Anisotropic Shear Strength Research Articles

Thermal effects on mechanical and failure behaviors of anisotropic shale subjected to direct shear

The stimulation of shale reservoirs frequently involves significant shear failure, which is crucial for creating fracture networks and enhancing permeability to boost production. As the depth of extraction increases, the impact of elevated temperatures on the anisotropic shear strength and failure mechanisms of shale becomes pronounced, yet there is a notable lack of relevant research. This study conducts, for the first time, direct shear experiment on shales at four different temperatures and seven bedding angles. By employing acoustic emission (AE) and digital image correlation (DIC) techniques, the evolution of damage and the mechanism of crack propagation under anisotropic direct shearing at varying temperatures is revealed. The results indicate that both shear displacement and strength of shale increase with temperature across different bedding angles. Additionally, shale demonstrates distinct brittle failure characteristics under various conditions during direct shearing tests. The types of anisotropic shear failure observed under the influence of temperature include central shearing fracture, central shearing with secondary fracture, and deflected slip along the bedding. Moreover, the temperature effect enhances shear-induced crack propagation along bedding planes. Shear failure in shale predominantly occurs during higher loading stages, which coincide with a substantial amount of AE signals. Finally, the introduction of the anisotropy index and temperature sensitivity coefficient further elucidates the interaction mechanism between thermal effects and anisotropy. This study offers a novel methodology to explore the anisotropic shear failure behavior of shale under elevated temperatures, and also provides crucial theoretical and experimental insights into shear failure behavior relevant to practical shale reservoir stimulation.

A General Method for Quantifying the Shear Strength Anisotropy of Soil

A General Method for Quantifying the Shear Strength Anisotropy of Soil

Anisotropy and cyclic loading characteristics of a stiff Bolders Bank glacial till at Cowden

Glacial tills are widespread across the world, common in northern Europe and found under the North and Baltic Seas. Their geological and geotechnical characterisation is important to the geotechnical design of a wide range of onshore and offshore structures. This paper reports outcomes from coordinated monotonic small-strain stress probing, cyclic and larger strain triaxial and hollow cylinder apparatus (HCA) testing on a natural low-to-medium plasticity, apparent highly over consolidated, stiff glacial clay till. Monotonic HCA and triaxial experiments investigated the till’s stiffness and shear strength anisotropy from its limited linear elastic range up to ultimate failure, showing that stiffnesses are higher in the horizontal direction than in the vertical and that higher undrained shear strengths develop under “passive” horizontal loading than “active” vertical loading. The cyclic triaxial tests revealed the impact of cycling on effective stress paths and strain development, and cyclic stiffness degradation. Stable, metastable and unstable patterns of behaviour are identified, along with different cyclic failure modes, which are related to the till’s static yielding behaviour. The experiment results provide the basis for developing, testing and calibrating monotonic and cyclic constitutive models and establishing simplified design procedures for piles installed in comparable tills to support offshore energy and other structures.

Research on clay shear strength anisotropy at Šenkovec clay pit

The paper deals with influence anisotropy to shear strength of clay and differences in test results of shear strength parameters, cohesion (c) and internal friction angle (φ) considering the direction of testing. Clay is very often the foundation soil of many buildings. There were performed three series of direct shear experiments for each of two different directions in relation to the horizontal, 0° and 90° (18 direct shear experiments in total). The results showed significant differences in the values of the shear strength parameters and to shear strength ultimately with considering to the test direction. Angle of internal friction (φ) is higher by 3.0° or 11.81% in the test at an angle of 0° in relation to the horizontal. A much more significant difference is visible in cohesion (c). The results showed that the cohesion (c) in the 90° test was higher by 14.92 kN/m² or 106.04%.

Stability evaluation of elliptical tunnels in natural clays by integrating FELA and ANN

The stability of tunnels in clayey soil is a major concern for underground space technology. Clay has anisotropy in shear strength induced by depositional and sedimentation processes. For the numerical analysis of geotechnical stability problems, the anisotropic undrained shear (AUS) model can account for this anisotropy of clayey soils. In this study, the stability of the elliptical tunnel (stability factor: σs-σt/suc) with varying elliptical shape (width-depth ratio: B/D) placed at different embedment depths (cover-depth ratio: C/D) in clay with different anisotropy (anisotropic strength ratio: re) and varying dimensionless overburden factor (overburden factor: γD/suc) is evaluated using finite element limit analysis and the AUS model. The failure planes are also evaluated for the above variations. Based on the numerical outcome, the artificial neural network (ANN) is utilized to establish the equation for predicting the stability of the elliptical shape tunnel with different shapes (i.e., width-depth ratio), and varying overburden, cover-depth ratio, varying anisotropic strength ratio of clay. The present study results are presented as design charts, tables, and equations so that they can be used in practice.

Three-dimensional undrained face stability of circular tunnels in non-homogeneous and anisotropic clays

The advanced 3D finite element limit analysis is employed to examine three-dimensional (3D) face stability of circular tunnels in undrained clays with the impact of strength non-homogeneity and anisotropy. The soil behaviour for all analyses is governed by the anisotropic undrained shear (AUS) strength model, which requires three anisotropic undrained shear strengths that are simply evaluated from conventional laboratory tests. The upper and lower bound solutions of 3D face stability of circular tunnels are accurately analyzed by considering four dimensionless parameters, including the cover-to-depth ratio, the overburden stress factor, the strength gradient ratio, and the anisotropic strength ratio. The significance of four dimensionless parameters on the stability load factor of the 3D face stability of circular tunnels, and the predicted failure mechanisms due to the influence of cover depth ratios, are also illustrated. A nonlinear regression analysis is employed to the computed data, which enables a new design equation of the factor of safety incorporating the conventional stability number. Thus, the new tunnel stability factors are proposed for the first time in the literature. A comprehensive application of the proposed safety factor equation is also presented and compared. The 3D effects of the tunnel face configuration between 3D geometry and 2D plane strain heading on the tunnel stability factors are also discussed. The new proposed design equation of the factor of safety provides an accurate and practical tool in assessing face stability of circular tunnels in non-homogeneous and anisotropic clays.

Undrained basal stability of braced circular excavations in anisotropic and non-homogeneous clays

This paper introduces the plastic stability solutions of braced circular excavations in anisotropic and non–homogeneous clays. Using the framework of Finite Element Limit Analysis (FELA) under axisymmetric conditions, the upper bound (UB) and lower bound (LB) solutions of the stability of excavations can be obtained. The clay is set to be anisotropic, where the Anisotropic Undrained Shear (AUS) model is used as a failure criterion of the surrounding soil. The results of this study are the proposed stability number which is the normalized parameter of the maximum unit weight and the anisotropic undrained shear strength of clay. Four dimensionless parameters are considered in the study: the anisotropic strength ratio, the depth–radius ratio, the depth–embedment ratio, and the strength gradient ratio. The impact of all considered dimensionless parameters on the results of the FELA solutions is examined. A machine learning regression approach, Multivariate Adaptive Regression Splines (MARS), is employed to develop an empirical design equation to predict the stability number of braced circular excavations in anisotropic and non–homogeneous clays. The proposed MARS equation can be a useful and reliable equation to estimate the basal stability of this excavation problem in practice.

Rate Effect on the Direct Shear Behavior of Granite Rock Bridges at Low to Subseismic Shear Rates

AbstractNatural faults are often heterogeneous with spatially variable cohesion caused by healing processes. Using rock bridges as laboratory analogs, we investigate how cohesion‐healed patches rupture at different shear rates. We performed direct shear experiments on granite rock bridges at shear rates from 1 μm/s to 10 mm/s (low to subseismic shear rates), and quantitatively characterized the failure surface morphology by integrating laser scanning with ArcGIS. The results show a notable dependence of both mechanical and morphological characteristics on shear rate. The failure mode transitions from tensile failure at low shear rates to shear failure at subseismic shear rates. At low shear rates, tensile failure forms curved coalescence patterns and undulating failure surfaces with significant roughness and morphological anisotropy. At subseismic shear rates, straighter shear‐formed coalescence patterns are observed. In addition, the peak shear strength, the roughness and morphological anisotropy increase with increasing shear rate within the subseismic rate regime. We explain the observed rate‐dependent behaviors by the energy and geometric features of micro‐cracking mechanisms. We suggest through the variations in 3D morphological parameters that rate effects should be incorporated into models of fault morphology. In addition, the failure surfaces at all shear rates are asymmetric, with asperities more inclined to the direction of the shear load. We find that this asymmetry results from both the fracture of rock bridges and the further shearing after failure, and it can be used to determine the sense of fault displacement.

Method for Preparing Hollow Cylindrical Specimens of Natural Granite Residual Soil

Abstract As an effective method of studying soil anisotropy, hollow cylinder torsional shear (HCTS) tests have been performed extensively on sedimentary soil, thereby establishing the anisotropic behavior of sand and clay. However, little is known about the anisotropic behavior of granite residual soil (GRS) formed by weathering, partly because hollow cylindrical specimens of natural GRS have yet to be prepared successfully, hence the lack of HCTS tests performed on GRS. The unique geotechnical properties of GRS, including high intact strength, susceptibility to disturbance, and the minor fissures in soil, pose great challenges when trying to prepare natural specimens. This paper proposes a new method to address this issue, which involves preparing a solid cylindrical specimen, drilling an initial inner cavity and then enlarging it, and finely trimming the specimen to its ultimate dimensions. Two methods are proposed for enlarging the inner cavity, both of which work well. The reliability of the proposed method is confirmed through specimen quality as indicated by the limited void ratio change due to reconsolidation, test repeatability, and comparison of the results of HCTS tests with those of triaxial tests. The HCTS tests reveal the shear strength anisotropy of natural GRS. Although specified for residual soil derived from granite, the proposed method could also be used for other weathered materials.

Shear strength anisotropy of rooted soils

The shear strength of rooted soils depends on the principal stress direction owing to the anisotropy in the soil structure and root system. Existing failure criteria cannot describe the strength anisotropy of rooted soils under general loading conditions because they are mainly based on the test results of direct shear. This study presents a new generalised three-dimensional anisotropic failure criterion for rooted soils. The model employs the projection of two independent microstructure fabric tensors (soil fabric and root network) on the stress tensor. Twenty-four drained triaxial compression and extension tests were carried out to measure the strength anisotropy of silty sand vegetated with vetiver grass (Chrysopogon zizanioides L.) at different overconsolidation ratios and calibrate the material parameters for the proposed criterion. Anisotropies of both cohesion and friction angle exist in rooted soil. Roots contribute mainly to the increase in cohesion (hence root cohesion) from most of the direct shear test data. Roots with predominant orientation aligning in the tensile strain direction contribute the most to soil strength. In the case of vetiver grass, which has a taproot system, their roots show the strongest reinforcement effect in the conventional triaxial extension path, in which the maximum portion of the roots is subjected to tension.

Upscaling Shear Strength of Heterogeneous Oil Sands with Interbedded Shales Using Artificial Neural Network

SummaryUnderstanding the shear strength of caprock shale and oil sands is important in risk assessment of slope stability in open-pit mining, caprock integrity of in-situ thermal recovery, and optimization of bitumen production from oil sands. A robust and efficient upscaling technique is essential to model the impact of heterogeneity on the deformation and failure of oil sands and caprock shale. Although conventional analytical and numerical upscaling techniques are available, many of these methods consider oversimplified assumptions and have high computational costs, especially when considering the impact of spatially correlated interbedded shales on the shear strength. A machine learning enhanced upscaling (MLEU) technique that leverages the accuracy of local numerical upscaling and the efficiency of artificial neural network (ANN) is proposed here. MLEU uses a fast and accurate ANN proxy model to predict the anisotropic shear strength of heterogeneous oil sands with interbedded shales. The R2 values of the trained ANN models exceed 0.94 for estimating shear strengths in horizontal and vertical directions. The deviation of upscaled shear strength from numerical upscaled results is improved by 12–76% compared with multivariate regression methods like response surface methodology (RSM) and polynomial chaos expansion (PCE). In terms of computational efficiency, the proposed MLEU method can save computational effort by two orders of magnitude compared with numerical upscaling. MLEU provides a reasonable estimate of anisotropic shear strength while considering uncertainties caused by different distributions of shale beddings. With the increasing demand for regional scale modeling of geomechanical problems, the proposed MLEU technique can be extended to other geological settings, where weak beddings play a significant role and the impact of heterogeneity on shear strength is important.

Bearing capacity of conical footing on anisotropic and heterogeneous clays using FEA and ANN

The paper presents axisymmetric FEA solutions for the bearing capacity of conical footings embedded in anisotropic and inhomogeneous clays using NGI-ADP model. The bearing capacity factor of conical footings is defined to be a function of three dimensionless parameters: (i) the cone apex angle, (ii) the increasing strength gradient ratio, and (iii) the anisotropic shear strength ratio. A series of parametric studies is carried out to portray the influence of all input parameters on the bearing capacity factors. Based on the FEA results of 216 investigated cases, the artificial neural network (ANN) is adopted to capture the relationship between input parameters and output bearing capacity factors. The complex relationship between input parameters and output bearing capacity is studied through the use of design charts and failure patterns. A new accurate equation with a high value of R2 is proposed and the sensitivity of each input parameter on the output results is scored by adopting the results from the ANN model. This study has provided some practical insights into the performance of conical footings embedded in anisotropic and inhomogeneous clays.

Closure to “Shear Strength Anisotropy of Natural Granite Residual Soil” by Xinyu Liu, Xianwei Zhang, Lingwei Kong, Song Yin, and Yiqing Xu

Closure to “Shear Strength Anisotropy of Natural Granite Residual Soil” by Xinyu Liu, Xianwei Zhang, Lingwei Kong, Song Yin, and Yiqing Xu

Discussion of “Shear Strength Anisotropy of Natural Granite Residual Soil” by Xinyu Liu, Xianwei Zhang, Lingwei Kong, Song Yin, and Yiqing Xu

Discussion of “Shear Strength Anisotropy of Natural Granite Residual Soil” by Xinyu Liu, Xianwei Zhang, Lingwei Kong, Song Yin, and Yiqing Xu

Study on the Anisotropic Shear Strength of Rough Joint via 3D Scanning, 3D Printing, and 3D Discrete-Element Modeling

The anisotropic shear strength of rough joints is examined from both experiments and numerical simulations in this paper. The rough joints in the experiment are prepared by combining 3D laser scanning and 3D printing technology. The direct shear tests are conducted in eight shear directions under constant normal load condition (CNL) with the normal stress of 0.2, 0.5, and 1.0 MPa. The dramatic variation of shear strength in different shear directions is observed from experimental results. Meanwhile, the numerical simulation based on the three-dimensional discrete-element method (DEM) is conducted, in which, the method to generate the identical rough joint by connecting triangular walls is proposed. With the calibrated microparameters from the unconfined compression test and direct shear test, the anisotropic shear strength is also observed in the numerical simulation, which is consistent with the experimental result. Accordingly, the topography anisotropy of the rough surface is analyzed, and the statistical parameter, Z2θ, is found closely related to the anisotropic shear strength of rough joints. Based on this, the Barton model is modified to estimate the shear strength of artificial joints with an anisotropic effect involved.

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.

Microscale evidence for and formation mechanisms of shear-strength anisotropy of a loess-paleosol sequence since the late Early Pleistocene: The case study of the Xiushidu profile, Southern Chinese loess Plateau

The shear-strength anisotropy of loess-paleosol sequences (LPS) provides vital information for engineering and construction activities and mitigation against geological disasters. The shear-strength anisotropy of undisturbed and disturbed samples from a profile from Xiushidu village, Jingyang County, in the Chinese Loess Plateau, and the formation mechanisms of such anisotropy were studied through direct shear tests and the microstructural analysis of their characteristics. Our results suggest that the shear strength of loess layers L1, L2, L3, L4, and L5 along the vertical direction was greater than that along the horizontal direction, under different values of axial compression. Under the same vertical pressure, the shear strength along the vertical direction was also generally greater for the layers above the seventh paleosol layer (S6), while the opposite result appears in the layers below S6. For the disturbed samples, their anisotropic characteristics are the same as for the undisturbed samples. The microstructure characteristics, including skeleton particles, contact relations, pore forms, cementation degree, and pore area ratio (PAR), explicated the reasons for the anisotropy of shear strength. Moreover, the influence of PAR, porosity (n), and dry density (ρd) on the shear strength in the vertical direction are greater than that in the horizontal direction, and the influence of water content is opposite to that of PAR, n, and ρd. Gravity, van der Waals forces, and hydrogeochemical reactions play important roles in the shear-strength anisotropy of the LPS. Therefore, research on the anisotropy of shear strength contributes to increasing our understanding of such layers, in preventing and mitigating against potential geological disasters and in implementing more appropriate engineering construction techniques.

Shear Strength Anisotropy of Natural Granite Residual Soil

AbstractA significant feature of residual soil is the presence of a cemented structure and fissures that dominate its anisotropic behavior. Although the anisotropy of some sedimentary soils is well...

Lining pressure for circular tunnels in two layered clay with anisotropic undrained shear strength

ABSTRACT In this paper, the required limiting pressure to be offered by the lining of circular tunnels formed in clay overlain by another clay layer under undrained condition has been computed. Both the layers are anisotropic in nature from the shear strength point of view. The upper layer is considered relatively either weaker or stronger than the lower layer i.e. undrained anisotropic shear strength of upper layer is relatively either smaller or greater than that of lower layer. Lower bound limit analysis coupled with finite element formulation and second order cone programming has been employed to perform the analysis. For different thickness of upper layer above crown of tunnel, the limiting lining pressure to maintain stability of tunnel has been obtained by varying thickness of lower clay layer, anisotropic parameters, normalised overburden pressure, undrained cohesion and unit weight of upper layer relative to lower layer. The solutions obtained from this analysis reveal that anisotropy in shear strength has significant effect on the required lining pressure to keep tunnel in stable condition.

Aspects of soft clay behaviour important for correct prediction of spudcan foundation penetration

An advanced hypoplastic constitutive model for soft clay has been developed with the following features: pre-yield non-linearity, strength anisotropy (different critical state friction angles in compression and in extension), stress- and OCR-dependency of undrained shear strength and soil structure and its degradation during loading. The model features are explored here through simulation of spudcan foundation penetration into a carbonate silty clay that was obtained from the Timor Sea. As different parameters control individual features of the model, model properties that are relevant to spudcan bearing pressure predictions can be separated from the properties that do not affect the results substantially. As expected, it is confirmed that pre-yield non-linearity is among the properties not important for the predictions. On the contrary, strength anisotropy, stress- and OCR-dependency of undrained shear strength and structure degradation are shown to be important aspects to be considered, indicating limitations of the use of simple elasto-plastic models for spudcan bearing pressure predictions. The conclusions can be adopted in the definition of a suitable experimental programme targeting the features of soil behaviour relevant to this geotechnical problem.