Study on mechanical characteristics and damage failure of coal mass from roadway side wall under loading and unloading stress path

To explore side wall breakage mechanism of coal roadway for excavation unloading,laboratory tests of coal mass in roadway side wall under loading and unloading stress paths were conducted,and its mechanical properties and damage failure were analyzed.The results indicated that in conventional triaxial compression test,with the raise of confining pressure,failure characteristics of coal mass would be changed from brittleness to ductility.Under stress path of confining pressure reduction,failures were more likely to happen and all of them were brittle failure.Coal mass strength was very sensitive to confining pressure variations and its peak value was lower under unloading stress path.Deformation modulus decreased with the reduction of confining pressure,change was gentle in the initial stage,but the downtrend was aggravated continuously by later.Poisson ratio increased as confining pressure conduction and its variation trend agreed with modulos of deformation.Under unloading stress path,coal mass failure was a composite state,in which shear failure was the chief form,but splitting failure also might appear.Fracture angle increased with the raise of initial confining pressure along a parabolic curve,but its value was obviously smaller under unloading stress path.Under unloading stress path,the relationship and variation trend between damage factor and confining pressure agreed with poisson ratio.Confining pressure difference ratio decreased with the raise of initial confining pressure under unloading stress path.