Study On Seismic Capability And Analysis Method Of Underground Subway Structures

With the rapid development of the global economic construction, the utilization of the underground space has become a significant research topic in academic field. More and more seismic damages suggest that the underground structures are not necessarily safe, and are even severely damaged in the earthquake. The M8.1 Mexico Earthquake in 1985, the M7.2 Kobe Earthquake in Japan in 1995 and the Wenchuan M8.0 Earthquake in China all caused damages of large number of underground structures. The research of stability of underground structures under earthquake action is one of the hot topics in geotechnical engineering researches, and embodies essential theoretical and practical significance. The subway is an important transportation means in modern cities and is relatively densely populated underground space; therefore the government and society show great concern for its safety and stability. This dissertation studies the subway underground structure, systematically summarizes the main methods of the anti-seismic researches of underground structures and also presents the problems and developing direction; it introduces in detail the basic theory, function and its utilization in the geotechnical engineering seismic resistance of the FLAC3D ;It studies the seismic performance, seismic damage mechanism and practical anti-seismic analytical methods of subway underground structure, the main research achievements are as follows:1. The dissertation applies the constitutive model of concrete damage into the analysis of seismic resistance of underground structures. Based on the concrete damage constitutive theory, it adopts the damage variables determined by evolution equation of concrete damage that is proposed by Mazars, on the basis of D-P constitutive model, it puts forward a damage constitutive model that can consider the comparative derogation of material strength and stiffness after the concrete damage. Based on the user redevelopment environment provided by FLAC3D, it develops the constitutive model using VC++, finally through the numerical simulation test it compares the difference between this model and the D-P model, and it shows the advantages of this damage constitutive model. Using this model, it analyses the damage mechanism of Daikai subway station, and the result well agrees the actual seismic damage, which therefore to some extent verifies the applicability of this damage constitutive model.2. By use of the model developed in this dissertation it successfully simulates the earthquake destruction process of the Daikai subway station and interprets the damage mechanism. The dissertation introduces the general situation of seismic damage in the Dakai subway station, on the basis of FLAC3D it describes the constitutive characteristics of the station by use of concrete damage constitutive model developed in this dissertation, and it analyses the characteristics of dynamic response and the damage process of the station. It establishes an interface element between the structure of the subway station and the surrounding soil layers, and it well simulates the contact, separation and sliding between the structure and the soil in the seismic process. It recognizes that the vertical ground motion increases the vertical stress maximum endured by the underground structure, accelerates the damage of the station interior column, which is the important element of the station damage. The interior column is the anti-seismic weak link in the subway station structure, and the damage mechanism can be explained as under the action of the horizontal and vertical ground motion, the damage of the station structure begins with the interior column, then the damaged column gradually loses the ability of supporting the upper load, and thus the upper plate of the station structure endures great upper soil pressure, the plastic failure takes place between the upper plate and the sidewall, and finally the upper plate and the whole structure collapses.3. It systematically studies the vibration characteristics and the influencing elements of underground structures, summarizes the basic features of the vibration of underground structures, compares the dynamic response characteristics of the underground and ground structures, recognizes that the restraining action makes the vibration characteristics of the underground and ground structures completely different; it specially researches the dynamic soil pressure endured by the underground structure and relative displacement situation of the structure and the soil layers in the earthquake, and discovers changing rules of the dynamic soil pressure and relative displacement of underground structures; It studies the vibration characteristics of underground structures under the action of vertical ground motion, discovers that the vertical stress in the upper and lower plates of the underground structure is much larger than that in the relative depth of the soil; It studies the influences of the overlying soil thickness on the horizontal displacement difference of the upper and lower plates of underground structures, and the result shows that when the thickness of the overlying soil exceeds that of the underground structure, the structural horizontal displacement difference shows linear increasing rules with the thickness of the overlying soil layers; finally it analyzes the influences of the existence of underground structures on the horizontal and vertical acceleration response of the surrounding sites.4. It proposes a practical anti-seismic analytical method-- equivalent lateral displacement stiffness ratio that suits the solution for the upper and lower horizontal displacement difference peak of the underground structures under the action of shear wave. It puts forward three different correction methods of displacement stiffness according to different simplified schemes of displacement stiffness, and they are rigid fixing modified mothod.hinged modified mothod. translation modified mothod. respectively. The application of equivalent lateral displacement stiffness ratio method requires the solution of the horizontal displacement difference peak of the free field in the corresponding depth of underground structure, then it multiplies the interaction coefficient ,and the horizontal displacement difference peak of the upper and lower plates of the underground structures can be obtained. Finally through the comparison between this method and the numerical simulation analytical method under different condition, it verifies the applicability of this method.