Compositional Variation in Component Layers in Natural Illite/Smectite
Published chemical data for suites of mixed-layer minerals from diagenetic sedimentary rocks, hydrothermally altered tufts, and a metasomatic bentonite bed indicate that the layer charge and composition of the different components of illite/smectite (I/S) differ from one geological environment to another. It appears that the composition of the elemental smectite and illite layers in the I/S is more or less constant for samples within each geologic setting. In the examples considered, the smectite layers are predominantly montmorillonitic in character (i.e., the charge is in the octahedral site), whereas the illite layers show different types of charge sites, depending upon the suite studied. Illite layers appear to have about the same charge in all three suites studied, slightly more than 0.7 per O10(OH)2 unit, whereas the smectite layers in the different suites range in charge from about 0.3 to 0.7 per O10(OH)2 unit. Cation-exchange capacities reflect these differences in charge, although not ideally. The differences in the composition of the component layers in each geologic suite of mixed-layer clays are probably due either to differences in the bulk chemistry of the rocks in the different suites or to differences in intensive variables, such as temperature and pressure, of the regime under which they have formed.
Smectite-group minerals in deep-sea sediments : Monomineralic solid-solutions or multiphase mixtures ?
Results of XRD, TEM, and chemical analyses are combined to characterize the true chemistry and structural formula of the smectite-group minerals present in selected samples of deep-sea sediments from the north-central Pacific and Angola, Lau, and North Fiji Basins. In most cases, the abundant Fe and K in the <0.2-ttm fraction is associated with accessory minerals such as mica, phillipsite, and iron oxides. Bulk analyticaldata obtained from the <0.2-pm fraction, corrected for the presence of these minerals, reveal a structural formula very close to that of ideal montmorillonite. The extensive chemical variability among members of the clay-mineral group usually is attributed to the occurrence of solid solution. The variability in smectite-group minerals also could be due to (l) the occurrence of mixedJayer structures, (2) the presence of compositionally distinct domains in the structure, or (3) a mixture of two or more distinct phases. A clear distinction between a monomineralic solid-solution phase and a mixture of coexisting end-members based on chemical composition or even conventional XRD and HRTEM techniques is of crucial importance. The data presented in the literature concerning the nature of the smectitegroup phase and related I/S mixed layers do not necessarily reflect the true nature of the individual clay-mineral phases. The application of the term "smectite" to a material characteized by a structural formula should be avoided.
On the use of the Hardening Soil Small Strain model in geotechnical practice
This article highlights the importance of using advanced constitutive soil models in numerical modelling as daily geotechnical practice. In this context, the Hardening Soil (HS) model is presented as the most advanced soil model implemented in the ZSoil finite element code. The article recalls the basic features of the HS model and it explains their role in numerical simulations. The role of pre-failure non-linear behaviour of soils is also discussed. An application of the HS model is illustrated on typical geotechnical problems such as retaining wall excavation, tunnel excavation and undrained loading followed by consolidation.