Application Of DC Electric Field To Restrain Capillary Water Rise In Moist Earthen Monuments

As a very important cultural sites, earthen sites are widely distributed in China, and its natural environments are also various. The extant earthen sites often have very significant historical and archeological values. And the Liangzhu Ancient City Site in Hangzhou is a typical representative of them, which is useful in the research of the origin of Chinese civilization. The climate of Liangzhu Ancient City Site is warm and wet, thus^ the site is a typical earthen site in humid environment. The site body chronically suffers from the erosion of groundwater capillary rising, consequently, it has a worse stability. Hence, how to effectively preserve it for a long time has become a very important research subject. Since the beginning of 21st century, with the support and emphasis on the cultural relic protection, there are a host of excellent studies in protection of outdoor sites. And the researchers continue to explore how to protect the damp earthen sites, like reinforcement using the modified material, laying bricks or stones to fill their surface, backfilling and displaying in situ, etc. However, these methods are just suitable for the specific types of diseases, and are not helpful in effective control of internal microscopic water environment, which is the key research topic for damp earth sites. The main causes of damp earthen sites to destroy are the rise of capillary water and transport of salts in soil, which destruct the original equilibrium structure of water-salt in soil. Therefore, a study to control the internal soil environment using electrochemical method was performed in this paper. That’s, a direct-current electric field was added into the earthen sites body, and the water was driven to negative pole from positive pole under the electric field force, hence, the rise of capillary water and transport of salts in soil were restrained, and the equilibrium structure of water-salt was maintained in the optimal state, thus, the internal soil environment was stable, which can resolve the protective problem of damp earthen sites radically.Firstly, the 2D plane simulation was used in this study, specifically, taking the filter paper with good hydroscopicity as research object, the study of the horizontal transport of water was conducted under the condition of direct-current electric field and the mechanism of direct-current electric field to drive water was analyzed. Secondly, the experiments of remoulded soil from Liangzhu Ancient City Site were conducted and the inhibiting effect of direct-current electric field on capillary water in soil was studied. Then, the capillary water rise tests of groundwater carrying salt in earthen monument were carried out, and the distribution and transport of water-salt was studied under the direct-current electric field. Lastly, regarding the underground palace of Porcelain Tower of Nanjing in the humid environment as object, the destroying mechanism of the capillary water rise and the mechanism of direct-current electric field to reinforce the earthen site were analyzed, and the corresponding technical ideas for reinforcement were put forward.In this paper, the main research contents and research results are as follows:1. The study of 2D simulation of direct-current electric field to inhibit the transport of capillary waterUsing the wetting mechanism of fiber structure to describe the water distribution, the water distribution of the infiltrated filter paper and the filter paper with capillary water supply are as follows:the water distribution of the infiltrated filter paper is even; Influenced by the adhesive attraction and capillary adsorption, the water distribution, in the filter paper with capillary water supply is not even and the water content decreases with the increase of distance from the porous stone. By using the Helmholtz-Smoluchowski model to analyze the transport mechanism of water in the filter paper, it concludes that after adding the direct-current electric field into the both ends of the filter paper, there is an extra osmotic pressure in the filter paper, which drives water to the end of filter paper, and the results indicate that the inhibition effect of direct-current electric field on capillary water is better when the water content of filter paper is more than 200%.2. The study of 3D simulation of direct-current electric field to inhibit the rise of capillary waterIn the case of no capillary water supply, the water in the control soil sample was only influenced by evaporation, and the whole water content decreased with time elapsing. The water content also decreased with the height.increasing at a certain time. With electroosmosis time elapsing, the change trend of water content in the soil samples in the ascending field group varied, specifically, the water content transformed from decreasing with the height increasing to increasing with the height increasing. And the effect of electroosmosis on promoting water upward is obvious. The decreasing rate of water content in the top of soil samples is greater than that in the base of samples in the descending field group. And the slope of curve of water content reduced rapidly with time elapsing. The inhibition effect of the electroosmosis on the transport of the water gradually enhanced from the bottom to up.In the case of capillary water supply, the water content in soil samples with dry density of 1.50g/cm3 in the ascending field group was about 5% more than that in no electric field 144h later, and was even 7.4% in the samples with dry density of 1.60g/cm3, which indicates that the effect of the electroosmosis on promoting the capillary water rise is obvious. In the descending field group, comparing the samples with density of 1.50g/cm3 with the control samples, the water content decreased with electroosmosis time elapsing at different distances, which illustrates that the electric field inhibits capillary water to transport upward.3. The coupling experimental research of direct-current electric field controlling the capillary migration of salt-waterIn the descending field group, the amount of capillary water inhibition in natural soil samples and saline soils samples decreased linearly with the height increasing, while in the ascending field group, the amount of capillary water promotion increased linearly with the height increasing. In the case of no capillary water supply, the amount of capillary water inhibition in natural soil samples and saline soils samples amounted to 1.56% and 4.68%, respectively, under the descending field, whilst the amount of capillary water promotion in natural soil samples and saline soils samples arrived to 2.09% and 1.93%, respectively, under the ascending field. In the case of distilled water supply, the amount of capillary water inhibition in natural soil samples and saline soils samples amounted to 5.38% and 6.30%, respectively, under the descending field, while the amount of capillary water promotion in natural soil samples and saline soils samples arrived to 3.39% and 8.81%, respectively, under the ascending field. In the case of saline solution supply, the slope of the curve of water content decreased, and the water content in the top of samples reduced while the water content in the bottom of samples increased under the descending field. Under the ascending field, the whole water content in natural soil samples increased by about 2%, while the whole water content in saline soil samples increased by approximate 4%. It was the first time to use the direct-current electric field to control the transport direction of Na+and SO42-, and then, combining with physical desalination method, removed the salts collected by electroosmosis, which can resolve the problem of salt weathering on the destruction of ancient earthen sites effectively.4. The mechanism research of direct-current electric field inhibiting the rise of capillary waterRegarding the underground palace of Porcelain Tower of Nanjing as object, and based on the capillary action physical model in the capillary mechanics and the shrinkable film’s (water-air interface) surface tension model in the unsaturated soil mechanics, the destroyed mechanism of the damp earthen site wall was analyzed. Based on the mechanism of electroosmosis, and combining with the experimental data, the effect of direct-current electric field inhibiting the rise of capillary water in damp earthen sites was analyzed theoretically. It concluded that under the direct-current electric field, the water in damp earthen sites redistributes, and the whole water content decreased, and the change of water content in unit height increased. The evaporation, capillarity, and electric field action achieved dynamic balance ultimately, thus, there will always be great matric suction in the interior of the earthen sites. The problem of wall collapse could be solved by applying the direct-current electric field to the earthen sites essentially. And the corresponding feasible plans to apply the direct-current electric field to the damp-proof of the damp earthen sites were put forward.