Oedometer Research Articles

Geotechnical investigation of soil bearing capacity a case study of Nile university of Nigeria

Geotechnical investigation was conducted within Nile University soil with the aid of the master plan showing areas awaiting future developments. This is as a result of the university planning for infrastructural expansion due to such in student’s population. Each time a building construction is to be taken place, the university management spent another set of money on conducting geotechnical investigation. There are no comprehensive geotechnical information of the soil bearing capacity that can represent the whole of Nile university space/area of 1,153,208,008?2. In-situ and laboratory test were carried out. Three boring point were established using map at a regular interval and drilled to a depth where basement was encountered. In each test sample were collected at 1.5m interval. The standard penetration procedure consist of split spoon and u-tube as set forth in ASTM-1990. The test involves counting the number of blows required to drive a spoon sampler by 300mm, using a weight of 64kg hammer with a free fall of 76mm on the anvil of a rod arrangement. An initial penetration of 150mm is allowable on the sampler known as sitting blows before counting the subsequent blow for the required 300mm. another six trial pit were evenly spread at regular interval at different location within the university. It was manually excavated by hand, undisturbed and disturbed sample were extracted by hand auger and shovel. Sample were extracted at 1.5m depth, properly labelled and taken to laboratory. The laboratory test of the extracted soil sample include moisture content, grain size distribution, liquid limit, plastic limit, consolidated drained test and unified soil classification system was adopted. It was observed that the results obtained from Standard Penetration test indicates variance in allowable bearing capacity with depth, between 0.0-0.5m is 56??⁄?2, 0.6- 1.0m is 112??⁄?2, 1.5- 2.0m is 215??⁄?2, 3.0 – 3.5 is 279??⁄?2, 4.5 – 5.0m is 273??⁄?2. The soil type were predominantly medium dense to very dense inorganic silty sand with clay content less than 5%. Point of refusal was encounter at 6m depth, boring was then terminated. The Direct shear box test analyses obtained from site BC and CH gave the allowable bearing capacity values of 250??⁄?2 at pit one, 211.2??⁄?2 at pit two, 798.1??⁄?2 at pit three, at pit four, 798.1??⁄?2, at pit five, 918??⁄?2 , at pit six 960.5??⁄?2, but with the results obtained from oedometer test show that we cannot adopt this allowable bearing capacity value due to it high settlement rate beyond the acceptable settlement rate set by Development Control in Nigeria, which is 100mm. therefore based on the result from oedometer test the allowable bearing capacity of 120??⁄?2 was adopted with a safe consolidated settlement of 52mm at site BC. While an allowable bearing capacity of 100??⁄?2 with safe consolidated settlement of 55mm was adopted for site CH. Site BC is predominantly underlain by Sandy Silt. While site CH the predominant soil type was Clayey Sand. Pad footing at 1.5m depth and 210??⁄?2 allowable bearing capacity should be adopted. It is recommended that for both site BC and CH pad foundation at 1.5m having maximum individual column load less than 600??⁄?2 with a ground beam adopted.

Shear strength and compressibility of reconstituted Boom clay, a stiff clay from the Paleogene

Boom clay is a stiff marine clay from the Oligocene epoch of the Paleogene period. It outcrops along the rivers Rupel and Scheldt in Northern Belgium, nearby the city and the port of Antwerp. Most of the existing geotechnical investigation around the Antwerp area, where the clay is located at relatively shallow depths, consists of standard laboratory tests on undisturbed samples performed at stress levels around the estimated in situ stress. Although the geological preconsolidation pressure of this clay is relatively well known, the clay yields at much higher stresses in 1D compression. Moreover, due to the brittle behavior of the stiff clay, it is often difficult to evaluate its shear strength behavior beyond peak strength. To this end, evaluating intrinsic parameters of compressibility, critical state shear angle and residual strength is of great relevance. In the present research, the mechanical behavior of reconstituted normally consolidated Boom clay was examined in a series of oedometer, CRS, CU triaxial, and multi-reversal direct shear tests. The mechanical behavior of the reconstituted clay was compared to the behavior of undisturbed clay samples. The results allow for a better evaluation of the structure effects of the natural samples on their mechanical response.

Multivariate probabilistic assessment of a regional database in Copenhagen

Extensive research has been carried out to establish empirical equations between two geotechnical properties and in spite of the increase in correlation studies, a construction of a multivariate distribution function of more than two parameters is still rare. The objective of this study is to investigate the possibility of constructing two multivariate distribution functions, each consisting of 5 geotechnical parameters based on a) triaxial and soil classification tests and b) oedometer and soil classification tests in stiff over-consolidated clay till from Copenhagen. For this purpose, laboratory measurements from twenty-six sites in Copenhagen are utilized and two multivariate databases are constructed. The correlations among the ratio of deviatoric and mean effective stresses (q/p´), over-consolidation ratio (OCR), secant modulus at reference stress 100 kPa (E50ref), initial void ratio (eo) and liquid limit (LL) are investigated in multivariate model A, while multivariate model B demonstrates the correlations among oedometer modulus at 100 kPa reference stress (Eoed(ur)ref), over-consolidation ratio (OCR), permeability change index (Ck), initial void ratio (eo) and liquid limit (LL). The Nataf transformation model is used for the construction of the multivariate distributions, which are then used to simulate the correlations between geotechnical properties by generating artificial samples. Finally, the artificial samples are compared with the original database for an initial validation of the model. The constructed multivariate models obtained as a result of this study can act as prior for Bayesian updating in multivariate distribution functions when additional geotechnical tests are carried out.

Laboratory study of a kaolinitic soil and sodium hydroxide interaction mechanisms and resulting swelling stresses

The study of prolonged exposure of natural soils to alkaline solutions has been conducted to analyze and control the local soil swelling behavior observed in industrial sites. The occurrence of foundation heave in a Brazilian industrial site prompted a laboratory investigation aimed at understanding the mechanism of interaction between soil and alkali that induced swelling behavior. The study was extended to determine the corresponding swelling stress to be used in reinforcement design, and to investigate the soil response to chemical stabilization with 5% ferric chloride solution. Previously, a comprehensive mineralogical and physical-chemical investigation was accomplished using industrial caustic liquor and four natural soils from local stratigraphy. All soils are mainly constituted by kaolinite and quartz, and one also presents some montmorillonite. Both kaolinite and montmorillonite were degraded by NaOH liquor, and newly formed minerals were identified. The expansion is related to the mechanism of the silica-NaOH reaction. In the study reported here, the local soil with the highest volume change in sedimentation tests with NaOH solutions was subjected to quasi-constant volume oedometer tests with water and with 4N NaOH solution to determine the swelling stresses resulting from the reactions until stabilization. The alternative of chemical treatment with 5% ferric chloride solution was investigated through the same testing procedure. X-Ray diffraction analyses were applied to evaluate the changes in mineralogy. The swelling vertical stress reached 40 kPa under confined conditions, and stabilization took up to 40 days. The chemical treatment was effective in controlling ongoing reactions, but not effective in prevention.

Index swelling prediction of clayey soils

In civil engineering, statistical studies are widely used in risk studies of landslides, seismic, swelling-shrinking and other hazard studies. This study deals with a volumetric deformation of the clayey soil by water absorption, which is the phenomenon of swelling. It is a serious problem for lightweight structures such as highways. This phenomenon is characterized by two mechanical parameters measured by an oedometric test carried out in the laboratory, which takes a long time and is expensive, why scientists have always looked for other quick and less expensive alternatives to estimate these parameters. They developed models between classical parameters, easily determined in the laboratory, such as water content, dry density, percent of clay particles, plasticity index and mechanical properties of clay soils. The developed models are based on descriptive statistics, a neural network, or multiple regressions. To estimate the risk of swelling and to verify the applicability of the models of the literature on the Algerian soils, a database was collected from the geotechnical reports carried out on the sites of certain projects in Algeria. The objective of the present study consists of estimating the mechanical parameter of swelling, which is the swelling index from a set of physical tests carried out in the laboratory.

Regulatory provisions for determining parameters of nonlinear mechanical behavior in hardening soil models

Presently, nonlinear mechanical soil models are widely used in the geotechnical practice. They are implemented in geotechnical finite-element software PLAXIS, MIDAS GTS NX, Z-Soil, Optum, etc. Notwithstanding the fact that International regulatory documents mainly recommend nonlinear models for calculating foundations, they do not specify methods for defining their parameters because nonlinear models are actually commercial products of various software manufactures and cannot be normalized in any country. For this very reason, specialists of Gersevanov Research Institute of Bases and Underground Structures (department of JSC "Research center of Construction") developed Standard of Organization STO 36554501-067-2021. The Standard is based on the world experience in assessing nonlinear model parameters defined with using advanced laboratory equipment. The article describes the most significant provisions of this Standard. Thus, depending on the problem to be solved, four groups of models are introduced: I - a model includes volumetric isotropic hardening (Soft Soil); II - a model includes volumetric isotropic hardening and rheological properties of the soil (Soft Soil Creep); III - a model includes double isotropic hardening (Hardening Soil) and IV - a model includes double isotropic hardening and stiffness at small strain (Hardening soil with small strain). Requirements to the necessary and sufficient types and scope of tests, which define model parameters, are reported. Taking into account that only laboratory tests define model parameters, Standard provides requirements for specimens' quality evaluation performed basing on measurements of the volumetric strain under effective natural stress. Specimens of poor and very poor quality cannot be considered. It is shown that power-law coefficient m , which indicates the relationship between soil stiffness and stress-strain-state in the models of III and IV groups can be received from triaxial compression and oedometer tests and shall be prescribed depending on prevailing deformation (shear or compression). Stiffness parameters at small strains can be defined wit in-situ tests (seismoacoustic profiling) or with laboratory tests (resonant column test, seismoacoustic profiling of specimens in triaxial cell equipped with bender element and strain gauges on the local base). The developed Standard specifies requirements for laboratory definition of stiffness at small strains.

SPH implementation of a critical state-based hypoplastic model for granular materials in large-deformation problems

In this work, an advanced hypoplastic model incorporating the critical state is integrated into smoothed particle hydrodynamics (SPH) for numerical investigation of problems with large deformation. This numerical method performs well in cases of element tests such as oedometer test, simple shear test and plain strain compression test, which usually own regular velocity fields. However, the standard SPH implementation will generate significant stress drift away from the failure surface in more complex geotechnical problems featured with dramatically-varying velocity field. This unrealistic stress state may deteriorate the stress state with a further step. As a result, a vicious circle takes place, which could give rise to a calculation failure. With this regard, a return mapping strategy is proposed to regulate the stress state locating beyond the failure surface. To this end, a closed-form solution to predicting the failure surface implicitly incorporated in the hypoplastic model is derived. Finally, the capability of the proposed numerical method is examined with two classical benchmark problems for granular materials, including the sand column collapse and rigid footing by comparing the numerical results with those from experiments and theoretical solutions.

Mechanical behaviour of C-S-H agglomerates at multiscale

Calcium-Silicate-Hydrate (C-S-H) is the primary binding phase in cement and plays a crucial role in lifecycle performance of concrete. Understanding its multiscale mechanical properties is essential for optimizing its performance. In this study, we employed atomic force microscopy (AFM), nanoindentation and oedometric test to characterize multi-scale mechanical behaviours of C-S-H with different chemical composition and microstructure. Moreover, a calculation framework is used to bridge inter-scale gaps. Our results reveal the complex hierarchical features of C-S-H and its impact on mechanical properties including elastic, plastic, and viscous responses. We also identify the critical role of chemical composition including calcium-to-silica ratio and water content on the observed mechanical behaviours. Additionally, our multiscale model provides a valuable tool for rational design of C-S-H-based materials with tailored mechanical properties at less material cost.

Determining parameters of peat soil for finite-element analysis using the Cam Clay model

A practical procedure for determining soil parameters when analysing the deformation of peat by finite-element analysis (FEA) using the Cam Clay model is presented. FEA has not been frequently used to predict the deformation of peat soil, even though it can be a powerful prediction tool. The main reason for this is that a practical method for determining parameters has not been established. In this paper, the remarkable correlation between the soil parameters required for FEA using the Cam Clay model and the natural water content (wn) and ignition loss (Li) of peat and organic clay is presented. Then, based on this correlation, a flow chart for determining the soil parameters of peat and organic clay is proposed. Using this chart, it is possible to determine soil parameters directly from triaxial compression, oedometer and other detailed tests or to estimate them from wn, Li and other physical indexes. A trial embankment on peat soil in Hokkaido, Japan, was also created. FEA was performed on the trial embankment using the proposed procedure for determining the soil parameters. The results of the FEA corresponded to the measured settlement behaviour in the trial embankment with an accuracy in the range −4% to +8%. The FEA results also modelled the measured behaviour of pore water pressure and lateral and vertical displacements in the trial embankment very well.

Mitigation Measures of Swelling Characteristics in Alkali-Contaminated Red Earth using Fly Ash and GGBS Blends

Alkali contamination induces swelling in the inherently non-swelling soils, resulting in heave severely damaging the sub and superstructures. Stabilization measures must be taken in vulnerable areas to prevent such colossal losses. Considerable research has been conducted regarding the swelling behavior of soils contaminated with alkalis and the impact of subsequent stabilization techniques utilizing different industrial byproducts and pozzolanic materials. However, there is a paucity of such studies on the Red Earth (RE) of the Visakhapatnam region. Furthermore, limited attention was given to the effects of extremely low concentrations of alkalis on the swelling behavior of soils. RE in the study area of the Visakhapatnam region predominantly consists of quartz, kaolinite, and hematite. In phase 1 of the experimental analysis, RE was contaminated with a highly dilute NaOH solution (0.05 N), and the resulting swell characteristics were investigated. A free swell of 10% was observed. One-dimensional standard oedometer tests revealed an equilibrium swelling of 5.6%, indicating significant potential for substantial heave and subsequent damage. X-ray diffraction (XRD) analysis revealed the formation of natrolite and analcime zeolites with a hollow structure, which caused the swell. Peaks of silicate minerals like paragonite and ussingite were also observed, indicating the dissolution of authigenic minerals and new precipitations even at lower concentrations of NaOH. In phase 2, swell tests were conducted on a mixture of contaminated red earth (50% w/w), fly ash (25% w/w), and ground granulated blast furnace slag (GGBS, 25% w/w), and it was observed that the swelling was completely arrested. XRD analysis unveiled that even though siliceous zeolites were formed, the development of chrysotile, attapulgite, tobermorite, and dicalcium silicate effectively inhibited the swelling with their pozzolanic behavior.

Thermo-mechanical behaviour of a kaolin-based clay soil

The thermal behaviour of KSS, a low-plasticity artificial clay made with kaolin clay, silt and sand, was investigated in a series of temperature-controlled oedometer tests, at temperatures between 5°C and 70°C, and at vertical pressures up to 2·4 MPa. The experiments investigated the effect of overconsolidation ratio (OCR), pressure level and repeated thermal cycling on thermally induced volume change. Thermal volumetric strains were found to be dependent not only on OCR but also on pressure level, contradicting previous experimental findings and highlighting the importance of even a small dependency of compression index Cc on temperature. Furthermore, thermal volumetric strains were irreversible on heating and cooling even for highly overconsolidated samples. Although irreversibility at high OCR values has been attributed to particle rearrangement and plastic accommodation in the past, an alternative explanation is put forward here, as yielding on the Hvorslev surface is expected to occur on unloading under one-dimensional (1D) conditions. The tests also revealed evidence of thermal creep for the initially normally consolidated samples. The influence of both current temperature and temperature history on the reloading response of mechanically overconsolidated KSS was tested and quantified in terms of their effect on the measured pre-consolidation pressure. The results from these tests were compared to results from the available literature referring to clays of similar and higher plasticity and the comparison highlighted that although soil plasticity can explain the observed quantitative differences between high-plasticity soils to a large extent, mineralogy, in addition to structure, may also play an important role for low-plasticity soils.

The compressive behaviours and microscopic characteristics of deep-sea soft soil with deionized or sea water

To investigate the effects of different pore water environments on compressive behaviours of the deep-sea soft soil from South China Sea, four kinds of soil samples including natural sample with sea water or deionized water, remolded sample with sea water or deionized water were prepared, respectively. Then, a series of oedometer tests and scanning electron microscopy tests were performed on the above samples to examine their compressive behaviours and microscopic characteristics, respectively. Additionally, the microscopic pore distribution of natural sample with sea water was observed by mercury intrusion porosimetry tests. The effects of different water environments on macroscopic compressive behaviours and microscopic particle characteristics are analyzed. The deionized water environment increases the compressibility and decreases the consolidation coefficient for natural samples after structural breakage and remolding. The microscopic particle characteristics corresponding to the macroscopic compressibility are also qualitatively analyzed. In deionized water environment, soil particles exhibit higher roundness, more orderly and lower surface roughness than that in seawater environment. In this study, the deionized water environment has no obvious effect on the structural yield stress and secondary consolidation coefficients of deep-sea soft soil.

The impact of paleoclimatic on the structural strength of loess paleosol sequences and its implications for tillage on the Loess Plateau: A case study from Luochuan profile

The structural strength of the Loess-Paleosol Sequence (LPS) and the presence of paleosols within the LPS have significant implications for tillage and understanding past climate conditions. This research investigation sought to examine the structural strengths of the Luochuan (LC) LPS via both triaxial shear and oedometer tests, with the microstructure being further characterized through scanning electron microscopy. Results indicate that the LPS's structural strength tends to increase as burial depth increases. Additionally, the loess layer's structural strength is typically lower than that of the adjacent paleosol layer. The LPS's microstructure experiences considerable transformations with increased burial depth, particularly regarding changes in particle contact relationship, degree of cementation, and pore volume. This shift is characterized by a transition from an overhead structure to a matrix structure. These findings suggest that the loess layers' structural strength is associated with weaker pedogenic weathering occurring under cold and dry climatic conditions, whereas the paleosol layer exhibits a higher structural strength due to intense weathering during a warm and humid climate. Overall, this study establishes a link between paleoclimate and mechanical properties, using microstructure as a mediating factor, and provides a theoretical basis for tillage on the Loess Plateau.

Uniqueness of the Normal Consolidation Line for Gold Tailings

Abstract Depending on the stress state, mine tailings are generally accepted to be susceptible to static liquefaction. A common method to assess the in situ stress state of tailings in relation to static liquefaction susceptibility involves the use of the state parameter. Because most tailings materials are normally consolidated (NC), this type of assessment requires knowledge about the normal consolidation line (NCL). It has been shown experimentally that the uniqueness of the NCL is vastly different for fine-grained and coarse-grained soils, with clays usually exhibiting a unique NCL and clean sands exhibiting an infinite number of parallel NCLs. Gold tailings, a sandy silt, fall between clays and clean sands, and there are limited experimental data regarding their compression behavior over a range of initial void ratios. This lack of data results in inconsistent interpretation of the uniqueness of the NCL for gold tailings in the industry. This can influence the results of designs and safety evaluations of tailings dams. In this study, a number of oedometer tests were conducted on gold tailings sourced from an active tailings dam in South Africa. Several specimens were prepared at various initial densities and were consolidated in small increments to a high effective stress. The oedometer tests were supplemented with triaxial compression tests, from which a unique critical state line was identified. Across the oedometer and triaxial tests, it was found that the behavior of the NC and overconsolidated samples was consistent with that typically observed for fine-grained soils. Therefore, for practical purposes, it appears that the gold tailings tested can be viewed in a framework with a unique NCL. No significant influence of particle crushing was noted.

Isotropic triaxial tests of equal strength concrete with different mix compositions

High intensity impact loading on a concrete structure induces three dimensional high intensity stresses, the analysis of which requires appropriate constitutive relationships describing the isotropic and deviatoric stress components. The isotropic component known as the hydrostatic relationship or the equation of state (EOS) is in the focus of this paper. The literature relates the EOS of concrete to its uniaxial compressive strength. This paper aims at refuting this relationship and validating the conjecture that the EOS depends on the concrete mixture. For that purpose, different concrete mixes of equal strength have been tested. The mixture parameters are the water content (with the same w/c ratio), the coarse aggregate content and the maximum aggregate size.Using a unique test setup, hydrostatic tests were carried out to extremely high pressures at the 1GPa level. The results clearly show the pronounced differences between EOS curves representing different mixtures having the same concrete strength, thus validating the above conjecture.As the EOS is sometimes derived from oedometric tests, the present investigation exploited oedometric tests results on exactly the same concrete mixtures and strength to compare the hydrostatic and oedometric resulted EOS curves. The comparisons show that for most cases the stress at a certain volumetric strain in the oedometric curves are somewhat lower than these in the hydrostatic EOS curves. However, for mixes with a large maximum aggregate size with a high aggregate content, the oedometric EOS merges with the hydrostatic EOS curves in the mid-to high pressure levels.Finally, statistical analysis using Analysis of Variance has been employed to identify the effect of different mixture parameters and their combination on the differences in the EOS.These new results enhance the insight on concrete behavior under triaxial high pressures and contribute to the development of a new generation of EOS curves.

Reliability of Methods for Determination of Stress History Parameters in Soils

Abstract Stress history acquired by any cohesive soil influences, to a large extent, three groups of fundamental properties indispensable in geotechnical design i.e. state of soil, shear strength, and stiffness characteristics. The basic stress history parameter (from which other parameters are derived) determined directly from laboratory tests is a preconsolidation stress σ′ p. Since the first method proposed by Casagrande in 1936, value σ′ p is determined in the oedometer test as a border between overconsolidated (OC) and normally consolidated (NC) zones. Approach based on division between predominantly elastic, (recoverable) strain, and plastic (irrecoverable) strain is a main principle of several methods of σ′ p determination, which have been proposed over the past eighty-six years. Accumulated experiences have revealed that any laboratory procedure based on the oedometer test does not provide realistic value of preconsolidation stress, especially in heavy preconsolidated soils. The major reason for that results from the fact that the mechanism responsible for natural overconsolidation is more complicated than mechanical preloading. Therefore, there is a necessity to reevaluate effectiveness of standard methods and look for another solution of evaluation yield stress σ′ Y in natural soils. This article presents the comparison between σ′ Y determined for various soils with use of standard methods based on conventional oedometer test and yield stress determined on the basis of alternative procedures. The latter are represented by various approaches as e.g. based on SHANSEP procedure or initial shear modulus and others. The most promising among these alternative methods is a new concept based on dilatancy phenomenon that takes place during shearing of a dense soil. The parameter reflecting stress history is derived from pore pressure response and is based on characteristic values of Skempton's parameter A record. Consistency of data concerning stress history parameters profile obtained for deep subsoil on the basis of various methods is shown for comparison.

Compressibility behavior of colemanite added bentonite under short and long-term high temperature

Introduction
 The increasing energy demand and the limited fossil fuel resources make searching for new sustainable clean energy sources. In this regard, it is vital to use energy geo-structures as a renewable and clean energy source. The geo-structures are in direct contact with the soil and cause temperature changes throughout the soil mass.
 Bentonite is considered suitable as an engineering barrier in deep geological disposal repositories for spent nuclear fuel, mainly because of its favorable swelling properties and extremely low permeability [1]. It was reported by many researchers that the engineering properties of clayey soils change at high temperatures [2,3]. Therefore, there is a need for soil materials that can maintain their long-term engineering properties under high temperatures. Borates are naturally occurring minerals. They can be found mainly in sediments and sedimentary rocks. The most commercially important boron minerals are tincalconite, colemanite, and ulexite [4].
 In the present study, it was tried to improve the compressibility behavior of bentonite by adding colemanite under high temperature. The oedometer tests were performed under a constant temperature (80 ºC) on the bentonite-colemanite mixtures. In this context, samples exposed to short and long-term high temperature (80 ºC) were used and the results were compared with the room temperature results.
 Material Characterization and Methods
 The Ca-bentonite sample used which is activated with sodium bicarbonate. Colemanite was added to bentonite at a rate of 10% of the bentonite by dry weight. In this context, B10C sample represents a 10% colemanite added bentonite mixture. The liquid limits of bentonite and colemanite are 270% and 37%, respectively. The samples (smaller than 75 μm) were obtained by mixing the mixture powder with tap water at a water content of 1.5 times the pre-determined liquid limit value of the mixtures. The slurries were consolidated under a vertical pressure of 12.5 kPa for 14 days. Samples 70 mm in diameter and 19 mm in height were obtained by trimming. The samples were placed in oedometers for tests at room and high temperature (80 ºC). The experimental system was modified for high-temperature tests. The modified system consists of a conventional apparatus, a heat ring, a thermostat, and a water tank. Thus, by heating the cell water, the temperature of the sample was indirectly increased to 80 °C. For long-term experiments, samples were placed in thermal pools in clamped molds (constant volume). These molds were kept in the thermal pools under a constant temperature of 80 °C for 6 months. Then, the consolidation tests of these samples were performed at 80 °C.
 Results and Discussions
 The result of the consolidation tests of bentonite mixtures at 80 °C after being kept in the thermal pool for 6 months is given in Figure 1. For comparison, the test results at room temperature and 80 °C are also shown. The compression amount of bentonite increased with the increase in temperature, while the swelling amount decreased. The total compression deformation of the bentonite sample, which showed 53% compression at room temperature, reached 65% when the temperature was increased to 80 °C. However, swelling deformation decreased from 7% to 3% when the temperature was increased. When the volumetric deformation behavior of bentonite (NC) is examined, thermal contraction behavior at high temperature (90 °C) was reported [2]. In other words, the vertical compression amount of NC bentonite increases at elevated temperatures. In addition, since the structure of the clay deteriorated at high temperature and the deformations were permanent, it was an expected result that the rebound (swelling) behavior would decrease at high temperature.
 There was a decrease in the amount of compression in the sample cured for 6 months, compared to 80 °C. However, although the compression amount of the 6-month cured sample decreased, the amount of compression still slightly increased compared to the room temperature. Compression-swelling deformation amounts of the samples cured for 6 months and tested at 80 °C are presented in Table 1 in detail. When the swelling behavior of additive-free bentonite was examined, increasing temperature decreased the rebound behavior of bentonite. In addition, the swelling amount of the sample cured for 6 months was determined to be almost the same as the sample at 80 °C. The reason for this was that the deformations that occurred at high temperatures were permanent. 
 When the temperature of the colemanite-added bentonite mixtures was increased, the amount of deformation increased. However, the amount of deformation of additive-free bentonite decreased with the colemanite addition at 80 °C, and returned to its room temperature value (approximately 53%). In other words, the addition of colemanite had a positive effect by reducing the deformation amount of bentonite at both short and long term high temperatures. The short and long term effects of 80 °C temperature on colemanite added samples were almost the same.

A Phase-Field Discrete Element Method to study chemo-mechanical coupling in granular materials

Geochemical reactions are ubiquitous for subsurface energy extraction and storage applications, like geothermal electricity production or hydrogen storage, but also for other applications aiming at reducing climate change like underground CO2 disposals [1]. Those reactions lead to mineral dissolution/precipitation that can modify the different properties of the reservoir rock and the caprock. For example, porosity of the porous material depends strongly on the dissolution/precipitation of the solid phase. It can thus affect the evolution of its permeability, a key parameter for most of those applications that involve the transport of a fluid through the reservoir. Moreover, the mechanical behavior and rupture of geomaterials is also strongly affected by dissolution/precipitation phenomena. In the case of cemented rocks, debonding can occur during weathering and strongly weakens the material [2]. This strength degradation has also been highlighted during oedometric tests in the case of granular materials presenting no cohesion [3]. In the case of underground storage applications, if a strength reduction is induced in the caprock, cracks can be created, leading to a migration pathway and leakage. It can also affect the behavior of faults in the vicinity of the reservoir and induce earthquakes [4]
 Granular material (like in fault) or sedimentary rock can be modeled by the discrete element method (DEM). This method was first developed by Cundall and Strack to model the micromechanical behavior by reproducing more accurately the interactions in an assembly of grains. In the classical approach, grains are modeled as disks (2D) and spheres (3D), however, real particles can be highly irregular. These complex shapes of the grains influence greatly the macroscopic mechanical behavior of the material [5] and accurate models should aim at capturing this complexity. To do so, different approaches have been developed in the frame of DEM like particles cluster, ellipsoids, polygonal (2D) or polyhedral (3D) particles [6]. The latter is the most accurate solution, but it tends to overestimate the roundness of the particles and show some limitations to reproduce experimental results [7]. Recently, a level-set discrete element model was developed and allowed to capture the complex shape of the grains and reproduce experimental results [8].
 To model grain dissolution/precipitation, discrete elements are often considered with a homogenous decrease/increase of the particle diameter. But in some cases, like pressure-solution, the dissolution/precipitation are localized. Hence, the dissolution occurs in the high-stress area, whereas the precipitation occurs in the low-stress area with a large solute concentration. This diffusive mass transfer is done within the pore fluid. Considering the granular material as a phase, the phase-field theory (PF) is a good candidate to model with physics-based laws an addition or reduction of the quantity of material locally. The dissolution at the contact is controlled by the introduction of mechanical and chemical energy into the Allen-Cahn formulation on the phase variables, whereas the precipitation and the mass conservation are verified by a coupled diffusion formulation on the solute concentration.
 In this study, an extension to the discrete element method is developed to simulate the irregular shapes of particles in a granular material and their heterogeneous change using the phase-field variable as a particle’s geometrical descriptor. This method, available on https://github.com/AlexSacMorane/PFDEM_ACS_MultiGrains, is applied to reproduce results from previous experiments on K0 evolution [9]. The influence of the grain shape has been highlighted. Then, the model is used to investigate on the pressure-solution phenomenon at several grains level [10]. For example, a so-called Andrade creep law has been reproduced.
 In conclusion, this new framework enables us to model chemo-mechanical couplings, considering the true shape of the grain. It is used to investigate the influence of the different physical phenomena (dissolution, precipitation or diffusion) controlling the rate of material compaction and of the macro properties evolutions, as the porosity, permeability or strength.

Minerals Dissolution Effect on the Mechanical Properties of Synthetic Carbonate Rocks

Injection of CO2 and water in saline aquifers or oil reservoirs causes changes of pressure, saturation and concentrations that affect the state of stress and promote chemical reactions in the host rock, resulting in porosity and permeability variations. It is therefore a coupled hydro-mechanical and chemical (HMC) problem. Numerical simulation of multiphase and multicomponent flow of CO2, oil and water with mechanical coupling allows realistic modeling of the reservoir and cap rocks. Carbonate reservoirs are geological formations composed mainly of minerals such as calcite and dolomite which can dissolve or precipitate in the medium when injecting a fluid of chemical composition and temperature different from those of the fluids initially contained in the rock. Water-weakening due to matrix acidification of carbonates is a well-known phenomenon that can be modeled by including mineral concentrations as state variables in the stress-strain behavior of the material.
 The objective of this work is to characterize synthetic carbonate rocks through microtomography and petrography techniques, focusing on a comparative analysis before and after load application and degradation with a reactive fluid [1]. The synthetic rocks were subjected to physical characterization (mineralogy, computed tomography and porosity) and mechanical characterization (uniaxial compressive strength and Brazilian tests) before and after the dissolution process. The petrographic analysis verified an increase in both intergranular and intragranular porosities after dissolution. The microtomography analysis quantified the maximum increase in porosity, from 11.8% to 41.3% in the two-dimensional analysis and 31.6% to 52% in the three-dimensional analysis of the porous structures. Furthermore, the pores were quantified according to their area, and data was obtained on the orientation of the pores, providing insight into the preferred paths of fluid flow. It was also observed that the microtomography technique was an effective tool for characterizing fractures in the samples before and after dissolution [1].
 Dissolution tests were also performed in a modified oedometer cell adapted to measure horizontal stress. The dissolution phase was conducted using water and an acid solution to evaluate the influence of the pH on the mechanical behaviour of the samples. When the sample in the oedometric cell is exposed to an acid solution under constant vertical load of 400kPa, vertical displacement takes place (volume decrease of the sample) and horizontal stress increases (Figure 1). The synthetic rock used in this experiment is mainly composed by calcite, with small additions of calcium hydroxide, and the reactive fluid is water acidified with acetic acid (with 10% concentration). This material is manufactured in laboratory in order to have greater control of its constituents and reproducibility of experimental results. During the acidification phase of the experiment, the sample was subjected to an acid flow with a pressure differential of 12kPa for 7h. The behavior in Figure 1 was also observed by [2] and [3]. The model for degradation of carbonatic soft rocks proposed by [2] was implemented in a finite element code capable of performing coupled THMC analyzes in porous media [4]. This model is based on the Critical State Theory with the introduction of a bonding variable that controls the size of the yield surface by modifying the tensile strength and the pre-consolidation stress of the material. In Figure 1 it is also possible to see that the HMC coupled simulation of the oedometric test was able to reproduce the experimental results in terms of volumetric strain and horizontal stress. In Figure 2 tomographic images of the synthetic carbonate rock before and after exposure to the acid solution in an oedometric cell under constant vertical load are presented. Such analyses are crucial for the injection of reactive fluids, such as CO2, at high depths. Dissolution of minerals will affect not only the porosity and permeability of the reservoir rock but also, possibly, the stress state in the vicinity of the injection well.

Effectiveness of self-sealing after gas transport in Boom Clay

The study of gas transport in low permeable materials is becoming a significant focus in energy-related geotechnics, particularly for managing deep geological disposal of long-lived and heat-emitting radioactive waste. Argillaceous rocks, studied to host the disposal, may present induced fractures caused by the excavation activities that increase the permeability to liquid and gas. The generation of gases can also lead to an excessive pressure build-up in these saturated media resulting in the development or reactivation of fractures/fissures creating preferential pathways for the gas flow [1]. Nevertheless, these rocks present the advantage of self-seal (via swelling of clay minerals due to re-saturation, consolidation or creep), reducing fissure permeability and potentially restoring the barrier function [2].
 This study focuses on Boom Clay, a Cenozoic clay candidate for hosting the repository in the Belgian programme, aiming at characterising the effectiveness of its self-sealing process after gas injection/dissipation tests due to the swelling of clay minerals during re-saturation. This poorly indurated rock presents sedimentary bedding planes that can act as preferential pathways during the gas invasion. In a previous experimental campaign, samples at two bedding orientations (parallel and orthogonal to the flow) were tested under oedometer conditions [3, 4]. The results indicate that gas transport induced slight expansion of the samples in both orientations, increasing their intrinsic permeability, which suggested the opening of preferential paths. Furthermore, the analyses of the pore network using mercury intrusion porosimetry (MIP) and micro-focus X-ray computed tomography (µ-CT) confirmed the development of gas pathways following the bedding direction or interconnecting bedding planes [4, 5].
 To assess the self-sealing capacity, oedometer tests were carried out at two bedding directions using an oedometer cell with lateral stress measurement to comprehensively understand the stress state and ensure that gas flow occurred through the sample rather than between the sample-ring interface. Prior to the gas injection stage, water permeability was measured. Subsequently, a gas injection/dissipation stage was performed at constant vertical stress. Immediately after, the sample was placed in contact with synthetic water to allow re-saturation. Finally, the water permeability was measured again. The self-sealing capacity of the clay was assessed by comparing the water permeability at both stages. If the obtained values are similar, the barrier function has been restored. Some tests also included a second gas injection to evaluate the potential pathway reopening. MIP and µ-CT data allowed for evaluating microstructural changes due to these processes.
 The experimental results demonstrate that gas can flow at pressures lower than the minor lateral stress, and the computed effective gas permeability is always higher than the initial intrinsic water permeability, pointing to the opening of preferential pathways (Figure 1a). Water permeability after re-saturation showed values comparable to the initial intrinsic permeability, recovering the hydraulic barrier function thanks to the clay minerals’ swelling (Figure 1a). This indicates the good self-sealing capacity of the Boom Clay. The gas permeability calculated during a second injection/dissipation stage is slightly higher than the initial one, suggesting some memory of the previously opened path (Figure 1a). The self-sealing effect was also observed in the CT images performed under unstressed conditions after water-undrained unloading of the tested samples. The fissures detected after the gas injection (Figure 1b left) are no longer visible after re-saturation within the technique resolution (fissures > 40µm) (Figure 1b middle). However, a small proportion of large and disconnected pores were identified in CT images, likely due to some gas exsolution. Regarding the subsequent gas injection, it again led to the development of fissures following the bedding direction detected with microstructural techniques (Figure 1b right).