Sand-non plastic fines mixtures show a transitional mechanical behaviour as the fines content increases from a low level to a high level. The transition fines content, which is the boundary between sand-dominated and fines-dominated behaviour, can be found by simple laboratory tests e.g. maximum/minimum void ratio or stress path tests e.g. location of the normal compression line. The small-strain stiffness of sand-fines mixtures is supposed to have a similar transitional trend when the fines content is high enough to reach the transition fines content. A series of tests with local LVDTs was performed on samples of Leighton Buzzard sand mixed with non-plastic silt. The tests were performed during isotropic compression and shear tests in a triaxial apparatus. The fines content range was large enough to determine how the small-strain stiffness changes with the fines content. The value of transition fines content found from the triaxial tests are compared with those found from the bender element test results. Possible similarities and/or differences are discussed.

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This paper presents a multiscale study on inherent anisotropy and strain localization in granular soils. We employ a newly developed hierarchical framework coupling FEM and DEM. The framework features a hierarchical coupling scheme to solve a boundary value problem by FEM while deriving the constitutive relation required at each FEM Gauss point from the DEM solution of the embedded representative volume element. It helps circumvent the need for phenomenological constitutive assumptions and provides direct correlation between macroscopic observations and microscopic mechanisms. The study employs elongated particles for the local DEM packings to simulate inherent anisotropy in sand. By varying bedding plane angles (particle orientations), specimens with different inherent anisotropies are generated and are then sheared with smooth loading platens under drained biaxial conditions. A strong correlation is found between inherent anisotropy and strain localization, and fabric evolution underpins the final pattern of shear band since both type-a and type-b shear bands are observed in the simulations. In particular, non-coaxiality is identified as a direct attributor to break symmetry and trigger strain localization in a sample, while the formation process of shear band hinges crucially on the interplay between two sources of fabric anisotropy, particle orientation based and contact normal based. Fabric anisotropy of the former source is more difficult to mobilize than that of the latter. Cross-scale analyses indicate the DEM packings inside the shear band undergo extensive shearing, fabric evolution and particle rotation and may reach the critical state, while those located outside the shear band may experience unloading.

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Almost 30 years have passed since the first oil was produced from the fractured granite basement offshore Southern Vietnam there are still many questions left to be answered for this special type of reservoir and its fracture system characterization. In this study, we tried to investigate the contributions of micro-fracture and macro-fracture porosity by revisiting and deriving the coefficient of porosity partitioning and applying for a case study in the Cuu long basin

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Although ground is inherently heterogeneous, layered geological structure is generally accepted. Due to the differential weathering process, geological structure is sometimes unexpectable and contains high uncertainties in it. Such micro heterogeneous geological system greatly affects macro tunnel behavior during and after construction, as well as in the long-term temporal responses. The geotechnical properties commonly used in numerical modeling tend to be representative or average of global-scale properties even though geotechnical properties are spatially distributed. Furthermore, the uncertainty and spatial variation of the geotechnical properties increase as the tunnel scale increases. Consequently, the probability of failure also increases. This study investigates the micro soil properties on macro tunnel deformation behavior. The numerical results show that the deformation around the tunnel increases with an increase in the variation of the geotechnical properties. It is found that the spatial variability of soil can affect the convergence of tunnel and also it can cause the increase in tunnel support pressure. The shear failure mechanism of tunnel can be significantly affected by a spatial correlation length. In order to achieve structural stability of tunnels, the design framework must take into account the quantitative effect of variation of soil properties on tunnel behavior.

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Breakage in granular material has gained extensive research attention recently. This grain scale behavior would cause significant change to the macroscopic response. The development of x-ray micro-tomography technique makes it possible to non-destructively examine the microstructure of an assemblage at different stages of loading. In this study, micro-focused X-ray CT scanned images are used to visualize the development of grain crushing under one-dimensional compression. Initially uniformly graded carbonate grains are prepared into a tailor-made oedometric cell such that X-ray CT scanned images can be taken at different stages (loading and unloading) of the test. Image processing is carried out to isolate individual grains from the assemblage. The evolution of grain size and shape, and void size is examined based on the processed 3D images. The results show that the crushed grains are more rounded. The void space becomes smaller and varies within a narrower range as the compression continues.

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In the present study, water-retention behaviour of sand during drying and wetting process has been observed microscopically using micro x-ray computed tomography (CT). CT images taken at different water-retention states during drying and wetting paths are trinarised to segment the images into the soil, water and air phases. The segmentation technique using region growing method takes into account uniform distribution of the mixels to reduce the inherent partial volume effect. Using the trinarised images, local porosity and degree of saturation are calculated for the cubic subsets centered at reference points, and then frequency maps of the two local values are drawn. Through the comparison of the frequency maps, the relation between the local porosity and the degree of saturation and the changes in the relation during drying and wetting process are discussed.

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