Study On Mechanical Properties And Constitutive Model Of Weak Cemented Sandstone In Xiaojihan Coal Mine

At present,the focus of coal production in China has gradually shifted to the western regions such as Shanxi,Inner Mongolia,Ningxia and Gansu provinces.Weakly cemented coal strata formed in the Jurassic and Cretaceous are widely distributed in tha above regions,among which weak cemented sandstone is the main component.The mechanical properties of weakly cemented sandstone have an important influence on the control of mine pressure during the pressure of coal mine construction and production.Therefore,it is of great theoretical and practical significance to study the mechanical properties for the failure mechanism of strata and the control of the roof and floor instability.This thesis takes the weakly cemented sandstone from the Xiaojihan coal mine in the Yuheng mining area of northern Shaanxi as the research object.Based on the technical methods of meso-structural test,various mechanical experiments,theoretical analysis and numerical simulation,the meso-structure,mineral composition of the weakly cemented sandstone are analyzed.The mechanical properties and deformation characteristics,as well as the energy evolution law and acoustic emission characteristics are studied.The constitutive model of weakly cemented sandstone is proposed and numerically implemented.The main research contents are as follows:1.The distribution and diagenesis of weakly cemented coal strata are analyzed,and the main diagenetic processes and cementation characteristics of weakly cemented sandstones are systematically studied.The mineral composition and meso-structure of weakly cemented sandstone are studied by polarizing microscope,XRD(X-ray diffraction)Diffraction analysis,SEM(scanning electron microscopy)and other mesoscopic methods,and then the main composition of mineral particles,types of cementations and skeleton structure between mineral particles and cements are studied.The above characteristics have a significant effect on the mechanical properties of weakly cemented sandstone.2.Based on the basic mechanical properties of weakly cemented sandstones,a method for determining the weak cementation characteristics of sandstones by "axial strain method"is proposed.According to the analysis of the length of compacted section in the stress-strain curve of weakly cemented coarse sandstone and medium sandstone,and compared with other types of sandstone,the characteristics of the length of compacted section in the stress-strain curve of weakly cemented sandstone form Xiaojihan coal mine under uniaxial compression condition are analyzed.A method for identifying weakly cemented sandstone is proposed based on the characteristic of "high proportion of strain in compaction stage to pre-peak strain".Furthermore,the effects of different confining pressures on the compaction section in the stress-strain curve of weakly cemented sandstone is studied.3.The uniaxial cyclic loading tests of dry and saturated weakly cemented sandstone specimens are carried out to study the mechanical response during cyclic loading,and the weakening mechanism of water-rock interaction on weakly cemented sandstones is analyzed.The evolution law of elastic strain energy and dissipative energy in dry and saturated weakly cemented sandstone are studied.The characteristics of energy accumulation and dissipation,the internal mechanism between energy and damage,and the sensitivity of weakly cemented sandstone to water action during rock damage and failure under dry and saturated conditions are studied from the viewpoint of energy.4.The uniaxial compression acoustic emission(AE)experiment of weakly cemented sandstone is carried out.The characteristics of AE count,energy rate curve and the spatiotemporal evolution of AE events are studied,and the influence of bedding dip(0°,45°and 90°)on the characteristics of AE and the spatiotemporal distribution of AE events is analyzed;Uniaxial AE experiments are carried on dry and saturated weakly cemented sandstones under cyclic loading.The effects of water-rock interaction on AE characteristics are studied,and the AE precursor characteristics before failure of weakly cemented sandstones were analyzed based on load-unload response ratio theory.5.Based on the deformation characteristics of weakly cemented sandstone,a segmentation constitutive model of "two parts Hook’s model-statistical damage" for weakly cemented sandstone is constructed.The digital image technique and statistical method are used to analyze the gray distribution characteristics of CT scanned digital images of weak cemented sandstone,and the rock homogeneity m based on gray characteristics of images is obtained.On this basis,the proposed constitutive model is validated by finite difference method,and the stress-strain curve is obtained,which is in good agreement with the experimental results.Finally,the characteristics of stress field and plastic zone during the failure of weakly cemented sandstone are analyzed.