Features And Causes Of The Extremely Water Softening Of The Xishanyao Formations At The Dananhu No.5 Coal Mine,Xinjiang,China

Xinjiang is an important strategic energy reserve area and energy base in China.The coal resources development of Dananhu Jurassic coalfield in Turpan-Hami region has been for nearly ten years.Xishanyao Formation of Middle Jurassic is the main coalbearing strata in Dananhu Jurassic coalfield.However,taking No.5 Coal Mine in Dananhu Coalfield as a typical representative,the self-developed coal mines in the coal field have been unable to get rid of the problems of water expansion and argillization of roof and floor in both roadway excavation and coal production,and have always been unable to solve the geological disasters such as serious deformation of roadway and pressure of support.According to the exploration data of coal resources in Xinjiang,the Zhundong coalfield and the Santanghu coalfield with larger resource reserves on the north side of the Dananhu coalfield have similar sedimentary environments and will also face similar disasters.Therefore,taking Dananhu No.5 Mine as an example,it is of great basic research value and engineering guiding significance to study the causes and disaster-causing mechanism of extremely weak strata in Xishanyao Formation.In order to study the weak degree of Xishanyao Formation in No.5 Coal Mine and the mechanism of water disaster,107 groups of samples were taken by using 4boreholes as coring boreholes to complete rock physical and mechanical properties test,rock thin section identification and X-ray diffraction clay mineral component analysis.At the same time,based on the physical and mechanical tests and logging results of 30 engineering geological holes in the exploration stage of No.5 coal mine,the petrological microscopic characteristics,sedimentary facies characteristics and water softening mechanism of the extremely soft strata of Xishanyao Formation in No.5 coal mine were studied by means of petrological observation,lithofacies paleogeographic analysis,mine hydrogeological conceptual model establishment and stope stress-deformation simulation.The main conclusions are as follows :(1)The extremely weak characteristics of Xishanyao Formation are characterized by low natural strength,rapid disintegration and softening and loss of strength after encountering water.The rock integrity is mainly broken or more broken,and only a small part of the relatively complete formation exists.The rock hardness of all strata is extremely soft rock,and the rock mass quality grade is V grade.(2)The diagenetic degree of Xishanyao Formation is low,mainly mechanical compaction.The main microscopic petrological evidence : First,clastic rock particles are mainly point-line contact with low compaction,and there are many macropores;second,the rock contains little or no cement,and the clay minerals are mainly filled in the particle pores in the form of matrix.(3)The clastics of Xishanyao Formation are mainly derived from the physical denudation of the parent rock area.The sedimentary environment has the characteristics of large topographic fluctuation,short transportation distance,fast accumulation speed and short weathering time.The characteristics of mechanical deposition and compression diagenesis are obvious.(4)The diagenetic degree of Xishanyao Formation is poor,the content of clay minerals in the rock is high,and the illite-montmorillonite mixed layer clay is the main.The characteristic that the Yimeng mixed layer is easy to expand after encountering water is the main internal reason for the extremely weak characteristics of Xishanyao Formation.The water-conducting fracture zone of the roof of No.18 coal seam leads to the overlying III-1 aquifer,which is the external cause of the mudding and disaster of the roof and floor of the roadway.(5)The extremely weak causes and engineering disaster-causing mechanism of Xishanyao Formation : Roof water seeps into the stope along the mining fissures,and the illite-smectite mixed layer in the roof and floor strata with low diagenesis has a strong hydrochemical effect on the main clay minerals,forming a strong clay expansion force,destroying the original weakly cemented rock structure,resulting in rapid argillization of the roof and floor,loss of strength and mining engineering disaster.It is further summarized as : low degree of diagenesis,less cement,more mixed base of illite and montmorillonite,roof water infiltration and mining induced fracture.The mechanism research provides basic research support for the development of hydrological and engineering mixed disaster management schemes for extremely weak strata in the coal mines and Zhundong area of Dananhu,Xinjiang.