Micromechanical interpretation of fines content effect on the K0-value of granular soils

This paper summarises the details of an experimental work and numerical simulation on the bearing capacity of geotextile-reinforced granular soils with different fine grain contents (10 and 15%) which are mostly used in pavement design. The effects of position and number of geotextile layer(s) on the bearing capacity of reinforced specimens were investigated. The standard laboratory California Bearing Ratio (CBR) test was conducted to investigate the load-penetration behaviour of the unreinforced granular soils as well as reinforced ones with nonwoven geotextile layer(s). The reinforcements have been placed in samples in seven different scenarios (one, two and/or three layers of reinforcement in the bottom, middle and/or top compacted soil layers). The results indicated an increase in bearing capacity for most of the scenarios due to placing the geotextile layer(s); however, it was seen that increasing the number of reinforcement layers will not necessarily increase the reinforced soil mass bearing capacity. Also it was found that the efficiency of placing the reinforcement layers in order to increase the bearing capacity is higher for the soil mass with higher fine content in compare to the soil mass with lower fine content; even in some scenarios for the soil with lower fine content, the geotextile would decrease the bearing in compare to the unreinforced sample. Besides this laboratory testing, Finite Element (FE) software was used to simulate the CBR tests. The FE analysis results showed that the ratio of predicted to measured CBR value is varied between the range of 1.06–1.20 for the soil with lower fine content and 0.86–1.086 for the soil with higher fine content. After the FE model was validated, it was tried by changing the properties of the nonwoven geotextile with a woven one in software to make a comparison between two reinforcement types.

In order to study the impact of internal erosion at the scale of an engineering structure, a hydro-mechanical continuous modelling approach considering suffusion is needed. It requires a relevant mechanical model for granular soils considering the -dependency ( fines content) and a hydraulic model for suffusion to control the changes in the fines content. To this purpose, the mechanical models for granular soil, the unified modelling approaches for -dependency of granular soils and the hydraulic modelling of suffusion are first reviewed. Then, a hydro-mechanical model considering both suffusion and mechanical loading is developed by combining the three components. For each component of the model, alternative choices are provided. Simulations of laboratory tests as well as an example of a dike-on-foundation problem demonstrated the reliability and applicability of this coupled numerical approach for internal erosion problems.

Seepage-induced failure may disable the bearing capacity of foundations in dams and embankments. However, the evolution mechanism of the seepage failure process in granular soils is not well understood. In this paper, a series of laboratory hydraulic tests were performed to investigate the seepage failure process in sandy gravels and fine-grained sands. Seepage behaviors of the hydraulic gradient, seepage flow velocity, and permeability coefficient were observed, and then, the Reynolds number was obtained to describe the seepage regime. By linking the hydraulic gradients with the Reynolds number, the seepage failure process was quantitatively divided into four phases: (i) incubation ( Re < 0.85 ), (ii) formation ( 0.85 ≤ Re ≤ 5 ), (iii) evolution ( 5 < Re ≤ 50 ), and (iv) destruction ( 50 < Re ). The findings of the study identified an approximately linear relationship between the hydraulic gradient and the seepage velocity in the phases of incubation and formation in which the viscous drag effects are not negligible, corroborating Darcy’s view. However, in the phases of evolution and destruction, the hydraulic gradient and the seepage velocity are nonlinearly related, indicating that the inertial force plays a leading role, and the quadratic equation is relevant for the regime transition from laminar flow to turbulent flow. Finally, the mechanism of each phase in the seepage failure process was clarified. Fine content and uniformity coefficient are internal factors that affect the potential of seepage failure, while the seepage force that drives the transport of fine particles is an underlying cause that promotes the development of seepage failure. This study will be quite useful in identifying the limits of applicability of the well-known “Darcy’s law,” in further improving the physical modelling associated with fluid flow through granular soils.

The aim of the current study is to provide evidence regarding the influence of particle size on the dynamic properties in granular materials. Experiments using a resonant column (RC) device were conducted on glass beads having similar coefficient of uniformity, but four different mean grain sizes to describe the tendencies in the magnitude of Gmax and Emax. It can be clearly seen that for similar particle gradation the influence of particle size is significant, in particular for larger mean grain sizes d50 exceeding 1·5 mm. The experimental data were fitted using existing empirical relationships with corresponding fitting parameters. The fitting parameters describing the void ratio and pressure dependence to estimate maximum stiffness were found to depend on grain size to a certain extent. This influence was diminished for the materials with a d50 < 1·5 mm. It was therefore possible to use a unique set of fitting parameters for these materials with a satisfying degree of accuracy. The results were compared with some previous studies to highlight the size dependency of Gmax and Emax for materials with mean size greater than 2 mm, but also the relatively small influence on materials with lower mean particle sizes (d50 < 2 mm) is acknowledged. The results are furthermore discussed through a micromechanical interpretation highlighting why larger grain sizes influence stiffness values.

An insight into the mechanical behavior of binary granular soils

Influences of buried depth and grain size distribution on seepage erosion in granular soils around tunnel by coupled CFD-DEM approach

It is well known that saturated silt and fine sand are susceptible to instability during undrained shearing. Coarse granular soils may also be unstable during undrained shearing or during rainfall infiltration starting at an unsaturated state. This paper investigates the instability of three saturated and unsaturated granular soils with different fines contents (f=50,32,and 10%). Isotropic consolidation tests and mercury intrusion porosimetry tests were first conducted to investigate the instability of the microstructures of these granular soils. Then drained and undrained triaxial tests were conducted to investigate the instability behavior under the saturated condition. Subsequently, wetting tests under the constant shear stress condition were carried out on unsaturated specimens to investigate the instability of the same soils during water infiltration. The instability in the microstructures is in accordance with the compressibility of the soils. The instability during undrained shearing under ...

Groutability of granular soils using sodium pyrophosphate modified bentonite suspensions

In order to understand differences in liquefaction behavior of well-graded gravelly soils compared to poorly-graded sands, a series of lab tests was performed on granular soils with different particle gradations or fines content having different relative densities reconstituted in laboratory. Large soil container tests indicated that SPT N-value of well-graded gravels of relative density higher than 50% is considerably larger than that of sand of the same relative density, resulting in lower liquefaction strength of gravelly soils than that of poor-graded sand under the same corrected N-value, N1, for N1 > 25−30. Cyclic triaxial tests on reconstitutes specimens indicated that relative density can serve as a proper index to uniquely evaluate liquefaction strength corresponding to 5% DA strain for variety of granular soils having different gradations. In contrast, post-liquefaction undrained residual strength for larger strain is not uniquely determined by relative density but largely dependent on particle gradations. Also found was that the liquefaction strength clearly reduces with increasing fines content Fc both in well-graded and poorly-graded soils but the reduction occurs in a smaller range of Fc in accordance with smaller critical void ratio for well-graded soils than for poorly-graded sand. Increase in Fc also reduces post-liquefaction residual strength of granular soils particularly for higher relative density. Greater reduction occurs in smaller Fc range for well-graded soils than for poorly-graded sand because of the difference in the critical void ratio.

Neural network based model for estimation of the level of anisotropy of unbound aggregate systems

Studying the behavior of surrounding soil in in situ environments during earth pressure balance (EPB) shield tunneling is deemed of great importance, and the discrete element method (DEM) has been commonly adopted for replicating the granular soil and capturing the qualitative behavior of ground response during tunnel advancement. The presence of fines has been proven to affect the soils' skeleton structures and may dominate their shear strength. However, a significant body of research in tunneling engineering does not often consider sand-fine mixtures, although the natural soil consists of some portions of fines. This numerical study focused on simulating the EPB shield tunneling in granular material containing different fines contents, and the advancement of EPB shield tunneling in the longitudinal direction was reproduced using two-dimensional DEM models. The macroscopic behavior, such as horizontal stress at the tunnel face, and the surface displacement after excavation were investigated. In addition, the micro-mechanical aspect of soil behavior (i.e., contact density) was computed and examined. Finally, the effect of muck discharge rate, which was simulated by deleting the particles within the inlet area, was analyzed.

Investigating the effect of particle angularity on suffusion of gap-graded soil using coupled CFD-DEM

Marketing in different countries: a natural research laboratory

In this paper, triaxial and cyclic direct shear behaviour of different sand mixtures were investigated by considering variations of shape, size and mixture content. In most studies, investigations on stress-strain properties of soils are carried out using clean sands. However, granular soils in the field may contain a considerable amount of grains in different physical characteristics (i.e., shape, size). Therefore, behaviour of the various sand mixtures in triaxial compression and cyclic direct shear testing apparatuses has received attention in this study. Two different sizes (0.25 mm-0.5 mm, and 1.0 mm-2.0 mm) of sands with distinct shapes (rounded and angular) were tested in triaxial and cyclic direct shear apparatuses. The mixtures of coarser and finer geomaterials were tested in various mix ratio values from 5% to 50% by weight. Based on the examinations during shearing of these materials, it was observed that behavior of the sand mixtures are closely related to the grain shape of host materials as well as fines content in both testing apparatuses, whilst size of the sands was not found to be significantly effective on the results.

Internal erosion in granular soils with different microstructures under cyclically increased hydraulic gradients