The effects of large sample spacing associated with evaluating the vertical strength variability of a clayey soil are discussed in this paper. The research concerns 0.2 m sample spaced cone penetration data profiles analyzed by means of statistical treatment. The aim is to verify the effects of this sample spacing, or that greater than 0.2 m, on trend definition and on parameters which describe the vertical strength variability of the soil. We consider the variance and the scale of fluctuation of the strength values compared to the trend. This investigation also involves the effects of large sample spacing on the location of geostatistical boundaries and the importance of the trend removal technique for obtaining a sound set of residuals. The results of this research provide both theoretical indications and engineering prescriptions for planning a site characterization program.

An investigation of clay microstructure and its evolution under one-dimensional (1D) and isotropic compression is presented for different clays. Data from the literature are compared to original results on two Italian clays, obtained using scanning electron microscopy, image processing, mercury intrusion porosimetry and on-purpose swelling tests. The effects of composition and loading history on clay microstructure, as well as its changes along the compression path (pre- and post-gross yielding) are analysed and a conceptual model of microstructure evolution is proposed for the clays under study. Normally consolidated clays at early virgin compression, either natural or reconstituted, are found to possess an open fabric of random to low orientation, complying with a prevailing inter-aggregate and a smaller intra-aggregate porosity, whose size and distribution depend on composition. Under 1D compression, either in the field or in the laboratory, the inter-aggregate porosity is lost, at a rate dependent on composition and loading history, and the dominant intra-aggregate micropore is progressively reduced. Accordingly, perfectly oriented stacks of domains are recognised which, however, embed preserved random particle arrangements even at large pressures, resulting in an increase of average orientation up to the reach of a steady orientation degree. Isotropic compression causes faster microstructure evolution, although large pressures are required to change 1D-induced fabric orientation.

Understanding the mechanical behavior of lunar regolith is of great importance to address the building of structures on the Moon, as well as for predicting the response of some equipment or facilities interacting with the lunar soil (i.e., rovers). Because the number of lunar regolith samples on Earth is quite low due to the difficulties of bringing it back during space missions, in the last decades, researchers started to develop similar materials, usually named lunar regolith simulants. This paper reports the geotechnical characterization of the De NoArtri (DNA) lunar simulant carried out in the laboratory. Compositional analyses and mechanical tests have been carried out to characterize this simulant; particle-size distribution analysis, chemical analysis, and scanning electron microscope (SEM) analysis have been performed to identify the DNA-1A from a compositional point of view, as well as triaxial compression, direct shear, and oedometer tests for depicting its mechanical behavior, with some comparison with original lunar soil and other simulants.

An investigation of the microstructural features of a high-plasticity clay, in both its natural conditions and reconstituted in the laboratory, is described. Scanning electron microscopy is used here to characterise the fabric at different magnifications, while image processing of the micrographs delivers a quantitative assessment of the fabric orientation. The results of energy-dispersive X-ray spectroscopy and swelling tests, as reported in a previous study by the authors, are used to characterise the bonding nature and strength, as well as mercury intrusion porosimetry to investigate clay porosimetry. Despite their identical composition, the natural and the reconstituted clay have experienced different deposition and loading history, generating different microstructural features that are shown to underlie their differences in state. For both clays, one-dimensional (1D) compression to medium–high pressures is seen to determine a well-oriented medium magnification fabric. However, larger-scale observations and the corresponding image processing results reveal non-uniform local fabric features, hence making fabric characterisation dependent on the scale of analysis and bringing about the issue of identifying the clay micro-scale representative element volume relating to the clay macro-behaviour. The micro-REV is identified for the clays under study and its connection with the macro-behaviour characterised. The microstructural evolution induced by 1D compression to very high pressures is shown to concern mainly the clay porosity and porosimetry, the fabric orientation being steady, thus explaining the isotropic hardening observed in laboratory tests.

The paper discusses the intrinsic properties, geological history, natural structure and mechanical behaviour of stiff Italian clays, part of the sub-Apennine Blue Clay formation, which are located at different sites within the marine basin where the clay formation was deposited in the early Pleistocene. For all the clay deposits being considered, diagenesis, unloading due to erosion, and weathering of the top strata represent the main stages of the geological history. However, the clays have undergone different levels of unloading, and have developed different levels of structure due to their different locations in the basin. These differences are accounted for in the analysis of their current states and behaviour. Based upon a large experimental database, the structure and mechanical behaviour of the clay samples from the different sites are compared and related to the clay history. In particular, the soil structure is characterised based upon SEM analysis results and compression–swelling test results. The comparison of the response to compression–swelling cycles of the natural clay with that of the same clay when reconstituted leads to the evaluation of parameters such as the stress sensitivity S_σ and the swell sensitivity C*_s/C_s, which are used to assess the structural strength of the natural clay with respect to that of the reconstituted clay. The stress–strain behaviour of the different clays along different constant stress-ratio compression paths and shear paths (from low to high pressures) is discussed in detail. The variations in stress–strain response of samples subjected to different pre-shear consolidation paths are outlined. The corresponding trends of the plastic strain increment ratios are investigated. As a result of the analysis, the features of the gross yield curves of the different clay horizons are also assessed. Finally, the study provides elements for the characterisation of the hardening properties of stiff clays, and therefore of the evolution of the material anisotropy during plastic deformation.