Mechanism Analysis Of Joint Shear Behavior Under Different Loading Rates

Rock joint is a type of complex engineering medium widely existed in geotechnical engineering.Joints cause complex stress distribution in the rock mass,which has a dominant influence on the stability of the engineering rock.The instability of rock joint along the joint surface and rock slope collapse induced by dynamic loads such as earthquakes and blasting loads are frequent disasters.Studying the mechanical properties of shear joints under dynamic load is important for preventing the slope instability in geotechnical engineering.The significant characteristics of seismic and blasting loads are variable loading rates.Accordingly,laboratory experiments to study the shear mechanics of joints under different shearing rates can not only provide advisory opinion for the construction,but also it can better improve the mechanics theory of shear.Four groups of rock-like artificial joints with different joint surface characteristics are subjected to direct shear tests under different normal stress conditions and different shearing rates,and the high-speed camera was used to collect the image of the shear deformation process of the joint sample.Specimen types include intact specimen,planar joint,and joint specimen with only a single undulating body with angles respectively of 15° and 30°.The test is conducted under four normal stress conditions and a range of shearing rates from 2.3 to 47.2mm/min.Two experimental schemes of single shear and multi-stage shear for specimens with different joint surface characteristics.The experimental data analysis is carried out from two aspects of joint shear mechanical indexes and damage mechanism.Summarizing the constitutive relationship of joints under different conditions by analyzing the shear stress-shear displacement curve of the whole shearing process of joints rock,and comparing the curve shapes of each group of samples.Then,based on the shear stress curve,the shear mechanical indexes of different sample groups such as shear strength,friction coefficient,shear residual strength and shear stiffness are further calculated,and the variation law of indexes with shearing rate is summarized.The results show that the shearing process of rock joint roughly obeys two models of shearing-yield and shearing-off.The undulation angle and the normal stress determines the type of the shear stress curve model.When the shear stress curve is closer to the shearing-yield type,the shear strength shows a negative correlation with the shearing rate.When the shear stress curve is closer to the shearing-off type shows the opposite law.Based on the experimental data,the segmented empirical formula with shearing rate as a variable is obtained.Combined with the conclusion of failure mode,it is considered that the sliding friction strength of joint surface decreases with the increase of shear rate,and the self bond strength is opposite.In addition,the shear mechanical index is also mainly affected by the normal stress and the undulation angle and the secondary influence of the shearing rate.Finally,combined with the image recording of the high-speed camera and the image of the joint surface after shearing,the damage appearance of the joint surface under different conditions is analyzed,the damage area and crack propagation laws of different sample groups are summarized.The results show that the joint surface damage is more serious with the increase of shearing rate,and the undulating joints with different failure modes obey different damage modes.With the increase of normal stress,the failure mode shows climbing effect under low normal stress and gnawing-off effect under high normal stress.With the increase of undulation angle,the failure mode shows shear abrasion with small undulation angle and shear Cut and grind with large undulation angle.The cracks of the intcat specimen are mainly developed in the unconstrained free zone,and the cracks of undulating joints are generally generated and developed in the concentrated area of climbing effect.At the same time,as the shearing rate increases,the more serious the damage is,the more cracks are generated in the rock joint,and the more fragmented destruction of rocks.