Dynamic Analysis And Countermeasure Of The Airblast Generated By Rock Avalanche

For the study of the dynamic behavior of airblast induced during the high-speed motion of rock avalanche along its travelling path, three typical avalanches generated by 5.12 Wenchuan earthquake are studied.Based on the field investigation, the geometry, movement and dynamic properties of rock avalanche are analyzed and the special attention is given to the dynamic catastrophic phenomenon caused by the airblast along the travelling path of the rock avalanches, including the destruction of woods and buildings. True two dimensional and three dimensional models of the Niujuangou rock avalanche, three dimensional model of the Wangjiayan rock avalanche and three dimensional model of the Xiejiadianzi rock avalanche are built with software FLUENT, respectively.Using the User-Defined Functions of the software FLUENT introducing the definition of resistance along traveling path under the law of Voellmy, and the soil mechanics parameters obtained during the field investigation, the dynamic behavior during the whole movement processes of the three typical rock avalanches and the characteristic of airblast induced by three of rock avalanches are inverted. The main results reached in these numerical simulations are as follows:The results of the three dimensional model of Niujuangou rock avalanche shows that: The whole travelling process of the sliding mass lasts 119 s, and the maximum speed at every moment appears in the part near the front zone of the sliding mass (except t=50 s, the maximum speed appears in the front zone of the sliding mass) with three ultimate values 52 m/s at 14th second,55 m/s at 27th second, and 49m/s at 50th second, respectively; The maximum velocity of the airblast appears at the 50th second with its value reached 38 m/s. The maximum pressure of the airblast,657 Pa, is generated at the moment when the sliding mass bursts out the scarp, which is equivalent to the violent storm. The squeezing effect of the sliding mass to the air ahead is most obvious when the sliding mass is moving fast and the leading height is higher; the topography along the sliding path displays a large influence on the distribution of airblast pressure. During the high-speed motion of the sliding mass, an obvious gradient variation of air pressure is induced when an obstruction appears along its sliding path, resulting in a destructive effect. When the sliding mass rushes down a scarp or ridge, the velocity of sliding mass will increase rapidly with a negative pressure generated inside of the sliding mass because of weightlessness, meanwhile, a sharply increase of the air pressure will also appear at some locations distributed ahead of the sliding mass due to the intensive compression induced by the sliding mass.The whole travelling process of the Wangjiayan rock avalanche lasts 12 s, and the ultimate value of speed is 36 m/s at 10th second; The maximum velocity of the airblast appears at the 10th second with its value reached 28 m/s; The maximum pressure of the airblast,600 Pa, is generated at the moment when the velocity of sliding mass reachs the ultimate value; The maximum pressure of airblast induced by Wangjiayan rock avalanche appearing at 8.5 s is 391 Pa. This pressure, which is equivalent to the whole gale, is generated at thelower windward side of the building and 110m away from the front zone of sliding mass.With the increment of airblast loading, the connected cracks at the lower part of the masonry structure has a trend of extending to the upper part. The structure failure at the lower part of the masonry structure can cause the building collapse from lower part to upper part. According to the masonry structure in the front of the deposition area of Wangjiayan rock avalanche which have been damaged by the earthquake before the airblast generated, the masonry structure in the front of the accumulation zone collapses before the moment of 8.5 s.The whole travelling process of the Xiejiadianzi rock avalanche lasts 80 s, and the ultimate value of speed is 18.3 m/s at 40th second; The maximum velocity of the airblast appears at the 40th second with its value reached 11.0 m/s; The maximum pressure of the airblast,385 Pa, is generated at the moment when the velocity of sliding mass reachs the ultimate value; Compared with the numerical simulation results between the model of Xiejiadianzi rock avalanche with rows of piles and the model without rows of piles on the traveling path, it can be found that: ①In the case of model with rows of piles, the majority part of the sliding mass deposit mainly stacked at the end of the travel zone near the piles, and only a small part of the sliding mass deposit leaves at the accumulation zone. The area of the accumulation zone is reduced by two-thirds compared with the model without rows of piles. ②The maximum speed in the part of the sliding mass near the rows of piles is changed from the nearby front zone to the middle zone of the sliding mass. ③Fach successive row of piles can slows the speed of the sliding mass and the speed of the airblast. At the same time, it reduced the pressure of the airblast, obviously. ④The impact pressure induced by the sliding mass acting on the front row of piles is higher than on the rear row of piles. In addition, the impact pressure induced by sliding mass mainly concentrated at the lower location of each row of piles.