Experimental And Numerical Investigation Of Rockfall Impact Fragmentation Mechanism

As a typical geological disaster,the scale of rockfall is relatively small,but it may also cause a great loss to the national economy and peoples's lives and property because it always occurs in sudden and exhibit great uncertainties in nature.It is of great significance to study the rockfall impact to explore the collision mechanism of rockfall and to analyze the effect of rockfall imapct fragmentation and to modify the rockfall-impact model.In this paper,the influence of impact velocity,the mechanical properties of rockfall,impact angle and the size of rolling stone on the rockfall impact fragmentation are studied through the system's slope impact test and numerical simulation analysis.The influence of the impact fragmentation of rockfall on the coefficient of restitution is analyzed.The main contents and basic understanding are as follows:(1)On the basis of literature survey,the advantages and disadvantages of various methods of rock specimen pre-damage were analyzed and compared,and high-temperature heating method was put forward to prepare rock samples,and the effect of high temperature damage was tested through a series of experiments.The results show that the high temperature heating will lead to a slight decrease in the quality and irreversible volumetric expansion of granite samples.The uniaxial compressive strength,splitting tensile strength,elastic modulus and longitudinal wave velocity of granite decrease with the increase of temperature.Generally,the effect of temperature is realtively slight within 400?.With the increase of temperature,the damage degree of rock properties is aggravated.Comparing all kinds of mechanical parameters synthetically,it is better to use rock integrity index based on longitudinal wave velocity as rock damage index.(2)A series of tests were carried out on the self-developed rockfall impact system for specimens after high temperature pre-damaget.The mechanism of impact fragmentation of rockfall under the coupling of the mechanical properties,velocity,incidence angle and the size of the rockfall is analyzed.The influence of impact fragmentation on the motion of rolling stone is discussed.The results show that the mechanical properties and velocity are the main factors that control impact fragmentation.The worse the mechanical properties of rockfall,the higher the impact speed,the greater of impact fragmentation.There is a normal velocity threshold for fragmentation of rockfall,and as the mechanical properties deteriorate,the normal velocity threshold is reduced.The incidence angle has a great influence on impact fragmentation of the rockfall and the energy coefficient of restitution.The impact fracture not only causes the total energy coefficient of restitution of rockfall to be slightly reduced,but also causes the individual fragments to have a large velocity.(3)The three dimensional dynamic finite element software LS-DYNA is uesed to simulate the fragmentation of rockfall upon impact.The numerical model and its parameters are adjusted and verified based on the experimental results and fragmentation characteristic.The results show that the optimoized soil/concrete model can reflect the characteristic of fragmentation.The tensile strength of rockfall has a great influence on the characteristic of fragmentation upom impact,While the structural form,the boundary condition and mechanical parameters of the slope may have a great influence on the coefficient of restitution.Based on this model,the influence of the fragmentation upon impact on the motion characteristics of rockfall is studied by using the ideal rigid body slope model.And the range of parameters such as the impact velocity,rock size and mechanical properties were extended.The numerical results show that the worse the mechanical properties of the rockfall,the smaller the energy coefficient of restitution.Before being fractured,the coefficient of restitution decreases slightly with the increase of the impact velocity,and it decreases sharply while the rockfall becomes fractured after impact.When the impact velocity is higher than a certain value,the energy recovery coefficient decreases gently.