Experimental Study On Failure Of Softening Base Unstable Rock In Limestone Area

Unstable rock collapse is a common type of geological hazards in mountainous areas of Southwest China,which has the characteristics of sudden,rapid and catastrophic.It is considered that the possibility of collapse is greater when there are thicker soft base under the unstable rock mass of steep slope.In limestone area,the structural plane of unstable rock is more complex than that of sandstone area.Analysis of the formation and failure mechanism of unstable rock in limestone area is helpful to promote the control of collapse of unstable rock in limestone area and the development of related disciplines.Based on the study of the softening base unstable rock in limestone area,using geomorphology,fracture mechanics,triaxial test and model test of immersion softening,the research results are as follows:(1)Based on the geological background of Zengziyan in Nanchuan,the geological environment and formation factors of unstable rock formation in limestone area are identified.Through the theory of geomorphological evolution,the five stages of formation and destruction of unstable rock in limestone area are revealed.The structural plane characteristics and formation factors of unstable rock with quasi-masonry structure in limestone area are analyzed,and the geological model of unstable rock with soft base and complex structural plane characteristics in limestone area is generalized.(2)Based on the geological model of soft base unstable rock,the fracture mechanics model is established.It is considered that the main fracture propagation forms of the control fissure of softening base unstable rock in limestone area are compression-shear fracture and tension-shear fracture.The expressions of fracture mechanics,failure discriminant and its contribution rate compared with the linear criterion of engineering application are derived.The example analysis shows that the fracture failure of this kind of unstable rock is caused by the crack penetration at the upper and lower ends of the control fissure,the crack propagation at the upper part is obvious under the condition of heavy rain,and the crack penetration at the lower part will aggravate the control fissure.The calculation of the fracture stability of unstable rock considering the crack at the lower part is more realistic and sensitive.(3)According to the mineral content of the soft rock of the base and the proposed mix ratio for construction,cement : medium sand : water : clay = 1:4.27:0.83:0.59,cement mortar blocks with simulated soft rock strength are allocated.Based on the indoor triaxial tests,the trixial tests of the soft rock under dry-water cycling condition is carried out to analyze the variation law of stress-strain curve under various working conditions.The deterioration law of mechanical properties such as axial compressive strength and shear strength with soaking time and the number of dry-wet cycles is obtained.The experiment shows that the failure of soft rock has typical plastic failure characteristics.The plastic characteristics tend to be obvious with the increase of confining pressure and dry-wet cycle.The wet-dry cycling process has a greater impact on the attenuation of cohesion,which is more sensitive to water-rock interaction.(4)Based on the geologic model of unstable rock in limestone area and the ratio of simulated soft rock,a small-scale test model of soft base unstable rock is made,and the immersion softening test of soft base unstable rock is carried out.The strain values of soft base changing with immersion under the action of continuous immersion are measured.The immersion softening process of unstable rock base can be divided into rapid softening stage,softening transition stage and softening stable stage.It is considered that the extension of tension cracks in the control fissure is aggravated by the superposition of self-weight effect and softening effect of soft rock immersion,and that the unstable rock mass will have the tendency of down-dip and slip.(5)Unstable failure test of unstable rock is carried out for the test model of soft base unstable rock.The test results show that the water pressure promotes the cracking of the upper crack of the control fissure,reduces the pressure zone of the base and enlarges the depth of the rock cavity,which aggravates the instability of the unstable rock.The failure process of unstable rock undergoes three stages: the whole collapse and instability stage,the disintegration and sliding stage and the secondary disintegration stage.By analyzing the acceleration time history curve of the whole process of the failure of the soft base unstable rock,the three peak accelerations of the whole process are obtained.Before the collapse of unstable rock,the acceleration increases gradually from the top to the bottom of the unstable rock,and after the collapse,the acceleration decreases gradually from the top to the bottom of the unstable rock.