# Investigation of Rock Joint and Fracture Influence on Delayed Blasting Performance
## 2 Current Status of Blasting on Fractured Rock Mass  
During open-pit blasting, rock joints are usually the primary locations prone to blasting damage, as they are the weakest points. At joint locations, a series of complex transmission and reflection waves can be generated along the transformation of blasting shock waves to stress waves. Subsequently, stress waves induce nonlinear dynamic responses in engineering bodies. [PERSON] et al. [1] used a time domain analysis and classical joint nonlinear normal deformation constitutive model (BB model) to calculate and analyze the force along joint ends and deduced the formula of stress change under the change in stress wave. [PERSON] et al. [2] analyzed the elastic longitudinal wave propagation in nonlinear rock joints based on the time domain analysis proposed from the wavefront momentum conservation theory and displacement discontinuity method. Subsequently, the researchers found the nonlinear deformation properties of joints have a great influence on transmitted and reflected waves. [PERSON] [3] established a one-dimensional analysis model of jointed rock mass and carried out a parametric analysis. In addition, [PERSON] [3] used continuous and discontinuous element methods to explore the influence of joint parameters on blasting and obtained a negative correlation between the extent of joints in a blasting area and the blasting effect. Hence, it was found that the influence of joint parameters on the blasting effect is also a crucial aspect of rock dynamics.  
As the blasting effect of highwall benches is also a multi-factor and complex system engineering topic, there are fewer related studies on the reasonable use of delayed initiation to achieve optimized shock wave collision in mine blasting. [PERSON] et al. [4] evaluated the spatial distribution of blast energy and the blasting effect at the initiation position based on blast energy transmission and blast stress field distribution combined with simulation and experiments. [PERSON] et al. [5] used the degree of tensile and compressional shear damage to calculate the crushing range and combined it with a step blasting field test to compare the blasting block size distribution under different blasting methods. The researchers then determined an optimized design for the number and position of blasting initiation points to significantly improve blasting efficiency. Therefore, the setting of delayed blasting in a hole is critical for efficient mining, slope stability, and capacity utilization improvement in open-pit mines.  
Most of the previous differential time calculation methods are based on semi-empirical formulas obtained from row initiation, and most of them have focused on reducing the blasting vibration. [PERSON] and [PERSON] [6] used a single-hole blasting curve as the waveform source and conducted regression analysis based on a Fourier series. Subsequently, they used a multi-level loop nested logic language to write a MATLAB program and obtained all possible syntheses of an eight-stage differential blasting. The vibration curve was consistent with the maximum calculated synthetic vibration velocity of 0.26 cm/s under the field design. With the continuous development of digital mining, the large-scale application of air spacers and digital electronic detonators has made deep-hole blasting technology more flexible and diverse [7]. However, there is still a lack of detailed research on joints and extensions.  
Based on the above research results, there are limited studies on the dynamic characteristics of particles around rock mass joints under blasting [8]. Therefore, this paper used the delayed blasting on a highwall bench of the Baiyunebo open-pit mine as a field study and investigated the dynamic response of jointed and fissured rock mass under blasting load to meet the blasting requirements of a modern, large open-pit mine as well as improve the utilization rate of blast energy.