Surface Weathering Characteristics And Mechanism Study On Dazu Rock Carving Niche Of Sakyamuni Entering Nirvana

UNESCO's world cultural heritage site Dazu Rock Carving Niche of Sakyamuni Entering Nirvana conservation and restoration project was listed in the Sichuan-Chongqing Grottoes conservation and demonstration project by the State Administration of Cultural Heritage of China in April 2016.It can provide reference and demonstration effect on conservation and repair of other similar types stone cultural relics.The present paper is based on field investigations carried out weathering diseases characteristics of sandstone surface at Niche of Sakyamuni Entering Nirvana.Characteristics of types and spatial distribution of sandstone weathering disease are compared and analyzed.Weathering degree is quantitatively assessed by combining the use of in situ non-destructive surface hardness measurement and ultrasonic measurement.On these basises,the weathering mechanism of sandstone is integratedly analyzed by environment,sedimentary and structural features of rock mass,indoor accelerated deterioration test,and composition testing of surface weathering sandstone samples.The primary conclusions of this paper are as follows:1)Eight kinds of diseases are mainly developed on sandstone surface of the statue.And among them,there are 62 cracks in total.Area of salt crystal,granular disintegration,biological colonization,seepage,scaling,blistering and missing part accounted for 40.55%,38.55%,26.21%,7.63%,1.81%,1.37%,and 1.0%of the developed area of the statue,respectively.Through the comprehensive analysis of surface hardness measurement and ultrasonic measurement shows that in the horizontal direction,surface weathering degree of the head and foot is more serious than that of the chest and abdomen;in the vertical direction,the surface weathering degree of the upper sandstone and the ground mudstone are severe than that of between them.2)Through the three single factors indoor accelerated deterioration test of wetting�drying cycles,freeze-thaw cycles and acid rain corrosion test,it can be found that the stratification dense rock samples are easier to weathering than bedding undeveloped rock samples.The main and secondary relationships of sandstone weathering are acid rain,freeze-thaw and wetting�drying.3)Micro-environment and salt crystallization are the main external factors that affect the weathering and destruction of sandstone surface.Changes in the micro-environment can promote frequent wet and dry alternation.Weathering forms such as granular disintegration,scaling and layer cracks may be significantly influenced by wetting�drying cycles.SO₂,nitrogen oxides and PM₁₀ landed on the surface of the statue by dry wet deposition,which corrode the statue and cause the accumulation of salts.Soluble salts such as gypsum on the surface of the statue may be frequently found wetting and drying conducive to repeated crystallization and expansion/contraction,which can change the crystal pressure in sandstone,resulting in granular disintegration and salt efflorescence.4)Sandstone properties lay a material foundation for different weathering,the sandstone mineral compositions of statue are mainly quartz,feldspar,calcite and clay minerals.Compared with fresh sandstone samples,the porosity of weathered sandstone is increased,the shale and calcareous cementation is lost,and the particles of quartz feldspar are seriously spalling.The weathered sandstone samples contained newly generated mineral such as kaolinite and gypsum,and the content of quartz is increased,the contents of feldspar,calcite,illite and chlorite are relatively reduced.Clay mineral crystallization swelling,osmotic swelling and separation pressure are the main mechanism of sandstone expansion and contraction.Feldspar can be dissolves,alters into kaolinite in acidic environment.Granular disintegration and scaling are induced by seepage wetting softening effect,mechanical erosion,chemical etching and soluble salt accumulation.