Dynamic Response Of Pile-Sheet Retaining Wall With Cushion Layer Under Rockfall Impact

Collapse is a major geological disaster frequently occurred in high mountainous canyon areas with high and steep slopes.The diameters of the rockfall range from several meters to more than ten meters,whose weight could reach as much as 100 tons and impact velocity may exceed 20m/s.When it comes especially to high-level rockfalls,the running rocks always have high suddenness,randomness and kinetic energy.The deep and steep valleys and frequent tectonic activities in the mountainous areas of southwest China create appropriate conditions for the collapse of rock mass in forms of single or group rockfalls.Once these potentially dangerous rocks collapse,they will inevitably cause huge disasters.Using classic contact theory in this paper,field prototype investigation and large-scale rockfall impact experiments based on the pile-sheet retaining wall with cushion layer and shed tunnel prototype structure were conducted to systematically study the dynamic characteristics parameters of the interaction between the rockfall and the protective structure and their variation law.The SPH-FEM coupled numerical analysis was used to compare and verify the model test and expand the related calculations.Finally,the finite element numerical simulation was used to verify the example and the dynamic response of rock impact of the new pile-sheet retaining wall with cushion layer structure in the impact surface.The main research results and innovations of this article are as follows:(1)Using the contact theory,the calculation formulas of the coefficient of restitution and impact force during the rockfall collision are derived,and the variation law of the coefficient of restitution and impact force with the initial incident velocity and incident angle as the variable parameters was obtained.A new model for calculating the tangential force was developed,and the movement form of the collapsible body that may fall is judged by the characteristics of the slope surface and the incident conditions of falling rocks.The main results include that: the restitution coefficient is positively correlated with the incident angle when the incident velocity is the same;the restitutionc oefficient is negatively correlated with the velocity when the incident angle is the same,and the impact force is linearly correlated with the velocity;the tangential behavior of falling rock is mainly controlled by the incident impact angle;the movement state of rock rebound depends on the initial incident conditions,when the incident angle is greater than the critical viscous-slip angle of incidence,there is slippage in the falling rock collision process,and on the contrary,stickiness occurs.(2)The large-scale outdoor rockfall impact test was carried out,the dynamic response of the reinforced concrete slab structure overlaid with rockfall impact was studied,the relationship between the impact force and the thickness of the cushion layer during the rockfall impact was furtherly analyzed;Based on the impact test on reinforced concrete slabs with different thicknesses,the deformation and failure modes of concrete slabs were analyzed.The main results include that the thickness of the cushion layer has a significant effect on the impact force.Within a certain range,increasing the thickness of the cushion layer can effectively reduce the impact force.At the maximum impact energy level of 20 k J in this paper,the optimal thickness of the cushion layer is 0.3m?0.4m;According to the measured data in experiment,the fitting method to calculate the maximum impact Depth method;The thickness of the reinforced concrete slab has a great influence on the failure mode of the slab.When the thickness of the reinforced concrete slab is 0.25 cm,the failure mode of the slab is characterized by bending failure and shear failure.But when it is 0.35 m in contrast,the final deformation mode is typical bending deformation.(3)On the basis of model test,SPH-FEM numerical simulation is used to verify the test results.The results show that the numerical simulation results agree well with the experimental results.On this basis,extended calculations were added.The impact energy was increased to 150 k J keeping other conditions unchanged,then the damage forms and mechanisms of reinforced concrete slabs were analyzed.When considering the rockfall impact force,concrete slab strain and failure characteristics comprehensively,it is finally determined that the optimal cushion layer thickness is 0.8m under the impact load of spreading energy.The SPH-FEM method adopted in this paper can provide a new simulation method on the impact response of the soil-structure combination.(4)The shape factor is introduced to simplify the falling stone into an ellipsoid shape,and the shape and size of the falling stone are determined by their triaxial length.The SPH-FEM coupling algorithm was used to analyze the influence of the falling rock shape and contact attitude angle on the impact force,impulse,energy consumption of the cushion layer and other characteristic parameters during the collision.It is found that when the shape factors are the same,the larger the attitude angle is,the larger the impact force amplification factor will be.The smaller the shape factor,the greater the influence of rockfall attitude angle on the impact force.In addition,when there is a certain angle between the impact speed of the falling rock and the rock axis,the falling rock will rotate around the contact point.(5)Relying on the pile-sheet retaining wall with cushion layer structure that has been implemented in Kongyu Town,Kangding City,Sichuan Province,an equal-scale numerical model is establish.In order to better simulate the energy dissipation of the infinite structure at the truncation boundary,the stress wave at the truncation boundary is absorbed to realize the wave dissipation,thereby increasing the calculation accuracy and reducing the amount of calculation.In the calculation,A variety of impact force calculation models is used to compare and verify the reliability of the results;The energy-dissipation law of the cushion layer is analyzed,and the cushion layer of soil is proved to be the most important energy-consuming component;When compared with soil granular materials,the plastic strain energy generated by the geogrids accounts for only about 6% of the total plastic strain energy;According to the definition of the concrete damage factor,the concrete damage volume is used to quantify the degree of structural damage.As a result,the tensile damage is much greater than the compressive damage and the damage at the center of the impact are very serious,where the concrete has failed and flaked forming a penetrating hole,which is consistent with the phenomenon observed in actual engineering.