Experimental Study On Deterioration Mechanism Of Cretaceous Weakly Cemented Red Sandstone Induced By Freeze-thaw Process During The Construction Of Mine Shafts In Western Area Of China

In western area of China,there are some soft strata,such as Jurassic and Cretaceous strata,which are characterized by late formation,poor cementation ability,low strength and easy to argillization.Meanwhile,there is no stable aquifuge in these strata.Thus,water penetration and shaft flooding accidents are always the major problems during the construction of mine shafts with artificial freezing technology.In this paper,experimental study was conducted on the deterioration mechanism of Cretaceous water-rich weakly cemented red sandstone induced by freeze-thaw process with consideration of the crustal stress during freeze-thaw process.Firstly,the mechanical parameters of Cretaceous weakly cemented red sandstone in freezing and post-thawing states were obtained based on mechanical tests.Also.the deterioration mechanism of weakly cemented red sandstone induced by freeze-thaw process as well as the influence of crustal stress during freeze-thaw process was elaborated.After that,the quantitative rules of micro-pore structure changes induced by freeze-thaw process were presented based on CT scanning and three-dimensional micro-structure reconstruction technology.Then,based on triaxial seepage tests and acoustic emission technology,the changes in seepage-stress coupling characteristics of Cretaceous weakly cemented red sandstone before and after freeze-thaw process were revealed,so as the damage evolution rules of red sandstone.At last,a statistical damage constitutive model of porous rock was established considering the coupling effect of stress and seepage water after freeze-thaw process.The detailed conclusions are as follows:1.The mechanical parameters of Cretaceous weakly cemented red sandstone in freezing and post-thawing states and the deterioration mechanism of weakly cemented red sandstone induced by freeze-thaw process as well as the influence of crustal stress during freeze-thaw process.The Cretaceous weakly cemented red sandstone is characterized as low strength,poor cementation ability,high porosity and sensitive to freeze-thaw process.After freezing at-15?,the strength of red sandstone increases by 124.86%.But after only one freeze-thaw process under the temperature range of-15 ??20 ?,the strength of red sandstone decreases by more than 33.50%.Compared with test results without confining pressure during freeze-thaw process,the existence of confining pressure during freeze-thaw process increases the parameters of frozen red sandstone further,but the parameters decrease further after thawing.The Cretaceous weakly cemented red sandstone consists of numerous pores and cracks with different sizes,meanwhile,its pore and crack network consists of numerous freeze-thaw damage elements.The deterioration caused by freeze-thaw process is a comprehensive effect of multiple damage mechanisms.It is considered that the crustal stress during freeze-thaw process enhances the restraining capacity of pores and cracks so as to make the freezing effect fully develop to the secondary micro-cracks.Due to the further development of the freezing effect,the parameters of frozen red sandstone increase further.Also,the frost heaving force increases and the damage of red sandstone exacerbates.Thus the mechanical parameters of red sandstone decrease further after thawing compared with conditions without crustal stress during freeze-thaw process.2.The quantitative rules of micro-pore structure changes of Cretaceous weakly cemented red sandstone induced by freeze-thaw process.The pore radius of Cretaceous weakly cemented red sandstone mainly falls in the range of 0?370 ?m,with more than 68%in the range of 0?60 ?m.With the increase of confining pressure during freeze-thaw process,the damage of red sandstone increases,e.g.after freeze-thaw process,the increase ratio of porosity increases from 3.88%to 8.53%as confining pressure increases from 0 MPa to 12 MPa.The deterioration caused by freeze-thaw process mainly results in the increase of micro-pores with a radius of 0?40 ?m,among which the proportions of micro-pores with a radius of 0?20 ?m and 20?40 ?m increase by 33%and 24%respectively.3.The changes in seepage-stress coupling characteristics of Cretaceous weakly cemented red sandstone before and after freeze-thaw process.During seepage tests,with the increase of confining pressure during freeze-thaw process,the permeability of Cretaceous weakly cemented red sandstone increases significantly after freeze-thaw process,the increase ratio is beyond 13%.What's more,while the permeability decreases with the increase of confining pressure during seepage test,it increases as seepage pressure.During triaxial seepage loading tests,the stress-strain curves of Cretaceous weakly cemented red sandstone are similar under different confining pressures with different deformation stages.But the permeability of red sandstone has two different evolution modes.When the ratio of confining pressure and uniaxial compression strength is less than 0.5,the permeability corresponds to the deformation stages well,but when the ratio of confining pressure and uniaxial compression strength is beyond 0.5,the permeability has a decrease tendency all the time.When the ratio of confining pressure and uniaxial compression strength is less than 0.5,the permeability decreases before crack initiation stress,after that,the permeability increases slowly,and after crack damage stress,the permeability increases rapidly to the peak permeability.Generally,the peak permeability occurs lagging behind the peak strength.After freeze-thaw process,before crack initiation stress,the permeability increases significantly compared with that before freeze-thaw process,but after crack initiation stress,the permeability increases not obviously,even it can decrease.When the ratio of confining pressure and uniaxial compression strength is beyond 0.5,the confining pressure limits the expansion of vertical cracks and the failure mode of red sandstone trends to be ductile.Due to the formation of locally compaction band,the permeability decreases all the time and the relevance between permeability and deformation stages decreases obviously.4.Damage evolution rules of Cretaceous weakly cemented red sandstone during seepage-stress coupling process before and after freeze-thaw process.Based on AE cumulative energy,the AE signals of Cretaceous weakly cemented red sandstone during triaxial seepage loading tests before and after freeze-thaw process can be divided into four stages,that is,tranquil stage,stable growth stage,outbreak stage and post-peak stage.The damage evolution of red sandstone can also be divided according to these four stages.With the increase of confining pressure,the ultimate damage of red sandstone decreases and the post-peak bearing capacity increases.But after freeze-thaw process,the ultimate damage of red sandstone increases and the post-peak bearing capacity decreases.During loading process,the newborn cracks are mainly micro-cracks.With the increase of confining pressure,AE signals in initial loading stage weaken,and even without AE signals under relatively high confining pressure,but the peak AE energy increases.Due to freeze-thaw process,the red sandstone deteriorates,and the peak AE energy is smaller than that before freeze-thaw process.Before crack initiation stress,the permeability has a negative correlation with AE signals.After that,while the permeability has a positive correlation with AE signals when the ratio of confining pressure and uniaxial compression strength is less than 0.5.it has a negative correlation with AE signals when the ratio of confining pressure and uniaxial compression strength is beyond 0.5.In post-peak stage,the permeability has a negative correlation with AE signals.With the increase of axial load,the newborn cracks of red sandstone trend to be shear cracks.Meanwhile,in relatively high confining pressure and after freeze-thaw process,the proportion of shear cracks in each loading stage increases.On macro scale,with the increase of confining pressure,the angle between the main crack surface and the maximum principle stress increases and the ductility of red sandstone increases.As a result,the permeability around peak strength increases not obviously,even in relatively high confining pressure,the permeability decreases all the time.5.Statistical damage constitutive model of porous rock considering the coupling effect of stress and seepage water after freeze-thaw process.A statistical damage constitutive model of porous rock was established considering the coupling effect of stress and seepage water after freeze-thaw process.The damage in freeze-thaw stage and the damage under seepage-stress coupling effect after freeze-thaw process were considered in the theoretical model.Meanwhile,the accuracy of the theoretical model was verified based on the test data.