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Despite the increasing understanding of bentonite behaviour, there is still missing evidence on how different hydro-mechanical loadings, including sequences of hydration and compression, affect the fabric and the volume change behaviour of the material. It is generally assumed that the interplay between the behaviour of clay assemblages and the overall fabric of the material is the reason of having final states that are dependent on the stress path followed. Here the results of an experimental campaign aiming to study these factors are reported and discussed. Free swelling and swelling pressure tests were performed, both followed by compression to a relatively high stress. The experimental program involved various samples that were dismantled at intermediate states in order to perform microstructural observations by means of mercury intrusion porosimetry and electronic scanning microscopy. It was observed that while the void ratio at a given stress level depends on the stress path, subsequent compression led to a unique virgin compression line. The data obtained at the microscale gave further insight for an interpretation of the volume change behaviour observed at the macroscale, showing that at high stress the material tends to recover the same fabric regardless of the path to saturation.

The purpose of this study was to explore the 3D printing process of fine-grained soils with a geomechanichal approach. In particular, identifying the factors (including geomechanical characteristics of the soil, geometry of the problem, and 3D printer settings) that influence the behavior of 3D printed samples. To answer this question, three groups of materials differing in mineralogy, consistency limits and consistency, and three types of equipment have been used – an extruder and two 3D printers – to widely investigate the behaviour of the fine-grained soils when subjected to the process. First, the extrusion process has been studied finding that the parameter to take into account to predict the outcome are: the coefficient of consolidation of the soil, the extrusion length and the time of extrusion. Then, a double-porosity model was developed to predict the index properties of 3D printed specimens based on the geotechnical characteristics of the fine-grained soil and the specimens' geometry. This model was validated through its application to specimens with different values of the adimensional spacing between parallel filaments. The research then focuses on the structure of 3D printed clayey soil, examining the relationship between particle arrangements, pore size, orientation, and shape, and the 3D printing process. An experimental campaign was conducted to investigate the evolution of soil tensile state before, during, and after the 3D printing process using oedometric tests. Finally, the experimental campaign, conducted at the Geotechnical Laboratory of the University of Los Andes, sought to accurately reproduce 3D models generated using random field theory using a custom multimaterial 3D printer, finding the relationship between printing velocity, extrusion velocity and area of the nozzle that would guarantee an optimum 3D printing.

In geotechnical engineering, physical and numerical models seek to shed light on multiphase phenomena that threaten earth structure stability. This is the case of river levees: when subjected to non-ordinary hydraulic loads, local and global failures with consequent floods could occur. If, on one hand, centrifuge models can replicate the real phenomena, exploiting the enhanced gravity, while scaling geometrical features and time, on the other, numerical models extend the possible case studies by capturing key elements, governing the hydro-mechanical behaviour of the earthworks. However, the two techniques could complement and benefit each other. In this research, a potential failure mechanism, induced by the development of uplift pressures beneath the toe of a levee characterized by a peculiar stratigraphic profile, is investigated. The foundation consists of a shallow weak low-permeability layer, overlying a coarser and more permeable one, this latter acting as a hydraulic preferential flow path between riverside and landside. Results of a preliminary numerical study carried out with different methods are presented and discussed. The study aims to improve understanding of complex failure mechanisms and to encourage the development of more robust forecasting methods. Indeed the results have provided fundamental guidance for a centrifuge experimental set up.

L’Opalinus Clay Shale è la formazione che ospiterà il deposito profondo di scorie radioattive di alta attività in Svizzera. A causa della sua composizione fortemente eterogenea, tale materiale è stato classificato in diverse facies. Inoltre, esso è caratterizzato da una struttura fissile e da un comportamento idro-meccanico anisotropo. Queste particolari complessità rendono particolarmente difficile la scelta di parametri geomeccanici unici per l’intera formazione. In questa memoria viene presentato uno studio sperimentale mirato alla caratterizzazione del limite inferiore dei parametri di resistenza al taglio, attraverso l’esecuzione di prove di compressione trias siale e prove di taglio anulare, eseguite su campioni completamente destrutturati appartenenti alle diverse facies. Inoltre, sono state condotte delle osservazioni al microscopio elettronico per valutare la disposizione delle par ticelle lungo le superfici di taglio. I risultati ottenuti hanno permesso di definire delle correlazioni tra la frazione argillosa e gli angoli di resistenza al taglio ultimo e residuo.

La nota presenta i risultati di un lavoro sperimentale condotto con lo scopo di analizzare il comportamento idro-meccanico di una sabbia quarzosa per un ampio intervallo del grado di saturazione. Per analizzare le caratteristiche di ritenzione del materiale e per investigare la relazione tra la tensione di snervamento e la suzione di matrice sono state eseguite prove edometriche a suzione controllata e a contenuto d'acqua costante. I risultati ottenuti mostrano che la tensione di snervamento della sabbia non aumenta monotonamente con la suzione. Tale andamento è caratterizzato da un punto di massimo e da valori della tensione di snervamento sostanzialmente coincidenti per la condizioni satura e asciutta. Questo aspetto, non considerato finora dai modelli elasto-plastici per i terreni non saturi, conferma una delle principali ipotesi della classica Meccanica dei Terreni, secondo cui il comportamento meccanico di mezzi granulari saturi e asciutti è lo stesso se descritto in termini di tensioni efficaci.

Compacted scaly clays are complex geomaterials widely used for the realization of dam cores and waste dump liners. This research aims to investigate the swelling/shrinkage behavior of these geomaterials subjected in an oedometric cell to complex hydromechanical stress paths, including different sequences of cyclic variations of suction and stress. Microstructure investigations, based on scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) techniques, highlighted the microstructural characteristics of the material, which are the distinctive microfabric features (particles assemblages and scales) and the double-porosity networks, where micro- and macropores coexist. Moreover, the MIP technique, applied to the tested oedometric specimens, provided significant elements supporting the microscale interpretation of the observed behavior on the basis of mechanical interaction between the different levels of microstructural arrangements. In all the cases analyzed, the suction cycles induce a microstructural rearrangement in the macropore domain, enhancing the stability of the fabric of the compacted clay. Suction-controlled oedometric test results were modeled with an elastoplastic constitutive model, which considers the role of a double structure in the behavior of unsaturated soils. The results obtained highlight the mutual influence between wetting/drying cycles and loading/unloading paths on the volumetric behavior of the geomaterial.

A map derived by rockfall analysis at Mount Pellegrino is presented herein. The study area is affected by several phenomena of rockfall which caused numerous damage and a strong social and economic impact. Official reports and maps that give a general assessment of rockfall hazard are available in this respect, however, it would be advisable to provide a more specific cartographic support useful for land management and planning. The drafting of new maps showing the rockfall runout areas is an additional tool that may be used in conjunction with the existing maps as a means of risk mitigation and reduction. On the basis of geological, geomorphological, and geomechanical analysis and exploiting the information relating to a landslides inventory obtained by using both analytical and empirical methods, two different rockfall propagation areas were reconstructed. The final thematic map permit to appreciate the differences and similarities between the obtained runout areas.

Two case-histories of the design of submerged breakwaters for coast protection are shortly reported in the paper. The first one refers to the stabilisation and protection works of the sand beach of S.Alessio Siculo village, in Sicily, that suffered severe erosion triggered, some forty years ago, by the construction along the shoreline of a vertical seawall in lieu of the pre-existing sand-dune system. As a consequence, long stretches of the seawall were undermined and collapsed, and the beach almost disappeared. A submerged breakwater was built to reduce the inten-sity of wave attack, the seawall was rebuilt where necessary and provided with a rock revetment at the toe, and then a “perched” beach was formed. The submerged breakwater option was necessary to pre-serve the aptitude for bathing activities of the vil-lage. The second case-history refers to the submerged breakwater built in front of a marine cliff in order to protect it against erosion and undercutting. The cliff is located on the eastern narrow promontory of Or-tigia Isle (Syracuse), on which rests the medieval Maniace Castle dating back to the age of Fedrerik II. The cliff as well as the nearby sea bottom consist of yellowish soft calcarenitic layers interbedded with weak or poorly cemented yellow calcarenitic sand lenses. The breakwater consists of two layers of unrein-forced concrete blocks connected to the rocky sea bottom by means of active steel bar anchorages. Blocks are spaced 1 m so to form a network of channels permitting water and fish fauna to pass through the breakwater. During construction some basal concrete blocks, not yet anchored to the rocky sea bottom, were dis-placed or overturned by heavy sea. Therefore it was necessary to modify the design of the damaged stretches of the breakwater and revert to a mixed so-lution composed of the displaced concrete blocks, rockfill and large stones. The behaviour of the breakwater has been fully satisfactory during its 28 years life span. The design of both breakwaters has been assisted by physical model tests carried out at Wallingford and at Delft Hydraulics for S.Alessio Siculo and Or-tigia breakwaters respectively. Finally it must be mentioned that the S. Alessio Siculo project explicity contemplated the observa-tional approach as far as the seaside rock armour of the barrier (i.e.”sacrificial layer”) was concerned. On the contrary, the initial design of the defense system of Ortigia cliff did not foresee the use of the obser-vational approach. Therefore the designer had to think all over again a new solution when heavy sea damaged the uncompleted sections of the breakwa-ter. Luckily, a valid and economically viable solution was found.

Reduction of pore water pressures is a useful strate-gy to improve the stability conditions of slopes. Deep draining trenches can be used for this scope. For the realisation of deep trenches, the usual con-ventional construction techniques are not adequate and the use of adjacent vertical panels, built by means of the methods well established for dia-phragm walls, is necessary. However, unbonded ma-terials (i.e. gravels) cannot be used, since the excava-tion of a panel adjacent to already built ones will cause instability. For this scope a bonded material such as the pervious concrete can be used. It must have high permeability, filtering capacity in order to prevent the internal erosion of the soil in which the trench drain is installed, sufficient shear strength af-ter a short curing time so to avoid the instability of adjacent previously built panels or piles. Results of an extensive laboratory experimental research on the mix-design, the filter capacity and residual hydraulic conductivity prove that proper mix-design can be devised meeting the above requirements. Experi-mental results show, in particular, that the shear strength of the concrete after a short curing time permits to excavate intermediate panels tens of me-ters deep without jeopardising the stability of previ-ously built ones. Moreover the hydraulic conductivi-ty of pervious concrete subjected to vertical stress level corresponding to the weight of a column of concrete of about 40 metres deep drops to a third of its value at low stress level. Nevertheless, it is still acceptable for draining trenches installed in many soils including fine-grained ones (fine sands, silts, clays).

Le trincee drenanti profonde rappresentano uno dei metodi più efficaci per la mitigazione del rischio da frana, in pendii con falda idrica. Esse sono realizzate mediante pannelli o pali secanti. Il riempimento è costituito di calcestruzzo poroso o materiale granulare. Se le trincee sono adeguatamente “innestate” nel terreno stabile e il materiale di riempimento ha sufficiente resistenza e rigidezza come il calcestruzzo poroso, si ha ulteriore in-cremento di resistenza a taglio per effetto shear keys, oltre a quello derivante dalla riduzione delle pressioni in-terstiziali. L’incremento di resistenza è dovuto sia alla resistenza all’interfaccia calcestruzzo–terreni sia a quella intrinseca del calcestruzzo in corrispondenza della superficie di scivolamento, e può essere molto significativo in relazione allo spessore e all’interasse delle trincee. Nella presente memoria si riportano i risultati di una ricer-ca sperimentale sulla resistenza a taglio di interfaccia calcestruzzo poroso – terreni a grana fina, che dimostra la significatività dell’effetto shear keys. Pervious concrete for deep trench drains, used to stabilise slopes, must simultaneously satisfy many requirements, namely, adequate hydraulic conductiv-ity, adequate shear strength a few days after pour-ing, capacity to act as a protective filter for soils in which the drain is installed, good resistance to clog-ging and adequate residual hydraulic conductivity. The pervious concrete with appropriated mix-design can effectively satisfy all the abovementioned requirements. If the trenches depth is such that they intersect the sliding surface, and if the trenches are adequate-ly "socket" in the layers of stable soil, there is a fur-ther increase in shear strength due to the shear keys effect. This latter is in addition to the increase in shear strength resulting from the reduction of inter-stitial pressures, that remains the principal scope of the draining trenches. Obviously, the increase of shear strength due to the shear keys effect occurs if the trenches are filled with material that have enough strength and stiffness, such as the porous concrete. In this case, the beneficial effects of the draining trenches on stability are also due to the resistance at the concrete interface of the trench - soils and to the intrinsic resistance of the concrete at the area of the trench intersected by the sliding surface taken into consideration.The increase in resistance due to the shear keys effect can be very significant in relation to the thickness and interspace of the trenches. Results reported in the paper demonstrated that the interface fine grained soil-pervious concrete is higher than the residual shear strength of the soil.

The testing procedure and results on saturated samples of Opalinus Clay in the work of Schuster et al. (Rock Mech Rock Eng https://doi.org/10.1007/s00603-021-02474-3, 2021) were conducted and presented using strain rates two to four orders of magnitudes higher than the rates needed to allow pore pressure equilibrium in the material, both in drained and undrained conditions. This leads to an erroneous estimation of the mechanical properties in saturated conditions. We discuss this aspect in the context of shale testing. We also discuss the effect of drying-induced fissuring on the mechanical properties of geomaterials tested in dry conditions.

When dealing with unsaturated soil conditions, the influence of matric suction on cone tip resistance of the soil above the ground water table is typically neglected in engineering practice, with consequent possible misinterpretation of soil features. In the last decades, various researchers have investigated the influence of suction on cone resistance for sands, whilst still little is known for silty materials, whose contribution can be significant and extended for many meters above the groundwater table. Such issue is especially relevant for compacted earth structures, like river embankments, typically made of a heterogeneous mixture of intermediate soils. With the aim of providing a contribution and stimulating its correct implementation into geotechnical practice, a set of miniature piezocone tests have been carried out in a centrifuge on both saturated and partially saturated silty sand models. The interpretation of CPT results is discussed, highlighting the effect of partial saturation on cone tip resistance.

Al fine di valutare le condizioni limite per la stabilità di un pendio in frana, sono state svolte delle analisi di stabilità all’equilibrio limite 2D e 3D in un’area ubicata nel bacino del fiume Imera Settentrionale (Sicilia). Tali analisi hanno permesso di ricostruire un modello geologico e geotecnico ottimale, di individuare il livello piezometrico che ha determinato le condizioni di instabilità e di definire i parametri operativi di resistenza a taglio dei terreni. Le recenti riattivazioni dei meccanismi di frana hanno fornito elementi che hanno permesso di validare in particolare i risultati delle analisi numeriche 3D. I risultati ottenuti hanno anche evidenziato che il metodo di analisi tridimensionale costituisce un valido strumento di analisi, capace di prevedere l’estensione areale dei dissesti e di fornire una migliore stima della resistenza a taglio operativa dei terreni, soprattutto nei casi in cui la morfologia del pendio è complessa e la direzione degli spostamenti non è omogenea.

This paper reports the results of a centrifuge test on a silty sand river embankment compacted in unsaturated conditions and subjected to flooding. The measures of pore water pressure from tensiometers and pore pressure transducers show that the saturation line reaches the landside only after an unrealistic long-lasting flood, proving that the assumption of steady-state seepage for the design or assessment of a river embankment is too much conservative approach. conservative approach.

Transversal applications of 3D-printing (or Additive Manufacturing) have been recently implemented in the field of Geomechanics. In a 3D-printing process, the printed volume is obtained from successive layering of adjacent soil filaments. In this work, the fabric of an as-printed soil has been carried out by combining Mercury Intrusion Porosimetry (MIP) tests and Scanning Electron Microscope (SEM) observations, with the aim to highlight how the particle arrangements and the orientation and shape of pores are linked to the printing operation. The microstructural analyses showed that macropores are the result of the relative position of the filaments and their initial distortion in quasi-undrained conditions. Particle arrangement within the soil filament is strongly anisotropic, due to the rotative movement of the soil in the extruder.

Il processo di estrusione dei terreni a grana fina rappresenta uno tra i processi alla base dei metodi di stampa 3D dei terreni. Nel seguente contributo si sono scelti dei terreni adatti a tale scopo e si è studiato come le variabili dipendenti dal processo di stampa, quali la pressione applicata e la dimensione dell’ugello influissero sul materiale estruso.

La nota presenta una selezione dei risultati di un ampio programma sperimentale condotto con l’obiettivo di caratterizzare, dal punto di vista della resistenza a taglio, dei campioni rimaneggiati di Opalinus Clay, attraverso l’esecuzione di prove triassiali non convenzionali. I risultati mostrano chiaramente l’influenza del grado di preconsolidazione sul comportamento meccanico del geomateriale e in particolare sulla resistenza a taglio, che risulta caratterizzata da un picco di resistenza poco marcato e da un comportamento dilatante. La presenza di giunti all’interno del provino altera completamente il comportamento originario del materiale, che diventa duttile e contraente. Inoltre, in tali condizioni, la resistenza a taglio si riduce drasticamente ai valori di resistenza residua.

Earthen river embankments are typically in unsaturated conditions during their lifetime and the degree of saturation within their bodies may vary significantly throughout the year, due to seasonalfluctuations of the river stage, as well as infiltrations of meteoric precipitation and evapotranspiration phenomena. Given the significant effects of partial saturation on the hydro-mechanical behaviour of soils, realistic assumptions on the actual water content distribution inside the embankments are essential forproperly modelling their response to hydraulic loadings. In this framework, centrifuge modelling is a useful tool to get insights into the evolution of saturation conditions of a water retaining structure during flood events. It allows for the direct observation of the groundwater flow process, which is hardly detectable at the prototype scale, enabling, at the same time, the validation and calibration of predictive numerical tools.In this paper, the results of a centrifuge test carried out on small-scale physical model of a compacted silty clayey sand embankment subjected to a simulated high-water event, at the enhanced gravity of 50-g, are presented and discussed. The physical model was carefully instrumented with potentiometers, miniaturized pore pressure transducers and tensiometers. Pore pressures and suctions measured during the experiment showed that the stationary flow conditions were reached only after an unrealistic hydrometric peak persistence. It therefore emerges that, for the design and/or the assessment of the safety conditions of a river embankment similar to the one tested, the simplified hypothesis of a steady-state seepage, in equilibrium with the maximum river stage expected could result, in many cases, an excessively conservative assumption.

Permeable concrete is effectively used in many fields of civil and environmental engineering, including some geotechnical applications such as drainage piles, deep drainage trenches, and as a permeable backfill material for retaining walls. The principal requirements that a pervious concrete must have for geotechnical applications are those that simultaneously guarantee enough hydraulic conductivity and good filter properties against internal erosion of the soil in which the drains are realised. These requirements can be effectively achieved through special mix-designs in which a non-negligible amount of sand is added to the aggregates, unlike the permeable concretes used for example in the road construction sector where the aggregates are essentially made only of gravel and the concrete is a “no-fines” concrete. The permeability of the pervious concrete depends on many factors among which the stress level due to the self weight of the column of fresh concrete when the depth of the same column is non negligible, as it is in the geotechnical applications. Results of an experimental study on this aspect are reported in the paper. These results show that the hydraulic conductivity reduced with the vertical stress level due to the self weight of the fresh permeable concrete. This reduction is very significant in the range of vertical stress σv of 1–100 kPa (that correspond to the self-weight of a column of “classical” fresh pervious concrete h* ranging from about 0.1 to 6 m). Subsequently this reduction is less marked up to the maximum stress level reached in the tests (σv=700kPa, h* about 40 m). The minimum hydraulic conductivity (hydraulic conductivity for σv=700kPa) is, however, still sufficient and adequate for draining medium to fine-grained base soils (soil near piles or trench drains) such as clays, silty-clays, sandy-silts, silty-sands, and fine sands.

The presence of minor details of the ground, including soil or rock masses, occurs more frequently than what is normally believed. Thin weak layers, shear bands, and slickensided surfaces can substantially affect the behaviour of foundations, as well as that of other geostructures. In fact, they can affect the failure mechanisms, the ultimate bearing capacity of footings, and the safety factor of the geotechnical system. In this research, numerically conducted through Finite Element Code Plaxis 2D, the influence of a horizontal thin weak layer on the mechanical behaviour of shallow footings was evaluated. The obtained results prove that the weak layer strongly influences both the failure mechanism and the ultimate bearing capacity if its depth is lower than two to four times the footing width. In fact, under these circumstances, the failure mechanisms are always mixtilinear in shape because the shear strains largely develop on the weak layer. However, the reduction in the ultimate bearing capacity is a function of the difference between the shear strength of the foundation soil and the layer. The presence of a thin weak layer decreases the ultimate bearing capacity up to 90%. In conclusion, this research suggests that particular attention must be paid during detailed ground investigations to find thin weak layers. Based on the obtained results, it is convenient to increase the soil volume investigation to a depth equal to four times the width of the foundation.

The presented experimental study, from micro to macro scale, assesses the effects of grain shape on the strength-dilatancy behaviour and physical properties of seven natural sands of different origins and mineralogical composition. The micro-scale has been characterised by processing two-dimensional image of grains, using the fractal analysis of particle contour to define their shape. The macroscopic material properties are evaluated performing direct shear tests at three different normal stresses, and measuring void ratios limit and angles of repose. An innovative method to interpret the experimental data obtained has pinpointed interesting relationships linking the morphological features of grains to the mechanical and physical properties of sands. Data confirm that the increase in grain shape irregularity produces an increase in all the variables investigated: void ratios limit, friction angles and dilatancy parameters.

The compression in gap-graded mixtures of sands with combined mineralogy has been investigated in recent research, focusing on the key factors that might imply the occurrence of convergent or non-convergent paths in compression (i.e., transitional or non-transitional behaviour). From previous work, the mineralogy of a matrix composed of larger grains seems to determine the possibility of the occurrence of transitional behaviour. Hence, if there is a strong and stiff matrix made of quartz sand particles, which are either larger than or at least of similar size to the other component, then non-convergent compression paths (i.e., transitional behaviour) are likely to occur. As a further confirmation of this hypothesis, this technical note presents the results of oedometer tests on the same range of mixtures of a quartz sand and a carbonate sand as was used in previously published work, but with the larger component being of the stiffer and stronger quartz sand. In agreement with the hypothesis, transitional behaviour occurred.

Opalinus Clay is the shale currently under investigation as the host formation for geological radioactive waste disposal in Switzerland. Its hydromechanical response has been widely studied, and the experimental results show a range of values whose dispersion needs to be clarified. This work aims to explain the dispersion in the literature results by correlating the hydro-mechanical response to the mineralogical variability of the tested specimens . Based on published microstructural studies, the Opalinus Clay shale is herein schematised as a sequence of two kinds of layers: the shaly (high in clay-mineral content) and the sandy (low in clay-mineral content) layers. The mineralogical composition , porosity and hydromechanical parameters are assigned to each layer type. The one-dimensional compressibility and the elastic response of combinations of layers are derived adopting an analytical solution for the stratified, transversely isotropic medium . The drained elastic properties are used to calibrate the approach, and the undrained elastic properties are derived and compared to literature data. Empirical correlations between the layer composition and the geomaterial strength are also drawn. It is shown that by adopting a layered structure with an alternation of two kinds of layers, most of the variability in the studied geomechanical properties of Opalinus Clay can be captured.