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This paper explores the micro characteristics of unsaturated sand in triaxial shearing by using X-ray computed tomography (X-ray CT). To obtain higher resolution, a mini-triaxial set-up is designed in which the sample is miniaturised to 1 cm in diameter and 2 cm long, allowing scans with a pixel size of 9 μm. Samples are sheared in the mini-triaxial set-up at different constant suction levels (therefore, different degree of saturation). In the meanwhile, the samples are scanned by X-ray CT at various deformation stages, about 0, 2, 5, 10 and 15% axial strain. The three-dimensional (3D) reconstructed image is trinarised based on a region growing technique, which gives access to the microstructure of the solid, liquid and air phases. Then, the 3D image is subdivided into representative volume elements, with length ≈3·8D50, which gives local information of degree of saturation and porosity. It is observed that the sample dilates and water drains out during triaxial test under constant suction condition. The local study shows that the porosity increase and water desaturation are more significant in the middle part of the sample, especially for a higher suction value. This work allows the emphasis of the coupling between dilatancy on shearing (highlighted by the evolution of local porosity) and the evolution of the local degree of saturation in unsaturated granular materials., info:eu-repo/semantics/published

The critical state is an important concept for saturated and partially saturated granular materials as the strength and volume become constant and unique under continuous shear. By incorporating the water bridge effect, the mechanical behaviours of wet granular matters can be studied by the discrete element method (DEM). A series of DEM simulations are performed following the conventional triaxial loading path for dry and wet granular materials, and different suction values are applied at various confining stress levels. Unique critical state behaviours have been observed in both macroscopic and microscopic scales. It shows that the confining stress level plays an important role in the critical state behaviour of wet granular materials. The critical stress ratio for a wet material is not a constant value at different stress levels, and it is found that both the critical stress ratio and void ratio in wet granular matters are also much higher with a low confining stress. A framework is proposed by considering both the contact stress and the capillary stress effects to model the critical state lines. At large strain, the coordination number, the mean inter-particle force and fabric anisotropies evolve to constant critical state values for both dry and wet materials. The macro-parameters formulating the critical state stress ratio are found to be associated with the critical state anisotropies in solid skeleton and water phase fabrics, respectively.

Increases in seepage force reduce the effective stress of particles and result in the erosion of particles, producing heave failure and piping. Sheet piles/cutoff walls are often employed in dams to control the seepage. In this study, a computational fluid dynamics solver involving two fluid phases was developed and coupled with discrete element method software to simulate the piping process around a sheet pile/cutoff wall. Binary-sized particles were selected to study the impact of fine particles on the mechanisms of seepage. The seepage phenomenon mainly appeared among fine particles located in the downstream side, with the peak magnitudes of drag force and displacement occurring around the retaining wall. Based on the particle-scale observations, the impact of seepage produced a looser condition for the region concentrated around the retaining wall and resulted in an anisotropic condition in the soil skeleton. The results indicate that heave behavior occurs when the drag force located adjacent to the boundary on the downstream side is larger than the corresponding weight of the bulk soil.

Wet granular materials are three-dimensionally simulated by the discrete element method with water bridges incorporated between particles. The water bridges are simplified as toroidal shapes, and the matric suction is constantly maintained in the material. A comparison with experimental tests in the literature indicates that the toroidal shape approximation may be one of the best choices with high practicability and decent accuracy. Mechanical behaviours of wet granular materials are studied by triaxial tests. Effects of particle size distributions and void ratios are investigated systematically in this study. The hydraulic limit of the pendular state is also discussed. It gives the capillary cohesion function which is not only determined by the degree of saturation but also positively correlated to relative density and particle size polydispersity and inversely proportional to mean particle size. Furthermore, the capillary strengthening effect is also analysed microscopically in aid of the Stress–Force–Fabric relationship, mainly in fabric anisotropy, coordination number and stress transmission pattern, which revealed the micro-mechanisms of the additional effective stress induced by capillary effect.

Estimating hydraulic conductivity from particle size distribution (PSD) is an important issue for various engineering problems. Classical models such as Hazen model, Beyer model, and Kozeny-Carman model usually regard the grain diameter at 10% passing ( d10) as an effective grain size and the effects of particle size uniformity (in Beyer model) or porosity (in Kozeny-Carman model) are sometimes embedded. This technical note applies the dimensional analysis (Buckingham's ∏ theorem) to analyze the relationship between hydraulic conductivity and particle size distribution (PSD). The porosity is regarded as a dependent variable on the grain size distribution in unconsolidated conditions. It indicates that the coefficient of grain size uniformity and a dimensionless group representing the gravity effect, which is proportional to the mean grain volume, are the main two determinative parameters for estimating hydraulic conductivity. Regression analysis is then carried out on a database comprising 431 samples collected from different depositional environments and new equations are developed for hydraulic conductivity estimation. The new equation, validated in specimens beyond the database, shows an improved prediction comparing to using the classic models.

This study proposed two pedotransfer functions (PTFs) to estimate sandy soil water retention curves. It is based on the van Genuchten's water retention model and from a semi-physical and semi-statistical approach. Basic gradation parameters of d60 as particle size at 60% passing and the coefficient of uniformity Cu are employed in the PTFs with two idealized conditions, the monosized scenario and the extremely polydisperse condition, satisfied. Water retention tests are carried out on 8 granular materials with narrow particle size distributions as supplementary data of the UNSODA database. The air entry value is expressed as inversely proportional to d60 and the parameter n, which is related to slope of water retention curve, is a function of Cu. The proposed PTFs, although have fewer parameters, have better fitness than previous PTFs for sandy soils. Furthermore, by incorporating with the suction stress definition, the proposed pedotransfer functions are imbedded in shear strength equations which provide a way to estimate capillary induced tensile strength or cohesion at a certain suction or degree of saturation from basic soil gradation parameters. The estimation shows quantitative agreement with experimental data in literature, and it also explains that the capillary induced cohesion is generally higher for materials with finer mean particle size or higher polydispersity.

Pervious concrete piles (PCP) have high permeability and strength. During installation by vibrating-sinking tube construction, liquified soil migrates into PCP, leading to clogging. To study PCP clogging, model tests and X-ray computed tomography analyses were carried out. The clogging depth, porosity, and excess pore water pressure dissipation were characterized. The effect of geotextile on PCP anti-clogging was evaluated. The results show an increase in target porosity and sampling depth significantly increased the PCP clogging depth. A preliminary model was proposed to evaluate the clogging of PCP. The results show that clogging can be solved by surrounding PCP with geotextile.

This paper explores the particle-scale origin of the additional shear strength of unsaturated granular materials in pendular states induced by the capillary effect by applying the stress–fo...

This paper investigates the force transmission modes, mainly described by probability density distributions, in non-cohesive dry and cohesive wet granular materials by discrete element modeling. The critical state force transmission patterns are focused on with the contact model effect being analyzed. By shearing relatively dense and loose dry specimens to the critical state in the conventional triaxial loading path, it is observed that there is a unique critical state force transmission mode. There is a universe critical state force distribution pattern for both the normal contact forces and tangential contact forces. Furthermore, it is found that using either the linear Hooke or the non-linear Hertz model does not affect the universe force transmission mode, and it is only related to the grain size distribution. Wet granular materials are also simulated by incorporating a water bridge model. Dense and loose wet granular materials are tested, and the critical state behavior for the wet material is also observed. The critical state strength and void ratio of wet granular materials are higher than those of a non-cohesive material. The critical state inter-particle distribution is altered from that of a non-cohesive material with higher probability in relatively weak forces. Grains in non-cohesive materials are under compressive stresses, and their principal directions are mainly in the axial loading direction. However, for cohesive wet granular materials, some particles are in tension, and the tensile stresses are in the horizontal direction on which the confinement is applied. The additional confinement by the tensile stress explains the macro strength and dilatancy increase in wet samples., SCOPUS: ar.j, info:eu-repo/semantics/published

Capillarity in wet granular materials induces cohesion and increases the material strength due to the attractive forces acting on capillary bridges. In the funicular state, water bridges may be not only formed between two grains but also binding three or more particles, which breaks the axial symmetry of the liquid bridge. This work presents a fundamental study on capillary forces and rupture behaviours of funicular water bridges between three spherical bodies at equilibrium (or static) configurations. Funicular water clusters are numerically solved by an energy minimization approach. Experimental comparisons are made by measuring capillary forces and these confirm the validity of the numerical solutions. Evolutions of capillary forces and rupture distances are investigated systematically by moving two spheres away from the centre. The fixed water volume condition and the constant mean curvature condition are studied respectively. Comparisons are made between the un-coalesced pendular liquid rings and the coalesced funicular bridge. For a same fixed total water volume, the capillary force is weakened by water bridge coalescence to a funicular bridge when the spheres are packed together, but the situation may vary for different contact angles and inter-particle distances. For the constant mean curvature condition, water bridge coalescence does not alter capillary force significantly when particles are packed closely, but the discrepancy is larger by increasing the gap. Funicular water bridge rupture criteria are also proposed based on the studied configurations. It is observed that in general the transmission from pendular to funicular state extends the rupture distance when it has a relatively high water volume or low air-water pressure difference.

Herein, a new electrochemiluminescence (ECL) strategy for enzyme-free microRNA-21 (miR-21) amplified detection was designed based on target-catalyzed hairpin assembly by combining the signal-amplification capability of both intramolecular and intermolecular ECL co-reaction. In this strategy, two hairpin DNA probes of H1 and H2 were designed as capture probes and detection probes, respectively. To be specific, the capture probes of H1 were immobilized on the multilayer interface of AuNPs and thiosemicarbazide (TSC) assembly on the single-walled carbon nanohorns decorated electrode, while the detection probes of H2 was anchored on the nanocarriers of gold nanoparticals functionalized reduced graphene oxide (Au-rGO) which were tagged with the self-enhanced ruthenium complex (PEI-Ru(ΙΙ)) in advance. Based on the target-catalyzed hairpin assembly, target miR-21 could trigger the hybridization of H1 and H2 to further be released for initiating the next hybridization process to capture a large number of H2 bioconjugates on the sensing surface. Herein, the TSC was used not only as a coupling reagent to attach the AuNPs via Au-S and Au-N bonds but also as a novel intermolecular coreactant to enhance the ECL intensity, and the PEI-Ru(ΙΙ) as emitters exhibited enhanced ECL efficiency. Therefore, a strong ECL signal was achieved by the dual amplification strategies of target recycle and the intramolecular/intermolecular co-reaction of PEI-Ru(ΙΙ) and TSC. The designed protocol provided an ultrasensitive ECL detection of miR-21 down to the sub-femtomolar level with a linear response about 6 orders of magnitude (from 1.0 × 10(-16)M to 1.0 × 10(-11)M) with a relatively low detection limit of 0.03 fM (S/N=3).