Critical State Friction Angle Research Articles

On the Behavior of Bauxite Tailings under a Wide Range of Stresses

Despite its vital importance to the contemporary economy, some drawbacks are mainly associated with waste derived from mining activity. This waste consists of tailings that are hydraulically disposed of in large impoundments, the tailings dams. As the dams are enlarged to accommodate higher amounts of materials, the stress states at which the deposited tailings are submitted change. This may be a concern for the stability of such structures once the geotechnical behavior of this material may be complex and challenging to predict, considering the existing approaches. Thus, the present study concerns the mechanical response of bauxite tailings under a wide span of stresses, ranging from 25 kPa to 4000 kPa. One-dimensional compression tests and isotropically drained and undrained triaxial tests were carried out on intact and remolded samples of the bauxite tailings. The after-shearing grain size distribution was characterized via sedimentation analysis. The results have shown a stress-dependency of the critical state friction angle for the intact material, which may be related to fabric alterations derived from structure deterioration and particle breakage. Overall, this research provides valuable insights into the response of structured and de-structured bauxite tailings, which are helpful for future constitutive modeling of such material.

Hybrid artificial neural network models for bearing capacity evaluation of a strip footing on sand based on Bolton failure criterion

This paper employs the Bolton failure criterion, incorporating strength-dilatancy relationships, to analyze the bearing capacity factor of a strip footing on dense sand. Utilizing finite element limit analysis (FELA) based on the lower and upper bound theorems, the study presents the results as average bound solutions. By using the Bolton model, the b parameter is first calibrated and found that it should be about 3.50 to align the ultimate bearing capacity (qu) from FELA to have a good agreement with that from experimental test results from previous studies. The influence of parameters relevant to the Bolton failure criterion is analysed, showing that an increase in relative density (DR) significantly affects the variation in the bearing capacity factor (Nγ) at higher Q values, while lower Q values inhibit dilatancy due to soil crushing. The width of the strip footing (B) has a decreasing effect on Nγ at higher Q values, and the unit weight (γ) changes minimally impact Nγ within the range of 16–22 kN/m3. Additionally, an increase in the critical state friction angle (ϕcv) consistently increases Nγ, highlighting its direct correlation with soil shear strength. A hybrid artificial neural network (ANN) model integrates machine learning with four optimization algorithms: Imperialist Competitive Algorithm (ICA), Ant Lion Optimization (ALO), Teaching Learning Based Optimization (TLBO), and New Self-Organizing Hierarchical Particle Swarm Optimizer with Jumping Time-Varying Acceleration Coefficients (NHPSO-JTVAC). Comparative rank analysis of hybrid ANN models based on the selection of the optimal number of hidden neurons demonstrates that the ANN-TLBO model excels in predicting the bearing capacity factor, achieving a score of 48. This conclusion is corroborated by an error heatmap matrix, which indicates a minimized percentage of error relative to other hybrid ANN models. Importance analysis identifies particle crushing strength (Q) as the most significant factor influencing the bearing capacity factor (Nγ).

On the mechanical behaviour of a coral silt from the South China Sea

During land reclamation on the reef islands, large amounts of silt-sized coral soils were created by segregation and degradation. The accumulation of silt-sized coral soils is fairy uncommon while the research on the geotechnical properties of the coral silt is very limited. In this study, a systematic experimental investigation on the mechanical behaviour of a coral silt obtained from a reclaimed reef island in the South China Sea has been performed, with comparisons to the coral sand collected from the same area. Similar to the coral sand, the coral silt particles also exhibit irregular particle shape and intra-particle pore due to their nature origin. According to the limiting water contents, the coral silt is classified as a low-plasticity clayey silt. Under one-dimensional compression, the coral silt exhibits a much quicker convergence compared to coral sand. Prior to convergence, the compressibility of coral silt is higher. After yielding, the compressibility of coral sand becomes higher due to significant particle breakage. The loose coral silt subjected to undrained shearing at low confining pressures exhibits obvious strain softening behaviour, indicating static liquefaction or flow failure. The peak and critical state friction angles of coral silt are lower than those of coral sand but much higher than those of the other terrigenous clayey silts. A curved critical state line with well-defined horizontal asymptote in the deviatoric stress – mean effective stress plane is identified for coral silt, again indicating higher potential to static liquefaction.

CPT-based design of pile foundations in sand and clay: Perspectives

This paper provides some perspectives and guidance for the design of pile foundations in sand and clay using cone penetration test (CPT) results. A key variable in the estimation of the limit unit shaft resistance qsL of piles in sand is the critical-state interface friction angle δc, which is a function of the critical-state friction angle ϕc of the sand. In the absence of direct shear or triaxial compression test results, which is often the case for routine infrastructure projects, engineers typically assume a conservative value for ϕc in pile design. In addition, effective stress-based methods for estimation of qsL of driven piles in clay rely on the residual interface friction angle δr, among other variables; however, in these methods, δr does not vary with the normal effective stress on the pile operative at the time of shearing. In this paper, we present relationships and approaches to address these issues. Finally, a relationship between the cone resistance qc and the corrected standard penetration test (SPT) blow count N60 was also developed so that engineers may obtain an estimate of qc for use in a CPT-based design method when only SPT blow counts are available for a site.

Effect of initial fabric from sample preparation on the mechanical behaviour of a carbonate sand from the South China Sea

Mega reclamation projects have been undertaken on the coral reefs. However, research on the geotechnical properties of the hydraulically filled carbonate sand is limited, and the impact of the initial fabrics from different densification methods on the mechanical behaviour of carbonate sand remains unclear. In this work, a systematic experimental investigation on the effect of initial fabric induced by sample preparation method on the mechanical behaviour of carbonate sand has been performed. Four sample preparation methods, including moist tamping, vibration, dry pluviation and water pluviation were adopted to reconstitute samples with distinct fabrics. It is found that, for a single preparation method, the particle breakage as well as the peak friction angle of carbonate sand is affected by the drainage condition significantly, and both the critical state friction angle (φ'cs) and the compressibility (λ) of carbonate sand are strongly stress dependent. For samples prepared by different methods, an obvious divergence of critical state points can be seen in the specific volume and mean effective stress plane at low stress levels. The dry pluviation samples show the lowest φ'cs and highest λ, due to the highest anisotropy. In contrast, moist tamping samples give the highest φ'cs and lowest λ, due to the most isotropic fabric.

Evaluating Sand Particle Surface Smoothness Using a New Computer-Based Approach to Improve the Characterization of Macroscale Parameters

The analysis of sands, and the foundation systems with which they interact, are largely dependent on macroscale behavioral parameters that represent the aggregated response of several microscale characteristics. This research paper examines the influence of surface texture, or smoothness, on the behavior of sands. The challenge of estimating or measuring smoothness, due to its microscale feature domain, is addressed through an examination of six artificially graded sand specimens. These specimens are evaluated both visually and numerically to characterize their surface smoothness. The first approach described is a simple visual method that uses a smoothness scale consistent with those of roundness and sphericity. This method, which can be performed with a tool as simple as a hand lens, evaluates a group of representative particles collectively. The second approach is also a visual evaluation, but it utilizes images obtained via scanning electronic microscopy, traditional optical microscopy, and newer low-cost digital microscopes that can be rapidly connected to a smartphone or laptop. To validate these visual estimates, a novel third approach is introduced. This approach is a more objective numerical analysis measurement technique that enables rapid and economic quantification of smoothness. This technique may assist both practitioners and academics in their understanding of the macroscale response of coarse-grained soils. In addition to the visual methods, this research also conducted several laboratory index tests to observe the mechanical behavior of the specimens, considering their particle shape and surface smoothness properties. The results indicate that angular sands have greater minimum and maximum void ratios, a larger difference between the minimum and maximum void ratios, greater critical state friction angles, and greater flow rates through an orifice of fixed size. When adjusted for surface smoothness using the proposed approach, the behavior of the sands—particularly the limit void ratio results—appears to be more predictable in some cases. These results provide additional evidence of particle smoothness contributing to the strength behavior of sand, which may be particularly useful in the domains of slope stability, land reclamation, soil–structure interaction, and soil dynamics.

Mechanical behavior and particle crushing of marine carbonate gravel in Xisha Islands, South China Sea

Carbonate gravelly soils are widely distributed in sub-tropical marine areas. During construction and engineering operations, carbonate gravel undergoes considerable particle crushing, which has remarkable effects on their engineering properties. A series of large-diameter oedometer tests, large-diameter drained and undrained triaxial shear tests were conducted on carbonate gravel specimens taken from the South China Sea. The carbonate gravel specimens exhibit significant particle crushing at common engineering pressure levels, and the particle crushing highly depends on the initial density, loading history and stress path. Three fragmentation modes, fracture, attrition, and abrasion, are notable in carbonate gravel specimens, which induces significant variations of PSD curves and greatly effects the mechanical properties. Particle crushing in carbonate gravel specimens increases the compressibility resulting in the compression index increases with the increase of surcharge pressure; decreases the pressure-hardening resulting in the initial Young’s and secant moduli slightly increase with the increase of initial confining pressure; and decreases the dilatancy resulting in the peak and critical state friction angles significantly deceases with the increase of initial confining pressure. Particle crushing also shows great effect on the CSL of the carbonate gravel specimens in both p ′ - q space and e - p ′ space.

Mechanical behavior of iron ore tailings under standard compression and extension triaxial stress paths

The disposal of mining tailings has increasingly focused on the use of dry stacks. These structures offer more security since they use filtered and compacted material. Because of the construction method and the heights achieved, the material that compounds the structure can be subjected to different stress paths along the failure plane. The theoretical framework considered in the design of these structures generally is the critical state soil mechanics (CSSM). However, the data in the literature concerning the uniqueness of critical state line (CSL) is still controversial as the soil is subjected to different stress paths. With respect to tailings, this question is even more restricted. This paper studies two tailings with different gradings due to the beneficial processes over extension and compression paths. A series of drained and undrained triaxial tests was conducted over a range of initial densities and stress levels. In the q-p' plane, different critical stress ratio (M) values were obtained for compression and extension stress paths. However, the critical state friction angle is very similar with a slightly higher critical state friction angle for extension tests. Curved stress path dependent CSLs were obtained in the ν-lnp' plane with the extension tests below the CSL defined in compression. Regarding the fines content, the studied tailings presented very similar M and critical state friction angle values. However, the fines content affects the volumetric behavior of the studied tailings and the CSLs on the ν-ln p' plane shift downwards with the increasing fines content for compression and extension tests. In relation to dilatancy analysis, the fines content did not present an evident influence on the dilatancy of the materials. However, different values of mean stress ratio N were obtained between compression and extension tests and can corroborate the existence of non-unique CSLs for these materials.

Critical State-Based Mohr–Coulomb Bounding Surface Model for Sand under Monotonic Shearing

This study proposes a new constitutive model to describe the smooth transition from an elastic to a plastic response in sands under monotonic shearing. The model modifies the conventional Mohr–Coulomb model by considering the concept of a bounding surface and critical state soil mechanics. The friction angle consists of the critical state friction angle and a portion of the dilatancy angle, which is determined from the distance to the critical state line. Incorporating the bounding surface and the dilatancy angle into constitutive relationships for Toyoura sand produces numerical simulation results that have good agreement with the experimental results.

Effect of gas-oil contamination on the mechanical behavior of sand-woven geotextile interface: Experimental investigation and constitutive modeling

The accurate estimation of the frictional resistance of interfaces between soils and geosynthetics plays a central role in stability and serviceability of geosynthetic reinforced earth structures. Contamination with hydrocarbons generally impairs soil geotechnical properties; however, its effect on the behavior of soil-geosynthetic interfaces has seldom been examined precisely. For this reason, an extensive series of direct shear tests was performed to investigate the consequence of gas-oil contamination on the mobilization of shear strength and volume change response of gas-oil contaminated angular sand in contact with woven geotextile (WGTX). Complementary tests on the interfaces between glass beads as a replicate for sands with high degree of sphericity and roundness in contact with WGTX were also performed to explore the effect of particle shape. Gas-oil contamination is observed to causes decrease of the peak and critical state friction and dilation angles in both the sand-WGTX and glass bead-WGTX interfaces. However, gas-oil contamination-induced decrease in the frictional efficiency in the glass bead-WGTX interfaces was greater than that in the angular sand-WGTX interfaces. Calibration of a state-dependent sand-structure interface model against the laboratory data of gas-oil contaminated soils-WGTX interfaces results in a reasonable agreement between the model simulations and the laboratory data.

A reinterpretation of the mechanical behavior of rubber-sand mixtures in direct shear testing

The mechanical behaviors and properties of rubber-sand mixtures are interesting and of practical importance but are not fully understood yet. In this study, 168 direct shear tests were performed to investigate the shear behaviors and strength of rubber-sand mixtures by considering various rubber content and rubber particle sizes, with an analysis made from the energetic and micromechanical perspectives to illuminate the underlying mechanisms. The experimental results indicate that increasing the rubber content helps reduce the shear strength of rubber-sand mixtures and restrain the mobilization of dilatancy, whereas it aids in promoting the ductility. The role of rubber particle size relies on the rubber content level: increasing the rubber particle size basically weakens the shear strength and dilatancy of rubber-sand mixtures with a relatively high rubber content, but it contributes to a minor increase in the shear strength at large shear displacements, given a low rubber content. The stress-dilatancy behaviors of rubber-sand mixtures can be characterized by a general linear function, in which the dilatancy coefficient ξ and critical state friction angle ϕcs, as two crucial function parameters, are both variables affected by the rubber content, rubber particle size and normal pressure. The energetic and micromechanical analysis based on the idealized conceptual model reveals that the impact of rubber content, rubber particle size, and normal pressure on the critical state shear strength is fundamentally linked with a combined effect stemming from the roughness of shear plane and relative number of particle sliding and rolling behaviors, the variation of which is in essence governed by the deformation of rubber particles.

Determination of the critical state of a silty sand iron tailings in triaxial extension tests using photographic correction

Determination of the critical state of tailings is essential to the stability analysis of tailings dams, especially to assess the susceptibility to flow liquefaction failures. In contrast to the peak resistance, which can be determined in triaxial tests at relatively small strains, the critical state is often reached at higher strains. Therefore, determining the critical state in triaxial extension tests is more difficult than in compression tests because the concentration of stresses and strains in extension tests is more severe and usually inevitable, resulting in necking that may greatly affect the computed deviatoric stress and void ratio (if the test is drained). This paper presents a simple and cost-effective method of necking correction using photography. The method was used in the determination of the critical state in drained triaxial extension tests. A sample of silty sand tailings obtained from the reservoir of the Fundão dam two years prior to its collapse was used. The critical state friction angle was approximately the same in compression and extension, showing that the proposed method is promising as many authors consider that the critical state friction angle is the same in these two conditions.

Investigating the Potential of Infrared Thermography to Inform on Physical and Mechanical Properties of Soils for Geotechnical Engineering

Knowledge of physical and mechanical properties of geomaterials is fundamental to characterise their response to external forcings (mechanical, climatic) at various scales. This is true, for instance, in slope stability assessments, civil engineering works, and agriculture. The direct evaluation of these properties in situ can be difficult, especially in inaccessible or vast areas, and so can be the sampling and subsequent testing in the laboratory—where ensuring the representativeness of the acquired data at the scale of analysis poses an additional challenge. Thus, empirical correlations with more readily determinable quantities remain a powerful and practical tool. Recently, several sensors, able to inform on various geomaterial properties, have been developed. However, applications have typically targeted rocks, while studies on uncemented geomaterials (soils, geotechnically speaking) are lacking. Here, we propose a simple method to evaluate the porosity and critical state friction angle of soils via infrared thermography, consisting of periodic acquisitions of images in infrared wavelengths. To demonstrate the method’s capability, we analysed the cooling behaviour of samples of bentonite, kaolin, and sand (for which an extensive characterisation exists in the literature), after compaction to different porosities and pre-heating in an oven. We interpreted the results by seeking the optimal time interval for which a cooling rate index (CRI) could be defined, which is best linked with the target property. We found that the CRI correlates very well with the critical state friction angle (R2 > 0.85) and that different materials show unique and strong (R2 = 0.86–0.99) relationships between their porosity and the CRI, which also varies in a material-specific fashion according to the explored time interval. Although a systematic investigation on a wide range of natural soils is warranted, we argue that our method can be highly informative and could be used to calibrate remote sensing-based full-scale implementations in situ for various purposes.

Studying effects of interface surface roughness, mean particle size, and particle shape on the shear behavior of sand-coated CFRP interface

Carbon fiber-reinforced polymer (CFRP) composites can be considered an ideal alternative for conventional structural elements due to their superior benefits, such as high durability, satisfactory strength, and cost-effectiveness. This study is focused on interface shear behavior between sand-coated CFRP [spark plasma sintering CFRP (SPSCFRP)] and different types of sand with varying mean particle sizes (D50), and particle shapes [well-rounded (glass beads or GB), well-angular (glass powder or GP), and sub-rounded to sub-angular (natural sand)] under two levels of initial relative density. Four types of SPSCFRP surfaces with different levels of surface roughness, ranging from very smooth (VS) to highly rough (HR) were selected to perform interface shear tests. Analysis of sand-SPSCFRP data highlights an increase in surface roughness, mean particle size, angularity, and initial relative density resulting in mobilization of higher peak and critical-state friction angles. Rough SPSCFRP shows the most effective performance of the tested configurations in interface shear response. A critical value for normalized roughness (Rn) is observed (0.2≤Rcr≤0.5), within which the highest value of peak and critical-state friction angles is obtained. A well-pronounced post-peak reduction in shear stress of interfaces with Rn≥ 0.3 is observed. The peak and critical-state friction angles of sand-rough SPSCFRP interfaces are 15–32% and 11–32% higher than those of sand-CFRP interfaces, respectively. Given the same surface roughness and mean particle size, smaller peak and critical-state interface friction angles are mobilized for well-rounded sand than for well-angular sand. For the sand-SPSCFRP and sand-CFRP interfaces, the sand-structure design ratio (D.R.) ranges between 1.04 and 1.16 and 0.79–0.96, respectively. The average efficiency factor (E.F.) of the sand-SPSCFRP surfaces are 1.02 and 1.06, respectively, for medium-dense and dense specimens, while this value is much lower for sand-uncoated CFRP (∼0.8).

Investigation on the Influencing Factors of K0 of Granular Materials Using Discrete Element Modelling

Earth pressure coefficient at rest K0 is commonly estimated by empirical equations, which to date has had insufficient accuracy and universality. For better prediction, the investigation on the factors influencing K0 is required. A series of discrete element method (DEM) simulations of oedometer tests are conducted to verify the key factors influencing K0 of granular materials. The influences of initial fabric anisotropy, particle shape, initial void ratio, inter-particle friction angle is investigated. The evolution of microstructure is monitored during the tests to reveal the relationship between the microstructure evolution and K0 values. The results show that the effect of fabric anisotropy exists but is limited. Particle shape, initial void ratio, and inter-particle friction angle all significantly affect the K0 values alone. According to the DEM results, an attempt is made to propose a more reasonable empirical equation in which K0 is a function of relative density, critical state friction angle, and “shape factor”. This new empirical equation has higher accuracy and can consider the effect of particle shape, inspiring the determination of K0 values in practical engineering.

Effects of internal erosion on the cyclic and post-cyclic mechanical behaviours of reconstituted volcanic ash

Internal erosion is the transportation of soil particles from within or beneath the main structure inside the ground due to seepage flow. Internal erosion impacts the mechanical and hydraulic behaviour of soil. The present study investigates the impact of internal erosion on the cyclic and post-cyclic mechanical behaviours of volcanic ash sampled from Satozuka, Japan. A series of experiments using volcanic ash with loose, medium, and dense conditions have been performed using an erosion triaxial apparatus. Further, reconsolidation after application of cyclic loading was considered to study the impact of reconsolidation on the post-cyclic mechanical behaviour. The results show that the cyclic resistance of eroded specimens is improved regardless of the percentage of eroded fines and the initial relative density. The higher cyclic resistance for the eroded specimens is found to be due to a decrease in the intergranular void ratio after the erosion, change in fabric, and pre-strain history during erosion. Reduction in the critical state friction angle (φcri) and peak stress ratio (Rpeak) for the monotonic loading due to cyclic loading is greater for the eroded specimens. However, reconsolidated eroded specimens show increases in φcri and Rpeak, which are lower than those of the non-eroded and eroded specimens without cyclic loading. This could be due to the change in the soil fabric during cyclic loading where the positive impact of erosion is lost after cyclic loading. Such limited recovery of deviatoric stress with deformation is density-dependent.

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.

Critical-State Shear Strength and Pore Pressure of Granular Materials

Abstract The plane-strain critical-state friction angle is an important soil parameter in the design of geotechnical projects. The static angle of repose, which is the same as the plane-strain crit...

Friction and maximum dilatancy angles of granular soils incorporating low plastic fines and depositional techniques effects

This paper addresses the effects of over consolidation ratio (OCR = 1–8), low plastic fines content (Fc = 0–40%) and depositional techniques (DFP and WD) on the stress-strain and volume change behaviours as well as maximum dilatancy, peak-state friction, critical state friction and excess friction angles through two series of drained triaxial compression tests. The first one, emphasizing the effect of the OCR and the second one analysing the influence of Fc on the mechanical behaviour of Chlef sand and Chlef sand-silt mixtures, respectively. All specimens were prepared at a medium density (Dr = 53%) and subjected to a confining pressure (σ′3 = 0.1 MPa). The obtained results indicate that the increase in the fines content of the DFP specimens leads to a decrease in the deviatoric stress and dilative character at the same axial strain, however, the reverse trend is observed for the WD ones. Moreover, it is observed that the deviatoric stress generally increases with the increasing of the OCR for both depositional techniques and the DFP specimens prepared exhibit higher values of the shear strength than the WD ones. Furthermore, the results show that Fc, OCR and depositional techniques influence significantly the mechanical characteristics (ϕ′ps, ϕ′cs, ψmax and ϕ′ex).

Strength–dilatancy and critical state behaviours of binary mixtures of graded sands influenced by particle size ratio and fines content

Binary granular soil mixtures, as common heterogeneous soils, are ubiquitous in nature and man-made deposits. Fines content and particle size ratio are two important gradation parameters for a binary mixture, which have potential influences on mechanical behaviours. However, experimental studies on drained shear behaviour considering the whole range of fines content and different particle size ratios are scarce in the literature. For this purpose, a series of drained triaxial compression tests was performed on dense binary silica sand mixtures with four different particle size ratios to investigate systematically the effects of fines content and particle size ratio on the drained shear behaviours. Based on these tests, the strength-dilation behaviour and critical state behaviour were examined. It was observed that both fines content and particle size ratio have significant influence on the stress–strain response, the critical state void ratio, the critical state friction angle, the maximum dilation angle, the peak friction angle and the stress–dilatancy relation. The underlying mechanism for the effects of fines content and particle size ratio was discussed from the perspective of the kinematic movements at particle level.