| The freezing method is the most important drilling method in deep and complex rock and soil layers,while frozen wall technology is one of its core technologies.How to scientifically determine mechanical parameters of artificial frozen soil under in-situ frozen condition is a major problem in the design of frozen wall in ultra-deep soil layers.In view of the lack of systematic and in-depth researches on mechanical properties of in-situ frozen sand under high pressure and mechanisms of strength change under high pressure,in this thesis,in-situ strength characteristics of artificial frozen saturated sand under high pressure with the influence of different factors are studied by using the methods of sample test,theoretical analysis,and numerical simulation,and the mechanism of influence of sand particle breakage on strength is revealed.First,the influence of the background stress field of the specimen during consolidation and freezing on the test results is analyzed for the established mechanical test methods for deep frozen soils;based on the analysis of size effect of frozen sand samples,a triaxial instrument for in-situ mechanical property test of frozen sand under high pressure applicable to high-resolution CT scan observation was developed;in-situ triaxial compression tests of frozen saturated sand under the influence of pressure at freezing 0≤σf≤40 MPa,confining pressure 0≤σ3≤55 MPa,pore water pressure 0≤σb≤10 MPa and strain loading rate 0.05≤εL≤1 mm·min-1 were carried out.The study found that:Unlike unpressured frozen sandy soils,pressured frozen sand have basically no strength reduction phenomenon under high pressure and only show a slowdown in the rate of strength growth,which is influenced by σf.As σ3 increases,the variability in strength due to σf is progressively more pronounced,with the maximum increase in strength up to 43%when σf ≤40 MPa.The strength is almost unaffected when σf>10 MPa,σb≤3 MPa,and then decreases with increasing σb.The strength increases with εL in a hyperbolic pattern,when εL>0.1 mm/min,the strength remains basically constant.The effects of the three factors σb、σ3 and σf on the initial elastic modulus ranged from strong to weak,and both the initial elastic modulus and the cut-line elastic modulus showed fluctuating growth with σf.When σf≤20 MPa,the maximum increase in cohesion with σf is 8%and the angle of internal friction increases from 20.24° to 25.59°,then remained largely unchanged.It can be seen that σb mainly affects the effective consolidation stress,and the effects of σf and εL on strength are mainly reflected through frictional strength and cohesion,respectively.Secondly,using the Shanghai Synchrotron Radiation Light Source and industrial CT,5.2 μm high-resolution CT dynamic scan of the in situ triaxial test of permafrost under high perimeter pressure of 45 MPa were successfully carried out,and obtained the breakage pattern of sand grains under different σf and σ3 by three-dimensional reconstruction.For the optical microscopic observation results,an efficient particle edge detection procedure was developed to achieve automatic threshold segmentation and fast size extraction of tiny particles in optical images.It was found that:the process of abrasion zone development in sand fissures was observed,and pressure melting and crushing of ice under high pressure were not observed,when σ3>20 MPa,the sand grains showed more obvious crushing phenomena such as penetration fractures and edge fragmentation and abrasion.The breakage of frozen in-situ sand with pressure is significantly higher than that of frozen sand without pressure under the same confining pressure.When σf ≤20 MPa,the value of relative crushing rate Br increases with the increase of σ3,the opposite behavior when σf>20 MPa.Br is positively correlated with σ3,σf and |σ3-σf|,the combinationσf-σ3 with the most sand breakage is 40 MPa-10 MPa,followed by 30 MPa-50 MPa.Third,transforming the effects of σf and σ3 into effects on crushing dissipation energy Eb and particle friction factor M,With the introduction of pore ice crushing energy dissipation,an energy equation for frozen sand that can take into account the effect of sand grain crushing is established.The calculations revealed that:In triaxial compression tests of sand frozen in situ under pressure,the work done by volume and shear forces is mainly transformed between the elastic energy,frictional dissipation energy,Eb and shear expansion dissipation energy of the specimen,the energy is mainly transfered between the sand grains after entering the yielding phase.The increase in friction factor due to σf counteracts the weakening of the strength due to particle crushing,so that high pressure freezing attenuates the further crushing of the sand particles during the loading phase.Eb is proportional to |σ3-σf| and grows non-linearly with increasing σf andσ3,the growth rate of Eb is maximum when 20≤σ3≤30 MPa.The growth rate of Eb of pressurized frozen sand is larger than that of unpressurized frozen sand under the same confining pressure,which is the main reason for the weakening of strength of pressurized frozen sand under high confining pressure than that of unpressurized frozen sand.Combined with the analysis of the results of the crushing test before loading the specimens,it was found that the strength weakness of the sand with pressure freezing depends on the combination of σf and σ3 values,weakening of frozen sand due to compression and thawing requires the involvement of particle breakage.At-20℃ temperature,the weakening of strength mainly originates from the damage of soil skeleton due to in-situ fragmentation of sand grains before shear loading,and the degree of strength weakening is related to σ3,σf and |σ3-σf|(stage and degree of breakage).Finally,based on the CT reconstruction technique,a three-dimensional flexible servo discrete element model considering the real ice-sand system,particle breakage and the three-phase contact action of sand-ice-flexible film is established.Triaxial compression test simulation of sand crushable model and sand non-crushable model under different confining pressures was carried out by using the proposed computational model which is closer to the actual working conditions.The response law of model strength and force-deformation characteristics to particle crushing,as well as the influence law of particle crushing on the fine structure of the model and the mechanical characteristics of particle contact are obtained.It was found that using randomly generated ice particles smaller than one-seventh the size of sand particles in modeling can improve the homogeneity of the model,compared to rigid side limits,the use of flexible servo films can better reproduce the deformation and shear damage of the specimen.Comparing the numerical simulation results of particle crushable frozen sand(Clsuter+Ball)and particle non-crushable frozen sand(Clump+Ball)under different confining pressures,it is found that the weakening of frozen sand strength under high confining pressure is accompanied by in-situ crushing of sand particles,the weakening effect of sand breakage on strength under high confining pressure is confirmed.The increase of broken sand particles leads to the change of particle coordination number and contact force size and distribution morphology,which is also the main reason for the difference of the model bearing capacity and damage morphology under different confining pressure.The research results of this thesis are of great value to the understanding of the mechanical properties of in-situ frozen artificial permafrost in ultra-deep soils,and can be used as a reference basis for the design and calculation of frozen walls in ultra-deep soils,and also contribute to the development of deep permafrost mechanics.There are 115 figures,24 tables and 202 references. |
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