Surface Weathering Mechanism Of Sandstone Of Grottoes In Longdong Area,Gansu Province

As an important role in our cultural heritage,grotto temples are also one of the most distinctive types of Chinese cultural heritage in World Cultural Heritage.Featuring a distinct context of inheritance,close correlation and an integrated system,they have witnessed the whole propagation process of Buddhist culture,from its introduction into China to sinicization.Besides,they spread along the Silk Road from west to east and south region of China,representing the exchanges between Chinese and Western civilizations.These valuable ancient sites,are treasures produced by our ancestors,the crystallization of wisdom,which also show that our Chinese nation has enduring vitality and creativity.Thus,the protection,inheritance and utilization of grotto temples play an important role in the construction of social material and spiritual culture.They are of great significance in inheriting and developing excellent traditional Chinese culture,building a strong socialist culture,developing“The Belt and Road”with high quality and promoting exchanges and mutual learning among civilizations.Therefore,their protection research has attracted extensive attention.Affected by natural forces and human activities for thousands of years,grotto temples in China are generally threatened by the instability of rock mass and cave structure,water seepage and weathering problem.Notwithstanding,current status of grotto temples in China has been improved effectively after more than 70 years of protection and research,conservation issue for small and medium-sized sandstone grotto temples are still prominent.In particular,weathering is the most common,direct,harmful and urgent problem for conservation practice.There are 5986 grotto temples and rock carvings in China,among which there are 2155 grotto temples,and about 80%of them,showing various weathering degrees,were carved in sandstone cliff.Based on the weathering characteristics of cretaceous sandstone grotto temples in Longdong Area,a detailed regional field investigation combing with environment and weathering forms were conducted in this present study.Through field sampling and analysis,this paper also quantitatively evaluated the weathering degree of sandstone in each region of Longdong Area.Relationship between environment and weathering forms was further explored in detail,taking North Grotto Temple（NGT）as an example.The weathering mechanism of sandstone in NGT from the surface to the interior was researched by quantitative assessment.The results showed that the micro-environment of caves would lead to different weathering forms.Temperature variation gradient of environment outside and inside the cave was selected as the variable,freeze-thaw deterioration model of cretaceous sandstone in NGT was discussed.Furthermore,according to soluble salt content of water samples and micro-environment of the caves in NGT,deterioration model of sandstone affected by the combined action of water-heat-salt was studied.The following main findings and conclusions are summarized:（1）For the first time,the regional weathering forms of sandstone grotto temples in Longdong Area were analyzed and studied by regional research method.Four areas could be further divided based on the distribution of sandstone grotto temples in Longdong Area:Heshui area（A1）,Xifeng and Zhenyuan area（A2）,Jingchuan area（A3）and Huating area（A4）.The main weathering forms for sandstone grotto temples in A2 and A3 areas are salt efflorescence and granular disintegration,while the main weathering forms in A1 and A4 areas are peeling and flaking.（2）By field investigation,sampling and analysis,weathering degrees of sandstone grotto temples in Longdong area was quantitatively assessed.The result shows that the weathering resistance of sandstone of grotto temples in four areas is A1>A4>A2=A3.During the transition from A1 and A4 to A2 and A3,the weathering resistance of the sandstone in the Grottoes Temple decreased significantly.NGT in A2 area is one of the most typical case considering surface weathering of sandstone,thus study of the surface weathering of the sandstone was carried out in NGT.（3）The surface weathering of sandstone in NGT is typical in Longdong Area,thus a detailed investigation was conducted as a case study.Surface weathering forms of sandstone in NGT were divided into five categories:crack,detachment,features induced by material loss,discoloration and deposit,and bioerosion.Furthermore,12 sub-categories could also be detected:superficial fissure（weathering fissure）,blistering,delamination,fragmentation,granular disintegration,alveolization,differential erosion,salt efflorescence and bioerosion induced by plant,animal and microorganism.According to the monitoring data of temperature and humidity in NGT,it is found that the biggest difference between the internal and external environments of caves is variation gradient.Maximum variation gradients of temperature outside and inside caves are±8℃/h and±4℃/h.（4）The weathering characteristics and mechanism of sandstone from the surface to the interior in the horizontal direction of NGT were investigated by in-situ sampling and laboratory test.The salts migration kinetics inside the cliff were concluded based on the distribution of salts.The result show that efflorescence and subflorescence were confirmed as the main weathering forms for litharenite and feldspathic litharenite strata,respectively.There is a“vaporization plane”,which is the primary area where soluble salts tend to accumulate in.The position of the evaporation plane is located at a depth of 1 cm and 3 cm below the surface of litharenite and feldspathic litharenite strata.Feldspar alteration also plays an important role in the weathering process.Unlike the salt accumulation,feldspar tends to alter in the surface layer and result in the increase of kaolinite and illite mineral.（5）The freeze-thaw deterioration mechanism of saturated cretaceous sandstone of NGT was studied by taking temperature variation gradient as variable,real weathering forms of sandstone surface were reproduced by laboratory test.The result shows that main weathering forms of sandstone samples under the outside environment conditions were more serious than inside conditions.The bedding plane of sandstone samples was the dominant surface for the development of superficial fissure,on account of larger pore size and earlier frozen of liquid water,which promoted greater freezing pressure.With the freeze-thaw cycle,the compressive strength,tensile strength and P-wave velocity decline exponentially.In addition,the initial integrity parameter is added to make the fitting equation more consistent with the actual situation.The compressive strength,tensile strength and P-wave velocity all decreased exponentially,and the degree of chemical weathering gradually increased.The pore inside the sandstone was enlarged after freeze-thaw cycles and the structure became loose.（6）The cretaceous sandstone weathering mechanism of NGT under combined action of water-heat-salt was explored based on soluble salt content of water samples and micro-environments of caves.The result shows that:（1）The dominated surface weathering forms of sandstone samples were superficial fissure,granular disintegration and differential erosion.（2）Under the inside environment conditions,Na₂SO₄ solution leaded to differential erosion of sandstone samples,which mainly developed in the area between adjacent bedding planes.However,under the inside environment conditions,the superficial fissure of sandstone samples was relatively developed.（3）In consideration of salt solution types,Na₂SO₄ solution and mixed Na₂SO₄-NaCl solution could promote the freeze-thaw deterioration of sandstone,while NaCl solution restrained the freeze-thaw deterioration.Due to that the lowest temperature accepted in this study was-18.5℃which was significantly higher than the eutectic point temperature（-21.1℃）of the NaCl-H₂O binary system.With the freezing of water,the concentration of salt solution in the pore space increased,and this process would reduce the freezing point temperature of the solution,so as to reduce the freezing-thawing deterioration.However,when the freezing/thawing rate is too fast,equilibrium state of the eutectic lines of NaCl-H₂O binary system,Na₂SO₄-H₂O binary system and Na₂SO₄-NaCl-H₂O ternary system will be broken,which will cause more serious weathering.