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This study used triaxial tests to examine the impact of the root diameter of Cunninghamia lanceolata (Chinese fir) on the mechanical behavior of sand, including stress–strain development, strength, volumetric strain, and failure envelope. It also revealed the reinforcement mechanisms of roots with different diameters based on root–soil interactions. The results showed the following: (1) The addition of roots significantly enhanced sand strength and reduced volumetric deformation. The average peak strength increased by 31.8%, while the average peak volumetric strain decreased by 34.3%. (2) Roots provided additional cohesion and increased the friction angle of the sand, causing the failure envelope to shift upward and deflect. (3) Smaller-diameter roots improved the mechanical properties of sand more significantly, leading to higher peak strength, shear strength parameters, and smaller volumetric deformation. As the root diameter increased from 1 mm to 5 mm, the peak strength ratio decreased from 1.78 to 1.13, and the peak volumetric strain increased from 0.48 to 0.79. (4) Smaller-diameter roots, which form denser networks, allowing more roots to resist loads, and have a higher elastic modulus providing greater tensile stress, also possess higher tensile strength and critical sliding tensile stress, making them less likely to fail, thereby making the mechanical reinforcement of sand more significant. [ABSTRACT FROM AUTHOR]

Asperities within pre-existing fractures of coals can experience local damage during the fracture closure due to external loading. Previous research postulates that this local asperity damage can lead to strain rate-dependency without causing permanent deformation to the bulk of the coal specimens. This study aims to comprehensively investigate this behavior by developing a theoretical model that characterizes the strain rate-dependency driven by fracture asperity damage in coal. To achieve this objective, an initial series of micro-scale mechanical tests are conducted on joint specimens to establish a model for effective stress acting on asperities. Building upon this model, a theoretical foundation is further developed to describe the strain rate-dependent asperity damage evolution and resulting energy dissipation. These frameworks are subsequently incorporated into elasticity and damage mechanics to capture the strain rate-dependent stress–strain relationships. To validate the proposed model across multiple scales, additional triaxial tests on core-scale specimen and micro-scale mechanical tests on joint specimens are performed. The experimentally measured strain rate-dependency aligns well with the predictions of the proposed model, indicating a successful development of a robust model. The results of the model developed in this study reveal that the strain rate-dependency in fractured coals is governed by several factors, including asperity damage, mechanical properties of the coal specimens and effective stress acting on asperities of pre-existing fractures within the bulk of coal. Moreover, it is shown that the effective stress acting on asperities is significantly affected by both applied normal stress and joint roughness coefficient (JRC). The insights derived from this study demonstrate that the strain rate-dependency induced by micro-scale asperity damage of pre-existing fractures leads to observable strain rate-dependency in bulk specimens at core-scale and the proposed model can adequately capture this behavior. Highlights: Micro-scale mechanical tests are conducted on coal joint specimens to establish the equation for effective stress acting on joint asperities. A multi-scale theoretical model is developed for the strain rate dependency within coal resulting from fracture asperity damage. The proposed model is validated by experimental data obtained from multi-scale experiments. The effective stress acting on joint (fracture) asperities is governed by both the applied normal stress and JRC. The theoretical model indicates that micro-scale asperity damage leads to the strain rate dependency at core-scale level. [ABSTRACT FROM AUTHOR]

Stress path is a key factor affecting the particle breakage of calcareous sand. In this study, the effects of stress path variations on calcareous sand particle breakage were investigated through triaxial compression tests across four distinct stress paths. Additionally, the gradation evolution of calcareous sand during particle breakage was analyzed. Furthermore, the correlation between the total input energy and characteristic particle size was investigated through energy dissipation analysis. The results indicated that the relative breakage index increases gradually with an increase in the maximum deviatoric stress and final volumetric strain, irrespective of the stress path. However, the dilatancy of calcareous sand is related to the relative breakage index as well as the stress path. Notably, the relationship between the relative breakage index and the total input energy can be represented using a power function. A gradation evolution model was formulated based on the results of energy dissipation analysis, and its validity was verified. The results confirmed the model's effectiveness in predicting the particle breakage evolution in both single-gradation and continuous-gradation calcareous sand specimens, accounting for the effects of the various stress paths. [ABSTRACT FROM AUTHOR]

For many civil and mining engineering structures, rock is the primary construction medium. At engineering scales, the material of interest is the "rockmass," which contains sections of intact rock separated by a network of pre-existing joints (i.e. fractures). The mechanical behavior of a rockmass is defined by attributes of its intact rock (e.g. strength, stiffness, dilatancy), in addition to attributes of the pre-existing joints (e.g. orientation, surface condition, persistence). Testing representative rockmass specimens in the laboratory is uncommon because of its difficulty due to the required scale. The present study uses laboratory-scale rockmass analog specimens to study the effects that the pre-existing joints have on rockmass mechanical behavior. Smooth joints were sawed into decimeter-scale cylindrical specimens of Carrara marble. Specimens with two joint sets and two different degrees of jointing were tested. Specimens were loaded in compression using a triaxial apparatus under varying levels of confining pressure, and mechanical attributes were calculated including peak strength, residual strength, and pre-peak damage threshold parameters. At the higher end of confining pressures tested (12 MPa), the peak strength of the Carrara marble specimens was not influenced by the presence of pre-existing joints. These results were compared to the results of previous jointed specimen testing on the more brittle Blanco Mera granite. Notably, the drops in peak strength with increasing degrees of jointing were smaller for Carrara Marble than for Blanco Mera granite, which suggests that the Generalized Hoek–Brown failure criterion may not be appropriate to estimate rockmass strength for less brittle rocks. [ABSTRACT FROM AUTHOR]

Reinforcement of soils with fibers generally increases the mechanical properties of the fiber-reinforced soil (FRS) system. However, published literature is limited to investigating the undrained response of clay and synthetic fibers, with few studies targeting natural clay and natural fibers under drained conditions. There is a need to study the response of fiber-reinforced clay systems under drained conditions to assess long-term stability. This paper investigated the drained shear strength and durability of clays reinforced with natural hemp fibers using isotropically consolidated drained triaxial tests, in which the fiber content, confining pressure, and compaction water content were varied. Results showed that the incorporation of hemp fibers improved the deviatoric stress at failure by up to 60%, which increased the drained cohesion and friction angle of the FRS by 7–10 kPa and 3–7°, respectively. The increase in cohesive intercept was not affected by the compaction water content, while the increase in friction angle was pronounced in specimens compacted at optimum water content (w = 18%). Durability tests showed that the improvement in strength due to hemp fibers diminishes after 3 weeks of curing prior to drained testing, indicating the dramatic negative impact of degradation of natural fibers on the mechanical performance of fiber-reinforced clay and the need for industrial treatment of the fiber. [ABSTRACT FROM AUTHOR]

In the field of geotechnical engineering, reinforced pond ash is an alternative building material that has the potential to be utilized. The effects of using woven coir geotextile and polymeric geogrid for pond ash reinforcement were studied through experimental investigations. A triaxial test was performed on reinforced the pond ash. Two different types of geotextiles such as polymeric geogrid (rigid biaxial and flexible biaxial), two types of woven coir geotextile (different mass per unit area), and two types of pond ash (coarse and fine) were considered. The geotextile reinforcements were placed in the triaxial samples in various combinations in one, two, and three layers. The effects of the type of geotextile, position, and number of geotextile layers on the strength of pond ash were investigated. Both unreinforced and reinforced pond ash were subjected to a series of unconsolidated undrained (UU) triaxial tests to examine the shear strength response. The present research examines how the shear strength response of pond ash is affected by confining pressure (σ3), the number of geotextile layers (N), and the types of geotextiles used. To maintain constant maximum dry density throughout the layers, varying compaction energies were applied to different layers. The findings show that with improvement in confining pressure and number of layers, the shear strength of pond ash i.e., value of cohesion (c), angle of shearing resistance (φ), and strength ratio enhanced significantly. [ABSTRACT FROM AUTHOR]

Successful prediction of unsaturated soil behavior is a complicated task as the simultaneous effect of net stress and matric suction on degree of saturation and void ratio needs to be considered. Although, numerous constitutive models are developed to capture the hydro-mechanical behavior, these models either lack sufficient accuracy in simulating the fundamental behavior of unsaturated soils or rely on a formulation that is too complex to yield accurate results. Therefore, there still need coherent theoretical models backed by efficient algorithms for understanding the hydro-mechanics of unsaturated soils. This study introduces such a sound hydro-mechanical model based upon elasto-plasticity including hydraulic hysteresis. The proposed model relies on a relatively straight forward yet powerful formulation compared to similar models of its kind. The model needs material parameters that require only a handful of triaxial tests to obtain. Particular attention is given to the effect of change in void ratio on the water retention behavior. An efficient explicit scheme is utilized in integrating the model, where a number of unsaturated triaxial tests at constant suction and constant water content are simulated under various net stress and suction paths. Verification analyses result in good accuracy showing the capability of the proposed model for unsaturated soils. [ABSTRACT FROM AUTHOR]

The impact of fabric anisotropy, fractal dimension, and breakage on the strength and deformation of granular materials were diminished by uniform-size spherical glass beads. Triaxial drained and undrained tests were performed on glass beads based on a novel method to substitute true triaxial stress paths with conventional triaxial apparatus equivalents with varying intermediate principal stress coefficients (b-values). The result indicates that all specimens manifested a noticeable strain-softening phenomenon. The peak strength decreased with increasing b-value, and the specimens showed more pronounced dilatancy. This pattern is similar to the results of the true triaxial test in current research. Compared to the undrained test, the peak friction angle in the drained test displayed a greater variation with varying b-values, which indicated that the mechanical response of glass beads is sensitive to water. This difference provides experimental evidence for comprehending effective stress in granular materials with constant friction coefficients. The experiments reflect the effect of b-value changes on the p-q stress path, as well as on the peak stress ratio, the state transition stress ratio, and the critical state stress ratio. The specimens exhibited a distinct shear band at different b-values ranging from 0.2 to 0.6, which is different from observations in conventional triaxial tests for granular materials. [ABSTRACT FROM AUTHOR]

The 57-km long, new Gotthard Base Tunnel (GBT) crosses over a 1.1-km long stretch containing kakirites, i.e. weak rocks which, depending on the degree of tectonisation, may exhibit severe squeezing behaviour upon tunnel excavation. This tunnel stretch was already identified as a key problem in the early planning phase, necessitating intensive geological exploration and laboratory investigations over a 14-year period, including research and development in experimental techniques. The laboratory investigations for the GBT provided not only a considerable amount of data about the geotechnical parameters of the kakirites, but also valuable information about their basic behaviour and the adequacy of various testing techniques for this type of rock. Specifically, by using specially designed testing equipment and procedures, it was shown that, (1) conventional rock mechanics triaxial tests cannot provide reliable results for kakirites; (2) the kakirites obey the principle of effective stress in Terzaghi's classical form and consequently it is indispensable to measure the pore pressure during triaxial testing, analogously to the consolidated drained (CD) and consolidated undrained (CU) tests in soil mechanics; (3) the effects of a possible loss of water and desaturation during sampling, transport, storage and preparation of specimens can be successfully mitigated by subjecting the specimens to flow-through before the actual triaxial testing; and, (4) CD tests on partially saturated specimens that are obtained by means of flow-through can provide reliable and reproducible results. This saves a significant amount of time, allows a greater number of tests to be performed, and allows results to be obtained more quickly, which is particularly important for decision-making during tunnel advance.

The present research focuses on developing alternate sustainable base materials for a high-speed slab track. In this study, a series of monotonic triaxial, cyclic triaxial and permeability tests were conducted on four types of materials, viz. mix-A (gravel soil), mix-B (soil mixed with rubber), mix-C (polyurethane foam adhesive (PFA)-treated soil), and mix-D (PFA-treated soil–rubber mixture). The influence of cyclic loading frequency, effective confining pressure, drainage condition and relative density on the deformation, excess pore water pressure, resilient modulus and damping ratio of these different mixes is evaluated. The monotonic triaxial test results indicate that the PFA treatment of mix-A and mix-B increased their shear strength and critical state strength. In contrast, incorporating rubber into mix-A and mix-C helped enhance their ductility. The cyclic triaxial test results show that the PFA treatment of mix-A and mix-B significantly reduced the magnitude of deformation and generation of excess pore water pressure, which caused these untreated mixes to fail prematurely under lower confinement to which a typical base layer is subjected. The influence of cyclic loading frequency and effective confining pressure on the material's response differed for untreated and treated soil. The permeability test results indicate good drainage for mix-D comparable to mix-A. [ABSTRACT FROM AUTHOR]

This paper describes a new simple and inexpensive method for accurately measuring the three-dimensional deformation of soil specimens during triaxial tests. The method incorporates a cost-effective image capture system that can be applied to any triaxial apparatus as well as an automated post-image analysis approach for processing the images acquired during triaxial tests. Image capture is performed using multiple low-cost Raspberry Pi cameras and accessories placed outside the triaxial chamber. The system employs particle image velocimetry, camera calibration and stereophotogrammetry to determine the spatial variation in axial and radial sample strains as samples are loaded. It is demonstrated that axial and radial displacements can be measured with an accuracy of 1–2 μm, respectively, and it is verified by comparison with data from triaxial tests on sand samples that the full nonlinear stiffness–strain degradation of soil samples from fully elastic conditions can be determined without the need for any additional sensors. [ABSTRACT FROM AUTHOR]

Large-scale rock formations, referred to as "rockmasses", consist of intact rock separated by pre-existing discontinuities (i.e., joints). The mechanical behavior of rockmasses is difficult to directly test in the laboratory due to the required specimen scale. Instead, Synthetic Rockmass Modeling (SRM) is often used to simulate field-scale rockmass behavior. SRM requires a calibrated discrete element model (DEM) of intact rock combined with a Discrete Fracture Network (DFN). While the SRM concept has been informally determined to provide reasonable results based on practitioner experience, detailed and peer-reviewed validation is lacking. The goal of this study was to evaluate the predictive capabilities of the SRM method. Previously available data on intact and rockmass analog laboratory specimens of Blanco Mera granite containing DFNs with two joint sets were used as a basis for the SRM created in this study. Specifically, the intact DEM was a Bonded Block Model (BBM), generated to match the grain structure and composition of Blanco Mera granite and the model's input parameters were calibrated so that the behavior of the BBM matched that of the intact laboratory specimens. The predictive capabilities of the model were evaluated by recreating the DFN from the jointed laboratory specimens within the intact BBM and comparing the behavior of the jointed models back to the jointed laboratory specimens, which has not been previously studied in the literature. The BBM was found capable of approximately predicting the behavior of rockmass analog specimens containing a pre-existing DFN without further calibration, which shows potential for the use of SRM in both industry and academia. Specifically, the BBM predicted the strength, dilatancy, and microfracturing behavior of the jointed laboratory specimens. [ABSTRACT FROM AUTHOR]

The adjusted porosity/cement index η/(Civ)a has proved its effectiveness in modeling the strength and stiffness of a variety of artificially cemented soils, being a useful tool for the dosage of soil–cement mixes. In this regard, an analogous mechanical response is expected by establishing a specific η/(Civ)a value, either by increasing the cement content and decreasing the density (or the opposite). Still, the role of this sort of dosage change has not yet been verified for triaxial testing. Particularly, the effect of different dosages assembled at the same η/(Civ)a value has not yet been evaluated considering the effective peak stress parameters. Thus, the present research fulfills this gap by conducting a series of CID triaxial tests in cemented clayey sand. Complementary small strain stiffness tests and unconfined compression strength tests were also carried out. This research comprehended five η/(Civ)a values, considering, at least, three different dosages assembled within each η/(Civ)a. The test data have revealed that both the stiffness and the strength outcomes could be properly correlated to the η/(Civ)a, regardless of the dosage type, resulting in power-type relationships having R2 greater than 98%. Yet, the effective peak stress parameters (ϕ´peak and c´peak) could not be related to the η/(Civ)a index owing to the dosage dependence of the strength mobilization mechanisms. That is, within the same η/(Civ)a, the most cemented and most porous samples presented a cement-dominant response whereas the least cemented and least porous exhibited a density-controlled behavior. [ABSTRACT FROM AUTHOR]

Discrete measurement targets were frequently utilized in image-based methods on the specimen's surface to monitor the soil specimen during triaxial testing. However, the required density of measurement targets that should be used in triaxial testing to achieve highly accurate volume measurement has not been investigated. To overcome this limitation, this paper presents a parametric study to determine the optimum target/point densities to be utilized on the triaxial soil specimen surface to achieve the desired level of volume measurement accuracy in image-based methods. LiDAR scanning was applied to establish the "ground truth" volume of the specimen. The effects of deformation and failure modes were investigated by calculating the volume measurement accuracy at different strain levels and for different undisturbed soil specimens of clay and sand with silt. An interpolation method was proposed to increase the number of discrete targets representing the triaxial specimen's surface. The analysis results show that a higher target density is required at a larger strain. Also, adding the number of interpolation points can only increase the accuracy to a certain level. As the volume measurement accuracy was different for each of the clay and sand with silt specimens, the non-uniform deformation, and failure mode of the specimen can affect the required optimum density of discrete measurement targets. In conclusion, it is recommended to choose the optimum density of targets based on the accuracy requirement, the maximum soil deformation level, and the expected failure mode of the specimen. [ABSTRACT FROM AUTHOR]

An experimental technique is presented to quantify the lateral restraint provided by a geogrid embedded in granular soil at the particle level. Repeated load triaxial tests were done on transparent sand specimens stiffened with geosynthetic inclusions simulating geogrids. Images of laser illuminated planes through the specimens were segmented into particles using a deep learning-based segmentation algorithm. The particle outlines were characterized in terms of Fourier shape descriptors and tracked across sequentially captured images. The accuracy of the particle displacement measurements was validated against Digital Image Correlation (DIC) measurements. In addition, the method's resolution and repeatability is presented. Based on the measured particle displacements and rotations, a state boundary line between probable and improbable particle motions was identified for each test. The size of the zone of probable motions was used to quantify the lateral restraint provided by the inclusions. Overall, the tests results revealed that the geosynthetic inclusions restricted both particle displacements and rotations. However, the particle displacements were found to be restrained more significantly than the rotations. Finally, a unique relationship was found between the magnitude of the permanent strains of the specimens and the area of the zone of probable motions. • Triaxial specimens of transparent sand (fused quartz) with geosynthetic inclusions were tested under repeated loading. • A deep neural network originally developed to segment biological tissue was trained to segment particles of fused quartz. • Lateral restraint provided by the inclusions was quantified in terms of reduced particle rotation and displacement. [ABSTRACT FROM AUTHOR]

Employing Expanded PolyStyrene, EPS has become common in the construction industry, however, their compressibility creates complexities in effective stress analysis. In this paper, the compressibility of EPS beads and the overall compressibility of sand–EPS beads mixtures have been evaluated. Drained and undrained one-dimensional compression tests along with CD and CU triaxial tests were performed on the mixtures to evaluate the classic effective stress equation. Modified versions of the effective stress equation were applied to results and pore pressure correction factors were obtained from both data series based on two different effective stress analysis approaches (volume change and shear strength related) and compared. Results showed that the two sets of correction factors are consistent. Therefore, correction factors obtained based on the compressibility parameters of the mixtures can be used in the effective stress analysis of CU triaxial tests. Pore pressure factors obtained from the oedometer data were used to analyze results of CU triaxial tests. It is shown that a better agreement between CD and CU stress path results is obtained when the EPS compressibility is considered, leading to similar effective strength parameters in both conditions. [ABSTRACT FROM AUTHOR]

Granular materials exhibit time- and rate-dependent behaviour resulting from micromechanical processes at the scale of individual particles. Elastic energy is released during these processes and can be detected as acoustic emissions (AE). Using multistage creep tests under isotropic and anisotropic pressure on medium dense samples of dry silica sand, the relationship between the number of AE events NAE and the axial creep strain Ɛa was determined. In addition, the dependence on the mean pressure p and the deviator stress q was investigated. The experimental results show that the development of AE and axial strain during creep are qualitatively comparable. Within the creep phases both the change in Ɛa and NAE can be described by a logarithmic trend with time. The time-dependent development of both measured quantities exhibit a dependence on q. Moreover, the evolution of NAE with time also shows a pronounced increase with increasing p. A time-dependent power law can be assumed to represent the rates of NAE and the rates of Ɛa with time during creep. The exponent m of the power law is similar for all experiments performed. The initial rates of NAE and Ɛa increase with increasing p as well as increasing q/p-ratio. Finally, a linear correlation between log Ɛa and logNAE was found depending on two state parameters a and b, with a seems to be independent on the stress state.

A deformation measurement system for laboratory soil testing based on stereophotogrammetric image analysis was developed with a purpose of characterising soil behaviour continuously from small to large strains. The system is applicable to triaxial tests with pressure chamber and confining fluid by correcting the refraction-induced distortion of images by ray tracing. In comparison to similar existing studies, the present study put priority in precision and accuracy in relative displacement tracking, which is the key feature in resolving small strain in soil specimen and characterising its stiffness. The displacement tracking is performed with a hybrid algorithm combining Particle Tracking Velocimetry (PTV) for primary target matching and subpixel Digital Image Correlation (DIC) for more precise image matching. The deformation of clay samples with and without end lubrication was compared by visualising the deformation at different levels of strain. This paper is basically a summary of Nishimura (2022). Another example of application in laboratory testing involving fine particle suffusion is also shown. Some notes on implementation of the technique for good practice and on future development are additionally described.

Well-graded soils can be found in nature and in engineered structures, such as dams and embankments. Prediction of their behavior is still an engineering challenge in part due to the lack of data in the literature, arguably due to difficulties associated in testing these soils in the laboratory and in situ. Particularly, there is still debate over the effect of the increased range of particle sizes (i.e., widening gradation) on the shear strength and dilatancy of coarse-grained soils. This paper presents the results of drained and undrained isotropically-consolidated triaxial compression tests on six soil mixes of varying gradation. These soils were sourced from a single natural deposit and selectively sieved and mixed to isolate the effects of gradation from those of particle shape and mineralogy. The results indicate that the critical state lines in void ratio – mean effective stress space move downward as the gradation becomes wider. For the same state parameter, the soils with a wider gradation exhibit greater dilatancy and generate negative excess pore pressures with greater magnitudes than the poorly-graded soils. In drained conditions, the greater dilatancy of the well-graded soils leads to greater peak friction angles, while in undrained conditions it leads to greater undrained shear strengths. The results show that these differences in behavior can only be captured when interpreting the results in terms of the state parameter and normalized state parameter, while comparing the results in terms of the void ratio or relative density obscures the effect of differences in gradation.

Few researchers have attempted to experimentally evaluate the low-strain shear wave velocity (Vs) of specimens undergoing large strain deformations. They report that the Vs is practically unaffected by the strains, and the reasons behind this behavior are not fully understood. This study presents the continuous measurement of low-strain Vs with bender elements (BE) during monotonic shearing of two sand specimens in a triaxial device. The results are analyzed using a micro-mechanical model based on contact theory. The results of this study confirm that the Vs values from BE measurements are unaffected by an increase in axial strains that are induced by a separate mechanism. The micro-mechanical model predictions of Vs agree well with the results of this study and with the results of previous studies. They show that the mean effective stress and increase in inter-particle stiffness controls the low-strain stiffness despite a global increase in strains during monotonic loading. [ABSTRACT FROM AUTHOR]

The drained and undrained response of soft clays reinforced with granular columns has been the subject of numerous geotechnical research efforts to date. Although these studies have been essential for the development of design methodologies, the actual/true response of reinforced clay systems under realistic loading rates is likely governed by partially drained conditions that have yet to be investigated. Partial drainage occurs as a result of inevitable radial flow from the clay to the columns throughout the loading phase. Ignoring the positive effect of partial drainage on the short-term stability of foundations and/or embankments on a clay substratum reinforced with free draining columns may lead to over conservative designs. The objective of this study is to investigate the impact of partial drainage on the response of soft clays reinforced with sand column groups. To address this issue, four partially drained triaxial tests were conducted at different strain rates on normally consolidated soft clays reinforced with dense sand columns in a square configuration. Results included the effect of partial drainage on the total deviatoric stress, excess pore water pressure dissipation, volumetric strain, and stress concentration ratio. The results were bracketed between the fully drained and fully undrained responses/graphs that served as upper and lower bounds, respectively. A plot of the normalized improvement index vs tfailure/t50 showed full mobilization of strength at a time ratio of 15. Numerical modeling using finite difference of the pore water pressure dissipation during shearing was conducted. The consolidation results obtained from the numerical analyses were compared to measurements from the experimental program. The calculated degree of consolidation from the finite difference analysis correlated very well with the measured levels of consolidation during the tests. The results from the test program and the associated analyses presented in this paper contribute to enhancing our current understanding of the response of reinforced clay systems and may be a step on the road towards improving existing design methodologies. [ABSTRACT FROM AUTHOR]

Tailing is an artificial waste material that is created after the extraction of a metal from mineral soils that seriously need to be investigated in various aspects, including from geotechnical engineering point of view. These silt sized materials are mostly influenced by the strain rate effects. For this, a series of triaxial testings has been perfomed on reconstituted saturated zanjan zinc tailing specimens. The tests were done under two confining pressures of 50 and 200 kPa with a relative density of 73% at four different strain rates of 0.5, 0.3, 0.1 and 0.05 mm/min. The testing results showed that as the strain rate increases 10-fold, the amount of the maximum deviator stress increases 1.90 and 1.33 times on average for CU and CD conditions, respectively. Also in both conditions, the higher the strain rate, the higher the friction angle. Moreover, the results showed that with increasing the strain rate, the values of pore water pressure decreased by 0.6 times. By reducing the strain rate 10 times, the volumetric strain increases 3 times on average. It was concluded that the strain rate has effective influence on the shear behavior of the zanjan zinc tailing. [ABSTRACT FROM AUTHOR]

Failure criteria for foam‐grouted gravel under multiaxial loading Foam grouting is an innovative method for improving the mechanical properties of cohesionless, coarse‐grained soils. Despite this, only a few experimental studies have been conducted to assess the failure of foam‐injected soils under multiaxial loading. This paper aims to fill this gap in knowledge. The stiffness and strength of the natural soil, which serve as reference values for the evaluation of the improvement effect due to the injection, are derived from drained triaxial tests. The mechanical behaviour of the foam‐injected soil is investigated using uniaxial, triaxial, and splitting tensile tests. The foam‐injected soil, exhibits significantly higher strength and stiffness but lower ductility than the untreated soil. The ductility of the foam‐injected soil increases with increasing lateral pressure. The splitting tensile strength is only a small fraction of the compressive strength. The adequacy of the failure criteria Mohr‐Coulomb, Hoek‐Brown and Lade for the description of the failure of foam‐injected coarse‐grained soils is verified. All three failure criteria are basically suitable to describe the state of failure in triaxial compression. For compressive and tensile loading, the Hoek‐Brown and Lade failure criteria seem to be more appropriate compared to Mohr‐Coulomb. [ABSTRACT FROM AUTHOR]

Geotechnical stability analyses of mine waste rock (WR) piles require the critical friction angle (ϕ cr) of the coarse blasted rock. However, due to the presence of oversized rock clasts, shear strength can only be characterized on small samples prepared using grading scaling techniques, such as scalping. Thus, considering a testing device able to handle samples of characteristic size D, the material should be scaled down to a maximum particle size d max given by the minimum sample aspect ratio α = D/d max. However, a practical concern about how far the size scale can be reduced while keeping representative results remains a matter of debate in the geotechnical community. International standards do not agree on the minimum recommended α, and its effects on the mechanical behavior remain poorly understood. This paper aims to investigate the grading effects and sample size effects on ϕ cr of WR materials using the scalping technique, to provide insights on the minimum recommended α. Triaxial tests were conducted on loose and dense samples of diameters D = 150 and 300 mm. Samples were scalped from field material having d max = 75 mm, to allow a range of α from 4 to 30. Additionally, one of the world largest in-situ direct shear boxes (120 × 120 × 38 cm3) was developed to test the same WR material. The results show that scalping is an appropriate technique to assess the critical shear strength of WR. The minimum α for ϕ cr assessment in triaxial testing is not sensitive to grading nor sample size, but it is affected by sample density. The aspect ratio was found to be α ≥ 12 and α ≥ 16 for loose and dense samples, respectively. This finding advocates that α values recommended by worldwide standards, such as ASTM D7181-20 , might be too low and should be revisited after comprehensive testing. [ABSTRACT FROM AUTHOR]

The rapid response to the COVID-19 pandemic has resulted in increased municipal waste in the form of used face masks (FMs), which pose a global threat to the environment. To mitigate this, the study explores the applicability of shredded FMs as alternative reinforcing material in sands. Laboratory-grade Ottawa sand and naturally collected sea sand are adopted as the base sands for testing. The primary physical properties of the base materials and the FMs are first examined, and the soil particles are imaged via scanning electron microscopy. Thirty consolidated undrained (CU) triaxial compression tests were conducted to evaluate the effects of the weight fraction of FM, FM length, and the initial effective mean stress on the undrained shear strength parameters of the sands. The experimental results proved that FM inclusion can lead to a substantial improvement in the undrained shear strength of the sands; however, such improvement was sensitive to the initial effective mean stress, with higher undrained shear strength gains associated with lower initial effective mean stress. For a given FM content, the critical state ratio and angle of friction at the critical state increased with the FM length. Finally, the results revealed that FM-reinforced sands exhibit dilative and strain-hardening behaviors. [ABSTRACT FROM AUTHOR]

Understanding the grain-scale processes leading to reservoir compaction during hydrocarbons production is crucial for enabling physics-based predictions of induced surface subsidence and seismicity hazards. However, typical laboratory experiments only allow for pre- and post-experimental microstructural investigation of deformation mechanisms. Using high-resolution time-lapse X-ray micro-tomography imaging (4D µCT) during triaxial deformation, the controlling grain-scale processes can be visualized through time and space at realistic subsurface conditions. We deformed a sample of Slochteren sandstone, the reservoir rock from the seismogenic Groningen gas field in the Netherlands. The sample was deformed beyond its yield point (axial strain > 15%) in triaxial compression at reservoir P–T-stress conditions (100 °C, 10 MPa pore pressure, 40 MPa effective confining pressure). A total of 50 three-dimensional µCT scans were obtained during deformation, at a spatial resolution of 6.5 µm. Time lapse imaging plus digital volume correlation (DVC) enabled identification of the grain-scale deformation mechanisms operating throughout the experiment, for the first time, both at small, reservoir-relevant strains (< 1%), and in the approach to brittle failure at strains > 10%. During small-strain deformation, the sample showed compaction through grain rearrangement accommodated by inter-granular slip and normal displacements across grain boundaries, in particular, by closure of open grain boundaries or compaction of inter-granular clay films. At intermediate and large strains (> 4%), grain fracturing and pore collapse were observed, leading to sample-scale brittle failure. These observations provide key input for developing microphysical models describing compaction of the Groningen and other producing (gas) reservoirs. Highlights: Time-lapse synchrotron micro-CT imaging reveals grain-scale deformation processes for compaction of Slochteren sandstone, from the Groningen gas field At small axial strains, digital volume correlation shows local strains by small displacements along grain boundaries, without intra-granular cracks After yielding, the sample deformed through pervasive grain failure and pore collapse [ABSTRACT FROM AUTHOR]

The investigation of the mechanical behaviour of swelling claystones and shales is challenging because of their very low permeability and of their high sensitivity to changes in water content. The former makes it difficult to carry out triaxial tests with controlled homogeneous water pore pressure fields, and the latter results in some possible effects of swelling when unsaturated extracted specimens are re-saturated prior to being tested. This work presents some data from drained triaxial tests performed on specimens of Opalinus Clay, designated as host rock for radioactive waste disposal in Switzerland, extracted at shallow depth close to the city of Lausen. The data are compared to those recently published from undrained triaxial tests on specimens from the same place, and also to two sets of recent data independently obtained on Opalinus Clay specimens sourced from greater depth at the Mt Terri Underground Research Laboratory. By comparing the data of our drained tests on Lausen specimens to those from specific undrained tests in which swelling has been prevented, quite a good comparability in shear strength is observed at confining effective stresses larger than 5–6 MPa. This is related to the small magnitude of the swelling occurring above this stress during the specimen hydration in drained tests. The question of the possible linearity of the shear strength criterion at low stress is also discussed with respect to both our data and other published ones. It is suggested that undrained tests be also carried out at low confining stresses to investigate whether some perturbations due to hydration swelling occur in this area that could result in a nonlinear shape of the criterion. The change in Young modulus and Poisson coefficient with stress is also discussed, and the data on Opalinus Clay are compared to those of the Callovo–Oxfordian claystone from France, evidencing interesting similarities in terms of shear strength properties and Young's moduli. [ABSTRACT FROM AUTHOR]

The increase in the volume of scrap tires has led to environmental concerns in different locations of the world, particularly in the coastlines. Managing this solid waste, including reuse in civil engineering applications (e.g. geotechnical projects) can be an effective solution to solve the problem. A series of drained triaxial tests was performed on Qeshm calcareous soil obtained from Qeshm Island. Specimens were prepared at loose and dense relative densities and mixed with different percentages of tire crumbs (TCs), including 0%, 10%, 20%, 30%, and 50%. The variation ratio parameter was introduced to evaluate the effect of TCs on the engineering properties of the soil-tire mixture. Based on the results, the stress–strain behavior of the calcareous soil was strongly influenced by TCs contents. As the tire content (TC) increases, the stress transfer mechanism changes from particle-particle to tire-tire, which results in a shear strength reduction. For an example, the addition of 50% of TCs to the calcareous soil resulted in a 64% reduction in the maximum deviatoric stress under a confining pressure of 600 kPa. In addition, the secant modulus at maximum deviatoric strength of the soil-tire mixture with 50% of TCs decreased about 80%. [ABSTRACT FROM AUTHOR]

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New sustainable ground improvement techniques have been recently required for applications in onshore and offshore geotechnical structures. The Microbiological Induced Calcite Precipitation (MICP) method is a ground improvement technique that results from a natural biogeochemical process in order to create a soil cemented framework. The biocementation resulted from this method has been shown to produce a reasonable increment in soil's mechanical properties such as strength and stiffness. This study focused on evaluating the mechanical behavior of a biocemented uniform sand by using the MICP method. Three different cemented levels were produced in a laboratory scale by varying the amount of a chemical solution (cementation solution) percolated through the specimens. The experimental outcome from a set of triaxial tests performed at three different confining pressures (100 kPa, 200 kPa, and 400 kPa) demonstrated that the MICP method produced an increment in strength with a peak in the deviatoric stress when compared to the uncemented soil, changing the cohesion intercept from nearly 0 kPa to 75 kPa. Furthermore, the stress-dilatancy response indicated that a fully structural degradation is achieved at large plastic strains, differently from what is usually observed in chemically artificially cemented blends, like soil–cement and soil–lime. [ABSTRACT FROM AUTHOR]

Particle breakage plays a relevant role on the stress-strain behavior of rockfills. As is widely recognized, the amount of particle breakage of coarse-grained soils depends on a combination of several factors, namely grain size, mineralogy, the shape of the particles, stress level, and degree of saturation and its variation. The results of monotonic triaxial tests carried out on a large apparatus on specimens of two types of rockfills and three different gradings highlighted the role of the coefficient of uniformity, grain tensile strength, and loading path on particle breakage and overall soil behavior. Starting from experimental evidence, a simplified conceptual model to predict the level of breakage during a generic stress path was developed. Experimental data supported the hypothesis of the influence of particle breakage on the position of the critical state lines (CSLs) in the compression plane. This implies that CSL parameters are state dependent, changing with current grading modifications, and are not intrinsic. A simplified procedure for predicting the position of the CSLs as a function of current particle breakage was, therefore, developed and calibrated against laboratory test results. [ABSTRACT FROM AUTHOR]

Gas hydrate filled fractures and veins readily occur in fine‐grained deep water marine sediments that increase sediment strength and restrict sediment consolidation. Subsequent hydrate destabilization can dramatically reduce sediment strength, which may lead to slope failures. To investigate the undrained behavior a series of consolidated undrained (CU) triaxial tests were carried out on fine‐grained soils containing cylindrical tetrahydrofuran (THF) hydrate veins of varying diameter to mimic naturally occurring hydrate‐bearing clays. Axial compressions tests on stand‐alone hydrate veins showed brittle failure with axial stresses reasonably independent of vein diameter and confining stress and thought related to development of bending stresses and tensile cracking. Reduced axial strain rates led to ductile behavior, potentially suppressing tensile crack development, resulting in slightly higher failure stresses. CU shear tests on hydrate‐bearing specimens showed increasing strength and stiffness with increasing vein diameter and confining stress, although the impact of confining stresses reduced for the largest diameter veins. Using a Mohr‐Coulomb failure criteria, increased strength was associated with increase in cohesion and reduction in friction angle. The enhanced strength would reduce consolidation processes that would lead to sediment instabilities if the hydrate was destabilized. However, for larger diameter veins, increasing lateral stresses from the soil reduced buckling stresses with significant plastic deformation of the THF veins being observed at the end of tests, suggesting a strain rate dependent behavior. Further research is required to fully understand this behavior and its impact on sediment consolidation to fully consider the relationship between hydrate dissociation and sediment instability. Plain Language Summary: Gas hydrate, an ice‐like material, grows in marine sediments that which increases the sediments strength and prevents its consolidation. If the hydrate disappears the loss of strength may lead to slope failures. Laboratory tests were carried out to understand the behavior of similar soils containing cylindrical tetrahydrofuran (THF) hydrate veins of varying diameters to mimic natural hydrate‐bearing sediments. Testing of stand‐alone hydrate veins showed that they failed rapidly at similar vertical stresses regardless of vein diameter and confining stresses. This behavior was thought related to bending stresses causing cracking of the vein. Reducing how quickly the stress was applied slightly increased failure stresses. Testing of hydrate‐bearing soil specimens showed increasing strength and stiffness with increasing vein diameter and confining stresses, although the impact of confining stress reduced for the largest diameter veins. This suggests that natural hydrate‐bearing sediments would experience similar behavior and therefore might fail if the hydrate disappeared. However, for larger diameter veins, increasing confining stresses reduced buckling stresses on the vein so that THF veins bent rather than broke. Further studies are required to understand how this might change sediment consolidation which would affect sediment instability if the hydrate disappears. Key Points: Consolidated undrained (CU) tests were carried out to determine the undrained behavior of fine‐grained soils containing cylindrical tetrahydrofuran veinsCU tests showed increased strength and stiffness of the hydrate‐bearing soil with increasing vein diameter and confining stressesIncreasing lateral soil stresses suppress bending stresses in the vein leading to ductile behavior and significant plastic deformation [ABSTRACT FROM AUTHOR]

A sound understanding of the water permeability evolution in fractured shale is essential to the optimal hydraulic fracturing (reservoir stimulation) strategies. We have measured the water permeability of six fractured shale samples from Qiongzhusi Formation in southwest China at various pressure and stress conditions. Results showed that the average uniaxial compressive strength (UCS) and average tensile strength of the Qiongzhusi shale samples were 106.3 and 10.131 MPa, respectively. The nanometre-sized (tiny) pore structure is the dominant characteristic of the Qiongzhusi shale. Following this, we proposed a pre-stressing strategy for creating fractures in shale for permeability measurement and its validity was evaluated by CT scanning. Shale water permeability increased with pressure differential. While shale water permeability declined with increasing effective stress, such effect dropped significantly as the effective stress continues to increase. Interestingly, shale permeability increased with pressure when the pressure is relatively low (less than 4 MPa), which is inconsistent with the classic Darcy's theory. This is caused by the Bingham flow that often occurs in tiny pores. Most importantly, the proposed permeability model would fully capture the experimental data with reasonable accuracy in a wide range of stresses.

Different particle properties, such as shape, size, surface roughness, and constituent material stiffness, affect the mechanical behavior of coarse-grained soils. Systematic investigation of the individual effects of these properties requires careful control over other properties, which is a pervasive challenge in investigations with natural soils. The rapid advance of 3D printing technology provides the ability to produce analog particles with independent control over particle size and shape. This study examines the triaxial compression behavior of specimens of 3D printed sand particles and compares it to that of natural sand specimens. Drained and undrained isotropically-consolidated triaxial compression tests were performed on specimens composed of angular and rounded 3D printed and natural sands. The test results indicate that the 3D printed sands exhibit stress-dilatancy behavior that follows well-established flow rules, the angular 3D printed sand mobilizes greater critical state friction angle than that of rounded 3D printed sand, and analogous drained and undrained stress paths can be followed by 3D printed and natural sands with similar initial void ratios if the cell pressure is scaled. The results suggest that some of the fundamental behaviors of soils can be captured with 3D printed soils, and that the interpretation of their mechanical response can be captured with the critical state soil mechanics framework. However, important differences in response arise from the 3D printing process and the smaller stiffness of the printed polymeric material. Artificial sand analogs were 3D printed from X-ray CT scans of sub-rounded and sub-angular natural sands. Triaxial compression tests were performed to characterize the strength and dilatancy behavior as well as critical staste parameters of the 3D printed sands and to compare it to that exhibited by the natural sands. [ABSTRACT FROM AUTHOR]

Granular columns are widely used as a soil improvement solution adopted in the case of soft clays in an effort to increase their bearing capacity and stiffness. Granular columns, whether sand drains or gravel columns, are designed and constructed in groups and not as individual elements, given that there is a clear need to study the group effects and load sharing under various drainage conditions. Toward this need, a new fully instrumented triaxial test setup was developed and utilized to investigate the overall response of the composite. Particular emphasis was placed on monitoring pore-water pressures and contact stresses over the sand columns and the surrounding clay. The results from clay specimens that were reinforced at area replacement ratios of 17.1% and 30.4% indicate that the test setup is capable of quantifying the distribution of the stresses in the columns and the surrounding clay at different levels of axial strain. As such, previous information about the dependency of the stress concentration ratio on the area replacement ratio, axial strain, rate of loading, and drainage conditions could be inferred. It is anticipated that the insights gained and reported in this paper will facilitate the development of design methodologies for soft clays reinforced with sand column groups. [ABSTRACT FROM AUTHOR]

This paper presents findings from a research study focused on calibrating and improving the robustness of a permanent deformation model for unbound aggregate base/subbase materials, recently developed at the University of Illinois and referred to as the UIUC rutting model. The model provides estimates for permanent strains as a function of load cycles, applied deviator stress, and the ratio of mobilized shear stress to strength. Sixteen aggregate materials were tested for permanent deformation both at their source gradations, and an engineered gradation. Based on multiple linear regression analyses, the UIUC rutting model parameters were determined to estimate the laboratory-measured permanent strains with high accuracy. Constrained and stepwise regression statistical approaches were employed to reduce variability in the regression model parameters and express the model parameters as function of material properties such as shear strength, morphological shape properties, gradation, and compaction characteristics. Results from stepwise analyses indicated that the most accurate predictions could be achieved by considering particle shape properties. Finally, a framework was established to predict unbound aggregate permanent deformation trends of flexible pavement base/subbase layers using the UIUC model parameter equations obtained from stepwise regression, along with the physical/mechanical properties of the aggregate materials determined from standard laboratory tests. [ABSTRACT FROM AUTHOR]

The characterization of the reservoir rock's geomechanical properties is critical to address wellbore instabilities and subsidence-related issues. To address these issues, lab-derived dynamic and static elastic properties are essential to match the in-situ rock properties. In this study, as part of a new integrated workflow P-wave and S-wave velocities are congregated using ultrasonic transducers for the core plugs, which constitutes mainly carbonates, shales, and both. Mineral composition, shale anisotropy, seismic velocities, and cross plots are studied to understand shear wave splitting. During this study, as a part of 1D mechanical Earth models, rock elastic properties are calculated for 60 wells using petrophysical logs (gamma, density, acoustic and caliper). Also, triaxial loading tests are conducted on 14 specimens collected from the same wells, static Poisson's ratio and static Young's modulus are computed from the stress-strain curves. The major differences are observed between static and dynamic elastic properties calculated from well logs and lab tests. Cohesion and friction angle for rock samples are estimated from the triaxial tests under different confining pressures. The objective of this study is to compare the elastic properties derived from the ultrasonic method with well logs and fill the gaps in the 1D geomechanical model. The combined analysis of elastic properties from different methods provides exciting insights on wellbore stability in anisotropic rock. [ABSTRACT FROM AUTHOR]

A significant amount of research has been carried out in recent years to investigate possible options on the reuse of scrap tyres in civil engineering applications. One of the sustainable options is to utilise scrap tyre as tyre shreds/tyre chips (generally called as Tyre Derived Aggregate, TDA) and sand mixture as a lightweight fill material in the construction of infrastructure. Utilising TDA in infrastructure projects has multiple benefits including environmentally sustainable recycling and reuse of the scrap tyre thereby easing the consumption of natural fills, reduced material costs and enhanced geotechnical properties of the soil. Understanding the shear and volume change behaviours of TDA and sand mixture is critical before recommending the mixture as a suitable lightweight-reinforcing structural fill. In this study, the effect of the addition of TDA on the shear behaviour of sand was investigated using large scale direct shear and triaxial apparatus. It has been observed that TDA has significant influence on the shear and volume change behaviours of sand. Also, overall improvements in the soil characteristics, such as enhanced shear strength, can be achieved by the addition of TDA in sand. [ABSTRACT FROM AUTHOR]