The Residual Shear Strength of the Shaly and Sandy Facies of the Opalinus Clay

The paper presents a comprehensive laboratory campaign carried out with the aim to asses the residual strength of the Opalinus Clay. Ring shear tests with vertical effective stress up to 1 MPa were performed on remoulded samples of two different facies of the Opalinus Clay. Test results show that the «Shaly» facies is characterized by a low range of variation of residual strength while the strength of the «Sandy» facies is very sensitive to the variation of grain size distribution. Microstructural analyses (SEM) suggest that the difference in the observed mechanical behavior of the two facies can be related to different particles arrangements along the shear surfaces.

Volume change behaviour of a compacted scaly clay during cyclic suction changes

The research presented in this paper focuses on the investigation and modelling of the volume change response of compacted tectonised clay samples subjected to several wetting and drying cycles in controlled-suction oedometers. Oedometer tests were carried out under different values of constant vertical net stress, and wetting and drying cycles were performed varying applied matric suction between 10 and 800 kPa. The investigation was complemented with a study of the material microstructure to support the interpretation of the overall mechanical response. At a microscopic level, the material is characterized by different types of particle assemblages, scales, and clay aggregates. One of th…

Monitoring and prediction in early warning systems for rapid mass movements

Rapid mass movements (RMM) pose a substantial risk to people and infrastructure. Reliable and cost-efficient measures have to be taken to reduce this risk. One of these measures includes establishing and advancing the state of practice in the application of early warning systems (EWSs). EWSs have been developed during the past decades and are rapidly increasing. In this paper, we focus on the technical part of EWSs, i.e., the prediction and timely recognition of imminent hazards, as well as on monitoring slopes at risk and released mass movements. Recent innovations in assessing spatial precipitation, monitoring and precursors of the triggering and deformation of RMM offer new opportunities…

1D Compression Behaviour of Opalinus Clay

One of the main concerns related to tunnel excavations, drilling operations and wellbore stability in shales is the generation of excess pore water pressure due to changes in mechanical stress; therefore the consolidation of shales is a fundamental process that must be considered. This paper presents a compre- hensive methodology for analysing the compression and consolidation behaviour of shales. An apparatus to perform high-pressure oedometric tests is presented and an analytical method is introduced to analyse the shale consolidation behaviour, which allows information to be gathered on the coeffi cient of consolidation, stiffness, poroelastic properties, and permeability of the tested m…

Hydromechanical behaviour of a volcanic ash

This paper presents experimental analysis and numerical modelling aimed at improved understanding and prediction of the hydromechanical behaviour of volcanic ash at various states of saturation. Results from a comprehensive experimental programme are presented in order to characterise the response of the material in terms of matric suction and confining stress changes. The evolution of the yield stress at different suction levels has been quantified. The volumetric response with suction variations allowed the analysis of the collapse-upon-wetting behaviour. Water retention and permeability are also addressed. Tests results are used to calibrate a constitutive model based on the effective s…

Advances in the Testing of the Hydro-mechanical Behaviour of Shales

The paper presents some of the advanced experimental techniques and apparatuses that have been developed by the authors for testing the hydromechanical behaviour of shales. A methodology established for the analysis of the water retention behaviour in non-isochoric conditions is introduced; the method allows for the determination of the main drying and wetting paths in addition to the volume change response upon total suction variations. A high-pressure oedometric cell is then presented; the apparatus allows for the analysis of the transition from the pre-yield behaviour to the normally consolidated state. The analysis of the settlement versus time curves yields information on the permeabil…

The void ratio dependency of the retention behaviour for a compacted scaly clay

Resistenza a taglio di campioni rimaneggiati di Opalinus Clay

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 ta…

Mechanical behaviour of compacted scaly clay during cyclic controlled-suction testing

Scaly clays are stiff and highly fissured soils due to their complex geological history. Owing to these characteristics, they may be placed and compacted as excavated, to obtain a material with adequate engineering properties, suitable as core material in earth dams and in waste isolation fills. With reference to these engineering applications, an exhaustive experimental programme is developed to characterise the hydro-mechanical behaviour of this compacted material under a variety of controlled-suction paths. This paper presents selected test results, focusing on the interpretation of specific behavioural features of the volume change of this complex material; namely, the strain accumulati…

Volumetric Behaviour of Lime Treated High Plasticity Clay Subjected to Suction Controlled Drying and Wetting Cycles

The paper presents some experimental results collected on samples recovered from an experimental embankment obtained by compacting a lime-treated clay. Samples were collected soon after the in situ compaction and they were cured in controlled environmental conditions for at least 18 months. Mercury intrusion porosimetry tests (MIP) were carried out on freeze-dried specimens to characterize the microstructure of the material. In order to assess the durability of the improved material, laboratory tests focused on the effects of cyclic variations of the degree of saturation on the water retention properties and the volumetric behaviour of the stabilized clay. Collected results show that the li…

The Role of Anisotropy on the Volumetric Behaviour of Opalinus Clay upon Suction Change

An experimental investigation to analyse the anisotropic volumetric response of shaly and sandy facies of Opalinus Clay upon suction variations is presented. Obtained results demonstrate the different behaviour of the tested facies to a wetting-drying cycle. The shaly facies exhibits higher water retention capacity and stronger volumetric response than the sandy facies. Anisotropic response is experienced by both facies with the strain perpendicular to bedding higher than in the parallel direction. The sandy facies exhibits a more pronounced anisotropic behaviour in particular during the drying phase. A detailed analysis of the response in the two directions with respect to the bedding orie…

The pore structure of compacted and partly saturated MX-80 bentonite at different dry densities

Compacted MX-80 bentonite is a potential backfill material in radioactive-waste repositories. Pore space in MX-80 has been the subject of considerable debate. 3D reconstructions of the pore space based on tomographic methods could provide new insights into the nature of the pore space of compacted bentonites. To date, few such reconstructions have been done because of problems with the preparation of bentonite samples for electron microscopy. The nanoscale intergranular pore space was investigated here by cryo-Focused Ion Beam nanotomography (FIB-nt) applied to previously high-pressure frozen MX-80 bentonite samples. This approach allowed a tomographic investigation of the in situ microstru…

Water retention behaviour of compacted and reconstituted scaly clays

The paper presents the results of an experimental research devoted to investigate the response to suction variations of a scaly clay in compacted and reconstituted conditions. Different experimental techniques (axis translation, vapour equilibrium, dew point psychrometer suction measurements) were combined in order to explore the water retention properties in a wide suction range (0 ÷ 110 MPa). Experimental results allowed to define the water retention domains for a constant reference void ratio, highlighting the significant role of the microstructure on the response of the investigated clays. In particular, the collected results showed that in the low-medium suction range, the peculiar mic…

Microstructural characterization of a 3D-printed soil

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-undr…

A Comparative Study of Soil Suction Measurement Using Two Different High-Range Psychrometers

An extensive experimental study was performed to compare the measurement capabilities within different ranges of two total suction measurement equipment: SMI transistor psychrometers and a chilled-mirror dew-point psychrometer (WP4 Dewpoint PotentiaMeter). The equipment were used in a systematic way to determine the relative humidity of the environment surrounding different compacted clayey soils along drying paths and covering a wide total suction range (0.1 to 70 MPa). The equipment description and the calibration protocols followed are presented, as well as suggestions regarding the improvement of their performance. The water retention curves of two different compacted clayey soils are p…

Fabric evolution and the related swelling behaviour of a sand/bentonite mixture upon hydro-chemo-mechanical loadings

This experimental study investigates the swelling behaviour of an 80/20 sand/bentonite mixture and the associated fabric evolution under different hydro-chemo-mechanical loadings. Free and confined swelling tests are performed on specimens compacted to different dry densities and wetted with different pore fluids. Controlled suction confined swelling tests are performed to determine the suction–swelling pressure relationship for two different densities. An extensive microstructural characterisation is performed to relate the observed swelling behaviour to the evolution of the different pore networks during wetting. Based on the microstructural analysis, an expression is proposed to determi…

Shot-clay MX-80 bentonite: An assessment of the hydro-mechanical behaviour

This study presents the results of an experimental study conducted to characterise the hydro-mechanical behaviour of shot-clay MX-80 bentonite. In the shot-clay process, granular bentonite was mixed continuously with water and shot on the walls of a tunnel section at the Grimsel Underground Research Laboratory (URL), in Switzerland. The shot-clay was placed to create a layer of bentonite in direct contact with the host rock to avoid preferential water and/or gas flow along the tunnel wall. Samples for an experimental programme were collected during the shooting. The index properties, microstructural features, swelling potential and water retention properties of the shot-clay were analysed. …

One dimensional consolidation of Opalinus Clay from shallow depth

First experimental results on Opalinus Clay from shallow depth (< 30 m depth) are presented and compared to results on cores from Mont Terri Underground Rock Laboratory (~ 300 m depth). Samples were tested in one dimensional condition using an advanced experimental technique. The samples from the two sites show similar properties in terms of geotechnical characterization and one dimensional compressibility/swelling indexes, despite the different source depths.

Hydro-mechanical behaviour of shot-clay bentonite

Experimental tests were conducted to characterise the hydro-mechanical behaviour of shot-clay MX-80 bentonite. Granular bentonite was mixed continuously with water and shot on the walls of a tunnel section at the Grimsel Underground Research Laboratory (URL), in Switzerland. The shot-clay was placed to create a layer of bentonite in direct contact with the host rock to avoid preferential water and/or gas flow along the tunnel wall. Samples for an experimental programme were collected during the shooting. Results from an experiment, conducted under controlled total suction to analyse the hydro-mechanical behaviour of the material, are shown.

Evoluzione del comportamento idro-meccanico di una sabbia di quarzo dallo stato saturo allo stato asciutto

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 satu…

On the reactivation of a large landslide induced by rainfall in highly fissured clays

Abstract The paper discusses the field investigations, geotechnical characterization and time evolution of horizontal displacements in a wide landslide. The latter, triggered by an earthquake occurring in September 2002, is located in Sicily and involves a thick body of stiff and highly fissured clays belonging to a Varicoloured Clay formation. In order to characterise the reactivation of landslide mechanisms induced by rainfall, a three-year monitoring programme (2008–2011) was implemented to measure rain, pore water pressures, and deep and superficial displacements. The monitoring data made it possible to recognize three distinct landslides, which evolve at variable rates in different dir…

Water retention and swelling behaviour of granular bentonites for application in Geosynthetic Clay Liner (GCL) systems

Geosynthetic Clay Liner (GCL) systems are used as efficient hydraulic barriers in landfills for the disposal of hazardous municipal wastes. Along with geotextiles, bentonite materials are chosen as one of the primary components of GCLs due to their high retention, adsorption, and swelling capacities. GCLs are manufactured using bentonites at a high total suction and hydrated through the uptake of liquid from the subsoil and the confined material as soon as they are installed. Bentonites may exhibit considerable volume change upon wetting. Depending on the confinement stress, the void ratio may significantly increase with a decrease in suction, particularly at higher degrees of saturation. T…

Modelling landslides in unsaturated slopes subjected to rainfall infiltration using material point method

This paper presents a dynamic fully coupled formulation for saturated and unsaturated soils that undergo large deformations based on material point method. Governing equations are applied to porous material while considering it as a continuum in which the pores of the solid skeleton are filled with water and air. The accuracy of the developed method is tested with available experimental and numerical results. The developed method has been applied to investigate the failure and post-failure behaviour of rapid landslides in unsaturated slopes subjected to rainfall infiltration using two different bedrock geometries that lie below the top soil. The models show different failure and post-failur…

Drivers, barriers and impacts of digitalisation in rural areas from the viewpoint of experts

Abstract Context: The domain of rural areas, including rural communities, agriculture, and forestry, is going through a process of deep digital transformation. Digitalisation can have positive impacts on sustainability in terms of greater environmental control, and community prosperity. At the same time, it can also have disruptive effects, with the marginalisation of actors that cannot cope with the change. When developing a novel system for rural areas, requirements engineers should carefully consider the specific socio-economic characteristics of the domain, so that potential positive effects can be maximised, while mitigating negative impacts. Objective: The goal of this paper is to sup…

One-dimensional compression and consolidation of shales

This paper presents a comprehensive methodology for analysing the compression and consolidation behaviour of shales. An apparatus was designed to perform oedometric high-pressure tests by applying a maximum vertical total stress of 100 MPa and simultaneously controlling the pore water pressure of the specimen. An analytical method was formulated to analyse the shale consolidation behaviour, which allows information to be gathered on the coefficient of consolidation, stiffness, poroelastic properties, secondary compression and permeability of the tested material as a function of the applied stress conditions. Results obtained on Opalinus Clay shale using the developed methodology are present…

Life cycle environmental assessment of retaining walls in unsaturated soils

Abstract Retaining structures above groundwater level support soils that are usually in a state of partial saturation and subject to the actions of atmospheric agents. The current design approach considers the possible extremes of soil conditions – either totally dry or totally saturated – but it neglects matric suction’s contribution to soil shear strength. This work aims to describe how unsaturated-soil mechanics of can positively influence the sustainability of retaining structures through a holistic, multidisciplinary, geotechnical, and environmental analysis. The geotechnical analysis allows to estimate the lateral earth pressure of a geostructure in both unsaturated and extreme soil c…

Techniques for the modelling of the process systems in slow and fast-moving landslides

This chapter reviews some of the current strategies for landslide modelling. Main physical processes in landslides are first recalled. Numerical tools are then introduced for the analysis of the behaviour of slow- and fast-moving landslides. Representative case studies are introduced through the chapter to highlight how different modelling strategies can be used depending on the physical processes that the modeller wants to take into account.

Discussion on “Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions: Mechanical Properties and the Influence of Rock Fabric” of Schuster, V., Rybacki, E., Bonnelye, A., Herrmann, J., Schleicher, A.M., Dresen, G.

AbstractThe 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.

Effect of climate change on landslide behaviour

Landslides represent a major threat to human life, constructed facilities and infrastructure in most mountainous regions of the world. Considering future climate scenarios and modified precipitation patterns, the landslide activity will most probably change too. It is expected that shallow slips and debris flows will take place more frequently as a consequence of more extreme weather events. For the local scale of Switzerland for example, there is no increasing trend visible for the total rainfall amount, but an increasing tendency is observed for stormy events and their intensity, especially in fall, spring and winter. Some precipitation is expected to fall as rain instead of snow at highe…

Shear strength of a compacted scaly clay in variable saturation conditions

Scaly clays are stiff and highly fissured clays often used as construction materials. This paper presents the results of triaxial compression tests carried out on saturated and unsaturated samples of a compacted scaly clay. Complementary investigation on the microstructural features and their evolution with the amount of water stored into the material are also presented in order to shed light on the evolution of the micro- and macroporosity with suction. The water retention behaviour of the compacted scaly clay is also addressed. The results from the controlled suction triaxial tests are used to discuss the applicability of a single-shear strength criterion to compacted double-structured cl…

Long-term performance and life cycle assessment of energy piles in three different climatic conditions

Abstract The main purpose behind the use of energy piles is to enable the exploitation of geothermal energy for meeting the heating/cooling demands of buildings in an efficient and environment-friendly manner. However, the long-term performance of energy piles in different climatic conditions, along with their actual environmental impacts, has not been fully assessed. In this paper, the results of a finite element model taking into consideration the heating and cooling demands of a reference building, and the intermittent operation of a ground source heat pump, are revealed to examine the long-term performance of energy piles. Furthermore, a life cycle assessment model is implemented to com…

On the hydro-mechanical behaviour of a lime-treated embankment during wetting and drying cycles

Abstract The paper presents some experimental results obtained on samples extracted from a full-scale embankment obtained by compacting a lime-treated clayey soil. A comprehensive test programme was carried out in order to highlight the improvement of mechanical behaviour induced by lime treatment as well as to assess the durability of the improved material, which may be affected by severe seasonal wetting and drying cycles. Direct shear tests, triaxial compression tests, swelling potential measurement and oedometric tests were performed on samples cured in controlled environmental conditions for at least 18 months. Wetting and drying cycles were applied in a very wide range of suction valu…

Experimental investigations of the soil–concrete interface: physical mechanisms, cyclic mobilization, and behaviour at different temperatures

Behaviour of the pile–soil interface is important to correctly predict the response of floating piles in terms of displacement and lateral friction. Regarding energy piles, which couple the structural roles of deep foundations with the principle of shallow geothermal energy, the response of pile–soil interfaces is influenced by seasonal and daily cyclic thermal variations. Accordingly, the goal of this paper is to experimentally investigate the response of the pile–soil interface at different temperatures. This experimental campaign aims to analyse (i) the cyclic mobilization of the shear strength of the soil–pile interface that is induced by thermal deformation of the pile and (ii) the di…

Numerical investigation on water exchange of shale samples

Interest in the hydraulic and mechanical characterization of shales has grown in recent years, because of their application in the context of energy geotechnics. In the frame of nuclear waste disposal shales are considered as host formations for the placements of nuclear waste at high depths. In the frame of hydrocarbon production they are considered as unconventional reservoirs, from which extracting natural gas. Understanding how fluids flow through shales is then a key aspect for both fields of application. This paper focuses on the analysis of the transport of water vapour through laboratory samples. After reviewing the balance and flow laws that govern the transport of fluid in unsatur…

Suction controlled oedometric tests: analysis of some experimental aspects

The results of suction controlled oedometric test on compacted clay samples are discussed with reference to some aspects of the axis translation technique. The air overpressure procedure, based on a very gradual increment of the pore air reference pressure, is proved suitable in tests on samples with high saturation degree even when high intensity air pressure increments are requested. The effects of the axis translation on the recorded soil water volume changes, due to air diffusion through the HAEV ceramic disk and soil water evaporation to air pressure line. Are analyzed using of suction controlled wetting and drying tests

Coupled hydro-mechanical analysis of compacted bentonite behaviour during hydration

Abstract This study analyses the response of compacted bentonites upon hydration based on a coupled hydro-mechanical elasto-plastic framework. As an alternative to multi-porosity interpretation, the framework was selected based on the experimental evidence of adsorbed water behaviour in bentonites and the volumetric response at saturated states, apparently independent of its initial state. Based on these premises, a water retention model was formulated using an explicit distinction between adsorbed water and free water, enabling the postulation of the water properties and behaviour depending on its state. In order to effectively account for the transition between unsaturated to saturated st…

Role of Stress History on the Swelling-Shrinkage Behavior of Compacted Scaly Clay

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. More…

Hydro-mechanical behaviour of a sandy silt from a river embankment

The paper presents the results of an experimental campaign aimed at characterizing the hydro-mechanical behaviour of a sandy silt from a river embankment. Due to continuous river level fluctuations and changing climatic and environmental conditions, flood embankment materials experience frequent variations in degree of saturation and suction values. Such variations strongly impact the earthwork performance both in terms of seepage and stability conditions. For these reasons, a detailed characterization of the material behaviour in unsaturated conditions was carried out. Experiments were designed in order to highlight the response of the involved soil in terms of changes in matric suction an…

Increasing understanding and confidence in THM simulations of engineered barrier systems

Previous studies on the modelling of coupled thermo-hydro-mechanical (THM) processes in bentonite-based engineered barrier systems (EBSs) showed the sensitivity of the output quantities to changes in the input parameters. To investigate the effects of uncertainties on the modelling results, to improve the understanding of the coupled processes active in the repository near field and to gain in-depth understanding of model uncertainties of different codes, a sensitivity analysis and code comparison of EBS simulations was performed within the Task Force on Engineered Barrier Systems. The analysis included variations in material parameter values, boundary and initial conditions, considered phy…

An experimental and constitutive investigation on the chemo-mechanical behaviour of a clay

Engineering issues for which the understanding of the chemo-mechanical behaviour of soils is relevant include wellbore stability problems, the salinification of groundwater, and nuclear waste storage. However, despite the vast number of situations in which couplings between chemistry and mechanics occur, the available constitutive models rely on limited experimental evidence. This paper presents the results of an experimental programme on the chemo-mechanical behaviour of a non-swelling illite. The osmotic suction is controlled through the ion concentration of sodium chloride in the pore water. Stress paths include mechanical loading at a constant osmotic suction, and an increasing osmotic …

Volume change response and fabric evolution of granular MX80 bentonite along different hydro-mechanical stress paths

AbstractDespite 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 exper…

Experimental assessment of the hydro-mechanical behaviour of a shale caprock during CO2 injection

Abstract The presented experimental study focuses on the hydro-mechanical characterisation of a shale caprock (Opalinus Clay) in contact with carbon dioxide. The objective of this paper, consists in the evaluation of the material's sealing capacity in terms of entry-pressure, mechanical behaviour and sensitivity of the transport properties to chemo-mechanical effects induced by gaseous and liquid CO2 injection. Two types of Opalinus Clay core samples are tested; shaly and carbonate-rich. The sealing capacity has been evaluated on the shaly OPA according to the stepwise and the residual methods and compared to the results from mercury intrusion porosimetry. The obtained results and the diffe…

One-Dimensional Transient Analysis of Rainfall Infiltration in Unsaturated Volcanic Ash

The paper presents a one-dimensional hydro-mechanical analysis of rainfall infiltration in a loose volcanic ash and the utilisation of a factor of safety for the implementation of an early-warning system. Three different rainy seasons with different rainfall patterns were analysed . The analysis aims to understand the influence of the antecedent rainfall on the wetting front, the pore-water pressures and the factor of safety. The analysis was carried out in the context of a Master project of the first author at the Laboratory for Soil Mechanics of EPFL.

Displacement Evolution of a Large Landslide in a Highly Fissured Clay

The paper presents an analysis and modelling of the reactivation mechanism of a large landslide caused by rainfall, located in Cerda (Sicily, Italy). The study is based on data collected by means of a field investigation, carried out in order to assess the geotechnical properties of the involved material, and a three-year monitoring programme implemented to measure rain and pore water pressures as well as deep and superficial displacements. Three distinct landslides, evolving at variable rates in different directions, were recognized within the overall landslide area. The data clearly show the mechanical role of pore water pressures in the stability of the slope and point out the dependence…

Effect of the mineralogical composition on the elastoplastic hydromechanical response of Opalinus Clay shale

Abstract 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 mineralogic…

An insight on the thermo-mechanical behaviour of a shale

In recent years, several shale formations have been employed in geo-energy related engineering fields that foresee the presence of high thermal loads or significant temperature variations. Such thermal conditions are related to a need to reach great depths (several hundred of meters), as in the case of CO2 sequestration and Enhanced Geothermal System (EGS) technologies, or to the presence of thermal sources, as in the context of nuclear waste geological disposal. In addition, thermal changes can occur in shale formations through the injection of drilling and fracturing fluids at great depths, where a higher temperature with respect to the one of the injected fluid is encountered as a result…

Microstructure and shear strength evolution of a lime-treated clay for use in road construction

The results of a comprehensive experimental programme are presented for assessing the long-term microstructural modifications and evaluating the effects of microstructural rearrangement on the stress–strain behaviour of a lime-treated high-plasticity clay for road embankments. The stress–strain behaviour at different lime content and curing time was investigated by means of direct shear tests; microstructural analyses were carried out combining Scanning Electron Microscope observations and Mercury Intrusion Porosimetry tests. The results show that the stress–strain behaviour of treated clay is strongly dilatant with a high peak of strength, which increases with time and lime content. Furthe…

Effects of the foot evolution on the behaviour of slow-moving landslides

The paper presents a time-dependent 2D numerical model which has been developed with the purpose of highlighting the effects of the slope foot evolution on the behaviour of slow-moving landslides. The model allows to quantitatively analyse how foot mass variations can influence the stability and the movement rates of the landslide. The landslide body is modelled as composed of two rigid blocks sliding on two different planes and interacting through a common boundary, which position is assumed fixed during the analysis. A finite difference approach is used to discretize the time. For each time increment, changes in model parameters are allowed, including variations in shearing resistances, g…

Experimental analysis of a thermoactive underground railway station

Little is known about the real energy potential of thermoactive underground infrastructures, such as railway stations, that can act as a heating/cooling provider for the built environment. This study presents the results of thermomechanical full-scale in situ testing and numerical analysis of a thermoactive underground train station. The thermal performance and related geostructural impact of a portion of the new underground energy infrastructure (UEI) installed at the Lancy-Bachet train station in Geneva (Switzerland) are analyzed. Heating and cooling tests simulating real operative geothermal conditions are considered. Particular attention is given to ((i) the monitored wall-tunnel hydrot…

On the hydro-mechanical behaviour of remoulded and natural Opalinus Clay shale

The geo-energy sector makes use of advanced technologies such as shale gas extraction, CO2 sequestration and nuclear waste geological disposal that rely on the exploitation of shale formations. Due to the great depths involved in these applications and the difficulties in retrieving intact samples, remoulded shale specimens are often adopted for hydro-mechanical testing. Remoulded and intact shales may substantially differ in their hydro-mechanical behaviour due to the particular structure of the natural material, which is the result of diagenesis and burial history. This paper presents an experimental campaign aimed at (i) characterizing the role of diagenesis and depth for Opalinus Clay s…

The impact of the volumetric swelling behavior on the water uptake of gas shale

Abstract Water uptake of gas shales is commonly considered one of the most important factors responsible for fluid loss during flowback operations after hydraulic fracturing. Imbibition experiments cover a key role in this context to analyze the impact of several factors (such as mineralogical composition, fluids composition, anisotropic structure, volumetric swelling) that contribute to the water uptake in these unconventional reservoirs. The aim of this study is the quantification of the impact of the volumetric response (swelling and shrinkage) of gas shales on the water uptake during imbibition and desiccation processes. An experimental methodology to quantify the volumetric response du…

The shear strength of Opalinus Clay shale in the remoulded state

Abstract The Opalinus Clay shale formation is considered as a potential host geomaterial for the Swiss deep geological repository for radioactive waste. It presents different facies and it is characterised by a multi-scale heterogeneous composition, by a typical fissile structure with well-defined bedding planes and by anisotropic hydro-mechanical behaviour. This peculiar complexity makes it difficult to assign a unique set of geomechanical parameters to the material. This paper presents an experimental study aimed at characterising the lowest values of the shear strength parameters. In this sense, the shear behaviour was investigated on remoulded samples where the fabric and the diagenetic…

Thermo-mechanical volume change behaviour of Opalinus Clay

The paper examines the thermo-mechanical volume change behaviour of Opalinus Clay in relation to different stress conditions and overconsolidation ratio (OCR) values and evaluates the impact of temperature on some hydro-mechanical properties of this material. To this aim, a focused experimental campaign consisting in high-temperature/high-pressure oedometric tests has been carried out. The results show that the thermo-mechanical volume change behaviour of Opalinus Clay is heavily affected by the OCR: thermal expansion is found when the heating is carried out at high OCR, whereas irreversible thermal compaction is observed when heat is applied at a vertical effective stress that is sufficien…

Early warning thresholds for partially saturated slopes in volcanic ashes

Rainfall-induced landslides in steep soil slopes of volcanic origin are a major threat to human lives and infrastructure. In the context of constructing early warning systems in regions where extensive data on landslide occurrences and associated rainfall are inexistent, physically-based tools offer the possibility to establish thresholds for measurable field quantities. In this paper, a combined finite element infinite slope model is presented to study the transient hydraulic response of volcanic ash slopes to a series of rainfall events and to estimate seasonal safety factors. Furthermore, analytical considerations of partially saturated infinite slopes are made to define capillary stress…

Volume change characteristics of fine-grained soils due to sequential thermo-mechanical stresses

Know-how of volume change characteristics, VCC, of the fine-grained soils, exposed to thermal stresses, is essential for design of various thermo-active structures. These stresses are known to induce excess pore-water pressure, Δu θ , in the saturated state of such soils, which in turn affects their compression and shear strength characteristics. In this context, through several experimental studies, the effect of thermo-mechanical stress-path, the over-consolidation ratio (OCR) and degree of saturation on VCC (viz., thermally induced volumetric strain, ε vθ , compression and re-compression indices, c c and c r ) of the fine-grained soils has been demonstrated by earlier researchers. Howeve…

Yielding of a quartz sand from saturated to dry state

The paper presents the results of an experimental work where we analyse the behaviour of an unsaturated quartz sand in a wide range of degree of saturation (from saturated to dry state). The possibility of anticipating the hydro-mechanical behaviour of the soils when they approach the dry state is fundamental in many areas. An extensive experimental program, including controlled-suction and constant water content oedometric tests, was carried out to deeply analyse the water retention behaviour and the relationship between the yield stress and suction (Loading-Collapse curve). All elasto-plastic models provide a monotonically increase of the yield stress with suction. This assumption implies…

Benchmark study of undrained triaxial testing of Opalinus Clay shale: Results and implications for robust testing

Triaxial testing of argillaceous rocks and shales is significantly more challenging than conventional rock mechanical testing. The challenges are mainly related to the very low permeability of these geomaterials, and their sensitivity to exposure of atmosphere and brines, which induces variations of water content, suction and effective stress. There are currently no international standards to guide service laboratories for robust testing procedures for shales. A benchmark study of undrained triaxial testing was therefore initiated with three leading service laboratories in shale testing, performing 13 tests and using two different methods of establishing sample saturation prior to deformati…

Suction controlled drying and wetting cycle effects on the volumetric behaviour of a lime-treated high plasticity clay

The paper presents some experimental results collected on samples recovered from an experimental embankment obtained by compacting a lime-treated clay. Samples were collected soon after the in situ compaction and they were cured in controlled environmental conditions for at least 18 months. Mercury intrusion porosimetry tests (MIP) were carried out on freeze-dried specimens to characterize the microstructure of the material. In order to assess the durability of the improved material, laboratory tests focused on the effects of cyclic variations of the degree of saturation on the water retention properties and the volumetric behaviour of the stabilized clay. Collected results show that the li…

Hydro-mechanical analysis of volcanic ash slopes during rainfall

Rainfall-induced landslides in volcanic ashes represent a major natural hazard in many regions around the world. Owing to their loose structure, volcanic ash slopes are prone to rainfall-induced landslides. The paper presents a continuum modelling approach for the analysis of wetting-induced instability phenomena at the onset of failure in loose volcanic ash slopes. A numerical simulation of a landslide-prone volcanic slope in Costa Rica is carried out with a two-dimensional hydro-mechanical finite-element slope model. A constitutive model based on the effective stress concept extended to partially saturated conditions is used to reproduce the volcanic ash hydro-mechanical behaviour. The m…

Coupled seismogenic geohazards in Alpine regions

COupled seismogenic GEohazards in Alpine Regions (COGEAR) is an interdisciplinary natural hazard project investigating the hazard chain induced by earthquakes. It addresses tectonic processes and the related variability of seismicity in space and time, earthquake forecasting and short-term precursors, and strong ground motion as a result of source and complex path effects. We study non-linear wave propagation phenomena, liquefaction and triggering of landslides in soil and rock, as well as earthquake-induced snow avalanches. The Valais, and in particular parts of the Rhone, Visper, and Matter valleys have been selected as study areas. Tasks include detailed field investigations, development…

Hydro-mechanical behaviour of shallow Opalinus Clay shale

In Switzerland, Opalinus Clay shale has been selected as the host formation for radioactive waste disposal. The minimum required depth of the repository is related to the long-lasting isolation required for the disposal (1 million years). During this period, possible erosion scenarios affecting the repository need to be analysed. Opalinus Clay from shallow depths (< 70 m) was sourced from a borehole in Northern Switzerland, where the formation was affected by a considerable exhumation process. This work aims to investigate the impact of the mentioned phenomenon on the hydro-mechanical behaviour of Opalinus Clay through one-dimensional consolidation and permeability measurements. Laboratory …

An effective stress based shear strength criterion for unsaturated compacted double-porosity clays

An effective stress based shear strength criterion is proposed taking into account the fundamental effects of microstructure characteristics on retention properties of compacted double-porosity clays and their overall coupled hydro-mechanical behaviour. Results of triaxial compression tests carried out on saturated and unsaturated samples of a compacted scaly clay show that points representative of failure conditions depict a unique failure envelope when the macropore degree of saturation is used in the definition of the effective stress and its evolution with the suction is taken into account. Furthermore, the proposed criterion is very effective to consider the most relevant aspects of su…

A physical model for the interaction between unsaturated soils and retaining structures

Temporary and permanent retaining structures interact with soils that are usually in unsaturated conditions. In this work, a 1g-scale physical model is presented to investigate the interactions between retaining walls and unsaturated soils. The physical model is equipped with a water-filled hydraulic cylinder connected to a pressure-volume controller to measure the horizontal component of the later earth thrust and high capacity tensiometers to measure soil matric suction. A system of low-friction linear guideways has been installed at the base of the wall-model. The failure surface is observed through a 3 cm thick glass wall on one side of the container. A series of images are acquired dur…

Anisotropic volumetric behaviour of Opalinus clay shale upon suction variation

Opalinus clay shale is under consideration to serve as the host geomaterial for the underground storage of nuclear waste in Switzerland. Water retention behaviour and anisotropic behaviour represent two main features of this geomaterial; however, a comprehensive understanding of the interaction between these two features is still lacking. This paper aims to provide a detailed experimental analysis of the coupling between the water retention capacity and the anisotropic behaviour of two facies (shaly and sandy) of the Opalinus clay shale. The response of the tested geomaterials is characterised by an unequal swelling and shrinkage response in directions parallel and perpendicular to the bed…

The void ratio dependency of the retention behaviour for a compacted clay

The paper presents the results of a comprehensive experimental programme aimed to characterize the retention properties of a compacted scaly clay by determining the retention curves in terms of main wetting and main drying branches at different values of void ratio, ranging between 0.45 and 0.80. Different experimental techniques were combined in order to explore the retention properties in a wide range of suction. Vapour equilibrium technique was used to impose total suction in the range 2 ÷ 110 M Pa, while the air overpressure technique was applied to control matric suction in the range 0.01 ÷ 0.80 M Pa. Collected results clearly pointed out a strong dependence of the retention curves on …

Anisotropic Volumetric Response of Shales upon Suction Changes

Anisotropic fabric and volumetric response to suction changes represent two main features of shales, which are involved in several engineering applications. However, a comprehensive understanding of the interaction between these two aspects is still missing. The present work aims to provide a detailed analysis of the coupling between the water retention capacity of shales and their anisotropic behaviour through an advanced experimental methodology. Among the various tested facies of the Opalinus Clay Shale (Mont Terri, Switzerland), the results from the shaly facies are presented in this study. The behaviour of the material is characterized by swelling and shrinkage respectively upon wettin…

TERRE project: interplay between unsaturated soil mechanics and low-carbon geotechnical engineering

The geotechnical construction industry is a major component of the overall construction sector and is strategically important in infrastructure development (transportation, flood and landslide protection, building foundations, waste disposal). Although industry and research in the overall construction sector have been investing significantly in recent years to produce innovative low-carbon technologies, little innovation has been created in geotechnical construction industry, which is lagging behind other construction industry sectors. This paper discusses the interplay between low-carbon geotechnical engineering and unsaturated soil mechanics based on the research carried out within the pr…

Nonstationary flow surface theory for modeling the viscoplastic behaviors of soils

Abstract This paper presents a three-dimensional elastic viscoplastic model that can describe the time-dependent behaviors of soft clays. The constitutive model is formulated based on the nonstationary flow surface theory and incorporates new developments, including (i) an improved definition of the nonstationary flow surface that is capable of capturing the stress–strain behaviors under different loading paths, (ii) a unique stress–strain—viscoplastic-strain-rate equation that is able to explicitly describe the nonstationary flow surface, and (iii) a final stable state concept that identifies the final equilibrium state at the end of creep and stress relaxation, which is also used to simpl…

Water retention behaviour and microstructural evolution of MX-80 bentonite during wetting and drying cycles

MX-80 bentonite used in engineered barrier systems would be subjected to wetting and drying cycles. To assess the response of the material under such circumstances, a comprehensive experimental characterisation of the water retention behaviour of compacted MX-80 granular bentonite was performed in this study. A new methodology is proposed to investigate this behaviour under a constant volume condition for specimens prepared at different dry densities. The material was subjected to different hydraulic paths, including cyclic variations of the water content. As a result, an irreversible modification of the retention behaviour was observed when the material approached a fully saturated state …

Hydro-Mechanical Behaviour of a Sand-Bentonite Mixture for the Confinement of Nuclear Wastes

The established concept for the protection of the biosphere from high-level nuclear wastes consists in the use of multi-barrier systems in deep geological repository, for which clayey geomaterials are key elements. Bentonite-based materials are selected as buffer and backfilling materials. Once in place, the barriers will be subjected to a series of severe loadings, including strong variations of suction (i.e. wetting and drying cycles). The paper presents selected results from a large ongoing research work to investigate the behaviour of bentonite-based materials in the context of nuclear waste disposal. In particular, the paper focuses on the response of a Sand/Bentonite mixture under dif…

Comportamento idro-meccanico di terreni estrusi

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.

Deformability of compacted tectonised clay in controlled-suction oedometer tests

Early-stage thermal performance design of thermo-active walls implemented in underground infrastructures

Abstract Energy geostructures (EGs) represent an innovative technology in the sustainable energy agenda and are useful for satisfying the energy needs of the built environment. They are usually involving geostructures such as piles, walls, tunnels, shafts, and sewers. The application of such technology to infrastructure projects may have considerable thermal potential because of the large surfaces that can be thermally activated. This study focuses on thermo-active walls (energy walls, EWs), which are retaining structures used to sustain the sides of excavations. Key features related to their thermal design are examined, and a design methodology is proposed. The heat-exchange modes involvin…

Experimental analysis of the water retention behaviour of shales

Shales are among the most commonly considered geomaterials in current energy-related geomechanical investigations, as they are involved in engineering applications such as the unconventional extraction of natural gas, CO2 sequestration and nuclear waste geological storage. A deep understanding of their behaviour with regard to variations in the degree of saturation is of primary significance for such applications. With the aim to establish a sound protocol for shale retention behaviour analysis, this paper presents an advanced experimental methodology that takes into account the specificities of shales such as small pore size dimensions and the large variations in water potential to be appl…

Testing of the hydro-mechanical behaviour of shales.

Shales are among the most commonly considered geomaterials in current energy-related geomechanical investigations, as they are involved in engineering applications such as the unconventional extraction of natural gas, CO2 sequestration and nuclear waste geological storage. A deep understanding of their hydro-mechanical behaviour is of primary significance for such applications. In this paper selected results on the water retention behaviour and on the high pressure oedometric compression behaviour of shales are presented.

The Permeable Concrete: A Low Energy Consumption Solution for Deep Draining Trenches

The reduction of pore water pressures is one of the very effective measures to improve the stability conditions of marginally stable water-bearing slopes or to stabilise landslides. For this purpose the trench drains have been used long since. Like filling material of deep trenches the permeable concrete can be effectively employed. It presents relatively high hydraulic conductivity, filtering capacity in order to prevent the internal erosion of the soil in which the trench drain is installed, enough residual hydraulic conductivity after possible clogging, sufficient shear strength after a short curing time to avoid the instabilisation of adjacent previously built panels or piles. Results o…

Generalized effective stress concept for saturated active clays

Experimental evidence shows that changes in pore-water chemistry can significantly affect the mechanical behavior of saturated active clays. Despite this evidence, how the chemical composition of the pore water can be considered in effective stress definition is questionable. This paper develops the concept of generalized effective stress for active clays. To this end, physicochemical studies on water–clay mineral interactions are used to clearly define the different types of ions and water present in an active clay. In particular, the presence of both movable and non-movable ions within the liquid water is highlighted. Taking this into account, thermodynamic and geochemistry principles ar…

Hydro-mechanical modelling of a natural slope affected by a multiple slip surface failure mechanism

A coupled hydro-mechanical formulation is presented for the analysis of landslide motion during crisis episodes. The mathematical formulation is used to model a natural slope affected by a multiple slip surface failure mechanism, in which pore water pressure evolution was identified as the main cause for movement accelerations. An elasto-plastic constitutive model is adopted for the behaviour of slip surfaces. Material parameters are obtained by combining the available laboratory tests and the back analysis of some crisis episodes. After being calibrated and validated, the model is applied to improve the understanding of the physical processes involved and to predict the landslide behaviour…

Characterization of gas flow through low-permeability claystone: laboratory experiments and two-phase flow analyses

For the characterization of gas migration through a low-permeability clay host rock for deep underground repositories, a comprehensive understanding of the relevant phenomena of gas and fluid flow through low-permeability clay is required. The National Cooperative for the Disposal of Radioactive Waste (Nagra) in Switzerland has developed a comprehensive programme to characterize gas flow in low-permeability Opalinus Clay through laboratory tests and detailed numerical analyses for developing appropriate constitutive models. Laboratory tests were performed on cores by two different laboratories, the Laboratory for Soil Mechanics at EPFL and the Department of Geotechnical Engineering and Geos…