Anisotropy Of Undrained Shear Strength Research Articles

Effect of Statistically Anisotropic Undrained Shear Strength on the Probability of Slope Failure

Due to large-scale geological deposition processes, slope structures are often stratified, which means that the spatial distribution of the parameters involved in slope reliability evaluation is statistically anisotropic. This paper studies the effect of the statistical anisotropy of undrained shear strength on the probability of slope failure (pf) based on the Monte Carlo simulation. The results show that for the horizontally layered slope, the larger the horizontal correlation scale of undrained shear strength (λx) is, the larger pf is, especially when λx is smaller than the slope length; for the vertically layered slope, the larger the vertical correlation scale (λy) is, the smaller pf is, especially when λy is smaller than the slope height. Additionally, the mechanism of the above results is discussed by analyzing the displacement distribution at different correlation scales. The findings indicate that in the reliability evaluation of undrained slopes in stratified structures, either underestimation of λx or overestimation of λy leads to an unconservative estimate of pf, resulting in an overestimation of the slope stability.

Undrained Shear Strength Anisotropy of Cohesive Soils Caused by the Principal Stress Rotation

The construction of geotechnical structures, e.g. diaphragm walls, tunnels, pad foundations or embankments, on cohesive soils, requires assessing the bearing capacity in undrained conditions. For this purpose, it is necessary to determine the value of undrained shear strength. Undrained shear strength is not the same in the subsoil, because it depends on many factors. One of these factors is principal stress rotation caused by changing the stress state due to loading or unloading the subsoil. During determination of undrained shear strength in laboratory tests the influence of principal stress rotation on undrained shear strength is often overlooked because of difficulty of reflecting this phenomenon in laboratory research. A device that allows to determine the anisotropy of undrained shear strength in soils due to the principal stress rotation is a Torsional Shear Hollow Cylinder Apparatus (TSHCA). The paper presents test results performed in a TSHCA on undisturbed sandy silty clays (sasiCl) characterized by different index properties. The main objective of the research was determining the undrained shear strength at wide range of angle of the principal stress rotation. The results of laboratory tests allow assessing the influence of the principal stress rotation on the value of undrained shear strength that should be used to determine the bearing capacity of the subsoil.

Self-boring in situ shear pressuremeter testing of clay from Dalian Bay, China

A new geotechnical in situ test technique using a self-boring in situ shear pressuremeter (SBISP) is developed to evaluate the initial state (horizontal at rest pressure Ph), deformation (yield pressure Pf and pressuremeter modulus EP) and strength characteristics (cohesion C and internal friction angle φ) of soil. The SBISP possesses a self-boring drilling functionality that can greatly reduce the disturbance of the tested soil. The multistage compound loading method, which applies normal stress incrementally to the soil in the test hole while repeatedly applying shear stress, was adopted.Relying on the investigation of the cross-sea traffic engineering of Dalian Bay, three sites in the sea area of Dalian Bay were selected for SBISP testing of clay. The field vane shear test (FVT), pressuremeter test (PMT) and unconsolidated-undrained triaxial compression test (UU) were also evaluated at each testing site to compare with the SBISP test. The results measured by the SBISP and comparative tests (FVT, PMT and UU) showed the same trends with an increase in depth and change in strata. The differences in the test results were discussed in terms of the disturbance effect, anisotropy of undrained shear strength, differences in loading methods and instrumentation used in the different tests. A comparison between the results measured by the SBISP and in comparative tests shows that the engineering characteristics of Dalian Bay clay evaluated by SBISP are fairly accurate and can provide reference for the investigation and design of the cross-sea traffic engineering of Dalian Bay.

The undrained shear strength anisotropy of four Jurassic to Eocene stiff clays

The shear strength of heavily overconsolidated, stiff-to-hard plastic clays is crucial to their stability and also influential on the ground movements they develop in many geotechnical engineering applications. This paper considers the shear strength anisotropy of the London, Gault, Kimmeridge and Oxford clays through advanced hollow cylinder experiments on multiple high-quality samples taken at similar depths from inland sites where the geotechnical profiles have been established by comprehensive laboratory and in situ testing. Suites of undrained tests are reported, which loaded specimens from their in situ stress states to reach ultimate failure at pre-defined final major principal stress axis orientations defined in the vertical plane, while also controlling or monitoring the intermediate principal stress ratio, b. Both stress path and simple shear tests were undertaken with the hollow cylinder apparatus, which offers key advantages over conventional simple shear equipment. The interpretation reveals patterns of marked shear strength anisotropy that impact significantly on numerous geotechnical engineering applications.

Effects of anisotropy in correlation structure on the stability of an undrained clay slope

Slopes are mainly naturally occurred deposits, so slope stability is highly affected by inherent uncertainty. In this paper, the influence of heterogeneity of undrained shear strength on the performance of a clay slope is investigated. A numerical procedure for a probabilistic slope stability analysis based on a Monte Carlo simulation that considers the spatial variability of the soil properties is presented to assess the influence of randomly distributed undrained shear strength and to compute reliability as a function of safety factor. In the proposed method, commercially available finite difference numerical code FLAC 5.0 is merged with random field theory. The results obtained in this study are useful to understand the effect of undrained shear strength variations in slope stability analysis under different slope conditions and material properties. Coefficient of variation and heterogeneity anisotropy of undrained shear strength were proven to have significant effect on the reliability of safety factor calculations. However, it is shown that anisotropy of the heterogeneity has a dual effect on reliability index depending on the level of safety factor adopted.

Anisotropy of Undrained Shear Strength Induced by K0 Consolidation and Swelling in Cohesive Soils

AbstractStrength anisotropy should be introduced into stability analyses, because the shear direction differs at each sliding point. However, few experiments evaluating the effects of anisotropic consolidation on undrained shear strength have been available for practical construction problems. Therefore, torsional shear tests were conducted on specimens consolidated and swollen in different directions to evaluate the anisotropy of undrained shear strength. Furthermore, to treat two-dimensional problems, the tests were also performed under the plane strain condition, where no radial strain occurs. Results show that the anisotropy of the undrained shear strength is closely related to the differences between consolidation and shearing in terms of directions and stress conditions. Finally, slope stability analyses were conducted to demonstrate how the anisotropy of the undrained shear strength obtained from the experiments can be practically used in field analyses.

NGI‐ADP: Anisotropic shear strength model for clay

SUMMARYMany geotechnical problems involve undrained behavior of clay and the capacity in undrained loading. Most constitutive models used today are effective stress based and only indirectly obtain values for the undrained shear strength. To match the design profiles of undrained shear strengths, in active (A), direct simple shear (D) and passive (P) modes of loading are complicated. This paper presents the elastoplastic constitutive model NGI‐ADP which is based on the undrained shear strength approach with direct input of shear strengths. Consequently, exact match with design undrained shear strengths profiles is obtained and the well‐known anisotropy of undrained shear strength and stiffness is accounted for in the constitutive model. A non‐linear stress path‐dependent hardening relationship is used, defined from direct input of failure strains in the three directions of shearing represented by triaxial compression, direct simple shear and triaxial extension. With its clear input parameters the model has significant advantages for design analysis of undrained problems. The constitutive model is implemented, into finite element codes, with an implicit integration scheme. Its performance is demonstrated by a finite element analysis of a bearing capacity problem. Copyright © 2011 John Wiley & Sons, Ltd.

Shear strength anisotropy of natural London Clay

The paper reports the shear strength anisotropy of the natural, highly overconsolidated, London Clay from Heathrow Terminal 5 as established by comprehensive hollow cylinder apparatus (HCA) testing. Multiple high-quality block samples from 5·2 m and 10·5 m below ground level provided samples for suites of undrained stress-path shear tests performed after consolidation to effective stress states similar to those estimated in situ. The direction of the major principal stress axis, α (or of the principal stress increment, αdσ), and relative magnitude of the intermediate principal stress, b = (σ2 − σ3)/(σ1 − σ3), were chosen as the controlled stress-related variables and their influence on the peak shear strength was investigated. Strong shear strength anisotropy was proven, and a potential effect of the parameter b was also detected. With b = 0·5, for example, a minimum was noted in peak q/p′ at around α = 45–67°, and the maximum value, which was larger by some 40%, developed at α = 0°. A limited set of data obtained with larger test specimens suggested a possibility that loading with α = 90° could also lead to relatively low shear strength. The pre-failure pore water pressure development reflected anisotropy of the deformation characteristics at smaller strains and contributed to the total-stress undrained shear strength anisotropy. Regarding the influence of b, the Mohr–Coulomb failure line passing through the data for b = 0 or 1 provided a lower bound of all the data for a given value of α. The investigation of anisotropy was extended to deeper horizons of the stratum through profiling tests involving triaxial extension and simple shear conditions. Although the results confirmed the general patterns of anisotropy observed at 5·2 and 10·5 m below ground level, the degree of anisotropy appeared to become stronger with depth.

Anisotropy of Undrained Shear Strength of an Over-Consolidated Soil by Triaxial and Plane Strain Tests

In order to find out the real cause leading to the anisotropy of undrained shear strength of clayey soils, two types of undrained shear tests were carried out; 1) undrained plane strain tests on lightly over-consolidated kaolin clay, and 2) undrained triaxial tests on heavily over-consolidated kaolin clay. Analysis of the data has shown that the anisotropy of undrained shear strength of over-consolidated kaolin clay is primarily caused by the direction-dependent (anisotropic) shear strength parameters c' and ϕ' in terms of effective stresses. The anisotropy of pore water pressure parameter Ay is artificially generated as a result of the anisotropic change of the undrained shear strength itself, rather than of the anisotropic excess pore-pressure development during shearing, and is of secondary, if any, importance to interpret why the undrained shear strength is anisotropic. Intensity of the anisotropy in the undrained shear strength is not altered much by either changing the boundary conditions, such as triaxial and plane strain or increasing the over-consolidation ratio up to 32. The minimum values of cohesion c' and friction angle ϕ' appear when a specimen is compressed with the major principal stress inclined at about 30° to the consolidation plane. When a specimen is compressed either normal or parallel to the consolidation plane, values of cohesion c' and friction angle ϕ' are high with no significant difference between them. The difference between the maximum and the minimum friction angle ϕ' is almost the same as that observed in the triaxial tests of sands. In the case of sands, however, a specimen compressed normal to the bedding plane yields the maximum friction angle ϕ', while a specimen compressed parallel to the bedding plane yields the minimum value in the triaxial tests and a value close to the minimum in the plane strain tests. These observations suggest that the micro-deformation mechanism leading to the strength anisotropy is quite similar in both clay and sand, but not exactly the same.

Undrained shear strength anisotropy of Ko-overconsolidated cohesive soils. Technical note : Kamei, T.; Nakase, A. Soils Found V29, N3, Sept 1989, 145–151

Undrained shear strength anisotropy of Ko-overconsolidated cohesive soils. Technical note : Kamei, T.; Nakase, A. Soils Found V29, N3, Sept 1989, 145–151

Undrained Shear Strength Anisotropy of K0-Overconsolidated Cohesive Soils

To investigate the undrained shear strength anisotropy of K 0 -consolidated cohesive soils, undrained triaxial compression and extension tests with K 0 -consolidated and K 0 -rebound were performed on two kinds of cohesive soils. As a result, the undrained shear strength anisotropy is approximately constant within the range of OCRs of 1 to 10. The reason for this may be considered that the inherent structual anisotropy in a soil specimen produced during initial K 0 -consolidation history before rebound is essentially maintained during swelling. The constant undrained shear strength anisotropy, however, seems to depend on the amount of clay fraction and plasticity index of the soils.

The Analysis of Failure of a Normally Consolidated Clay Foundation Under Embankment Loading

In this research an attempt is made to derive the following two approaches of arithmetic methods of evaluating ultimate bearing capacity of normally consolidated clay foundation subject to partial loading such as embankment with finite width. The two approaches are as follows: (1) Finite element deformation analysis of elasto-plastic clay foundation including consolidation behavior, and (2) Limit state analysis by rigid plastic finite element method. In the former analysis, the stiffness factor of clay foundation in terms of effective stresses is newly introduced, from which the definition of total failure of clay foundation is freshly made. In the latter analysis, stress induced anisotropy of undrained shear strength of clay is newly introduced so that the consistency be held between the limit analysis and load deformation analysis. Main conclusions obtained in the present study are as follows:(1)The solution of rigid plastic finite element analysis is shown to be very close to the solution obtained in a deformation analysis as far as undrained condition is concerned.(2)The effect of partial drainage expected during gradual load application is found to be less than 30% of undrained bearing capacity when the loading rate is within the range commonly used in engineering practice. The solution of rigid plastic finite element analysis is shown to be very close to the solution obtained in a deformation analysis as far as undrained condition is concerned. The effect of partial drainage expected during gradual load application is found to be less than 30% of undrained bearing capacity when the loading rate is within the range commonly used in engineering practice.

Influence of Strain Rate on Undrained Shear Characteristics of K0-Consolidated Cohesive Soils

In order to investigate the influence of strain rate on undrained shear characteristics of normally consolidated cohesive soils, K0-consolidated undrained triaxial compression and extension tests were performed on three kinds of cohesive soils with different plasticity index IP under different strain rate. The conclusions from this study are: The maximum principal stress difference and axial strain at failure increase with the increase of strain rate, whereas the pore pressure coefficient Af decreases. The magnitude of excess pore pressure, both in the compression and extension tests, at the same strain is found to be smaller as the strain rate increases in the case of plastic clay. This trend does not become clear as the plasticity index decreases. Initial portion of effective stress path progresses approximately elastically from the point of K0-consolidation for all the specimens, irrespective of different strain rates. The undrained shear strength anisotropy at high strain rate becomes smaller than that at low strain rate.

Anisotropy of Undrained Shear Strength of Clays under Axi-Symmetric Loading Conditions

ABSTRACT Experimental investigations indicate that an anisotropically consolidated clay changes its undrained shear strength depending on the experimental procedures and/or direction of shear. It is needed to formulate the anisotropy of undrained shear strength of clays for use in reliable analyses of shear failures of foundations, embankments, retaining walls etc. In this paper, the anisotropy of undrained shear strength is formulated assuming the anisotropy induced by the initial anisotropic stress state. Theoretical considerations on the undrained behaviour of normally and overconsolidated clays are made on the basis of a constitutive equation. Two equations describing the undrained state and the failure state are derived and, applying these equations to axv-symmetric stress state, derived are some equations which give the theoretical values of the undrained shear strength to be obtained from the triaxial tests on normally and/or overconsolidated clays. The validity of the equations are examined by the comparison of the calculated strengths and the experimental strengths reported in the past.

Undrained Shear Strength Anisotropy of Normally Consolidated Cohesive Soils

ABSTRACT In order to investigate the undrained shear strength anisotropy of cohesive soils of intermediate grading between sands and clays, a series of consolidated undrained triaxial compression and extension tests is performed on soil samples which are prepared by mixing Kawasaki clay and Toyoura sand (with crushed fraction) so as to obtain a wide range of soil type expressed in terms of plasticity index Ip. Four kinds of cohesive soil samples with Ip of 30, 20, 15 and 10 are tested. The conclusions from this study are: 1) Undrained shear strength anisotropy increases with decrease in Ip. And difference between values of (cu/p)E/ (cu/p)c obtained by CIU and CK0U tests increases as the plasticity index increases. 2) Relationship between cu/p values obtained by four types of consolidated undrained triaxial tests and Ip is quantitatively shown. 3) In view of either shape of effective stress paths and the undrained shear strength anisotropy, shear properties of cohesive soils of Ip of 20 and 30 are seen to be those of clays, however, soils of Ip of 15 and 10 are considered the intermediate soils. Based on results of the present study, a way of accounting for the anisotropic undrained shear strength in stability analysis is proposed, with assumption that the in situ undrained compression strength of vertical sample is known.

Shear-strength anisotropy variations in weathered Oxford clay

The undrained shear-strength anisotropy of samples from weathering profiles in Oxford Clay was determined using a modified shear vane apparatus. The value of strength anisotropy increased from approximately 0.7 in unweathered material to approximately 2.0 in highly weathered clay. This variation is related to changes in moisture content in the weathering profiles, along with chemical weathering, causing preferential weakening of horizontally oriented interparticle bonds.

An approximate correction for the influence of strength anisotropy on conventional shear vane measurements used to predict field bearing capacity

The author derives a correction to be applied to shear vane measurements to allow the more accurate estimation of the field bearing capacity of anisotropic soils. He first outlines the need for a correction factor and discusses bjerrum's empirical factor that has been used to obtain a more realistic forecast of field stability. He points out that an ideal factor should compensate for the effects anisotropy and progressive failure as well as the effects of testing rate, size of test zone and any other effects resulting from differences between the test and field conditions. He argues that because bjerrum's correction is based on the result of plasticity index measurements on remoulded samples it cannot correct for inherent anisotropy, only for the other factors and that a further correction is necessary. A mathematical relationship is derived between this additional correction and the degree of anisotropy of undrained shear strength and includes a curve depicting this relationship. He concludes by setting out a relationship between field and vane strengths that is similar to bjerrum's correction, but incorporates the additional correction based on the degree of anisotropy. The latter may be estimated from undrained compression tests made at different orientations on block samples or, more directly, by tests with diamond-shaped vanes. It is pointed out that, although useful, the correction is only approximate as it assumes equal strengths in the three shearing modes. /TRRL/

A new technique of vane testing

Summary A means of detecting sample disturbance and the anisotropy of undrained strength of clay soils by use of the standard vane is presented. If the sample is appreciably disturbed it is not possible to derive any values for the anisotropy of undrained shear strength from the analysis. The technique permits the determination of this anisotropy using only one vane by considering the elliptical distribution of strength in a plane perpendicular to bedding. The standard vane, with a length/breadth ratio ( k ) of 3/2, theoretically permits the determination of an average strength within 18 per cent of the true value for real soils depending on the angle of insertion. A vane with k = 2/3 is theoretically the least sensitive to anisotropy of strength and would consequently be the best shape for determining an average value in preliminary investigations. For real soils the average strength recorded would be within 2 per cent of the true value no matter what the angle of insertion of this vane.