| Yangfanggou Hydropower Station is located in the middle reaches of Yalong River in Muli County,Liangshan Prefecture.The topography of the dam site is undulating and the slope is high and steep.It is located in the reach with strong weathering and unloading of rock mass.Based on the understanding of the abutment slope of Yangfanggou Hydropower Station,combined with the engineering geological conditions and unloading characteristics of the project area,it is found that there is a serious unloading phenomenon in the excavation process of the slope rock mass,and the excavated rock mass will inevitably lead to the change of the stress field of the slope,and its mechanical characteristics are different from those under loading.When rock mass is excavated and unloaded,its mechanical properties change,which leads to the deterioration of rock mass quality,and may affect the stability of slope and the safety of engineering.Therefore,considering the unloading factor is necessary for the mechanical characteristics and quality of excavated rock mass,so as to truly reflect the engineering characteristics of excavated rock mass.In this paper,the jointed rock mass of Yangfanggou Hydropower Station is selected as the main research object,and the mechanical properties and parameters of jointed rock mass under unloading and unloading conditions,as well as joint connectivity,are analyzed by comparing and analyzing the strength and deformation characteristics of jointed rock mass under unloading and unloading conditions through laboratory tests.The influence of inclination angle on mechanical parameters in unloading process.Combining with the existing SMR classification method of slope rock mass quality evaluation system,considering the deterioration of rock mass quality caused by unloading,the XSMR rock mass quality evaluation method considering unloading factors is established.In the process of research,the connectivity and inclination of jointed rock mass are determined as research variables by analyzing the structure and unloading characteristics of rock mass in situ.Simulated material is used to pour specimens with different connectivity and dip angle to simulate jointed rock mass in site,and triaxial unloading test is used to simulate excavation and unloading of rock mass in site.The triaxial loading and unloading tests of jointed rock mass are carried out.Based on the analysis of mechanical characteristics and parameters of jointed rock mass under unloading conditions and the evaluation of rock mass quality under excavation unloading conditions,the overall route and method are studied.(1)Joint rock mass with different connectivity and inclination is made indoors,and triaxial loading and unloading tests are carried out.(2)The strength and deformation characteristics of jointed rock mass under loading and unloading conditions are compared and analyzed,and the mechanical properties of jointed rock mass under loading and unloading conditions are analyzed.(3)Comparing the strength and deformation parameters of joints with different connectivity and inclination,and analyzing the influence of connectivity and inclination on the mechanical parameters of jointed rock mass under unloading conditions.(4)Based on the established variation law of mechanical parameters under unloading conditions and existing rock mass quality evaluation methods,a rock mass quality evaluation method considering excavation unloading is established.Through the above research,the main achievements are as follows:(1)Under triaxial loading,the greater the confining pressure,the higher the strength and the stronger the plastic characteristics of jointed rock mass.Compared with the loading test,the strength of joint specimens under unloading condition is lower,the stress drop is very obvious,the damage is more intense,the damage degree is higher,showing strong brittle characteristics.The cohesion obtained from unloading test is 16.7% lower than that under loading,while the internal friction angle is 5.6% higher,indicating that the mechanical properties under unloading failure are worse.(2)By comparing and analyzing the strength parameters under loading and unloading conditions,with the increase of joint connectivity,the strength of specimens decreases,and the cohesion and internal friction angle decreases accordingly.Moreover,with the increase of connectivity,the degree of cohesion decreases gradually,but there is no obvious law between the degree of increase of internal friction angle and connectivity.(3)According to the strength parameters of loading and unloading tests at different joint inclination angles(30,60,90),the strength of specimens at inclination angle 60 is the smallest,and the corresponding cohesion and internal friction angle are the smallest,while the strength at inclination angle 90 is the largest,and the corresponding cohesion and internal friction angle are also the largest.For the reduction of cohesion of specimens,the dip angle is the smallest at 60 degrees,while the dip angle is the largest at 90 degrees,showing strong anisotropy.(4)In the unloading test of jointed rock mass,the deformation modulus decreases with the increase of unloading quantity,the larger the connectivity of joints,the smaller the deformation modulus of specimens,and the decrease of the deformation modulus increases obviously.The Poisson's ratio increases with the increase of unloading load,and the rate increases.The larger the connectivity of joints,the larger the Poisson's ratio of specimens,and the increase of Poisson's ratio is more obvious.It shows that the greater the connectivity,the faster the deterioration of rock deformation parameters and the higher the deterioration degree of rock mass.(5)According to the deformation parameters of loading and unloading tests at different joint inclination angles(30,60,90),the deformation modulus at inclination 60 is the smallest while Poisson's ratio is the largest,and the deformation modulus at inclination 90 is the largest and Poisson's ratio is the smalle.For the change trend of specimen in unloading process,the modulus of deformation decreases most at inclination 60 degrees,and the Poisson's ratio increases the most,while the modulus of deformation decreases least at inclination 90 degrees,and the Poisson's ratio increases the least.It shows strong anisotropy.The above analysis establishes the relationship between the connectivity of joints and the decrease of cohesion under unloading test,and the relationship between the connectivity of joints and the decrease of deformation modulus under unloading test.Based on the mechanical properties and parameters obtained from the above experiments,and combined with the existing slope rock mass quality assessment method SMR,the XSMR rock mass quality assessment method considering excavation unloading is established. |
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