Study On Movement Characteristics Of Rockfall At Mogao Grottoes In Dunhuang

Mogao Grottoes is located in the hinterland of the Northwestern China,where features harsh natural conditions and strong weathering forces.Under a variety of natural and human factors,serious weathering erosions occurred on the face of the cliff and the slope near the top of the slope,which lead to a series of rockfall events.This problem not only affects the stability of the caves and the safety of cultural relics in the caves but also puts the safety of tourists into a risk.Previous researches on rockfall mainly focus on slopes by railways or highways both at home and abroad,and the large-scale rockfall caused by collapse is their stress.The research on small-scale rockfall in cultural heritage sites is not deep;therefore,it's necessary to study this issue specially.Based on the previous research,this paper used many methods such as rockfall field test,resistant coefficient field test,and numerical simulation on trajectory,which aims to study the causes of rockfall at Mogao Grottoes,main source areas of rockfall,the movement characteristics of rockfall,and risk monitoring.Following conclusions were made.1.There are two categories of rockfall source areas at Mogao Grottoes,which are the slope near the top of the cliff and the erosion step near the cliff brow.The slope near the top of the cliff shall cause falling-out rockfalls,which is the main source of rockfall disasters now.Different rock groups and different interbedding in same rock group can lead to differential weathering.The erosion step near the cliff eyebrow shall develop exfoliation rockfalls,which is sort of small-scale collapse.Exfoliation rockfalls could cause serious and harmful consequences,but it is not the main source of rockfalls at Mogao Grottoes.2.In field test,according to different shapes of potential rockfalls on the slope,four categories were divided,which are rough ball,bluff body,cylinder and prism.These four types of rockfalls have common movement characteristics,and the main movement types are rolling and bouncing.Most of rockfalls regardless of shapes stayed on the slope,only about 20% of rockfalls moved down to the scenic area at one time.The movement of rockfall is subject to micro-landforms of the slope significantly.In the process of the field test,rockfalls eventually moved down gullies or grooves.3.Different masses of rockfalls have different anti-interference abilities.Vegetation and frictional resistance have limited influence to rockfalls with the large mass,and so does terrain.Therefore,rockfall with a large mass has larger kinetic energy and run-out distance than rockfall with a small mass.The rockfall with a small mass is subject to vegetation and frictional resistance,its whole movement process shows obviously random.Rough ball blocks have smaller offsets,compared with other shapes of rockfalls.Flat body blocks have maximum movement range.The offset values of prism and cylinder blocks increased with their fineness ratio.4.By comparing the relationship between the endpoints and run-out distance,simulation results using RocFall 4.0 fit the results of field test.Simulation results show that about 20% of rockfalls moved down to the base of the cliff at one time,and became threats to tourists.Nearly 80% of rockfalls were stopped on the slope of which half moved 5m.On the slope,the main movement types are rolling and sliding with some bouncing,and the bouncing height is very small which is in a range of 0.1-0.2m,therefore,the total kinetic energy and translational velocity don't change too much.When rockfalls flew down the slope,the main movement type is falling,and the total kinetic energy and velocity increased rapidly.