Research And Mitigation Of Rockfall Hazards At Tunnel Entrance And Exit

With financial support of the National Natural Science Foundation (ID: 50678182), the author chooze rockfall hazards at tunnel entrance as research object, and aimed at its effective mitigation, throw systemic rockfall field experiments, tunnel entrance rockfall hazard rating, rockfall trajectories and movement parameters calculation, rockfall impact forces calculation, tunnel entrance arrangement, rockfall mitigation at tunnel entrance and so on, get the following concludings:(1) Through different quality, different shapes rockfall filed experiments, the auther found that the movement mode of rockfalls are mainly rolling and bouncing, only a small number of rockfall has a short phase sliding at start, and the more spherical the the law is more obvious. For different shape rockfalls, spherical, square, and short cylindrical rockfalls have greater threaten scope, faster speed and stronger jumping ability. But the rectangular and sheet rockfalls have the worst movement ability, and the greater the Slenderness Ratio or the Flat Degree, the rockfalls have worse movement ability. No matter what shape , quality and size of rockfalls, up to the threaten scope, a high bouncing hight, lateral distance, velocity, and so on, have shown great randomness, and the smaller the quality of rockfall, the randomness are more obvious. From the movement mode and ability of rockfalls, the cylindrical rockfalls' movement features like spherical and square rockfalls. and the cylindrical rockfalls' like rectangular rockfalls. But no matter what shapes, the inpact on the slope will be a sharp change of rockfall movement trajectories and mode.From the Offset Ratio of rockfall movements, test results have been greater than some proposals value of under 0.1, but all Offset Ratio value is smaller than 0.3. And about 96% of rockfalls Offset Ratio is less than 0.25, about 89% less than 0.2, about 57% less than 0.1.(2) In view of the random characteristics of rockfall movement, as well as the needs of rockfall mitigation decision-making at tunnel entrance, the hazards rating ystem was established. The author evaluated measured the hazard rating by the possibility of rockfall avalanche, the possibility of rockfall reached the tunnel entrance after its avalanche and seriousness of rockfall hazards. A quantitative empirical rating system was founded and divided the risk dgreed into 4 grades named Grade 1 to Grade 4 tunnel entrance, for a tunnel entrance thretened by rockfall hazards Grade 1 means the most serious state, and Grade 4 means the tunnel ertrance has no rackfall hazard.(3) Through the homogeneous cylinder assumption, the author decomposed the rockfall movement mode into flight, collision and rolling, and introduced kinematics and dynamics principles to express movement parameters of rockfall(bouncing hight, movement speed, the total kinetic energy, etc.). Through choosing random value of sensitive parameters such as the normal coefficients of restitution( e_n), the tangential coefficients of restitution(e_t), and rolling friction coefficient(μ) in a identified range, and through a number of calculating by different parameter value to reflect the movement characteristics parameters random range of rockfalls. And through statistical analysis we can get the necessary parameters for the mitigation of rockfalls at the tunnel entrance.The author recommends that, for Grade 1 tunnel entrance, should choose the most adverse value as rockfall mitigation design basis. Accordingly, 95% ensuring rate parameter for Grade 2 tunnel entrance and mean value for Grade 3 tunenel entrance.(4) The threaten area by rockfall hazards is basis of tunnel entrance design, land planing and rockfall mitigation structure design. By 2D rockfall trajectory and movement parameter calculation and combination with the Offset Ratio, the 3D rackfall threaten area can be delineated. The author recommends that, for Grade 1 tunnel entrance, the Offset Ratio is 0.3. Accordingly, 0.25 for Grade 2 tunnel entrance and 0.2 for Grade 3 tunenel entrance.(5) Based on the Impulse Principle, considering gravity and rebounding effect in the rockfall impacting process, throught field experiment data, different formulas calculating results and the 3D dynamic numerical simulation, the author presented rockfall impacting force calculation method. The method can apply in solving impacting forces problem for different impact speed, different thickness of the bufferlayer, different impact angle. And then the impacting force calculation method provided load basis for the design of rockfall mitigation structure such as cut-and-cover tunnel and shed-tunnel. The calculation of impact impacting force and its comparative analysis showed that the smaller the impact angle the corresponding impact force is also smaller, and the thicker the buffer layer, the impact force is smaller. and after the distribution of buffer layer, the impacting force on structure is much more smaller. But with the increasing of the thickness of buffer layer, the weight on the stucture is also increaed. And to enhance the anti-impact capacity only by increasing the thickness of buffer layer is usually expensive. In the actual engineering, the buffer layer design need considered rockfall size, impact force and structure optimization. The adverse location by impacting force for cut-and-cover tunnel is arch crown or arch back. But for shed tunnel the adverse location is usually at mid span.(6) The cut-and-cover tunnel and shed tunnel not only can serve as tunnel entrance or exit, but also one type of rockfall mitigation techniques for linear engineering such as roads. To tunnel entrance and exit which threatened by rockfall hazards, we can design the tunnel based on forecasts of rockfall threaten area, rockfall trajectory and impacting force calculation. And also can be designed like a tunnel in general area in the premise of ensuring safety by mittigation the rockfall hazards using systemtic active and passive mitigation techniques. The active control techniques are applicable to the large scale and well investigated rockfalls. And the passive protection system can be used as supplementary to the active control techniques, or can also be used to prevent and treatment of small, bad investigated, linear distributed rockfalls. The safty net and semi-rigid rockfall barrier wall are two kinds of good passive mitigation system. And the design of passive mitigation techniques or tunnel entrance must be based on the results of rockfall threaten forecasting, trajectories, bouncing hight, velocity, kinetic energy, and impact force calculation.