Experimental Research And Theoretical Analysis Of The Time-depedent Splitting Phenomenon Of The Deep Caverns

With national support to the development of water resources and hydro-power project, the construction of hydro-power station in southwest China are being carried out in full wings. During the construction of the hydro-power station, due to the more high mountains in the southwest region, the underground cavern unearthed for construction was at high temperature, high crustal stress, high osmotic pressure and excavation disturbance, i.e. "Three High and One Disturbance" which means an extremely severe geological condition. Therefore, some distinctive damage modes which were similar to the partition damage, strong flow and large deformation, etc. occurred in the deep cavern. Among these damage modes, the time-depedent splitting attracts more attention. This splitting is a distinctive engineering geological phenomenon caused by cavern wall rocks when they are under higher crustal stress. Its influence mainly includes reducing the durability of the bolt of cavern, causing water seepage, even undermining the stability and bearing capacity of crane beams. Hence, a considerable number of experts and scholars at home and abroad have done a lot of systematic research on the time-depedent splitting phenomenon but they still can’t reach an agreed conclusion on its formation mechanism.Based on a National Natural Science Foundation Project named "Damage evolution mechanism research of time-dependent splitting failure induced by excavation unloading in high-side wall", under the engineering background of underground caverns of Jinping Hydro-power Station Class I, this article at first analyzes the causes of the time-depedent splitting failure and factors about time-depedent damage degree by doing indoor geo mechanical model tests under four kinds of working condition. Then we do theoretical analysis combining with fracture and damage mechanics, rock rheological mechanics, rheological element model, etc. to establish a model which can reflect the time-depedent phenomenon of wall rock and get corresponding constitutive model. The main research contents of this article are as follows:(1)We did geo mechanical model tests under four kinds of working condition in laboratory to research the influence of different stress condition to the time-depedent splitting phenomenon in wall rocks in deep cavern and to get the generating conditions of this phenomenon and the influencing factors in degree. And we recorded the lateral strain of wall rocks changing with time and compared and analyzed the differences of strain and time variation curves and generating factors under three kinds of working condition.(2)Then we did theoretical analysis combining with fracture and damage mechanics, rock rheological mechanics, rheological element model, etc. First, we analyzed the changing process of wall rock stress in the process of the deep cavern from beginning to unearth to unloading to achieve stability and then described the whole process of wall rock splitting cracks from beginning to split to ending with stabilizing at a fixed value or son and last according to some stages of wall rock cracks appearing and developing, used different rheological elements to characterize different stages of the time-depedent splitting and connected the elements to establish the model of describing the whole process of wall rock time-depedent splitting phenomenon. In this process, the concept of split plastic model was put forward creatively and was used to characterize wall rock splitting phenomenon and cracks splitting process at first time. Then we got the constitutive equations to characterize this phenomenon by analyzing the models we just established.(3)We used nonlinear fitting software 1stOpt to identify the parameters of constitutive equations we got earlier. Comparing theoretical curve of the equations to practical curve by fitting the equations and the data obtained from tests for lateral strain changing with time, we could find that the trend of the two curves are roughly the same. The rock parameters identified finally by software were broadly in line with actual situations and the fitting results were rather accurate.