Abstract:With continuous urbanization, China,s annual new construction area is of about 2 billion m², as a result of which the theoretical and technical innovations for large-scale infrastructure construction are increasingly demanding. Scholars have found that organisms have unique external morphology and organizational structure by studying the phenomena of reinforcement and toughening, underwater adhesion, drilling and tunneling, light weight and high strength in nature. By imitating external morphology, structural characteristics or motion mechanism of organisms, they can provide new ideas, principles and theories for innovation and sustainable development of civil engineering. This paper mainly expounds the application of bionics in civil engineering from three aspects: bionic materials, bionic structures and bionic apparatus and constructions, and lists typical application cases. Finally, the bio-inspired civil engineering is summarized and prospected, which provides basic principles for future research. As an emerging research area, the basic theory of bio-inspired civil engineering is promising not yet perfect. Meanwhile, bio-inspired civil engineering involves the interdisciplinary research of biology, materials, structure and other disciplines, and it is necessary to carry out cooperation between different disciplines.

Abstract:Root content has significant influence on the saturation permeability coefficient of soil, but there is a lack of quantitative research on the effect of coarse tree roots on soil saturation permeability coefficient. In this study, a self-developed infiltration experimental box with constant head method was used to conduct quantitative research on the influence of different root diameters （1 mm≤d≤3 mm、3 mm<d≤5 mm, and 5 mm<d≤8 mm） root content (root volume content: 0.4%-2.4%), and root distribution forms (horizontal and vertical) on the saturation permeability coefficient of remold root-soil. The results indicate that, under the same root diameter conditions, the saturated permeability of the rooted soil is linearly and positively correlated with the root volume content. However, as the root diameter increases, the slope of this linear relationship continuously decreases. There is also a significant correlation between the saturated permeability coefficient and the root surface area content, and the variance analysis shows that the relationship is not affected by root diameter. Meanwhile, the linear relationship between the root surface area content and its saturation permeability coefficient was determined under different distribution types of roots (horizontal and vertical). In addition, it was found that the change rate of the saturation permeability coefficient of the rooted soil with the root surface area content in the vertical distribution was about 1.8 times that of the horizontal distribution. Based on the results above, it can be seen that the presence of roots can significantly improve the saturation permeability coefficient of soil, and the surface contact of root soil is the fundamental reason affecting its hydraulic characteristics.

Abstract:The purification degree of heavy metal contaminated soil directly affects its reuse. This paper took the soil contaminated with Cd, Cu, Ni, Zn and Pb as the research object, and the improved surround electrode arrangement as the testing method. The purification degree and soil properties of contaminated soil under phytore-electric coupling remediation were used as indicators to examine the key factors and reveal the enhancing mechanism. The results demonstrate that the removal rate of phytore-electric coupling remediation is higher than that of single one, and the occurrence form of heavy metals and the response of plants to heavy metals are the key factors. The method of improved surround electrode arrangement can actively mobilize the migration and then accumulation of heavy metals in a larger range of soil to plant roots, and then solve the problem of low accessibility and biological activity of heavy metals in sole phytoremediation. The assistance of phytoremediation is beneficial to relieve the adverse effects of electrokinetic remediation on soil properties, and solves the problem of concentrating effect and high energy consumption. The coupling remediation combined the improvement of spatial distribution and bioavailability of heavy metals, metabolism of plant, and life activities of soil microbial, which effectively improve the purification of heavy metal contaminated soil. After remediation, the soil can maintain a stable pH of 6.27-7.91, and a conductivity of 108-159 μs/cm. In addition, the energy consumption is only 13.76-18.81 kW·h/m³. The phytore-electric coupling remediation method helps to promote the sustainable use of contaminated soil.

Abstract:Hydraulic conductivity of soil decreases significantly after MICP treatment, but there is a lack of theoretical calculation of the hydraulic conductivity of MICP-treated soil. This paper presents theoretical analysis and experimental study on hydraulic conductivity of MICP treated granite residual soils, and proposes theoretical expressions for CaCO₃ contents and hydraulic conductivity. CaCO₃ mass contents were derived based on the kinetic equation of the enzymatic reaction that decays linearly. Moreover, the particle size and number of CaCO₃ crystals were calculated using the SEM image, and then void ratio, tortuosity and average specific surface area of the grain were deduced. These factors were substituted into Kozeny-Carman equation to propose a theoretical expression of the hydraulic conductivity of MICP-treated soils. Compared to the experimental data, the results show that CaCO₃ contents increase sharply at the beginning and then become stable, while hydraulic conductivity exhibits the features of an early rapid decrease and subsequent tendency to stabilize. Hydraulic conductivity of the specimens with the concentration of 0.50 kmol/m³, 0.75 kmol/m³, 1.00 kmol/m³ and 1.25 kmol/m³ of cementitious solution decreased by 35%, 40%, 45% and 55% respectively. The expression of CaCO₃ mass contents and the model of hydraulic conductivity agree well with the testing data. The findings provided valuable insights into prediction of CaCO₃ contents and hydraulic conductivity of MICP treated granite residual soil.

Abstract:Microbial geotechnical technology improves the mechanical properties of basic materials through the process of biological mineralization, making it more suitable for building or solving environmental problems in multi-disciplinary fields. Calcium carbonate produced by microbial mineralization is the main component that improves the mechanical properties of basic materials, but the production efficiency and form are affected by many factors. By means of ion meter, pH meter, SEM, EDS and XRD, the influences of the concentration and proportion of calcium chloride and urea and the volume ratio of bacteria liquid and cementation liquid (calcium chloride, urea) on the morphology and crystal composition of precipitates in natural seawater and deionized water environment during the mineralization process were investigated. The results showed that: production of calcium carbonate is poorer when calcium ion concentration and the ratio of Ca²⁺ to urea are lower, however, the ion has an inhibitory effect on the production of urease by bacteria when the initial calcium ion and urea concentration exceeds 1.0 mol/L. The more obvious inhibition effect was observed at a higher ion concentration, and thus a lower precipitation product rate.The amount of precipitates and precipitation rate in the seawater environment were higher than those in the deionized waterenvironment. When the volume ratio of bacterial to cementation solutions are 1∶4 in deionized water and less than 1∶10 in seawater, the amount of calcium carbonate is higher. The crystal type in deionized water is mainly calcite, while the crystal type in seawater is mainly of magnesium salt type.

Abstract:Microbially induced carbonate precipitation (MICP) technique is one of the promising methods that can improve the mechanical behaviors of the sand materials via the precipitated calcium carbonate crystals. However, the uniformity of bio-cemented sand materials is a vital problem and hinders the engineering applications of MICP technique. This study explores the effects of calcium chloride and calcium acetate on the MICP process and the uniformity of bio-cemented sand columns (3 cm in diameter and 11 cm in height, 5 cm in diameter and 100 cm in height) treated one batch of MICP injection. The calcium carbonate content and unconfined compressive strength results of bio-cemented sand samples demonstrate that the calcium acetate can enhance the length of the cementation path, improve the calcium carbonate distribution along the grouting direction, and reduce the strength differences among the bio-cemented sand materials. Combining with the results of ammonium and calcium ion concentration in the solution environment, when the calcium acetate acts as the calcium source, the rate of MICP reaction is slower than that by calcium chloride, benefitting the transmission path of the undecomposed urea and the free calcium ions, thereby improving the uniformity of bio-cemented sand materials.

Abstract:In this paper, the effect and mechanism of microbial induced carbonate precipitation (MICP) combined with carbon fiber for curing calcareous sand were investigated. The effect of carbon fiber on the effect of MICP on the consolidation of calcareous sand was investigated by a low-round grouting test, and the optimal amount and length of carbon fiber was determined. Subsequently, high-round grouting tests were carried out to study the sand column strength when MICP consolidation fluid could no longer be injected. Through comparative analysis of experimental results and scanning electron microscopy (SEM), the interactions among microorganisms, carbon fibers, and calcareous sand were analyzed. The test results show that the addition of carbon fiber can significantly improve the strength of microbial reinforced calcareous sand. In the low-round grouting test, the addition of carbon fiber can effectively improve the calcium carbonate production and specimen strength, and the calcium carbonate production in the fiber group increase by 15%-34% and the specimen strength increase by 135%-217% compared with the control group without fiber. In high-cycle grouting tests, when MICP consolidation fluid could no longer be injected into the sand column, due to differences in the number of grouting cycles between the two groups, the final calcium carbonate generation of the fiber group decrease by 4% compared to the control group without fiber, while the specimen strength increase by 11%.

Abstract:MICP is an emerging environmentally friendly sand reinforcement technique in the field of geotechnical engineering that can be used to improve the engineering performance of calcareous sands. However, due to the complex structure of MICP-treated calcareous sand specimens, more work needs to be carried out at the fine-scale to explain the macroscopic phenomena. This paper employs CT scanning to perform three-dimensional reconstruction of MICP-treated calcareous sand specimens before and after triaxial compression, and analyzes the fine-scale changes of the specimens. The results show that the two-dimensional grey-scale images obtained using CT scanning can demonstrate the pore distribution of the specimen, and the non-local mean filtering algorithm has a good noise reduction effect in the reduction process of the 2D grey-scale images. Based on this, 3D reconstruction was conducted on the MICP-treated calcareous sand specimens. The results showed that before loading, the pores of the specimens mainly distributed at one-third, two-thirds, and both ends of the specimens in the vertical direction, and the pores in these parts of the specimens would increase after loading. It is indicated that the damage to the specimen during loading mainly concentrated in the existing weak areas of the specimen itself. Segmentation of significant damage surfaces on the basis of 3D reconstructions shows that the damage mode of the specimen at low stress levels is X-conjugate shear damage, while at high stress levels the damage mode is of single bevel shear damage.

Abstract:The microbial coral sand pile composite foundation is composed of coral sand and coral sand piles solidified using MICP, which has the advantages of the ability to obtain local materials, less disturbance to the ground and marginal effects to the ecological environment of islands and reefs, and has good application prospects for construction of islands and reefs. Through the model test of single pile composite foundation of biocemented coral sand pile under different working conditions, the influence of relative density (50%, 65%, 72%), area replacement rate (8%, 14%, 20%), water content (dry and saturated) and water level fluctuation on the bearing characteristics was revealed. The test shows that in the dry state, with increase of the relative density and the area replacement rate, the bearing capacity and the pile side friction of the single pile composite foundation of biocemented coral sand pile increase. The axial force of the microbial coral sand pile and the stress ratio of pile and soil decrease with increase of the relative density, and increase with the area replacement rate. The bearing capacity of composite foundation can be effectively improved by increasing the relative density and area replacement rate. The ultimate bearing capacity of the single pile composite foundation of biocemented coral sand pile in the saturated state is 49%-66% of the bearing capacity in the dry state. Under the same upper load and area replacement rate, compared with the dry state, the pile side friction resistance and the maximum pile axial force of the composite foundation in the saturated state decrease, the stress ratio of pile and soil increases. The twice water level rises and falls have marginal effect on the bearing characteristics of single pile composite foundation.

Abstract:Three Gorges Reservoir Area is prone to natural disasters; microbial induced carbonate precipitation (MICP) is a soil consolidation technique with the advantages of low energy consumption, less pollution and great sustainability. The clayey purple soil is the main soil type in the Three Gorges Reservoir Area with small soil pores, and the effect of MICP on its reinforcement is unclear. in this study, the MICP-cured soil specimens, which conformed of different Bacillus bacterium concentrations (OD₆₀₀=0, 0.5, 1.0, 1.5) and curing fluid concentrations (0, 0.5, 1.0, 1.5 and 2.0 mol/L) were subjected to unconsolidated undrained triaxial shear tests to examine the stress-strain relationship, elastic modulus and shear strength index (cohesion and internal friction angle) while their microstructures were analyzed by SEM tests. The results indicated that the shear strength, elastic modulus and cohesion increased and then decreased with the increase of the bacterium concentration or the curing fluid concentration under the same curing fluid concentration or the same bacterium concentration. The best combination existed when bacterium concentration is OD₆₀₀=1.0 and the concentration of cement solution is 1.5 mol/L. The average internal friction angle was characterized as increase followed by decrease with the increase of curing fluid concentration, the highest value of internal friction angle was obtained at the bacterium concentration with OD₆₀₀=0.5 or OD₆₀₀=1.0 under the same curing fluid concentration. Compared with no treatment, the maximum values of the cured specimens were increased by 62.59%, 50.18%, 119.50%, and 10.33% (226.00 kPa, 6.44 MPa, 48.30 kPa and 26.70°), respectively. The SEM revealed that the MICP-reinforced purple soil formed a large number of spherical and flaky calcium carbonate crystals, which distributed on the surface and in the interstices of soil particles. The crystals present on the surface of soil particles increased the surface roughness and soil particle size, which in turn increased the friction angle of the soil, while the crystals present in the interstices of soil particles produced cementation, contributing to the cohesion. The MICP can effectively improve the strength of cohesive purple soil, and the optimal reinforce performance occurred at the combination of the bacterium concentration with OD₆₀₀=1.0 and curing fluid concentrations with 1.5 mol/L.

Abstract:The soil solidification technology based on microbial- or enzyme-induced carbonate precipitation process has the characteristics of high strength and environmental friendliness. However, it has limitations such as low treatment efficiency and time-consuming. This paper studies the treatment method, effect and mechanism of another biological soil solidification technology based on the urease-enhanced magnesia carbonization process. Comparisons were made against pure magnesia carbonation, enzyme-induced calcium carbonate precipitation (EICP) and Portland cement. The results show that when the urease-enhanced magnesia carbonization treatment method is used, a higher strength (0.92 MPa) can be obtained after one treatment, and the strength of the sample is significantly higher than that of the samples treated with pure magnesia (0.30 MPa), EICP (0.28 MPa) or Portland cement (0.69 MPa). The preparation method of the urease-enhanced magnesia carbonization treatment sample also has great influence on the strength. Compared with the one-phase injection method, the strength of the sample treated by the pre-mixed method was 3 times higher. In addition, the strength of the urease-enhanced magnesia carbonization treatment sample was further improved by about 70%, and the highest strength reached 1.30 MPa, with the addition of small amount of non-fat milk powder. Through microstructural and mineralogical analysis, it was found that after urease-enhanced magnesia carbonization treatment, the solidified materials filled the pores between the sand particles, cemented the sand particles together, and formed a stable spatial structure. It could also be seen that soil treatment by urease-enhanced magnesia carbonization led to higher hydrated magnesium carbonates content, and lower brucite content.

Abstract:In order to explore the physical and mechanical properties of weak expansive soil improved by urease Enzyme-Induced Calcium Carbonate Precipitation (EICP) technique, the expansion characteristics and mechanical properties of weak expansive soil treated by EICP were tested by soybean urease. The effects of urease concentration, initial Ca²⁺ concentration, enzyme-binder ratio, urine-calcium ratio and curing time on the formation rate of calcium carbonate and free expansion rate were studied by orthogonal test of EICP multi-factor ratio, and the optimum ratio of EICP reaction solution was determined. On this basis, the expansive soil was improved by different dosages of EICP reaction liquid, and the effect was tested by free expansion rate test, unconfined compressive strength test and triaxial compression test. The results show that when the content of EICP reaction solution is 20%, the inhibiting is the best, and the mechanical strength and calcium carbonate formation rate of soil are the highest. The precipitate induced by soybean urease is calcite calcium carbonate, which is attached to the surface of soil particles. It not only fills the pores of soil particles, but also cements the skeleton particles of soil, hinders the contact between soil and water, improves the compactness and bonding strength of soil, and finally improves the expansibility and mechanical properties of expansive soil.

Abstract:China has a large number of informal landfills that are operating under excessive pressure, which has led to a number of environmental issues that need to be resolved immediately. High heavy metal content landfill leachate can easily contaminate the water near the landfill, and the aged waste produced by landfill mining expansion also contains excessive amounts of heavy metals such as Cd, Pb, Zn, and Cr (Ⅲ), which can easily result in secondary pollution during resource use. Therefore, the issue of heavy metal pollution in landfills requires urgent attention. The efficacy of Microbial Induced Carbonate Precipitation (MICP) technology to immobilize heavy metals has recently been thoroughly investigated. Sporosarcina pasteurii has drawn interest due to its high expression of urease and excellent environmental adaptability. However, in related investigations, the principal remediation materials are contaminated solutions and regular soil, which are visibly distinct from aged refuse in terms of the causes of contamination and chemical make-up. As a result, this study conducted experiments on the bioremediation of heavy metal contaminated solutions and aged refuse, investigated the viability of heavy metal bioremediation by S. pasteurii, and examined the changes in the heavy metal fraction before and after bioremediation as well as the remediation mechanism. The findings indicate that Cd, Pb, Zn, and Cr (Ⅲ) remediation rates from the solutions by S. pasteurii could be as high as 95%, 84%, 5% and 98%, respectively. Additionally, exchangeable Cd, Pb, and Zn remediation rates from aged refuse could reach 74%, 84%, and 62%, respectively, while the exchangeable Cr(Ⅲ) concentration before remediation is virtually 0. After the bioremediation, the amount of residue-bound and iron-manganese oxide-bound heavy metals in the aged refuse increased whereas the amount of exchangeable and carbonate-bound heavy metals dropped. Meanwhile, the MICP process,s calcium carbonate precipitation and the Fe and Al content of the aged refuse both support the conversion of heavy metals into a more stable fraction.

Abstract:In the process of microbial solidification and stabilization of heavy metal contaminated soil, the rapid reaction rate and the inhibition of high concentration of salt ions on bacterial activity are the key problems that hinder the application of single stirring process. The solidification and stabilization tests of zinc-contaminated silt were carried out by using the single-stage microbial stirring method. In the tests, low-solubility gypsum (calcium sulfate dihydrate) was used as the calcium source for the microbially induced mineralization reaction, and the calcium salt content and zinc pollution concentration were adopted as the control variables to investigate the changes in soil strength, zinc ion leaching concentration and chemical forms. The results show that the solidification strength of 500 mg/kg zinc-contaminated silt is 77.3% higher than that of untreated zinc-contaminated silt. Meanwhile, the concentration of zinc ion leaching is 87.3% lower than that of untreated zinc-contaminated silt. After solidification and stabilization, the strength of zinc-contaminated silt dramatically improved and the migration of heavy metal zinc in soil significantly decreased. Calcium carbonate, zinc carbonate and basic zinc carbonate are formed in the solidified and stabilized zinc-contaminated silt, and carbonate crystals aggregate to form porous clusters. The single stirring method microbial mineralization treatment based on gypsum calcium source can effectively solidify and stabilize zinc-contaminated silt. This technique has potential application in solidifying/stabilizing zinc-contaminated silty soil site.

Abstract:Microbial induced carbonate precipitation can achieve the crack self-healing in concrete. Bacterial carrier can effectively improve the survival rate of bacteria in concrete matrix and thus improve the crack self-healing effect in concrete. However, the potential decrease in mechanical properties, poor compatibility with the cementitious matrix and relatively high cost of the current bacterial carrier cannot be ignored. The self-healing concrete based on enhanced recycled concrete aggregate as bacterial carrier is proposed in this paper. The influence of enhancement time of recycled concrete aggregate on compressive strength and self-healing capacity of concrete was studied to determine the reasonable mineralization enhancement time of recycled concrete aggregates, and the enhancement mechanism of recycled concrete aggregates and self-healing mechanism of concrete are revealed. The experiment result shows that the reasonable enhancement time of recycled concrete aggregates is 7 days, the water absorption and crushing index of enhanced recycled concrete aggregates decrease by 20.5% and 9.5%, the compressive strength of concrete prepared with the enhanced recycled concrete aggregates increased by 8.6%. After 56d of curing and healing, the average value of healed crack widths and completely healing percentages of the concrete prepared with enhanced recycled concrete aggregates are 0.44 mm and 73%, respectively. The surface precipitates on the enhanced recycled concrete aggregates exhibit regular cubic shapes and the crystals of these precipitates are calcite. The crack-filling precipitation crystals of the self-healing concrete exhibit regular cubic shapes and cluster shapes. The crystals of these precipitations are calcite and aragonite.

Abstract:The torsional structure design of RC members has been widely concerned in recent years. However, the existing design codes are extrapolated based on the test data of small size specimens without considering the impact of size effect, so the safety degree of the predicted values of large size specimens remains to be further discussed. In this paper, the calculation formulas of pure torsional bearing capacity at home and abroad were compared and analyzed. Based on the results of three-dimensional mesoscopic numerical simulation of pure torsional loading of reinforced concrete columns, the size effect law of nominal torsional strength of RC columns was summarized, and the size effect law formula of nominal torsional strength which can quantitatively reflect the effect of stirrup ratio was established. Furthermore, by introducing the size effect coefficient α_h, a suggestion predicted formula for the pure torsional capacity of RC members considering the influence of size effect was proposed, and α_h is divided into three regions, which comprehensively considers the safety and economy of structural design, so as to ensure the safety of the predicted values of large size specimens. By comparing the existing 90 test data, it was proved that the modified formula can effectively improve the safety reserve of the predicted bearing capacity of large size specimens.

Abstract:The protection of sacrificial piles is one of the main measures to reduce the local scour on bridge piers. Existing research on the protection of local scour by sacrificial piles mainly focuses on circular sections and pays less attention to the influence of the section characteristics of sacrificial piles on local scour. In this study, physical model experiments with sacrificial piles of different diameters and cross-sections were carried out to protect a single bridge pier from local scour in a unidirectional flow circulating flume. During the experiment, the development of the local scour depth and the shape of the scour pit at the scour balance were accurately measured by an underwater high-definition camera and tilt photogrammetry. The study investigated the influence of sacrificial pile diameter and section shape on the protective effect of sacrificial piles for a single pier against local scour. The results show that the diameter and section shape of the sacrificial pile are key factors affecting the local scour protection effect of the bridge pier. The protective effect of the sacrificial pile on the rear pier increases with increasing diameter of the sacrificial pile. Compared with square and circular cross-section sacrificial piles, rhombus section sacrificial piles have a better scour protection effect on bridge piers. The sacrificial pile protection will affect the severity of the scour pit change around the rear pier, and the better the protection effect is, the gentler the change is in the rear scour pit. Additionally, the protection of the sacrificial pile shortens the time for the scour to reach the equilibrium state, and the larger the diameter of the sacrificial pile, the shorter the time is to reach the scour equilibrium state. Therefore, when using sacrificial piles to protect against local scour of bridge piers, the section characteristics of sacrificial piles should be reasonably considered to improve the scour protection effect.

Abstract:A hydraulic cementitious material was prepared by using rice husk ash, CFB desulfurization ash and steel slag, which can replace cement for soil solidification. In order to improve the activity and other properties of this cementitious material, the performance change and mechanism of the clinkerless cementitious material after mechanical grinding were studied. The results show that with the increase of grinding time, the particle size distribution curve of the cementitious materials changes from multi-peak distribution to single-peak distribution, the water consumption of standard consistency decreases and the setting time shortens. The end time of the hydration induction period and the appearance time of the second exothermic peak were significantly advanced, and the cumulative heat release increased. The early compressive strength and autogenous shrinkage of mortar specimens increased significantly. The initial flow expansion degree of the fluidized solidified soil prepared by the cementitious materials increases with the increase of grinding time, and the time-dependent loss accelerates. Mechanical grinding had no significant effect on the early strength of the solidified soil, while the later strength increases first and then decreases. Using this cementitious material without clinker to prepare fluidized solidified soil can meet the strength requirements of general backfill projects.

Abstract:This paper studied the influence of temperature increase rates (5, 10 ℃/min), duration at target temperatures (1, 2 hours) and cooling methods (natural cooling, water cooling) on the residual compressive strength of alkali activated slag concretes after high temperature. The microstructural change of the alkali activated slag matrix with temperature elevation was also studied to analyze the strength change. The results show that when the higher temperature increase rate was used, within 400 ℃, the influence of thermal stress was not sufficient, and a shorter duration in heat could remain the strength of concretes at a considerable level; with temperature elevation to 800 ℃, residual strength decreased dramatically with the increase of thermal stress; afterwards, thermal stress has achieved a certain extent, and a shorter duration in heat remained the strength. Increase in duration at target temperature has intensified the deterioration of alkali activated slag matrix and matrix-aggregates ITZ, resulting in the decrease of the residual strength. Water cooling has lowered the temperature of C-S-H hydrates to decompose, as well as resulted in thermal stress in concretes, and consequently limited the remain of the residual strength.

Abstract:In this paper, in order to explore the fracture propagation mode of rubber self-compacting concrete, combined with RA-AF characteristics and Gaussian mixture model, three-point bending test was carried out on semi-circular bending specimens with prefabricated cracks. Rubber content (0 %, 10 %, 20 %, 30 %) and span ratio (0.45, 0.54, 0.72) were selected as test variables to analyze the crack mode and variation of rubber self-compacting concrete. The results show that when the rubber content is 20 %, the rubber self-compacting concrete shows better working performance. With the increase of rubber content, the proportion of tensile cracking AE events increases, indicating that the cracking mode transform into type Ⅰ tensile cracks, and the cracks are continuous. The increase of the span ratio will lead to the decrease of the bearing capacity of the specimen, but the damage degree gradually decreases at the same loading stage, and the fracture mode inside the specimen changes. When the span ratio is 0.54, the specimen shows better working performance. The GMM method shows that the tensile crack AE and the shear crack AE are not simply divided by a straight line, but coexist in some areas. The GMM method can more reasonably describe the variation of the proportion of tensile crack AE events and shear crack AE events under different working conditions.

Abstract:There is a serious degradation effect on reinforced concrete by chloride ions. Cracks provide a fast channel for diffusion and accelerate the failure of structural durability. In the available publications, scholars focus on studying the influence of a single fracture characteristic factor on the chloride diffusion coefficient, the fitting formula based on the experimental results has limitations. Therefore, in this study, machine vision technology was used to extract and quantify multiple fracture characteristic factors. The representative elementary volume (REV) model of cracked concrete was constructed, and its equivalent diffusion coefficient was calculated by combining the influence of cracking density, interfacial roughness and orientation degree of cracks. Finally, the experimental results in the literature are compared with the calculated results based on the REV model. The conclusions show that the average relative error is 1.6% under different conditions of salt freeze, and 2.9% under different initial fracture conditions after 28 times of salt freeze. The results agree well. This indicates that the proposed chloride ion equivalent diffusion coefficient for cracked concrete has good reliability.

Abstract:With the successful application of swarm intelligence algorithms in the back analysis of pavement structural parameters, the problems of complex multivariate nonlinear optimization have been solved, while how to choose an appropriate algorithm is always the urgent problem in the back analysis of pavement structural parameters. In view of the characteristics of the back analysis of pavement structural parameters，such as complex models, numerous inversion parameters, and quite time-consuming forward calculation procedures, eight common swarm intelligence algorithms are selected in this paper. Related researches on the performance of the algorithms under the limited number of forward calculation calls are carried out. In this paper, the swarm intelligence algorithm is further tested by taking the inversion problem of the pavement structure parameters considering the material transverse isotropy and the contact state between layers as an example. The research results show that different algorithms have their own characteristics. Among them, particle swarm optimization (PSO), genetic algorithm (GA), brain storm optimization (BSO), artificial bee colony (ABC) and fireworks algorithm (FWA) work better in multi-peak problems. The firefly algorithm (FA) has a faster rate of convergence when solving the problem of a flat area near the optimal solution. For genetic algorithms , the later rate of convergence of the real number coding method is higher than that of the binary coding method, but the search ability for multi-peak problems is weaker. Artificial fish-school algorithm (AFA) and shuffled frog leaping algorithm (SLA) have better optimization ability only under a larger number of forward calculation calls. For inversion of pavement structure parameters, PSO, GA, BSO and FA have good inversion results in deflection curve matching, while BSO can get the best inversion result in the view of correlation coefficient.

Abstract:The damping ratio of soil is an essential and important dynamic parameter in soil dynamic analysis and evaluation. For the diversity and complexity of the hysteretic curve, it is difficult to calculate the damping ratio by the standard method for the excessive amount of data. The simplification of the hysteretic curve is helpful to process and analyze the test data quickly. To investigate the characteristics of the damping ratio of muddy silty clay in the Yangtze River floodplain in Nanjing under the cyclic load of the subway train, the triaxial dynamic test was carried out, and the damping ratio under different confining pressures and dynamic amplitudes was analyzed. For the geometric characteristics of the stress-strain hysteretic curves and the physical significance of the cyclic loading, the calculation method is simplified by assembling the hysteresis cycles as a group. The results show that with the increase of dynamic strain, the damping ratio shows a three-stage trend, i.e., rapid, slow and stable growth. With the increase of confining pressure, the damping ratio decreases gradually. When dynamic amplitude increases, the ratio also increases. In the simplified method, the greater the number of cycles in the representative cycle element, the smaller the calculated damping value compared with the standard method. Under the condition of appropriate accuracy, it can be used as a practical method to calculate the damping ratio, so as to reduce the amount of data processing and calculation.

Abstract:Exploring the mechanical response and damage mechanism of fractured rocks under hydro-mechanical coupling is one of the important ways to solve the safety and stability problems of rock engineering under hydro-mechanical coupling conditions. Based on Biot porous media theory and elastic theory, a numerical simulation method of hydro-mechanical coupled phase field is developed, and a staggered time integration scheme is proposed to obtain stable solutions of fluid pressure and solid deformation, in which the obtained control equations adopt the volume strain separation and partial strain separation of elasticity theory of fully saturated porous media. Two different numerical examples of fluid permeability tests and hydraulic fracturing with natural fracture interaction are used for validation. By the comparing of the numerical model with the analytical solution, the numerical results and the previous data are in better agreement, which verifies the validity and correctness of the model method. In addition, in order to investigate the unloading damage mechanism in the actual engineering excavation process, a hydro-mechanical coupled unloading damage model was established by combining numerical examples of borehole injection tests to simulate the whole process of fracture rock propagation damage under the dual conditions of hydro-mechanical coupling and surrounding pressure unloading. The study shows that mixed tensile-shear crack expansion and connection dominate the final damage mode during coupled hydro-mechanical unloading damage. Confining pressure and initial unloading stress states have a promoting effect on the development of shear cracks, while water pressure is reversed.