Mobilized Friction Angle Research Articles

Nonlinear analysis of laterally loaded single pile in layered soil based on conical strain wedge model

The nonlinear analysis of piles subjected to lateral load is of significance for pile foundation design. In this paper, a novel conical strain wedge (CSW) model is proposed to simulate the pile-soil interaction and a nonlinear analysis of laterally loaded monopiles in layered soils is performed. The superiority of the CSW model is that it has higher accuracy in simulating stress states of the soil by taking into account the tangential shear stress around the front surface of the wedge soil and its circular cross-section is more consistent with the failure pattern. The efficiency and accuracy of the proposed CSW method are verified by comparison with the measurement results of four field tests, including sandy soil, normally consolidated (NC) clay and over-consolidated (OC) clay cases. Finally, a comprehensive parametric study is conducted to investigate the sensitivity of some input parameters. The results indicate that (1) the correction factor of the mobilized friction angle for different soil types is recommended as 0.2. (2) as the lateral load rises, the soil mobilized friction angle at various depths increases and tends to the ultimate friction angle. Under the same load situation, the soil mobilized friction angle decreases as depth increases. (3) the pile deformation is significantly influenced by the burial depth ratio of the pile.

A Simple Breakage Model for Calcareous Sand and Its FE Implementation

This paper reports the development of a particle breakage model within a finite-element (FE) framework based on the modified Mohr–Coulomb model (MMC). A modified form of the strength-dilatancy relationship is used to incorporate additional energy consumed due to particle breakage. A new particle breakage index is introduced and constitutively linked to the mobilized friction angle. The model is verified against carefully conducted drained triaxial tests on medium-dense calcareous sand obtained from offshore Australia as well as other published data. The model is implemented in the FE program ABAQUS for both small strain and large deformation FE analyses in practical field problems. Several important microscopic observations can be made from the proposed model that are not limited to identifying soil regions where particle breakage is maximized. Due to its inherent simplicity, the newly developed model can provide reasonable predictions for soil-structure interaction problems in calcareous sands.

Monotonic and cyclic behaviour of sand in direct simple shear test conditions considering low stresses

Many geotechnical problems, such as buried pipeline–soil interaction and wave loading on sand seabeds, require the soil behaviour at relatively low effective stresses as compared to that in typical geotechnical practice. Monotonic and cyclic direct simple shear (DSS) tests were conducted on dry sand under constant normal stress, ranging from 12.5 kPa to 400 kPa, to investigate the effects of confining pressure on stress–strain behaviour. The tests were conducted using a combined advanced dynamic cyclic simple shear apparatus, which has a high-precision feedback system for controlling and measuring the forces and displacements while maintaining a high level of accuracy, which is essential, especially for tests at low normal stresses. The monotonic test results with this advanced system show an increased shear to normal stress ratio, thereby the mobilized friction angle, at the low-stress level compared to that of high normal stresses, as observed in some previous studies. The cyclic test results show that the load–displacement behaviour and cyclic compaction are governed by the normal stress. The sand becomes densified when the applied shear strain amplitude is greater than its threshold value. In constant strain amplitude cyclic loading, the lower normal stresses result in higher compaction; however, an opposite trend exists in constant stress amplitude cyclic tests.

Numerical simulation of direct shear test on granular materials composed of breakable angular particles: A DEM-XFEM approach

A combined DEM-XFEM novel approach is used to simulate the direct shear test on assemblies of breakable two-dimensional angular particles in order to study the macro- and micro-mechanical behavior of granular materials and the effect of particle breakage phenomenon on it. Therefore, the direct shear test simulations are conducted on two different particle assemblies, a breakage-disabled (BD) sample and a breakage-enabled (BE) one, and the results are analyzed and compared. In this approach, the particle assembly under direct shear loading is simulated through DEM, while the fracture within individual particles is modeled using XFEM, in which the Hoek-Brown criterion (HB) and an interaction-integral-based LEFM maximum circumferential tensile stress criterion are utilized to model the crack initiation and its propagation, respectively. Overall, the presented results demonstrate that particle breakage in granular materials reduces the amount of anisotropy, the dilative behavior, and the mobilized friction angle of the sample.

Arching effect and displacement on theoretical estimation for lateral force acting on retaining wall

A new approach is proposed to evaluate the non-limit active earth pressure in c–φ soil based on the horizontal slices method and limit equilibrium method, which takes into account arching effect, displacement, mobilized friction angle, tension cracks, and shear stress of the horizontal layer. The accuracy of the proposed method was demonstrated by comparing the experimental results and other theoretical methods. The comparison results showed that the proposed approach was appropriate for calculating the non-limit active earth pressure in c–φ soil and cohesionless soil. A parametric study was undertaken to access the effects of cohesion, mobilized friction angle, and shear stress of the horizontal layer for the lateral earth pressure, as well as the effects of displacement for the rupture angle and tension cracks. Moreover, by comparing calculated results of the different theoretical methods and numerical model, it indicated that the empirical formulations of the mobilized internal friction angle and soil–wall interface friction angle used to cohesionless soil were still suitable for c–φ soil.

The microscopic origin of K0 on granular soils: the role of particle shape

The at-rest coefficient of lateral pressure, $$K_0$$ , is a critical macroscopic parameter for evaluating stress transmission in granular media for engineering practice. This paper revisits the microscopic origin of $$K_0$$ and its corresponding underlying physics based on micromechanics-based theories and numerical simulations, with a focus placed on the effect of particle shape. Two typical kinds of distortion (elongation and blockiness) in particle shape are considered, modeled by ellipsoids and superballs in the discrete element method. One-dimensional compression tests are performed on numerical specimens with dense and loose initial states for different particle shapes. An analytical relationship between $$K_0$$ and anisotropy of fabric measures (i.e., contact normal, contact force, and shape-related anisotropy) is established within the stress-force-fabric framework. It is found that the analytical $$K_0$$ is consistent with the measured one directly from the simulation regardless of particle shape, verifying a well-established relationship between $$K_0$$ and fabric. It is further found that contact force partition of $$K_0$$ plays the most prominent role in $$K_0$$ compared to contact normal and shape-related partitions. Results also reveal the different influence of shape distortion on $$K_0$$ and the corresponding mechanical properties related to $$K_0$$ , such as void ratio, mobilized friction angle, and coordination number. More specifically, $$K_0$$ has a ‘W’-shape relationship with elongation for dense ellipsoids and an ‘M’-shape relationship with blockiness for superballs.

Post-erosion mechanical response of internally unstable soil of varying size and flow regime

One of the leading causes of dam failure is internal erosion. The impact of erosion of non-plastic fine particles, known as suffusion, on the soil structure and strength has been studied experimentally. However, influences including sample size have not been thoroughly investigated. Internally unstable gap-graded cohesionless soil samples with various sizes were investigated using an erosion-triaxial apparatus. Samples were subjected to downward inflows of different seepage velocities. The results indicated that the potential for clogging increased with an increase in specimen length, leading to less fine particle erosion. Internal erosion changed the mechanical soil behaviour even after the loss of fines equal to 5% of the overall sample volume. Eroded specimens with similar intergranular void ratios showed similar undrained post-erosion behaviour. However, the magnitude of the post-erosion initial undrained peak shear strength is a function of coarse particle interlocking, residual fine content, and equivalent intergranular contact index. It was also found that the steady state line remained unchanged after erosion of fine particles and the mobilized friction angle at the steady state line is independent of the residual fine content.

Undrained true triaxial response of initially anisotropic particulate assemblies using CFM-DEM

Abstract While simulation of the drained true triaxial response of particulate assemblies using discrete element method (DEM) has caught the attention of researchers in the recent years, simulation of the undrained condition under generalized stress condition has not yet been well addressed in the literature. This treatment suggests an applicable coupled fluid-discrete element scheme to simulate the undrained response of initially anisotropic particulate assemblies under true triaxial condition. Coupled fluid-discrete element simulations highlight the combined impact of Lode angle and anisotropic microstructure of the assemblies on the effective stress paths, mobilization of shear strength, and accumulation of pore-water pressure under undrained condition. For loose assemblies suffering from flow instability, it is shown that stress ratio, mobilized friction angle, and pore-water pressure at the onset of instability depend on Lode angle.

Experimental investigation of influence of clay in soil on interface friction between geotextile and clayey soil

Although the soils are not always pure in nature and are found as a mixture of clay, silt, sand, or coarse soil grains, pure clay or sand soils are used in all studies related to this subject in the literature. In this study, the influence of clay inclusion in sand on the mechanical behavior of nonwoven geotextile interfaces over clayey and soil and interfacial stress is studied. A relatively well-graded angular sand and a clear smectite type clay were selected as granular material. In all clay-sand mixing percentages, shear box experiments were performed in both dry and water-saturated. When the clay ratio is high, the maximum interfacial stress is around 15 kPa, whereas in the case of the sand ratio is highest, the interfacial stress varies between 30 and 35 kPa. However, as sand grains increase, the failure mechanism becomes more brittle. Geotextile-reinforced samples were investigated while varying the amount of clay in sand layers of saturated and dry condition. Variation of the amount of clay in the sand at the shear box tests showed that the mobilized friction angle between mixed soil and geotextile has a key factor in evaluation of failure in shear strength of mixed soil.

Dilatancy and Critical State of Calcareous Sand Incorporating Particle Breakage

Particle breakage has a crucial impact on the dilatancy and critical state of calcareous sand. This study performed several triaxial experiments on calcareous sand samples to describe the stress–dilatancy relationship of calcareous sand. The experimental results revealed that critical friction angle always is lower than the mobilized friction angle for calcareous sand. The shear resistance correlates with dilatancy and particle breakage. In addition, particle breakage highly affected the critical state line in the conducted tests. By introducing the concept of breakage parameters, this study established a modified dilatancy equation considering particle breakage under the theoretical framework of state parameters. The experimental results of this study exhibited a good correlation with the model results employing unique parameters.

On the role of geofoam density on the interface shear behavior of composite geosystems

Expanded polystyrene (EPS) geofoam has been increasingly used in geotechnical engineering applications to replace conventional backfill material or to reduce earth pressure on retaining and buried structures. In most of these applications, geofoam blocks are installed in direct contact with different construction material (soil, concrete, steel, wood, etc.). This results in a composite system that is subjected to loading in both the normal and shear directions. Therefore, successful analysis and design of these composite structures require a detailed knowledge of interface strength characteristics of geofoam under a combination of normal and shear stresses. In the current research, an attempt has been made to study the interface shear characteristics of geofoam with selected materials, including concrete, wood and steel. Three different geofoam densities namely, 15, 22 and 39 kg/m3 were used in this study. A series of direct shear tests (DST) was conducted with a shear box of dimension 100 mm × 100 mm × 50 mm under three different normal stresses 18, 36 and 54 kPa. Test results revealed that geofoam density and applied normal stress have significant effects on the vertical compression and interface strength properties (i.e. adhesion and friction angle) of the investigated geosystem. Surface roughness was found to play a significant role in the shear resistance at the contact. Geofoam–concrete interface showed maximum strength as compared to wood and steel materials. Care should be exercised when placing geofoam against steel or wood surfaces as the mobilized friction angle is generally small and sliding may develop depending on the applied normal stress level.

Effects of gradation on the mobilized friction angle for the instability and steady states of sand-silt mixtures: experimental evidence

Effects of gradation on the mobilized friction angle for the instability and steady states of sand-silt mixtures: experimental evidence

Modeling the shear response of granular materials with discrete element assemblages of sphere-clusters

Numerous high-performance computations of discrete element simulations of the direct-shear test investigate macroscopic and microscopic responses of granular materials. Volumetric dilation and mobilized friction angles are compared with spheres and sphere-clusters to investigate the influence of irregularly-shaped particles. Sphere-clusters are required to obtain realistic strengths and dilation rates. The absence of particle crushing in the simulations causes negligible void ratio reduction with increasing stress, but coordination number and initial stiffness are systematically stress-dependent. The development of the shear zone is shown through particle rotations and displacements, and the fluid-like nature of granular materials in shear is shown through particle velocities.

Diameter effect on uplift capacity of horizontal circular anchor embedded in sand

ABSTRACT Variation of uplift capacity of horizontal circular anchor plate in sand with its diameter has been studied by using lower bound finite element limit analysis. The soil friction angle is a function of stress level. For plates with similar embedment ratios (H/D) but different diameter (D), the in-situ stress experienced at the top of the plate due to self-weight of the overlying soil would be different which resulting in varying degrees of the mobilized friction angle of soil. This would lead to varying capacity factors for plates of different diameters but similar H/D. The present numerical study has been conducted for wide range of D and H/D of rough anchor plates embedded in both loose Hostun and dense Toyoura sands. The uplift capacity factor has been found to increase with decreasing value of anchor diameter. The average uplift pressure, however, decreases with decreasing value of diameter of plate anchor.

Sand type effect on the behaviour of sand-granulated rubber mixtures: Integrated study from micro- to macro-scales

The mechanical behaviour of rubber mixed with different types of host sands is investigated from micro- to macro-scales. The change of the mobilized friction angle because of rubber inclusion is associated to the change of inter-particle friction angle and the friction angle due to particle rearrangement. The mobilized friction angles at the peak and critical states were obtained from triaxial testing data, and the inter-particle friction angles at different sand and sand-rubber interfaces were measured by means of grain-scale experiments. It is found that: (i) The peak friction angle decreases after adding rubber particles for all host sands, which is more significant for dilative sands. (ii) For smooth and non-breakable sands, adding rubber increases the friction angle at the critical state, due to the increasing inter-particle friction from sand-rubber interfaces, but the reduction of sand-sand contacts weakens the interlocking, leading to easier particle rearrangement. For rough and breakable sands, adding rubber decreases the friction angle at the critical state, since the inter-particle friction of sand-rubber interfaces is lower than that of pure sand interfaces, but leads to less particle breakage and thus harder particle rearrangement. (iii) State parameter works well in assessing the effect of rubber inclusion on the peak strength.

Sand instability under constant shear drained stress path

Experimental findings have revealed that granular soils are vulnerable to pre-failure instability when subjected to constant shear drained (CSD) stress path. Using the concept of loss of uniqueness, it is shown that two dissimilar scenarios and accordingly, two independent criteria for granular soils in loose and dense states may lead to instability under CSD. The instability criteria are then applied in association with a state-dependent elastoplastic constitutive model taking into account possibility of elastic-plastic coupling. Numerical simulations have confirmed that instability in loose sands triggers prior to complete mobilization of the critical state stress ratio. However, instability of sands in dense state instigates at peak mobilized friction angle. A nearly unique linkage between stress ratio and state parameter at the onset of instability is predicted.

On the Micromechanics of True Triaxial Test, Insights from 3D DEM Study

In this paper, three dimensional discrete element method simulations of true triaxial test are presented. During simulations, the major, the intermediate and the minor principal strains are monitored to maintain the b-value constant. The effect of intermediate principal stress on the frictional parameters of granular materials is studied. Furthermore, the micromechanics of samples using the Stress–Force–Fabric (SFF) relationship is investigated. The SFF relationship is comprised of anisotropy tensors of fabric, contact normal and tangential forces. Results show that, the mobilized friction angles in simulations with different b-values have very good agreements with results of the so-called general stress tensors, which are made of force and position vectors at contact points. It is observed that the evolution of anisotropy in the sample is at the origin of different responses of granular materials in macro scale. The frictional properties of granular materials is observed sensitive to the intermediate principal stress both at macro and grain scale. Our observations highlight the important role of intermediate principal stress, and are consistent with experimental studies of true triaxial tests on granular materials.

Effect of Stress Direction on the Undrained Monotonic and Cyclic Behaviour of Dense Sands

Geotechnical design may be unsafe if the anisotropic behaviour of soil is not considered. The behaviour of anisotropic materials depends on the principal stresses and their directions. A detailed experimental programme was conducted to study the effect of stress direction on the monotonic and cyclic behaviour of dense sand. A total of 20 undrained tests were performed at a constant mean confining stress (σ'0m) constant intermediate principal stress ratio (b= (σ2-σ3)/(σ1-σ3)), and principal stress directions (α). Two fine sands, Babolsar and Toyoura, were selected as the test materials. The isotropic consolidated specimens were prepared using the wet tamping technique. The results showed that the major principal stress direction had little considerable effect on the mobilized friction angle at steady state or phase transformation. The results showed that stress direction had a significant effect on the non-coaxiality between the principal strain increment direction and the principal stress direction. The soil fabric was led to significant non-coaxiality value before the peak shear strength. Increasing the octahedral shear strains decreased the non-coaxiality value due to destruction of the soil particle interlock (soil fabric). The effect of stress direction on non-coaxiality and excess pore water pressure generation was also investigated.

Predicting the initiation of static liquefaction of cross‐anisotropic sands under multiaxial stress conditions

SummaryExperimental evidence has shown that the liquefaction instability of sands can be affected by its material density, stress state, and inherent anisotropy. In order to predict the initiation of the static liquefaction of inherent cross‐anisotropic sands under multidimensional stress conditions, a rational constitutive model is needed. An elastoplasticity model able to capture the influences of intermediate principal stress ratio (b = (σ2 − σ3)/(σ1 − σ3)) and loading direction on stress–strain relationships and volumetric properties was proposed. The yield function was formulated to be controlled by Lode angle, loading direction, and material state; the stress–dilatancy was a material state‐dependent function. After using the existing drained hollow cylinder tests to validate the proposed model, this model was used to simulate the existing undrained hollow cylinder tests. During this simulation, the second‐order work criterion was used to determine the initiation of static liquefaction. The results showed that an increase in both the intermediate principal stress ratio and the loading angle induces a decrease in the second‐order work. Static liquefaction is initiated more easily at a stress state with a large intermediate principal stress ratio and a large loading angle, and the mobilized friction angle at the instability points decreases with the intermediate principal stress ratio and the loading angle. Copyright © 2017 John Wiley & Sons, Ltd.

Micromechanical formulation of the mobilized stress ratio under principal stress rotation in granular materials

In this paper the mobilized stress ratio is investigated by using the static equilibrium at the micro scale level analysis. The mobilized stress ratio is obtained in the distribution of the contact normal, E (β ), and the inter-particle mobilized friction angle, ϕμ . The second invariant of the fabric tensor, α is used to account for fabric and its evolution. There is a difference between the theoretical rotation of principal stress axes that is obtained by the classical equations of the mechanics of materials and the experimental rotation of principal stress axes. The experimental rotation is related to the anisotropic parameter, α . The noncoincidence between the theoretical and experimental is related to the relative direction of the mobilized plane and the bedding plane. A comparison with experimental tests demonstrates the validity of this formulation.