Research On Structure Characteristics And Mechanical Properties Of Rockmass For Underground Water-Sealed Oil Storage Caverns

Underground water-sealed oil storage cavern is an important way of strategic petroleum reserve in the world today.However,the construction of underground water-sealed cavern is in its infancy,facing many difficulties caused by the safe and rapid construction of super-large-section caverns and the design of water curtain systems.Discontinuity is a key factor for guaranteeing the effectiveness of water-sealed system and the cavern stability.Therefore,study on the structure characteristics and mechanical properties of rockmass has the important theory significance and engineering application values.Taking one large underground water-sealed oil storage facility as engineering background,the paper conducts the systematic studies on the method for the rockmass classification and mechanical parameter estimation,the identification of rock discontinuity sets and fracture network construction technology,geometrical connectivity and scale effect of rockmass structure,groundwater seepage characteristics and mechanical effects during the construction process of caverns.The main research contents are as follow:(1)On the basis of engineering geological survey with digit recognition technique of discontinuity photogrammetry,the distribution characteristics of discontinuities in typical sites of cavern were accurately obtained.The rockmass quality evaluation was conducted by adopting a combination of RMR,Q and GSI,whose difference and correlation were also analyzed.The differences among a series of empirical formulas estimating the rockmass strength and deformation modulus related to the rockmass classifications were discussed and then the reliability method of estimating the mechanical parameters was proposed.(2)For the conventional K-means clustering algorithm,this method is sensitive to the initial cluster centers and is easily trapped in a local optimum.Therefore,a modified affinity propagation algorithm was first proposed for identifying rock discontinuity sets based on discontinuity orientations.The discontinuity sets in an underground water-sealed oil storage cavern were successfully identified.The research results show that this algorithm can achieve good clustering results with a faster convergence speed and higher robust than those of Shanley and Mahtab method,FCM method,the spectral clustering method,K-means method based on particle swarm optimization(KPSO).On this basis,the statistical distributions of some parameters of discontinuities were then obtained.Based on the Monte Carlo method,the visualization of three dimension fracture network were realized by adopting the MATLAB programing.(3)According to the fracture network construing technology,the studies on the geometrical characteristics and size scale of rockmass structure were conducted based on the line of intersections among fracture disks.The change law of the connectivity parameters with the different rockmass sizes was statistically analyzed and the effects of fracture orientation and size on the fracture connectivity were discussed.Based on the modified depth first search algorithm and Dijkstra algorithm,the three dimensional connectivity path and the shortest path were obtained.Based on the connectivity parameters,i.e.the structural plane area per unit volume,the number of intersection per unit volume and the intersection length per unit volume and the fractal dimension of fracture structures,the structural representative elementary volume(REV)was studied.(4)Based on the discrete medium theory,the study on the scale effect and anisotropy of the equivalent permeability tensor of the fractured rockmass was conducted and thus the seepage REV and the corresponding equivalent permeability tensor were obtained.Then,the continuous medium numerical simulation was adopted to study the anisotropic seepage characteristics of underground water-sealed cavern and the underground water inflow of caverns was calculated.These research results are helpful to further understand the water-sealed mechanism.(5)The study on the spatial effects of construction behavior of two adjacent caverns with three bench excavation under solid-fluid interaction was carried out,and the evolutions of principal stress and deformation with the longitudinal excavation during the construction were obtained.The interaction of the two caverns was also studied.These results provide some theory foundations for the cavern support design under the action of the water curtain system.