A study of pore properties in reconstituted Lucera clay at states of the critical state framework

The critical state behaviour of granular material in triaxial and direct simple shear condition: A DEM approach

The discrete-element method (DEM) has gained popularity for developing a qualitative understanding of soil behaviour under a critical state soil mechanics (CSSM) framework. Most studies with a three-dimensional assembly of particles have used spheres as representative granular material to reduce computational demands. However, most granular materials – for example, sands – are not rounded, but possess features of angularity. Therefore, ellipsoid and cluster particles with different degrees of eccentricity were used in this study to evaluate the effect of the particle shape on the drained and undrained triaxial loading behaviour after isotropic and K0 consolidation. The particle numerical properties and grain size distributions were kept the same for all specimens, irrespective of particle shape. The critical state data points for spheres and ellipsoids plotted on almost the same critical state line (CSL) in e–log(p′) space, whereas the CSLs of clusters plotted above them. Additionally, M lines shifted downward with increasing sphericity. It was also found that the stress ratio at the triggering of static liquefaction (ηIS = q/p′) in ηIS–ψ space was affected by particle shape and consolidation path. The dilatancy (d = dεvp/dεqp) was also affected by particle shape. It was found that dilatancy parameters for the SANISAND constitutive model are affected by particle shape, which may contribute to an improved understanding of particle shape in constitutive modelling.

Discrete element modelling of strength and critical state characteristics of granular materials under axial compression and axial extension stress path tests

This paper describes a series of tests carried out to examine triaxial compression and shearing of a high-plasticity compacted clay. Reconstituted and compacted samples which were saturated are used as a basis for interpreting the unsaturated test results within a critical state soil mechanics (CSSM) framework. The shear strength behaviour of unsaturated soils have previously been found to be reasonably well captured in a CSSM framework, whereas the volume change behaviour has been more difficult to rationalise. Based on test results presented using the Bishop effective stress, the volume change behaviour during shear suggests that a unique critical state line is approached, independent of the applied suction. The normalised shearing behaviour of the compacted unsaturated soil is interpreted to be analogous to that of saturated-structured soils.

On the critical state characteristics of methane hydrate-bearing sediments

Silty sand in the offshore Yuedong area exhibits static mechanical behaviour strongly influenced by fines content (FC) and density. To characterize this behaviour and inform geotechnical design, a series of monotonic triaxial compression tests under drained and undrained conditions was conducted on reconstituted Yuedong silty sand with FC from 0–50%. The results show that increasing FC consistently reduces the shear strength and friction angle with no evidence of a threshold fines content (TFC) for strength minima within the tested FC range of 0–50%, which is believed due to the low particle size ratio of the Yuedong silty sand. A high FC suppresses dilatancy and increases liquefaction susceptibility under undrained conditions. The mechanical behaviour is governed by the interaction of FC and void ratio (e 0). All specimens ultimately converged to a unique critical state line, indicating a constant critical state friction angle (∼31°) independent of fines content. Crucially, the CSL in the e–log p′ plane shifts downward with increasing FC, resulting in a growing state parameter (ψ = e−e cs) and reduced dilatancy potential at the same void ratio. These findings demonstrate the applicability of the critical state soil mechanics (CSSM) framework to high-FC silty sands, and unify the influence of FC, e 0, and stress level into a coherent interpretative model. This study advances the fundamental understanding of silty sand behaviour and provides valuable guidance for strength parameter determination and geotechnical design in Yuedong silty sand and similar materials.

Characterizing the process of liquefaction initiation in Anzali shore sand through critical state soil mechanics

The disposal of mining tailings has increasingly focused on the use of dry stacks. These structures offer more security since they use filtered and compacted material. Because of the construction method and the heights achieved, the material that compounds the structure can be subjected to different stress paths along the failure plane. The theoretical framework considered in the design of these structures generally is the critical state soil mechanics (CSSM). However, the data in the literature concerning the uniqueness of critical state line (CSL) is still controversial as the soil is subjected to different stress paths. With respect to tailings, this question is even more restricted. This paper studies two tailings with different gradings due to the beneficial processes over extension and compression paths. A series of drained and undrained triaxial tests was conducted over a range of initial densities and stress levels. In the q-p' plane, different critical stress ratio (M) values were obtained for compression and extension stress paths. However, the critical state friction angle is very similar with a slightly higher critical state friction angle for extension tests. Curved stress path dependent CSLs were obtained in the ν-lnp' plane with the extension tests below the CSL defined in compression. Regarding the fines content, the studied tailings presented very similar M and critical state friction angle values. However, the fines content affects the volumetric behavior of the studied tailings and the CSLs on the ν-ln p' plane shift downwards with the increasing fines content for compression and extension tests. In relation to dilatancy analysis, the fines content did not present an evident influence on the dilatancy of the materials. However, different values of mean stress ratio N were obtained between compression and extension tests and can corroborate the existence of non-unique CSLs for these materials.

The relation between the state indices and the characteristic features of undrained behaviour of silty sand

The discrete element method (DEM) has been extensively used to capture the macroscopic and particulate response of granular materials. Although particle rolling (i.e. controlled by rolling resistance) has been acknowledged as a major contributing factor towards micro-mechanical behaviour of idealized spherical granular material, its influence on characteristic behaviour has not been thoroughly investigated within critical state soil mechanics (CSSM) framework. For instance, the influence of particle rolling on characteristic features of undrained and drained behaviour (e.g. phase transformation, characteristic state, instability, dilatancy, critical state) and the state parameter, (ψ) has not been captured. In this study, a series of constant volume (CV) and drained triaxial compression simulations were undertaken using a rolling resistance linear contact model, deployed within a DEM software. The CSSM framework was centrally used to assess the influence of particle rolling tendencies/resistance on CV and drained behaviours from both a macro- and micro-mechanical standpoint. The study advanced the current understanding of the influence of rolling resistance on CS-related behaviour.

A series of undrained cyclic triaxial tests were conducted on coal ash specimens prepared by moist tamping to investigate factors affecting the cyclic behaviour. One-way and two-way cyclic loadings were employed to investigate the effect of stress reversal on forms of cyclic behaviour. The linkage of static and cyclic behaviour of coal ash was investigated by comparing behaviour of replicate specimens under cyclic and monotonic loadings. It was found that there existed a unique critical state line (CSL) for coal ash irrespective of whether the critical state was achieved from monotonic test or post-cyclic shearing. When specimens with initial states at different positions relative to the CSL in e- log(p') were subjected to different types of cyclic loading, plastic strain accumulation and cyclic liquefaction in the forms of cyclic instability and cyclic mobility were observed. Cyclic behaviour of coal ash can be synthesized by the framework of critical state soil mechanics (CSSM).

The formation of layering during the sedimentation process of tailings makes it of great significance to investigate tailings and to analyze their susceptibility to flow liquefaction. In this study, homogeneous iron ore tailings (IOTs) specimens were reconstituted with pure coarser grains and pure finer grains sampled from a typical tailings storage facility. Additionally, an improved sample preparation method was developed to create heterogeneous IOTs samples containing a fine-grained interlayer with different thicknesses and dip angles using the above two materials. A series of standard drained and undrained triaxial compression tests were conducted to investigate the effects of the presence of a layered structure and its geometry on the stress–strain responses, and the properties of the IOTs under the critical state soil mechanics framework, which has been widely adopted in the analysis of liquefaction in mine tailings. The results showed that for the two homogeneous specimens, unique critical state lines (CSLs) can be identified, but they have different degrees of curvature in the e-ln p′ plane, causing a decrease in the susceptibility to liquefaction with increasing fines content. With increasing fine-grained interlayer thickness (FGLT) within 0–40 mm, the critical state friction angle (φcs) decreased steadily, while the CSLs in the e-ln p′ plane translated upward. This may be because the morphology of the microstructure within the fine-grained interlayer restricted the compression of the intergranular pores. With increasing fine-grained interlayer dip angle (FGLA) within the range 0–30°, φcs decreased until a discontinuity occurred at a dip angle of 15°, while the CSLs in the e-ln p′ plane rotated clockwise through a pivot point. Different FGLAs could change the contact area between the different layers and the axial distribution of the fine-grained interlayer and thus may further contribute to the rotation of the CSLs.

The uniqueness of the Critical State Line (CSL) of silty and sandy geomaterials, on which the Critical State Soil Mechanics is based, has often been the subject of debate. Results from the literature seem to show a dependency of the CSL on the stress path followed and, for reconstituted samples, on the sample preparation method used. In the research described in the present paper, a series of triaxial tests was performed on sandy and silty tailings collected from the Stava tailings dams in Italy. The samples were prepared using different techniques commonly used in the laboratory, and then they were sheared using different stress paths. No dependency of the Critical State Line location on the sample preparation method was observed. This might result from the physical characteristics of particles of the tailings studied. While triaxial tests performed on samples isotropically and anisotropically consolidated indicated a robust Critical State Line that was independent of stress history, tests performed in extension did show a much more contractive behaviour than tests performed in compression. However, the final states that the samples sheared in extension reached cannot be defined as Critical States because strong strain localisation always appeared at low strains, and consequently the void ratio was inhomogeneous throughout the samples.

The characterization and modelling of geomaterials at the microscale have typically been focused on soil particles although voids constitute a significant part of soils and play an important role in soil behaviour. Departing from the recent studies on particle arrangement and discrete element modelling of clays, this lecture highlights how clay behaviour can be effectively described at the microscale in terms of pores, and how this can be linked to the mechanical behaviour observed at the mesoscale. To that purpose, a series of compression and shearing tests was carried out on reconstituted clay, and samples of the tested soil analysed by mercury intrusion porosimetry, gas adsorption and scanning electron microscopy in order the map the micro-porosity with recognised states in the critical state soil mechanics (CSSM) framework. Initial results suggest that it is possible to explain some features of CSSM observed at the mesoscale with mechanisms observed at the microscale.

Stress–strain behaviour of a loosely compacted volcanic-derived soil and its significance to rainfall-induced fill slope failures