Anisotropic Shear Strength Research Articles

New peak shear strength criteria for anisotropic rock joints

In general, roughness profiles of rock joints consist of non-stationary and stationary components. At the simplest level, only one parameter is sufficient to quantify non-stationary joint roughness. The average inclination angle I, along with the direction considered for the joint surface, is suggested to capture the non-stationary roughness. Most of the natural rock joint surface profiles do not belong to the self similar fractal category. However they may be modelled by self-affine fractals. Using a new term called specific length, it is shown that even though the fractal dimension D is a useful parameter, it alone is insufficient to quantify the stationary roughness of non-self similar profiles. Also, it is shown why contradictory results for the estimation of D of non-self similar profiles appear in the literature. To estimate D accurately for non-self similar profiles, it seems necessary to use scales of measurement less thanthe crossover length of the profile. Because the crossover dimension of joint roughness profiles can be extremely small, in practice it may be quite difficult to measure roughness at scales of less than the crossover dimension and thus to estimate D accurately. To overcome the aforementioned problems, it is suggested to combine D with a parameter which is negatively correlated to D and also has the potential to compensate for the errors caused by an inaccurate D, and to use the combined parameter to quantify stationary roughness in practice. Four new strength criteria which take the following general form are suggested for modelling the anisotropic peak shear strength of rock joints at low normal effective stresses (0–0.4 times unconfined compressive strength): τ = σ tan φ + a(SRP) c log 10 σ J σ d + I where σ, τ, σ J, φ and SRP denote, respectively, the effective normal stress on the joint, peak shear strength, joint compressive strength, basic friction angle, and the stationary roughness parameter. The following four options are suggested to represent the term a(SRP) c: az 2 ′c, aK d bD c, aK s bD c or aK ν bD c. Joint roughness data should be used to estimate the roughness parameters I, z 2 ′, k d, K s, K ν and D in different directions on the joint surface. Parameter D reflects the rate of change in length in response to a change in the scale of measurement r. Because z 2 ′, K d, K s and K ν are scale-dependent parameters, they can be used to model the scale effect. The coefficients a, b, c and d in the strength criteria should be determined by performing regression analysis on experimental shear strength data. In practice, to allow for modelling uncertainties, the new equations should be used with a factor of safety of about 1.5.

Anisotropic shear strength of rock joints : Huang, T H; Doong, Y S Proc International Symposium on Rock Joints, Loen, 4–6 June 1990P211–218. Publ Rotterdam: A A Balkema, 1990

Anisotropic shear strength of rock joints : Huang, T H; Doong, Y S Proc International Symposium on Rock Joints, Loen, 4–6 June 1990P211–218. Publ Rotterdam: A A Balkema, 1990

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.

Yield Function for Soil with Anisotropic Fabric

A fabric tensor \IF\di\dj\N for three-dimensional assemblies of granular soils is introduced as an index showing the anisotropy due to the preferred orientation of constituent particles and is actually determined by using data derived from a material science approach of soils. Using the fabric tensor, a Drucker–Prager type of yield function is extended so as to take into account the anisotropic yielding behavior of granular soils. Plane strain tests on Toyoura sand are analyzed with a result that the anisotropic shear strength is well fitted by the extended Drucker–Prager yield function. Based on this, it is concluded that this study provides a step to link the material science approach of soils, in which the spatial arrangement of particles and associated voids plays an important role, to the continuum theory of plasticity.

GEOTECHNICAL PROPERTIES OF PEATS AND PEATY GROUNDS

A series of physico-chemical and mechanical tests were performed on the soils sampled from peaty grounds in the suburbs of Iwamizawa city, Hokkaido and Ohmiya city, Saitama, and the geotechnical properties of peats and peaty grounds were investigated. The results of various physico-chemical tests were mainly summarized as a function of organic matter content involved. Also, using the samples of peats and humused chipes, the changes of pore volume and pore size distributions with the one-dimensional compression were investigated with the porosimeter apparatus. From the results of mechanical tests, it was found that the coefficients of secondary compression and permeability of soils were strongly influenced by the amount of organic matter and fibrous peats remarkebly showed anisotropic shear strength.

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.

Three-dimensional stability analyses of four embankment failures : Azzouz, A S; Baligh, M M; Ladd, C C Proc 10th International Conference on Soil Mechanics and Foundation Engineering, Stockholm, 15–19 June 1981V3, P343–346. Publ Rotterdam: A. A. Balkema, 1981

This paper presents four case histories of embankments rapidly loaded to failure on saturated clay foundations. Two- and three-dimensional total stress circular arc stability analyses are performed utilizing two strength models: (1) the (uncorrected) field vane strength, and (2) the stress history and normalized soil engineering properties strength after accounting for strain compatibility along the shear surface and strength anisotropy. End effects are shown to increase the conventional two-dimensional factors of safety by as much as 30%, and hence, if neglected, can lead to significant overestimation of backfigured strengths. (TRRL)

Undrailed shear strength anisotropy of normally consolidated cohesive soils : Nakase, A; Kamei, T Soils Found, V23, N1, March 1983, P91–101

Undrailed shear strength anisotropy of normally consolidated cohesive soils : Nakase, A; Kamei, T Soils Found, V23, N1, March 1983, P91–101

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 of clay (in Italian) : Nova, R Riv Ital Geotech, V14, N4, Oct–Dec 1980, P234–246

Shear strength anisotropy of clay (in Italian) : Nova, R Riv Ital Geotech, V14, N4, Oct–Dec 1980, P234–246

Shear strength anisotropy variations in weathered Oxford clay : Russell, D J; Denness, B; McCann, D M Engng Geol, V12, N4, Dec 1978, P337–344

Shear strength anisotropy variations in weathered Oxford clay : Russell, D J; Denness, B; McCann, D M Engng Geol, V12, N4, Dec 1978, P337–344

Experimental study of anisotropic shear strength of sand by plane strain test : Oda, M; Koishikawa, I; Higuchi, T Soils Found, V18, N1, March 1978, P25–38

Experimental study of anisotropic shear strength of sand by plane strain test : Oda, M; Koishikawa, I; Higuchi, T Soils Found, V18, N1, March 1978, P25–38

Bearing capacity of anisotropic cohesionless soils

Previous test results of the anisotropic shear strength of cohesionless soils are reviewed. The theory of the ultimate bearing capacity of shallow foundations on homogeneous isotropic soils is extended to anisotropic cohesionless soils. The proposed method of analysis is compared with the results of some load tests on anisotropic sand. An extension of this method to foundations under inclined load is briefly discussed.

Experimental Study of Anisotropic Shear Strength of Sand by Plane Strain Test

ABSTRACT Anisotropic parallel alignment of particles is universally observed not only in river, beach and coastal dune sands but also in artificially deposited sands. Anisotropic shear strength caused by the anisotropic parallel alignment of particles can be observed more clearly in the plane strain condition e2 = 0 than in the symmetrical stress condition σ2 = σ3. This must be chiefly due to the fact that re-arrangement of particles during shear deformation can be more easily performed in the latter condition than in the former one. Shear strength of sand in a plane strain test has been generally believed to be 10% to 20% greater than that obtained in a triaxial compression test. When the specimen is compressed at a small angle to a bedding plane, however, it is possible that the shear strength in the plane strain test is smaller than that in the triaxial compression test. The effect of anisotropic shear strength cannot be ignored when stability problems and earth pressure problems in plane strain condition are analyzed.

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/

Slope Stability Analysis in Anisotropic Winnipeg Clays

Two major aspects of an investigation dealing with the stability of natural and cut slopes in the Winnipeg area are described. The first concerns an experimental investigation of the anisotropic shear strength characteristics of the layered Lake Agassiz clays and the second is a study of the mechanism of failure for slopes in these layered clays.Four main types of clay were investigated and it was found that the effective shear strength parameters were greater for failure across the layers than for failure along the layers. Residual shear strength parameters were also determined.Stability analyses were carried out for representative slope cross sections using circular and noncircular failure surfaces, taking into account the anisotropic shear strength effects measured in the laboratory tests.Factors of safety were found for the conventional method of analysis assuming circular arc failure surfaces and isotropic shear strength properties. These factors of safety were found to be up to 0.5 greater than those obtained from noncircular failure surfaces which partially passed along the layers and so mobilized the lower shear strength properties of these layers.