Mechanism And Effect Of Disturbance In High Dam Foundation Excavation

In the construction of hydropower engineering,a very effective utilization of rock mass is required,while the utilization of rock mass is closely related to blasting excavation.During blasting,cracks would arise and gradually growth until penetrating.With the excavating contour surface formed,the transient unloading of geostress would be finished.The excavation would produce certain disturbance in rock,including external damage of rock,relaxation of jointed rock.It would finally lead to the degradation of mechanical parameters of rock in disturbed area.Therefore,it is required to investigate the working mechanism and mechanical response of rock foundation under coupled load of blast and transient unloading in excavation.It has significant values in theory and engineering for the disturbed rock in the aspect of relaxation law,mechanical degradation,optimization of excavation design and control of engineering disasters.In this paper,theoretical analysis,numerical simulation and engineering data verifying are used to investigate the disturbance in excavation for rock foundation of high dams.The main achievements are as follows.Based on the six cases about rock utilization in high dam engineering,some analysis have been done which included the use of classification of rock quality,unloading phenomenon in excavation,related reinforcement measures and changes in mechanical parameters after excavation.It has been found that,in the area with high geostress,the rock relaxation at structure surface after unloading and the degradation of mechanical parameters(two typical parameters are deformation modulus and shear strength)are the two main problems in the utilization of high dam foundation.The mechanism of blasting relaxation and the geostress unloading relaxation were analyzed by stress wave model.The initial and final status of jointed rock during transient unloading and quasi-static unloading were analyzed by the energy method.During the blasting excavation of rock mass at the lower elevation of slope,the geostress on the excavating contour surface would decrease along with the growth of blasting crack.When the free face is formed,the transient unloading is achieved.Then the course curves of the coupling effect of transient unloading and blast load can be obtained.Given that,the relaxation mechanism coupling blast and unloading in the area with high geostress can be obtained.The relaxation process of jointed rock was also described in the aspect of stress wave.It was found that the relaxation process is related to loading time,peak load and load path.A computational model was performed for the coupled effect of blast load and transient unloading.Then it was used to analyze the relaxation of jointed rock under the coupled load in the aspect of opening displacements and strain displacements.The results showed that the disturbance of jointed rock in excavation is produced collectively by dynamic blast load and dynamic stress in transient unloading.As to the disturbance from transient unloading,the two key points are unloading time and initial geostress.With a shorter unloading time or larger initial geostress,the disturbance from unloading is larger,otherwise it is smaller.As to the disturbance from blast load,the two key points are the peak load and load-decaying time.With a higher peak load,the disturbance is larger,otherwise it is smaller.The disturbance is not monotone increasing or monotone decreasing with load-decaying time.It is related to the length of jointed rock.The coupled effect of blast load and geostress unloading makes more relaxing displacement than their simple effects.As to the displacements,strain displacements are mainly caused by geostress unloading,while opening displacements are mainly caused by blast load.The jointed rock at forward-slope and counter-slope would be relaxed under the coupling effect of geostress unloading and blast load.The properties of the relaxation were studied by using numerical simulation.After the transient displacement along the loading direction,the jointed rock at forward-slope would become relaxed because of rebound resilience.During the relaxation,the jointed rock may have rotations because the jointed rock at forward-slope is wedge-shaped which would lead to an unbalanced force.The rock rotation is related to angle at forward-slope.As to the jointed rock at counter-slope,it would have a displacement along the direction of free face.With the increase of angle at counter-slope,the area of loading face would decrease.As a result,the jointed rock would rotate during relaxation.P wave could reflect the characteristics of rock mass as it can be influenced by lithology,structural plane and geo-stress.By using dimensional analysis,linear equations for estimating rock mass mechanical parameters(deformation modulus and shear strength)based on the P wave modulus were proposed.F-check method,together with the field tests data of several projects,was used to verify the universality and applicability of these linear equations.Considering the mechanical degradation in excavation and using the Hoek-Brown law,a method based on the P wave modulus was presented to estimate the mechanical parameters of rock mass in the disturbed zones.Therefore,the empirical equations using P wave modulus are feasible for estimating the mechanical parameters of rock mass in disturbed zones.