Physical Model Test On Rockburst Of Tunnels Under Thermo-mechanical Coupling Effect

As tunneling and underground engineering proceeds to greater depth,the high geo-stress and high geothermal backgroundshas evolved into some kind of critical problem facing tunnel engineering.The original rock stress field is one of the inner factors which influence the rock burst occur,and temperature effect under high ground temperature may also affect the rock burst.So it is very necessary to study the regularity and mechanisms of the rock burst in brittlesurrounding rock of tunnel under the effect ofthermal-mechanical coupling field.Based on the Similarity Theory,this study initialed from the fabrication of experimental facilities and operation skills,and then made test models by utilizing the best proportion of rock burst similarity materials which most effectively reflects the brittleness and toughness.Through laboratory physical tests we could simulate the excavation process and the time effect and also the way how the load affects,thento study the failing process,the mechanismsand the characters of the failure in surrounding rock of tunnel,especiallythe brittle failure such as rock burst.Physical model testresults show the main research results are as follows:(1)Selecting quartz sand,plaster,cement,water reducing agent,retarder and water as the similar material of rock burst,and evaluating the rock burst tendency by brittleness coefficient K and impact energy indexW_B based on the physical and mechanical parameters test.Moreover,analyzing the mechanical parameterssuch as elastic modulus E,Poisson’s ratio?and stress-strain curve of similar material,determiningthe optimal proportionof similar materials and the best rockburst-potential particle diameter to make good preparation for the great scale of physical model with great hardness and brittleness.(2)Fabricating the tunnel excavation system and temperature heating-control system in dependently,and applied them to 4 physical model tests.Those tests verified that the originally fabricated system successfully achieved the tunnel excavation,unloading,and temperature heating and controlling.It also accurately simulate the particular method of“excavation after loading”during the physical model test,and simulating the thermal-mechanical coupling field under the combined action of high ground stress and high ground temperature.(3)On the basis of the Two Dimensional Geological Mechanics Simulation Test Stress Loading System,and based on the similarity theory and similar materials.Using the strain measuring system,the temperature control system,the acoustic emission system,the tunnel excavation system and the camera system,designing the physical model test of tunnel rock burst under the action of the thermal-mechanical in details.Then,to carry out the physical model test of tunnel rock burst with four different temperature(20℃and 40℃,60℃,80℃)under the same stress condition.Eventually,the paper summarized a set of comprehensive physical model test schemes of whole test process range the models fabrication to the end of the test.(4)The test results show,In the same stress field(the lateral pressure coefficientλ=2)and four different temperature(20℃and 40℃,60℃,80℃),after tunnel excavation,the surrounding rock of vault and arch shoulder prone to brittle cracking destruction caused by pressure.And the surrounding rock nears the arch wall tend to manifest bending fold,cracking and other brittle destruction.Moreover,the arch bottom is easy to fail as cracking and shearing,etc.(5)With the increase of temperature,the strain and stress concentration occurred soon and elastic strain energy concentration quickly in surrounding rock after tunnel excavation,the time of energy accumulation will be shorter with the increase of temperature.As the stress,strain and energy of surrounding rock growth to a certain limit,the energy of the surrounding rock suddenly released by the form of rock burst.The stress and strain show wave phenomenon during the process of brittle failure,such as rock burst occurred in surrounding rock,The higher the temperature is,the larger the stress and strain fluctuation will be in the surrounding rock.Higher temperature improved the surrounding rock hard and brittle,the higher the temperature the smaller strain of surrounding rock is before tunnel excavation.In the temperature of 20℃and 40℃,60℃,the acoustic emission energy of the brittle failure of tunnel surrounding rock will go up as temperature increases,and the number of three-dimensional positioning acoustic emission source points increases along with the rise in temperature,and distributes mostly on the tunnel wall.(6)Physical modeling tests show that the rockburst is progressive,it starts from small range,small grain size of granular drop and ejection,then the scope of flake and fragmental peeling ejection,and a massive rock burst in the end.This progressive characters of rock burst relates to the temperature field,the higher the temperature,the highe ragglomeration level of stress and energy in surrounding rock,also more abrupt the rockburst is,so as shorter the process from tunnel excavation to rock burst of surrounding rock.As temperature increases,the diameter and quantity of the stripped and ejected fragments become much larger.At the same time,the surrounding rock’s brittle failure occurs more suddenly on account of the rise of temperature.(7)In the situation of the four temperature field of 20℃and 40℃,60℃and80℃,The brittle failure of the tunnel is very severe in 60℃and 80℃,even big fault blocks jumped out under the 60℃field.Under the temperature of 80℃,firstly appears some small brittle failure in a small scale,and then after the surrounding rock’s bend and bulge failure,surrounding rock directly failure by brittle crack in a very short time without warning.The brittle failure of surrounding rock is comparably not that severe under 20℃and 40℃field,but the damage aera of tunnel under 40℃field is larger than the former,so as the peeling ejection debris amount.After brittle failure as rockburst,the surrounding rock keeps in stable state under the temperature of 20℃and 40℃,while it keeps self-stabilized under the temperature of 60℃and even occurs serious failure when the temperature rise to 80℃.To sum up,as the temperature increases the characteristics of brittle failure of surrounding rock is more obvious,and the higher the temperature is,the rock burst of tunnel is more intense under the thermal-mechanical coupling effect.(8)The existence of temperature field haschanged both physical and mechanical parameters of materials,leading to the distribution of additional stress field induced by temperature.Under the same ground stress condition,the higher the temperature,the higher additional stress induced by temperature,the higher concentration of stress level and energy accumulated in surrounding rocks;At the same time,the temperature affects both the physical and mechanical properties of rock,enhancing the hard brittle property of surrounding rock,which is helpful to higher elastic strain energy concentration.Test results have proved that the increase of the temperature boosts the intensity of rock burst in surrounding rock.