Search Results (990)

The mechanical response of unstabilized rammed earth (URE) depends on a chain of factors spanning from soil composition to compaction conditions and specimen geometry and manufacturing conditions. This paper proposes a multiscale experimental framework for the physical and mechanical characterization of URE, structured around three hierarchical scales—soil, fabric and specimen—and demonstrates it on a single soil sample used consistently across more than a decade of experimental campaigns. At the soil scale, mineralogical composition, particle size distribution, Atterberg limits and linear shrinkage are determined. At the fabric scale, Proctor compaction tests establish the optimum moisture content and maximum dry density, and cohesion tests quantify the tensile cohesion of the material. At the specimen scale, monotonic and cyclic uniaxial compression tests reveal that compressive strength is essentially isotropic with respect to loading direction, while stiffness exhibits a pronounced anisotropy, with an anisotropy coefficient of 2.6. A Proctor-based specimen manufacturing procedure is used to reduce the coefficient of variation of compressive strength from 11.8% to 1.8%, demonstrating the critical role of compaction control in result reproducibility. Diagonal compression tests yield a shear strength of approximately 10% of the compressive strength, consistent with the tensile-to-compressive strength ratio commonly reported for URE. The proposed framework highlights the limitations of single-parameter characterization and provides methodological guidance applicable from soil evaluation to full mechanical characterization of URE. Full article

Bentonite slurry is frequently used as a support fluid in the construction of drilled shafts. During the piling process, the slurry acts as a sealant and slightly penetrates the nearby soil. However, the degree to which bentonite slurry penetrates the soil affects the resulting frictional capacity of the bored piles. This experimental study examines the extent of this phenomenon, arising from the formation of what is typically known as the bentonite filter cake or mud cake. The frictional properties of the filter cake are examined through three groups of direct shear tests, employing three pre-cast concrete blocks positioned on a sand layer that has been subject to bentonite slurry for varying durations. To ensure comparison, a similar pre-cast concrete block was utilized in each test series, resulting in uniform surface roughness in the concrete. A handheld surface roughness device was utilized to measure the roughness profile of each concrete block, assessing the surface roughness of all concrete surfaces. The outcomes of the direct shear test performed were subsequently normalized based on the assessed roughness of the concrete surface. Experimental results showed that the friction capacity of the soil–concrete interface for granular materials (“sand–concrete interface”) decreases with longer exposure to bentonite slurry. Specifically, the shear strength is inversely proportional to the square root of the bentonite slurry exposure time. Tests on the internal friction angle of Québec Valcartier granitic sand and the friction angles at sand–concrete interfaces with and without bentonite slurry exposure revealed that the non-exposed sand–concrete interface achieves a peak friction angle equal to 77% of the peak internal friction angle of Québec Valcartier granitic sand. This value represents 69% and 60% of the peak friction angle of the sand tested for bentonite exposure durations of 2 and 4 h, respectively. Full article

With deep coal mining in China, high in situ stress frequently causes severe floor deformation, bolt-cable support failure, and excessive floor heave, which critically threaten mine safety. In this study, we use physical experiments, numerical simulation, and theoretical analysis to explore how hydraulic fractures with different azimuth angles affect stress transfer in roadways under floor dynamic pressure. Prefabricated fractures simulate weak planes induced by hydraulic fracturing. Uniaxial compression tests and PFC2D fluid–solid coupling simulations analyze mechanical properties, failure modes, acoustic emission behavior, and stress distribution. Results show that fracture azimuth significantly controls rock damage and failure modes. As the angle increases from 0° to 90°, failure changes from gradual degradation to sudden instability. Peak strength first decreases then increases, reaching the minimum at 22.5°, while roadway damage is minimal at 45°. Small-angle fractures lead to shear failure with clear precursors, and large-angle fractures cause sudden tensile failure. Hydraulic fractures form directional stress-relief zones and enable effective stress transfer and pressure relief. The results support parameter optimization of hydraulic fracturing and stability control for deep roadways under floor dynamic pressure. Full article

FDO’s wide boiling range and complex composition hinder controlled synthesis of high-performance mesophase pitch. Here, FDO was separated into light, middle, and heavy narrow fractions by vacuum distillation. Multi-scale characterization traced molecular evolution and mesophase development. The light fraction consists of three-ring aromatics with short alkyl side chains and shows the lowest reactivity, yielding limited condensation and poor stacking with isotropic regions and dispersed spheres. The middle fraction contains four-ring aromatics with moderately extended chains, exhibiting enhanced reactivity and undergoing nucleation, growth, coalescence, and disintegration of mesophase spheres. However, insufficient volatiles restrict shear orientation, forming a mosaic texture. The heavy fraction has four-ring aromatics with the longest alkyl chains and the lowest substitution degree, giving the highest reactivity. During thermal cracking, long chains release abundant radicals and volatiles; directional escape generates shear, promoting rapid growth and ordered alignment of aromatic lamellae. At 440 °C for 12 h, this fraction yields high-quality mesophase pitch with small-domain texture, a low softening point (295 °C), and high anisotropic content (98.8%). The pitch shows excellent spinnability, and derived carbon fibers (tensile strength ~1.45 GPa, modulus ~151 GPa) outperform a commercial reference processed under identical conditions. This study reveals molecular-level regulation of mesophase evolution by narrow fraction structures. Full article

The mechanical properties of soil–concrete interfaces directly impact the bearing capacity and structural stability of underground projects. Characterizing mechanical responses and quantifying multi-factor influence mechanisms are fundamental to geotechnical design, numerical simulation, and safety assessment. To reveal the mechanical properties of the unsaturated loess–structure interface, this study conducted a series of direct shear tests on loess–concrete interfaces under varying moisture contents. The effects of interface roughness, soil dry density, normal stress, and soil moisture content on the interfacial shear strength were quantitatively evaluated. The results show 20–35% shear stress variation with dry density, up to 35% shear strength reduction upon wetting, less than 10% shear stress difference due to interface roughness, and normal stress controls, shear stress magnitude, and initial failure sliding displacement. Based on the test results, moisture content was introduced as an additional variable to establish a modified hyperbolic model for unsaturated soil-structure interfaces. This model contains six parameters, all of which can be determined through interface direct shear tests at different moisture contents. These findings advance the quantitative understanding of unsaturated loess–concrete interface mechanics and provide a critical theoretical foundation for the design, numerical analysis, and stability assessment of unsaturated loess–structure interfaces under multi-factor coupled conditions in practical geotechnical engineering. Full article

The increasing demand for sustainable materials in additive manufacturing has driven the development of bioplastics derived from renewable biomass, including microalgae. In this study, the rheological behavior of a 20 wt.% aqueous gel prepared from native Chlorella vulgaris (C. vulgaris) starch, plasticized with 30 wt.% glycerol, was investigated to assess its suitability for extrusion-based 3D printing (direct-ink-writing, DIW). Steady shear analysis revealed a pronounced yield stress (${\tau }_0$ = 271.93 Pa) and strong shear-thinning behavior, described by the Herschel–Bulkley model (K = 59.47 Pa·sⁿ, n = 0.67), indicating structural stability at rest and efficient flow under shear. Oscillatory measurements confirmed a predominantly elastic response, with storage modulus (G′ $\approx$ 13,500 Pa) greatly exceeding loss modulus (G″) and a low loss factor (tan $\delta \approx$ 0.1), demonstrating gel integrity and shape retention. Temperature-dependent analysis indicated enhanced network strength without thermal softening, while thixotropic recovery tests showed rapid structural rebuilding after shear removal. Notably, a $~$50% increase in G′ during recovery highlights improved interlayer adhesion potential. These results show that C. vulgaris starch exhibits the key rheological characteristics required for DIW-type extrusion printing, including yield stress, shear-thinning behavior, viscoelastic stability, and rapid recovery, making it a promising candidate for this application. Full article

The structural strength requirements for timber buildings have been significantly tightened in the second generation of Eurocodes (EN 1990:2023, EN 1991-1-7), which poses a particular challenge for solid timber frames with a beam-and-column structure, where the transfer of tensile forces via dowel connections is inherently limited. Existing multiscale frameworks for timber post-and-beam robustness lack operational detail at each scale, and no validated workflow currently bridges joint-level continuum damage mechanics and frame-level progressive failure analysis in compliance with the second-generation Eurocodes. This paper addresses this gap by proposing an effective two-scale finite element method (FEM) modelling framework for assessing the strength of such frames during column removal. Existing multiscale models describing the strength of timber structures with beam-and-column systems lack the operational details necessary to integrate failure mechanics at the joint level and progressive failure modelling at the frame level within a single, validated workflow. In this paper, this gap is addressed through three specific contributions: a physically modified quadratic Hashin-type failure criterion for timber, which eliminates the non-physical increase in shear strength under combined stress states perpendicular to the grain; a two-scale structure based on the finite element method (FEM), in which the results of continuous damage mechanics at the joint level directly parameterise non-linear joint elements with six degrees of freedom at the frame level, taking into account coupled directional wear and erosion of the elements; and quantitative validation of both scales against experimental data and the conversion factors for characteristic values of the second generation of Eurocode 5 (prEN 1995-1-1:2023). At the connection level, the simulated strength and stiffness values agree with the experiments to within an error of no more than 5%. At the frame level, the model correctly reproduces the non-linear ‘load–displacement’ relationship, the sequence of joint failure, and the axial forces in the chain line for vertical displacements up to 390 mm, which corresponds to experimental observations. Full article

The shear strength–suction (here suction induced from unsaturation) relationship is inherently challenging, yet this difficulty is compounded for cemented crushable sands, whose behavior fundamentally diverges from classical clay-centric theories. This paper presents an innovative teaching case study focusing on colloidal-silica-cemented calcareous sand, based on direct shear tests across a full saturation range (0–100%). Our experimental findings reveal two unconventional characteristics that challenge textbook models: (1) suction strength exhibits a positive dependency on normal stress—an inverse trend to conventional expectations; and (2) strength near desiccation drops below saturated values, contradicting the monotonic increasing function typically observed in clays. A review of 20 existing models confirms that none of them can simultaneously capture both features, highlighting a clear gap in both theory and instruction. To address this gap pedagogically, the core novelty of this work lies in the development of a classroom-friendly predictive model that introduces two physical innovations: first, it incorporates normal-stress-dependent suction strength by modifying capillary condensation probability—departing from constant-angle assumptions; second, it accounts for desiccation-induced strength deterioration through a gel crack size effect, which is absent in conventional unsaturated strength formulations. The model retains clear physical interpretability and demonstrates strong agreement with experimental data. By integrating unconventional behavior, model limitations, and novel physically inspired formulations into a coherent case study, this work equips students not only to recognize deviations from classical unsaturated strength theory but also to construct their own mechanistic models in geotechnical engineering education. Full article

This study investigates the interlayer shear performance of 3D-printed concrete (3DPC) using direct shear tests. Three nominal layer heights, 5 mm, 10 mm, and 15 mm, were considered, and specimens were loaded parallel to the printing path (x direction) and perpendicular to the printing path (y direction). The results show that the interlayer nominal shear strength decreased with increasing layer height. When the layer height increased from 5 mm to 10 mm and then to 15 mm, the nominal shear strength decreased from 9.18 MPa to 7.01 MPa and 4.88 MPa in the x direction, and from 7.87 MPa to 5.29 MPa and 2.68 MPa in the y direction. At the same layer height, the x-direction specimens exhibited higher nominal shear strength than the corresponding y-direction specimens, with increases of approximately 17%, 33%, and 82% for the 5 mm, 10 mm, and 15 mm series, respectively. DIC analysis indicated that tensile–shear damage was the main local failure characteristic. The loading-direction effect was related to different shear-transfer paths: the L-x specimens mainly followed a “continuous filaments-mortar matrix-interlayer bonding” path, whereas the L-y specimens were more controlled by weak interlayer-edge regions and local stress concentration. The effective shear-area analysis showed that the effective bonded area decreased with increasing layer height. After area correction, the corrected shear strengths of the x-direction specimens were 9.18 MPa, 8.76 MPa, and 8.13 MPa for L-5-x, L-10-x, and L-15-x, respectively, while those of the y-direction specimens were 7.87 MPa, 6.61 MPa, and 4.47 MPa, respectively. This indicates that a larger layer height not only reduced the effective bonded area but also weakened filament compaction and bonding quality. The findings provide a mechanism-oriented basis for understanding the anisotropic interlayer shear behavior of 3DPC. Full article

To support the seismic optimization of long-span bridges in regions of high seismicity, this study evaluates the seismic performance of continuous rigid-frame box-girder bridges with different web configurations. A continuous box-girder bridge with corrugated steel webs (CSWBGB) having a main span of 105 m was analyzed and compared with two control models: a continuous box-girder bridge with flat steel webs (FSWBGB) and a conventional prestressed concrete box-girder bridge (PCBGB). Finite element models of the three web types were developed using MIDAS/Civil, and seismic responses were evaluated using the response spectrum method with geometric nonlinearity incorporated; the analyses were conducted under E1 and E2 ground motion intensities (corresponding to a 63% probability of exceedance in 100 years and a 2% probability in 50 years, respectively, as specified in the Chinese seismic design code). Displacement, axial force, and shear force responses were systematically compared among the three configurations. The results show markedly different seismic responses despite the bridges having similar fundamental frequencies. In the longitudinal direction under seismic excitation, the CSWBGB exhibited larger axial displacement than the FSWBGB, yet its peak axial force and shear force decreased by 13% and 18%, respectively, indicating that the greater axial deformation helps relieve internal force demands. Under transverse E1 seismic action, the CSWBGB displayed smaller lateral displacements than both the FSWBGB and the PCBGB. Compared with the CSWBGB, the PCBGB experienced an 11% larger longitudinal displacement and a 43% higher peak axial force, reflecting its relatively limited seismic performance. These findings demonstrate that the CSWBGB not only provides lighter self-weight than the PCBGB but also offers enhanced transverse stiffness, which results in smaller lateral displacements and lower peak shear forces—thus achieving an optimal balance between lightweight design and structural strength. Although the CSWBGB shows strong potential for practical application, its longitudinal displacement response should be carefully controlled in design. Full article

Polymeric geosynthetics serve as fundamental components of engineered composite liners in waste containment facilities. The interface shear behavior between a coextruded textured geomembrane (GMTC) and a bentonite–polymer composite geosynthetic clay liner (BPC-GCL) was investigated under both dry and hydrated conditions, with varying polymer content (0%, 3.5%, and 5.5%), using large-scale direct shear tests. Hydration of BPC-GCL was found to significantly reduce GMTC/BPC-GCL interface shear strengths, with the magnitude of reduction increasing with normal stress. For the BPC-GCL with 3.5% polymer content, the peak strength at 400 kPa decreased by 36% from 272 kPa (dry) to 175 kPa (hydrated), which was attributed to bentonite softening and reduced frictional resistance. Polymer content non-linearly influenced shear behavior. At 400 kPa, the 3.5% BPC-GCL exhibited an 18% higher peak strength than the conventional GCL, while the 5.5% BPC-GCL showed a 9% reduction compared to the 3.5% specimen, attributed to internal structural damage and interfacial lubrication. Visual post-shear inspections revealed that dry conditions promoted interfacial friction-dominated failure, while hydration induced significant internal BPC-GCL damage, including fiber break and bentonite extrusion. The failure mode shifted with polymer content, and conventional GCL failed through internal bentonite deformation, while BPC-GCL exhibited a composite mechanism combining internal reinforcement and interfacial friction, with the 3.5% BPC-GCL demonstrating a more favorable composite effect than the 5.5% BPC-GCL. The study underscored the critical roles of hydration conditions and polymer modification in governing the shear mechanisms and strength at the GMTC/BPC-GCL interface. Full article

Cementitious materials in the fresh state are commonly regarded as viscoplastic. That is, below a given yield stress, they exhibit solid-like behavior, whereas above this threshold, they behave as fluids. In this context, the shear strength of such materials has traditionally been analyzed from a rheological standpoint, considering them as fluids and using time as the primary state variable. From a structural perspective, however, relatively few studies have treated the material as a solid. With the advent of 3D printing technology, this trend has persisted. Within this framework, the present research aims to evaluate the shear strength of a structural mortar for 3D printing in its solid-like regime, by applying the Mohr–Coulomb failure criterion. Furthermore, in a novel approach, the degree of hydration of Portland cement is proposed as a state variable to replace time, enabling a more comprehensive and objective description of the material’s mechanical evolution. Thus, addressing this gap in the state of the art, a chemo-mechanical coupling is developed. To obtain the necessary data, direct shear, uniaxial compression, and isothermal calorimetry tests are performed. The results indicate that the friction angle remains constant, at approximately 33°, and that cohesion, the parameter governing strength gain, exhibits the same linear rate of increase with hydration in both mechanical tests, indicating an intrinsic relationship within the material. Full article

Joints, as inherent weak structural planes within rock masses, interact with bedding planes and govern the stability of layered rock slopes. Numerical models incorporating different levels of joint persistency and bedding dip angles were developed, followed by direct shear simulations under varying normal stresses. The coupled effects of multiple factors on mechanical response and failure mechanisms were systematically analyzed. The results show that shear strength increases with normal stress and decreases with joint persistency, exhibiting pronounced anisotropy. Microcrack evolution exhibits three distinct stages: elastic, initiation, and coalescence. The synergistic evolution of shear cracks along bedding planes and tensile cracks within the matrix primarily drives macroscopic failure. In contrast, tensile cracks along bedding planes and shear cracks within the matrix play a secondary role. The final failure is dominated by the concentration, expansion, and coalescence of shear microcracks, which penetrate bedding planes and form continuous failure zones. The bedding dip angle controls the geometric orientation of microcracks, normal stress governs the failure mode, and joint persistency affects the continuity of the failure path. The combined effects of these three factors determine the ultimate failure pattern and engineering stability of layered rock masses. These findings provide new insights into how joint persistence governs the shear behavior and failure characteristics of layered rock masses, offering both theoretical and technical support for engineering practices such as slope stability analysis. Full article

Against the background of global carbon reduction initiatives and ongoing energy transition, this study addresses the technical challenges of constructing salt cavern storage facilities in bedded salt formations. Typical bedded salt rocks in Southwest China were taken as the research object, and systematic core sampling and multi-dimensional laboratory tests were conducted to investigate their geomechanical and petrophysical properties. The tests included mechanical experiments such as direct shear, uniaxial and triaxial compression, as well as physical property measurements including permeability, porosity, SEM, XRD, and mercury intrusion porosimetry (MIP). The results show that halite exhibits excellent plasticity and tight sealing performance, interlayers have high compressive strength, and mudstone is characterized by significant brittleness. All lithologies possess low permeability and dense internal structures. For this reason, they are well suited for salt cavern energy storage utilization. Furthermore, the research findings provide key basic data and a solid scientific basis. This study supports the construction of salt cavern gas storage and compressed air energy storage (CAES) plants in bedded salt rock areas. Full article

High-purity copper features excellent electrical conductivity but generally low mechanical properties. Adding a three-dimensional graphene network as reinforcement to make a copper–graphene metal matrix composite is promising for a wide range of applications with better mechanical performance and functional capabilities. However, direct application in a metal matrix is difficult due to unfavorable wetting, which causes poor dispersion and weak interfacial bonding in the graphene–metal system. Here, the powder metallurgy method was used to construct a three-dimensional continuous graphene network in the copper matrix combined with high-pressure torsion. Optimized deformation/thermomechanical treatment enhanced the microstructural development processed by the severe plastic deformation method of high-pressure torsion. The primary advantage of this hybrid process is that it enables us to achieve grains with a size in the ultra-fine or even nanoscale. A homogeneous equiaxed nanostructure without segregation was observed during microstructural characterization, with a grain size of ~300 nm. This study investigated structural development during progressive deformation, and the samples were evaluated from the viewpoint of grain size and grain boundaries. The process significantly increased the microhardness of the copper–graphene composite. The tensile strength reached ~500 MPa at room temperature. The interpenetrating structural feature of graphene promoted interfacial shear stress to a high level, whereas plastic deformation increased the dislocation density and grain boundaries, thus resulting in significantly enhanced load transfer strengthening and crack-bridging toughness simultaneously. Full article