Laboratory Tests Of Salt Damage To Ancient Earthen Sites In Northwest Arid Area Of China

Earthen sites are ancient architecture with historical, cultural, and scientific value, which are based on soil as the main building materials, the place where ancient people are engaged in production, cultural exchange, religion and other activities.The number of national key cultural relics protection units has amounted to378in the sixth published national key cultural relics protection units,and the proportions of which accounted for22%in three places of Xinjiang Uygur Autonomous Region, Gansu province, Shanxi province in northwest area of China. A large number of earthen sites in northwest of China are important part of our excellent precious cultural heritage with high academic value, with the improvement of living standards of Chinese people, the rich tourism resources of the region will become an important symbol to enhance China’s cultural soft power.Large diurnal temperature, concentrated rainfall and intense evaporation are one of the environmental characteristics in the arid area, it is the extreme weather conditions that make ancient earthen sites suffer from damages of wind erosion, rain erosion, repeated wet and dry cycles, freeze-thaw cycles. According to on site survey and related references, we found that earthen sites suffered from damages of crisp base, wind erosion, weathering, rain erosion, cracking and collapse, and salt damage plays an important role in these damages.Therefore,exploring salt damage is of great significance for the interpretation of causes of earthen sites.then appropriate protection measures of earthen sites could be made.This thesis are based on references and field investigation of earthen sites. Firstly, the soil taken from Dunhuang City, Gansu Province, are mixed with different percentage (1%,3%,5%) of NaCl、Na2SO4and the combination of both to form5×5×5cm test block, and laboratory weathering tests are carried out. Then, on the basis of the laboratory weathering test results and related references, we improve experiment design, the percentage of salts added are controlled below3%, and wet and dry cycle test, freeze-thaw cycle test are carried out, mainly judged through macro and micro photos and mechanics changes in performance. Finally we explore the mechanism of salt damage to earthen sites by the test of water and salt transport in test block and the test of the effects of temperature on saline soil.The results showed that the occurrence of salt damage to earthen sites is related to many factors, such as salt content, salt types, moisture content and the environment. We propose threshold of salt damage to earthen sites:under certain conditions, when salt content of soil exceeds a certain value, the surface and the structure of the soil change obviously, the mechanical properties declined greatly, the salt content in this condition is called salt damage threshold. Based on this, the paper argues that in this study, under indoor weathering conditions, the thresholds of salt damage to soil are:1%<NaCl<3%, Na2S04,0.1%, complex salt FA (C:S=1:1)1%,0.2%<complex salt FB (C:S=1:2)<0.4%,1%<complex salt FC (C:S=2:1)<3%,(where C represents NaCl, S represents Na2SO4); in this study, after wet and dry cycle, freeze-thaw cycle tests, the appearance of test block change greatly, the physical properties declined greatly compared with controlled subjects. It is the repeated wet and dry cycles, freeze-thaw cycles that make the soil damaged, the salts in the cycles play a catalytic role. Water and salt transport experiments show that:the salts moved from inside of the soil to outside of it, water play an important role in the salt transport process. Infrared thermal imaging tests showed that the greater the salt content, the higher the temperature of the soil, the stronger the ability of the test block to absorb heat. This temperature difference of soil containing different salts play a important role in water-salt transport. The change of saline soil has connection with the types of salts, salt content, water and other factors.This study investigated the extent of salt damage to soil under different conditions, provided a method salt damage research, propose the threshold of salt damage to soil and the corresponding values and ranges. The results can provide scientific theoretical basis for earthen sites preservation, and guide the corresponding measures to be made to preserve earthen sites. At the same time, it can provide references to the Cave Temple murals mechanism of salt damage in northwest of China and related research on salts.