Microstructure And Mechanical Behavior Of Consolidated Earthen-site Soils

As one of the important cultural relics,earth sites are distributed in many regions of the world.However,various forms of decay have developed owing to exposure to the elements and human engineering activity.This has threatened the preservation of earthen sites.The research and development of consolidants is the key and difficult point in the consolidation of earth sites.Conservation attempts carried out thus far have proven that single consolidants cannot appropriately consolidate earthen sites.The method of combination of existing consolidants is expected to provide a new idea for the consolidation of earthen sites.The change of mechanical properties of soils after consolidation is one of the core problems of earthen sites protection,and the mechanical behaviors of soils are closely related to its microstructure and mechanical parameters.However,it is difficult to directly observe the effect of consolidants on earthen sites through laboratory tests,and there is a lack of understanding of the relationship between microstructure,mechanical parameters and mechanical behavior of soils.In addition,it is hard to solve the propagation theory of simple harmonic wave in complex soils.The numerical simulation method will solve these problems.The purpose of this paper is to study the consolidation effect of composite materials on earthen sites,and explore a new multiscale modeling technique to reveal how microstructures of consolidated soils affect their physical and mechanical behaviors under static and dynamic loads.Specifically,a micromechanics-based finite element method is applied to simulate the homogenized constitutive relations of the untreated and consolidated soils,based on the microstructures from scanning electron microscopy images and image-processing procedures.Multiscale models are used to quantify the stiffening,strengthening and acoustic effects of consolidants to earthensite soils,and predict the macroscopic mechanical behaviors of soils.Laboratory tests and analytical models were used to verify the accuracy of simulation.This paper quantifies the mechanical response of pore characteristics,consolidants and microscopic mechanical parameters to linear axial displacement load and harmonic acceleration pulse load.The main research contents and achievements of are as follows:(1)The distributed soils were collected from the Great Wall of Ming Dynasty in Yongchang County,Gansu Province,China.The remolded samples were treated by dropping infiltration with five kinds of materials,namely,inorganic consolidant,organic consolidant,composite materials with different combinations of organic and inorganic materials,and ethanol.A new technique is developed to improve the consolidation effect on earthen sites by comparing the changes of the physical and mechanical properties of earthen–site soils treated by single materials and composite materials.(2)Uniaxial compression and ultrasonic tests were carried out to study the deforming,strengthening and acoustic responses of consolidants to earthen-site soils.Scanning electron microscope and energy dispersion spectroscopy were employed to analyze the microstructure and elemental changes of consolidated soils.A micromechanics-based finite element method,based on the microstructures from scanning electron microscopy images and image-processing procedures,was proposed to obtain the changes of microscopic pore characteristic parameters of untreated and consolidated earthen-site soils.Results show that the peak stress,secant modulus and ultrasonic wave velocity of the samples consolidated by composite materials all increased in different amplitude.The average equivalent diameter,average sphericity and porosity of the untreated samples were larger than those of the consolidated samples.The element composition of the untreated and consolidated earthen–site soils were basically the same.(3)A cross-scale modeling technique was developed to integrate scanning electron microscope images and image processing techniques into the finite element simulation.Uniaxial compression tests were performed to verify the simulation results of untreated and consolidated earthen-site soils.The effects of micro-mechanical parameters and microstructures on the static behaviors of soils under linear axial load are quantitatively studied by using the micromechanics-based finite-element method.In addition,the accuracy of the finite element simulation method was confirmed again by the reverse calculation of the microscopic static elastic coefficient from the macroscopic mechanical parameters through the ensemble microscopic mechanical analytical model.The results of finite element simulation are in good agreement with those of uniaxial compression test.The technique of integrating scanning electron microscopy and image processing technology into the finite-element modeling is feasible to predict the static behaviors of soils under linear axial displacement loads.(4)A micromechanics-based cross-scale model was proposed to explore the propagation characteristics of sinusoidal acceleration pulses in earthen-site soils.The scanning electron microscope image and image processing technology were integrated into the finite element simulation,the dynamic elastic parameters were estimated according to the static elastic coefficient of the soils,and the simulation results of the untreated soil and consolidated earthen-site soils were verified by ultrasonic test.The effects of dynamic elastic parameters and microstructure on the acoustic characteristics of soils under sinusoidal loading were quantitatively studied by using the micromechanics-based finite element method.Results show that the acoustic parameters predicted by the finite element model of the untreated and consolidated earthen-site soils are roughly consistent with the experimental results,which proves that it is effective to quantitatively investigate the propagation characteristics of simple harmonics in the soils by integrating scanning electron microscope images and image processing techniques into the finite element simulation.(5)The relationship between microstructures and macroscopic mechanical behaviors of earthen-site soils was established by micromechanics-based finite element method.Specifically,the influence of micro-pore parameters and micromechanical coefficients on the strength,stiffness and acoustic characteristics of soil was analyzed by using multi-scale model,and the relationship between micromechanical coefficients and macro-mechanical properties was studied.In addition,the sensitivity of macroscopic mechanical behavior of soils to the variation of microscopic mechanical parameters was studied for the binary medium model with matrix and inclusion.It is found that the strength and stiffness characteristics of medium and the propagation velocity of sinusoidal pulse in medium have different responses to pore characteristic parameters of medium,and the sensitivity of macroscopic mechanical behavior to different pore characteristics is different.The micro-mechanical parameters of medium have different effects on the strength,stiffness and acoustic behavior of soils.The technique of microstructure image processing and analysis,combining with the calculation and visualization functions of the finite element software,can be employed to conduct parameters optimization analysis and quantify the mechanical response of microscopic elastic coefficient and the microscopic structure.This can provide reference for the prediction of the change of mechanical properties in laboratory tests and field monitoring,and will help to improve the detection accuracy of mechanical properties of soils under the action of static and dynamic load.It can better serve the evaluation and implementation of practical projects.Therefore,this study has a broad application space in the field of cultural heritage protection.