Experimental Study On The Difference Of Fracture Extension In Different Kinds Of Rocks

Rock is a kind of natural object formed by the accumulation of minerals and cuttings according to certain rules under geological action.Different kinds of rocks show different physical and mechanical properties,and the engineering stability of rock mass mainly depends on the lithology,its internal structural plane and the stress state environment of rock mass.Secondly,the fracture propagation of different types of rocks has not been systematically studied and summarized in the past.Therefore,it is of great practical value and theoretical significance to study the fracture propagation of different types of rocks with permeable fractures.In this research,single-fissure specimens of sandy mudstone,sandstone and limestone with five dip angles(15°,30°,45°,60° and 75°)were produced.A series of conventional load tests,seepage tests and creep tests were carried out using these three types of single-fissure rock specimens to investigate the influences of confining pressures,initial moisture contents,water pressures,fissure dip angles,loading types and rock types on the stress-strain curves,strength and deformation parameters,the crack propagation and failure modes.In addition,numerical simulation studies were performed to investigate the effects of rock types on the cracking behaviors of single-fissure rock samples(1)Under the action of uniaxial load,sandy mudstone and red sandstone show a high similarity in fracture propagation.Except for sandy mudstone specimens with the dip angle of 15° and 30° and red sandstone with the dip angle of 30°show the wing crack propagation mode and the anti-wing crack propagation mode,the other specimens all show the wing crack propagation mode and the limestone shows the columnar splitting failure.The wing crack precedes the anti-wing crack in the upper and lower tip of the permeable crack,and the initiation Angle of the wing crack decreases with the increase of the crack Angle.When the fracture angles are the same,the fracture initiation angles of the sandy mudstone specimens are all larger than that of the red sandstone specimens,indicating that the fracture initiation angles are related to the types of rocks,that is,rocks with high uniaxial compressive strength have significantly smaller fracture initiation angles.(2)Under the action of triaxial load,the growth mode of sandy mudstone is wing crack or anti wing crack,while that of red sandstone is "X" type.However,the main crack is anti wing crack,and that of limestone is anti wing crack.With the increase of confining pressure,the crack initiation angle of the reverse wing of sandy mudstone,red sandstone and limestone specimen decreases.It is found that with the increase of the uniaxial compressive strength of the rock,the angle of crack initiation increases.(3)Under the action of seepage load,the sandy mudstone specimen adopts the wing crack or anti-wing crack propagation mode;The red sandstone shows the anti-wing crack propagation mode.The fracture growth of limestone specimens under the action of seepage shows no obvious regularity,and there are wing cracks,anti-wing cracks and shear cracks not along or near the permeable crack tip.There are few secondary cracks in red sandstone,and the number of them is significantly less than that of sandy mudstone and red sandstone.Moreover,with the increase of pore pressure,the number of cracks in sandy mudstone and limestone after failure generally increases.No obvious regularity of crack propagation Angle was found in sandy mudstone and limestone.Under different pore pressure conditions,the average initiation Angle of the anti-wing crack in red sandstone specimens was between 125.5°-126°,indicating that the pore pressure had no influence on the initiation Angle of the anti-wing crack.(4)Under uniaxial creep load,the sandy mudstone and red sandstone specimens show the wing crack or anti-wing crack growth mode;when the crack dip Angle is ?45°,it shows the wing crack growth mode;when the crack dip Angle=75°,it shows the anti-wing crack growth mode.The limestone is the "H" shape crack propagation mode of the wing crack or the coexistence of the wing crack and the anti-wing crack.The initiation Angle of the wing crack of sandy mudstone,red sandstone and limestone decreases with the increase of the crack Angle.With the increase of fracture dip Angle,the number of cracks in sandstone,red sandstone and limestone specimens after failure decreases on the whole.When the crack dip is the same,the number of cracks generated after the failure of limestone specimen is the highest,followed by sandy mudstone,and the number of cracks generated after the failure of red sandstone specimen is the lowest.(5)Under the action of triaxial creep load,the sandy mudstone specimens under different confining pressures show a growth mode dominated by wing crack,wing crack or wing crack and wing crack.The specimen of red sandstone is in the mode of wing crack or anti-wing crack propagation.The limestone specimens all have inverse wing crack propagation mode.When the confining pressure is higher,sandy mudstone,red sandstone and limestone are more likely to show the anti-wing crack propagation mode.Observation of the anti-wing crack Angle at the permeability fracture tip of sandy mudstone,red sandstone and limestone shows that the initiation Angle of the anti-wing crack decreases with the increase of confining pressure.Secondly,with the increase of uniaxial compressive strength under the same confining pressure,the initiation Angle of anti-wing crack increases.(6)The crack types generated by the failure of sandy mudstone,red sandstone and limestone specimens under different loads can be divided into 11 types,including six tensile cracks,two shear cracks,one composite crack,far-field crack and surface spalling.The crack initiation Angle calculated by the maximum circumferential tensile stress theory is not completely consistent with the conventional uniaxial and uniaxial creep test values of the three rock specimens,but the relationship between the crack initiation Angle of the wing crack and the prefabricated crack initiation Angle is the same as the theoretical value,and the crack initiation Angle of the wing crack decreases with the increase of the crack initiation Angle.(7)Conventional uniaxial numerical simulation of three rock specimens with 45°see-through cracks showed that the sandstone mudstone and red sandstone were dominated by wing cracks,while the limestone specimens had both wing cracks and anti-wing cracks,and the wing cracks occurred before the anti-wing cracks.The generation and propagation of wing cracks are mainly influenced by tensile shear stress,while the generation and propagation of anti-wing cracks are mainly influenced by compression shear stress.