Study On Weathering Mechanism Of Sandstone On The Cliff Of North Grottoes Temple

As the largest grotto group in Longdong,Gansu,China,the North Grotto Temple is an indispensable and precious heritage in terms of Buddhist art and cultural exchange and dissemination along the Silk Road.More importantly,it carries the historical and cultural information of the Longdong area and has extremely high Historical,artistic,scientific and social values.However,the North Grottoes Temple,which has been in existence for thousands of years,has been severely weathered under the influence of changes in the natural environment and human activities,and urgently needs to be studied and protected.Based on the literature review of domestic and foreign sandstone weathering research,this article investigates and analyzes the characteristics of environmental and disease of the North Grottoes Temple.Based on the classification of the sandstone,the similarities and differences of the basic properties of the two kinds of sandstones were studied and compared.Focusing on simulating the climate characteristics of the North Grottoes Temple area,setting boundary conditions,carrying out seven sets of environmental coupling simulation indoor weathering simulation experiments,using various means to monitor the change law of sandstone samples during the experiment,and qualitatively and quantitatively analyze the change characteristics.Using AHP to calculate the weights of various degradation effects to explore the weathering mechanism of the sandstone on the cliff of the North Grottoes.The main conclusions are as follows.(1)The North Grottoes Temple is based on Cretaceous sandstone and overlaid with loess.There are many and abundant groundwater sources in the area.The monsoon features are obvious,the temperature and humidity environment changes drastically,and the precipitation is greater than 400mm.The complex and changeable climatic conditions provide the conditions for the weathering of the sandstone on the cliffs of the North Grottoes Temple Sandstone weathering provides conditions.The weathering rate of sandstone on the cliff is relatively fast,and the surface weathering diseases are widely distributed and various,such as hole-like weathering,surface salt flooding,surface powdering and spalling,scaly warping and spalling,etc,which seriously affects the safety and viewing of the sandstone statues on the cliff of the North Grottoes Temple.(2)The sandstone of the North Grottoes Temple can be divided into medium-grained and fine-grained sandstone according to the gradation of sand grains,and there are differences in the microstructure and macroscopic properties of the two.The fine-grained sandstone particles are small,the cement is mainly calcareous,supplemented by mud,and the cementation mode is mainly basement type.The cementation degree is good,the number of pores is small,the pore diameter is small and the degree of connectivity is poor;The medium-grained sandstone has larger particles and only a small amount of argillaceous cement.The cementation method is mainly contact type,the degree of cementation is extremely poor,the porosity is high and the diameter is large.Compared with medium-grained sandstone,fine-grained sandstone has higher density and specific gravity,weaker water absorption,lower acoustic anisotropy.The uniaxial compressive strength and elastic modulus of the former are only 10%to 15%of the latter.The longitudinal wave velocity of the latter is about 1.5 times that of the former.Generally speaking,medium-grained sandstone has strong softness,weak resistance to weathering,and is more susceptible to weathering in the natural environment.(3)The degradation performance of sandstone samples in seven groups of indoor weathering simulation tests is different,the common point is that the coarser particles on the surface fall off and show differential weathering,the surface hardness decreases,the surface porosity and the volume of connected voids increase,the wave speed decreases and the color difference increases.The difference is that each group of weathered sandstone samples has different attenuation degrees and rates.Except for the water-salt and hydrothermal-salt cycles,other weathered sandstone show an increasing trend in volume and differential expansion on the surface.(4)With the help of Analytic Hierarchy Process,the influence size of seven kinds of weathering is analyzed and obtained.The order is hydrothermal salt cycle,salt cycle,saturated freeze-thaw cycle,dry-wet cycle,unsaturated freeze-thaw cycle,high-low humidity cycle,temperature and humidity cycle.The total weights of the seven weathering effects are compared pairwise,and the analysis shows that among the three weathering influencing factors(salt,water,and temperature),salt has the greatest impact on sandstone weathering,followed by water and temperature.In addition,the existence of water is related to the weathering resistance of sandstone,and the effect of liquid water on the weathering of sandstone is greater than that of gaseous water;the water content of sandstone is proportional to the intensity of weathering;the addition of temperature factors when the sandstone is saturated can significantly weaken the weathering resistance.This conclusion effectively verifies the comparison results of indoor weathering simulation test.(5)Under the high and low humidity cycle,the relative humidity environment of the sandstone changes repeatedly and violently,which destroys the equilibrium state.Under the action of cyclic stress,the sandstone has a local permanent cumulative loss.Especially when the relative humidity of the environment is high for a long time,the weathering resistance of sandstone will be reduced.Under the temperature and humidity cycle,the internal minerals of the sandstone exhibit thermal expansion and contraction effects under the action of changing temperature.The tension and pressure between the particles are continuously converted,resulting in the weakening of the connection between the particles and the continuous expansion of internal cracks.The sandstone under the dry-wet cycle is mainly affected by water-rock interaction,such as hydrolysis,oxidation and hydration.Under the unsaturated freeze-thaw cycle and the saturated freeze-thaw cycle,the interstitial water in the surface of the sandstone freezes and phase changes,and the phenomenon of freeze-up causes a large tensile stress inside the sandstone,causing squeezing and relative displacement between sandstone particles.At the same time,the original micro-cracks of the sandstone expanded under the effect of frost heave.The unsaturated freeze-thaw cycle mainly acts on the surface and the shallow surface,while the saturated freeze-thaw cycle has a stronger force and a greater range of depth.Under the water-salt cycle,some sand particles on the surface of the sandstone fall off due to the reciprocal effects of sulfate dissolution and crystal expansion cycles.The acid solution generated by the oxidation of SO₄^2-generated by sulfate dissolution and ion exchange dissolves a large amount of calcareous cements,and the pressure generated when the Ca SO₄ solution is supersaturated is greater than the tensile strength of the sandstone body.Under the hydrothermal-salt cycle,the sandstone suffers from the combined effects of freezing and thawing and the water-salt cycle.The former creates a larger space for the latter,while the latter expands the effect of the former.