Research On Rock Mass Structures And Stability Analysis Of High And Steep Rocky Slope On The Right Bank Of A Bridge Over The Nu River

In recent years,the road network in our country has been constantly improved,with the continuous development of China’s social economy and the continuous improvement of its international status.A large mountain road project has attracted much attention due to its extremely complex engineering geological conditions,extreme geological disasters and great engineering difficulties,and is under construction in full swing.Under the influence of the rapid uplift of the Qinghai-Tibet Plateau,the neotectonic activities are intense,the valley is rapidly cut down,the unloading action is strong,the earthquakes are frequent,the fractures in rock masses are developed well,and the geological disasters are frequent.A bridge over the Nu River will be the highest bridge in the large mountain road project,with a height of over600 m and a length of about 1.5 km after completion.This ultra-high and ultra-long span bridge puts forward higher requirements for the stability of rock masses on both banks.Therefore,it is essential to carry out research on characteristics of complicated rock mass structures and stability analysis of rocky slopes to ensure the safe and economical construction and operation of the bridge.This thesis takes the rocky slope of the right bank of a bridge over the Nu River as the engineering background.Based on the field geological survey and combined with the fracture data obtained by unmanned aerial vehicle(UAV)photogrammetry technique,this thesis focuses on the characteristics of fracture development.Furthermore,the connectivity of the fracture system and the size effect and spatial effect of rock mass structures were studied on the basis of the discrete fracture network(DFN)model.Then,the mechanical characteristics,model construction,deformation behavior and stability under natural conditions of jointed rock masses on the bridge right bank were systematically studied,with discrete element method as the main numerical means.And different discrete element models were used to further reveal the mechanical characteristics of jointed rock masses and deformation failure mechanism of rocky slope.The main research contents and achievements of this thesis were summarized as follows:1.Based on the field geological survey and a large amount of fracture data obtained by UAV photogrammetry technique and measured manually by compass and tape,this thesis conducted a comprehensive study on the topographic features,the development of collapse,rock mass structures and bank slope structures of the right bank of a bridge over the Nu River.In addition,this thesis focuses on the comparative analysis of the characteristics of fracture development and finds that it is not only related to the geological genesis but also related to the distribution elevation.Based on the comprehensive analysis of the engineering geological characteristics of the study area,a macro-qualitative understanding of the rock mass structural stability of the right bank has been obtained.2.Based on a large amount of fracture data obtained by UAV photogrammetry technique,a 3D DFN model was constructed to characterize the complicated rock mass structures of the right bank of a bridge over the Nu River.To overcome the defects that the computations of the traditional algorithm for accurate intersection test are enormous and time-consuming,a stepwise approach combing the bounding box technology and the Gilbert-Johnson-Keerthi algorithm was proposed to accelerate the analysis and calculation of fracture intersection.Then,the connectivity of the DFN was further analyzed.The results show that facture set 1 with a high dip angle and fracture set 3with a medium dip angle constituted the main flow pathways and played a predominantly combined cutting role on the rock masses.Based on the connectivity of the fracture network,the size and spatial effects of rock mass structures and the representative elementary volume(REV)were further investigated.3.The mechanical characteristics of jointed rock masses and stability of the rocky slope were investigated.A multiscale DFN-DEM modelling approach was proposed by introducing the concept of the REV and the synthetic rock mass modelling technique,which is used to improve the model accuracy and computational efficiency of discrete element simulation of jointed rock masses.Dividing the full-scale DFN model into multi subscale DFN models based on fracture diameter,the REV size of the jointed rock mass defined by each subscale DFN model was comprehensively estimated by geometric,hydraulic,and mechanical parameters,and the corresponding equivalent mechanical properties were obtained using numerical uniaxial and triaxial compression tests in the 3DEC.A two-scale DFN-DEM model was constructed for the rocky slope of the right bank of a bridge over the Nu River,and the stability of the rocky slope under natural conditions was assessed by adopting the strength reduction method and displacement catastrophe criterion.4.The comprehensive parameters study of the smooth joint contact model(SJCM)and the comparative analysis of rocky slope stability based on different discrete element models were conducted.Firstly,the influence of SJCM’s parameters on mechanical properties and failure mechanism of jointed rock masses was studied comprehensively,and a systematic approach for calibrating smooth joint parameters was proposed.According to the presented multiscale DFN-DEM modelling approach,the framework for the construction of the two-scale DFN-SJCM-PFC model on the rocky slope of the right bank of a bridge over the Nu River was illustrated.The applicability of the SJCM in a large-scale rocky slope stability analysis was validated,and the deformation and failure mechanism of the right bank rocky slope was further revealed from the microscopic level.Notably,it provides a new insight for the application of the SJCM in the modelling of a large-scale jointed rock mass.