Compression and Strength Characteristics of Two Silts of Low and High Plasticity

The potential for enhanced strength or stiffness arising from the microstructure of natural silty soils compared with their reconstituted state is examined using the framework for clays, to assess the extent to which it can be used for silts. A detailed study performed on two clayey silts of low and high plasticity, respectively, in their intact and reconstituted state, is used to illustrate the different response of the silts compared with clays in terms of compressibility and strength, e.g., multiplicity and shape of the normal compression lines and strong dilative tendencies during undrained and drained shearing. This paper shows that the response of the reconstituted silts can be used as a reference to identify the influence of the microstructure on the compressibility and strength of the intact silts. The results are found to give clear signs of structure in both compression and shearing only for the low-plasticity silt. Stiffness measurements corroborate these findings

On the time-dependent behaviour of glacial sediments: a geotechnical approach

Soft sediments can deform under both changes in stress and changes in strain rate, as well as during sustained loading (e.g. creep). In glacial conditions, soft subglacial sediments can contribute to glacier movements, and when the ice/sediment system is coupled, the rate-sensitivity of the sub-layer can affect the velocity of the glacier. These issues have been the object of studies by several geologists. In parallel, the effects of time and strain rate on the behaviour of soils have been studied for many years by geotechnical engineers, to estimate the long-term performance of ground structures. This paper applies the knowledge acquired in soil mechanics to a geological problem: results on two glacial sediments of different origins, obtained from advanced geotechnical laboratory tests, are presented and analysed in the light of recent advances in soil mechanics. Independently of time effects, the test data show that the pre-failure behaviour is not elastic but that the sediments develop plastic strains before reaching their peak strength, with the stiffness gradually degrading. Results from tests performed with variable strain rates suggest that the time-dependent (viscous) behaviour of the sediments may be governed by their mineralogy, in particular the presence of plastic fines. The viscous behaviour observed in the plastic tills seems to affect the pre-failure behaviour only, for example the yield surface, stiffness, strength, but the criteria of failure, e.g. critical or residual angle of failure are found to be constant and independent of strain rate. The test data also suggest that the viscous behaviour may be related to stress level, being more prevalent at low stresses. These results, obtained under laboratory-controlled conditions, over a large range of strains, pre- and post-peak strength, are a necessary step towards developing constitutive models for subglacial sediments rigorously. It is only after this has been achieved that such models should be implemented in finite element code to analyse ice/sediment systems, with the aim to understand glacier movements better. © 2008 Elsevier Ltd. All rights reserved.link_to_subscribed_fulltex

Corrigendum to "On the time-dependent behaviour of glacial sediments: A geotechnical approach" [Quaternary Science Reviews 28 (2009) 693-707] (DOI:10.1016/j.quascirev.2008.07.016)

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Minimum image quality for reliable optical characterizations of soil particle shapes

Remarkable advances have been seen in image-based methods for automating soil particle shape characterizations in the last decade. However, the accuracy and reliability of image-based methods has rarely been questioned. This study shows that image quality affects the computational results of particle shape descriptors, including aspect ratio, sphericity, convexity, circularity, and roundness. These descriptors display a hierarchy of resistance to the effects of low image quality. The particle length, perimeter, and area are used as controlling parameters for quantifying the influence of image quality. The minimum requirements for ensuring reliable image-based shape characterization of these parameters are established

Triaxial compression on semi-solid alloys

Multi-axial compression of the mushy zone occurs in various pressurized casting processes. Here, we present a drained triaxial compression apparatus for semi-solid alloys that allow liquid to be drawn into or expelled from the sample in response to isotropic or triaxial compression. The rig is used to measure the pressure-dependent flow stress and volumetric response during isothermal triaxial compression of globular semi-solid Al-15 wt pct Cu at 70 to 85 vol pct solid. Analysis of the stress paths and the stress–volume data show that the combination of the solid fraction and mean effective pressure determines whether the material undergoes shear-induced dilation or contraction. The results are compared with the critical state soil mechanics (CSSM) framework and the similarities and differences in behavior between equiaxed semi-solid alloys and soils are discussed

Effect of Particle Shape on Monotonic Liquefaction: Natural and Crushed Sand

This study investigates the effect of particle shape on monotonic liquefaction of the soil by performing a series of static triaxial compression tests. The tests conducted on three types of natural sand (NS), crushed sand (CS), and mixed sand (MS) (i.e., 50% natural sand +50% crushed sand by dry weight of the soil) with different particle shape descriptors consist of roundness (R), sphericity (S), and regularity (ρ). The shearing responses showed that the CS and MS specimens showed a dilative response whereas the natural sand had a strain-softening contractive behaviour. Also, the interpreted results based on a framework of the critical state of the soil mechanic (CSSM) showed that in e-p′ plane, the specimen with a higher amount of crushed particles have a greater strength due to a higher packing characteristics. The investigations on critical state locus on q-p′ plane showed that by increasing the roundness, sphericity and regularity of the specimens the critical friction angle (ϕcs) decreased. Also, studies on flow liquefaction in undrained instability state (UIS) showed that by increasing the particle shape descriptor values, the specimens are more prone to be liquefied