A micromechanical study of the equivalent granular void ratio of soil mixtures using DEM

The concept of intergranular void ratio has become more popular in characterising the behaviour of soil mixtures of sand and fine particles up to a threshold transitional fines content. The transitional fines content at which these mixtures change from a sand-dominated to a fines-dominated behaviour is usually defined as the densest mixture. For samples having a less-than-transitional fines content, the fine particles can fall inside the void spaces created by the larger sand particles. Assuming all fine particles are inactive and hence treated as void, the compression curves of different sand-dominated mixtures can be represented by a single curve in term of the intergranular void ratio. More recently an additional ‘b’ parameter was defined as the fraction of the active fine particles out of the total fine content and only the inactive fine fraction was counted as void. The value of b was usually obtained from back-analysis or predicted using a semi-empirical approach. In this numerical study using the Discrete Element Method (DEM), various definitions of the intergranular void ratio e∗ are investigated and discussed, together with the micromechanical data showing the actual involvement of the fine particles in the force transmission. The results show that the value of b is related to the fraction of the fine particles involved in transmitting the strong, larger-than-average contact forces. The value of b is not constant but increases with stress level (decreasing void ratio) and fines content for samples having fines content less than the threshold value

A 2D DEM mono-pile model under combined loading condition

This paper presents a 2D-Discrete Element Method (DEM) model that is used to study situations when vertical, lateral and combined loads are applied to a rigid mono-pile. At present, mono-pile foundations are widely used to support tall and heavy wind turbines, which are subjected to significant wind and wave actions. A safe design must address issues such as rotations and changes in soil stiffness subject to these loading conditions. Design guidance on the issue is limited, as well as the availability of laboratory and field test data. The interpretation of these results in sand, such as the relation between loading and displacement, relies mainly on empirical correlations to pile properties. Regarding numerical models, only data from Finite Element Method (FEM) can be found. They are not comprehensive enough, and most of the FEM results are sensitive to input parameters. Micro scale behaviour, such as movement and densification of discrete particles near a pile could change the mechanism of the soil-structure interaction. A DEM model was used in this paper to study the combined loading behaviour. By explicitly considering the particulate nature of the granular sand around a model pile, the micro-mechanism governing the complex soil structure was investigated. Verification of the DEM model was carried out by comparing simulation data against a model pile. Analyses of the model pile under pure vertical, pure lateral and combined loads are presented. A discussion about the permanent accumulated pile displacements caused by the combined loads is presented, together with analyses on the sand micromechanics. They should offer insights on further research to optimise the design of mono-pile foundations to resist live loads in service

Effect of modulus and dosage of waterglass on early age shrinkage of sodium silicate activated slag paste

In recent years, alkali-activated slag (AAS) as an alternative low-carbon emission and high strength cementitious material has received increased attention. However, high shrinkage and crack tendency of AAS limit its wider industrial applications. Although considerable researches have already been carried out on Portland cement (PC) based systems, the information and understanding on the early age shrinkage of AAS is still limited. On the other hand, early age shrinkage occurs mainly due to a high rate of water evaporation from the surface of fresh concrete. Therefore, this study seeks to explore the potential of two sodium silicate activated slag mixtures variables, namely, waterglass moduli and activator content, on moisture loss and early age shrinkage properties of AAS. A control AAS mix was formulated with water glass (silica modulus of 1.5) as an activator at the dosage of 4% (Na2O equivalent) by mass of slag and fixed waterbinder ratio (w/b) of 0.4. Two mixtures with modulus of 0.75 and activator dosage of 6% were compared in this study. The most significant findings to emerge from this study is that, AAS paste with moduli of 0.75 and activator content of 4% showed lower amount of early age shrinkage. However, further study still needs to be carried out in order to establish a better understanding of the current results

A DEM method for simulating rubber tyres

Recently, recycled rubber tyres were found to be an economical and environmental-friendly reinforcement material in geotechnical engineering. Although the use of rubber tyre-reinforced soil has become increasingly popular, there is still a lack of a robust and systematic method to model rubber tyres when using the discrete-element method (DEM) to investigate the stress-strain responses. In this paper, DEM rubber tyres are simulated by bonding regular-packed balls, and numerically tested under tensile force using the particle flow code in three dimensions. When comparing the effects of different packings on the sample, using Young's modulus and Poisson's ratio, it was found that only body-centred-cubic packing could achieve a Poisson's ratio of 0·5 representing no volume change during the deformation of rubber. The difference between uniaxial compression and tension simulations was also compared as well as the influences of particle overlapping, particle radius and sample aspect ratio on the mechanical response of the tyre model. Finally, the DEM parameters were set to match the experimental Young's modulus data. This proposed DEM rubber tyre strips model could be a basis to study other rubber reinforcements such as tyre chips and shreds, irregular rubber buffings and granulated rubber

Compression behaviour of hydrate bearing carbonate sand - fines mixtures

High gas hydrate content has been found in fine-grained sediments containing substantial amount of foraminifera in the South China sea. One of the possible hydrate accumulation habits is filling the intra- particle voids in the foraminifera. To understand the effects of this hydrate accumulation habit on the compression behaviour of the fine-grained sediment, two series of isotropic compression tests were conducted. Due to high intra-particle porosity, carbonate sand (CS) was mixed with the fines to mimic the hydrate formation in the intra-particle voids in the laboratory prepared soil specimens. The compression test results revealed that the mixtures of fines with as high as 40% CS content can exhibit the transitional behaviour such that non-convergent compression lines are observed at the high stress level. It is evident that breakage of CS grains is negligible in these mixtures. Hence, the initial fabrics are not erased under the high stress level resulting in non-convergent compression lines. The compression curves of the hydrate bearing CS-fines mixtures can be classified into three different stress regimes. There is no significant difference in the compressibility of the soil mixtures with and without hydrate in the low stress regime. As the stress increases further reaching the medium stress regime, the hydrate reduces the compressibility of the soil mixtures with increasing hydrate saturation. Upon reaching the high stress regime, the bond breakage at the inter-particle contacts becomes significant leading to the convergence of compression curves between the hydrate bearing and host soil mixtures. It is also found that a new effective hydrate saturation, representing the amount of hydrate in the inter-particle voids, is better correlated to the compressibility of the hydrate bearing soil mixtures

DEM study on the mechanical behaviours of methane hydrate sediments: hydrate growth patterns and hydrate bonding strength

Natural methane hydrate soil sediments attract worldwide interest, as there is huge commercial potential in the immense global deposits of natural gas hydrate that lies under deep seabeds and permafrost regions. However, the geomechanical behaviour of methane hydrate soil is poorly understood. In this study, Discrete Element Method (DEM) was employed to provide insights into the mechanical behaviour of hydrate-bearing sediments with different hydrate patterns in the pores: the pore-filling case and the cementation case. A series of drained triaxial compressional tests were performed, and the results were analyzed in terms of stress-strain response and volumetric response. In both pore-filling and cementation cases, the presence of hydrates caused an increase in the strength and dilative tendency of the simulated hydrate-bearing soil samples, and the strength and dilation both increased with hydrate saturation (or amount of hydrates in the pores). In addition, at the same hydrate saturation, the cementation case showed higher values of strength and dilation than the pore-filling case. In the cementation case, two typical hydrate growth patterns were considered: soil surface coating (hydrates form around the grain surface) and soil-soil contact gathering (hydrates preferentially form at the grain contacts). Results showed that hydrate growth patterns greatly influenced the mechanical behaviour of the simulated hydrate-bearing samples, especially when the bonding strength and hydrate saturation were increased. In both patterns, strength and dilation were enhanced as bonding strength increased, and the enhancement was greater in the soil-soil contact model than in the soil surface gathering model. At high hydrate saturation, as bonding strength increased, a larger axial strain was needed to reach the peak strength, and the development of dilation was delayed

Sliding stability analysis of a retaining wall constructed by soilbags

Model tests were conducted to analyse the sliding stability of a retaining wall constructed by soilbags. The aim was to obtain an equation that calculates the active resultant earth pressure of sand acting on the wall in the ultimate state. Additionally, shear tests on multi-layers of vertically stacked soilbags were designed to investigate how the interlayer friction resistance varied with the height of the wall. The results show that the active earth pressure acting on the soilbag-constructed retaining wall in the ultimate state is non-linear, but it can be calculated from the force equilibrium of a differential element. The interlayer friction resistance of soilbags is found to be related to the shape of the sliding surface. Based on the obtained equation and the unique shear test results, the sliding stability of the retaining wall constructed by soilbags could be appropriately analyse

Investigation of the ultimate particle size distribution of a carbonate sand

A series of ring shear tests were conducted to investigate the ultimate particle size distribution of a carbonate sand. The tests were carried out under different stress levels, on three types of specimens: 1) uniformly graded specimens made of dry natural sand 2) remoulded specimens of the crushed sand after first shearing to large strains 3) specimens made of natural sand grains but with the same grading as in (2). The first series of tests on type (1), carried out to very large strains, led to apparently stable gradings, distinct for each stress level. Only limited additional particle breakage could be induced by remoulding the specimens after shearing (type (2)) and subjecting them to more shearing. Tests on specimens created at the apparently stable gradings (type (3)) but from the intact sand particles however led to significantly greater breakage. For the three types a stable, fractal grading was achieved. Analyses of the soil particles’ shape showed that the aspect ratio, sphericity and circularity reach a steady value at large strains, in parallel to reaching a stable grading. The mobilized angle of shearing resistance however was not significantly different in the different types of samples, suggesting the final grading dominates the behaviour

DEM investigation of sand response during displacement pile installation

Previous experimental and numerical studies indicate that the stress state of sand at a specific depth changes significantly during the installation of a displacement pile. At a given depth level, the horizontal stress in sand increases as the end of pile approaches and reduces as the pile continues to penetrate and go past the sand element. This horizontal stress reversal, together with the large-strain deformation of the sand at the pile shaft, may cause a reduced accuracy in the calculation of pile capacity. In this paper, the micro-mechanical behaviour of sand developed around pile shaft during the installation of a closed-ended pile was studied using the two-dimensional Discrete Element Method (DEM). Sand assembly was modelled as uncrushable discs, and the closed-ended pile was modelled as a rigid clump which was made of a large number of overlapped discs with a fixed distance. The sand responses in terms of stress, strain and volume changes during the monotonic jacking of the closed-ended pile were investigated. Simulation results revealed micro-mechanical behaviours of the sand in both the interface zone “B” adjacent to the pile shaft and the far field zone “A” away from the pile. It was shown that the sand along the pile shaft at a small normalised distance to pile tip was subjected to a volume reduction as the pile goes past. As the pile drives deeper, the sand at a larger normalised distance to pile tip exhibited dilation. This captured process will give insights to the degradation of shaft friction at a given sand horizon

p38 Mapk signal pathway involved in anti-inflammatory effect of chaihu-shugan-san and shen-ling-bai-zhu-san on hepatocyte in non-alcoholic steatohepatitis rats

Background: Traditional Chinese Medicine (TCM), has over thousands-of-years history of use. Chaihu-Shugan-San (CSS), and Shen-ling-bai-zhu-San (SLBZS), are famous traditional Chinese herbal medicine formulas, which have been used in China, for the treatment of many chronic diseases.Materials and Methods:This study investigated the anti-inflammatory effects of CSS and SLBZS on signaling molecules involved in p38 mitogen-activated protein kinase (p38 MAPK), pathway on hepatocytes of non-alcoholic steatohepatitis (NASH), rats induced by high fat diet. SD male rats were randomly divided into 8 groups: negative control group, model control group, high (9.6g/kg/day)/low (3.2g/kg/day)-dose CSS group, high (30g/kg/day)/low (10g/kg/day)-dose SLBZS group, high (39.6g/kg/day)/low (13.2g/kg/day)-dose integrated group. The rats of NASH model were induced by feeding a high-fat diet. After 16, wks, Hepatocytes were isolated from 6, rats in each group by collagenase perfusion. The liver histopathological changes and serum inflammatory cytokines TNF-α, IL-6 were determined. The proteins of TLR4,  phosphor-p38 MAPK and p38 MAPK involved in p38 MAPK signal pathway were assayed.Results: The statistical data indicated the NASH model rats reproduced typical histopathological features of NASH in human. CSS and SLBZS ameliorated lipid metabolic disturbance, attenuated NASH progression, decreased the levels of TNF-α and IL-6 in serum, as well as inhibited TLR4 protein expression, p38 MAPK phosphorylation, and activation of p38 MAPK. In conclusion, CSS and SLBZS might work as a significant anti-inflammatory effect on hepatocyte of NASH by inhibiting the activation of TLR4, p-p38 MAPK and p38 MAPK involved in p38 MAPK signal pathway.Conclusion: To some extent, CSS and SLBZS may be a potential alternative and complementary medicine to protect against liver injury, alleviate the inflammation reaction, moderate NASH progression.Key words: p38 mitogen-activated protein kinase; Toll like receptor 4; Hepatocytes; Non-alcoholic Steatohepatitis; Traditional Chinese medicine