| Coal resource reserve of depth more than 1000 m of North China coal fied takes a percentage of 53% of the total of resources reserves, focusing on the mining depth of coal-producing regions in the -600 m level and extending down at a rate of 8-12 m percent a year. With deep mining coal named lower group coal, Ordovician karst water, posing a major threat to the lower group coal, become the main aquifer.With mining depth increasing, the water inrush coefficient have greatly exceeded safe limits which have been prescribed in mining norms. How to reduce Ordovician limestone water inrush coefficient to achieve safe mining and to release deep coal becomes the most important issue of deep mining.Taking Yanzhou coalfield as main study area, by means of outcrop survey, surface pumping test, underground drainage test, indoor physical similarity test, numerical model, microstructure analysis, hydrodynamic section zoning features of the top of Ordovician limestone have been analyzed and its filling structure classification, field judging indexes and waterproof threshold given, have also been proposed.The above achievement has been widely used in Yanzhou coalfield and Feicheng coalfield. The main results of this paper are as follows:From the study of geological formation background of paleokarst crust, its hydrogeological significance have been explain clearly. The stratum whose structure surface was filled the same or later period fine particles or chemical precipitate in the top ancient karst weathering crust is named filling zone. The zone is belong to hydrogeological leaching and burial closed karst period and its formation process is influenced by tectonic movement, paleoclimate and paleogeomorphology.From the perspective of the karst dynamics, hydrological dynamic cross-section can be divided into four zones, named vertical zone, alternating with the season zone, horizontal runoff zone, and deep slow flow zone. Based on the overall study of fissure connectivity and filling degree, the boundaries of the filling zone were proposed clearly with the upper limit named residual soil and the lower limit named horizontal runoff. Besides that, the filling is gradually from clay fines ingredient to calcite.Based on these studies, combined with a large number of outcrop statistical survey results, the four macroeconomic indicators evaluating the filling zone, joint opening, filling ration, the structure of surface contact form and continuity factor, have been proposed. And based on the indicators the filling zone structure can be divided into three types, completely filling structure, incompletely filling structure, no filling structure. Combined with a lot of site ground and underground drainage pumping test, project evaluation of filling zone have been proposed, which are permeability coefficient, drilling units inflow q, drilling water inflow Q and their waterproof threshold of filling zone have been also given.Based on permeability tests of remolded soil from filling zone, filling zone permeability is mainly affected by two factors which are porosity and filled content. A two-dimensional diagram of filling zone affected by the above factors can be divided into two zones. Permeability of the first region, the two factors are both sensitive, changes from strong permeability to micro-permeability with a single factor. The above result verified a fact that permeability of the filling zone also will experience the same changes with the structure changing. Permeability of the second region, the two factors are both non-sensitive, maintained at the micro-level.Using micro analysis ways of SEM, microscope image analysis, mercury injection test, two micro void types, pore and fracture, of the filling zone have been proposed. There are three types of pores, including intergranular dissolved pore, intercrystalline dissolved pore and solution pore. There are also two types of microfissure, including dissolved fracture, diagenetic contraction joints. Mercury injection test showed that medium and coarse voids content showed a gradual increasing trend with sample’s depth increasing and the area median also showed a similar rule, which shows gradually increased permeability of Ordovician limestone. This is because the top of Ordovician limestone fissures have been filled and median-coarse pores significantly smaller than the lower part, which is the microscopic evidence for impermeability of the filling zone.In order to make discontinuous fractured media simply being equivalent to continuous and uniform anisotropic medium, three orthogonal homogeneous network model and permeability expression of the filling zone structure, completely filling structure, noncompletely filled, no filling structure, have been established. Combined with the field statistics of fracture geometry parameters, permeability coefficients of three structures have been also calculated. The results show that no filling structure has strongest permeability, incompletely filling structure has moderate permeability and completely filling structure has the weakest permeability, which reveals the mechanism of the completely filling structure can being utilized as a aquifuge. The thermodynamic model of carbonate of the filling zone have been established, which have revealed the mechanism of chemically filling in the view of karst thermodynamics.Based on the above studies, some conclusions can be acquired that completely filling structure can be used directly as aquifuge; Incompletely filling structure can be used as aquitard which can be applied as aquifuge after grouting reconstruction; No filling structure has no different with the underlying Ordovician limestone.The above results have been applied in Ordovician limestone karst water prevention and treatment in Feicheng and Yanzhou mining area and have also achieved good application effect, which provided technical support and new theoretical guarantee for Ordovician limestone karst water prevention treatment technology of deep mining. |
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