Mechanism And Redundancy Study Of Some Collapse Problems In Underground Engineering

In the process of urbanization, the use of underground space becomes a trend.However, due to the inherent complexity of underground engineering, engineeringaccidents, such as collapses of deep excavation and excavation face of tunnel, etc.,occurred frequently. However, few studies have been conducted on the evolutionmechanism, simulation method and control theory of those collapse problems, andthis limits the development of underground engineering. In this thesis, two kinds oflarge deformation simulation methods, i.e. Discrete Element Method (DEM) andMaterial Point Method (MPM) were applied to the simulations of some collapseproblems in underground engineering to study the collapse mechanism, and theconcept and design methodology of redundancy is raised to control the progressivecollapse problems in underground engineering. Main contents of this thesis are listedbelow.(1)Considering the catastrophic sequence of collapse of deep excavation, it isnecessary to introduce the concept of redundancy into the design of retaining structureand develop the design methodology based on redundancy. In this paper, redundancyof deep excavation retaining structure is classified into five aspects. Necessity andimportance of each aspect are explained. Two typical case histories are studied toreveal the redundancy problems that may exist. Finally, a series of measures areproposed to increase the redundancy of deep excavation retaining structure.(2)Ring beam supporting structure was adopted as representative example, anddifferent plan configurations of this kind of structure were designed. Further on,progressive collapses of partially damaged supporting structures were simulated usingDEM. A kind of new redundancy index was proposed, and the redundancy in terms ofsystem redundancy index of two ring beam supporting systems were analyzed andcompared in detail. The research results show that the number of load paths is animportant factor that can influence redundancy of retaining structure obviously.(3)The influence of prop connection on the retaining structure redundancy wasselected as an example of redundancy research. The failure processes of twoexcavations with different types of prop connections were simulated by DEM, and thephenomena of progressive collapse in them were compared and analyzed. It isindicated that improvement of the retaining structure redundancy can effectively resist the progressive collapse in deep excavation and then the evolution of the local damageto entire collapse can be avoided.(4)A large strain simulation method MPM is introduced into the research ofexcavation stability, and in conjunction with FEM, the failure process and the shearslip surface propagation of a two-stage excavation project were analyzed from bothsmall deformation and large deformation aspects. Some conclusions about thedeformation, stability, failure evolution, and cooperative work mechanism ofnon-prop and multi-stage retaining structure are obtained. These results lay afoundation for further systematic analysis of this new support method.(5)The traditional approach to assess the risk of tunnel collapse is to evaluate thefactor of safety against failure. However, such analysis only gives whether the tunnelwill collapse or not and does not provide information on the magnitude of thepost-failure behaviour (for example, catastrophic or progressive) if tunnel collapseoccurs. In this study, MPM was used to investigate the post-failure behavior of tunnelheading collapse in two dimensional plane strain conditions. The capability andaccuracy of MPM were verified by comparing the results to centrifuge test data andanalytical solutions from limit state methods. MPM simulations were conducted withdifferent soil conditions (clay or sand) and profiles (homogenous or linear increasingstrength) as well as different tunnel geometries (i.e. tunnel depth and unlined length).The differences in post-failure behavior and mechanisms are examined and reported.