Effect Of Strength Anisotropy Research Articles

Effects of Strength Anisotropy and Strain Softening on Soil Bearing Capacity through a Cosserat Nonlocal Finite-Element Method

Effects of Strength Anisotropy and Strain Softening on Soil Bearing Capacity through a Cosserat Nonlocal Finite-Element Method

Effect of strength anisotropy on schist rocks and slope stability analysis

Effect of strength anisotropy on schist rocks and slope stability analysis

Effect of strength anisotropy on strain localization in natural clay

Effect of strength anisotropy on strain localization in natural clay

Analysis of Anisotropic Strength and Wellbore Unstable Zone in Shale Formation Relating to Water Content

During oil and gas exploitation, wellbore stability has always been the focus of many scholars. Instability of wellbore will lead to washouts, breakout, stuck pipe, mud loss, and so on, even the abandon of the well. Shale rock is widely distributed in nature and must be taken into account in some rock engineering applications. In order to evaluate the strength anisotropy and the influence of moisture content of the wellbore integrity, the LMX outcrops are divided into three groups to carry out triaxial strength experiment. Based on the fitting method of collaborative search, the strength parameters of different moisture contents are determined; then, the effect of strength anisotropy and the moisture content of the LMX reservoir on wellbore collapse pressure and wellbore integrity are investigated. The results show that shale strength, especially cohesion, is more sensitive to the influence of water. With the increase of water content, the inclination angle of the minimum value obtained by shale strength is deviated to the left, and the collapse pressure and instability area increase significantly. At the same time, it is necessary to adjust the direction of borehole drilling to avoid the occurrence of the zone of bedding slip instability in the direction of the minimum in situ stress, and the inversion of in situ stress should be corrected by considering the effect of bedding on the location of the maximum in situ stress. The study on the effects of bedding and water content on the instability zone can provide suggestions and guidance for the optimization of borehole trajectory, the control of bottom hole safety pressure, the inversion of in situ stress, and the drilling and completion of wells.

Effect of Strength Anisotropy on the Stability of Natural Slopes

It is well known that low plasticity soft clays have large variations in undrained shear strength with the direction of loading. Laboratory experiments show typically a cross anisotropic behavior, where the undrained shear strength in compression is significantly larger than the undrained shear strength in extension. The total stress based NGI-ADP model, as available in PLAXIS, captures such shear strength anisotropy well, when applied to embankments on or excavations from a horizontal or almost horizontal terrain. However, for non-horizontal terrain the direction of the in-situ principal stresses are inclined, and the axis of anisotropy are hence expected to be somewhat inclined too. As for natural slopes, the effect on the calculated factor of safety due to this inclination is not yet well documented. Previous studies show that this effect might not be a negligible effect and can increase the factor of safety by about 10% for gentle natural slopes. In order to study this, a new ADP model has been implemented in PLAXIS. The formulation is inspired by results from DSS laboratory testing where samples were consolidated under inclined effective stresses before shearing in the same or the opposite direction of the initial shear stress. As expected, the model shows higher factors of safety when applied to a slope than a conventional analysis. The paper then discusses to what extent this represents a real safety margin that has previously been neglected.

Undrained stability of plane strain active trapdoors in anisotropic and non-homogeneous clays

Despite a large number of studies on the active failure of trapdoors conducted in the literature, there are very limited studies examining the combined influences of undrained strength anisotropy and non-homogeneity of clays on the problem. This paper presents the new lower bound solutions of the active failure of plane strain trapdoors in clays with linearly increasing anisotropic strengths. To achieve this purpose, the lower bound (LB) finite element limit analysis (FELA) of an anisotropic undrained strength criterion using second-order cone programming (SOCP) is developed to investigate the undrained stability of the problem under plane strain conditions. The anisotropic strengths of clays are modelled by employing an elliptical strength envelope requiring strengths in plane strain compression, extension and direct simple shear. The LB FELA is formulated by using a discrete form of the LB theorem of classical plasticity and a resulting optimization problem of anisotropic clay is cast and solved using the framework of conic quadratic optimization. A dimensionless technique is adopted to express the undrained stability of the problem in terms of adhesion factors of trapdoor roughness, cover-depth ratios, strength gradient ratios, and anisotropic strength ratios. The effects of these parameters on the trapdoor stability factor and associated failure mechanisms of trapdoors are also examined and discussed. Employing a nonlinear regression analysis to the computed LB solutions, the new design equations for the estimation of the undrained stability of trapdoors are proposed, where the effects of strength anisotropy and non-homogeneity as well as width and depth of trapdoor and its roughness can be considered for practical stability applications.

Elastoplastic Cosserat continuum model considering strength anisotropy and its application to the analysis of slope stability

Aiming at solving the problems of strength anisotropy and strain localization of cohesive soil, Pietruszczak’s method, in which the microstructure tensor is combined with stress invariance, is developed to analyze the cohesion anisotropy and is introduced into the Drucker-Prager constitutive model under Cosserat continuum theory. A consistent algorithm of the corresponding constitutive model is derived. The characteristics of strength anisotropy and the reliability of the developed numerical method are verified by the experiments in laboratory. The importance and necessity of developing the numerical model with strength anisotropy under the framework of Cosserat theory are evaluated via simulation of a plane strain compression model. It indicates that the degree of the cohesion anisotropy has an important influence on the bearing capacity, and that the numerical model can overcome the ill-posedness of the mesh sensitivity and maintain the well-posedness of the strain localization problem. Furthermore, the effects of strength anisotropy and strain softening on the safety factor of the slope are analyzed via the gravity increase method. It is demonstrated that the Cosserat continuum model can effectively overcome the problems of mesh-dependence encountered by the classical continuum model and yield a reasonable safety factor with mesh refinement.

Lower bound solutions for undrained face stability of plane strain tunnel headings in anisotropic and non-homogeneous clays

In this paper, the face stability of plane strain tunnel heading in anisotropic and non-homogeneous clays is investigated by the lower bound (LB) finite element limit analysis (FELA) with second-order cone programming (SOCP). New LB solutions of the problem are numerically derived and described by the set of the dimensionless parameters consisting of the stability load factor, the cover-depth ratio, the normalized overburden pressure, normalized strength gradient, and anisotropic strength ratio while failure mechanisms associated with these parameters are also discussed and compared. The computed LB solutions are validated with the existing results of isotropic strengths confirming the accuracy and correctness of the solutions. Based on a nonlinear regression, the new design equations for the stability load factor and factor of safety for plane strain headings in anisotropic and non-homogeneous clays are first time proposed for assessing the undrained stability of relatively long-wall tunnels particularly considering the effects of strength anisotropy and non-homogeneity.

An Investigation of Failure Envelope for Skirted Foundations in Soft Clay Based on Computer-Aided Analyses

The stability of skirted foundations subjected to combined vertical, horizontal and moment loading is of importance to the offshore engineering. The paper reports a computer-aided study on the effects of strength anisotropy and foundation embedment depth on the combined bearing capacity of skirted foundations. The results show that when the traditional load reference point, i.e. the base center, is utilized, the shape of failure envelope for skirted foundations is strongly dependent on embedment ratio, however independent of soil strength anisotropy. To gain a unified equation to describe the shapes of failure envelopes for skirted foundations with a range of embedment ratios, the searching algorithm for the optimal load reference point is implemented. Based on the optimal load reference point, the failure envelope obtained can be approximated as an ellipse in normalized moment-horizontal load space. Then a closed-form equation is proposed, which can be used to evaluate the bearing capacity of skirted foundations in soft clay considering strength anisotropy effect.

Effect of Strength Anisotropy on Failure Envelope of Offshore Shallow Foundations under Combined Loading

Wu, K.; Fan, Q., and Zheng, J., 2015. Effect of strength anisotropy on failure envelope of offshore shallow foundations under combined loading.The bearing capacity of offshore shallow foundations subjected to combined vertical, horizontal and moment loading is of interest particularly to the offshore geotechnical engineers. This paper reports a finite element (FE) study on the effects of strength anisotropy and foundation embedment depth on the combined bearing capacity. The Tresca failure criterion modified to allow for soil strength anisotropy and coupled with an elasto-perfectly plastic stress-strain relationship is assumed in the analysis which is conducted using the FE software ABAQUS. The undrained bearing capacities of shallow strip foundations under uniaxial vertical (V), moment (M) and horizontal (H) loadings are predicted and compared with available solutions. The V-M-H capacity of embedded foundations under combined loading is represented by failure envelopes in H-M space for various vertical loading levels. The results presented in this paper show that the embedment ratio significantly affects the shape of the failure envelope expressed in terms of loads normalized by the pure moment or pure horizontal load-carrying capacity of the foundation. However, the shape of failure envelope defining the undrained capacity of shallow foundations under horizontal and moment loads is independent of soil strength anisotropy.

Strength Anisotropy of a Tailings Sand and its Effect on Stability of a Tailing Dam

In current engineering practice, the effect of strength anisotropy on stability analysis of tailings dam is often ignored. This paper presents direct shear test results on a tailing sand with different angles between shear direction and bedding plane. It is found that shear strength of this soil is significantly directional dependent. The variations in peak friction angle are about 11o, which is approximately 28.3% of the minimum value. The effect of strength anisotropy on stability of a tailings dam is also investigated. The analysis considering anisotropic strength shows a smaller factor of safety and shallower most critical slip surface as compared with isotropic strength model that is used in current engineering practice. To achieve more accurate stability analysis of tailings dams, it is suggested to take into account shear strength anisotropy of tailings soils.

Estimation of intact and remoulded undrained shear strengths from penetration tests in soft clays

Difficulties in obtaining high-quality soil samples from deepwater sites have necessitated increasing reliance on piezocone, T-bar and ball penetration tests to determine soil properties for design purposes. This paper reports the results of an international collaborative project in which a worldwide high-quality database of lightly overconsolidated clays was assembled and used to evaluate resistance factors for the estimation of intact and remoulded undrained shear strength from the penetration resistance of each device. The derived factors were then compared with existing theoretical solutions to evaluate the influence of particular soil characteristics. The overall statistics showed similar levels of variability of the resistance factors, with low coefficients of variation, for all three types of penetrometer. However, correlations of the resistance factors with specific soil characteristics indicated that the resistance factors for the piezocone were more influenced by soil stiffness, or rigidity index, than for the T-bar and ball, while the effect of strength anisotropy was only apparent in respect of resistance factors for the T-bar and ball relative to shear strengths measured in triaxial compression. In the correlation between the remoulded penetration resistance and remoulded strength, the resistance factors for remoulded strength were found to be higher than those for intact strength and with a slight tendency to increase with increasing strength sensitivity but insensitive to soil index properties. Based on an assessment of the influence of various soil characteristics, resistance factors are recommended for the estimation of intact and remoulded undrained shear strength from the penetration resistances of each device for soil with strength sensitivity less than six.

Evaluation of short-term stability for sea-wall structure at Kobe Airport

Prior to the huge reclamation work for constructing Kobe Airport, the sea-wall structure resting on a 30m-thick soft Holocene clay layer in the 16m-deep sea was constructed at stages over the period of three years. Regarding the design of short-term stability of the sea-wall structure, the ratio of undrained shear strength of the normally-consolidated seabed clay to in-situ yield stress was estimated to take the value of 0.35 in order to satisfy the factor of safety in excess of 1.3. This design strength in the clay foundation was checked with the unconfined compression strength of the fresh clay samples retrieved from boreholes during the construction. In this paper, the short-term stability of the embankment in this well-documented case history was re-evaluated after-the-event with the results of comprehensive series of constant-volume direct shear box tests in which the effects of strength anisotropy as well as the shearing rate on undrained shear strength were both examined. Discussion is made how to estimate the short-term stability of the embankment by considering the effects of strength anisotropy and shearing speed on undrained shear strength and how it is linked with the current design method in Japan in which a half of the undrained compression from unconfined compression test is employed.

The effect of strength anisotropy on the bearing capacity of spudcan foundations

The vertical bearing capacity of spudcan foundations in strength anisotropic soils is investigated numerically using the MIT-S1 model implemented in the AFENA finite element package. The model in AFENA is validated against existing laboratory test data of normally consolidated soil. The bearing capacities of spudcans in soils with isotropic and anisotropic strengths are compared. Soil with isotropic strength is simulated using an elasto-plastic model. It is found that the bearing capacity of a spudcan in an anisotropic soil is reduced by about 9% for a rough spudcan and 3% for a smooth spudcan on average. There is a combined effect of soil anisotropy and spudcan roughness on the spudcan bearing capacity. Moreover, the effect of the pressuremeter strength of an anisotropic soil on foundation capacity can’t be ignored.

The effect of strength anisotropy on the behaviour of embankments on soft ground

The need to build embankments on soft clay deposits is widespread, including flood defences, rail and motorway embankments. The difficulties in constructing embankments on these deposits are well known. The permeability of the ground is usually low, so that construction is generally performed under undrained conditions and, as a consequence of the low undrained shear strength of the clay, these structures are of a limited height, usually only 3–4 m. The classical design of an embankment is often based on a limit equilibrium approach, in which the soil is considered as an isotropic material. However, soft clays are almost always anisotropic, having both stiffness and strength dependent on the orientation of principal stresses. This complicates any design analysis, and as sufficient data are not usually available to define the magnitude of anisotropy, empirical factors are introduced into conventional design procedures. This paper attempts to demonstrate the effect of soil strength anisotropy on the behaviour of soft clay embankments through finite element analysis using an anisotropic soil model. The finite element predictions are compared with data from a full-scale test embankment brought to failure. The dangers of ignoring anisotropy, or treating it empirically, are highlighted.

The effect of strength anisotropy on the behaviour of embankments on soft ground

The need to build embankments on soft clay deposits is widespread, including flood defences, rail and motorway embankments. The difficulties in constructing embankments on these deposits are well known. The permeability of the ground is usually low, so that construction is generally performed under undrained conditions and, as a consequence of the low undrained shear strength of the clay, these structures are of a limited height, usually only 3–4 m. The classical design of an embankment is often based on a limit equilibrium approach, in which the soil is considered as an isotropic material. However, soft clays are almost always anisotropic, having both stiffness and strength dependent on the orientation of principal stresses. This complicates any design analysis, and as sufficient data are not usually available to define the magnitude of anisotropy, empirical factors are introduced into conventional design procedures. This paper attempts to demonstrate the effect of soil strength anisotropy on the behaviour of soft clay embankments through finite element analysis using an anisotropic soil model. The finite element predictions are compared with data from a full-scale test embankment brought to failure. The dangers of ignoring anisotropy, or treating it empirically, are highlighted.

Influence of Strength Anisotropy on Piezocone Resistance in Clay

Since different undrained shear strengths may be obtained from different laboratory or field tests, different cone factors are needed to calculate the undrained shear strengths from the results of a piezocone test in clay. Based on the anisotropic undrained shear strength criterion, a set of cone factors for calculating undrained shear strength is proposed in this paper. These cone factors are interrelated with each other in terms of the strength anisotropy ratio of the clay. The suitability of the proposed cone factors has been verified using the test results generated from different tests and from clays with different origins. By comparing with the cone factor for isotropic clay, it was found that the effect of strength anisotropy on the cone factor is not always significant. In fact, the effect of strength anisotropy becomes significant only in lightly overconsolidated to normally consolidated clay with moderate to high strength anisotropy. Furthermore, the maximum difference in cone factor is not expected to exceed 20% with or without taking into account the strength anisotropy of clay.

Cavity expansion and cone penetration resistance in anisotropic clay

Due to strength anisotropy, the undrained shear strength of naturally deposited clay can vary with its position in the coordinate. When subjected to a spherical expansion, the undrained shear strength of clay yields the lowest value in the horizontal direction and the largest value in the vertical direction. To represent the nature of clay, a model based on the anisotropic undrained strength criterion has been developed in this paper to determine the anisotropic undrained shear strength under a spherical expansion condition. To simplify the model, the effects of in‐situ initial stresses, anisotropy, and the anisotropy of soil deformability are not taken into account here. Combining the spherical cavity expansion theory and the anisotropic undrained shear strength under spherical expansion conditions, a simplified limit pressure of spherical cavity expansion in anisotropic clay has been established. The magnitude of limit pressure is related to its position in the spherical cavity. Based on the limit pressure in spherical cavity expansion derived from anisotropic strength, the cone factor of an advancing cone has been derived to correlate the cone resistance and undrained shear strength of clay. The calculated cone factors uniquely correspond to the undrained shear strengths determined from different tests and are interrelated to each other in terms of strength anisotropy ratio Ar of clay. The accuracy of the calculated cone factors has been satisfactorily verified with laboratory and field CPTU results. However, it has also been found that the effect of strength anisotropy only becomes obvious when clay has a low rigidity and high strength anisotropy. If the strength anisotropy of clay is not considered, a less than 15% error on the value of the cone factor will result.

Limiting resistance of a spherical penetrometer in cohesive material

The shear strength of soft sediments may be assessed by field tests, principally either vane tests or cone penetrometer tests. The continuous profile of strength obtained from the cone penetrometer is an advantage over the vane test, although the significant corrections for overburden stress, and pore pressure acting on the back face of the cone, limit its accuracy in soft soils. Alternative shapes for penetrometers have been suggested, for example a cylindrical T-bar and a spherical ball, both of which allow full flow of soil around the probe, thereby obviating the need for corrections due to the ambient stress level. Plasticity solutions have been available for some while relating the (average) shear strength of the soil to the net bearing pressure for plane strain flow around the cylindrical T-bar. This paper documents a corresponding solution for a spherical penetrometer, using upper and lower bound approaches supported by finite element analysis. Effects of strength anisotropy are discussed in the light of example results comparing the measured resistance of cone, T-bar and ball penetrometers.

Study of the effect of soil anisotropy on slope stability using method of slices

Abstract Naturally occurring cohesive soil deposits are inherently anisotropic with respect to different properties amongst which is the shear strength. This anisotropy is primarily due to the process of sedimentation followed by predominantly one-dimensional consolidation. The effect of strength anisotropy on the stability of slopes has commonly been investigated using the mass procedure in which the mass of soil failed is taken as a one unit. The analysis is conducted by utilizing either the conventional limit equilibrium method or the upper bound technique of limit plasticity. It is, however, the method of slices that is most widely employed in the analysis of slope stability problems, especially with the advent of the digital computer. In this study, the influence of cohesion anisotropy on the stability of slopes in homogeneous soil, with internal friction angle greater than zero, was investigated using the method of slices. Analysis was made for the case of two-dimensional circular slip surfaces. It has been found that soil anisotropy has a significant effect on the stability of slopes when the slope angle is less than 53°. However, such an effect becomes insignificant as the soil angle of internal friction exceeds about 10°. The results also indicated that the geometry of the slip surface is only slightly affected by the degree of anisotropy in the soil cohesion.