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Mobilized Shear Strength of Overconsolidated Seattle Clays
Water infiltration can cause softening of compacted structural fill and a reduction of the shear strength from the peak compacted strength to the fully softened strength (FSS) with an accompanying reduction in drained factor of safety (FoS). This study presents two-dimensional (2D) and 3D stability analyses of a compacted fill slope failure that occurred 6 years after construction due to water leaking from a connection between the main and lateral water pipes in the water supply system. The compacted fill material primarily consists of high plasticity fine-grained soil. The 3D FoS at the end of construction is 2.44 using the peak compacted strength envelope. However, the 3D FoS is close to unity (1.0) when the FSS is assigned to the compacted fill material with the appropriate piezometric surface, which means the 2 H:1 V compacted fill slope softened to the FSS within 6 years. This is an interesting FSS case because the failure surface is 4 m deep and semi-circular, which differs from infiltration cases that exhibit a shallower and more planar surface.
This study reports a fundamental experimental program that aims to propose an approach towards developing a descriptive framework that extends the concept of effective stress and viscoplasticity to frozen states. A series of triaxial compression tests was conducted on reconstituted Kasaoka Clay at different temperatures (three temperatures below the freezing temperature and one above it) and strain rates. The specimens were isotropically normally consolidated to 100, 200 or 400kPa before freezing and sheared at constant strain rates of 10-3, 10-4 and 10-5 min-1 after the test temperature got stabilized. The observed behavior was interpreted through comparisons with that of unfrozen specimens, which were subjected to a same strain path as that for the frozen specimens through consolidation and swelling corresponding to the volumetric changes due to pore water phase changes. The strength of frozen specimens, as well as being far larger than that of unfrozen specimens, showed much more significant dependence on the strain rate, which is probably due to the presence of the ice. In light of Ladanyi and Morel’s (1990) hypothesis on the uniquely related strain path and effective stress path of both frozen and unfrozen states of the same soils, the “effective stress paths” followed by the soil skeleton of the frozen soil were probed by the carefully controlled parallel tests on the frozen and unfrozen states. The Critical state lines (CSLs) at different temperatures were thus plotted based on the envisaged effective stress paths. The above characterization of frozen and unfrozen soils is expected to lead to construction of a unified framework for describing the behaviors of both states.
The relationships between Ko and OCR are investigated for primary loading - unloading - reloading conditions. The study reviews laboratory data from over 170 different soils and presents an approach common to clays, silts and sands. Simple empirical methods for predicting Ko for normally consolidated and overconsolidated soils are evaluated. The validity of the methods is supported by statistical analyses. On the basis of the findings, only the effective stress friction angle (O') and prior stress history (OCR and OCRmax) are needed to predict approximate values of Ko.
Results of torsional ring shear tests on cohesive soils reveal that the drained residual strength is related to the type of clay mineral and quantity of clay-size particles. The liquid limit is used as an indicator of clay mineralogy, and the clay-size fraction indicates quantity of particles smaller than 0.002 mm. Therefore, increasing the liquid limit and clay-size fraction decreases the drained residual strength. The ring shear tests also reveal that the drained residual failure envelope is nonlinear, and the nonlinearity is significant for cohesive soils with a clay-size fraction greater than 50% and a liquid limit between 60% and 220%. This nonlinearity should be incorporated into stability analyses. An empirical correlation for residual friction angle is described that is a function of liquid limit, clay-size fraction, and effective normal stress. Previous residual strength correlations are based on only one soil index property and provide a residual friction angle that is independent of effective normal stress. For slope stability analyses, it is recommended that the residual strength be modeled using the entire nonlinear residual strength envelope or a residual friction angle that corresponds to the average effective normal stress on the slip surface.
The main factor affecting the drained residual strength measured in the Bromhead ring shear apparatus is the magnitude of wall friction developed along the inner and outer circumferences of the confined specimen. The magnitude of wall friction increases with the depth of the remolded specimen, and thus the plane of least wall friction occurs at or near the soil/top porous stone interface. As the top porous stone settles into the specimen container, the wall friction influencing the shear plane increases, causing an increase in the measured residual strength.
A new specimen container is proposed for the Bromhead ring shear apparatus that allows a remolded specimen to be overconsolidated and precut prior to drained shearing. This minimizes settlement of the top platen and the horizontal displacement required to reach a residual strength condition. As a result, a multistage test can be conducted without excessive settlement and thus wall friction occurring in the new specimen container. The use of a multistage test significantly reduces the time required to establish a drained residual failure envelope. The use of an overconsolidated and precut specimen also provides a better simulation of the field conditions that lead to a large post-peak decrease in drained strength in clayshales, claystones, and mudstones, and residual strengths that are in excellent agreement with field case histories.
This paper presents a review of long-term stability of stiff clay and clay shale slopes, and detailed reanalyses of 99 case histories of slope failures in 36 soft clays to stiff clays and clay shales. We analyzed 107 sections using the observed actual slip surface. In a first-time slope failure in clay or shale, part or all of the slip surface is unsheared prior to the occurrence of the landslide. Most stiff clays and clay shales contain stratigraphic discontinuities such as bedding planes and laminations. The fully softened shear strength is shown to be the lower bound for mobilized shear strength in first-time slope failures in homogeneous soft to stiff clays and on the slip surfaces cutting across bedding planes and laminations. For many of the first-time slope failures it appears that part of the slip surface is at the residual condition. For excavated slopes, the residual condition could be present before the final slope is formed, or it may develop in response to excavation by progressive deformation along nearly horizontal surfaces including bedding planes or laminations. In addition to the permeability dependent rise in porewater pressure, and softening, delayed first-time failure of slopes in stiff clays and clay shales is caused by propagation of the residual condition into the slope, on horizontal or subhorizontal surfaces including stratigraphic discontinuities. The residual condition is present on the entire surface of reactivated landslides.
Results of torsional ring shear, direct shear, and triaxial compression tests on cohesive soils reveal that the fully softened shear strength is stress-dependent and related to the type of clay mineral and quantity of clay-size particles. An empirical relationship for the fully softened friction angle is presented that is a function of liquid limit, clay-size fraction, and effective normal stress. Studies of first-time slides, i.e., slopes that have not undergone previous sliding, in stiff fissured clay with a liquid limit between 41 and 130% suggest that the mobilized shear strength along the failure surface can be lower than the fully softened shear strength. Recommendations are presented for estimating the mobilized shear strength in first-time slides based on soil plasticity. Soils with a liquid limit greater than 30% exhibit a large difference between the fully softened and residual friction angles. In these soils, the presence or absence of a pre-existing shear surface should be clarified.
This paper presents recommendations for selecting the type and magnitude of drained shear strength parameters for analysis of landslides. In particular, the importance, existence, and use of the cohesion shear strength parameter is reviewed. For slope stability analyses, it is recommended that the shear strength be modeled using a stress dependent failure envelope or a friction angle that corresponds to the average effective normal stress acting on the slip surface passing through that particular material instead of using a combination of cohesion and friction angle to represent soil shear strength. Other recommendations for stability analyses include using an effective stress cohesion of zero for residual and fully softened strength situations. To facilitate selection of shear strength parameters for landslide analyses, empirical relationships for the drained residual and fully softened strengths are updated from the previous empirical relationships presented by Stark and Eid. Finally, the paper presents torsional ring shear test results that indicate that pre-existing shear surfaces exhibit self-healing that results in increased shear resistance. The magnitude of healing appears to increase with increasing soil plasticity, and this increase could have implications for the size, timing, and cost of landslide remediation.
This volume brings together case studies and summary papers describing the application of state-of-the-art engineering geologic methods to landslide hazard analysis for the Seattle, Washington, area. An introductory chapter provides a thorough description of the Quaternary and bedrock geology of Seattle. Nine additional chapters review the history of landslide mapping in Seattle, present case studies of individual landslides, describe the results of spatial assessments of landslide hazard, discuss hydrologic controls on landsliding, and outline an early warning system for rainfall-induced landslides.
Channels of inferred subglacial fluvial origin ("channelways') are conspicuous across the recently deglaciated eastern Puget Lowland of Washington State. Exploiting the unusually well defined geometry of the former Puget-lobe ice sheet, we have reconstructed the subglacial hydraulic head in this region to better understand the location and orientation of these channels. Our study suggests that the formation and preservation of these landforms require both the concentration of subglacial water, notably near abalation-zone ice margins, and favorable preglacial bed-rock topography. Where these elements occur together, landforms of subglacial fluvial origin should be recognizable across the now-exposed beds of other former ice sheets as well. -from Authors
The innovative engineering approach adopted for the new Terminal 5 at Heathrow Airport called for an advanced investigation of the London Clay strata, including detailed in situ profiling and stress path laboratory testing on high-quality rotary-cored samples. Although the scope of the investigations exceeded that normally specified for conventional design, questions relating to the structure and anisotropy of stiffness and strength of the clay remained that could not be answered. Further research was required, and the deep excavations at the site provided the opportunity for a team from Imperial College to take multiple block samples from three depths, supplemented by two additional dedicated rotary-cored boreholes. Intensive research was performed at Imperial College on these samples, as described in three companion papers by Gasparre et al. and Nishimura et al. This overview paper integrates the findings from the recent research with those from the commercial investigation and with earlier studies to extend our understanding of the geology and key characteristics of this stiff clay, and their variation with depth. The influence of lithology, structure and destructuring is examined, and the practical implications of the work are discussed.
Using previously proposed values of standard deviation (σ) for the drained fully softened and residual strength correlations, the probability of failure (P f) can be calculated to supplement the drained factor of safety (FoS). Using the standard deviation (σ) of soil strength, e.g., fully softened (FS) and residual, the standard deviation of FoS (σ FoS) and coefficient of variation of FoS (V FoS) can be calculated readily using slope stability software to estimate P f. Being able to calculate P f allows owners, regulators, and/or engineers to select a frequency of failure for a particular slope given the consequences of failure instead of a nebulous value of FoS. For example, it is easier for owners, regulators, and/or legal representatives to understand a frequency of failure, e.g., 1 failure in 1,000 years, than a drained or effective stress FoS of greater than 1.5 with some level of uncertainty. Even more confusing to owners, regulators, and/or legal representatives is the difference between various values of drained and undrained FoS, which is simplified via the selection of a suitable value of Pf.
Drained residual strength is the controlling shear strength for slopes that have experienced prior movement, contain colluvium, or have a continuous zone of slickensided material. This study provides correlations between power function coefficients a and b and soil index properties, e.g.,liquid limit, plastic limit, and clay-size fraction, to represent the drained residual strength envelope. These correlations provide a prediction of the residual secant friction angle, ϕ’r, at any desired effective normal stress, which can be used to establish the residual strength envelope for stability analyses. The standard deviation of a and b coefficients and the corresponding trendlines were found using two statistical methods, with the Graphical Three-Sigma Rule Method providing a better representation of the data scatter than the Computational Method. Data from the literature were compiled and compared to the proposed a and b coefficients and there is favorable agreement.
I wish to thank the American Society of Civil Engineers for the privilege of presenting the sixth Terzaghi Lecture. For the past 20 yr I have been deeply interested in the measurement of ground movements related to slope instability. This topic appeared appropriate for this paper since much of my early interest was stimulated both by Terzaghi and by Casagrande, who was the second Terzaghi Lecturer. Indeed, the previous five Terzaghi lectures have each been closely related to slope stability problems of various types.
All evidence indicates that slides in slopes of overconsolidated clays and clay shales are preceded by the development of a continuous sliding surface by progressive failure. Requisite conditions for progressive failure are discussed and it is proposed that a progressive failure may result from a large content of recoverable strain energy of the clay resulting from its geological history. In clays with weak diagenetic bonds the strain energy is immediately recovered upon a reduction in load. In clay shales diagenetic bonds were formed when the clay carried its maximum consolidation pressure, with the result that the recoverable strain energy is locked-in and will only be liberated when the bonds are gradually destroyed by weathering near the surface. This is the Third Terzaghi Lecture presented at the ASCE Structural Engineering Conference, at Miami, Fla., February, 1966.
To measure the fully softened strength, the normally consolidated and reconstituted specimen must be drained, i.e., no development of shear-induced pore-water pressures, during shearing. This article investigates (1) typical shear displacement rates in torsional ring shear tests that yield drained shearing conditions for a range of fine-grained soils and (2) the problems that can develop if the porous discs in a torsional ring shear device are not clean and saturated prior to placement of the reconstituted soil paste in the specimen container. The data presented herein show that using a clogged or unsaturated porous disc will require a drained shear displacement rate that is one to two orders of magnitude slower than required, e.g., 0.018 mm/min, if the porous discs are clean, maintained, and saturated as recommended by the original equipment manufacturer and the ASTM D7608, Standard Test Method for Torsional Ring Shear Test to Measure Drained Fully Softened Shear Strength and Stress Dependent Strength Envelope of Fine-Grained Soils.
The fully softened strength (FSS) is usually the controlling shear strength for cut slopes in stiff-fissured fine-grained soils, shales, and claystones, engineered fill slopes, and compacted embankments with no prior slope movement. The FSS is used to model the shear strength of shallow slope soils because it represents the shear strength remaining after the effects of overconsolidation, compaction, desiccation, or other strengthening processes have been removed because of wet–dry cycles, applied shear stresses, stress relief, progressive deformation or failure, swelling, freeze–thaw cycles, weathering, or all of these. An empirical correlation is frequently desired for such slopes because the slopes can be long, such as a levee system, and the borrow material is not completely defined, which makes comprehensive testing difficult. This article presents the mode of shear for the FSS condition, a comparison of existing FSS correlations, power function coefficients to estimate the FSS envelope, and recommendations for modeling the stress-dependent FSS envelope in stability analyses.
Pleistocene glaciers in Washington and Oregon consisted of the Cordilleran ice sheet, which originated in western Canada and invaded northern Washington on both sides of the Cascade Range, and alpine glaciers in the mountain ranges of both States. Glacial episodes of early(?) to middle(?) Pleistocene age are best known in the Puget Sound lowland of western Washington, where two major glaciations are recorded, each consisting of two or more advances of the Puget lobe of the Cordilleran glacier into the area south of Seattle, Washington. In addition, three glacial episodes occurred during late Pleistocene time, the last during the Fraser Glaciation between 25,000 and 10,000 years ago. During the earlier two glaciations of late Pleistocene time, large valley glaciers and broad piedmont glaciers were fed by vast ice fields and ice caps in the Cascade Range of both Washington and Oregon and in the Olympic Mountains of Washington. Alpine glaciers were smaller during the Fraser Glaciation, although ice fields again mantled much of the higher part of the Cascade Range in Oregon. In the mountains of western Canada, continued growth of alpine glaciers during the Fraser Glaciation formed the Cordilleran ice sheet, which expanded into northern Washington after 22,000 years ago and which reached its maximum stand 50 miles south of Seattle between 15,000 and 13,500 years ago. Retreat of the glacier was accompanied and followed by glaciomarine conditions in the Puget Sound lowland from 13,500 to about 11,000 years ago. A readvance of the Cordilleran glacier in the Fraser Lowland of northern Washington about 11,000 years ago was followed by disappearance of the ice sheet. Two glacial episodes in post-Altithermal time are recorded at Mount Rainier, Washington. The older episode occurred between 3,500 and 2,000 years ago, and the younger within the last thousand years.
Seattle lies within the Puget Sound Lowland, an elongate structural and topographic basin bordered by the Cascade and Olympic Mountains. The geology of the Seattle area is dominated by a complex, alternating, and incomplete sequence of glacial and interglacial deposits that rest upon an irregular bedrock surface. The depth to bedrock varies from zero to several kilometers below the ground surface. Bedrock outcrops in an east-west band across the lowland at the latitude of south Seattle and also around the perimeter of the lowland. Numerous faults and folds have deformed both the bedrock and overlying Quaternary sediments across the lowland, most notably the Seattle fault. During an earthquake on the Seattle fault ca. 980 A.D., 8 m of vertical offset occurred. The Seattle area has been glaciated at least seven times during the Quaternary Period by glaciers coalescing from British Columbia. In an area where each glacial and interglacial depositional sequence looks like its predecessor, accurate stratigraphic identification requires laboratory analyses and age determinations. The modern landscape is largely a result of repeated cycles of glacial scouring and deposition, and recent processes such as landsliding and river action. The north-south ridges of the lowland are the result of glacial scouring and subglacial stream erosion. The last glacier reached the central Puget Sound region ca. 15,000 years ago and retreated past this area by 13,650 ¹⁴ C yr B.P. Post-glacial sediments are poorly consolidated, as much as 300 m thick in deep alluvial valleys, and susceptible to ground failure during earthquakes.
The relevance of the triaxial test to the solution of stability problems was discussed. The test was used to measure the actual properties of the cohesive soils, and the results were applied to the solution of the more important classes of stability problem. The logical relationship of the various standard tests to the different classes of the stability problem was also discussed.
The hitherto expensive and tim-consuming nature of ring shear tests to determine the residual strength of soils has prevented the test from becoming a routine procedure in commercial laboratories. However, ring shear testing cannot become a routine procedure outside the research laboratory until a simple, robust, inexpensive apparatus which has a fairly large potential through-put of tests in the working week is developed. This paper describes such a device, which has recently been built and evaluated at the School of Civil Engineering, Kingston Polytechnic and which in its fully developed form is now available commercially. The paper describes the apparatus, its development, application and test results.
Case histories and probable causes of slope failures in stiff-fissured clays and shales are reviewed, and analyses are described which were performed to determine the influence of the initial stress conditions on the stresses around excavated slopes. These analyses show that the shear stresses around excavations are much larger for conditions representative of heavily overconsolidated clays (high initial horizontal stresses) than for conditions representative of normally consolidated clays (low initial stresses). Shear stresses large enough to cause failure at some points may develop even when the factor of safety calculated by the ∖a = 0 method of analysis is much larger than unity. The higher the horizontal stresses before excavation, the higher the factor of safety corresponding to development of local failure.
Empirical correlations provide estimates of parameter values for preliminary design, verification of laboratory shear test data, and confirmation of back-analysis of a failed slope. The empirical correlations presented herein use liquid limit, clay-size fraction, and effective normal stress to capture the variability and stress-dependent nature of drained residual and fully softened strength envelopes. This paper describes the testing and analysis used to increase the number of data points in the existing correlations, expand the residual strength correlation to include an effective normal stress of 50 kPa, and develop correlations between values of liquid limit and clay-size fraction measured using sample processed through a No. 40 sieve (ASTM procedure) and values derived using ball-milled/disaggregated sample. In addition, equations are presented to express the empirical correlations used to develop a spreadsheet that estimates the residual and fully softened friction angles based on entered values of liquid limit and clay-size fraction. (C) 2013 American Society of Civil Engineers.
The influence of stress history on the coefficient of earth pressure at rest of remoulded cohesive soils was studied experimentally. A one-dimensional compression test cell and auxiliary controls which enabled the measurement of radial stresses under a condition of zero lateral strain was developed for this purpose. Radial pressures were measured throughout all stages of consolidation up to a maximum axial stress of 2,200 psi and subsequent rebound to zero stresses.Five cohesive soils, the properties of which are well documented, were selected for this study. These soils were: Chicago Clay, Goose Lake Flour, Weald Clay, London Clay, and Bearpaw Shale. 'The selected soils range in plasticity from low to high and cover a range of drained angles of shearing resistance from 15.5 degrees to 27.5 degrees. For testing purposes all soils were remoulded at a liquidity index of 0.5.For a pre-consolidation load of 2,200 psi and various values of the over-consolidation ratio, it was found that the coefficient of earth pressure at rest is related to both the drained angle of shearing resistance and the plasticity index. The coefficient of earth pressure at rest during rebound was found to be a function of the stress history of the soil as defined by the pre-consolidation load and over-consolidation ratio.
Pressuremeter testing was conducted for the State Route (SR) 99 Bored Tunnel project in Seattle, Washington, to estimate in situ soil stress-deformation parameters along the tunnel alignment. Many of the tests were conducted in a very stiff to hard glaciolacustrine clay known as Seattle clay. This unit is historically known for deep-seated slope failures and many of these failures have been attributed to the release of high, locked-in lateral stresses. Estimation in situ lateral stresses along the tunnel alignment was a primary focus of the exploration program. Due to the hard consistency of this unit and the potential for cobbles, neither self-boring pressuremeter nor dilatometer testing was feasible; therefore, prebored pressuremeter testing was used. Using several lateral stress estimation techniques, including a novel in situ creep testing approach, the in situ lateral stresses in the Seattle clay were estimated to be significantly higher than what would be expected by assuming a simple, laterally constrained, vertical loading and unloading stress path due to glaciation. Deformational features commonly encountered in Seattle clay indicate its stress history also has included significant lateral shearing. The memory of this shearing within the fabric of the clay may influence the in situ stress state and response to lateral unloading.
This Wiley classic presents both theoretical and practical knowledge of soil mechanics in engineering. Written by Karl Terzaghi, universally recognized as ``the father of geotechnical engineering,`` it has long been the standard in the field. It offers a fundamental understanding of how to determine and use soil properties needed for design and construction; points out appropriate nature and benefits of exploration and soil tests under various conditions; and discusses most suitable methods and types of equipment for fills, excavations, and foundations. It also features expanded coverage of vibration problems, mechanics of drainage, passive earth pressure, and consolidation. This book contains information on the following. Part 1: physical properties of soils; index properties of soils; soil exploration; and hydraulic and mechanical properties of soils. Part 2: theoretical soil mechanics; hydraulics of soils; plastic equilibrium in soils; and settlement and contact pressure. Part 3: problems of design and construction; ground improvement; earth pressure and stability of slopes; foundations; settlement due to extraneous causes; and dams and dam foundations.
Proglacial lake shorelines in the Puget Sound area were distorted by the isostatic response associated with recession of the Cordilleran ice sheet about 18 000-13 500 yr ago. The shorelines, which are highest near the east-west midpoint of the elongate lowland, are upwarped northward, with a regression slope of 0.85 m km-1; departures from the trend may have been caused by Holocene tectonic warping. Calibrating this lacustrine record with data from the Everson marine limit permits the actual (rather than relative) uplift to be estimated and contoured. -from Author
Synopsis
The sensitivity of clays is defined as the ratio of their undisturbed and remoulded strengths, and varies from about 1·O for heavily over-consolidated clays to values of over 100 for the so-called extrasensitive or “quick” clays. As a result of experimental work it is shown that thixotropy can account for low or medium sensitivity but not for high sensitivity. Both laboratory and field evidence, however, show that a reduction in the salt concentration in the pore water, caused by leaching clays of marine or estuarine origin, can result in high sensitivities. Moreover, of the small number of clays so far investigated, those which have not been leached are found to exhibit only low or medium sensitivities of the order of magnitude explicable in terms of thixotropy. But it is, nevertheless, not suggested that leaching is the sole cause of high sensitivity and other possibilities are briefly discussed. Reasons are given, supported by experimental work, for the insensitivity of heavily overconsolidated clays with water contents approximating to the plastic limit.
A result of particular interest from the scientific point of view is that the water content and undisturbed strength of a clay remain unaltered by learhing, although the liquid limit and remoulded strength are appreciably reduced. It is suggested that this implies effectively “solid” contact between the particles of an undisturbed clay' Finally some evidence is given which indicates that the fissured structure present in many clays may be due to syneresis.
La sensitivité des argiles est définie comme étant le rapport entre leur résistance à l'état brut et leur résistance après remoulage, et varie de 1,0, environ, pour les argiles trop fortement tassées, jusqu'à des valeurs de plus de 100 pour des argiles dites extrasensitives ou “sables mouvants”. A la suite de travaux d'expérience, on montre que la thixotropie peut entre en ligne de compte pour les basses ou moyennes de sensitivités, mais non pas pour les fortes sensitivités. Cependant, les travaux de laboratoire et sur les lieux montrent qu'une réduction de la concentration en se1 dans l'eau de porosité, causée par le lavage des argiles d'origine maritime ou alluvionaire, peut provoquer de fortes sensitivités. Qui plus est, du petit nombre d'argiles qui ont fait l'objet de recherches jusqu'à maintenant, celles qui n'ont pas été lavées n'ont présenté seulement que des sensitivités faibles ou moyennes de l'ordre de grandeur explicable en fonction de la thixotropie. Cependant, il n'est pas suggéré que le lavage soit la seule cause de haute sensitivité et d'autres possibilités font l'objet d'une discution rapide. On donne des raisons, s'appuyant sur des travaux expénmentaux, pour le manque de sensitivité des argiles trop fortement tassées dont la teneur en eau approche la limite plastique.
Un résultat d'un intérêt tout particulier du point de vue scientifique est que la teneur en eau et la résistance à l'état brut restent inchangées par le lavage, alors que la limite liquide et la résistance après remoulage soient sensiblement réduites. II est suggéré que ceci signifie qu'il y ait un contact “solide” effectif entre les particules d'une argile à l'état brut et s'oppose à la théorie communément acceptée que les particules sont séparées par des couches relativement épaisses d'eau adsorbée. Finalement il est donné des preuves qui indiquent que la structure fissurée qui se rencontre dans beaucoup d'argiles puisse être due à une synérèse.
Synopsis
Within the framework of limit equilibrium methods of stability analysis, no restriction need be placed at the outset upon the shape of the possible slip surface. In many cases, the critical surface may deviate significantly from a circle or a plane and therefore a method that facilitates the analysis of surfaces of arbitrary shape is of interest. A method for doing this is presented. The assumptions necessary to make the problem statically determinate are discussed. The solution of the governing equations ensures that all equilibrium and boundary conditions are satisfied. The method has been programmed for a digital computer and some examples of its application are given. Comparisons are also made with other methods of analysis.
Dans la cadre des méthodes d'équilibre limite d'analyse de stabilité, il n'y a pas besoin d'imposer de restrictions au départ sur la forme de la surface de glissement éventuelle. Dans bien des cas, la surface critique peut dévier d'une manière significative d'un cercle ou d'un plan at par conséquent une méthode qui facilite l'analyse des surfaces de formes arbitraires présente un intérêt. On présente une méthode pour accomplir cela. On discute des hypothèses nécessaires pour que le problème soit déterminé au point de vue de la statique. La solution des équations dominantes garantit que toutes les conditions d'équilibre et de limite soient satisfaites. La méthode a été programmée pour une calculatrice digitale et on donne quelques examples de ses applications. D'autres méthodes d'analyse y son comparées.
Delayed failure of railway cutting slopes in stiff clays excavated in the nineteenth and early twentieth centuries has been studied for some forty years. Known failures have generally been deep-seated. More recently failures have been a problem in the slopes of motorway cuttings and embankments, although generally these have been shallow. The average operational strength at failure in these slides has been significantly less than the peak strength, and progressive failure has been postulated as the probable cause of this. Progressive failure can now be analysed using advanced numerical techniques. A series of coupled finite element analyses have been conducted assuming strain-softening soil with properties based on the Brown London Clay, and the results are reported. They show that progressive failure is considerable, and fully explain the observed field behaviour. The delays experienced in the field are also recovered by the analyses. Progressive failure is generated primarily by the high lateral stresses in the soil prior to excavation. The rupture surface generated spreads horizontally from the toe as the soil swells, and differs significantly from the critical surface predicted by limit equilibrium analysis. The average strength on it at collapse is significantly lower than that obtained by back analysis by limit equilibrium methods. It becomes lower as lateral stress increases, but the effect is compensated by the increasing depth of rupture. The roll of the hydraulic surface boundary condition, which is controlled by climate, is important in controlling collapse. The implications of the findings for monitoring and for remedial works in existing slopes is discussed.
The post-peak drop in drained shear strength of an overconsolidated clay may be considered as taking place in two stages. First, at relatively small displacements, the strength decreases to the 'fully softened' or 'critical state' value, owing to an increase in water content (dilatancy). Second, after much larger displacements, the strength falls to the residual value, owing to reorientation of platy clay minerals parallel to the direction of shearing. If the clay fraction is less than about 25% the second stage scarcely comes into operation; the clay behaves much like a sand or silt with angles of residual shearing resistance typically greater than 20 degree . Refs.
Failure of slopes cut in overconsolidated clay may be delayed by the rate of equilibration of pore pressure after excavation. Other mechanisms, such as decrease in drained strength with time, have been suggested as reasons for delayed failure. Previous studies of cutting slopes in the London Clay have suggested that equilibration of pore pressure is relatively rapid and that other mechanisms control eventual failure. Recent observations of pore pressure in a cut slope nine years after excavation suggest that equilibration of pore pressure may be the principal mechanism controlling delayed failure in the London Clay. The studies made of failures in London Clay slopes have played a significant part in the general study of long-term failure of cut slopes in clay, so this conclusion has some general implication.
La rupture des talus découpés dans de l'argile surconsolidée peut être retardé par la vitesse d'équilibrage de la pression interstitielle après excavation. D'autres mécanismes tels que la réduction de résistance drainée avec le temps ont été suggérés pour expliter les ruptures à long terme. Des études antérieures de talus découpés dans l'argile de Londres ont suggéré que l'équilibrage de la pression interstitielle est relativement rapide et que d'autres mécanismes régissent une rupture éventuelle. Des observations récentes de pression interstitielle dans un talus neuf ans après son excavation suggèrent que l'équilibrage de la pression interstitielle peut être le principal mécanisme régissant la résistance à long terme dans l'argile de Londres. Etant donné que les études des ruptures de talus dans l'argile de Londres ont joué un rôle important dans l'étude générale de la résistance à long terme de pentes taillées dans l'argile, cctte conclusion s'avère être d'un intérêt général.
The processes of weathering of the Jurassic Lias clay (a clay shale) at four different sites are shown to result in a progressive change of the relationship between strength and water content, with successively shallower, more weathered layers showing higher strengths at a given water content. A weathering scheme, recognizing four weathering zones, based on the degree of oxidation, appears to correspond well with the strength-water content data. The progressive changes of fabric observed in petrological thin sections of the clay can also be related to these weathering zones. It is concluded that weathering of the Lias clay after excavation is so slow on an engineering time scale that it is probably not an important factor leading to long term slope failure. In such heavily overconsolidated clays the scatter of strength may be as much associated with small variations in water content as with the presence of fissures.
A fifth site provides an example of severe disturbance of the Lias clay resulting from permafrost conditions during the Pleistocene, and it is shown that not only does this disturbance largely mask the effect that weathering has on the strength-water content relationship, but also probably results in a considerable reduction in effective stress strength parameters.
On montre que les processus d'altération de l'argile du Lias Jurassique (un schiste argileux) en quatre sites différents, résultent d'un changement progressif dans la relation entre résistance et teneur en eau, au tours duquel les couches les moins profondes, plus altérées, ont des résistances plus élévées pour une teneur en eau donnée. Un schéma d'alteration, qui distingue quatre zônes d'altération en fonction de leur degré d'oxydation, semble bien correspondre aux relations de la résistance et de la teneur en eau. On peut également faire une corrélation entre les changements progressifs dans la structure, tels qu'observés sur des lames minces pétrographiques de l'argile, et ces zônes d'alteration. On en conclut que l'altération de l'argile du Lias après excavation est si lente à l'échelle des travaux de construction, que ce facteur n'est probablement pas important en ce qui concerne la rupture à long terme des talus. Dans ce argiles très largement surconsolidés, la dispersion des résistances peut être aussi bien attribuée à faibles variations de la teneur en eau qu'a la présence de fissures.
Un cinquième site donne un exemple d'un remaniement très important de l'argile du Lias, résultant de conditions de gel permanent au tours du Pléistocène, on montre que ce remaniement a pour conséquence, non seulement de masquer les effets de l'alteration sur la relation résistance-teneur en eau, mais également de réduire, probablement dans une proportion considérable, les paramètres contrainte effective résistance.
A detailed study is presented of the effect of weathering on the undrained and effective stress strengths of London Clay from a site at South Ockendon, Essex. A limited investigation of the chemistry and clay mineralogy was also carried out.
It is concluded that the effect of weathering is to reduce the apparent degree of overconsolidation. This effect is seen in the undrained strength-water content relationship of the most weathered clay, which shows a 4% higher water content at a given strength than the unweathered clay. There is a corresponding reduction of the overconsolidation ratio from about 40 to about 20. In effective stress terms the corresponding strength change is given by a reduction of c ′ from 28 kPa to a minimum value of about 9 kPa, assuming that ø′ (= 20°) remains unchanged by the weathering.
Synopsis
A number of physical and mechanical properties of the London Clay are given based on observations made in several tunnel excavations, and from laboratory tests on specimens. These specimens have been carefully prepared by hand trimming from blocks of clay, which were extracted from tunnels over a large area of the London Basin. Earlier studies of someof these properties (Cooling and Skempton, 1942; Bishop, 1947; Skempton and Henkel, 1957)have been restricted to measurements on samples obtained from boreholes or involving tube-sampling operations.
Three important structural features are recognized in the clay, namely, fissures, laminations, and “backs”. The latter two features are rarely considered. The laminations render the clay orthotropic with one axis of symmetry perpendicular to the bedding and specimens have been orientated in mechanical tests so as to demonstrate this feature. The “backs” appear to be associated with minor folding (or faulting) of the stratum, but their extent and importance requires wider exploration.
Stress-strain characteristics of the block specimens measured in undrained triaxial tests show that the clay is stronger and less compressible than measurements on borehole samples have suggested.
Measurements of the longitudinal wave velocity on clay samples readily detect the direction of the laminations, and attenuation of the ultrasonic signal is indicative of the fissuring and particularly of disturbances caused by sampling operations.
Consolidation and swelling characteristics were measured in the oedometer and again showed the orthotropic properties of the clay. The measured swelling pressures appear to provide an approximate guide to the existing effective pressures in the ground; these are also indicated by a distinct change in the pressure-void ratio relationship.
I1 est donnéun certain nombre de propriétés physiques et mécaniques de l'argile londonnienne basées sur des observations faites dans plusieurs fouilles de tunnels, et d'expériences en laboratoires sur des échantillons. Ces échantillons ont été soigneusement préparés par dégrossissage à la main de blocs d'argile qui furent extraits de tunnels sur une vaste superficie du bassin de Londres. Des études antérieures de quelques unes de ces propriétés (Cooling et Skempton, 1942; Bishop, 1947; Skempton et Henkel, 1957) ont été limitées á des mesures sur des échantillons prélevés au tours de forages ou d'opérations comprenant l'echantillonnage par tube carottier.
Trois caractéristiques de structure importantes se reconnaissent dans l'argile, à savoir: les fissures, le feuilletage et les failles “backs”. Les deux derniéres caractéristiques sont rarement prises en considération. Les feuihetages rendent l'argile or thotropique avec un axe de symetrie perpendiculaire à la couche et au tours d'épreuves mécaniques on a orienté des échantillons de mane à faire ressortir cette caractéristique. Les failles “backs” semblent étra associées à de petites failles de la couche, mais leur étendue et importance demande de plus amples recherches.
Les caractéristiques de tension-deformation des blocs-échantillons mesurées lors d'essais triaxiaux drainés montrent que l'argile est solide et moins compressible que des mesures faites sur échantillons de forage le sugérent.
Les mesures de la vélocité d'ondulation longitudinals sur des échantillons décélent facilement la direction des feuilletages, et l'atténuation du signal ultrasonique indique des fissures et particulièrement des remaniements causés par les opérations d'échantillonnage.
Des caractéristiques de consolidation et de gonflement sont mésurées à l'cedometre et de nouveaufont apparaitre les propriétés orthotropiques de l'argile. Les pressions de gonflement semblent donner un guide approximatif sur les pressions existant effectivement dans le sol; celles-ci sont aussirévélées par un changement distinct de la relation du rapport entre la pression et le vide.
The Puget lobe, the southwesternmost extension of the Cordilleran ice
sheet, last advanced into the Puget Lowland of western Washington at
about 15 ka. The advancing ice sheet deposited voluminous sediment on a
prograding, proglacial outwash plain that extended from the Olympic
Mountains to the Cascade Range, herein recognized as the "great Lowland
fill." Subsequent overrunning by the ice sheet excavated deep linear
troughs now occupied by the large lakes and marine waters of Puget
Sound. Excavation of these troughs and valleys of the Puget Lowland
required the net transport of about 1000 km3 of sediment,
almost entirely during ice occupation and primarily by subglacial water.
These landforms of glaciofluvial deposition and erosion define the
modern landscape here, emphasizing the importance of these processes in
the region's geomorphology.