Dynamic Response of Underground Pipeline to Chimney Ground Shock Vibration During Chimney Demolition by Blasting Longyuan Lou Jianwu Xu Quanjun Qi Shifu (Department of Field Engineering, School of Engineering and Engineering, PLA University of Science and Technology, Nanjing 210007) The impact of shock vibration waves on the walls of the pipeline and the possible forms of damage were discussed; measures to reduce vibration damage when the chimney collapsed and touched the ground were proposed. With the development of economic construction, due to the requirements of urban construction and factory technical transformation, there are often some Buildings (structures) need to be demolished. Some tall chimneys built in the past cannot meet the requirements of production, technology and environment, especially the chimneys built in the 1950s and 60s have entered the phase of elimination. For such tall structures, the demolition by controlled blasting method often has very obvious advantages. However, in the process of demolition of the chimney using the blasting method, the vibration of the ground medium caused by the explosion of the explosive and the impact vibration of the chimney on the ground when it touches the ground may cause the vibration damage of the surrounding buildings (structures), so the relevant must be considered The practice and research of safety issues have shown that the impact vibration of the taller chimney on the ground when it collapses and touches the ground is even greater than the ground vibration caused by the explosive explosion itself. Therefore, the former is usually the first thing we should pay attention to in order to further study the blasting When the chimney is demolished, the vibration caused by the collapse of the chimney to the ground may damage the underground pipeline. This paper mainly studies the dynamic response of the underground pipeline around the chimney to the impact vibration of the chimney when it touches the ground.
Fund Project: Project supported by the National Natural Science Foundation of China (19872075) 1. The impact of the chimney on the ground when the chimney touches the ground. After the chimney blasts to form a cut, the total energy E of the cylinder begins to fall. It includes its initial kinetic energy E and initial potential energy E potential. Two parts: 1.1 Impulse / s on the ground when the chimney touches the ground; d is the density of the chimney material (kg / m3); R is the outer radius of the chimney top (m); Rr is the outside of the chimney at the blast cut , The inner radius (m); H is the height of the center of gravity of the dumped part of the chimney (m); h is the height of the part above the blasting cut of the chimney (m); the gravity of the dumped part of the chimney (kN); see the general chimney blasting demolition In order to reduce the excessive consumption of explosive energy, the amount of drilling work, and prevent the chimney from generating large vibration damage when it is dumped and touched the ground, the cut length is usually required to be as small as possible on the premise of ensuring the directional dumping of the bearing surface and the chimney The cut size chimney was at rest when it began to dump, and the initial angular velocity k was zero. Therefore, it can be considered that the initial total energy of the chimney when it starts to dump includes only the initial potential energy. The kinetic energy of the chimney after it is completely dumped is converted from its potential energy, and it is approximated that the linear velocity of the chimney when it is in contact with the ground is perpendicular to the ground. Therefore, the angular velocity when the chimney is in contact with the ground during the dumping process is as follows: a unit body is taken along the chimney axis, the instant the chimney is in contact with the ground, and the impulse of the unit body to the ground is the diameter. Vx is the ground contact linear velocity of a unit body along the chimney axis, and Vx = xk, x is the axial distance of the unit body from its support point when the chimney is dumped. Integrate the formula (3) to obtain the chimney ground collapse Impact impulse on the ground 1.2 Observation and analysis of the attenuation effect of the medium around the pipeline on the vibration wave shows that when the chimney collapses and touches the ground, the possible impact damage to the underground pipe fittings is mainly due to the impact of the medium around the pipe forming an active fault and displacement The shear failure of the pipe fittings must be considered. Therefore, when analyzing the stress of the pipe fittings, the role of the medium layer around the pipe fittings must be considered. For the convenience of analyzing the problem, on the basis of confirming the medium around the pipe as a three-phase medium, it is approximately regarded as an elastic homogeneous body, so it can be considered that when the chimney is completely tipped and touched the ground, its kinetic energy will all be transformed into the elastic deformation energy of the medium If the stiffness coefficient k (N / m) of the medium is known, when the maximum compression AW is generated in the medium around the pipe, the maximum force Pm between the medium around the pipe and the collapsed chimney can be calculated: the chimney will fall to the ground When the ground surface shock vibration wave propagates in the medium, it will cause frictional movement between some media particles and cause its permanent deformation. The attenuation of the shock vibration wave on the ground surface is due to the radiation caused by the continuous increase of the wave front area. Attenuation, and attenuation caused by permanent plastic deformation of the medium due to absorption attenuation and shock vibration caused by viscous internal friction.
1.3 The form of load acting on the outer wall of the pipe shell The impact load acting on the ground surface when the chimney falls to the ground and then acts on the underground pipe through the surrounding medium of the pipe. For the axially symmetrical cylindrical pipe, the dynamic response of the medulla is the same when the collision is moved. The types of impact loads on the ground when the chimney collapses and touches the ground are generally of the following types: (1) vertical concentrated harmonic force (2) instantaneous sudden jump platform load (3) linear attenuation load P (t) = upper Pm is The maximum impact force on the ground when the chimney is dumped and touches the ground. 2 Stress analysis of pipe fittings At present, industrial and civil underground pipe fittings mainly include steel welded between pipes (such as gas pipes and oil pipelines). Quality structural pipe fittings, reinforced concrete pipes, petroleum cement pipes, cast iron pipes and other pipe fittings (such as water pipes with lower pressure), etc. Due to the differences in materials of underground pipe fittings, pipe fittings of different materials exhibit different mechanical properties, such as the elastic properties of reinforced concrete pipes, which are much worse than those of steel pipe fittings. And even for pipes of the same material, due to the size and structure, the mechanical properties will show a large difference. Under the impact load, the pipe materials often produce destructive deformation, showing triple nonlinearity, that is Geometric nonlinearity (large deformation), material nonlinearity, and boundary nonlinearity 1 Basically assume that the pipe as a whole is rigid when subjected to impact loads, that is, during the impact of the chimney collapse on the ground, the pipe has no displacement deformation; The ideal liquid flow in the pipe, and when the stress analysis of the pipe fitting is carried out, the influence of the liquid velocity and the viscous force of the liquid on the pipe force is not considered.
2.2 The equation of motion of the pipe fitting under the external load is established at the impact load of the underground pipe fitting. Take a micro-element cross section to analyze the force of the pipe fitting as shown. . Due to the axial symmetry of the pipe fitting, in the cylindrical coordinate system, the axial displacement is zero, and the plane strain state equation is established by Hooke's theorem: and, X = the elastic modulus and Poisson's ratio of the pipe wall shell material Above, R is the outer diameter of the cylinder shell h is the thickness of the cylinder shell; d is the density of the shell material; t / is the radial acceleration. At d0, sin (d3) and for a cylinder with an inner radius r and an outer radius R, the radial stress of the pipe when subjected to an external load P is: Medium, the thickness h of the underground pipe is much smaller than the inner radius One tenth, that is, at the upper f, the radial stress can be approximated as zero: the simplified form of the tangential stress value e equation can be solved from the upper few, and the corresponding e also obtains the maximum value: when emax is greater than the pipe fitting When the material strength limit is multiplied by the safety factor exn, the pipe must be completely destroyed, that is, the scoop is 1.5 ~ 20; for brittle materials: concrete n = 3. In (24), when V1 is regarded as a function of kT It can be seen from the calculation that when kT> 1, with the increase of kT, it will tend to be constant and wide. In fact, the chimney ground contact time T is always greater than a certain constant, and the natural vibration frequency of the pipe is R 2). In engineering practice, the product of the two can achieve kT> 11. Therefore, the main influencing factor of e is The maximum static displacement generated by the equivalent static load of the chimney ground impact force is PmRlEh), so the final influencing factor of ex is the maximum impact force Pm on the ground when the chimney collapses. From the previous analysis, we can see that Pm = 2PHk In order to reduce Pm, the height of gravity center H of the collapsed part of the chimney and the stiffness coefficient k of the ground can be reduced. The effect of the vibration on the vibration damage of the surrounding underground pipes is not very large, and the main reason for the damage comes from the impact vibration of the chimney when it collapses and touches the ground.
When the chimney collapses and touches the ground, the maximum tangential stress em generated on the underground pipe is proportional to the square root of the product of the stiffness k of the soil surrounding the pipe and the potential energy PH of the chimney. , Underground pipe fittings will be destroyed. To reduce the impact vibration of the chimney on the ground, some soft media can be laid on the ground contact area to reduce the stiffness coefficient k; when designing the blasting and demolition scheme of the chimney, it can be considered to use segmented destack Blasting demolition to reduce the impact damage of the potential energy to the ground. In addition, because the above analysis is carried out under the condition that the initial angular velocity of the chimney starts to be zero, this requires the designer to use reasonable blasting parameters to control Make it as close as possible to the actual situation 1 Gao Jinshi, Zhang Qi. Blasting theory and blasting optimization. Xi'an: Xi'an Map Publishing House, 1993 2 Huang Yi. Fundamentals of elastic mechanics and its finite element method. Beijing: Metallurgical Industry Press, 1983 3 Li Shouju, Zhang Liguo, He Qingzhi, etc. Analysis of dynamic response of thin cylindrical shell structure by hydraulic blasting. See: Engineering Blasting Collection (Fifth Series). Wuhan: 4 Yang Shanyuan from China University of Geosciences. Rock blasting dynamics foundation. Beijing: Coal Industry Press, 1993 5 Liao Zhenpeng. Guide to engineering wave theory. Beijing: Science Press, 19966 Tian Huili, Tian Yunsheng. Experimental study on external blasting to remove scale in the tube. Blasting, 1999 (2): 23-277 Liu Hui. Research progress on the impact of adjacent blasting on the tunnel. Blasting, 1992 (2): (Editor Xiong Yun Li Zhuoqing)
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