Numerical Simulation And Inversion Analysis Of Unsteady Seepage In Underground Caverns

Groundwater seepage is inevitable during the construction and operation of underground powerhouses of hydropower stations.Groundwater can cause seepage failure in the surrounding rocks of caverns,subsequently resulting in problems of seepage stability,and it brings hidden dangers to the safety and stability of the projects.Due to the complexity and instability of seepage boundaries,the underground seepage field is often in an unstable state.Therefore,studying the law of unsteady seepage and analyzing the stability of the surrounding rock under the effect of unsteady seepage is of great significance.Based on the theory of unsteady seepage,this thesis analyzed the evolution of the unsteady seepage field in the excavation process of underground caverns.Additionally,the improved particle swarm optimization algorithm was applied to the three-dimensional unsteady seepage inversion.Finally,Tianhuangping Pumped Storage Power Station was used as an example,and corresponding unsteady seepage inversion analysis was carried out to further evaluate the long-term seepage stability.Specifically,the main contents are as follows:(1)Based on the variational inequality theory of unsteady seepage,the variable bandwidth iterative method was proposed to calculate the overflow surface,and the corresponding calculation program was compiled.The correctness of the program was verified by the classical drainage sand trap test.(2)The variation characteristics of seepage boundaries in the excavation process of underground cavern were analyzed.The calculation method of unsteady seepage field in the excavation process was proposed,and subsequently verified by the simulation of underground seepage fields during tunnel excavation.On this basis,the method of applying unsteady seepage load to the surrounding rock of the cavern is proposed.Taking the underground powerhouse of Azade Hydropower Station as an example,the calculation of the unsteady seepage field in the excavation process was carried out,and the stability of the excavation under the seepage load was analyzed.The results showed that the free surface of the groundwater gradually decreased during the excavation of the cavern,and the flow rate of the cavern increased gradually.In addition,although the seepage load has an adverse effect on the stability of the cavern,the impact is not remarkable,mainly due to the relatively good rock mass quality and the small amount of seepage load.(3)Based on the biological evolution principle,four strategies,grouping,optimization,improvement of flight mode and probability control,were used to improve the basic particle swarm optimization algorithm and enhance the global searching and local exploring ability.Further,unsteady seepage inversion calculation program was compiled.By solving several complex functions,the effectiveness of the improved particle swarm optimization algorithm was verified.Then,the efficiency of the inversion program was proved by a simple finite element calculation model.The results showed that the improved particle swarm optimization algorithm is far better than the traditional particle swarm optimization algorithm in search speed and precision,and the inversion program can be applied to the inversion analysis of unsteady seepage field correctly.(4)The emptying period of the Tianhuangping Pumped Storage Power Station was used as an example for unsteady seepage inversion analysis.The monitoring data of the reservoir emptying period was used to analyze the permeability characteristics of the underground rock mass.By analyzing the variation trend of groundwater,the results showed that there is a certain degree of seepage hysteresis effect in different parts of the reservoir during the emptying period.Meanwhile,the underground powerhouse can maintain seepage stability under normal operation and reservoir emptying conditions.Seepage pressure in the inclined shaft area is consistent with the change of the upper reservoir water level,seepage pressure in the drainage corridor area lags for half a month approximately,and that in the underground powerhouse area lags about a month.This paper provides a calculation method for the simulation of the seepage field of underground cavern excavation and the stability analysis of surrounding rock considering seepage load.It also provides a way of thinking for the determination of geotechnical permeability parameters in the process of unsteady seepage.Finally,it provides guidance for analyzing long-term seepage stability of underground caverns.