Stability Analysis Of Dense Caves And Risk Assessment Of The Dangerous Rock Body At The Southern Area Of Mogao Grottoes,Dunhuang

Our country,as the only one in the world with a long history that has never been uninterrupted,has abundant cultural heritages.At present,there have been 52 sites of cultural heritages and natural landscapes registered in the World Heritage List.In published seven batches of National Key Cultural Relics Protection Units,grotto temples(including stone carvings)take up a total of 266.These grottoes temples have been mostly excavated in sandstone(or glutenite)cliffs in intensive amounts.Mogao grottoes,as a world cultural heritage site,a typical representative of glutenite grottoes and also a model of heritage protection and management,are a well-preserved Buddhist art resort with the largest existing scale,the longest duration,the most abundant contents,exquisite art and huge influences.As the Silk Road has successful applied for a world cultural heritage site,along with national "The Belt and Road" initiative and Gansu Huaxia Cultural Heritage Innovation Zone have been implemented,protection and research of Mogao grottoes have attracted much attention.Since excavation,the cliffs of Mogao Grottoes have suffered from threatens including partial collapse,falling rocks,quicksand,leakage and salt damage,which have been controlled by lithology and geological structure under some influences of wind and sand,rainfall,earthquakes and tourists,etc.Their stability has also been seriously threatened,especially for weakly consolidated glutenite grottoes,and there is still no reasonable analysis method for this.Especially for dangerous rock bodies existed locally,stability and risk assessment are urgently needed.In recent years,the number of tourists has been increasing dramatically,which has posed a serious threat to densely distributed caves and wall paintings.Meanwhile,since the unpredictability and strong destructiveness of the earthquake,seismic influence on the dense caves still lacks of effective research.This paper arranges the protection history of Mogao Grottoes in detail,and through the investigation and collection of the environment in Mogao Grottoes,the stratigraphic features of the Mogao Grottoes are refined.The analytic hierarchy process is used to evaluate the overall stability of the cliff of the Mogao Grottoes.The results show that the cliff is stable and there are dangerous rock bodies locally.The impact of tourist loads and earthquakes on the stability of the cliff is crucial,and these two factors are selected to study the influence on the stability of the densely distributed caves by numerical simulation method.Finally,the Fuzzy-AHP method is used to evaluate the hazard of potentially dangerous rock bodies within the cliff and its influence on the elements at risk such as people and caves.Finally,the risk of these dangerous rock bodies is evaluated and the risk map of Mogao Grottoes' cliff is obtained.The following main conclusions are summarized:(1)For the protection history of Mogao Grottoes,it has experienced the period of guardianship,the period of reinforcement,and the period of scientific protection to the present stage of preventive protection,and all monitoring and research work on heritage is part of preventive protection.The risk theory was introduced to identify the risk source,assess the hazard and consequence,and successfully applied in the risk assessment of the cliffs in Mogao Grottoes.(2)Through field investigation,128 sections are selected to make a more detailed division of the cave stratum,which can be divided into:(1)slope type of cliff without sand layer;(2)step type of cliff body without sand layer;(3)vertical type of cliff without sand layer;(4)step type of cliff with obvious sand layers;(5)slope type of cliff with inconspicuous sand layers.The strata containing the sand layer are still the main form of the cliff.(3)Using the three physical exploration methods of geological radar,high-density resistivity method and surface wave method,the stratum of cliff top is detected in detail.The results show that the surface of the cliff top is loose,especially the thickness of the sand layer in the surface of the gully is clear.It is identifiable that the water content is relatively high.Along the top of the cliff,the stratum is gradually dense,and the sand layer is partially contained.The overall stratification of the stratum is good,some sections are slightly undulating,and some sections have water vapor accumulation.(4)Through a large number of field investigations and data collection,the “distribution of caves”,“shape of cliff” and “fractures” were selected as evaluation factors,and the overall hazard of the cliff of Mogao Grottoes was evaluated by analytic hierarchy process.The evaluation results show that the cliff of Mogao Grottoes is generally stable and there is no high-hazard and medium-hazard zones.At the same time,tourist loads and earthquakes are dynamic factors and are the most important factors affecting the stability of the cliff,which should be conducted more special studies.There are many potentially dangerous rock bodies within the cliff,which have the greatest impact on the stability of the cliff and personnel safety,and should be assessed its risk.(5)A relatively high hazard area was selected to analyze the influence of tourist load on the stability of the caves under different working conditions by using numerical simulation.The calculation results show that:(1)The roof of the densely distributed caves and the roof of the thin-topped caves will appears tensile stress concentration(0.14 MPa),but the tensile stress does not increase significantly under the load of the tourist(0.15 MPa).(2)The displacement of densely distributed caves is large(0.55 mm).The displacement of the caves is slightly increased under the load of tourists(0.6 mm).(3)The increase of displacement under the working condition 3 is small compared with other two working conditions.This type of tourist position can effectively reduce the impact of tourists on the displacement of the caves.In short,the static load of tourists has not obvious influence on the stability of the caves.(6)The numerical simulation method is used to analyze the stress,displacement distribution characteristics and acceleration of the caves under the self-weight and earthquake.The calculation results show that:(1)The densely distributed caves in the middle floor are more likely to have tensile stress concentration,and the seismic load significantly increases the tensile stress concentration effect,and increases the maximum tensile stress value(0.16 MPa).(2)The displacement of the densely distributed caves and top floor caves is large.Under the effect of earthquake,a horizontal relative displacement of approximately 9.0 mm and a vertical permanent displacement of 3.0 mm were produced eventually.(3)The acceleration amplification effect of the densely distributed caves in the middle layer is most obvious.For all caves,the low-frequency part of the acceleration is significantly enlarged,and all frequency ranges of the acceleration of the middle floor caves are amplified,especially for the high-frequency part,the effect is more obvious.(4)Although there is tensile stress concentration in densely distributed caves,there is no tensile failure.However,nonuniform horizontal relative displacement and large acceleration response will have a negative impact on caves' stability and the protection of wall paintings.(7)The risk assessment of 42 potentially dangerous rock bodies within the cliff of the Mogao Grottoes was carried out by using the Fuzzy-AHP method.The evaluation results show that:(1)it is indicate that Fuzzy-AHP method is scientific and can be used for risk assessment similar to the cultural heritage site of Mogao Grottoes.(2)In the process of hazard assessment,through a large number of field investigations and data collection,"dangerous body","cliff shape","lithology","fracture" and so on were selected as evaluation indicators,and the results of risk assessment are in consistent with the field survey results.(3)In the process of consequence assessment,the selected elements at risk mainly include personnel,buildings,infrastructure and other property.These indicators cover almost all the people and property that can be affected.(4)According to the hazard and consequence assessment results,the risk division of the southern cliff can be obtained.The results show that the risk of the south side of the cliff is relatively low.The middle part of the cliff is relatively high risk due to its high cliff and large threat.It is hoped that the evaluation results will have a certain guiding significance for the constructing of the monitoring and early warning system and long-term preventive protection of Mogao Grottoes,and can provide some certainly reference value for other researchers and managers.