Research On Mechanical Properties And Damage Evolution Of Coal And Rock Under True Triaxial Unloading Condition

In order to simulate the excavation process of deep underground engineering more truly,the change law of mechanical properties of surrounding rock caused by deep coal mining is studied.Based on true triaxial test equipment and acoustic emission monitoring equipment,this paper takes the intermediate principal stress,unloading rate and unloading path as research variables to conduct true triaxial unloading tests on coal and rock.Meanwhile,the damage theory is introduced to conduct in-depth research on the mechanical characteristics and damage evolution law of coal and rock under three conditions.The following research results were obtained:(1)When the intermediate principal stress is increased from 4 MPa to 10 MPa,the maximum principal strain value of coal and rock under unloading is slightly increased.The intermediate principal strain value and volume strain value decreased significantly.The minimum principal strain value decreased slightly.The failure mode of coal rock is shear failure when the intermediate principal stress is small,and tensile failure when the intermediate principal stress is large.The mechanical data of coal and rock unloading failure are fitted and analyzed by three strength criteria.From the goodness of fit,the three-parameter octahedral shear stress strength criterion can better represent the strength characteristics of coal and rock unloading failure under different intermediate principal stress conditions.(2)When the unloading rate increases from 0.05 MPa/s to 0.4 MPa/s,the maximum and minimum principal strain values of coal and rock during unloading decrease significantly,while the intermediate principal strain values increase slightly and the volume strain values decrease significantly.The failure mode of coal rock at low unloading rate is shear failure,while that at high unloading rate is tension-shear compound failure.In terms of goodness of fit,the modified Drucker-Prager strength criterion can better represent the strength characteristics of coal and rock under true triaxial unloading failure at different unloading rates.(3)Three unloading paths are designed according to different unloading directions: Path 1is single-sided unloading in the direction of ;Path 2 is single-sided unloading in the direction of and .Path 3 is single-sided unloading in the direction of and loaded in the direction of .The maximum principal strain value of coal rock during unloading is path 3 >Path 1 > Path 2,and the intermediate principal strain value,minimum principal strain value and volume strain value are path 1 > path 3 > path 2.The unloading failure of coal and rock under path 1 is shear failure,the unloading failure under path 2 is plate tensile failure,and the unloading failure under path 3 is tensile shear compound failure.In terms of goodness of fit,the modified Drucker-Prager strength criterion can better represent the strength characteristics of coal and rock when unloading failure under different unloading paths.(4)The larger the intermediate principal stress is,the larger the cumulative ringing count and the higher the energy of coal rock during unloading failure;The larger the unloading rate,the larger the cumulative ringing count and the higher the energy.Path 1 has the smallest cumulative ringing count and the lowest energy,followed by path 3.Path 2 has the largest cumulative ringing count and the highest energy.By calculating and analyzing the characteristic values of acoustic emission parameters,the internal fracture types of coal and rock are identified.It is found that the internal failure characteristics of coal and rock are consistent with the surface failure characteristics under different unloading conditions.(5)When the intermediate principal stress increases from 4 MPa to 10 MPa,the total energy,elastic strain energy,energy storage limit and dissipated energy increase by 68.9%,165.3 %,50.7 % and 59.8 % respectively.When the unloading rate increases from 0.05 MPa/s to 0.4 MPa/s,the total energy,elastic strain energy,energy storage limit and dissipated energy of coal and rock samples during unloading are reduced by 52.4 %,24.1 %,21.6 % and 76.1 %.Path 3 has higher total energy,elastic strain energy,energy storage limit and dissipated energy during unloading than the other two paths,followed by path 1 and path 2.(6)Based on the damage mechanics theory,the damage variables and were defined by the cumulative ringing count and dissipated energy parameters respectively,and the damage evolution equation was derived to quantitatively analyze the damage evolution process of coal and rock under various unloading conditions.The damage evolution process of coal rock under different unloading conditions can be roughly divided into three stages: the early stage of slow growth,the middle stage of steady growth and the late stage of rapid growth.(7)Through comparative analysis of the unloading failure model derived from the two damage variables,it is found that the two models have certain applicability to the failure process of coal and rock under different unloading conditions.From the point of view of average deviation rate,the model based on is more suitable to describe the unloading process of coal and rock than that based on .