Equivalent Structural Geomechanical Model And Structural Stability Analysis Of Fractured Rock Masses

With the rapid improvement and development of infrastructure,the scales of various projects are also gradually increasing.The rock masses are the essential carrier of the crucial strategic projects,for example,hydropower,transportation,and nuclear power,whose stability is significant to the safety of the engineering.Influenced by the geologic process,rock masses often developed structural surfaces with different nature and scale,such as fracures,weak interlayers,faults,beddings,etc.The cutting action of these structural surfaces greatly degrades the quality of the rock masses,and increasing the possibility of deformation and instability failure of rock masses.Therefore,fully considering the impact of structural characteristics on its mechanical properties is the necessary task of rock engineering design,and it is also an important technical support to ensure the long-term stability of rock engineering.At present,in the field of rock masses mechanical analysis,especially for the overall stability analysis of rock masses project,there are few effective technical means to directly establish a large number of structural plane systems in the analysis model.As for the existing difficulties,the equivalent mechanical parameters of rock masses can implicitly include the weakening effect of structural planes on rock masses quality,and then establish the corresponding equivalent structural geomechanical model,so as to realize the analysis of rock masses engineering controlled by complex structural planes,which is in line with the current theoretical and actual engineering needs.It can be seen that the acquisition of structural characteristics and equivalent mechanical parameters of rock masses is key to establish equivalent structural geomechanical model.This thesis takes the No.28 sluice of the Datengxia Hydropower Station in Guangxi as the engineering background.The purpose is to analyze the structural characteristics of dam foundation rock masses and the influence of structural planes on equivalent mechanical parameters of rock masses,and then establish a multi-scale equivalent structural geomechanical models considering the deterministic structural planes and random structural planes.Meanwhile,the equivalent model is constructed using numerical and physical model test,and the deformation process and instability mode of the equivalent model are analyzed and discussed,which offers the scientific guidance for the building and support scheme of the project.Based on the synthetical analysis of the above contents,the conclusions of the thesis are drawn:(1)Based on the detailed field investigation,the geometric characteristics of the structural planes developed in the dam foundation the No.28 sluice are identified.Faults,beddings,weak interlayers and structural fractures are developed in the rock masses.Among them,the beddings of each stratum are extremely developed,showing parallel bedding structure.Weak interlayers in Nagaoling(D₁n)and Yujiangian D₁y¹-1formations are also developed,and their occurrence is basically the same as that of the rock stratum,with a spacing of 2?5m.The structural fractures are connected between the adjacent weak interlayer in D₁n and D₁y¹-1 formations,while it is intermittent in D₁y¹-2 and D₁y¹-3 formations.The fractures are roughly divided into two groups.For the two-dimensional stability study,the fracture group whose strike is nearly perpendicular to the section direction plays an important role in the analysis model.This fracture group inclines upstream,with dip angle of about 79°and an average spacing of about 2 m.(2)For the complex intermittent structural fractures developed in the rock masses of D₁y¹-2 and D₁y¹-3 formations,their distribution characteristics and intersecting state in three-dimensional space of rock masses can be reproduced by means of three-dimensional network simulation based on the principle of probability statistics.Furthermore,the generated network model is cut to obtain the two-dimensional trace distribution of structural planes with similar statistical characteristics.Based on the Dijkstra shortest path search algorithm,the maximum connectivity in the potential failure direction and its corresponding fracture geometry features can be obtained,which provides important data support for the subsequent reasonable evaluation of the equivalent shear strength of intermittent fractures.(3)Based on the principle of equivalent strain,an analytical solution of the equivalent elastic modulus of rock masses considering confining pressure effect is derived for the first time from the perspective of mechanics,which can not only obtain the equivalent elastic modulus of rock mass conveniently and reliably,but also quantitatively characterize the stress-dependency of equivalent elastic modulus.At the same time,the quantitative values of key parameters in the generalized Hoek-Brown criterion are systematically expounded,and then the equivalent shear strength could be estimated according to the generalized Hoek-Brown criterion.Mechanical test and geotechnical test can reduce the subjective influence of artificial value,and can effectively improve the reliability of obtaining the equivalent shear strength of rock masses using H-B criterion.(4)Through the direct shear test of indoor rock-like materials,the effects of the connectivity,fluctuation angle and the number of fractures on the potential failure path of intermittent fractures on the equivalent shear strength of intermittent fractures are clarified,and the weakening mechanism of rock bridges in the process of rock masses shear failure is explored.Based on the test results,a modified Jennings strength criterion considering multi factor fractures geometric characteristics and mechanical property damage of rock bridges is proposed for the first time.Compared with the traditional Jennings criterion,the modified Jennings criterion is more in line with the real failure of rock masses,can effectively improve the accuracy of evaluating the equivalent shear strength of intermittent fractures,which has remarkable significance for engineering practical.(5)Based on the study of rock mass structural characteristics and equivalent mechanical parameters,an equivalent structural geomechanical model of dam foundation rock masses of sluice 28#dam section considering the influence of multi-scale structural planes is constructed.In order to explore the deformation and failure mode of the equivalent model,the equivalent model involves double equivalence in the construction:the first equivalence is that the influence of dense beddings on the properties of rock masses is implicitly expressed in the equivalent mechanical parameters.By giving the equivalent mechanical parameters to the rock block elements,the analysis problem of discontinuous rock masses controlled by complex beddings can be transformed into the analysis problem of continuous medium model;The second equivalence is that the most dangerous path composed of intermittent fractures and rock bridges is equivalent to through-out fractures,which can not only consider the influence of intermittent fractures,but also reflect the potential instability mode of rock masses engineering under load.(6)Numerical simulation is an important means to analyze the deformation and failure modes of rock masses engineering.The equivalent model of No.28 sluice is constructed by the numerical simulation,and the stability and failure mode of the dam foundation equivalent structure are analyzed by using the complete fluid-solid coupling mechanism.Meanwhile,the physical model test is used to reproduce the geological structure and hydraulic structure of the equivalent model based on the similarity theory,and the comprehensive stability and failure mode of the dam foundation rock masses are studied,which can verify and supplement each other with numerical simulation.The results show that the overall through-out sliding failure surface is not formed under the action of external load,and the deformation failure modes are mainly the cracking failure of the structural planes near the dam heel and toe key trench,and the buckling failure of near surface rock masses in downstream caused by the through compressive plastic zone in the downstream rock masses.