Chemo-mechanical behaviour of non-expansive clays accounting for salinity effects

Sr isotopes and pore fluid chemistry in carbonate sediment of the Ontong Java Plateau: Calcite recrystallization rates and evidence for a rapid rise in seawater Mg over the last 10 million years

Interactions between clay particles and organic pore fluids occur frequently in chemical waste disposal site environments. In a study conducted at Syracuse University laboratory experiments have identified the formation of clay tactoids in organic-saturated clay materials. At low pressures, tactoids impart granular fabrics and corresponding granular geotechnical behaviors to clays. As pressures increase, tactoidal clay materials pass through several unique stages of deformation: dilatant stage, pore space compression stage, tactoid breakage stage, and individual particle behavior stage. The response of a tactoidal clay to stress is dependent upon clay mineralogy and pore fluid chemistry; each stage of deformation for several organic fluid/clay mineral combinations has been quantified by compressibility, void ratio, and permeability measurements. Scanning electron microscopic examination of stressed samples confirms this multistage behavior. The occurrence of tactoids imparts a more open fabric to clay materials, resulting in increased permeability and greater friction angle. Due to the increase in permeability of an organic-saturated clay, clay liner materials should be tested at all stages of organic fluid inundation. (Author/TRRL)

Laboratory investigations on the swelling behavior of composite expansive clays stabilized with shallow and deep clay-cement mixing methods

Expansive soil is the most predominant geologic hazard which shows a large amount of shrinkage and swelling with changes in their moisture content. This study investigates the macro-mechanical and micro-structural behaviours of dredged natural expansive clay from coal mining treated with ordinary Portland cement or hydrated lime addition. The stabilised expansive soil aims for possible reuse as pavement materials. Mechanical testing determined geotechnical engineering properties, including free swelling potential, California bearing ratio, unconfined compressive strength, resilient modulus, and shear wave velocity. The microstructures of treated soils are observed by scanning electron microscopy, x-ray diffraction, and energy dispersive spectroscopy to understand the behaviour of the expansive clay blended with cement and lime. Test results confirmed that cement and lime are effective agents for improving the swelling behaviour and other engineering properties of natural expansive clay. In general, chemical treatments reduce the swelling and increase the strength and modulus of expansive clay, subjected to chemical content and curing time. Scanning electron microscopy analysis can observe the increase in formation of particle clusters with curing period, and x-ray diffraction patterns display hydration and pozzolanic products from chemical particles. The correlations of mechanical properties and microstructures for chemical stabilised expansive clay are recommended.

The micromechanical behaviour of clays cannot be investigated experimentally in a direct fashion due to the small size of clay particles. An insight into clay mechanical behaviour at the particle scale can be gained by way of virtual experiments based on the discrete-element method (DEM). So far, most DEM models for clays have been designed on the basis of theoretical formulations of inter-particle interactions, with limited experimental evidence of their actual control over the clay's macroscopic response. This paper presents a simplified two-dimensional DEM framework where contact laws were inferred from indirect experimental evidence at the microscale provided by Pedrotti and Tarantino in 2017 (particle-to-particle interactions were probed experimentally by varying the pore-fluid chemistry, and the resulting effect was explored by way of scanning electron microscopy and mercury intrusion porosimetry). The proposed contact laws were successfully tested against their ability to reproduce qualitatively the compression behaviour of clay with pore fluids of varying pH and dielectric permittivity. The DEM framework presented in this work was intentionally kept simple in order to demonstrate the robustness of the micromechanical concept underlying the proposed contact laws. It is anticipated that a satisfactory quantitative prediction would be achieved by moving to a three-dimensional formulation, by considering polydisperse specimens and by refining the contact laws.

Volume change behaviour of expansive clays has been one of the leading causes of damage to civil infrastructures worldwide. Contributing factors that lead to failures relate to changes in water content within the soil. Variations of water content can vary significantly based on an area’s climate regime. Trinidad has two seasons, the dry season (January to June) and the wet season (July to December). This variation leads to volume changes of expansive clay, where they exist mainly within the central and south regions of Trinidad. These areas are densely populated by residential and commercial buildings, which can be susceptible to damages from unsaturated expansive clays. The Soil Water Characteristic Curve (SWCC) for expansive clays is critical to estimate their unsaturated properties for the analysis of water flow movement. This study investigates the SWCCs for two expansive clay soil types in Trinidad. A WP4-T (Water Potential Measurement) is used to measure soil suction. The shrinkage curve (SC) test is conducted to consider the volume change of soil. The Fredlund and Xing (1994) SWCC equation and Fredlund and Zhang (2013) SC equation are used to fit the measured data. The SWCCs in terms of gravimetric and volumetric water contents and degree of saturation are compared. It is found that the normalised degree of saturation SWCC can provide a better display of the SWCC and estimation of the air-entry value.

In general, dispersive soils may cause some geotechnical problems, although in a geo-environmental context they can be used as an excellent contaminant absorbent due to their large surface area and well exposed clay particles to soil pore fluid. Because of this environmental application, using dispersive clay is common in many landfill projects. On the other hand, the dispersivity behavior of soils is known to be a function of clay mineral and concentration of salt in pore fluid. Most previous studies have based their experimental research on pure clayey soils such as montmorillonite, illite or kaolinite. Considering the fact that from a realistic point of view, there are very few cases of single clay mineral application in geo-environmental projects, there is a need to pay more attention to the behavior of soils which are composed of different minerals. This research investigates the role of mineral type and concentration of pore fluid on dispersivity performance of smectite, illite, kaolinite and their mixtures by means of turbidity, XRD and atomic adsorption tests. In addition, in this article a fuzzy set method based on laboratory tests is developed, with an aim to predicting the geo-environmental behavior of clay minerals mixtures. The system introduced is verified by a database which is compiled from laboratory tests. The results indicate that this system provides extensive knowledge concerning the necessary salt concentration required to disperse pure clay minerals and their mixtures. Furthermore, the numerical results generate some applicable graphs to estimate the required concentration of salt to disperse clayey soils with a view to using it in geo-environmental projects.

Interactions of expansive clay soils with transportation facilities have caused billions of dollars of damage. In the process of determining agents and methodologies to overcome expansive clay behavior, multiple injections of chemical agents into the subgrade to modify clay behavior are most promising. A study was conducted to optimize a potassium-rich solution of four parts. The modifying solution has been injected into expansive clay subgrades with success since the early 1970s. A laboratory methodology was developed to inject specimens multiple times and to conduct swell tests. Screening tests were conducted to evaluate linear effects that each of the four constituents had on swell response through interpretation of a factorial design. Analysis of replicated tests, randomly arranged in blocks, revealed that one constituent had no effect on swell behavior. A central composite experimental design then was used to optimize the remaining constituents. The resulting response surface equation led to an optimal combination for swell abatement, confirmed during short- and long-term verification testing. The optimal blend of constituents was determined to reduce swell from 7.2 percent to 0.84 percent under injection situations simulating the field, and it was found to improve the reduction in swell by 2.6 percent compared with the original blend. In addition, XRD confirmed that modification of clay occurred. This study was unique in its use of laboratory simulation of multiple field injections and in its use of statistical analysis to minimize the number of tests needed to ensure confidence in the optimization sought.

Due to its low hydraulic conductivity, high swelling capacity, and good adsorption properties, the Gaomiaozi (GMZ) bentonite has been selected as potential buffer/backfill materials for construction of engineered barriers in the deep geological repository for disposal of high-level nuclear waste (HLW) in China. Investigation of salt solution effects on the water retention properties of compacted bentonite is of great importance in the context of geological disposal of HLW based on the multibarrier concept. In this study, amended specimens were obtained through a spray of different concentrations of salt solutions to reach target salt contents, respectively. With employment of the vapor phase technique for suction control, water retention tests were conducted on densely compacted GMZ bentonite (1.7 Mg/m3) with different salt contents under confined conditions. Corresponding soil water retention curves (SWRCs) were obtained. Analysis indicates that, for a given suction, the measured water content of GMZ bentonite specimen increases as the salt content (or pore fluid concentration) increases. The influencing rate depends on suction. For lower suctions (lower than 38 MPa), the water retention capacity increases as the salt content increases, while for higher suctions (higher than 38 MPa), the influence can be negligible. Based on the Fredlund and Xing (1994) equation, a soil water retention model was proposed for simulation of the SWRCs of compacted GMZ bentonite with consideration of pore fluid chemistry. Parameters were analyzed and determined with consideration of influences of the pore fluid concentration. Verification indicates that the SWRCs simulated by the proposed model are well agreed with the measured ones.

Exploring the impact of aquaculture sludge on the swell-shrink behavior of expansive clays

Centrifuge model studies on desiccation cracking behaviour of fiber-reinforced expansive clay

The stabilisation of expansive clay subgrades using recycled glass (RG) was proposed, as a sustainable ground improvement technique. Previous studies mainly focused on using RG powder with contents up to 10%, while the current study utilised sand-size particles and up to 40% RG content. Physical properties, compressibility, strength characteristics, and long-term climatic effects on the volumetric behaviour of stabilised clay were investigated. Volumetric responses of stabilised clay were analysed through a constitutive model developed for environmentally stabilised clay. The experimental results revealed that the plasticity of mixtures decreased by 30% as RG content increased to 40%. By using larger RG particles, the strength and bearing capacity increased by about 45% and 130% with the addition of 25% RG. However, adding about 6% of glass powder was sufficient to increase the strength and bearing capacity to about 100% and 200%, respectively. The swell-shrink results suggested that the maximum swelling was achieved in the second cycle in which the clay classification was converted from medium to high expansive clay. The experimental results were also compared and discussed with corresponding data collected from the literature. The outcomes of this study advance the prediction and understanding of the mechanical behaviour of RG-stabilised clay.

Clayey soils which are expansive will swell and can cause lifting of buildings or other structures through the ingress of high moisture. Conversely, during evaporation, it will shrink and can result in building settlement. The vertical cylindrical cells reinforced with geogrid are referred as geopiles in which expansive soil is compacted at 95% optimum moisture content in rigid tank. This research work presents the heave behavior of expansive clay through the provision of geogrid encased with different materials, viz. fine sand, coarse and gravel. We studied that the heave behavior of expansive clay is by using single, two and four geopiles of same diameter filled with above said materials. The results indicated that heave decreased with increasing diameter particles size of the fill materials. The comparisons between unreinforced clay and reinforced clay using geogrid were analyzed.

Expansive clay in semiarid area behaves features of typical unsaturated soil, which is sensitive to climate and moisture, appears multi-fissures, deterioration of mechanical performances with the change of moisture. Consequently, to ensure the rational design of foundation treatment, and safe operation, research on moisture migration mechanism of expansive clays is of great significance. Herein, test models of mixed water migration and vaporous water migration were presented to investigate the mechanism. Series of tests, in which the moisture content in left soil column was same as 24%, and in right soil column was 0%, 6%, 12% and 18% respectively, were conducted under constant temperature. It is concluded that the moisture content changed with increase of migration time in both migration models, and the migration behaves nonlinear when the initial water content increase up to 6% for mixed water migration model and 12% for vaporous water migration model. But the migration characteristic of mixed water and vaporous water were different. The initial 30 days the change of moisture water was larger in mixed migration model, however in vaporous water migration model within 60 to 90 days was larger. These results will provide a basis for reveal evolution mechanism of engineering behaviors of unsaturated expansive clays.

A model considering solid-fluid interactions stemming from capillarity and adsorption mechanisms in unsaturated expansive clays