Investigation on microstructure evolution of clayey soils: A review focusing on wetting/drying process

Variability in moisture content is a common condition in natural soils.It influences soil properties significantly.A comprehensive understanding of the evolution of soil microstructure in wetting/drying process is of great significance for interpretation of soil macro hydro-mechanical behavior.In this review paper,methods that are commonly used to study soil microstructure are summarized.Among them are scanning electron microscope(SEM),environmental SEM(ESEM),mercury intrusion porosimetry(MIP)and computed tomography(CT)technology.Moreover,progress in research on the soil microstructure evolution during drying,wetting and wetting/drying cycles is summarized based on reviews of a large body of research papers published in the past several decades.Soils compacted on the wet side of op-timum water content generally have a matrix-type structure with a monomodal pore size distribution(PSD),whereas soils compacted on the dry side of optimum water content display an aggregate structure that exhibits bimodal PSD.During drying,decrease in soil volume is mainly caused by the shrinkage of inter-aggregate pores.During wetting,both the intra-and inter-aggregate pores increase gradually in number and sizes.Changes in the characteristics of the soil pore structure significantly depend on stress state as the soil is subjected to wetting.During wetting/drying cycles,soil structural change is not completely reversible,and the generated cumulative swelling/shrinkage deformation mainly derives from macro-pores.Furthermore,based on this analysis and identified research needs,some important areas of research focus are proposed for future work.These areas include innovative methods of sample preparation,new observation techniques,fast quantitative analysis of soil structure,integration of microstructural parameters into macro-mechanical models,and soil microstructure evolution charac-teristics under multi-field coupled conditions.

Effects of Periodical Soil Drying and Leaf Water Potential on the Sensitivity of Stomatal Response to Xylem ABA

The study on the changes of stomatal sensitivity in relation to xylem ABA during periodical soil drying and the effect of leaf water status on the stomatal sensitivity has confirmed that xylem ABA concentration is a good indicator of soil water status around roots and the relation between xylem ABA concentration and predawn leaf water potential remained constant during the three consecutive soil drying cycles based on the slopes of the fitted lines. The sensitivity of stomata to xylem ABA increased substantially as the soil drying cycles progressed, and the xylem ABA concentration needed to cause a 50% decrease of stomatal conductance was as low as 550 nmol/L in the next two soil drying cycle, as compared with the 750 nmol/L ABA in the first cycle of soil drying. The results using the split_root system showed that leaf water deficit significantly enhanced the stomatal response to xylem ABA and the xylem ABA concentration needed to cause a 50% decrease in stomatal conductance was 2 to 4 times smaller in the whole_root_drying treatment than those in the semi_root_drying treatment. These results suggested that the sensitivity of stomata to xylem ABA concentration is not a fixed characteristic.

Influences of different modifiers on the disintegration of improved granite residual soil under wet and dry cycles

The disintegration of granite residual soil is especially affected by variations in physical and chemical properties. Serious geologic hazards or engineering problems are closely related to the disintegration of granite residual soil in certain areas. Research on the mechanical properties and controlling mechanisms of disintegration has become a hot issue in practical engineering. In this paper, the disintegration characteristics of improved granite residual soil are studied by using a wet and dry cycle disintegration instrument, and the improvement mechanism is analyzed. The results show that the disintegration amounts and disintegration ratios of soil samples treated with different curing agents are obviously different. The disintegration process of improved granite residual soil can be roughly divided into 5 stages:the forcible water intrusion stage, microcrack and fissure development stage, curing and strengthening stage, stable stage, and sudden disintegration stage. The disintegration of granite residual soil is caused by the weakening of the cementation between soil particles under the action of water. When the disintegration force is greater than the anti-disintegration force of soil, the soil will disintegrate. Cement and lime mainly rely on ion exchange agglomeration, the inclusion effect of curing agents on soil particles, the hard coagulation reaction and carbonation to strengthen granite residual soil. Kaolinite mainly depends on the reversibility of its own cementation to improve and strengthen granite residual soil. The reversibility of kaolinite cementation is verified by investigating pure kaolinite with a tensile, soaking, drying and tensile test cycle. Research on the disintegration characteristics and disintegration mechanism of improved granite residual soil is of certain reference value for soil modification.

Effect of cyclic drying and wetting on engineering properties of heavy metal contaminated soils solidified/stabilized with fly ash

Solidification/stabilization （S/S） is one of the most effective methods of dealing with heavy metal contaminated soils. The effects of cyclic wetting and drying on solidified/stabilized contaminated soils were investigated. A series of test program, unconfined compressive strength （UCS） test, TCLP leaching test and scanning electron microscopy （SEM） test, were performed on lead and zinc contaminated soils solidified/stabilized by fly ash. Test results show that UCS and the leaching characteristics of heavy metal ions of S/S contaminated soils are significantly improved with the increase of fly ash content. UCS of S/S soils firstly increases with the increase of the times of drying and wetting cycles, after reaching the peak, it decreases with it. When the pollutant content is lower （1 000 mg/kg）, the TCLP concentration first slightly decreases under cyclic drying and wetting, then increases, but the change is minor. The TCLP concentration is higher under a high pollutant content of 5 000 mg/kg, and increases with the increase of the times of drying and wetting cycles. The results of scanning electron microscopy （SEM） test are consistent with UCS tests and TCLP leaching tests, which reveals the micro-mechanism of the variations of engineering properties of stabilized contaminated soils after drying and wetting cycles.

Strength weakening and its micromechanism in water–rock interaction,a short review in laboratory tests

Water–rock interaction(WRI)is a topic of interest in geology and geotechnical engineering.Many geological hazards and engineering safety problems are severe under the WRI.This study focuses on the water weakening of rock strength and its infuencing factors(water content,immersion time,and wetting–drying cycles).The strength of the rock mass decreases to varying degrees with water content,immersion time,and wetting–drying cycles depending on the rock mass type and mineral composition.The corresponding acoustic emission count and intensity and infrared radiation intensity also weaken accordingly.WRI enhances the plasticity of rock mass and reduces its brittleness.Various microscopic methods for studying the pore characterization and weakening mechanism of the WRI were compared and analyzed.Various methods should be adopted to study the pore evolution of WRI comprehensively.Microscopic methods are used to study the weakening mechanism of WRI.In future work,the mechanical parameters of rocks weakened under long-term water immersion(over years)should be considered,and more attention should be paid to how the laboratory scale is applied to the engineering scale.

Theoretical study on stability evolution of soft and hard interbedded bedding reservoir slopes

Soft and hard interbedded bedding rock slopes,which is prone to failure,are widely distributed in the Three Gorges Reservoir,China.Limit equilibrium method(LEM)is commonly used to analyze the stability of bedding rock slopes that have a single failure plane.However,this method cannot accurately estimate the stability of soft and hard interbedded bedding reservoir slopes because the strength parameters of a soft and hard interbedded rock mass vary spatially along the bedding plane and deteriorate with time due to periodic fluctuations of reservoir level.A modified LEM is proposed to evaluate the stability evolution of soft and hard interbedded bedding reservoir slopes considering the spatial variation and temporal deterioration of shear strength parameters of rock masses and bedding planes.In the modified LEM,the S-curve model is used to define the spatial variation of shear strength parameters,and general deterioration equations of shear strength parameters with the increasing number of wettingdrying cycles(WDC)are proposed to describe the temporal deterioration.Also,this method is applied to evaluate the stability evolution of a soft and hard interbedded bedding reservoir slope,located at the Three Gorges Reservoir.The results show that neglecting the spatial variation and temporal deterioration of shear strength parameters may overestimate slope stability.Finally,the modified LEM provides useful guidance to reasonably evaluate the long-term stability of soft and hard interbedded bedding reservoir slopes in reservoir area.

The Effect of Basalt Fiber on Concrete Performance under a Sulfate Attack Environment

To enhance the sulfate attack resistance performance of concrete,Sulfate erosion test was carried out on basalt fiber concrete with different contents,selecting a concentration of 5%sulfate solution and using a dry−wet cycle mechanism attack of basalt fiber-reinforced concrete(BFRC).Every 15 dry−wet cycles,the mass,compressive strength,splitting tensile strength,and relative dynamic elastic modulus of BFRC were tested,and the SO_(4)^(2−)con-centration was measured.This work demonstrates that the mass,relative dynamic elastic modulus,compressive and splitting tensile strength of BFRC reveal a trend of climb up and then decline with the process of the dry−wet cycle.Basalt fiber can enhance the sulfate corrosion resistance of concrete by delaying the erosion of concrete induced by SO_(4)^(2−)and increasing the bearing and anti-deformation capacities of concrete by improving its inter-nal structure.Additionally,when mixing 0.2%basalt fiber into concrete,the strength deterioration rate will be reduced when the peak values of splitting tensile and compressive strength appear at 60 and 75 times the alter-nating dry−wet cycles,respectively.Adverse effects will occur when the fiber volume fraction exceeds 0.2%.The research in this paper can provide a foundation for the engineering applications of basalt fiber concrete.

Chloride Resistance of Concrete under Complex Stress and Environment

The presence of stress is shown to have a significant impact on chloride ions in concrete. Reinforced concrete is usually durable and cost-effective which has resulted in its widespread use for construction, however, the concrete subjected to environment and load has become increasingly apparently that attacked by aggressive agents such as chloride ion. In this study, the coupling influences are stress effects and environmental problems on the coastline concrete durability have been investigated. A series of cyclic of a wet-dry cycle and submersion tests were performed onto the stressed concrete to obtain an understanding of the physical mechanisms causing the accumulation of chlorides in the interior pores of concrete under different stress types and exposure environments, based on the same duration. Specimens were prepared and subjected to NaCl solution in a wet-dry cycle and submersion, the chloride in the tension zone is gradual with increasing the stress level, as well as the chloride ion in the wet-dry cycle, is increasing the number of cycles. The apparent diffusion coefficient of each specimen was calculated respectively, the profile of concentration at a different section of tension and compression zones were presented in influence factors of the number of cycles, the length of drying phase, and periodic wetting cycles with sodium solution was discussed. After employed Fick’s second law, the results suggested Da in a wet-dry cycle is much higher than the Da in submersion zones.

Evolution model of concrete failure surface under coupling effect of seawater freeze-thaw and erosion

In order to effectively assess the mechanical properties of concrete with freeze-thaw and seawater erosion, tests about basic mechanical properties of concrete after freeze-thaw and seawater erosion are conducted based on the large-scale static and dynamic stiffness servo test set. 50, 100, 200 and 300 cycles of freeze-thaw cycling are made on normal concrete, and the artificial seawater is produced. The reasonable wet and dry accelerate system is selected. 10, 20, 30, 40, 50 and 60 cycles of wet and dry cycling are made to concrete after freeze-thaw cycling. The degeneration law of the concrete elastic modulus and compressive strength is studied. The Ottosen tri-axial strength criterion considering cycles of freeze-thaw and wet and dry cycling is deduced based on uniaxial mechanical properties of concrete and damage theory. Experimental results show that with the increase in the number of wet and dry cycles and freeze-thaw cycles, the concrete axial compressive strength and the elastic modulus decline gradually. Tensile and compressive meridians of concrete shrink gradually. The research can be referenced for anti-crack design of actual structures eroded by seawater at cold regions.

Effects of seasonal water-level fluctuation on soil pore structure in the Three Gorges Reservoir,China

Inundation of the Three Gorges Reservoir has created a 30-m water-level fluctuation zone with seasonal hydrological alternations of submergence and exposure, which may greatly affect soil properties and bank stability. The aim of this study was to investigate the response of soil pore structure to seasonal water-level fluctuation in the reservoir, and particularly, the hydrological change of wetting and drying cycles. Soil pore structure was visualized with industrial X-ray computed tomography and digital image analysis techniques. The results showed that soil total porosity(? 100 ?m), total pore number, total throat number, and mean throat surface area increased significantly under wetting and drying cycles. Soil porosity, pore number and throat numberwithin each size class increased in the course of wetting and drying cycles. The coordination number, degree of anisotropy and fractal dimension were indicating an increase. In contrast, the mean shape factor, pore-throat ratio, and Euler-Poincaré number decreased due to wetting and drying cycles. These illustrated that the wetting and drying cycles made soil pore structure become more porous, continuous, heterogeneous and complex. It can thus be deduced that the water-level fluctuation would modify soil porosity, pore size distribution, and pore morphology in the Three Gorges Reservoir, which may have profound implications for soil processes, soil functions, and bank stability.

Suction variations and soil fabric of swelling compacted soils

This study addresses firstly the soil fabric variations of loose and dense compacted soil samples during a single wetting/drying cycle at suctions between 0 and 287.9 MPa using mainly the mercury intrusion porosimetry（MIP） tests.Two suction techniques were employed to apply this wide suction range：the osmotic technique for suctions less than 8.5 MPa,and the vapor equilibrium or salt solution technique for suctions higher than 8.5 MPa.Secondly,the soil water retention curves（SWRCs） were predicted by the MIP test results for both loose and dense soil samples.A reasonable correspondence between MIP results and SWRCs was found on the wetting path at lower suctions close to saturation and on drying path at higher suctions.

Chloride Ion Critical Content in Reinforced Concrete

Chloride ion critical content was studied under soaking and cycle of dry and wet conditions, with three electrochemical nondestructive measuring techniques, i e, half-cell potential, A C impedance, and time potential. The experimental results show that chloride ion critical content is primarily determined by the water cement ratio, while for the same concrete mixture the chloride ion critical content in soaking conditions is larger than that in a cycle of dry and wet conditions.

In-Situ Corrosion Monitoring of Scratched Epoxy Coated Carbon Steel in Saturated Ca(OH)2 with or without 3% NaCl by Scanning Electrochemical Microscopy and Electrochemical Impedance Spectroscopy

The present work is investigated the in-situ monitoring of local corrosion process of scratched epoxy coated carbon steel in saturated Ca(OH)2 with and without 3% NaCl using SECM and correlated with EIS. The results obtained from EIS analysis showed that the corrosion resistance of scratched epoxy coated carbon steel decreases in Cl- containing solution as the increase in wet/ dry corrosion cycles. This was indicated by decrease in film resistance (Rf) and charge transfer resistance (Rct), while the coated steel maintain the resistance values in saturated Ca(OH)2, most of which recovered after drying. The corrosion process was monitored using SECM by setting the tip potential at -0.70 V vs Ag/AgCl, where the consumption of dissolved oxygen occurred at the surface of test sample. The consumption of dissolved oxygen current (I’oxy-c) values was increased during the immersion in a solution with 3% NaCl. However, in wet/dry corrosion cycles, I’oxy-c was decreased due to the coverage of hydroxides/oxides at scratch area which suppressed the consumption of dissolved O2. It was found that the continuous decrease in corrosion was mainly attributed to continuous formation of corrosion products at anodic spots.

Shrinkage Characteristics of Lime Concretion Black Soil as Affected by Biochar Amendment

Studies have reported that biochar is a sustainable amendment that improves the chemical and physical properties of soil.In this study,an incubation experiment was conducted to investigate the effects of different application rates of biochar on the cracking pattern and shrinkage characteristics of lime concretion black soil after three wetting and drying cycles.Biochar derived from the corn straw and peanut shell mixture was applied to the soil at rates of 0,50,100,and 150 g kg^(-1)dry weight,representing the treatments T_(0),T_(50),T_(100),and T_(150),respectively.During the wetting and drying cycles,the cracking pattern and shrinkage characteristics of the unamended and amended soil samples were recorded.Application of biochar significantly increased soil organic carbon content in the samples.During soil desiccation,biochar significantly reduced the rate of water loss.Cracks propagated slowly and stopped due to the relatively higher water content in the soil applied with biochar.The cracking area density(ρ_c),equivalent width,fractal dimension,and cracking connectivity index decreased during the drying process with increasing application rate of biochar.Theρ_(c )value of the T_(50),T_(100),and T_(150) treatments decreased by 33.6%,52.1%,and 56.9%,respectively,after three wetting and drying cycles,whereas the T_(0) treatment exhibited a marginal change.The coefficient of linear extensibility,an index used to describe onedimentional shrinkage,of the unamended soil sample(T_(0))was approximately 0.23.Application of 100 and 150 g kg^(-1)biochar to the soil significantly reduced the shrinkage capacity by 41.45%and 45.54%,respectively.The slope of the shrinkage characteristics curve,which indicates the ralationship between soil void ratio and moisture ratio,decreased with increase in the application rate of biochar.Furthermore,compared with the T_(0) treatment,the proportional shrinkage zone of the shrinkage characteristic curve of the T_(50),T_(100),and T_(150) treatments decreased by 5.8%,13.1%,and 12.1%,respectively.Differences were not observed in the moisture ratio at the maximum curvature of the shrinkage characteristic curve among the treatments.The results indicate that biochar can alter the cracking pattern and shrinkage characteristics of lime concretion black soil.However,the effects of biochar on the shrinkage of lime concretion black soil are dependent on the number of wetting and drying cycles.