| As a multi-component material with complex material composition and internal structural characteristics,the deterioration mechanism and mechanical properties of earthen ruin’s soils are of great interest.In the case of the Kaifeng Zhouqiao site,for example,the current state of the Zhouqiao earthen site,located in the downtown area of Kaifeng,is not optimistic as modernisation of the city intensifies.The site is dominated by chalky clay,which is a low strength,water-sensitive material,and is located in an area with a complex natural environment,where the surface of the site has cracked extensively and the stability of its body has gradually decreased.Due to the constraints of heritage conservation,it is difficult to carry out experimental studies on the surface of earthen sites,considering the uniqueness of the sites.Therefore,it is a priority in the field of site conservation to analyse the deterioration mechanisms and evolutionary patterns of the soil and to establish the links between the physical and mechanical properties of the earthen ruin’s soil at macro and micro scales,while meeting the principle of minimal intervention on the earthen site.Based on molecular dynamics and microstructure,this paper chooses to investigate earthen ruin’s soil by means of numerical simulations to establish the relationship between the microstructure,composition and properties of earthen ruin’s soil materials,and then to analyse the deterioration mechanism.By assuming the earthen ruin’s soil to be a two-phase composite composed of soil particles and pores,the link between the microstructural characteristics of the earthen ruin’s soil and the macro-mechanical properties is constructed,and the damage and destruction processes of the earthen ruin’s soil are investigated using ABAQUS software.The aim is to track and analyse the deterioration progress of the earthen site in real time,and to evaluate the ontological safety of the earthen site,providing a reference basis for the conservation and restoration of the earthen site.The main research contents and conclusions are as follows.(1)Based on the molecular dynamics(MD)simulation method,a method was proposed to analyze the mechanical properties of the earthen ruin’s soil at the nanoscale.A nano-structural model of the soil of the Zhouqiao site was developed using the Molecular Dynamics method with the aid of Material Studio 7.0 simulation software.The main components are clay-based minerals(montmorillonite)and water molecules,where water and soil are structurally characterized by a stacked distribution.By applying vertical displacements to the periodic boundary molecular simulation box,LAMMPS software was applied to analyze the mechanical properties of different nanoscale models of the earthen ruin’s soil,and the stress-strain relationships of the site granular materials with different water contents were obtained.(2)Based on the scanning electron microscope(SEM)images of the earthen ruin’s soil,the SEM images with 500 times magnification were selected for microstructure reconstruction,and polygonal and circular pore models were established based on the porosity size of the earthen soil and image binarisation processing techniques,taking into account the computational efficiency,structural composition,and engineering requirements.By thresholding the SEM images of the earthen ruin’s soil,the soil samples were divided into a two-phase structure composed of soil samples and pores,and ABAQUS software was applied to reconstruct the microstructure of the soil samples.Based on the assumption that the particles around the pores of the unsaturated earthen ruin’s soil are a homogeneous material,the soil around the pores is given the mechanical properties obtained from the molecular dynamics model.Through uniaxial compression simulations,the effects of different pore shapes on the mechanical response of the microscopic model properties and the deterioration patterns produced were compared and analyzed.It was found that the simulation results of the polygonal pore model were in better agreement with the experimental results and better reflected the changes in the microstructure of the soil sample under load.(3)In order to investigate the influence of the environmental effects of dry shrinkage on the cracking of the surface layer of the site,orthogonal tests of dry shrinkage of the earthen ruin’s soil were carried out to provide an experimental basis for the validation of the microstructural model results.Polar difference analysis,image processing techniques,surface strength determination,and microstructural characterization were used to investigate the effects of soil thickness,moisture content,and time on the morphological characteristics of the surface fractures at the site.Under the effect of dry shrinkage,the thickness of the soil layer is the most unfavorable factor affecting the cracking of the earthen ruin’s soil,the thinner the thickness of the soil layer,the lower the moisture content,the more serious the cracking;while the initial moisture content is the most unfavorable factor affecting the reduction of the surface strength of the earthen ruin’s soil,the surface soil sample has lower strength and more cracks than the interior.Analysis of the SEM image results reveals that under dry shrinkage conditions,the shape characteristics of the internal fissures of the soil sample are mostly intergranular fissures.As the particle size of the particle agglomerates inside the soil sample decreases with the decrease of the fissure rate,based on the action mechanism of surface tension when the particle size of the soil sample keeps increasing,the water-holding capacity of the soil sample becomes weaker and its intergranular microscopic force gradually increases,which eventually leads to fissures on the surface of the soil sample.(4)By simulating the evolution of microscale fractures in the earthen ruin’s soil,the deterioration mechanisms and evolutionary patterns of the earthen ruin’s soil were revealed.By simulating and analysing several microscopic numerical models of earthen ruin’s soil established,the deterioration in several directions was analysed using rose diagrams and displacement field vector diagrams to simulate the cracking position of soil samples more accurately,and the deterioration mechanisms and evolutionary laws of earthen ruin’s soil were analysed at the microscopic scale.The fracture evolution process of the soil sample can be summarised as follows: tensile stress field,single primary fracture,secondary horizontal,tertiary vertical fracture,and formation of the fracture network.The upper soil is easier to produce large shrinkage deformation than the deep soil and cause the development of longitudinal fissures.Due to the different lengths of water molecule migration paths,horizontal fissures become the main factor in the development of fissures in soil samples as the water migration paths tend to stabilize.Analysis of the fissure rate rose diagrams shows that the fissure rates in the upper part of the soil samples are generally higher than those in the lower part.The results of macroscopic tests and numerical simulations are in better agreement indicating that the model can better reflect the generation and evolutionary development of microscopic damage in chalky clays. |
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