Study On Mechanical Properties Damage Effect And Constitutive Model Of Jointed Sandstone Under Dry-wet Cycles

The rainfall,rising and falling of the underground water and the reservoir level might give rise to the alternate dry and wet for the rock and soil mass in a long time.This long-term dry-wet cycles will inevitably lead to the mechanical properties deterioration of rock and soil mass structure,and finally cause a series of geological disasters.The Three Gorges Reservoir has formed riparian zones that is seriously affected by the dry-wet cycles because of its operation.Due to the combined effects of external loads,joints,and dry-wet cycles,the rock mass in the riparian zone will failure ultimately.Therefore,it is great theoretical and practical value to deeply explore the dry-wet cycle mechanical properties damage effect of the rock mass in the riparian zones,reveal the mesoscopic damage mechanism of the rock mass,and establish a macro-mesoscopic coupled damage constitutive model of the jointed rock mass under dry-wet cycles.As a result,the stability of rock mass on the bank slope can be improved,at the same time,the geological disaster in reservoir area can be controlled.To clarify the macro and micro damage mechanism of the jointed rock mass under dry-wet cycling conditions,a series of systemic studies were conducted on the bases of General projects of NSFC:study on macro-mesoscopic damage mechanism and constitutive model of jointed rock mass under dry-wet cycles.In this paper,the mechanical properties deterioration mechanism of rock mass by the dry-wet cycles was investigated based on the laboratory test(macro and meso),numerical simulation,and theoretical analysis(rock mechanics,damage mechanics,and linear elastic fracture mechanics).Then the macro mechanical characteristics,damage effect,failure model,strain field and acoustic emission characteristics of damage evolution,fracture criterion,energy evolution mechanism,micro damage evolution mechanism,macro-mesoscopic coupling damage variables and damage constitutive model of jointed sandstone under dry-wet cycles were reveal.The main contents and conclusions are as follows:(1)The macro mechanical behaviour of jointed sandstone under dry-wet cycles were studied through the indoor dry-wet cycles test on jointed rock mass.The obtained results show that the plastic deformation of the jointed sandstone is not obvious at the initial stage of dry-wet cycles.However,when the number of dry-wet cycle reaches a certain value,the plastic deformation of the jointed sandstone becomes obvious.In addition,the post-peak failure of jointed sandstone exhibits a brittle feature in the experiment.With the increase of the number of wet-dry cycles,the rock ejection of jointed rock during the failure process becomes less violent,and the cracking noise is gradually reduced.Besides,with the increase the number of dry-wet cycles,the tensile strength,compressive strength and elastic modulus of the jointed sandstone shows a law of logarithmic function decreasing.The strength envelope for the Mohr-Coulomb and Hoek-Brown criterion gradually decreases with the increase of the number of dry-wet cycles,and the reduction rate for the intact rock specimen is more than that for the jointed rock specimen.(2)By using acoustic and optical analysis methods,the crack evolution and failure characteristics of jointed sandstone after dry-wet cycles were quantitatively investigated.Subsequently,a fracture criterion for jointed sandstone under dry-wet cycling conditions was established.The obtained results show that the dry-wet cycles have an important influence the failure mode of jointed sandstone.In detail,with the increase of the number of dry-wet cycles,the failure mode of jointed rock specimens changes from cracking to the combined failure of cracking and buckling and spalling with different forms.Especially,the degree of buckling and spalling of the specimen is continuously increased.After the dry-wet cycles,acoustic emission events of jointed rock specimens can be divided into four phases:initial active period,stable active period,exponential active period,and peak active period.In addition,the activity of acoustic emission becomes much less active.With the increase of the number of dry-wet cycles,the proportion of small acoustic emission events in jointed rock samples gradually increases.It is also found that the fracture toughness calculated by fracture criterion considering the dry-wet cycling is consistent with the fracture testing results,which shows that the fracture criterion can be used to estimate the fracture toughness of rock under the condition of dry-wet cycles.(3)The energy evolution mechanism of jointed sandstone under dry-wet cycles was studied by the theory of energy dissipation in rocks,and the dry-wet cycle effect and confining pressure effect of peak point energy index were analyzed.The energy damage volution of jointed sandstone under loading can be divided into six stages(initial damage stage,stable damage stage,damage stationary stage,accelerated damage stage,mutational damage stage and the damage and failure stage)according to the law of the dissipated energy evolution in the process of deformation and failure of rock sample.The total energy and elastic strain energy of rock sample at the peak point decrease with the increase of the number of dry-wet cycles,and the fitting formula with the dry-wet cycles conforms to the law of logarithmic decline.The ratio of dissipation energy to elastic strain energy show the continuous mutation and the violent mutation,which indicated crack propagation and strength failure criterion of the jointed sandstone.Both the energy U₁ and the elastic strain energy U^e of the rock specimen increase linearly with the increase of the confining pressure,which generated by the axial pressure at the peak point.The strain energy U₃ that consumed by the confining pressure increases linearly with the increase of confining pressure.The energy U₀ that producted by hydrostatic stress also increases linearly with the increase of confining pressure;besides,U₀ gradually increases with the increase of the fissure number.(4)In this paper,the meso test of jointed sandstone under the dry wet cycle was carried out,and the meso damage evolution mechanism of jointed sandstone under dry-wet cycles is analyzed by particle flow simulation.The internal damage of rock samples was becoming more and more serious due to the dry-wet cycles,and the low-grade pores were gradually changing to the high-grade pores.The overall transition path of the pore structure is the small pore to the medium pore and then to the large pore,the proportion of small pores and small pore throats gradually decreased with the increase of wet-dry cycles while the proportion of medium pores,medium pore throats,large pores,and large pore throats gradually increased with increase of wet-dry cycles.The crack evolution of jointed rock specimens mostly experienced four stages:slow increase under dry-wet cycles,namely,uniform velocity increase,step increase and sudden increase.The number of micro cracks at the peak point increased with the increase of dry-wet cycles.The compression crushing,tensile fracture and dissolution of the clastic particles in the jointed sandstone lead to the development of the micro pore structure,which results in the increase of the porosity and permeability of the rock sample.The evolution of these meso damages within the rock sample is the essential reason for the deterioration of jointed sandstone structures.(5)Based on the continuum damage mechanics,linear elastic fracture mechanics and indoor testing results,the macro joint damage variables,the loaded meso damage variables and the dry-wet cycles damage variables were deduced respectively.The macro-mesoscopic coupling damage tensor was established by using the strain equivalent principle,which can comprehensively consider the macro joint damage,the loading meso damage and the dry-wet cycles damage.Finally,the macro-mesoscopic coupling damage constitutive model of jointed sandstone under the dry-wet cycle was established,at the same time,the initial compaction stage of the constitutive model was modified.The modified constitutive model was used to predict the evolution of stress-strain curve of jointed sandstone under dry-wet cycles,and compared with the stress-strain curve obtained from laboratory test.The reliability of the macro-mesoscopic coupling damage constitutive model of jointed sandstone under the dry-wet cycle was verified in this paper.