resistance by a factor of 3 0 6 6 with a six fold increase in ploughing velocity characteristic timescales of pore pressure diffusion and compaction down glacier from ploughing particles depend on till diffusivity ploughing velocity and sizes of ploughing particles these timescales accurately predict the ranges of these variables over which excess pore pressure and velocity weakening occurred existing ploughing models do not account for velocity weakening a new ploughing model with no adjustable parameters predicts ploughing resistance to no worse than 38 but requires that excess pore pressures be measured velocity weakening by this mechanism may affect fast glacier flow sediment transport by bed deformation and basal seismicity

flow mechanism of the des moines lobe of the laurentide ice sheet rapid flow of the des moines lobe of the laurentide ice sheet may have been related to its unlithified substrate new reconstructions of the lobe based on moraine elevations sediment subsidence during moraine deposition and flow direction indicators indicate that the lobe may have been 3 times thicker than in previous reconstructions nevertheless implied basal shear stresses are 15 kpa so internal ice deformation was not significant instead the lobe likely moved by a combination of sliding plowing of particles through the bed surface and bed shear consolidation tests on basal till yield preconsolidation stresses of 125 300 kpa so effective normal stresses on the bed were small a model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed resulting in too small a shear traction on the bed to shear it at depth consistent with this prediction measurements of orientations of clasts in basal till yield a weak fabric implying pervasive bed shear strain less than 2 although some stronger fabrics have been reported by others we infer tentatively that movement was principally at the bed surface by plowing

shear resistance variations in experimentally sheared mudstone granules a possible shear thinning and thixotropic mechanism we present results of ring shear frictional resistance for mudstone granules of different size obtained from a landslide shear zone little rate dependency of shear resistance was observed in sand sized granules in any wet or dry test while saturated gravel sized granules exhibited significant and abrupt reversible rate weakening from 0 6 to 0 05 at about 2 mm s repeating resistance variations occurred also under constant shear displacement rate mudstone granules generate mud as they are crushed and softened shear thinning and thixotropic behavior of the mud can explain the observed behavior with the viscosity decreasing the mud can flow through the coarser soil pores and migrate out from the shear zone this brings new granules into contact which produces new mud thus the process can start over similarities between experimental shear zones and those of some landslides in mudstone suggest that the observed behaviour may play a role in some landslide kinematics

evidences of landslide earthquake triggering due to self excitation process the basin like setting of stiff bedrock combined with pre existing landslide masses can contribute to seismic amplifications in a wide frequency range 0 10 hz and induce a self excitation process responsible for earthquake triggered landsliding here the self excitation process is proposed to justify the far field seismic trigger of the cerda landslide sicily italy which was reactivated by the 6th september 2002 palermo earthquake ms 5 4 about 50 km far from the epicentre the landslide caused damage to farm houses roads and aqueducts close to the village of cerda and involved about 40 106 m3 of clay shales the first ground cracks due to the landslide movement formed about 30 min after the main shock a stress strain dynamic numerical modelling performed by fdm code flac 5 0 supports the notion that the combination of local geological setting and earthquake frequency content played a fundamental role in the landslide reactivation since accelerometric records of the triggering event are not available dynamic equivalent inputs have been used for the numerical modelling these inputs can be regarded as representative for the local ground shaking having a pga value up to 0 2 m s2 which is the maximum expected in 475 years according to the italian seismic hazard maps a 2d numerical modelling of the seismic wave propagation in the cerda landslide area was also performed it pointed out amplification effects due to both the structural setting of the stiff bedrock at about 1 hz and the pre existing landslide mass in the range 3 6 hz the frequency peaks of the resulting amplification functions a f fit well the h v spectral ratios from ambient noise and the h h spectral ratios to a reference station from earthquake records obtained by in situ velocimetric measurements moreover the fourier spectra of earthquake accelerometric records whose source and magnitude are consistent with the triggering event show a main peak at about 1 hz this frequency value well fits the one amplified by the geological setting of the bedrock in correspondence with the landslide area which is constituted of marly limestones and characterised by a basin like geometry

rate weakening friction characterizes both slow sliding and catastrophic failure of landslides abstract catastrophic landslides cause billions of dollars in damages and claim thousands of lives annually whereas slow moving landslides with negligible inertia dominate sediment transport on many weathered hillslopes surprisingly both failure modes are displayed by nearby landslides and individual landslides in different years subjected to almost identical environmental conditions such observations have motivated the search for mechanisms that can cause slow moving landslides to transition via runaway acceleration to catastrophic failure a similarly diverse range of sliding behavior including earthquakes and slow slip events occurs along tectonic faults our understanding of these phenomena has benefitted from mechanical treatments that rely upon key ingredients that are notably absent from previous landslide descriptions here we describe landslide motion using a rate and state dependent frictional model that incorporates a nonlocal stress balance to account for the elastic response to gradients in slip our idealized one dimensional model reproduces both the displacement patterns observed in slow moving landslides and the acceleration toward failure exhibited by catastrophic events catastrophic failure occurs only when the slip surface is characterized by rate weakening friction and its lateral dimensions exceed a critical nucleation length h h that is shorter for higher effective stresses however landslides that are extensive enough to fall within this regime can nevertheless slide slowly for months or years before catastrophic failure our results suggest that the diversity of slip behavior observed during landslides can be described with a single model adapted from standard fault mechanics treatments

post failure stage simulation of a landslide using the material point method abstract traditional numerical methods such as the finite element method are usually used to analyse the slope response in the pre failure and failure stages under the assumption of small deformations however these methods are generally unsuitable for simulating the post failure stage of landslides due to the occurrence of large deformations the material point method mpm is a numerical technique capable to overcome this limitation in the present study mpm is used to carry out a two dimensional analysis of the run out process of the senise landslide that occurred in southern italy in 1986 accuracy of the method is assessed by comparing the final geometry of the landslide observed just after the event and the magnitude of the measured displacements to those provided by the numerical simulation the calculation results match fairly well the observed ones when two slip surfaces detected by the inclinometers are accounted for in the analysis an additional improvement of simulation is achieved taking into account the presence of some buildings and retaining walls existing in that area kinematics of the landslide is also investigated the results show that the senise landslide was a translational slide moving along two planar slip surfaces located in thin layers of clayey silt in addition these results demonstrate that a suitable analysis of the post failure stage can lead to a better understanding of the complex mechanical processes that characterise some landslides and thereby help in establishing the most effective stabilization measures

a ring shear device for the study of till deformation tests on tills with contrasting clay contents abstract deformation of subglacial sediment may have had a profound influence on the geomorphic effects and dynamics of pleistocene ice sheets but data that bear on the rheology of such sediment are few and contradictory a means of studying systematically the mechanical properties of glacial tills is provided by a ring shear device that shears a large annular specimen to high strains at variable rates special features of the device allow continuous observation of the distribution of shear strain measurement of local stresses normal to the direction of shear and isolation of wall effects thus far a linear viscous putty and two tills with different clay contents 4 and 32 by weight have been tested the experiments with till indicate as expected that the presence of dispersed clay minerals reduces markedly both the residual strength and hydraulic diffusivity of till clay also reduces spatial variations in normal stress associated with grain bridges the walls bounding the specimen support only 9 17 of the total resistance to shearing strain was localized in both tills but not in the linear viscous putty strain localization in the tills indicates that their theology departs significantly from linear and bingham viscous models

interface shear box tests for assessing axial pipe soil resistance the clay interface shear resistance is an important parameter for the design of offshore pipelines which slide on the seabed as a result of thermally induced expansion contraction and lateral buckling this paper presents a methodology for characterising the clay interface resistance and quantifying the effect of drainage and consolidation during or in between shearing episodes models for describing the clay interface resistance during planar shearing are presented and compared to test data for a range of drainage conditions from drained to undrained and including the case of episodic consolidation the test data are from two series of interface shear box tests carried out on marine clays the effects of normal stress level in the low stress range overconsolidation and interface roughness are also examined

shear zone structures and stress fluctuations in large ring shear tests abstract large ring shear tests are carried out in order to investigate the naturally drained shear behavior of slip zone soils of three giant landslides in the three gorges project area twenty seven specimens are sheared at three shearing rates 0 1 1 and 10 mm s shear zone structure and stress fluctuation at residual state are examined with reference to sample properties and test conditions it is observed that a shear stress continues to increase at large displacements presumably due to slow dissipation of pore pressure within the shear zone b the presence of coarse particles impedes development of a slickensided shear surface c increasing shearing rate tends to positively influence residual shear stress by allowing rougher shear surfaces but this effect is dimmed by increasing coarse fraction and density d the amplitude and wavelength of the most dominant first order component of complex shear stress fluctuation pattern are measures of roughness of shear surfaces and turbulence within the zones and e these fundamental sinusoidal wave parameters correlate well with atterberg limits particle shape and shearing rate increasing atterberg limits ip and ll and or decreasing particle elongation lead to larger wavelength and smaller amplitude favoring relatively smooth shear surface whereas increasing shearing rate produces a decrease in both amplitude and wavelength

temperature dependent residual shear strength characteristics of smectite bearing landslide soils this paper presents experimental investigations regarding the effect of temperature on the residual strength of landslide soils at slow to moderate shearing velocities we performed ring shear tests on 23 soil samples at temperatures of 6 29 c the test results show that the shear strength of smectite rich soils decreased when temperatures were relatively low these positive temperature effects strength losses at lower temperatures observed for smectite bearing soils are typical under relatively slow shearing rates in contrast under relatively high shearing rates strength was gained as temperature decreased as rheological properties of smectite suspensions are sensitive to environmental factors such as temperature ph and dissolved ions we inferred that temperature dependent residual strengths of smectitic soils are also attributed to their specific rheological properties visual and sem observations of ca bentonite suggest that slickensided shear surfaces at slow shearing rates are very shiny and smooth whereas those at moderate shearing rates are not glossy and are slightly turbulent indicating that platy smectite particles are strongly orientated at slow velocities the positive temperature effect is probably due to temperature dependent micro friction that is mobilized in the parallel directions of the sheet structure of hydrous smectite particles on the contrary the influence of micro viscous resistance which appears in the vertical directions of the lamination is assumed to increase at faster velocities our results imply that if slip surface soils contain high fractions of smectite decreases in ground temperature can lead to lowered shear resistance of the slip surface and trigger slow landslide movement

laboratory investigation on reactivated residual strength recent research has outlined that mobilized in situ strength at the reactivation of displacements along a preexisting sliding surface may be slightly greater than laboratory residual strength although geotechnical literature explains this difference in various ways for instance strength gain due to soil aging little work has been devoted to this topic bromhead ring shear tests were carried out on remolded samples of the cormons flysch a typical geological formation in northeastern friuli venezia giulia italy the specimens came from the shallow weathered crust of the cormons flysch near the areas of montona and rosazzo this experimental study aimed at highlighting the effect of a limited number of parameters on strength gain at the reactivation of displacements along aged sliding surfaces the results of this research confirm that aging may cause a rising of reactivated shear strength above the residual value depending on time of aging and vertical stress the longest aging period about a month showed a 20 strength increase for the rosazzo flysch and a 30 increase for the montona flysch reactivated strength increases according to a power law almost 50 develops over a short time i e after 24 h of aging in the long term strength growth is characterized by a deviatoric creep rate gradually approaching zero

objective oriented multistage ring shear test for shear strength of landslide soil ring shear tests were conducted on five samples of different nature with a modified imperial college type ring shear machine the three different testing methods used 1 individual sample testing for each normal stress 2 increasing load multistage ring shear test and 3 reducing load multistage ring shear test all showed similar effective residual internal friction angle for the samples irrespective of testing method however effective residual shear intercept was different according to the testing methodology the internal friction angle did not vary particularly after the first minimum point in the stress displacement curve although the residual shear intercept decreased with increase in the displacement the thickness of the shearing zone increased along with the displacement the remolded peak shear strength for saturated conditions at field dry density varied with the consolidation history measurement of remolded peak shear strength was possible in a single sample using the increasing load multistage ring shear test at normal consolidation the equilibrium water content of the sample after the ring shear test was nearly equal to the plastic limit

residual and postliquefaction strength of a liquefiable sand abstractseismic design of geotechnical structures often requires estimates of how shearing resistance of liquefiable soil is reduced to its minimum value residual strength sr as pore pressures build up and is subsequently regained as pore pressures dissipate it was envisioned that the shear strength of liquefying sands could be measured in flight in a seismic geotechnical centrifuge model by pulling thin coupons plates horizontally through the sand models before during and after shaking to simulate the large strains and strain rates associated with liquefaction flow failures this paper presents the results of seismic centrifuge tests that were used to make such measurements companion ring shear rs tests also are described although centrifuge and rs residual strengths were generally similar and increased with relative density the centrifuge sr and sr vo sr divided by preshaking effective vertical stress increased significantly with small changes in relative density while the rs test sr a

the experimental basis for interpreting particle and magnetic fabrics of sheared till particle fabrics of basal tills may allow testing of the bed deformation model of glacier flow which requires high bed shear strains 100 field studies however have not yielded a systematic relationship between shear strain magnitude and fabric development to isolate this relationship four basal tills and viscous putty were sheared in a ring shear device to strains as high as 714 fabric was characterized within a zone of shear deformation using the long axis orientations of fine gravel and sand particles and the anisotropy of magnetic susceptibility ams of small 5 8 cm3 intact samples results indicate that till particles rotate toward the plane of shearing with long axis orientations that become tightly clustered in the direction of shear 0 78 s1 0 94 for three dimensional data these strong steady state fabrics are attained at shear strains of 7 30 with no evidence of fabric weakening with further strain regardless of the specific till or particle size fraction under consideration these results do not support the jeffery model of particle rotation which correctly describes particle rotation in the viscous putty but not in the tills owing to fluid mechanical assumptions of the model that are violated in till the sensitivity of fabric development to shear strain magnitude indicates that for most till units where shear strain magnitude is poorly known attributing fabric variations to spatial differences in other variables such as till thickness or water content will be inherently speculative attributing fabric characteristics to particular basal till facies is uncertain because shear strain magnitude is unlikely to be closely correlated to till facies weak or spatially variable fabrics in the absence of post depositional disturbance or major deviations from unidirectional simple shear indicate that till has not been pervasively sheared to the high strains required by the bed deformation model strong flow parallel fabrics are a necessary but insufficient criterion for confirming the model copyright 2008 john wiley sons ltd

excess pore pressure and grain crushing of sands by means of undrained and naturally drained ring shear tests an investigation of excess pore pressure generation of a weathered granitic sand taken from the source area of a typical landslide caused as a result of liquefaction and a fine silica sand was conducted in which grain crushing within the shear zone of ring shear test specimens was examined as the key phenomenon of rapid long runout motion of landslides in order to investigate and explain the low average apparent friction angle mobilized in a liquidized landslide speed controlled ring shear tests were conducted under undrained conditions on weathered granitic sand specimens formed under a wide range of initial void ratios it was revealed that very small steady state shear resistances were obtained irrespective of the initial void ratios which can explain the low average apparent friction angle in addition two series of ring shear tests on weathered granitic and fine silica sands were conducted under naturally drained conditions by keeping the upper drain valve of the shear box open during the tests the first series of tests was performed under differing total normal stresses but at the same shear speed and the second series was conducted at differing shear speeds but under the same total normal stress in order to investigate and analyze excess pore pressure generation and dissipation within the shear zone that is associated with grain crushing permeability analyses were conducted by passing water through the sample box of the ring shear test apparatus before and after shearing in addition grain size distribution analyses of samples taken from the shear zone after shearing were carried out for the weathered granitic sand samples a significant change in bulk permeability and large amount of grain crushing were observed in these tests on the above soil a considerable reduction of shear resistance which increased proportionally to the total normal stress and shear speed were obtained it was observed that due to grain crushing finer grains that lowered the permeability of the soil in the shear zone were formed it is likely that the decrease in permeability facilitated the generation of high excess pore pressures by reducing the pore pressure dissipation rate from the shear zone thus flow behaviour was exhibited even under naturally drained conditions

experiments on the dynamics and sedimentary products of glacier slip abstract experimental work in glacial geomorphology is focused almost entirely on processes in the thin shear zone at the beds of sliding glaciers where ice at its pressure melting temperature moves over either rigid rock or deformable till experiments with rotary shear devices illuminate constitutive behavior there central to the dynamics of fast moving glaciers and provide a foundation for interpreting the sedimentary record of glacier slip and associated sediment transport results from experiments designed to study ice sliding over a rigid wavy bed shear deformation of till and plowing of clasts across the surface of a till bed point to a common conclusion drag at the bed can decrease with increasing slip velocity thereby concentrating driving stress elsewhere and promoting rapid glacier flow this rate weakening behavior at glacier beds is in contrast to the viscous slip resistance assumed in ice sheet models and most efforts to determine distributions of basal drag from glacier surface velocities ring shear experiments in which various basal tills and more idealized materials are sheared to high strains provide quantitative insight into grain size evolution mixing at contacts between basal tills microstructure development particle fabric development and development of anisotropy of magnetic susceptibility preferred orientations of principal magnetic susceptibilities provide the most dependable and complete description of till shear patterns when applied to basal tills of the geologic record magnetic till fabrics measured along thick till sections and calibrated experimentally indicate that deformation of the bed by two lobes of the laurentide ice sheet was shallow

magnetic fabric of sheared till a strain indicator for evaluating the bed deformation model of glacier flow wet based portions of ice sheets may move primarily by shearing their till beds resulting in high sediment fluxes and the development of subglacial landforms this model of glacier movement which requires high bed shear strains can be tested using till microstructural characteristics that evolve during till deformation here we examine the development of magnetic fabric using a ring shear device to deform two wisconsin age basal tills to shear strains as high as 70 hysteresis experiments and the dependence of magnetic susceptibility of these tills on temperature demonstrate that anisotropy of magnetic susceptibility ams develops during shear due to the rotation of primarily magnetite particles that are silt sized or smaller at moderate shear strains 6 25 principal axes of maximum magnetic susceptibility develop a strong fabric s 1 eignevalues of 0 83 0 96 without further strengthening at higher strains during deformation directions of maximum susceptibility cluster strongly in the direction of shear and plunge up glacier consistent with the behavior of pebbles and sand particles studied in earlier experiments in contrast the magnitude of ams does not vary systematically with strain and is small relative to its variability among samples this is because most magnetite grains are contained as inclusions in larger particles and hence do not align during shear although processes other than pervasive bed deformation may result in strong flow parallel fabrics ams fabrics provide a rapid and objective means of identifying basal tills that have not been sheared sufficiently to be compatible with the bed deformation model

the residual shear strength of tropical soils the paper discusses the residual shear strength of tropical soils ring shear tests have been performed with tropical soils from southern brazil using the bromhead apparatus the data from the study are compared with data obtained in a comprehensive literature review of the subject the residual friction angle of tropical residual soils from sedimentary formations decreases with increasing plasticity index following a general trend presented in the literature most of the residual soils described herein have also shown this trend however some of these residual soils particularly basalt saprolitic soils have shown quite low residual friction angles despite having low plasticity index and clay fraction values it has been confirmed that the residual shear strength of tropical soils depends chiefly on mineralogy particle size distribution effective stress parent rock weathering degree pedogenetic evolution and when partly weathered minerals are present degradation upon shearing the substantial

fast shearing of pre existing shear zones in soil the paper presents results from a laboratory investigation into the influence of fast rates of displacement on the residual strength of soil this influence was studied in the ring shear apparatus shear zones were formed by slow drained shearing and then tested at alternately fast and slow rates of displacement results from a wide range of natural soils are presented all soils show initially an increase in strength when resheared at fast rates of displacement the strength then tends to decrease with fast displacement to a minimum value the fast residual strength three types of rate effects on the residual strength are identified a positive rate effect in soils showing a fast residual strength higher than the slow residual a neutral rate effect in soils showing a constant fast residual strength equal to the slow residual irrespective of rate of displacement and a negative rate effect in soils showing a significant drop in fast residual strength below the slow residual when sheared at rates higher

sediment entrainment by a soft bedded glacier a model based on regelation into the bed experimental results indicate that wet based soft bedded glaciers may penetrate their substrates by regelation melting and refreezing and thereby entrain sediment in principle there should be a steady depth of penetration at which the downward regelation speed driven by the interfacial effective pressure equals the basal melt rate herein the magnitude and distribution of penetration are estimated for the case of a glacier resting on a deformable bed with channelized basal drainage the distance between channels and the distribution of effective pressure across the bed are calculated and special attention is paid to the interdependence of the basal melt rate and effective pressure a major uncertainty is whether fine grained tills may impede or prevent regelation as a result of high surface tension at the ice water interface predicted penetration depths range from millimetres to a few decimetres and thus dirty basal ice layers of such thicknesses might be expected predicted distances between channels agree well with those indicated by borehole measurements at ice stream b west antarctica effective pressures and hence penetration depths increase toward channels therefore the edges of interfluves bounded by anastomosing subglacial channels should be eroded preferentially this motivates the testable hypothesis that such erosion contributes to the formation of streamlined landforms such as drumlins copyright 2000 john wiley sons ltd

shear strength of clay at different shear rates abstracta test procedure that involved shearing soil specimens at different shear rates was proposed to study the effect of shear rate on the strength recovery of three natural clays the obtained results indicated that an increase in shear strength occurred when the shear rate decreased this strength regain was more pronounced for higher plasticity clays which were sheared at a relatively lower confining pressure of 100 kpa

laboratory study of the frictional rheology of sheared till 1 deformation of till produces power law creep for low strain at stresses high enough to cause permanent deformation but below the shear strength experiments were conducted on till a mixed size granular material from matanuska glacier alaska and the scioto ohio lobe of the laurentide ice sheet caesar till we deformed till in double direct shear under fixed shear velocity or shear stress creep normal stress ranged from 50 kpa to 5 mpa at shearing rates ranging from 1 to 300 m s for 1 cm thick samples creep was induced via small step perturbations in stress fabric development within till layers was investigated by varying shear strain prior to creep tests in velocity controlled experiments till deforms as a nearly coulomb plastic material with slight velocity strengthening corresponding to a stress exponent n 60 creep experiments conducted well below the shear strength indicate lower n values increasing as shear stress increases with increasing initial strain and inferred fabric development the creep strain rate decreases while n increases experiments at a normal stress of 1 mpa and no initial strain show n 6 8 increasing to n 17 5 at higher shear strains however strain rate was still decreasing and thus these values represent maximum estimates our data show that in the absence of dilatant hardening till exhibits rate sensitivity at strain of order 1 or less at low strains n likely depends on consolidation state permeability and dilation deformation is nearly rate insensitive coulomb plastic at shear stress near the shear strength or at high strain

long runout mechanism and landsliding behaviour of large catastrophic landslide triggered by heavy rainfall in guanling guizhou china a large catastrophic landslide was triggered by a heavy rainfall on 28 june 2010 in guanling guizhou china the landslide buried two villages and killed 99 people along the runout path the lands