Study On Mechanism And Prevention Technology Of Water Seepage Of Typical Grottoes In Gansu Province

The Grottoes are a cultural and artistic treasure trove that integrates architecture,sculpture,murals,and Buddhist culture.As an important component of China’s cultural heritage,as an important part of Chinese cultural heritage,they bear witness to the continuous innovation,prosperity,and development of cultural exchange between China and foreign countries.They showcase the cultural spirit,cultural breadth,and cultural confidence of the Chinese nation,and possess extremely high historical,artistic,scientific,social,and cultural value.The safe preservation and long-term utilization of grottoes are always threatened by various damages and deteriorations which developed under the comprehensive influence of long-term natural factors and human activities.The prevention and control of water damage is the long-term problems and challenges in the field of grotto conservation due to diverse geological conditions,various hydrological environments,significant climatic differences,abundant disease types,and complex disease mechanisms.In this paper,based on a comprehensive review and understanding,the characteristics of the coexisting environment of grotto temples in China was analyzed,a classification system for water damage in grottoes was established,the main factor affecting the water damage was determined.Typical representatives,such as Yulin Grottoes,a gravel grotto in an arid environment in Gansu Province,Beishiku Temple,a sandstone grotto in a humid environment,and Tiantishan Grottoes,a gravel grotto in reservoir bank environment,are selected.Through on-site investigations,long-term monitoring,laboratory experiments,and other methods,the characteristics on type of water damage,distribution and change of surrounding rock moisture in 3 typical grottoes was analyzed,the mechanism of seepage in surrounding rock of the 3 grottoes was revealed,the prevention technology for water damage in different environments was proposed and applied,and the effect of measures was evaluated.The main research contents and results of this paper are as follows:（1）Based on the analysis of characteristics on climatic condition,geological condition,water environment of grottoes in the list of major historical and cultural protected at the national level in China,most of grottoes are distributed in the area with average annual precipitation greater than 600mm,average annual temperature between10-20℃,and average annual relative humidity above 60%.The excavated strata of the grottoes involve Paleozoic,Mesozoic and Cenozoic eras,with the Jurassic and Cretaceous periods accounting for nearly half.The lithology is mainly sandstone,conglomerate,glutenite and carbonate rock,as well as silt,mudstone,tuff,marble and metamorphic sandstone.The landforms of the grottoes mainly include river terraces,gully cliffs,low mountains and hills.The water damaged grottoes caused by the change of reservoir water table involve every protection level in China,and the environment characteristics and deterioration types are different from those of other grottoes.Therefore,a special type of grotto temple coexistence environment,the reservoir bank environment,was proposed based on the dry,humid,and wet environments.（2）Through the combinative approach of field investigation and literature survey,more than 50 representative grottoes in different existing condition were investigated,and the classification of water damage was divided into 2 categories and 11subcategories according to the mode of action method,manifestation and cause of those damages,including primary water erosion deterioration such as rain erosion,seepage erosion,condensation water erosion,water accumulation erosion,flood erosion,dissolution erosion,surface pollution,and secondary water erosion such as weathering,salting damage of wall paintings,shedding of ground support layer,biological deterioration.4 categories of 20 factors affecting water damage of grottoes were calculated by analytic hierarchy process,the main factors were determined as rainfall,permeability of surrounding rock,condensate water,groundwater,eaves,and salt content of surrounding rock.（3）Yulin grottoes in arid environment,Beishiku Temple in humid environment,and Tiantishan Grottoes in reservoir bank environment were selected as research subject of Gansu Province,the study for the characteristics of water damage and surrounding rock water content in those grottoes was implemented by the means of pit exploration,drilling,long-term monitoring,laboratory test.Yulin grottoes were excavated on the sandy conglomerate cliff of Yulin River on the Quaternary alluvial-proluvial fan in the fourth series,with an average annual precipitation of 72.2 mm.The lithology of the strata is poor and the structure is loose.The characteristics of water damage are as follows:the salting damage in the top of the second-floor caves and the root of the first-floor caves in the east cliff of Yulin grottoes are obviously developed,and the domes of the sixth grottoes are re-wetted during the rainfall process.The water content of the cliff is from 0.13%t0 3.69%,the water content increases first and then decreases with the boundary of 7m from the top of the cliff to the bottom,and the water content of the cave wall increases from the outside to the inside.The Beishiku Temple,which was excavated in the cliff of Cretaceous sandstone,has an average annual precipitation of563.4mm.Structural fissures,unloading fissures and monoclinic strata are developed.The water damage characteristics are obvious in the alternating development of water seepage-biological diseases-pan salt during every year.The average weathering rate of sandstone in typical parts of the cave is 1.36mm/a.The water content of the cliff is 0.17-2.49%,which decreases with the increase of the height of the cliff,and the water content increases from the surface to the inside.The water content surface layer of the cave wall is higher than that of the interior in the wall of caves,and the water content in the last6 months of each year is higher than that in the first 6 months.The Tiantishan Grottoes were excavated on the Paleogene sandy conglomerate cliff on the right bank of the Huangyanghe Reservoir.The highest water level of the grottoes was 12 m higher than that of the 13^th grottoes.The water damage characteristics of the grottoes were as follows:the water damage was obviously controlled by the reservoir water level,and the soluble salt in the grottoes was significantly enriched.The volumetric water content of the cliff is 1.8-18.6%,and decreases with the increase of the cliff height.The water content in the cave is higher than that outside the cave.（4）Based on a combination of long-term field rainfall monitoring and indoor simulated rainfall experiments,the source of moisture in the surrounding rock of the Yulin Caves was determined to be the water vapor present in the original rock mass and the water vapor supplied by the surrounding Gobi,through an elimination process.It was found that the water vapor migration of Yulin Grottoes is under the combined action of temperature,air pressure and rainfall,which causes the change of air pressure within the surrounding rock and drives the water vapor migration in the hole driven by the pressure gradient,temperature gradient and water content gradient.Through on-site investigations,moisture content monitoring,and permeability tests in different directions,it was discovered that the water infiltration in the 267^th Cave of the Beishiku Temple occurs in the rainy season,with water entering through structural fissures and sandstone pores,the precipitation infiltrate into the cave along the inclined and horizontal bedding planes.Based on geological lithology and structural features,combined with water infiltration patterns and moisture content monitoring on both sides of unloading fractures at different locations,it was determined that water infiltration into the 13^th Cave of the Tiantishan Grottoes occurred during fluctuations in the Huangyanghe Reservoir,with water entering through fissures in the water level fluctuation zone and infiltrating into the cave along dominant pathways,such as pores,under the combined effects of hydrostatic and pore water pressure.（5）Combined with the water damage characteristics and deterioration mechanism of 3 typical grottoes,the overhead cover-TPO waterproofing technology on the cliff top of Yulin Grottoes in arid environment,the temporary rain shelter technology for cliff of Beishiku Temple in humid environment,and the ground replacement and drainage technology of Tiantishan Grottoes in reservoir bank environment were developed.These protection technologies have been applied through relevant protective projects and have demonstrated good water damage prevention effects based on long-term observation and annual monitoring data comparisons.The results in this paper can provide a systematic solution for the investigation and monitoring of water damage in grottoes under different environments.The air pressure of surrounding rock is introduced firstly time as an influencing factor into the analysis of water vapor migration mechanism in grottoes.The water vapor migration effect of surrounding rock of low water-bearing gravel rock grottoes under the action of environmental variables in arid environment is scientifically expounded,and the water damage prevention and control technology for cliff top-cliff surface-ground is proposed,which can provide theoretical basis and technical support for water damage control of similar grottoes.