Forming process of subsided column in coalfields of North China

The subsided column formed possibly in runoff and discharge straps of groundwater systems and also nearby second-level syncline axis or turn-off areas or small faults or ring-shaped faults if surface or underground paleo-Karstic caves or gypsum caves provide conditions for strata falling.Forming processes of subsided columns can be divided into several steps which happened during collapse,intermission,dissolve erode,convey,subsidence,fall and so on happend again and again,under three kinds of dynamical action,rock mass gravity,concentration of ground stress and vacuum negative pressure,based on caves developed,groundwater violence flow and water-rock reaction.