Water-resisting Property and Key Technologies of Grouting Reconstruction of the Upper Ordovician Limestone in North China’s Coalfields

Coal is an important energy resource in China. The coal production concerns the national economy and the people’s livelihood as well as the sustainable and stable development of the economic society. The North China Coalfields (NCCs) are important coalfields in China, also the coalfields that have been most seriously threatened by groundwater hazards. The major coal seams in these coalfields are located in the Permo-Carboniferous strata. Because the Lower Carboniferous, Devonian, Silurian and Upper Ordovician formations are generally absent in the coalfields, the Permo-Carboniferous strata overly directly the massive Middle Ordovician limestone. Mining practices in these coal mines demonstrate that the Middle Ordovician limestone is a strong aquifer with very heterogeneous karst fractures and characterized by intense recharge and high water pressure. The Ordovician limestone constitutes the major threatening water source from the coal seam floor. In order to accomplish safe mining of the deep coal resources under water pressure, the research on utilization and grouting reconstruction technology of the TOL is extremely urgent and imperative.

NCCs are important coalfields formed during the Permo-Carboniferous coal-forming period in China. The coalfields are distributed in a large region bounded by Yinshan Mountains in the north, Qingling Mountains in the south, Helan Mountains in the west, and seacoast in the east. They encompass Shanxi, Hebei, Shandong, Henan, Jiangsu, Anhui and Shaanxi provinces. The coal distribution area is over 1.1 million km². This thesis includes on-site investigation in 15 mines in 6 large mining areas in NCCs such as Xingtai, Hancheng, Jincheng, and Yanzhou. Approximately 153 copies of data on different types of boreholes, 84 copies of geological and hydrogeological data, and 37 sets of technical reports were collected. The collected data concerned typical coal mines such as Dongpang and Xingdong mines in the Xingtai mining area, Caocun and Tuanbai mines in the Huoezhou mining area, Chengzhuan and Shihe mines in the Jincheng mining area, Panlong, Sangshuping, Xiangshan and Xinghuo mines in the Hancheng mining area, Baodian, Dongtan, Nantun, Xinglongzhuang mines in the Yanzhou mining area, and Dongjiahe mine in the Chenghe mining area. Tectonic movements, sedimentary environments and karst development characteristics of Ordovician limestone in NCCs were studied in this chapter.

In this chapter, the technical thinking that combines planar division with vertical zoning was adopted to study water-bearing and water-resisting properties of the TOL in NCCs. The comprehensive evaluation method of the water abundance of TOL in coal mining areas was presented. Three vertical zones in the TOL were put forward, which set the foundation for field implement of utilization and grouting reconstruction of the TOL.

The water resistance performance of the weathered and filled zone of the TOL as well as the feasibility of using the zone as an aquifuge were comprehensively studied using laboratory tests, in-situ tests, and spatial distribution features of the regional strata. The research results show that the weathered and filled zone has competent strength and reliable water resistance performance and can be used as a water barrier where the weathered and filled zone is ubiquitous with little influence of tectonic structures.

The research results on the utilizability of the weathered and filled zone of the TOL as an aquifuge, as discussed in Chap. 4, verify that the weathered and filled zone exists in the TOL with good water-resisting performance in some mining areas and can be used as an aquifuge. However, the aquifuge between the coal seam and the Ordovician karst fracture water-bearing interval may not stand the action of the water pressure in the seam floor if the total thickness is not sufficient. It is imperative to conduct additional research on the grouting reconstruction in the water-bearing interval of the TOL. The objective is to transform the water-bearing interval into the water-resisting interval when needed to ensure safe coal mining.

The integral technical system of utilization and grouting reconstruction of the TOL was constructed. The integral system includes optimal formula which uses pulverized coal ash as the grouting material, dispersion properties of grout in karst fractures of the TOL, and surface automatic grouting system. Theoretical analysis, laboratory experiments, and equation derivation were conducted to study the technical system.

The utilization and grouting reconstruction of the TOL were applied to working face #3105 of Sangshuping Coal Mine, Hancheng, China. Safe mining was realized without dewatering, and approximately 450,000 tons of “idle coal reserves” that were once seriously threatened by the Ordovician limestone aquifer were emancipated. The successful case study verified the importance of understanding the water-bearing and water-resisting properties of the TOL and proved the reliability of the proposed integral technical system in the utilization and grouting reconstruction of the TOL.

The thesis put forward new understanding on the water-bearing and water-resisting properties of the TOL based on the field investigation in six large mining areas in NCCs, the study on the paleo-sedimentary environments of the Ordovician strata and karst development characteristics of the TOL in typical mining areas. The technical thought of “plane partition and vertical zonation” was applied to evaluate the lateral water yield property and the vertical zonation of the TOL. The major difficulties related to grouting reconstruction in the TOL were studied from a holistic aspects including grouting material proportion experiments, dispersion mechanisms of grouting materials, grouting system optimization, and development of automation control software. An engineering case study was carried out in Sangshuping Mine of Shaanxi Coal and Chemical Industry Group.