Criterion Of Plastic Zone Shapes Of Roadway Surrounding Rock And Its Application

Underground roadways in coal mines in China have a huge amount of engineering,with newly excavated roadways reaching tens of thousands of kilometers every year.At present,the incidents of roadway roof accidents account for the highest proportion of all kinds of accidents in coal mines,which seriously threatens the safe and efficient production of coal mines.The occurrence of roadway roof disaster is caused by the deformation and destruction of roadway surrounding rock under stress.The stability of roadway surrounding rock is directly related to the shape and scope of the destruction zone.The stability control of roadway surrounding rock is also based on the shape and scope of the destruction zone of roadway surrounding rock.It is very important for the stability control of roadway surrounding rock to grasp the shape and extension laws of roadway surrounding rock failure area under different stress conditions.Plastic zone is a theoretical failure zone determined by rock strength criterion.Within the precision requirement of mining engineering field,the stability of surrounding rock in underground engineering is usually studied by considered plastic zone as failure zone.Based on the elastic-plastic theory,this thesis studied the evolution laws and the extension direction laws of the plastic zone in the surrounding rock of roadway in the non-uniform stress field by means of theoretical analysis,numerical simulation,comparison with the existing rock mechanics test and field measurement.This thesis deduced the criterion for determining the shapes of the plastic zone of the surrounding rock of circular roadway in the non-uniform stress field and the criterion for determining the direction of the expansion butterfly leaf angle of the plastic zone.And the formulas were applied to the stability analysis of surrounding rock of roadway in high deviating stress environment.The main conclusions and innovative achievements were as follows.1.The criteria forjudging the shape of plastic zone of surrounding rock of circular roadway in non-uniform stress field was established,and three shapes of plastic zone of surrounding rock of circular roadway(circular,elliptical and butterfly)were theoretically defined.(1)The shapes evolution laws of plastic zone of surrounding rock of circular roadway in non-uniform stress field were obtained:when the confining pressure ratio was 1,the shape of plastic zone was circular under bidirectional isobaric condition.With the increase of confining pressure ratio,the plastic zone gradually changed from circular to elliptic.When the confining pressure ratio reached a certain value,butterfly-shaped plastic zone would appear in surrounding rock,and then the shape of plastic zone would expand in butterfly shape.(2)The shape coefficient τ was proposed to determine the shapes of plastic zone of surrounding rock of circular roadway in non-uniform stress field,and the theoretical formula of shape coefficient was deduced:τ=m2/2m1 Where:(3)The criteria for determining the shapes of plastic zone in surrounding rock of circular roadway in non-uniform stress field were obtained.(4)The shapes evolution laws of plastic zone in surrounding rock of roadways with different cross-section shapes showed consistency.When the stress environment and mechanical parameters of surrounding rock satisfied the conditions of butterfly plastic zone,no matter what the shapes of roadway cross-section was,the surrounding rock of roadways would form butterfly plastic zone.Once the butterfly plastic zone was formed,the shape of the roadway section had little effect on the butterfly shape of the plastic zone.The rule of the butterfly plastic zone of the surrounding rock under circular condition was still applicable under the condition of non-circular section.(5)Compared with the regular form of homogeneous surrounding rock conditions,butterfly-shaped failure zones would appear the phenomenon of butterfly leaf missing and penetrating in heterogeneous rock strata combination rock mass.Although these failure zones were different from the standard butterfly-shaped ones intuitively,they still conformed to the mathematical definition of butterfly-shaped failure zone.The butterfly leaf was still the maximum failure depth of surrounding rock.The butterfly criterion was still applicable in the stratified surrounding rock environment.2.The directional expansion characteristics of butterfly leaf in the butterfly-shaped plastic zone were found,and the azimuth criterion for the risk of butterfly-shaped roof roof in the roadway was established.(1)In the non-uniform stress field,the distribution law of the circumferential stress field around the circular roadway leaded to the inverted "V"-shaped angular distribution of the failure strength of the stress at the circumferential position,which first increased and then decreased.Any rock material would be prior to failure at the inverted "V" sharp angle with high stress failure strength(based on the mohr-coulomb strength criterion),and eventually formed the butterfly plastic zone.(2)High stress level and high confining pressure ratio were two necessary stress conditions for forming the butterfly-shaped plastic zone of roadway surrounding rock.(3)The angle between the maximal radius line of the butterfly-shaped plastic zone and the direction of the maximal confining pressure was the angle of the butterfly-shaped leaf,which was expressed as θt.In theory,the formula for calculating the distribution range of the angle of the butterfly-shaped plastic zone was obtained by virtue of the squeeze criterion:(4)The law of butterfly leaf directional expansion in the butterfly-shaped plastic zone was found.Through theoretical calculation,numerical simulation and comparison with existing rock mechanics tests,it was found that when there was a butterfly-shaped plastic zone in the roadway surrounding rock,the plastic zone would extend along the direction of the butterfly-shaped leaf.Based on the analysis of the above three methods,this thesis confirmed the direction of butterfly leaf Angle between 38° to 45°.(5)The azimuth criterion for the risk of butterfly roof fall in roadway was established as follows:β={α-θt}Where,β was the azimuth of the butterfly leaf,a was the azimuth of the maximum principal stress,and θt was the angle of the butterfly leaf.The direction of the main stress would affect the orientation of the butterfly leaf.When the butterfly leaf was located in the roof of the roadway,the risk of butterfly roof roof was the highest3.Based on the mechanical characteristics of different morphological plastic zones,the qualitative relationship between the morphology of the plastic zone and the stability of roadway surrounding rocks was established.(1)Compared with the circular and oval plastic zones,the distribution of the butterfly-shaped plastic zone was not uniform,and the plastic zone was mainly concentrated in the butterfly leaf,with a wide range.On the one hand,more support resistance was needed to bear the broken rock mass.On the other hand,the broken rock in the butterfly leaf area was equivalent to the weak body being squeezed by the surrounding elastic rock,which increased the stress on the support body.After the emergence of the butterfly-shaped plastic zone,the requirements for the length,strength and elongation of the support body were higher and more difficult to control.(2)Compared with the circular and oval plastic zones,the butterfly-shaped plastic zone was sensitive to stress changes.If the change of the underground stress field was equivalent to loading in the direction of the maximum confining pressure or unloading in the direction of the minimum confining pressure,the expansion of the butterfly plastic zone might be induced.At this time,the range of support needed was increased,and the impact force on the support body was caused,which increased the risk of roadway roof fall.(3)Compared with the circular and oval shapes,the appearance of the butterfly-shaped plastic zone was accompanied by the characteristics of non-uniform failure,large scale and stress sensitivity.It increased the requirement of surrounding rock support and increased the risk of surrounding rock roof.Therefore,it was necessary to be alert and take corresponding disaster prevention measures after the occurrence of butterfly-shaped failure of surrounding rock.4.A method to analyze the hidden danger of butterfly roof in roadway based on the shape of plastic zone and the orientation criterion of butterfly leaf was established.(1)This thesis revealed the mechanism of roadway butterfly roof in high deviatoric stress field.In the high deviated stress field,a butterfly-shaped plastic zone was formed in the roadway surrounding rock.When the butterfly leaf azimuth β=0°,butterfly leaf rotation to roadway roof,butterfly roof fall at highest risk at this time.The rock inside the butterfly leaf of roof was seriously damaged,accompanied by huge expansion pressure and strong deformation.When the anchor rod(cable)could not bear the weight of the surrounding rock inside the butterfly leaf,the roadway would have a butterfly-shaped roof fall.(2)High deviatoric stress field provided conditions for the formation of butterfly-shaped plastic zone and increased the risk of surrounding rock butterfly-shaped roof fall.Based on the shape criterion of the plastic zone and the orientation criterion of the butterfly leaf,the shape of the plastic zone and the orientation of the butterfly leaf in the roadway surrounding rock in a certain stress environment were determined and analyzed,so as to master the stability of the roadway surrounding rock in advance.In the roadway layout,try to avoid the location where the butterfly-shape plastic zone occured.If the butterfly-shaped failure occurs in the roadway surrounding rock,timely measures should be taken to strengthen the support.5.The physical processes of butterfly roof fall in mining roadways of Baode Mine and heading roadway under goaf of Guotou Mine were revealed according to butterfly roof fall hazard analysis method.(1)The stress field characteristics and plastic zone shape characteristics of roadway surrounding rock before and after mining influence were studied.The mechanical mechanism of asymmetric large deformation and roof fall of roadway surrounding rock under mining influence in Baode Mine was revealed.The high deviatoric stress environment was formed in a certain range of the side of goaf,the mining roadway was in the high deviatoric stress field,and the surrounding rock formed butterfly-shaped plastic zone.The butterfly leaf rotated with the direction of principal stress.When the butterfly leaf was directly above the roof of the roadway,the risk of butterfly roof fall in the roadway reached the highest level.At this time,the surrounding rock of the roadway was seriously deformed and destroyed,and the roof fall would occur if the support was improper.(2)The stress field characteristics and the shape characteristics of plastic zone in different positions on the side of coal pillar under goaf were studied.This thesis revealed the mechanism of asymmetric deformation and roof fall under the influence of goaf above in Guotou Mine.Affected by coal pillar and goaf above,a high deviating stress environment was formed in a certain range.The original roadway was in a high deviating stress field,and the surrounding rock formed butterfly-shaped plastic zone.The butterfly leaf rotated with the direction of principal stress and the surrounding rock of the roadway roof was seriously deformed and destroyed.At this time,the roof fall would occur if the support was improper.(3)Due to the existence of underground mining space,the stress field was redistributed around the goaf,leading to the formation of a high deviatoric stress environment in the local area.Based on the shape of plastic zone and the orientation criterion of butterfly blade,the method of roadway butterfly roof hazard analysis had a good application effect on the study of roadway layout and surrounding rock stability in high deviated stress field.