Study On The Characteristics And Development Mechanism Of Erosion On The Upper Part Of The Ming Great Wall In Gansu Province

The Great Wall is one of the key representatives of immovable cultural relics in China,among which the Ming Great Wall is the most time-consuming,the most rigorous military defense system level,the most perfect overall facility structure of the Great Wall project in Chinese history,and has important status and effect.The Ming Great Wall in Gansu Province starts from Jiayuguan City pass and passes through Jiuquan City,Zhangye City,Jinchang City,Wuwei City,Lanzhou city and other cities,which is an important part of the Ming Great Wall.Under the destruction of natural forces for hundreds of years,the Ming Great Wall appeared many deterioration,among which the erosion is one of the most common deterioration.In the past studies,scholars mainly focused on the base erosion of earthen sites,but did not study the upper wall erosion in depth.In this study,typical sections of the Ming Great Wall in 8 regions of Gansu Province were taken as representatives to carry out field investigations to analyze the characteristics of erosion in the upper part of the wall,and the effects of climate and soil properties of Ming Great Wall sites.For the key factors of rainfall and sunshine,an indoor rainfall-sunshine coupling experiment was carried out to record the changes of physical and mechanical properties,salt migration and microstructure of the remolded soil during the process.The development process and internal mechanism of upper erosion were investigated by combining the macroscopic and microscopic experimental phenomena.The conclusions of the study are as following.（1）In general,the climate along Ming Great Wall of The Dynasty in Gansu Province changed from northwest to southeast.Annual rainfall and days of rainfall gradually increased,the average annual temperature gradually increased,and the number of days of sandstorm and the average wind speed gradually decreased.For the erosion of the upper wall,from northwest to southeast,the linear erosion rate,the average depth and the average height from the top gradually increased,while the average height gradually decreased.（2）Climate factors and soil properties of the Ming Great Wall site have obvious effects on the development degree of upper erosion.The linear erosion rate has a positive linear correlation with rainfall and days of rainfall,and a negative linear correlation with sunshine intensity,daily temperature range and dry density of site soil.There was a positive linear correlation between the upper erosion depth and rainfall,days of rainfall,sunshine intensity,dry density,plasticity index and Na₂SO₄content of the site soils.The mean height of upper erosion was positively correlated with the number of strong wind days in the region.The most important factor is rainfall followed by diurnal temperature range and sunshine.（3）Under the rain-sunshine cycle,soluble salts were redistributed in the reconstructed soil samples of site.Due to the infiltration of rain and evaporation of radiation,most soluble salts,including SO₄^2-,were enriched in the area 2-3 cm away from the top of the sample.The increase in radiation intensity would accelerate the phenomenon of soluble salt enrichment.（4）Under the rain-sunshine cycle,cracks and epidermal exfoliation occurred in the upper part of the sample,and the change of surface color,surface hardness and unconfined compressive strength weakened.The main reasons for the destruction of the samples were the deterioration of the dry-wet cycle caused by the rain-sunshine cycle and the shortening and alkalization of the soil caused by the enrichment of soluble salt.The improvement of sunshine radiation intensity would accelerate the destruction process of the samples and the deterioration of the properties of the site soil.（5）Combined with the experimental results,it could be preliminarily inferred that the development of the upper erosion of the Great Wall had gone through five steps:rain and sunshine cycle,dry-wet deterioration effect,salt migration,fissure generation,epidermal detachment in the saline area,and epidermal exfoliation.The five steps were carried out in a continuous cycle.After years of damage,the erosion depth in the saline area kept deepening,and the upper erosion phenomenon finally occurs in the upper part of the Great Wall.