An investigation of breakage behaviour of single sand particles using a high-speed microscope camera

Much research has focused on the micro-mechanics of sand particles. The single particle uniaxial compression test is a common way to study breakage behaviour. However, there is still little agreement on particle breakage criteria and the mechanisms of breakage remain uncertain, partly because of the often rapid brittle failure of sand particles. In this study, a series of single particle uniaxial compression tests on different kinds of sand particles were carried out, using a high-speed microscope camera to capture the processes of breakage. This enabled a maximum of 2000 frames to be obtained per second to identify clearly the failure processes and crack propagation. Four failure modes have been proposed based on the rapidity of failure and the size and number of particle fragments created during the breakage: splitting, explosive, explosive–splitting and chipping. The relationship between the particle strength and the breakage mode has then been explored, investigating also whether immersion would affect the breakage mode and strength. The morphologies of the sand particles including local roundness, particle size, aspect ratio, regularity and two-dimensional sphericity were measured to investigate their influence. With assistance of the high-speed microscope camera, the crack initiation locations were identified and the frequency of different locations obtained, comparing the results with existing particle micro-mechanical theories

Mechanisms of compression in well-graded saprolitic soils

Soils originating from weathering processes present considerable heterogeneity in their composition, which can make it difficult to analyse their behaviour in a systematic way. For the granitic saprolites discussed in this paper, based on a trend between soil density and weathering degree, there appears to be two different domains of behaviour, a granular domain and a clay matrix one, according to the degree of weathering reached. Recognition of these domains can reduce the apparent scatter of data for the engineering behaviour of weathered soils. A number of one-dimensional compression tests are presented for saprolitic soils from Hong Kong having different weathering degrees. In addition, isotropic and one-dimensional compression tests from the literature on other saprolites from Hong Kong and around the world were reanalysed and used to identify possible trends in the mechanisms of compression for these two domains. From practical considerations, the trends considered were between compressibility and common engineering grading descriptors. An attempt was also made to provide the physical explanations behind the behaviour observed, and the particle breakage was investigated in detail, both from a quantitative and qualitative point of view. It was found that the values of relative breakage (Hardin in ASCE J Geotech Geoenviron Eng 111(10):1177–1192, 1985), for a same stress level, might be very similar for soils having different compressibility values and different initial gradings. When studying particle breakage in further detail, it can be observed that it is linked to the amount of large particles and their characteristics. The maximum particle size, rather than the amount of fines in a mixture, may be a better predictor for differences in compressibility and breakage

A study of completely decomposed volcanic rock with a transitional mode of behaviour

A transitional mode of behaviour is seen in some soils where the specific volumes of samples do not converge to the same values at the same stress state within the ranges of strains that can be achieved in laboratory tests so there are no unique normal compression or critical state lines. This type of behaviour has been found in different soils but not previously in soils resulting from decomposed igneous rocks. In order to investigate the possibility of transitional mode of behaviour in a decomposed volcanic rock, an extensive series of one-dimensional compression and triaxial tests were conducted on samples in reconstituted and intact states. The important features of transitional mode of behaviour in soils have been identified, that is, the presence of non-unique and parallel normal compression and critical state lines. The behaviour of the soil is therefore dependent on the initial specific volume. The degree of transitional behaviour is strong, particularly in the reconstituted samples, but it is less clearly identified in the intact samples due to the small range of initial specific volumes available. These observations indicate that the transitional mode of behaviour, previously seen in sedimentary soils and artificial soil mixtures, can be extended to some weathered igneous rocks. Determining the effects of structure is difficult due to the non-unique intrinsic properties, but nevertheless, the effects of structure have been identified and discussed

Influence of structure on the behavior of a saturated clayey loess

An intensive experimental investigation by means of triaxial and oedometer tests was performed on a clayey loess that was retrieved from two depths at a location on the southern Chinese Loess Plateau. Intact and reconstituted samples were used to identify the effects of the natural structure on soil behavior in saturated conditions. The behavior in compression was clearly affected by structure with the intact samples reaching well-defined gross yield points outside the intrinsic compression line of the reconstituted soil, after which the compression paths converged towards the intrinsic compression lines, behavior which is consistent with destructuration. However, very high stresses were required to give complete convergence. Similarly the triaxial tests that were carried out at lower stress levels also did not give convergence of the critical states so that different critical state lines could be defined for the intact and reconstituted soils. This was consistent with qualitative observations from scanning electron micrographs that natural elements of fabric and possibly bonding persisted even after triaxial shearing. The effects of structure at the two depths on the compression and shearing behavior were slightly larger for the shallower samples. Despite the very different genesis of the soils, it was found that similar frameworks could be applied to those used for sedimentary clays and that the degree of structure was equivalent to a clay of medium sensitivity

The constitutive modelling of London Clay

Advances in understanding real soil behaviour, achieved over the past twenty years, and subsequent developments of relevant soil constitutive models, have greatly enhanced the predictive ability of numerical analyses of geotechnical structures. Of particular relevance, especially for serviceability limit state (SLS) design, has been the understanding of soils' small strain behaviour. For the behaviour of London Clay, initially developed models were simple curve-fitting techniques that reproduce stiffness dependence on stress and strain level, which has shown some successes in predicting SLS behaviours. Modelling failure of London Clay is significantly more complex due to its overconsolidated and fissured nature. More recent developments in soil constitutive modelling have therefore tried to encompass most important aspects of clay behaviour within the unified framework of critical state. However, even these advanced models have insufficient features to simulate the behaviour of overconsolidated clays. In this paper, a study has been made, comparing high quality laboratory triaxial data on London Clay with the predictions of two constitutive models, highlighting these deficiencies. It is shown how relatively simple modifications of advanced models, to capture the anisotropy of small strain stiffness and to impose a strength curtailment to represent strain localisation, can significantly improve the predictions of laboratory experiments

The micromechanical behaviour of a biogenic carbonate sand

An experimental investigation of the micromechanical behaviour of a biogenic carbonate sand from the Philippines was conducted. Mechanical parameters of sands at the micro-scale are required in order to simulate the particle interactions in numerical analyses carried out using the Discrete Element Method (DEM). The tests were performed on particles obtained from the sedimentation of coral fragments and they were carried out by means of a custom-made inter-particle loading apparatus that enabled the investigation of both the normal and tangential loading behaviour at the particle contacts. The normal loading tests showed a reversible behaviour after the first loading, while the tangential loading behaviour of this sand appears to be dependent on the vertical confinement and mainly reversible for small displacement cycles. Also, the inter-particle friction coefficients at larger displacements are substantially higher than those calculated for other sands using the same experimental procedures

Quantifying “Transitional” Soil Behaviour

The last decade has seen an increasing amount of research on so called “transitional” soils that are characterised by incomplete convergence to unique normal compression lines and/or critical state lines in simple laboratory tests. This topic has often provoked reaction, perhaps because some have seen it as a challenge to critical state frameworks of soil behaviour. A particular issue is whether incomplete testing or other test defects might cause such an apparent behaviour. Confusion around the topic has not been helped by the wide range of degrees of convergence seen for different materials and differences seen between convergence in compression and shearing. This paper proposes a unifying means of plotting laboratory test data from such soils that will hopefully provide a rational framework for such discussions, since it makes explicit the degree of convergence towards unique volumetric states for different forms of loading. Data are examined for three “transitional” soils. The results show that bringing about convergence for these soils would require strains beyond those that may easily be applied and that the lack of convergence cannot solely be an artefact of test defects. Plastic volumetric strain was found to cause much faster convergence than plastic shear strain

Development of inter-particle friction in a railway ballast

A detailed experimental programme investigated how the coefficient of friction at particle contacts for a typical UK railway ballast varied during cyclic loading. Despite a decrease of roughness, the friction coefficient increased steadily, stabilising in the region of tens of cycles. In contrast to previous work on the contact behaviour of sands, water inundation caused a significant but reversible decrease of the coefficient, while the generation of a significant amount of abraded fines did not affect it, nor did the load level