| In recent years, with the further rapid development of economy in city and countryside, the amount of engineering construction and scope of human activities are increasing. Then it is inevitable to build roads, plants and structures in district with karst. The problem of instability of karst roof frequently occurs during construction and a big safety coefficient is adopted to confirm the safety of structures. However, the ultimate bearing capacity under all kinds of probabilistic geology patterns is not taken into account, which probably brings about numerous loss even serious safety misadventure.As we know, the thickness of karst roof is a crucial factor to the stability of karst. So it is essential for us to study the thickness of karst roof in projects. On the basis of previous research, the main research contents and results are as follows:(1) Having collected project cases and researched success experience and engineering accidents over decades in districts with karst. Some practical measurements for karst problems are summed up and the concept of safe thickness of karst roof is put forward. Besides, the patterns of karst are divided into three probabilistic geology forms, under which the mechanism of cause of instability of karst is analyzed.(2) Corresponding mechanical constitutive patterns are established according to safe thickness under probabilistic geology patterns and the relationship between ultimate bearing capacity and its influence factors. In addition, the software FLAC 3D that is based on the method of finite difference is employed to conduct numerical simulation. Using the market of plastic zone, the market of deformation, the market of shear strain increment as three characteristics, the ultimate bearing capacity of karst under all probabilistic geology patterns and the principle of judging stability of karst roof are confirmed after calculation and analysis.(3) The relationships between ultimate bearing capacity and its different influence factors such as the size of loading zone, physical and mechanical parameters of rock roof, span and height of karst, thickness of soil roof, are explored. The influence relationships of the influence of each factor on the safe thickness of karst roof are found and the influencing trends of each factor are confirmed.(4) The problem of instability of thin roof in karst zone is selectively analyzed in the case of uneven settlement occurred in commodity house with 37 stories in karst district in Qinyuan City. The main causes of the roof being instable are karst that is not discovered before, a bit thin karst roof, long span, and incompact sand used as filler. So the above mentioned case is a part of cases whose roof pattern is soil texture plus rock texture, which is same as the conclusion of the theoretical analysis above, proving the validity of theory. |
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