Abstract:To study the relationship between the microstructure and the macroscopic mechanical parameters of the acid-corroded sandstone, the mechanical parameters of sandstone corroded by hydrochloric acid were examined based on the physical property testing and the uniaxial mechanical testing. The characteristic parameters and the fractal dimension of the pore structure were obtained via SEM-EDS. The chemical damage variable based on porosity was established and the constitutive model of the corroded sandstone was derived based on the extended Lemaitre strain equivalence principle.The results show that the mass loss of sandstone immersed in hydrochloric acid at pH=1, 3 and distilled water with pH=7 for 180 days increased by 1.22%, 1.08%, and 0.57%, respectively.The uniaxial compressive strength decreased by 51.93%, 41.91%, and 27.11%, respectively. It has obvious tempo effect. The average pore size and the fractal dimension of pores of the acid corroded sandstone increase with the corrosion time; The macroscopic mechanical parameter deterioration degree of sandstone iscorrelated with the mass loss rate and the microscopic porosity of sandstone. The Feasibility of the proposed model is validated via the numerical and laboratory testing results.

Abstract:In order to investigate the mechanical properties of the basalt fiber-reinforced cemented soil subjected to the action of salt (Na₂SO₄) and drying-wetting cycles, a certain amount of fiber was added into the cemented soil. The specimens were soaked in different concentrations of salt solution, and then placed into an oven for drying subjected to a series of drying-wetting cycles. Based on the unconfined compressive strength tests, the characteristics of compressive strength under the curing condition of low and normal temperature were studied. And then, the effect of salt solution concentration and number of drying-wetting cycles on surface quality, compressive strength and stress-strain relationship of specimens were discussed. The results show that the curing condition of low temperature prevents the development of strength of cemented soil, however, basalt fiber can still improve the mechanical properties of cemented soil. The inclusion of fiber can effectively improve the resistance of cemented soil to the combined effect of salt solution and drying-wetting cycles, postpone the propagation of cracks on the surface of cemented soil, reduce the strength loss rate, and improve the residual strength. Results also indicate that there is an obvious peak in the testing curve.

Abstract:In the engineering practice, the retaining wall and foundation pit could be regarded as a plane strain condition. Considering the influence of intermediate principal stess on the strength, we obtained the new plane strain strength criterions by substituting the intermediate principal stress relationship of Lade-Duncan strength criterion into SMP and Lade-Duncan, AC-SMP and the generalized Mises strength criterion. Based on these modified strength criterions, active and passive earth pressure models were established, and then extended to cohesive soil. The theoretical and measured values were compared and analyzed. The results show that the effect of intermediate principal stress on earth pressure is significant. The Mohr-Coulomb strength criterion fails to consider the influence of intermediate principal stress, and therefore the calculation results are more conservative than measurement. Considering the influence of the principal stress, the results of SMP and Lade-Duncan strength criterion are much closer to measurement than MC. The AC-SMP and generalized Mises strength criterion can describe the earth pressure of the retaining wall only within a certain range of friction angle, but when it exceeds the applicable range, both are no longer suitable. The calculations of the generalized Mises strength criterion, within the applicable range, are closer to actual data compared with the SMP and Lade-Duncan strength criterion.

Abstract:Artificially synthesized transparent soil materials combined with particle image processing technique has been widely used in model testing. However, further study of pile foundation scour and research on its characteristics is essential. Based on the transparent sand prepared by the mixture of fused silica sand, twelve alkane and 15# white mineral oil, the basic physical properties, angle of repose and starting velocity of the transparent sand were studied through laboratory experiments. The results show that the transparency of the mixture is not much different except for the 1-2 mm particle size group which is significantly worse; the bulk density of transparent sand in each particle size group is of slight difference; the bulk density of transparent sand in each particle size group is also of neglectable difference; dry bulk density increases first and then decreases with the increase of particle size; the pore liquid shear rate and shear stress show a good linear relationship, which is a typical Newtonian fluid; the submerged angle of repose increases with increase of the median diameter; the initiation velocity of transparent sand with a particle size group of 0.1-0.2 mm increases with increase of liquid depth, fitting by Shamov's formula, indicating the better anastomosis effect.

Abstract:The stability of intersection part of tunnels is greatly affected by structural planes of surrounding rockmass.Physical model testing is an effective method to study the failure mechanism of structural planes.This paper presented special section tunnel models with discontinuities by 3D printing technique to overcome the difficulty of model preparation. The failure process of the model under uniaxial loading is observed based on digital image correlation technique and endoscope. The influence of printing path on the failure mode of tunnel model is evaluated, and the influence of the dip angle of structural planes on the failure mechanism of special section tunnel is studied. Results show that printing path has marginal effect on the failure mode of the tunnel model, and it is effective and feasible to prepare the tunnel model that its failure mode controlled by structural planes using 3D printing. The non-through structural plane with weak filler transfer the surrounding rock stress to the rock pillars on both sides, which protects the surrounding rock from over-loading and improves the stability of the tunnel. When the tip of the structural plane is far away from the tunnel, the model shows the failure of the rock column caused by the through cracks. When the tip of the structural plane is near the tunnel, the model shows the separation failure of the roof block. The failure mechanism of the special section tunnel formed by the expanding excavation of the tunnel cross section is different from that of the conventional tunnel, and its arch structure effect is obviously weakened.

Abstract:To study the change of the pile foundation displacement during the dynamic excavation of the double-route shield tunnel, three-dimensional simulation of shield tunneling under-crossing pile foundation is performed with a finite element package, PLAXIS 3D. Numerical simulation realizes the step-by-step advancing during shield tunnel excavation by taking into account the influence of earth pressure in soil warehouse, grouting pressure, and friction between the shield and the soil. Apart from that, using consolidation calculation of PLAXIS 3D, the paper evaluate the ground settlement caused by consolidation of the shield's own weight on the soil, and further evaluated the impact of the excavation speed on the pile displacement. The calculation results show that the tunnel excavation will cause the settlement, lateral displacement and tilt of the pile foundation, and the overall displacement and tilt of the pile foundation can continue to increase with progress of the shield tunnel excavation. The sensitivity analysis of construction parameters reveals that: increasing the excavation speed can effectively control the displacement of the pile foundation, but when the excavation speed increases to a certain extent, the impact of the excavation speed on the pile foundation is significantly reduced; The effect of simultaneous excavation of the double-route tunnel on the pile foundation is less significant than that of the separated double-route excavation.

Abstract:There are many mountainous areas in China, with complex terrain, weak planes and geological structures and wide distribution of geohazards. Landslides are one of the most catastrophic natural hazards occurring in mountainous areas, leading to economic loss and casualties. Landslide susceptibility models are capable of quantifying the possibility of where landslides are prone to occur, which plays a significant role in formulating disaster prevention measures and mitigating future potential risk.Since expert-based models are difficult to quantify and generally depend on the subjective judgments, the accuracy and precision of landslide susceptibility models are now evolving from expert models and statistical learning toward the promising use of machine learning methods. This study presented critical reviews on current machine learning models for landslide susceptibility investigation, an extensive analysis and comparison between different machine learning techniques (MLTs) from case studies in the Three Gorges Reservoir area was presented. In combination with field survey information as well as historical data, machine learning models were used to map landslide susceptibility and help formulate landslide mitigation strategies. The advantages and limitations of several frequently employed algorithms were evaluated based on the accuracy and efficiency of landslide susceptibility forecasting models. As the result shows, the tree-based ensemble algorithms models achieved better compared with other commonly methods of papping landslide susceptibility. Furthermore, the effect of database quality and quantity is significant, and more applications of some advanced methods (i.e., deep learning algorithms) are yet to be further explored in further researches.

Abstract:When caissons are used to support offshore wind substation platforms, the platform and electrical equipment are installed in two phases, with a waiting period of several weeks to months between the two events. To evaluate the peak resistance during secondary loading phase, this ‘penetration-consolidation-penetration’ process in clay is reproduced by the coupled effective stress large deformation finite element analysis to avoid the severe distortion of elements during installation. The numerical model is first verified by simulating a centrifuge test.A comparison of the initial penetration resistance with the penetration resistance after consolidation shows that the waiting period can cause a dramatically increase in soil strength. And the depth of peak penetration resistance during secondary loading phase is almost equal to the initial installation, implying that only a small displacement is required to achieve the compressive ultimate limit state of the caisson after the waiting period. Parametric study is subsequently carried out to investigate the effect of consolidation. A normalized time parameter considering foundation size, soil permeability and consolidation duration is proposed, and a trend line describing the increase of peak caisson resistance with normalized time is provided.

Abstract:Based on the load transfer method, considering the pile-pile interaction and introducing Pyke criterion to simulate the loading and unloading behavior on the pile-soil interface, as well as to determine the model parameters of pile-soil interaction, settlement characteristic analysis method for the double-pile foundation of energy pile was established and extended to the energy pile group. The reliability of the proposed method was verified by comparing with the experimental data in the literature. Combined with an example, this method was used to analyze the settlement characteristics of energy pile group under the action of pure mechanical loading, pure temperature effect and thermal coupling. The results show that the displacement ratio of pile group decreases with increase of pile top load under mechanical loading. Under the action of temperature effect, the displacement direction of pile group top is consistent. Under the action of thermal coupling, the displacement of pile top of pile group is relevant with the specific load level of pile top and pile body temperature increment. When the load of pile top is small and the temperature rises, it is very probable that the top of single pile will be lifted while the top of pile group will sink.

Abstract:The quality problem of prefabricated laminated slab (PLS) is one of the important factors that lead to the failure of prefabricated components in construction.A method of intelligent detection of PLS based on machine vision is presented in this paper. First, the image is collected through the camera system on the production line of PLS, and then the image is preprocessed through noise removal. The Canny algorithm is used to extract the edge features, and Harris corner detection algorithm is used to extract the internal features of the image.The extracted information is compared with the stored information.This method is used to identify and analyze the features of three PLS. The results prove that intelligent detection method can be used for image acquisition and image preprocessing of PLS, and the characteristics of the statistics, the size of PLS, the number, size and location information of reserved holes and embedded parts.Intelligent detection method can quickly detect and judge whether the PLS is qualified. It can improve the pass rate of factory components and reduce the return rate of components, and hence reduce the construction cost and the risk of project delay.

Abstract:Based on the differential element method, considering transfer of bond stress between corroded steel bar and concrete, and the compatibility of cross-section strain before and after bond stress transfer is considered, the deflection calculation model of corroded RC beams strengthened by steel plates and FRP is proposed in this paper. To validate it, the testing results in existing literatures and this study, are compared and analyzed. The results show that under different loading levels, compared with the strengthened beam without considering bond stress transfer, the deflection calculation results via proposed model considered the bond stress transfer of corroded RC beam strengthened by bending is more closer to the testing values. The relative errors of the former ultimate loading and the corresponding ultimate deflection is -0.6%~1.8% and -10.5%~9.1%, respectively, while the relative errors of the latter ultimate loading and the corresponding ultimate deflection is 1.1%~6.1% and -11.4%~-2.2%, respectively. The proposed model can also accurately predict the load-deflection curve of corroded RC beams strengthened by FRP materials (including CFRP and BFRP). The improved relative error of ultimate loading value is -3.6%~4.2%, and the that for ultimate deflection is -4.2%~8.3%.

Abstract:In order to explore the deformation performance of the prestressed partially clad steel composite beams under vertical loading, a vertical two-point loading test is carried out on 12 prestressed partially clad steel composite beams to explore the deformation performance such as crack development, deflection and ductility. In this paper, the fractal dimension theory is used to analyze the crack development of each specimen. Based on the Euler beam theory and Timoshenko beam theory, the flexural equations of beams are derived respectively, and then the mid-span deflection of beams is calculated. The displacement ductility coefficient, section curvature ductility coefficient and energy ductility coefficient of prestressed partially clad steel composite beams are calculated. Influence factors of three ductility factors are analyzed by gray correlation analysis. The results show that the cracks develop most fully when the partially clad steel composite beams with a certain degree of pre-compressive stress are applied. The deflection equation based on the theory of Timoshenko beam and the calculation value of mid-span deflection are closest to the real value. The section area is the most important factor affecting the ductility coefficient of displacement and the ductility coefficient of curvature, and the anchoring form of section steel is the most important factor affecting the ductility coefficient of energy.

Abstract:The long-span suspension bridges, which are notable flexibility, are sensitive to wind load. Using the finite element method, considering the geometric nonlinearity and displacement-load nonlinearity, the aerostatic stability of suspension bridges is studied when the non-uniform wind attack angle flow and non-uniform wind speed flow distributes symmetrically and asymmetrically along the main girder. The results show that the flow of positive angle of attack will reduce the critical stability wind speed of the bridges, the flow of negative angle of attack is conducive to the bridges to resist the instability of static wind. The impact of negative angle of attack on the static wind stability of the bridge is greater than that of positive angle of attack. When the flow is asymmetrically distributed, the critical stability wind speed can be determined by small wind angle of attack. When the flow is symmetrically distributed, the mean angle of attack can be selected to measure the aerostatic stability of the bridge. Incoming flow with non-uniform wind speed has an adverse effect on the aerostatic stability of the bridge. Moreover, the influence of symmetric distribution of non-uniform wind speed is greater than that of asymmetric distribution of non-uniform wind speed.Under different initial angles of attack, this kind of incoming flow has similar effects on the static wind stability of the bridge. That's to say, the critical wind speed of aerostatic stability of the bridge decreases with the increase of the unevenness of the wind speed.

Abstract:At present, most of the studies on the wind field characteristics of downburst mainly focus on the wind speed model and partial wind field characteristics. Few studies on the relationship between wind field characteristics and mean wind speed, the correlation between different wind field characteristics or the comparison with the boundary layer surface wind characteristics are carried out.Based on the measured wind speed data of downburst, the time-varying mean wind speed of downburst is extracted by utilizing the moving average method, and then the fluctuating wind speed is obtained. On this basis, the characteristics of turbulence intensity, turbulence integral scale and gust factor of downburst are studied. The relationships between the above three characteristics and mean wind speed, as well as the correlations among the above three characteristics are analyzed. The results show that: 1) the turbulence intensity, turbulence integral scale and gust factor of downburst are greater than the corresponding wind field characteristic of atmospheric boundary layer surface wind at the same height under the corresponding site category; 2) there is a negative correlation between the turbulence intensity of downburst and the average wind speed, as well as between the turbulence integral scale and the turbulence intensity while there is a positive correlation between the turbulence integral scale and the average wind speed, as well as between gust factor and turbulence intensity; 3) the wind resistance design for low-rise buildings should focus on the influence of fluctuating components of downburst, while the threat of airflow vortexes of downburst should be paid more attention for high-rise buildings and high-rise structures; 4) compared with the atmospheric boundary layer surface wind, downburst is prone to produce larger instantaneous extreme wind speed, which could pose greater threats to engineering structures.

Abstract:In order to study the dielectric properties of nano-carbon fiber-reinforced concrete, and find the law of reflection and loss of nano-carbon fiber-reinforced concrete to electromagnetic waves, the waveguide method was used to test the dielectric properties of nano-carbon fiber reinforced concrete with a fiber content of 0, 0.1%, 0.2%, 0.3%, 0.5% in the frequency range of 1.7 GHz to 2.6 GHz constant. The effects of fiber content and frequency on the dielectric properties of nano-carbon fiber-reinforced concrete were analyzed from the aspects of relative dielectric constant real part, imaginary part and loss tangent.At the same time, the effects of nano-carbon fiber and ordinary carbon fiber on the dielectric properties of concrete materials with 0.3% fiber content were compared and analyzed. The test results show that: nano-carbon fiber can increase the real part and imaginary part of the complex dielectric constant and loss tangent of the concrete material, and enhance the loss ability of the concrete material to electromagnetic waves; the greater the amount of nano-carbon fiber, the stronger the dielectric properties of nano-carbon fiber-reinforced concrete and the greater the loss of electromagnetic waves; the effect of nano-carbon fibers on the improvement of the dielectric properties of concrete materials is stronger than carbon fibers.

Abstract:In order to study the influence of rubber content on the fracture mechanical properties of concrete under cyclic loading, monotonic and cyclic loading fracture tests are carried out on five kinds of notched concrete beams with different rubber content (0, 5%, 10%, 15% and 20%).According to the load-crack mouth opening displacement (P-CMOD) curves obtained by fracture tests, the fracture energy and dissipated energy of rubber concrete are calculated. Combined with the failure modes of typical rubber concrete specimens, the influence of loading conditions and rubber particle content on the mechanical properties and energy dissipation of concrete is analyzed comprehensively.The results show that with the increase of rubber content, the strength and elastic modulus of rubber concrete decrease gradually, while the fracture energy increases gradually.Under cyclic loading, the continuous and gradual accumulation of concrete dissipated energy with cyclic loading is similar to the accumulation of crack mouth opening displacement, the damage and failure process also presents a "three-stage" characteristic.The total dissipated energy increases with the increase of rubber content, but it does not exceed the fracture energy.

Abstract:Through the mechanical properties and rapid freeze-thaw cycle test of self compacting recycled aggregate concrete, the influence of recycled coarse aggregate slurry on the properties of self compacting recycled aggregate concrete is studied from the aspects of compressive strength, splitting tensile strength and flexural strength, mass loss rate, compressive strength loss rate and relative dynamic elastic modulus of specimens after freeze-thaw. Combined with SEM electron microscopy test, the mechanism of improving the frost resistance of concrete by wrapped slurry aggregate is analyzed microscopically.The results show that, the conversion relationship between the split tensile strength, flexural strength and compressive strength of ordinary concrete and recycled concrete is not applicable to self-compacting wrapped slurry recycled aggregate concrete and the new relationship proposed in this paper is in good agreement with the experimental data.Wrapped slurry recycled coarse aggregate optimizes the physical performance of the aggregate, improves the transition area between the old and new concrete interfaces, enhances its compactness, and improves the frost resistance.When 42.5R cement slurry is used to wrap the aggregate, the frost resistance of concrete is the best. Although it is not as good as ordinary concrete, the relative dynamic elastic modulus of concrete increases by 18.6% after 150 cycles of freeze-thaw compared with the concrete of aggregate without wrapping slurry.The freeze-thaw damage model established based on the compressive strength and relative dynamic elastic modulus after freeze-thaw has a higher fitting accuracy, and can better reflect the freeze-thaw damage failure law of self-compacting wrapped slurry recycled aggregate concrete.

Abstract:Phosphorus, as an essential element in life organism, plays an important role in life activities. Recently, with the increasingly stringent discharge standards of wastewater treatment plants (WWTPs), ferric salts have been widely used in WWTPs as chemical agents for phosphorus removal. Although a large number of studies have been carried out, dueto the diversity and complexity of the sewage system, the efficiency of chemical-assisted biological phosphorus removal and side-stream phosphorus recovery is still facing many challenges. Therefore, this article reviews the basic characteristics of the ferric salt dependent phosphorus removal process, sorts out the interaction mechanism between ferric salt and phosphorus.The influence and trend of iron salt on microbial community structure in the two processes are summarized, and the research direction of the process in the future is prospected.It is found that continuing to systematically study the symbiotic relationship between microorganisms and different phosphorus minerals, optimizing the separation and purification conditions of downstream products of side stream phosphorus recovery, and establishing a dynamic model to reasonably predict and feed back the concentration of iron and phosphorus in the sewage treatment system are inevitable measures to improve the resource recovery efficiency of the sewage treatment plant and promote sustainable development.

Abstract:Dissimilar iron reduction coupled with anaerobic oxidation (Feammox) is a common pathway to oxidize ammonia nitrogen in natural environment, which however has been rarely reported in sewage treatment. In this study, the sludge digester liquor was used as the NH₄⁺-N source to study the NH₄⁺-N removal during Feammox driven by adding ferrihydrite under anaerobic conditions. The results showed that after 48 days of anaerobic incubation, the NH₄⁺-N removal efficiency and total nitrogen removal efficiency were 72.4% and 57.5%, respectively, while the NH₄⁺-N removal efficiency and total nitrogen removal efficiency in the reactor without adding ferrihydrite were 7.6% and 2.5%, respectively. The continuous production of Fe(Ⅱ) was detected in the ferrihydrite-added reactor. After feeding with isotope-labeled ¹⁵NH₄⁺-N as the NH₄⁺-N source, ³⁰N₂ was detected in the headspace of the ferrihydrite-added reactor, which proved the occurrence of Feammox. Real-time quantitative PCR(qPCR) results showed that Acidimicrobiaceae A6 expressed actively. High-throughput 16S rDNA showed that a variety of iron-reducing bacteria were significantly enriched. After simultaneously adding ferrihydrite and humus into the reactor, the removal efficiency of NH₄⁺-N further increased to 88.35%, and the removal efficiency of total nitrogen reached 73.75%. The analysis of the composition and morphology of iron oxides added in the reactors showed that humus weakened the passivation rate of iron oxides, which was beneficial for ferrihydrite to maintain activity and continue to participate in Feammox.

Abstract:Diclofenac sodium is one of the new pollutants widely distributed in water, and its removal has attracted much attention. In order to improve the removal efficiency of diclofenac sodium and obtain cost-effective and environmentally-friendly adsorbents, the chitosan polydopamine modified hydrogel (CS-PDA) was prepared from chitosan by introducing polydopamine onto chitosan by photocatalytic method. The adsorption properties of diclofenac sodium were studied. The CS-PDA was characterized by SEM, FTIR, XPS and BET. The results showed that the CS-PDA had a network structure, porous structure and a specific surface area of 22.46 m²·g^-1. Explored the effects of time, temperature, initial concentration and pH value on the adsorption effect, the results showed that the adsorption capacity of CS-PDA was the largest at pH=4.3, the adsorption kinetics of CS-PDA for diclofenac sodium was in accordance with the pseudo second order kinetic model, and reached the adsorption saturation at 240 minutes.The adsorption isotherm was in line with the Langmuir isotherm model and the Langmuir theoretical maximum adsorption capacity of diclofenac sodium at 25 ℃ was 333.2 mg/g. The adsorption capacity of diclofenac sodium by CS-PDA was higher than 70 mg/g after 7 regeneration cycles.

Abstract:In order to study the effect and mechanism of extracellular polysaccharides on the stability of Anammox granular sludge, amylase which can enzymolyze polysaccharides was used to enzymatically hydrolyze granular sludge. The results showed that the outer edge of the granular sludge in the α-amylase treatment group swelled, while the surface of the granular sludge in the β-amylase treatment group did not change significantly, but it was broken and the stability was obviously reduced. The surface properties and XDLVO theoretical analysis showed that enzymatic hydrolysis reduced the hydrophobicity of granular sludge and increased the repulsive force between microorganisms, thereby affecting the stability of granular sludge. The results of Fourier infrared spectroscopy showed that the content of hydrophobic functional groups in extracellular polymers of sludge was significantly reduced after enzymatic hydrolysis. Confocal scanning found that the content of α-D-glucopyranose polysaccharide at the outer edge of the granular sludge in the α-amylase treatment group was significantly decreased, while the β-D-glucopyranose polysaccharide in the β-amylase treatment group was distributed in fragments. Therefore, the α-amylase treatment group showed that the hydrophobic effect of extracellular polysaccharides, binding to each other through O—H functional groups or bridging with cations can promote the aggregation between microorganisms. The β-amylase treatment group showed that the entanglement between the long backbones of extracellular polysaccharides and the bridging of abundant binding sites to form a skeleton enhanced the adhesion between microorganisms and was beneficial to the stability of granular sludge.

Abstract:Exhaust emission from the diesel engine in transportation is one of the essential sources that cause air pollution, of which the primary pollutants include particulate matter, nitrogen oxide, hydrocarbon, carbon and oxygen compounds, which have significant differences in emission characteristics under different working conditions. We collected, organized and summarized the research on diesel exhaust pollutants emission and emission reduction measures in the past five years, and discussed the emission characteristics of diesel exhaust pollutants under different working conditions(altitude, speed, engine speed and load). The results showed that the pollutant emission was the most serious under the high speed of engine conditions in high altitude areas. Moreover, the analysis of the influencing mechanism found that different working conditions mainly affect the combustion temperature, oxygen intake, oil and gas mixture, combustion time and other factors, and then affect the formation of different pollutants. Furthermore, we summarized the measures and influencing mechanisms to improve the pollutant emission from exhaust emission of the diesel engine. This paper will provide theoretical guidance for the study of pollutant emission characteristics of a diesel engine under different working conditions. So far, there are many studies on typical pollutants in diesel engine exhaust, but few studies on the emission characteristics and generation mechanism of toxic and harmful substance such as persistent free radicals in particulate matters and gas-phase substances. However, the environmental risk of persistent free radicals may be higher than that of typical pollutants, which will be a major content in the study of diesel engine exhaust pollutants in the futures.