Study On Mechanism Of Seepage Diseases In Maijishan Grottoes

Maijishan Grottoes,located in the southeast of Tianshui City,Gansu Province,China,is about 45km away from the urban area,geographically on the north side of the western extension of the Qinling Mountains.Been directly built on the vertical cliff in the south side of Maijishan mountain during later Qin Dynasty,Maijishan Grottoes now has a history of more than 1500 years.Maijishan Grottoes is famous for its rich and exquisite Buddhist statues,known as the"Oriental Sculpture Exhibition Hall",and is the first batch of national key cultural relics protection units in China.On June 22,2014,Maijishan Grottoes was successfully listed as a site in the“Silk Roads:the Routes Network of Chang'an-Tianshan Corridor of World Heritage List”.Due to its special topography and climate,Maijishan Grottoes has suffered seepage disease for a long period.In 1970s,China government conducted a called“Spray and Anchor”reinforcement project.The project built a concrete cover on the cliff of Maijishan Grottoes.Nowadays,the reinforcement project has been completed for nearly 40 years,the water diseases of the grottoes have always threatened the preservation of the grottoes.Water disease management needs to be carried out to meet the objective needs of the protection of cultural relics.This research aims to find the mechanism of water seepage in Maijishan Mountain and provide a basis for future conservation works.The main works of this research are as follows:?1?the flow rate of seepage water at each seepage outlet in Maijishan Grottoes were monitored in Jul.and Aug.,2019.The response characteristics between the flow rate at seepage outlets and rainfall were analyzed,and the seepage patterns of water in the mountain were inferred;?2?The chemical characteristics of the water seepage diseases were determined by sampling and analyzing the water,sampling in the same period with monitoring works,and crystalline salt,sampling in dry season,at each outlet;?3?Simulated immersion tests were carried out to measure the pH,electrical conductivity?EC?,and soluble salt content of the rock and concrete immersion solution,the chemical processes of water from entering the mountain to seepage outlets were comprehensively analyzed.The response characteristics between the outlets'flow rate and rainfall show that the rainfall has a significant effect on the water seepage in grottoes.After a rainfall,there will be two peaks of flow rate in seepage outlets,causing by two independent seepage paths.The response characteristics between the flow rate and rainfall of all outlets can be divided into three basic types,which are:fast lag response,delayed lag response,and continuous zero response.Multiple response characteristics could superimpose on each other to form a more complex response system.The first two types are consistent with the characteristics of fissure drainage,and the third response type is consistent with the characteristics of pore drainage.The dominant cation in the seepage water of all outlets is Na+,and the dominant anions are HCO₃^-and CO₃^2-.The pH of seepage water ranges from 7.80 to 11.69.The EC of the seepage water ranges from 513 to 3540?S/cm.As the flow rate increases,there are downward trends for both pH and the EC of the seepage water.That is,in the relationship between flow rate and pH or EC,the lower limits of pH and EC keeping the same?7-8 of pH and 500?S/cm of EC?,the upper limit of which decrease with the increase of flow rate.It can be inferred that the seepage water has characteristic convergence values of pH between 7 and 8 and EC around 500?S/cm,which totally different with which in rain water.Therefore,the chemical processes of rain water from entering the mountain to discharging can be divided into two stages.The first stage of chemical processes is where the pH and EC of the water reach the characteristic convergence value,and the second stage of chemical processes is where the pH and the EC of the water further increase.The results of the simulated immersion test of rock and concrete show that:the first stage of chemical processes is mainly water-rock interaction,occurs in the aquifer far away from the surface;the second stage of chemical processes is mainly water-concrete interaction,occurs when the water close to the surface.The processes of water from the aquifer to the seepage outlets meet the description of three modes:low fracture seepage mode,high fracture seepage mode,and closed seepage mode.The three modes orderly correspond to three types of response characteristics between flow rate and rainfall,which are fast lag response,delayed lag response,and continuous zero response.