Dynamic Failure Mechanism Of Shed-tunnel Structure Under Rock-fall Impact

As a typical passive protection structure,shed tunnel has good mechanical properties and is widely used in highway engineering,Railway Engineering and tunnel engineering in mountainous areas.However,the research on the dynamic response and failure mechanism of the shed-cave structure impacted by falling rocks has always been the difficulty and focus of the research.Based on the National Natural Fund Project(No.51408084),this paper studies the dynamic response,frequency variation and failure mechanism of shed-tunnel structure under different rock-falling shapes,different rock-falling heights and different impact points by means of indoor model test,numerical simulation and spectrum analysis.The main research results are as follows:(1)Through the indoor model test,it is concluded that the order of acceleration of different shapes of falling rock impact shed is sphere,cube,cylinder,and the acceleration decreases with the increase of the distance from the impact point.(2)Based on the numerical simulation method,when the falling height of the falling rock is the same,the maximum displacement of the node 10385 directly below the impact point is the largest,and the maximum displacement of each node decreases with the increase of the distance from the impact point;for the two nodes with the same distance from the impact point,the maximum displacement of the node along the short-side direction is greater than that of the node along the long-side direction.The maximum displacement of the node in the side direction and the node on the upright column of the shed cave have obvious displacement in the three directions of X,Y and Z.(3)When the falling rocks impact the roof of the shed cave at different heights,part of the energy is converted to the energy of the shed cave,and the proportion of the energy converted to the shed cave in the initial energy of the falling rocks increases with the increase of the falling height.Specifically,when the falling height increases from0.5m to 2.0m,the energy of the shed cave accounts for 40.0% to 60.41% of the initial energy of the falling rocks.?(4)The roof deflection,maximum impact force and energy of the shed tunnel decrease with the increase of cushion thickness.Compared with the case without cushion,the maximum deflection of the roof of the cushion tunnel with thickness of3cm-9cm is reduced by 62.93%-71.48%,the maximum impact force is reduced by70.01%-76.56%,and the energy of the shed tunnel is reduced by 89.96%-93.92%.(5)When the falling height increases from 0.5m to 2.0m,the peak velocity of vibration increases with the falling height.When the falling height is 0.5m,1.0m,1.5m and 2.0m,the peak value of vibration velocity decreases by 62.7%,64.4%,65.6% and72.8% respectively when 3 cm thick sand cushion is installed.(6)When sand cushion is set up,the frequency spectrum of vibration velocity based on FFT shows obvious "single peak".Compared with no sand cushion,the frequency component is more single.Without sand cushion,the spectrum shows obvious "multi-peak".(7)Under the conditions of short side center,long side center and roof center,the instantaneous frequency of HHT concentrates in the initial period of impact contact;under the condition of setting buffer cushion(sand cushion),the instantaneous frequency of HHT distributes in the whole period of time,and the instantaneous frequency develops to low frequency.(8)When rockfall impacts on different parts of the roof of ordinary shed cave,the energy proportion of frequency band No.1(0-39.0625Hz)is the largest,which is close to 80%,and the energy proportion of other frequency bands is less than 5%.When the cushion cushion(sand cushion)is set,the frequency bands with larger energy share are:No.1(0-39.0625 Hz),No.3(78.125 Hz-117.1875 Hz),No.4(117.1875 Hz-156.25 Hz),No.7(234.375 Hz-273.4375 Hz)and No.8(273.4375 Hz-312.5 Hz).(9)When the falling rock impacts the shed tunnel at a speed of 20 m/s,the peak value of impact force when the impact point is the center of the roof is 2.55 times that of the center of the long side of the roof and 2.61 times that of the center of the short side of the roof.The deflection of the impact point at the center of the roof is larger than that at the center of the long and short side of the roof.Under the working conditions,the difference between the sum of the peak kinetic energy and the peak internal energy is small,and the proportion of the peak kinetic energy and the peak internal energy at each impact point is obviously different.