Analysis Of The Deterioration Mechanism And Characteristics Of Drying-Wetting Cycles On Gypsum Rock

Gypsum rocks are distributed in most basins in Northwest China and in the Three Gorges area.In recent years,with the large-scale investmenet of the national engineering construction,gypsum rocks were encountered frenquently in many engineering construction processes,which bring many problems to the construction.Gypsum rocks were found in the surrounding rock of the Shiziya tunnel in Badong County of Hubei Province,which cause serious damage to the structures in only 2 years after the completion of the tunnel.The additional investment is increased by about twenty million yuan.During the construction of the tunnel,the excavation breaks the balance state of the natural stress of the rock mass,and produced the secondary fissure in the surrounding rock.The fissure provides the channel for the infiltration of groundwater,and the ground water has varius deterioration on the mechanical properties of the rock.Drying-wetting cycling is one of the most common water-rock interactions.At present,the researches on the deterioration characteristics of rocks under drying-wetting cycles are mostly focused on sandstone at home and abroad.Many engineering diseases indicate that it is also of great theoretical and practical significance to research the deterioration characteristics of gypsum rocks under drying-wetting cycles.Based on this,this paper takes gypsum rocks in Jialingjiang formation near Shiziya tunnel in Badong County as research object.The deterioration characteristics of physical and mechanical properties of gypsum rocks under drying-wetting cycles are studied by using laboratory tests,theoretical analysis and PFC simulation.The main contents and results of this paper are as follows:1.The failure characteristics of the Shiziya tunnel and the distribution of gypsum rock layers are briefly described.The regional geological background of the study area is summarized.Through basic physical tests,X diffraction and water absorption/dehydration test,the basic physical properties,mineral composition and water absorption and dehydration characteristics of gypsum rocks are clarified.Samples are selected for subsequent drying-wetting cycling deterioration test.2.Drying-wetting cycling deterioration tests for gypsum rocks are carried out.The water absorption/dehydration test,mercury injection test,uniaxial compression test,acoustic emission test,Brazil splitting test and triaxial compression test is conducted on the gypsum rocks under different drying-wetting cycles,in order to analyze the degradation effect of drying-wetting cycles on the water absorption/dehydration characteristics,pore structure characteristics and mechanical properties of gypsum rocks.The results showed that:（1）With the increase of drying-wetting cycles,the cumulative water absorption of gypsum rock increased,and the maximum pore size and average pore size increased,and the separation became better,and the pore distribution gradually inclined to coarse pores.（2）In general,the uniaxial compressive strength,elastic modulus,tensile strength,cohesion and internal friction angle of gypsum rocks tends to decrease with the increase of drying-wetting cycles.The influence of drying-wetting cycles on the mechanical parameters of gypsum rock is different.The order of influence is elastic modulus>tensile strength>uniaxial compressive strength>cohesion>internal friction angle.（3）During the process of drying and wetting,the interaction and promotion of hydro-physical and hydro-chemical influencing effects changed the crystal structure and pore structure of gypsum rock,decreased the strength of crystal,and increased the porosity,resulting in deterioration of mechanical properties of gypsum rock.3.Through acoustic emission test of uniaxial compression of gypsum rock under different drying-wetting cycles,the influence of drying-wetting cycles on acoustic emission characteristics of gypsum rock is analyzed from stress-strain curves,acoustic emission events,ringing counts,and acoustic emission energy.The results showed that:（1）With the increase of drying-wetting cycles,the post-peak stress-strain curve changes from a steep drop to a slow drop,which indicats the gypsum rock property changes from brittle to ductile.The form of cumulative quantity（number of acoustic emission events,ringing counting,AE energy）-time curves changes from growth curve form to logarithmic curve form.（2）In the 0 and 5 drying-wetting cycles,the"peak"phenomenon in compaction stage and the early elastic stage of stress-strain curve is not obvious,the surge phenomenon of AE characteristic quantity in post peak stage appears in the"centralized";in the 10,15 drying-wetting cycles,the"peak"phenomenon in compaction stage and the early elastic stage of stress-strain curve is obvious,the surge phenomenon of AE characteristic quantity in post peak stage appears in the"scatter".（3）When there is no drying-wetting cycle,the distribution of each characteristic quantity in each stage is different.After 15 drying-wetting cycles,the order of all the characteristic quantities in each stage is compaction stage<elastic stage<accelerated failure stage,and all the characteristic quantities are dramatically different in compaction stage and accelerated failure stage.4.Based on the theory of statistical damage,the statistical damage constitutive model of strain softening for rock is established and applied to the fitting of stress-strain curves of gypsum rock.The main research contents and conclusions are as follows:（1）The rock deformation and failure process is divided into three stages of initial compaction,elastic and stress damage,and the three parameter Weibull distribution is adopted to represent the probability distribution of micro-unit strength,then a statistical damage constitutive model which can fit the whole press of the stress-strain curve is established,and its fitting effect is good.（2）According to the evolution characteristics of damage variable D,Richards growth curve model is adopted to fit the damage variable D.Then,a statistical damage constitutive model based on evolution equation of damage variable is established.The fitting effect of this model is better.（3）According to the relationship between the fitting parameters of the damage variable and the number of drying-wetting cycles,the construction method of the stress and water-rock coupling damage constitutive model of gypsum rock is proposed.（4）Accumulative ringing count is used as a damage variable to establish a statistical damage model of gypsum rock under uniaxial compression.However,compared with the statistical damage constitutive model based on damage variable evolution equation,its fitting effect is slightly worse in the post-peak stage.（5）Based on the statistical damage theory,a nonlinear fitting formula of stress-strain curves is established,which can not only directly abtain the fitting results of stress-strain curves,but also obtain the fitting results of damage variables.The stress-strain curves of gypsum rock under triaxial compression are fitted,and it is proved that the fitting effect of this method is good and can be applied in practice.5.Based on two dimensional particle flow model(PFC^2D),PFC^2D models of gypsum rock under different drying-wetting cycles are established.The degradation of meso-mechanical characteristics of gypsum rock under drying-wetting cycles and the deterioration of drying-wetting cycles on gypsum rock with single fissure are studied.The main conclusions are as follows:（1）The confining pressure and the number of drying-wetting cycles have an influence on the mesoscopic damage characteristics of the numerical samples.With the increase of confining pressure,the distribution of meso-bond gap and meso-shear cracks tends to be"dispersed"from"concentration".With the increase of the number of drying-wetting cycles,the distribution of meso-bond gap and meso-shear cracks tends to be"concentrated"from"dispersion".（2）Both under the conditions of uniaxial compression and triaxial compression,the number of microcracks decreases with the increase of drying-wetting cycles.The confining pressure has an inhibitory effect on the growth of the meso-tensile crack,and is beneficial to the growth of the meso-shear crack.（3）The deterioration law of the micro-parameters of gypsum rock is basically the same,which is the micro-parameters decrease with the increase of drying-wetting cycles.The effect of drying-wetting cycles on the meso-parameters of gypsum rock is different,and the order of influence is:joint modulus>joint internal friction angle>joint cohesion>joint tensile strength>joint friction coefficient.（4）Both the fissure angle and the number of drying-wetting cycles have effect on the uniaxial compressive strength of gypsum rock with single fissure.The uniaxial compressive strength of single fissured gypsum rock with fissure angleα=30°always is the minimum,but the deterioration effect of drying-wetting cycles on the single fissured gypsum rock with fissure angleα=60°is the most strong.The order of the deterioration effect of drying-wetting cycles on the single fissured gypsum rock sorted by fissure angle is:α=60°>α=0°>α=30°>α=45°>α=75°>α=90°.