Multi-scale Study On Influencing Factors Of Rammed Earth Permeability Coefficient Of The Great Wall Of Ming Dynasty In The North And South Of Qilian Mountains

The permeability of rammed earth is closely related to the degree of development of various diseases in earthen sites,and is a key issue of general concern in the field of geotechnical engineering and cultural heritage protection.Rammed earth is widely used in various ancient buildings,and still retains many rammed earth buildings.Along the border of the Qilian Mountains,a large number of rammed earth sites of the Great Wall of the Ming Dynasty were visited on the spot.The earthen sites in various regions suffered from different degrees of disease infestation.The occurrence and development of the disease are not only affected by external environment,but also related to the characteristics of rammed earth itself,and the permeability of rammed earth is an important feature related to it.In addition,combined with engineering examples,it is found that the permeability of rammed earth has great influence on the depth and breadth of protective grout infiltration in remediation measures,which has far-reaching significance for the study of the permeability of rammed earth.This paper expects to show the permeability of rammed earth by studying the permeability coefficient of undisturbed rammed earth.The rammed earth sites of the Great Wall of Ming Dynasty in 18 districts(counties)of Gansu and Qinghai provinces in the southern and northern foothills of Qilian Mountains were selected as the research objects,and samples were taken in the collapsed sections of the sites as far as possible to ensure that the sites would not be damaged again.The suitable undisturbed soil samples were selected using a South Type permeameter of 55,and falling head permeability test was carried out to determine the soil permeability coefficient.According to this value,the rammed earth was classified into a strong permeability type and a weak permeability type.Through the experiments,the influencing factors of permeability coefficient of rammed earth are explored from the macroscopic,mesoscopic and microscopic scales.The main research results are as follows:(1)Based on the density test and specific gravity test,the macroscopic scale starts from the dry density of grain index and void ratio of pore index indicators,indicating finally that the undisturbed rammed earth is denser,the pore content is less.The correlation analysis between dry density,porosity and coordination number obtained from void ratio and permeability coefficient shows that they are well correlated with permeability coefficient.(2)From the perspective of fractal dimension,the granularity fractal dimension value and pore fractal dimension are obtained based on particle analysis test and matrix suction test.At the same time,it is also found that the undisturbed rammed earth belongs to the category of fine grained soil.The clay and colloid content of the rammed earth with weak permeability are higher than that of the rammed earth with strong permeability,and the VG model for soil-water characteristic curve of undisturbed rammed earth is more suitable.The correlation analysis and regression analysis of two kinds of fractal dimension values and permeability coefficient show that both of them have a great correlation with permeability coefficient,and there is a boundary point to distinguish the permeability of rammed earth.(3)From the micro-scale point of view,the micro-images obtained from scanning electron microscopy experiments are defined as granular-mosaic-contact and granularmosaic-cementation structures according to the morphology,arrangement and connection of particles,which is consistent with the division of permeability of rammed earth.The microstructural characteristic parameters which are related to the particle and pore such as equivalent particle diameter,equivalent pore diameter,oblate degree,filling ratio,area ratio,average shape factor and probability entropy are extracted by image processing software.Some parameters are closely related to permeability coefficient,and the permeability of rammed earth can be preliminarily determined from the values of parameters.(4)The influencing factors affecting the permeability coefficient of rammed earth at different scales have been determined,but there are single and one-sided problems.Therefore,the relationship between multi-factor and permeability coefficient at various scales is further considered.The prediction formulas of rammed earth permeability coefficients at macroscopic,mesoscopic and microscopic scales are obtained by various methods and compared with the experimental values.It is better to predict the permeability coefficient of the undisturbed rammed earth with the fractal dimension as the variable at the mesoscopic scales.