Pore-scale mechanisms underlying the behavior of enhanced bentonites exposed to aggressive inorganic solutions

Understanding the mechanisms that control the hydraulic and semipermeable membrane behavior of enhanced bentonites (EBs), which comprise natural montmorillonite amended with polymers or organic compounds, is important for assessing the long-term performance of engineered barriers manufactured from these materials. Accordingly, the available experimental evidence for the hydraulic and semipermeable membrane behavior of EBs was critically interpreted through a theoretical framework, which allows the macroscopic transport and swelling properties of chemically active clays to be related to a limited number of intrinsic, state, and fabric parameters. Four commonly evaluated EBs were interpreted, namely, Multiswellable Bentonites (MSBs), Dense Prehydrated GCLs (DPH-GCLs), HYPER Clays (HCs), and Bentonite Polymer Composites (BPCs). Osmotic swelling, which is the primary mechanism for significant swelling and low hydraulic conductivity of unenhanced (natural) sodium bentonite, is not significantly influenced by polymer amendment. The primary mechanism controlling the conductive porosity and the flow path tortuosity upon permeation of BPCs with concentrated electrolyte solutions is intergranular pore clogging by sodium polyacrylate, whereas the mechanism for the same behavior of DPH-GCLs and, probably to a lesser extent, of HCs is preservation of a dispersed clay fabric via intercalation of sodium carboxymethyl cellulose between the montmorillonite unit layers. Similar to BPCs, direct exposure of MSBs to liquids with aggressive chemistries induces the fabric to flocculate to a greater extent than that of prehydrated natural bentonites. However, unlike BPCs, the decrease in the conductive porosity of MSBs is due to a greater compressibility of the solid skeleton rather than to a pore-clogging mechanism

Relative contribution of chemico-osmosis and electro-osmosis to the experimental determination of the reflection coefficient in semipermeable clay soils

The containment performance of bentonite-based barriers is known to be influenced by the semipermeable membrane behaviour of the bentonite, which arises from the electrical interactions between the clay particles and the ionic species dissolved in the pore solution. Most of the experimental research conducted to date has provided evidence of the clay membrane behaviour, the extent of which is typically quantified through the reflection coefficient, , when the permeant (electrolyte) solution contains a monovalent or divalent salt. Under such conditions, the osmotic counter-flow of solution is controlled to a great extent by the solute exclusion, which is also referred to as the chemico-osmotic effect. However, theoretical simulations of coupled solute transport and solvent flow suggest that, when two or more cations with different diffusivities are contained in the permeant solution, the electro-osmotic effect, which stems from the condition of null electric current density, can be comparable to or even greater than the chemico-osmotic effect. The relative importance of the aforementioned contributions to multi-electrolyte systems is examined herein through the interpretation of laboratory test results from the literature pertaining to a bentonite-amended clay soil permeated with aqueous mixtures of potassium chloride (KCl) and hydrochloric acid (HCl)

Strain-controlled oedometer test for the measurement of the chemico-osmotic properties of bentonites

The possibility of relating the macroscopic transport properties and the swelling behaviour of bentonites to a limited number of physico-chemical and fabric parameters has been investigated by means of a new laboratory apparatus, which allows the reflection coefficient, which is also known as the membrane efficiency coefficient, and the swell coefficient to be simultaneously determined on the same clay specimen. The results of two multi-stage tests, which were performed on a natural sodium bentonite under fully saturated conditions, while varying both the specimen porosity and the salt concentration of the equilibrium solutions, have been interpreted through a physically-based model in which the pore-scale electro-chemical interactions be-tween the solid skeleton, the aqueous phase and the ion species are explicitly taken into account. The efficiency of the tested bentonite in acting as a semi-permeable membrane and its swelling behaviour have been found to be accurately simulated when a single fabric parameter, referred to as the solid charge coefficient, is calibrated on the available experimental dataset, thus suggesting that the containment performance of bentonite-based barriers, used for a number of geoenvironmental applications, may be predicted on the basis of the results of a strain-controlled oedometer test

The use of dynamic probing tests and cone penetration tests to verify the effectiveness of expanding polyurethane resin injections for ground improvement

Injection of expanding polyurethane resins is a popular method to improve both the stiffness and the shear strength of the ground below existing foundations. The effect of the polyurethane resin expansion is to increase the soil confining stress and density around the injection holes. An estimation of the horizontal stress and volumetric strain changes that are induced within the ground is derived from the theory of cavity expansion in elasto-plastic materials. A series of case-histories is presented to document the feasibility of different in-situ tests to evaluate the achieved ground improvement. The tests have been performed before and after the injection of polyurethane resins and the obtained results have been compared with theoretical predictions. The considered investigation methods include the dynamic probing tests and the cone penetration tests. The preliminary results that have been achieved using an experimental miniature cone penetration test are also illustrated. The advantages and limitations of different test methods are discussed and practical indications for conducting such verifications of polyurethane resin injection effectiveness are provided

Improvement of foundation soil behavior for Gründerzeit buildings in Austria using polyurethane resin injections

The second half of the XIX century signed a period of comprehensive industrial and cultural development in Austria known as Gründerzeit. The architecture was deeply influenced during this epoch by the construction of large masonry buildings, called “Gründerzeit Häuser”, which represent today the historical heritage of the most important Austrian cities. Due to the increasing demand for apartments in the centre of these cities, renovations and adaptations of existing buildings has been performed in the last decades. These modifications of the buildings structure represent, besides a business opportunity for the real estate economy, an increase/variation of the load for the foundations. Hence, the renovation and preservation of this architectural heritage often involve the improvement of the foundation soil. This article illustrates how the soil treatment with expanding polyurethane resin represents an efficient solution to achieve this goal. A case study of a historic building in the city of Salzburg is presented

Proprietà osmotiche delle bentoniti in equilibrio con miscele di elettroliti

Il comportamento a membrana semipermeabile delle bentoniti, impiegate nei sistemi di contenimento degli inquinanti, è stato finora oggetto di ricerca sperimentale impiegando soluzioni acquose di un singolo sale, non contemplando l’effetto della compresenza di due o più elettroliti nella soluzione interstiziale. A motivo della rilevanza di queste ultime condizioni operative nel momento in cui le bentoniti vengono a contatto con liquidi contaminati, è stata eseguita una prova osmotica multi-stage su una bentonite sodica naturale in equilibrio con miscele acquose di cloruro di sodio e cloruro di potassio, e i risultati sono stati interpretati tramite un modello teorico che permette di decomporre il controflusso osmotico di soluzione in una componente chimico-osmotica, che trae origine dalla parziale esclusione degli anioni dai pori della bentonite e si manifesta anche in presenza di un singolo sale, e una componente elettro-osmotica, che trae origine dalla condizione di assenza di corrente elettrica attraverso il mezzo poroso in presenza di cationi caratterizzati da diversa diffusività e determina l’insorgenza di valori del coefficiente di riflessione, w, sia superiori all’unità (w = 1.064) sia inferiori a zero (w = - 1.168)

Determinazione delle proprietà osmotiche delle bentoniti tramite un edometro a deformazione controllata

Allo scopo di verificare la possibilità di correlare le proprietà di trasporto e il comportamento rigonfiante delle bentoniti a un numero ristretto di parametri fisico-chimici e microstrutturali, è stata messa a punto una nuova attrezzatura di laboratorio che consente di misurare simultaneamente, sullo stesso campione di argilla, il coefficiente di efficienza chimico-osmotica e il coefficiente di rigonfiamento. Sono state condotte due prove multi-stage su una bentonite sodica naturale variando sia la porosità sia la concentrazione salina delle soluzioni di equilibrio, e i risultati sono stati interpretati alla luce di un modello meccanicistico, nel quale sono tenute esplicitamente in conto le interazioni elettrochimiche che hanno luogo tra lo scheletro solido, la fase acquosa contenuta nei pori e le specie ioniche in soluzione. Si è osservato che l’efficienza a membrana semipermeabile e il comportamento rigonfiante della bentonite testata possono essere simulati in modo accurato nel momento in cui un singolo parametro microstrutturale è calibrato sui dati sperimentali disponibili

Assessing the influence of chemico-osmosis on solute transport in bentonite membranes based on combined phenomenological and physical modeling

The ability of bentonite-based barriers to act as semipermeable membranes that inhibit the passage of solutes (ions) is well documented. This behavior induces chemico-osmotic liquid flux that can improve the performance of such barriers by reducing solute mass flux. This paper explores the potential significance of chemico-osmosis on solute transport through bentonite membranes using a phenomenological transport framework combined with a physical model relating the macroscale transport properties (membrane efficiency coefficient, w, and hydraulic conductivity, kh) to the microscale physicochemical and fabric properties of the bentonite. The model was used to simulate the coupled transport of monovalent salt (KCl) through a geosynthetic clay liner. The results indicate that the influence of chemico-osmosis is dependent upon the void ratio of the bentonite and the extent to which clay platelets are aggregated to form tactoids. Chemico-osmosis is predicted to have an increasingly more significant impact on solute transport with increasing source concentration (Cs0), despite decreasing w with increasing Cs0