Analysis of microscopic pore structures of the silty clay before and after freezing–thawing under the subway vibration loading

With the rapid development of the subway rail transit, the effect of the cyclic loading on the surrounding foundations and buildings has drawn wide attention. In addition to the in situ tests and the laboratory triaxial tests, microscopic tests also provide an effective way to clarify the physical and mechanical characteristics of soils. On the other hand, the characteristics of the soft silty clay before and after freezing–thawing has been less studied. In this paper, the scanning electron microscope (SEM) tests following the cyclic triaxial tests of silty clay layer were performed to investigate the variations of the microscopic pore structures of the layer before and after freezing–thawing. The corrected Otsu method was used to obtain the binary SEM images of silty clay. The porosity results demonstrate that the magnifications from 1000× up to 5000× were suitable for observation of the silty clay microstructures. The binary SEM images of soil pore structures were quantitatively analyzed, including the porosity, the size distribution, the pore shape coefficient, the pore orientation distribution and the fractal dimension. The pore orientation of samples without loading is arranged in the horizontal direction, while the pores of samples under cyclic loadings are arranged in the vertical. After freezing–thawing, the mean anisotropy value of the microscopic pore structures increased about 12% and the porosity of samples without loadings increases about 11.24%. The lower the freezing temperature is, the larger the porosity within the samples becomes. However, the freezing–thawing has little effect on the pore shape coefficient of the silty clay. The porosity of the silty clay increases with an increase in pore diameter, but it decreases with the increase in excess pore pressure. In addition, the microscopic pore structures of the silty clay exhibit fractal characteristics. The fractal dimension is reduced by the disturbance from the effect of freezing–thawing, coupled with the effect of cyclic loading.     

Coal pore characteristics at different freezing temperatures under conditions of freezing–thawing cycles

Most coal reservoirs in China have low permeability, which causes gas drainage to be inefficient. The method of cyclic cryogenic fracturing basing on freezing–thawing (F–T) fracturing effects is proposed to break coal to increase its permeability. An F–T experiment was carried out at different freezing temperatures using a nuclear magnetic resonance (NMR) test and an ultrasonic test. We investigated the evolution of coal pore structure under different freezing temperature F–T cycles and the mechanism of F–T fracturing. The results indicate that the frost force and the thermal stress in the cyclical process of F–T work together to cause fracture formation and fatigue damage. When the freezing temperature decreases, the network of pores and fissures becomes more developed and the number of mesopores, macropores and micro-fractures increases. This suggests that the network of pore-fractures in coal will be more interconnected and the space available for gas seepage will be larger. Ultimately, this significantly increases the efficiency of gas drainage.     

Microscale analysis to characterize effects of water content on the strength of cement-stabilized sand–clay mixtures

Acta Geotechnica - Cement stabilization is a useful and widely adopted method to improve the engineering properties of soils. However, characterization of the unconfined compressive strength, a...     

Toward the understanding of the treatment of textile industries’ effluents by clay: adsorption of anionic dye on kaolinite

Clays, particularly kaolinite, are promising adsorbents for the treatment of textile effluents, but there is a need of better understanding the mechanisms of adsorption, especially in the case of anionic dyes. Thus, the removal of RR120 anionic dye was investigated using Tunisian raw clay (TBK) composed of kaolinite and illite, and a standard kaolinite (KGa-2), and conducting batch experiments by varying different parameters (contact time, ionic strength, concentration, temperature). We investigated the clays’ surface charges by electrophoretic mobility measures and the dye-clay interactions during adsorption, by the streaming-induced potentials (SIP). The results showed that KGa-2 has higher adsorption capacity for RR120 dye than TBK clay, moreover enhanced by increasing the ionic strength and/or lowering the pH of the aqueous. The SIP results showed an increase of negative charges for both clays, reflecting the adsorption of the anionic dye on the positive charges of the amphoteric surfaces of the clays. The SIP magnitudes indicated a higher adsorption rate for KGa-2 in accordance with the kinetic study. The Sips model that described the best adsorption isotherms indicates lateral interactions of the dye molecules, stronger in the case of KGa-2 than TBK. Also, the dye molecules form a thinner layer on KGa-2 surfaces. In addition, the dye molecule’s structure was not altered, as verified by mass spectrometry. The adsorption process was feasible and spontaneous and favored at ambient temperature. Thus, kaolinite-rich clays are effective in the removal of anionic dyes in aqueous solution and potential good adsorbents in wastewater treatment.     

Competitive adsorption of metal ions onto goethite–humic acid-modified kaolinite clay

A binary mixture of humic acid and geothite was prepared and used to modify kaolinite to produce geothite–humic acid (GHA)-modified kaolinite adsorbent useful for the adsorption of Pb²⁺, Cd²⁺, Zn²⁺, Ni²⁺ and Cu²⁺ from Single and Quinary (5) metal ion systems. The cation exchange capacity (CEC) and specific surface area of GHA-modified kaolinite clay adsorbent were found to be 40 meq/100 g and 13 m²/g, respectively, with the CEC being five times that of raw kaolinite clay (7.81 meq/100 g). The Langmuir–Freundlich equilibrium isotherm model gave better fit to experimental data as compared with other isotherm models. In Quinary metal ion system, the presence of Zn²⁺ and Cu²⁺ appears to have an antagonistic effect on the adsorption of Pb²⁺, Cd²⁺ and Ni²⁺, while the presence of Pb²⁺, Cd²⁺ and Ni²⁺ shows a synergistic effect on the adsorption of Zn²⁺ and Cu²⁺. The GHA-modified kaolinite showed strong preference for the adsorption of Pb²⁺ in both metal ion systems. Brouers–Weron–Sotolongo (BWS) kinetic model gave better fit to kinetic data compared with other kinetic models used. Data from BWS kinetic model indicate that adsorption of metal ions onto GHA-modified adsorbent in both metal ion systems followed strictly, diffusion-controlled mechanism with adsorption reaction proceeding to 50 % equilibrium in <2 min in the Single metal ion system and <1 min in the Quinary metal ion system. Adsorption of metal ions onto GHA-modified kaolinite is fairly spontaneous and endothermic in nature in both metal ion systems although the rate of metal ion uptake and spontaneity of reaction are reduced in the Quinary metal ion system.     

Impact of pore fluid chemistry on the thermal conductivity of bentonite–sand mixture

Thermal conductivity is an important parameter to consider when designing clay-based barriers for use in deep geological repositories (DGR). In the DGR environment, the infiltration of local saline groundwater can potentially change the pore fluid chemistry of a barrier over its lifetime. This change in chemistry is known to alter the thermal properties of the barrier materials. In order to examine the impact of pore fluid salinity on thermal conductivity, experiments were conducted under both distilled water and saline pore fluid conditions. The material mixtures were prepared at two different dry densities using two different salt types. Furthermore, five different thermal conductivity prediction models were selected and evaluated on their performance with respect to the experimental outcomes. In general, these results indicated that an increase in the constituent pore fluid’s salt concentration leads to a decrease in the thermal conductivity of the material. Additionally, the thermal conductivity values of the materials prepared at a high dry density were greater than of those compacted at a low dry density.     

Sorption of phenanthrene on the complexes of kaolinite and different organic component

Sorption of hydrophobic organic chemicals by various components influences their behavior and fate in environment. In the natural environment, mineral components, organic matter and microorganism didn＇t exist alone. They combined or reacted one another and formed the mineral-humic, mineral-microorganic and mineral- humic- microorganic complexes. A clear understanding of the sorption of organic chemicals by the complexes of mineral and humic acid and/or microorganism will help to determine their sorptive mechanisms in environment. In this paper, the sorption patterns of phenanthrene on the complexes of kaolinite and different organic component （humic acid and microorganism） have been carried on. The results show that the combination of HA and kaolinite not only changed the structure of HA, but also modified the surface chemistry of clay mineral. Interaction between HA and kaolinite is presumably ascribable to coulombic interactions and ligand exchange between the -COOH groups of HA and OH groups at the kaolinite surface. During the sorption on mineral surface, aliphatic fractions of HA were preferentially sored by kaolinite while aromatic fractions were left in the solution. More linear isotherms and higher Koc values were observed for kaolinite-HA complex in comparison of the pure HA. The sorption capacity of kaolinite-HA complex increased with increasing ionic strength and pH, and showed more nonlinear character. Kaolinite, microorganism and kaolinite-microorganism complex can all sorb phenanthrene, but the sorption capacity significantly differed. Bacterial cell sorbed more phenanthrene than kaolinite. The biofllm coating of kaolinite affected its sorption to phenanthrene. Kaolinite with biolfilm coating sorbed more phenanthrene than that without biofilm coating. The sorption capacity of kaolinite-microorganism complex decreased with increasing ionic strength and decreasing pH, but showed more nonlinear character. Both HA and microorganism can alter the nature of kaolinite sorbing phenanthrene respectively.     

Influence of the initial water content and dry density on the soil–water retention curve and the shrinkage behavior of a compacted clay

The paper presents the results of an experimental study on the effects of the initial water content and dry density on the soil–water retention curve and the shrinkage behavior of a compacted Lias-clay. The initial conditions after compaction (initial water content and initial dry density) have been chosen on the basis of three Proctor tests of different compaction efforts. According to the eight chosen initial conditions clay samples have been compacted statically. The relation between total suction and water content was determined for the drying path starting from the initial conditions without previous saturation of the specimens. A chilled-mirror dew-point hygrometer was used for the suction measurements. For the investigation of the shrinkage behavior cylindrical specimens were dried to desired water contents step-by-step without previous saturation. The volume of the specimens was measured by means of a caliper. Based on the test results the influence of different initial conditions on the soil suction and the shrinkage behavior is analyzed. The soil–water retention curves obtained in terms of the gravimetric water content are independent of the initial dry density. At water contents above approximately 11–12.5% a strong influence of the compaction water content is observed. At smaller water contents, the soil–water retention curve is independent of the compaction water content. The results of the shrinkage tests show that the influence of the compaction dry density on the shrinkage behavior is negligible. Similar to the drying behavior of saturated samples a primary and a residual drying process could be distinguished. The primary drying process is strongly influenced by the initial water content. In contrast, the rate of the volume change of the residual drying process is unaffected by the initial water content.     

Effect of thermal stress,condensation and freezing–thawing action on the degradation of stones on the Castle of Chambord,France

This work consists in estimating the role of climatic conditions in the degradation of two French limestones, tuffeau and Richemont stone, used in the construction and the restoration of the Castle of Chambord, the largest castle in the Loire Valley, France. Meteorological data, air temperature, air relative humidity and rainfall were statistically analysed in combination with stone data from thermal–humidity sensors inserted into the walls. The climatic conditions of the surrounding area were described to assess their role in enhancing the degradation of the stones through three weathering processes: thermal stress, condensation and freezing–thawing. The damage risks due to the weathering processes were taken into account not only through the bulk effects on the stone surfaces, but also their effects were extended to investigate the damage that occurs within the porous structure of the stone. Field observations showed that the main patterns of degradation affecting the stones of the castle are biological colonization and stone detachment in the form of stone spalling and exfoliation. The results of the analysis show that there is no risk of damage to the stones due to thermal stress. Moreover, the two stones experience similar overall trends against freezing–thawing processes. Finally, this study clearly highlights the important role of condensation in the degradation of the stones of the castle.     

Resilient and plastic strain behavior of freezing–thawing mucky clay under subway loading in Shanghai

Depending on artificial freezing method applied in subway tunnel construction, a series of stress-controlled cyclic triaxial tests were conducted on freezing–thawing mucky clay to investigate their resilient and plastic strain behavior. In terms of practical engineering, this study focuses on three significant influencing factors which are artificial freezing temperatures, dynamic stress amplitude and loading frequency. This study demonstrates how these influence factors effect on the resilient strain or dynamic elastic modulus and accumulated plastic strain which are crucial to better understanding the strain behavior of freezing–thawing soil. The results indicate that the value of freezing temperature has slight influence on dynamic elastic modulus, but the freeze–thaw action can truly decrease the dynamic elastic modulus of soil, and soil with higher freezing temperature possesses larger accumulated axial strain. Besides, the dynamic elastic modulus decreases remarkably with the increasing of the cyclic stress amplitude, while the accumulated plastic strain behaves adversely. In addition, loading frequency has the least effect compared with other two factors, but lower frequency can generate larger accumulated plastic strain.     

Influence of the halokinesis on the clay mineral repartition of upper Maastrichtian–Ypresian in the Meknassy-Mezzouna basin,centerwestern Tunisia

The Meknassy-Mezzouna basin is affected by a fault system, assembling two main directions, northsouth and eastwest. The Triassic outcrops are widely noticeable at Jebel Jebbes El Meheri and the Mezzouna link. During the late Maastrichtian–Ypresian, the sedimentation in the basin is influenced by halokinetic events, which are clearly manifested either by the thickness of El Haria formation along the Triassic outcrops (rim syncline) or by an alteration surface at the top of the Abiod formation. Such events also confirm the emersion of the basin from the late Maastrichtian to the early Lutetian. However, the present work tries to highlight the effects of halokinetic uplift on the clay mineralogical variations at that area. In harmony with this halokinetic activity, the clay minerals of this time interval (during the late Maastrichtian–Ypresian) show a trend of variation which corresponds to the evolution from illite and kaolinite, indicating a strongly hydrolytic marine environment, to smectite, sepiolite, and palygorskite, reflecting a very rapid evolution from this marine environment to a lagoon environment, then to a more confined continental environment. In fact, the appearance of sepiolite and palygorskite on the top of El Haria formation and the Paleocene–Eocene transition can be explained by a transformation or neoformation mechanism in an alkaline environment, rich in silica and magnesium, under arid to semi-arid climatic conditions. Moreover, the palygorskite can be formed in continental deposits as well as in close marine environment, which displays a limited communication with the open sea. Under these conditions, the evaporation leads to high ionic concentration of alkaline pH, which is favorable to the formation and stability of this mineral.     

Variations of the temperatures and volumetric unfrozen water contents of fine-grained soils during a freezing–thawing process

Acta Geotechnica - In this study, the variations of the temperatures and volumetric unfrozen water contents for two fine-grained soils (i.e., silty clay and silt) with high degrees of saturation...     

Soil–projectile interaction during penetration of a transparent clay simulant

Visualization of soil structure interaction during projectile penetration of clay is made possible by use of a surrogate composed of magnesium lithium phyllosilicate combined with high-speed photography and digital image correlation. A free-falling penetrator striking at 5.5 m/s simulated a projectile. Penetration resistance was constant within the resolution of the experiment; it was mainly due to the bearing resistance of the soil in contact with the nose, rather than skin friction. Bearing resistance in dynamic penetration for a hemispherical-nose rod was about 20% higher than quasi-static tests using a sphere. Bearing resistance was also about 20% higher for a hemispherical nose compared to a conical nose. Cavitation behind the nose is dependent on its shape with soils rebounding toward the projectile for conical noses but not hemispherical ones.

     

Study of role of basin shape in the site–city interaction effects on the ground motion characteristics

Natural Hazards - In this paper, the role of basin shape in the site–city interaction (SCI) effects on the ground motion characteristics is documented. The effects of city type and city...     

Effect of sand grain size and boundary condition on the swelling behavior of bentonite–sand mixtures

Deep geological repository is a favorable choice for the long-term disposal of nuclear wastes. Bentonite–sand mixtures have been proposed as the potential engineered barrier materials because of their suitable swelling properties and good ability to seal under hydrated repository conditions. To investigate the effects of sand grain size on the engineering performance of bentonite–sand mixtures, we prepare five types of bentonite–sand mixtures by mixing bentonite with sand of varying particle size ranges (0.075–0.25 mm, 0.25–0.5 mm, 0.5–1 mm, 1–2 mm and 2–5 mm, respectively). We carry out sequential oedometer tests under different simulated repository conditions, including constant vertical stress (CVS), constant stiffness (CS) and constant volume (CV) conditions. The microstructural heterogeneity and anisotropy of these soil mixtures are characterized through the quantitative analysis of micro-CT scanning results. Experimental results reveal that both sand grain size and boundary condition significantly influence the swelling of soil mixtures. Under three conditions, the temporal evolutions of swelling stress and strain follow similar trends that they increase faster at the beginning and gradually stabilize afterward. Comparing the ultimate values, swelling strains follow CVS?>?CS?>?CV, while swelling stresses follow CV?>?CS?>?CVS. Under CS boundary conditions, as the stiffness coefficient increases, the swelling pressure increases and the swelling strain decreases. CT results further indicate that mixtures with larger sand inclusions are more structurally heterogeneous and anisotropic, resulting in increased inter-particle friction and collision and a higher energy dissipation during the swelling process. Moreover, the non-uniform distribution of bentonite in local zones would be intensified, which plays an important role in compromising swelling behavior. Therefore, soil samples mixed with larger sand particles present a smaller swelling stress and strain values. This study may guide the choice of engineered barrier materials toward an improved design and assessment of geological repository facilities.

     

Influence of size of granulated rubber and tyre chips on the shear strength characteristics of sand–rubber mix

Use of scrap tyres in isolation systems for seismic damping, requires a knowledge of the engineering properties of sand–rubber mixtures (SRM). The primary objective of this study is to assess the influence of granulated rubber and tyre chips size and the gradation of sand on the strength behaviour of SRM by carrying out large-scale direct shear tests. A large direct shear test has been carried out on SRM considering different granulated rubber and tyre chip sizes and compositions. The following properties were investigated to know the effect of granulated rubber on dry sand; peak shear stress, cohesion, friction angle, secant modulus and volumetric strain. From the experiments, it was determined that the major factors influencing the above-mentioned properties were granulated rubber and tyre chip sizes, percentage of rubber in SRM and the normal stress applied. It was observed that the peak strength was significantly increased with increasing granulated rubber size up to rubber size VI (passing 12.5 mm and retained on 9.5 mm), and by adding granulated rubber up to 30%. This study shows that granulated rubber size VI gives maximum shear strength values at 30% rubber content. It was also found that more uniformly graded sand gives an improved value of shear strength with the inclusion of granulated rubber when compared to poorly graded sand.     

Evaluation of the effects of uncertainty on the predictions of landslide occurrences using the Shannon entropy theory and Dempster–Shafer theory

Natural Hazards - One of the requirements for planning and decision-making to develop the infrastructures is to prepare the landslide occurrence hazard map. For this purpose, in this article, the...     

Damming effect on the distribution of mercury in Wujiang River

Seasonal changes in total mercury concentrations in surface water were observed for the Wujiang River, with higher values at the time of greater flow. The total mercury in this river was mostly associated with suspended particles, particulate mercury accounting for 84% of the total mercury flux on average during the high flow period, and 52% of the total mercury flux on average in the low flow period. Significant losses of Hg from the water were observed in the downstreams of the reservoir. In addition, the concentrations of particulate mercury in the downstreams of reservoir appeared to have been enhanced by sediment re-suspension and shoreline erosion caused by flood discharge, while the filtered portion decreased. These observations suggested that reservoirs played an important role in controlling the transport and fate of mercury in the Wujiang River.