Granular Disintegration Characteristics And Mechanism Study Of Niche Of Sakyamuni Entering Nirvana At Dazu Rock Carvings

Niche of Sakyamuni Entering Nirvana has exposed to the natural environment for a long time.Under the influence of environmental factors,e.g.,rain,environmental temperature,humidity,biological factor and so on,efflorescence,granular disintegration,biological colonization,cracks,seepage,scaling,blistering and missing parts were produced.We found that the most serious disease was granular disintegration.Therefore,the sandstone of Niche of Sakyamuni Entering Nirvana was selected as the research object.The disease characteristics and formation mechanism of sandstone on granular disintegration was studied.The findings could provide useful insights for the protection of the studied statues.The corresponding methods and theories could also provide reference and demonstration for the conservation and restoration of other similar types of sandstone cultural relics in Sichuan and Chongqing.In this paper,through the investigation of granular disintegration of Niche of Sakyamuni Entering Nirvana sandstone,the range distribution and disease characteristics of granular disintegration were defined.The formation mechanism of sandstone was studied by means of fine investigation of granular disintegration,long-term monitoring of in-situ sample weathering and indoor simulated degradation experiment.The main results of the paper were as follows:(1)According to the disease characteristics of granular disintegration,the total area of the granular disintegration area before water control was 211.36 m2,and the total area of the granular disintegration area after water control was 130 m2,which was 38% less than that before water control.The disease characteristics of granular disintegration were manifested in the phenomenon that the sandstone surface became �crisp powder� due to a large number of spalling particles.Through comparative analysis of the disease characteristics before and after water treatment in typical areas of granular disintegration,we found that water stains,salt crystals and pollutants were greatly reduced after water treatment,and the degree of granular disintegration was reduced.(2)The fine investigation of granular disintegration showed that when the groundwater moved upward along the sandstone with the action of capillary water,the surface hardness decreased gradually from top to bottom along the longitudinal section.XRD showed that the content of calcite and feldspar decreased from top to bottom along the longitudinal section,and the content of quartz increased from top to bottom along the longitudinal section,and the weathering degree increased from top to bottom along the longitudinal section.SEM showed that the structure was looser from top to bottom along the longitudinal section,and the porosity was the greater,and the particle damage and fracture expansion were more serious.(3)The long-term monitoring of in-situ sample weathering showed that the main failure mode of sandstone was granular disintegration,and the deterioration degree of the sample at the seepage position was greater than that at the relatively dry position.XRD showed that potassium feldspar,plagioclase,calcite,smectite,illite and chlorite in the samples showed a small loss with the passage of time,especially at the site of water seepage,where the mineral content decreased significantly.The quartz increased as a whole,and the secondary minerals(i.e.,kaolinite and gypsum)also appeared.(4)The indoor simulated degradation experiment showed that the apparent characteristics of the samples were mainly granular disintegration,and the damage to the stratified samples was obvious,and it was easy to form weathering cracks.The mass,surface hardness and P-wave velocity of the samples gradually decreased with the increasing periods.The decline trend was different in different periods,but they all tended to be stable at the later stage.Under the influence of water-rock interaction,the surface layer of sandstone began to degrade first,then passed to the middle layer,and finally to the inner layer.The deterioration was intensified with the increase of periods.Compared with the non-stratified sandstone,the stratified sandstone was more likely to form granular disintegration.Acid rain cycle was the most important factor influencing the formation of granular disintegration,followed by freezing-thawing cycle,with the least influence of dry-wet cycle;the degree of sandstone deterioration was proportional to the pH value of acid rain,and the greater the pH was,the greater the deterioration was.(5)The formation of granular disintegration was due to the mineral dissolution,stratification structure,cleavage surface,the influence of smectite and the comprehensive effect of external environmental factors.The influence of environmental factors was the main driving force for formation of granular disintegration,among which environmental temperature,environmental humidity,acid rain and capillary water were the main causes.Through the dissolution of microbial exudates,the action of dry-wet cycle and acid rain caused the cement to continuously lose,and the surface of mineral particles was continuously eroded.Under the combined action of freeze-thaw cycle and salt crystallization,secondary extrusion pressure was generated on mineral particles,resulting in particle spalling.As a large number of particles spalling occurred,the surface of sandstone became crisp and powdery,resulting in granular disintegration.