Undrained Shear Tests Research Articles

Study on Pore Water Pressure Model of EICP-Solidified Sand under Cyclic Loading

Under traffic load, earthquake load, and wave load, saturated sand foundation is prone to liquefaction, and foundation reinforcement is the key measure to improve its stability and liquefaction resistance. Traditional foundation treatment methods have many problems, such as high cost, long construction period, and environmental pollution. As a new solidification method, enzyme-induced calcium carbonate precipitation (EICP) technology has the advantages of economy, environmental protection, and durability. Through a triaxial consolidated undrained shear test under cyclic loading, the impacts of confining pressure (σ3), cementation number (Pc), cyclic stress ratio (CSR), initial dry density (ρd), and vibration frequency (f) on the development law of pore water pressure of EICP-solidified sand are analyzed and then a pore water pressure model suitable for EICP-solidified sand is established. The result shows that as σ3 and CSR increase, the rise rate of pore water pressure of solidified sand gradually accelerates, and with a lower vibration number required for liquefaction, the anti-liquefaction ability of solidified sand gradually weakens. However, as Pc, ρd, and f rise, the increase rate of pore water pressure of solidified sand gradually lowers, the vibration number required for liquefaction increases correspondingly, and its liquefaction resistance gradually increases. The test results are highly consistent with the predictive results, which show that the three-parameter unified pore water pressure model is suitable for describing the development law of A-type and B-type pore water pressure of EICP-solidified sand at the same time. The study results provide essential reference value and scientific significance in guidance for preventing sand foundations from liquefying.

Undrained triaxial tests of underconsolidated overlying sediments of hydrate deposits in the South China Sea

The South China Sea (SCS) is an important repository for natural gas hydrates (NGHs), where some NGH deposits and their overlying sediments (OSs) are in an underconsolidated state. During the exploitation of NGHs, the stability of the OSs of the NGH deposits is one of the key factors in ensuring mining safety. Especially under different consolidation degrees, the mechanical response and stability of the OSs will vary, which directly affects the safety and efficiency of the NGH mining. This research explores the influence of different consolidation degrees on the mechanical properties of the OSs of NGH deposits in the SCS. The strength and pore pressure characteristics were analyzed through undrained triaxial shear tests. The findings indicate that an increase in consolidation degree enhances the strain softening behavior, and an increase in initial effective stress enhance the strain hardening and shear contraction behaviors of the samples. The failure strength, modulus, and maximum excess pore pressure increase with the consolidation degree and initial effective stress, and there is an approximately linear correlation between failure strength and consolidation degree. Therefore, the exploitation of NGHs in the SCS must comprehensively consider the burial depth and consolidation degree of the OSs. The findings of this study will assist in optimizing mining plans and enhancing safety during the exploitation process.

The role of liquefaction in the evolution of shallow submarine canyon heads from a geotechnical perspective: A case study of the Garrucha Canyon (SE Mediterranean)

This work aims to establish the role of liquefaction in a shallow submarine environment defined by a canyon head reaching the coast. The study area is the Garrucha submarine canyon head, which is located in the western Mediterranean Sea.The potential of liquefaction is approached empirically by two methods in parallel, undrained cyclic direct simple shear (UCDSS) test and piezocone penetration test (CPTu) analyses. For both approaches, considering the regional earthquake records, a cyclic load linked to Mw ≤ 6.5 earthquake events or a maximum ground surface acceleration amax of 0.25 g is considered.The sediment samples analysed are nonplastic sands with low silt/clay contents and can be defined as liquefiable. Geotechnical analysis reveals a high probability of triggering liquefaction in this kind of sediment at depths greater than 3 m below the seafloor. CPTu records are used to assess and improve the liquefaction model for the study area by defining 3 different stratigraphic configurations or liquefiable conditions: uniformly liquefiable, interbedded liquifiable and nonliquefiable.This work highlights the importance of liquefaction—a process normally underestimated in submarine environments—in the downslope transport of sediment from the upper part of a canyon and, more generally, in canyon head evolution with different potential morphosedimentary consequences.

Liquefaction behavior and shear strain-based pore pressure model of fiber reinforced calcareous sand

The generation mechanism of pore pressure plays an essential role in understanding the liquefaction behavior of sand under cyclic loading. Extensive undrained simple shear tests were undertaken to study the pore pressure and shear strain development characteristics of calcareous sand reinforced by fibers. The results show that the deformation patterns of the tested calcareous sand gradually shift from brittle to ductile failure as fiber content increases. The mechanism of pore pressure generation in calcareous sand subjected to cyclic loading is quite distinct from that of siliceous sand, exhibiting more pronounced accumulation in the initial stage of cyclic loading. Fiber reinforced calcareous sand exhibits reverse shear contraction behavior when liquefaction is imminent. A remarkable finding is the establishment of a unique correlation between pore pressure ratio and shear strain, irrespective of the fiber reinforcement. Consequently, a shear strain-based pore pressure generation model of reinforced calcareous sand is then developed to predict the pore pressure built-up trend under varying fiber content and length conditions. This model is also applicable to various testing conditions and soil types.

Study on mechanical properties of unsaturated silt in the Yellow River flood area

The alluvial deposits of the Yellow River led to the formation of less clay content, loose soil particles, and low compactness of the unsaturated silt, which makes it complicated to determine the mechanical properties of the Yellow River flood area soil. To investigate the mechanical characteristics of unsaturated silt, the relationship between unsaturated silt saturation and matric suction is analyzed through the soil-water characteristic test, and the soil-water characteristic curve is fitted by the Van-Genuchten model. A series of consolidated undrained shear tests were conducted on silt in Kaifeng using the GDS triaxial apparatus, the influence of moisture content, matric suction (u a-u w), and net confining pressure (σ 3-u a) on the strength characteristics of unsaturated silt was discussed. The results indicated that the net confining pressure, moisture content, and matric suction significantly influenced the shear characteristics of unsaturated silt. The shear strength of unsaturated silt increased with increasing net confining pressure at the same moisture content. The strain-hardening state of silt at high confining pressure was more obvious, while the silt at low confining pressure showed a weak strain-hardening state. Under a constant net confining pressure, the deviatoric stress versus deviatoric strain curves of silt with low moisture content exhibited a strain-softening state, while other moisture content showed a weak strain-hardening state. At the same matric suction, the shear strength increased with increasing normal stress. The cohesion increased and then decreased while the internal friction angle decreased and then increased with the increasing moisture content and matric suction.

Thermo-mechanical response of sand-silt mixtures

The thermo-mechanical response under undrained conditions is crucial in evaluating the mechanical behaviors of sand-silt mixtures. A series of consolidated undrained triaxial shear tests were conducted to investigate the mechanical response of sand-silt mixtures with different fines contents, temperatures, and initial mean effective stresses. The results showed that the volume shrinkage was observed in all specimens during heating. The amplitudes of volume shrinkage were dependent on the fines content and initial mean effective stress. Moreover, the fines content and temperature significantly influence the peak strength, flow behavior, stress ratio at the undrained instability state, and collapsibility index. Furthermore, a unified critical state line has been established in the compression plane for both clean sand and binary mixture under different temperatures and fines contents. It is reliable and suitable to use the equivalent skeleton void ratio to illustrate the thermo-mechanical response of sand-silt mixtures under undrained conditions.

Strength and deformation characteristics of waste mud–solidified soil

The treatment, disposal, and resource utilization of waste mud are challenges for engineering construction. This study investigates the road performance of waste mud–solidified soil and explains how solidifying materials influence the strength and deformation characteristics of waste mud. Unconfined compressive strength tests, consolidated undrained triaxial shear tests, resonant column tests, and consolidation compression tests were conducted to evaluate the solidification effect. The test results show that with an increase in cement content from 5 to 9%, the unconfined compressive strength of the waste mud–solidified soil increased by over 100%, the curing time was extended from 3 to 28 days, and the unconfined compressive strength increased by approximately 70%. However, an increase in initial water content from 40 to 60% reduced the unconfined compressive strength by 50%. With the increase of cement content from 5 to 9%, the cohesion and friction angles increased by approximately 78% and 24%, respectively. The initial shear modulus under dynamic shear increased by approximately 38% and the shear strain corresponding to a damping ratio decay to 70% of the initial shear modulus decreased by nearly 11%. The compression coefficient decreased by approximately 55%. Scanning electron microscopy and X-ray diffraction tests showed that a higher cement content led to the formation of more hydration reaction products, especially an increase in the content of AlPO4, which can effectively fill the pores between soil particles, enhance the bonding between soil particles, and form a skeleton with soil particles to improve compactness. Consequently, the strength of the waste mud–solidified soil increased significantly while its compressibility decreased. This study can provide data support for dynamic characteristics of waste mud solidified soil subgrade.

Experimental Study on Cyclic Simple Shear Test of Coastal Tidal Soft Soil

Based on undrained cyclic simple shear tests conducted on coastal tidal soft soil under various conditions of cyclic stress ratios and moisture contents, this study investigated the influence of these factors on the dynamic properties of the soil. The findings indicated that with increasing moisture content and stress cycle ratio, the stress–strain hysteresis loop gradually expanded, resulting in a higher strain difference and a transition from a dense to a sparse curve pattern. Moreover, the symmetry of the hysteresis loop was lost in the later stages of shearing. With an increase in the number of cycles, the cumulative shear strain gradually increased, and the increase in the cyclic ratio of water content to stress reduced the number of cyclic shear cycles required to achieve failure, thereby accelerating the soil’s failure rate. A predictive formula was developed based on the experimental results to estimate the failure cycles as a function of the cyclic stress ratio and moisture content. Furthermore, the softening index decreased gradually with an increasing number of cycles, and a higher moisture content and cyclic stress ratio accelerated the soil’s softening process. It was observed that under the conditions of optimal moisture content, the soil exhibited a slower softening rate during the initial stage of shearing.

Experimental study on the influence of cyclic loading frequency on liquefaction characteristics of saturated sand

A series of undrained cyclic torsional shear tests was conducted to investigate the effect of cyclic loading frequency on the liquefaction characteristics of saturated sand using a hollow cylinder apparatus. The test results show that the dilative and contractive tendencies of various saturated sands are not only related to the physical properties of the sand, but are also affected by loading frequency. Under low-frequency loading, the saturated sand has a dilative behaviour, excess pore water pressure fluctuates after initial liquefaction and the soil maintains the ability to resist liquefaction to some extent after the initial liquefaction. The liquefaction mode in terms of the stress–strain relationship generally performs as the ‘cyclic mobility’. However, under high-frequency loading, the saturated sand has a contractive behaviour, and excess pore water pressure generally remains stable after the initial liquefaction. The liquefaction mode in terms of stress–strain relationship generally exhibits as ‘cyclic instability’. The deformation caused by low-frequency loading is significantly larger compared with that caused by high-frequency loading. At higher loading frequencies, the phase transformation stress ratio increases with the increase of loading frequency, and gradually approaches the failure stress ratio.

Experimental Study on Enhancing the Mechanical Properties of Sandy Soil by Combining Microbial Mineralization Technology with Silty Soil.

Currently, coastal sandy soils face issues such as insufficient foundation strength, which has become one of the crucial factors constraining urban development. Geotechnical engineering, as a traditional discipline, breaks down disciplinary barriers, promotes interdisciplinary integration, and realizes the green ecological and low-carbon development of geotechnical engineering, which is highly important. Based on the "dual carbon" concept advocating a green and environmentally friendly lifestyle, Bacillus spores were utilized to induce calcium carbonate precipitation technology (MICP) to solidify coastal sandy soils, leveraging the rough-surface and low-permeability characteristics of silty soil. The mechanical-strength variations in the samples were explored through experiments, such as calcium carbonate generation rate tests, non-consolidated undrained triaxial shear tests, and scanning electron microscopy (SEM) experiments, to investigate the MICP solidification mechanism. The results indicate that by incorporating silty soil into sandy soil for MICP solidification, the calcium carbonate generation rates of the samples were significantly increased. With the increase in the silty-soil content, the enhancement range was 0.58-3.62%, with the maximum calcium carbonate generation rate occurring at a 5% content level. As the silty-soil content gradually increased from 1% to 5%, the peak deviator stress increased by 4.2-43.2%, enhancing the sample shear strength. Furthermore, the relationship between the internal-friction angle, cohesion, and shear strength further validates the enhancement of the shear strength. Silty soil plays roles in adsorption and physical filling during the MICP solidification process, reducing the inter-particle pores in sandy soil, increasing the compactness, providing adsorption sites, and enhancing the calcium carbonate generation rate, thereby improving the shear strength. The research findings can provide guidance for reinforcing poor coastal sandy-soil foundations in various regions.

Effect of Moisture Content and Wet–Dry Cycles on the Strength Properties of Unsaturated Clayey Sand

Based on the actual situation of the project on the Weihai–Yanhai Expressway section of Rongwu Expressway, the effects of water content change and the dry–wet cycle on the mechanical behavior of unsaturated clayey sandy soil were analyzed in this study. In this study, ventilated undrained triaxial shear tests were carried out on unsaturated clayey sandy soils with different water contents (6%, 8%, 10%, 12%, 14% and 16%). Concurrently, the soil samples were subjected to three distinct wet and dry cycle pathways (2~22%, 2~12%, and 12~22%) to gain an understanding of how the mechanical features of the soil changed under the different conditions. The test findings demonstrate that when the water content increases, the unsaturated clayey sandy soil’s cohesiveness and shear strength diminish. The strength of shear decline exhibits a pattern of first being quick, followed by sluggish. The strength of shear and cohesiveness of clayey sandy soil declined under the influence of the dry and wet cycles, with the first cycle primarily affecting variations in cohesiveness and strength of shear. Furthermore, the strength of shear and cohesiveness of clayey sandy soil diminish more with increasing wet and dry cycle amplitude and upper water content limits. Lastly, the drying shrinkage and hygroscopic expansion of clay particles in clayey sandy soils during wet and dry cycles are not significant, resulting in less structural damage and deterioration of the mechanical properties of the soils. The study’s findings have a significant impact on the durability of roadbeds made of unsaturated clayey sandy soil in both wet and dry situations.

Static and dynamic strength properties of highly structured clay-rich diatomite in Shengzhou, China

Diatomite is a widespread geological substance found in coastal and lake areas worldwide. Although diatomites are increasingly used in geotechnical engineering construction, a significant knowledge gap exists regarding their mechanical responses to static and dynamic loadings, particularly in high-speed railway projects. This study investigates the static and dynamic behaviors of diatomites through extensive experiments, considering both natural and remolded states. It focuses on representative diatomite specimens obtained from the Shengzhou region in China, an area intersected by the Hang-Shao-Tai high-speed railway. The results of uniaxial compression and undrained triaxial shear tests show that the structural degradation of diatomite significantly impacts its stress-strain responses, failure patterns, and mechanical parameters under static and dynamic loading conditions. The structural effects of natural diatomite tend to degrade as the confining pressure level increases, resulting in corresponding changes in its mechanical parameters. Undrained triaxial shear tests conducted on natural diatomite reveal that the inclinations of the shear band closely correspond to those predicted by the Mohr-Coulomb solution. Additionally, we present a modified hyperbolic-type model to evaluate the damping ratio of diatomites. Furthermore, a power-law model based on Hardin's formula was used to predict the maximum shear modulus of diatomites under varying stress levels and void ratios. Microstructural and chemical analyses were conducted using scanning electron microscopy and X-ray diffraction to gain insights into the observed structural effects at the microscale.

On the undrained shear behavior of sand mixtures with non-plastic fines subjected to large shear displacement

Fluidized landslides pose significant hazards owing to the catastrophic failure and flow-like behavior with high mobility and long traveling distances. Non-plastic fines can be abundant in many fluidized landslides with 30%–40% content of relatively fine-grained particles. However, the role of fine particles in the initiation and movement of these landslides is not well understood. To investigate the effects of non-plastic fines content (FC) on the undrained shear behavior of the mixture, undrained ring shear tests were performed on mixtures of silica sand (D50 = 0.16 mm) with varying weight proportions of non-plastic fines (D50 = 0.035 mm) with different initial densities. We showed that liquefaction potential increased as fines content increased and initial density decreased. Peak strength (τp), steady-state strength (τss), and total dissipated shear energy to the steady state (Wss) increased against increasing density of the sample. Given the same initial density, τp, τss, and Wss decreased with the increases of FC. Using intergranular void ratio (es) and equivalent intergranular void ratio (es⁎), we found that plots of τp, τss, and Wss against es⁎ merged to a relatively narrow domain, indicating es⁎ can serve as a comprehensive indicator for evaluating the FC's effects on undrained shear behavior. An optimum es⁎ existed for the mixtures, which corresponds to the minimal brittleness index IB, that is used for analyzing liquefaction associated with post-failure behavior. The Wss in the log scale continuously decreased with increasing es⁎. We inferred that the addition of fine particles could change the inter-particle force transfer. When the FC increases, a greater proportion of fines can actively engage in the forces transferring structure as active fine particles, resulting in increasing liquefaction potential and reducing the energy dissipation throughout the entire shearing process of the saturated mixtures, thereby facilitating the fluidization and mobility of landslides. This study provides an evaluation of the full liquefaction process of fines-rich mixtures and further explains the fines' role in the high mobility of fluidized landslides based on the energy dissipation view.

Deformation and Strength of Unsaturated Loess—Hydraulic Coupling Effects under Loads

The volumetric change in unsaturated loess during loading causes serious damage to the foundation and structure, accompanied by changes in hydraulic conditions. Therefore, quantifying the change in the load effect of loess under hydraulic coupling is of great significance for revealing the mechanism of hydraulic interaction. This study conducts isotropic compression and undrained shear tests on unsaturated compacted loess, simultaneously introducing the strength parameter η to enhance the Glasgow coupled model (GCM). The objective is to elucidate the hydraulic and mechanical coupling mechanism, where saturation increases under mechanical effects lead to strength degradation. The results show that saturation increases under mechanical effects improve the compressibility of the sample, and saturation has a direct impact on the stress–strain relationship. The increase in water content and confining pressure increases the trend of the critical state stress ratio M decreasing, and the strain softening trend increases. The compression of volume during shear tests increases the saturation, changes the hydraulic characteristics of loess, and affects the deformation and strength of loess. The modified GCM improves the applicability and prediction accuracy of unsaturated loess under the same initial state. The research results are of great significance for revealing the hydraulic and mechanical behavior of loess.

Effect of thermal cycling on the isotropic consolidation and triaxial shear behavior of lateritic clay

It is crucial to comprehend soil thermomechanical behavior while designing underground energy structures to ensure safety. Studies on the soil response to thermal cycles in terms of the generation of thermal-induced volume change and pore water pressure are rare, and relevant research on how these responses might affect soil consolidation parameters and shear strength is very limited. To experimentally investigate the effect of thermal cycling under drained and undrained conditions on the isotropic consolidation parameters and triaxial shear strength of lateritic clay, this paper employs a temperature-controlled triaxial apparatus to conduct a series of isotropic mechanical consolidation and thermal consolidation tests, as well as undrained triaxial shear tests. The thermal response in volume change and pore water pressure are discussed, and the changes in the consolidation parameters, the preconsolidation pressure, and the shear strength are identified. It is concluded that increments of irreversible contraction of lateritic clay are observed during thermal cycling under drained conditions and further lead to a slight increase in the preconsolidation pressure. Nevertheless, thermal cycling hardly affects the swelling and compression index. The shear strength increases after being subjected to thermal cycling under drained conditions, which can be attributed to the increase in cohesion. When drainage is not allowed during thermal cycling, the generation of pore water pressure occurs during temperature variations and completely dissipates after the thermal cycling phase, and its reversibility is unaffected by the stress level and number of cycles. Furthermore, thermal cycling has little effect on the consolidation parameters, preconsolidation pressure, and shear strength. This study provides new insights into the mechanisms controlling the response of clay to thermal cycling.

A novel unresolved/semi‐resolved CFD‐DEM coupling method with dynamic unstructured mesh

AbstractThe greatest challenge when performing large deformation simulations using the CFD‐DEM coupling method lies in the dynamical update of the fluid meshes. To address this problem, a novel CFD‐DEM coupling method integrated with the dynamic unstructured grid is proposed in this work. The mesh initialization and reconstruction are performed by the Constrained Delaunay triangulation (CDT) implemented by the tetgen algorithm. Moreover, the equation of state (EOS) is introduced to consider the impact of fluid compressibility. The reliability of this coupling method is preliminarily verified by the particle sedimentation test. Additionally, undrained triaxial shear and one‐dimensional consolidation tests are conducted to examine the applicability of the proposed method in simulating geotechnical cases. From our analysis, it is found that the shear responses in the undrained triaxial shear test obtained from the proposed method with dynamic mesh are in direct accordance with those led by the constant volume (CV) method. However, difficulties arise in accurately describing the moving boundary by the fixed mesh, leading to unreliable results. Moreover, in the one‐dimensional consolidation test, both the dynamic and fixed grids can capture the Mandel—Cryer effect, which is more pronounced under the dynamic grid, mainly due to the dual impact of the loading device and particle skeleton on the fluid. A greater compressibility specified under the dynamic grid led to a weakening of this effect, whereas its duration is increased. Our work provides valuable insights for effectively dealing with dynamic fluid boundaries.

Effect of gradation and relative density on shear strength of coral sand

ABSTRACT Due to the wide particle size distribution of coral sand, There are significant differences in mechanical behaviour of coral sand with different particle size distribution. It is of great significance to propose a quantitative analysis method for mechanical properties considering the effect of gradation. In this study, 27 groups of coral sand specimens with particle size ranging from 5 mm to 0.125 mm were prepared. The consolidated undrained (CU) triaxial shear tests were carried out and the influence of gradation and relative density on the shear characteristic were discussed. The grading and relative density of coral sand are controlled by the median particle size (d50) and relative density (Dr), The results show that the peak deviator stress decreased, and the effective internal friction angle first increased and then decreased with the increase of d 50. Both the peak deviator stress and effective internal friction angle of coral sand increase with the relative density. A parabolic relationship was founded between the effect internal friction and d 50 as D r was constant. The effective friction angle peaked when d 50 was 2 mm. Finally, a calculation method of shear strength parameter considering the gradation and relative density of coral sand was proposed.

Effects of initial static shear on liquefaction behaviour of Toyoura sand in undrained multi direction dynamic cyclic simple shear tests and DEM

Effects of initial static shear on liquefaction behaviour of Toyoura sand in undrained multi direction dynamic cyclic simple shear tests and DEM

Characterisation of the behaviour of liquefaction triggering in a cyclic undrained simple shear test setting

Characterisation of the behaviour of liquefaction triggering in a cyclic undrained simple shear test setting

Evolution of excess pore water pressure in undrained claystructure interface shear tests

Recent studies focused on the shear behaviour of clay-structure interfaces have shown the importance of the shearing rate on the strength of these interfaces. In normally-consolidated clays, increasing the shearing rate results in a decrease in the interface strength, while the trend is opposite in heavily overconsolidated clays. While analytical and empirical interpretation methods indicate that the generation of shear-induced excess pore pressures are responsible for theaforementioned trends, experiments with pore water pressure measurements at the clay-structure interface are rare. In this paper, we first describe a modified interface shear box testing setup that is equipped with a pore water pressure sensor. For this equipment, the fully rough structural surface was manufactured with a port at the centre of the clay-surface interface to measure the pore water pressure. We present the results of undrained clay-structure interface tests on normally consolidated (NC) and overconsolidated (OC) specimens of kaolin clay. The results agree with the expectations, where the NC specimens generate excess pore pressures with greater magnitudes and heavily OC specimens generate negative excess pore pressures. Measurements of the pore water pressures allow calculating vertical effective stresses, which can be used to investigate the effective stress paths followed by the clay-structure interface during the tests. This paper also provides a comparison of the measured values of beta and adhesion factors with previously published results and relationships used for the design of deep foundations.