| Due to prolonged physical and chemistry weathering,a large number of discontinuous and uneven joints exist in rock mass,which makes rock mass in the natural state a geological body with complex characteristics.When the stress wave generated by impact load or explosion propagates in the rock mass,discontinuous joints influence the stress wave propagation law and leads to wave attenuation.In addition,the existence of joints change the strength and stiffness of rock mass structure,which causes a great threat to the safety of geotechnical engineering.It is meaningful for people to deeply understand the influence of joint on the rock mass,evaluate the dynamic response of rock mass structure.Thus,the geotechnical engineering risk assessment and prediction can be properly carried out,so as to reduce the hazards of the geotechnical engineering disasters.Previous studies have shown that the properties of joint morphology have the great effect on the physical and mechanical properties of the rock mass structure.Joint matching coefficient(JMC)is an important parameter to describe the morphology features of joint surface.This study takes the rough joint samples with different JMC as the research object to investigate the influence of joint structures,such as joint matching condition,joint surface fluctuation,joint distribution and joint thickness.The dynamic mechanical properties of joint sample are analyzed and the dynamic damage effect of joint surface is systematically investigated.In order to simulate and quantify joint samples with different morphology features,the casting method which is simple and repeatable is applied to prepare the samples.Considering the probability of joint surface damage under impact loading,the rough joint of samples is designed as consist of a certain height and a different number of ladders.There are four parameters concerened with joint characteristics discussed in this paper,including JMC,joint distribution,joint surface fluctuation and joint thickness.The first two parameters are controlled by adusting the number and top surface size of ladders.And the latter two are related to the ladder angle and ladder height,repectively.Split Hopkinson Pressure Bar(SHPB)apparatus is adopted to conduct impact test on joint samples.Multiple low amplitude impact loads are applied to specimen in order to avoid the overall failure of samples before the damage of joints.The pendulum device could generate a relatively stable dynamic load with low amplitude by adjusting the angle of pendulum,which makes the device as the dynamic loading component in SHPB instead of nitrogen equipment.The experiment results are analyzed in two ways which includes the stress wave propagation and dynamic mechanical properties.First of all,the strain data is processed to obtain the transmission coefficient and stress wave energy.The influence of joint properties on the stress wave attenuation is studied in this part.Secondly,the analysis of mechanical properties starts from the stress-strain relationship of joint sample.The resistance to deformation of different samples are compared based on the dynamic secant modulus.The overall deformation of joint specimen is regarded as containing the joint part and intact rock part.Based on this assumption,the stress-closure curve of sample is thus obtained and the joint stiffness coefficient is introduced to evaluate the normal closure behavior of joint.According to a series of parameter studies,it is shown that the morphology characteristics and geometric features of joints have influence on both the stress wave attenuation and the deformation behavior of joint sample.Finally,the joint damage effect is emphasized in the dynamic mechanical analysis.3D scanner is used to record the actual morphology change of rough joint surface.The digital model of joint surface is analyzed in the software and results indicate that the damage mainly concentrates on the position of ladders.The increase of joint roughness as well as the decline of joint fluctuation can also be found.In addition,a new parameter called joint volume change rate is introduced to compare the damage condition of joints.The characteristics of joint surface are confirmed to be related to the failure pattern of amples and damage degree of joints. |
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